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Experimental investigation of sand particle erosion in a 90° elbow in annular two-phase flows
Wear 438-439 (2019) 203048
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Experimental investigation of sand particle erosion in a 90° elbow in annular two-phase flows
T
Peyman Zahedia,∗, Mazdak Parsib, Alireza Asgharpoura, Brenton S. McLaurya, Siamack A. Shirazia a b
Erosion/Corrosion Research Center, The University of Tulsa, Tulsa, OK, United States National Oilwell Varco, Houston, TX, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Solid particle erosion Multiphase flow Annular flow Gas-liquid flow Elbow
Upward gas-liquid-solid flows are often observed in oil and gas transmission pipelines. The presence of solids in such flows is a concern, as it can cause serious damage to pipe fittings, which are prone to erosion. These are normally components such as elbows and tees that can change the flow direction. The current work presents the results of an experimental erosion campaign including 56 tests in a 101.6-mm standard elbow (radius of curvature of 1.5) under gas-sand and gas-liquid-sand flow conditions. The latter included 38 tests (30 distinct conditions) mostly in the range of annular and unstable annular flows. Thickness loss measurements were made using a non-invasive ultrasonic technology instrument at 8 different points along the outer bend of the elbow to investigate effects of various factors on erosion. Additionally, flow visualization and paint erosion studies were performed to examine liquid distribution as well as the particulate phase behavior. Generally, when superficial liquid velocity increased, erosion ratio decreased for low liquid rate conditions. This trend, however, reversed for higher liquid rates. Also, the paint erosion experiments revealed that the location of the maximum erosion was around 40°–50° on the middle outer bend, for all cases examined.
1. Introduction
can acquire a high velocity close to that of the gas in the core and cause erosion, when impacting a wall. The complexity of the erosion phenomenon in gas-liquid-solid flow has led researchers to conduct experiments and collect data on the erosion distribution and magnitude in 90° elbows. These data can be ultimately used to develop erosion prediction models and validate Computational Fluid Dynamics simulations. Next, a review of previous studies on erosion in multiphase flow is provided.
When gas and liquid are present in a pipe, they distribute themselves in a specific configuration called the flow pattern. In vertical upward gas-liquid flow, the flow patterns are generally considered as dispersed bubble, bubbly, slug, churn, annular and transition between some of these patterns. Annular flow is normally observed in gas-condensate production pipelines and characterized by the presence of high-velocity gas flow in the pipe core and a thin liquid film around the pipe circumference. The high-velocity gas exerts a high interfacial shear on the liquid film close to the pipe wall, which in turn causes the formation of the so-called “disturbance waves” at the interface of the gas and liquid. The high shear also causes the detachment of some droplets from the disturbance waves and these droplets become entrained into the gas core. When solid particles are present, it is believed that the same mechanism that entrains liquid droplets into the gas core can also cause the entrainment of the solid particles. This is based on the experimental observation of McLaury et al. [1]. They demonstrated that the liquid and solid particle entrainment ratios were similar, when particle and droplet sampling along the radial axis of a pipe under annular flow conditions was employed. Understandably, the entrained solid particles
∗
2. Background 2.1. Electrical resistance (ER) probe An ER probe is an intrusive instrument, which measures the thickness loss using two elements namely sample and reference. The sample element is in contact with the flow, whereas the reference element is protected inside the probe from impact of solid particles. When impacted, the sample element experiences metal loss, which changes its electrical resistance. Thickness loss of the sample element is calculated by comparing its electrical resistance to that of the reference element. Pyboyina [2] utilized an ER probe flush mounted at 45° in a 2-inch standard elbow to measure thickness losses. Experiments were
Corresponding author. Currently with Bechtel Oil, Gas & Chemicals, Inc., Houston, TX, United States. E-mail address: [email protected] (P. Zahedi).
https://doi.org/10.1016/j.wear.2019.203048 Received 25 June 2019; Received in revised form 8 September 2019; Accepted 9 September 2019 Available online 11 September 2019 0043-1648/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 2. Maximum erosion under gas-liquid-sand annular flow conditions: 300μm sand, 76.2-mm vertical elbow; data extracted from Refs. [7,8].
Fig. 1. Effect of pipe diameter and superficial liquid velocity on erosion: 300μm sand, Vsg = 30 m/s, vertical elbow; data extracted from Ref. [3].
found that the location of the maximum erosion was around 45° for both gas-sand and gas-liquid-sand tests. Moreover, erosion rates were observed to be about 7 times greater in some cases in the vertical elbow compared to those in the horizontal elbow. Samples of Vieira et al. erosion data are shown in Fig. 2. Flow velocities cover superficial gas velocities of 27, 41, and 49 m/s and superficial liquid velocities between 0.005 and 0.04 m/s. As liquid rate increases, the erosion ratio decreases, which can be attributed to the increase in the liquid film thickness that can reduce the particle impact velocity. However, this trend reverses, when the erosion ratio reaches a minimum. It is also observed that the higher the superficial gas velocity, the larger the erosion magnitude. Parsi et al. [10] carried out experiments to investigate sand particle erosion in a 76.2-mm standard elbow under vertical churn flow conditions. They showed that unlike in annular flow, where a localized erosion pattern could be observed, in churn flow, a larger portion of the elbow experienced high values of erosion and erosion hot spots were on the sides of the elbow. This was further investigated through characterizing gas-liquid flow with a Wire-Mesh Sensor [11] and Computational Fluid Dynamics (CFD)-based erosion modeling [12]. It was shown that in churn flow, the so-called huge wave structures demonstrated a chaotic behavior and were larger than the disturbance waves of annular flow, and consequently, particles were distributed over a larger portion of the pipe cross-section before impacting the elbow.
conducted in two orientations of vertical upward-horizontal and horizontal-horizontal, with the sand size being 150 μm . For annular flow experiments, he found that erosion ratio decreased by increasing the liquid flow rate. Dosila [3] also used ER probes flush with the elbow outer radius and conducted experiments in 50.8-mm and 76.2-mm vertical upwardhorizontal elbows under annular and slug/churn flow conditions and investigated effects of pipe diameter and superficial liquid velocity on erosion. As illustrated in Fig. 1, erosion rates are higher in the smaller elbow. Moreover, the injection of a small amount of liquid to gas-sand flow causes a significant decrease in erosion ratio. There is also a critical superficial liquid velocity (≈ 0.008 m/s) above which erosion shows an increase with the increase in liquid rate. Fan [4] extended Dosila's [3] experiments to a 101.6-mm elbow under gas-low liquid flow conditions. Experiments were performed in vertical upward-horizontal and horizontal-horizontal orientations with superficial gas velocities of 15 m/s and 23 m/s and two sand sizes of 150 μm and 300 μm . Higher erosion rates were observed in the vertical orientation compared to the horizontal configuration. Although this previous pioneering research provided useful information for multiphase erosion magnitudes, the ER probes are intrusive and can affect the interaction of particles with the thin liquid film. 2.2. Ultrasonic technique
3. Experimental program
Highly accurate and non-intrusive, this testing method includes ultrasonic transducers that utilize high frequency sound waves traveling through the target wall material. Thickness of a wall at the location of an installed sensor can be obtained by computing the sound wave velocity in the material and the travel time of the wave through the material. Bourgoyne [5] used this technique and studied effects of various factors on erosion. He concluded that generally the erosion rate was about two orders of magnitude higher in gas-sand mixtures in comparison to liquid-sand mixtures. Kesana et al. [6] studied erosion in a 76.2-mm elbow under horizontal pseudo-slug flow conditions. Effect of sand size on erosion was investigated using two sand sizes of 150 μm and 300 μm . Results indicated that increasing the particle size increased the metal loss. Effects of particle size, orientation, and gas and liquid velocities were further investigated by Vieira et al. [7–9] in a 76.2-mm elbow under annular and low-liquid flow conditions. Erosion experiments were performed with ER probe and ultrasonic instruments, and it was
3.1. Flow loop The E/CRC flow loop at the North Campus research complex of The University of Tulsa was used for conducting erosion experiments in a 101.6-mm standard elbow (r/D = 1.5). A schematic of the flow loop including associated components is shown in Fig. 3. The major components of the loop were air compressors, pneumatic pumps, slurry tank, test section, and separator tank. The loop was equipped with two Ingersol Rand air compressors (shown by 1 and 2) each with a capacity of 400 standard cubic feet per minute. To reach a greater gas flow rate, the compressors were connected in parallel. The loop also had pumps P2b and P3a, which were larger capacity pumps compared to P1a, P2a, and P1b. The latter three pumps were used when lower rates of liquid were required. Temperature and pressure gauges were installed right after the outlet of compressors 1 and 2. High pressure gas provided from the 2
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Fig. 3. Schematic of the experimental facility.
steady-state condition, before each flow measurement. Afterwards, the liquid flow rate was measured. This was executed using a stopwatch and accounting for the liquid volume loss inside the slurry tank over a certain period. In all cases, air and water were used as the carrier fluids. After the required gas and liquid flow rates were obtained, sand was added to the liquid in the slurry tank. Once gas was mixed with the slurry, the mixture of gas-liquid-sand was sent to the test section and the elbow became eroded due to sand particle impacts. Experiments were performed for a sufficient amount of time to attain a measurable thickness loss in each test. Thickness losses were measured using a temperature compensated ultrasonic technique. Details about this technique can be found in our previous studies [6–8,10]. Eight ultrasonic transducers were mounted at different locations on a 101.6-mm elbow (Fig. 4). Two of these transducers were mounted between 40 and 50° on the outer bend of the elbow, where usually the maximum wear takes place. Two transducers were mounted at 60°, since a V-shaped erosion pattern is usually seen in vertical tests and four other transducers were distributed at the locations, where they could be hot spots in horizontal tests. Before and after each experiment, wall thickness data were gathered from the ultrasonic data acquisition. Besides the erosion experiments, flow visualization tests were conducted using an acrylic 90°standard elbow encompassing two pieces bolted to each other (Fig. 5). To record the flow pattern under various flow conditions and observe the phase distributions inside the elbow, three cameras were mounted on the acrylic elbow. The same elbow was further used for paint erosion studies. In this case, inside the elbow was painted and exposed to sand impacts which caused the paint to be removed, and one could easily observe the erosion pattern. Note that the same testing procedure was used for gas-sand experiments except that 1- sand and air were not recirculated and 2- sand was directly injected to the gas through a sand feeder and nozzle mounted at the inlet of the test section.
Fig. 4. Locations of the transducers mounted on the 101.6-mm elbow.
compressors was sent to the test section through one of the two pipes with 25.4-mm and 50.8-mm diameters at the gas entrance section. Depending on the required gas flow rate, either Valve-1 or Valve-2 was opened and adjusted to reach the desired flow rate. Gas flow rate was measured using one of the two TRIO-WIRL V gas vortex flow meters indicated as CFM-1 and CFM-2. The former was mounted on the 25.4mm pipe and could measure a flow rate as low as 7 CFM, while the latter was installed on the 50.8-mm pipe and was used when the flow rate was higher than 20 CFM. Another component of the loop was a 250-gallon slurry tank, which was equipped with a propeller mixer. The mixer prevented sand particles from depositing at the bottom of the tank and created a homogenous slurry. Liquid from the slurry tank and gas from the compressors were mixed and sent to the test section. The test-section housed two pipes of 76.2-mm and 101.6-mm diameters both of which were made from Stainless Steel-316. There were also some clear PVC observation sections along the two pipes. The testsection was 18 m long promoting fully-developed flow before the elbow. All experiments in this study were conducted using the 101.6mm pipe set. Another important component of the loop was the gas separator. Gas was exhausted from the top of this separator, while the liquid-sand mixture was pumped back from the bottom of the separator to the slurry tank to be recirculated through the loop.
3.3. Test matrix Fig. 6 plots the experimental test conditions on a flow pattern map for the gas-liquid-sand erosion tests using FLOPATN. Superficial liquid velocity ranged from 0.01 m/s to 0.28 m/s and superficial gas velocity varied from 15.1 m/s to 35.7 m/s. The test matrix was designed to cover annular flow and transition between annular flow and unstable annular flow with falling films. It should be noted that for velocities greater than 27 m/s, annular flow pattern was observed in the experiments.
3.2. Testing procedure The first step of performing gas-liquid-sand erosion experiments was to reach the desired gas flow rate by adjusting Valve-1 or Valve-2. It was ensured that both temperature and pressure of the gas reached a 3
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liquid on erosion magnitude and pattern, some erosion experiments were performed under gas-sand conditions. In this case, gas velocities in the range of 10 to m/s 37 m/s were employed. 4. Results and discussion 4.1. Gas-sand experiments 4.1.1. Effect of gas velocity on erosion Eighteen experiments were conducted in the vertical elbow under gas-sand conditions with two sand sizes of 75 and 300 μm and gas velocities of 15, 23, 31 and 37 m/s. Results of these experiments are demonstrated in Fig. 8. Generally, erosion increases with gas velocity and there is no significant difference between erosion caused by the two different sand sizes. These erosion data were further examined to find a parametric trend (Fig. 9). The recent work by Parsi et al. [14] shows that erosion rate (ER ) with a unit of m/s nondimensionalized by gas velocity (Vg ) has the following relationship with dimensionless penetration ratio, Pnr:
ER = 2.2855 × 10−12 (Pnr )1.0612 Vg Fig. 5. Acrylic elbow and cameras used for flow visualization and paint erosion studies.
Dimensionless penetration ratio, Pnr, is a power-law combination of four dimensionless groups: Reynolds number (Re ), the particle diameter (dp ) to the pipe diameter (D ) ratio, the particle density ( ρp ) to the fluid density ( ρf ) ratio, and a particle diameter nondimensionalized by a reference particle diameter (dPref ).
However, the flow pattern model predicts churn flow for those tests. In all gas-liquid-sand erosion experiments, the concertation of sand added to the slurry tank was kept at a constant value of 1% by mass of water. Therefore, the sand flow rate for each test is calculated as
3.41
dp Pnr = Re1.372 ∗ ⎛ ⎞ ⎝D⎠ ⎜
m˙ sand = C × ρl × Vsl × Apipe
(2)
(1)
where m˙ sand is the sand flow rate, C is the sand concentration fraction (i.e. 0.01), ρl is the liquid density, Vsl is the superficial liquid velocity, and Apipe is the pipe cross-sectional area. This means that sand flow rate changes with superficial liquid velocity. Therefore, to be able to compare erosion magnitudes for different superficial liquid velocities, thickness loss measurements are reported in mm/kg, which denotes millimeter of wall thickness loss per kilogram of sand. Microscopic images of sands used in the experiments are shown in Fig. 7. These were 75 μm sand and 300 μm sand (California #60) with sharp edges. Sharpness plays an important role in erosion damage, where small sands with sharp edges can cause severe erosion damage [10]. According to the E/CRC database for 75 μm sand size distribution, about 70% (by mass) of this sand is in the range of 44 μm to 105 μm . Also, about 60% (by mass) of the California sand is in the range of 250 μm to 425 μm . To reach a good understanding of the influence of the presence of
⎟
1.45
⎛ ρp ⎞ ∗ ⎜ ⎟ ρ ⎝ f⎠
3.1
dPref ⎞ ∗ ⎜⎛ ⎟ ⎝ dp ⎠
(3)
Fig. 9 shows that the erosion data for different particle diameters and gas velocities are similar. Here, however, a factor of 3.8854 has been used in Eqn. (2) (instead of 2.2855). In general, the dimensionless erosion rate data ( ER ) is proportional to Pnr to the power of 1.0612. Vg
4.1.2. Paint erosion study Three cameras were mounted on the elbow to record paint removal sequences and identify the location of maximum erosion. Fig. 10 demonstrates such sequences for a test with the sand size of 300 μm and gas velocity of 31 m/s. Two observations can be made here. First, the location that the paint is initially removed is approximately at 40–50° outer bend. This can be explained considering the gas velocity profile in the straight section of the pipe. Understandably, particles traveling at the pipe center acquire high velocities comparable to the gas velocity. These particles can directly hit the elbow at 40–50° outer bend and cause the
Fig. 6. Test conditions for a) vertical and b) horizontal erosion experiments. 4
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Fig. 7. Microscopic images of sand particle used in the tests a) 75 μm and b) 300 μm Sand.
“inadequate for predicting erosion” and rather particle–wall collision models accounting for the wall roughness were required. The discrepancy between the current work and the work mentioned might be because of the very high concentrations of sand used in their experiments, as they injected 200 kg and 300 kg of sand over the course of 3hr. To examine effect of sand size on erosion pattern, another experiment was conducted with 75 μm sand and gas velocity of 31 m/s. A pattern like that caused by 300 μm sand was observed (not shown here); however, the rebounds of 75 μm particles did not cause a paint removal as significant as that by 300 μm sand. This could be due to the lower Stokes number of the smaller sand causing it to follow the flow streamlines after the first impact. Fig. 8. Effect of sand size and gas velocity on erosion magnitude under gas-sand flow conditions in the vertical configuration.
4.2. Gas-liquid-sand experiments 4.2.1. Effect of flow rate on erosion Effects of superficial gas and liquid velocities on maximum erosion in unstable annular and stable annular flow conditions are presented in this section. Fig. 11a and b show samples of maximum erosion results for superficial gas velocities of 23 m/s, and 31 m/s, respectively. The superficial liquid velocity of zero refers to gas-sand experiments. The sand size here is 300 μm and the elbow orientation is vertical. Addition of liquid to gas-sand flow significantly reduces erosion. With further increase in the superficial liquid velocity, the reduction in erosion persists up to the superficial liquid velocity of 0.02 m/s. Then, there is a range of superficial liquid velocities, where erosion may increase as superficial liquid velocity increases. To understand this, three events must be considered. 1- as superficial liquid velocity increases, the liquid film thickness in the straight pipe before the elbow increases. This results in a decrease in the gas passage area, which in turn increases the gas core velocity (conservation of mass). Of course, the higher the gas core velocity becomes, the entrained sand particles can impact with higher velocities. 2- as superficial liquid velocity increases, droplet entrainment and consequently sand entrainment rate increases. Therefore, more sand particles can impact the elbow. 3- as superficial liquid velocity increases, the liquid film around the elbow, which can act as a protection, becomes thicker. Therefore, when liquid flow rate increases, whether erosion magnitude decreases or increases is determined by the prevailing factor among the followings: increase in particle velocity in the gas core, increase in particle entrainment, and increase in thickness of the protective liquid film around the elbow. Effect of flow rate on erosion can also be looked upon from flow regime point of view, as illustrated in Fig. 12. Here, variation of erosion
Fig. 9. Parametric trend: erosion rate data nondimensionalized by gas velocity plotted against dimensionless penetration ratio, Pnr.
maximum paint removal. The second observation is the paint removal between 60° and 90°, which is due to the particle rebounds. Overall, the results indicate the presence of a bunny-like paint removal pattern; a circular region at 40–50° plus a V-shape region between 60 and 90°. Unlike the current work, the experimental-numerical work by Solnordal et al [13] does not report the V-shape region under gas-sand condition; gas velocity of 80 m/s and sand median diameter of 184 μm. They used a surface profiler to provide erosion data on a 40 × 20-point grid for a 101.6-mm elbow. Their experimental data indicated the presence of a circular erosion pattern on the elbow. They further showed that smooth wall assumption in CFD was 5
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Fig. 10. Sequences of paint erosion patterns in vertical gas-sand experiments; Vg = 31 m/s and 300 μm sand. The last two figures show the erosion pattern from two different views. The red arrow shows the flow direction. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
is more pronounced under annular flow conditions (Fig. 13). Erosion caused by 300 μm sand is about 3–4 times higher than that caused by 75 μm sand in lower gas velocities of 22 m/s and 23 m/s. At higher superficial gas velocities of 27 m/s and 35.5 m/s, however, erosion caused by 300 μm sand and 75 μm sand is comparable.
with superficial gas velocity is demonstrated for two superficial liquid velocities of 0.02, and 0.04 m/s. Surprisingly, transition from co-current upward annular flow to unstable annular flow with liquid flow reversal appears to have an impact on maximum erosion magnitude. At a gas superficial velocity of 23 m/s, where unstable annular flow was the flow regime observed, erosion is slightly greater compared to that measured in annular flow condition with a superficial gas velocity of 27 m/s. This behavior has also been reported by Vieira et al. [8] whose experiments were conducted in a 76.2-mm elbow. Note, however, that under annular flow conditions, erosion increases, as superficial gas velocity increases from 27 m/s.
4.2.3. Location of maximum erosion Samples of erosion values and locations of maximum erosion in gassand and gas-liquid-sand experiments in the vertical orientation and for various superficial gas and liquid velocities are shown in Fig. 14. The values on each transducer indicates erosion in mm/kg. Vieira et al. [8] and Parsi et al. [10] observed that as flow pattern changes from annular flow to slug/churn flow, location of maximum erosion moves towards the sides of the elbow. This location transmission was attributed to the periodic liquid structures existing in churn
4.2.2. Effect of sand size on erosion Compared to gas-sand flow conditions, effect of sand size on erosion
Fig. 11. Effect of gas and liquid flow rates on maximum erosion magnitude in vertical annular flow; a) Vsg = 23 m/s, and b) Vsg = 31 m/s. Sand size is 300 μm. The superficial velocity of zero indicates gas-sand flow. 6
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Fig. 12. Effect of flow regime on erosion; variation of erosion magnitude with superficial gas velocity at constant superficial liquid velocities of (a) 0.02 m/s and (b) 0.04 m/s.
orientation to understand the liquid film behavior inside the elbow. In the experiments, water dyed blue and tests were performed with a constant superficial liquid velocity of 0.06 m/s for different superficial gas velocities of 26 m/s, 34 m/s, and 42 m/s (Fig. 15). Of course, the liquid film on the elbow is originated from the liquid film around the circumference of the straight pipe before the elbow. The latter film moves towards the center of the outer bend (as shown by small triple-arrows) and generates a relatively thick liquid layer at the center of the outer bend. As gas velocity increases, liquid entrainment rate from the liquid film into the gas core increases and consequently, the thickness of the film at the center decreases. Additionally, effect of superficial liquid velocity on liquid film behavior at a constant gas velocity was investigated (Fig. 16). It is seen that as superficial liquid velocity increases, the liquid film becomes thicker. Secondary flow inside the elbow drains the liquid from the outer bend towards the inner bend causing streaks of water to appear on the sides of the elbow. Further exploration of the videos revealed that liquid droplets moved in the gas core, impacted the outer bend of the elbow, and coalesced with the liquid present at the outer bend. Under gas-liquidsand flow conditions, these droplets may contain sand; therefore, the sand must penetrate through the liquid film to impact the bend. Fig. 17 examines the flow visualization results in the context of erosion. As indicated earlier, there is a range of superficial liquid velocities, where erosion may increase with an increase in liquid velocity, and then erosion decreases afterwards. It is seen that for experiments with Vsl ≤ 0.06 m/s, the liquid films on the outer bend are not significantly different. Yet, in the straight pipe
Fig. 13. Effect of sand size on erosion magnitude under vertical gas-liquid-sand flow conditions.
flow. However, in the current work, it was observed that regardless of gas and liquid velocities, location of maximum erosion was approximately between 40° and 50° on the outer bend of the elbow. For the location of the maximum erosion (not the pattern) under gas-sand flow conditions, the results of the current work are consistent with the observations of Solnordal et al [13]. 4.2.4. Liquid film behavior Flow visualization studies were performed in the vertical
Fig. 14. Samples of erosion values in gas-sand and gas-liquid-sand experiments in the vertical orientation. 7
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Fig. 15. Effects of gas velocity on liquid film behavior in the vertical orientation. Shown are pictures from top and side views.
before the elbow, as superficial liquid velocity increases, the liquid film becomes thicker compressing the gas path. Consequently, a small increase in the superficial liquid velocity also increases the gas core velocity. This in turn increases the particle velocities in the gas core and thus erosion increases. Further increase in the superficial liquid velocity (e.g. Vsl = 0.10 m/s), however, creates a thicker liquid film on the elbow, which can protect the wall from the particle impacts and thus erosion decreases.
vertical orientation is higher than that in the horizontal inclination. The difference is very significant for the pair with superficial gas velocity of 23 m/s and superficial liquid velocity of 0.15 m/s, where a difference of a factor of 40 can be noticed. Although 8 transducers were mounted on the elbow to have the maximum coverage on the outer bend, it is possible that in horizontal tests transducers are not in appropriate location to measure the maximum thickness loss. To compare the erosion results in the vertical and horizontal elbows, there are two main factors to be considered; liquid film and entrainment rate distributions due to gravity. The effects of these two factors on erosion were investigated through flow visualization and paint erosion tests in the horizontal elbow. Let us first explore the liquid film behavior in the horizontal elbow. Fig. 20 displays flow visualization results for different superficial gas and liquid velocities. Considering Fig. 20a, in stratified wavy flow (Vsg = 15 m/s, Vsl = 0.04 m/s), the liquid film merely flows in the lower half of the elbow, where it slides over the outer bend. However, as superficial gas velocity increases to 35 m/s and annular flow regime is generated (Fig. 20b), a uniform film forms on the outer bend in both the upper and lower halves of the elbow. Fig. 20c and d reveal the effect of superficial liquid velocity on the film behavior. For the same superficial gas velocity of 20 m/s, as superficial liquid velocity increases from 0.04 m/s to 0.08 m/s, liquid on the bottom half of the outer bend slides towards the upper half. As a result, a wider area of the outer bend is covered by liquid protecting the elbow from entrained particles that are mostly concentrated near the liquid gas interface due to gravity.
4.2.5. Paint erosion study Fig. 18 demonstrates the (top view) sequences of paint removal for two experiments with superficial gas velocities of 23 m/s and 31 m/s. The superficial liquid velocities were 0.08 m/s and 0.05 m/s, for the former and latter, respectively. Both experiments were performed in the vertical orientation with 300 μm sand. There are two eroded regions. The first region is where the elbow experiences the maximum erosion; the circular area about 40–50° at the outer bend center. The second eroded region is the single stripe-like area about 60–90°. The erosion here occurs due to impingements of rebounded sand as well as abrasion by sand present in the liquid film sliding on the outer bend. 4.2.6. Effect of flow orientation on erosion Some erosion experiments were performed in the horizontal orientation to investigate the influence of flow orientation on erosion. Fig. 19 compares the results of those experiments with erosion data obtained in the vertical elbow. In all cases examined, erosion in the
Fig. 16. Effects of superficial liquid velocity on liquid film behavior in the vertical orientation. Shown are pictures from top and side views. 8
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Fig. 17. Effects of liquid velocity on erosion magnitude at a constant gas velocity equal to 31 m/s (vertical elbow).
Besides the liquid film behavior, the effect of gravity must be considered. This effect can be spotted in Fig. 21, where an asymmetrical erosion pattern is observed; on the upper-half of the outer bend, more paint has been removed. Because of the gravity force, most of the particles are carried by the liquid film at the bottom of the straight pipe,
When covering the elbow, the liquid film can play a role of protection, as it can decelerate particles impacting. This protection can be seen in Fig. 19, where erosion is much lower for a high superficial liquid velocity of 0.15 m/s compared to the case with Vsl of 0.02 m/s (horizontal elbow).
Fig. 18. Paint removal sequences in vertical annular tests with 300 μm sand; left column: Vsg = 31 m/s and Vsl = 0.05 m/s, right column: Vsg = 23 m/s and Vsl = 0.08 m/s. 9
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horizontal orientation, particles mostly flow with the low-velocity liquid film before entering the elbow, while in the vertical orientation, particles can entrain into the high-velocity gas core and impact the elbow. Note that at the lowest superficial velocities (Fig. 19), the difference between erosion magnitudes in the two orientations is the lowest. This is expected, since as the liquid amount decreases, the gas-liquid flow becomes more like gas-only flow, where flow orientation has no impact on erosion. 5. Conclusions Different experiments were performed to study solid-particle erosion under gas-sand and gas-liquid-sand conditions in a 101.6-mm elbow. Ultrasonic measurements were conducted to evaluate wallthickness loss during each experiment. In summary:
Fig. 19. Comparison of maximum erosion in vertical and horizontal experiments under annular flow conditions; 300 μm sand.
• Under gas-sand conditions, a rabbit ear-like erosion pattern was
which has a low velocity. As a result, the bottom half of the outer bend is exposed to particles with low velocities. Revisiting Fig. 19, in the horizontal elbow, there is always a liquid film protecting the elbow from the impacts, whereas in the vertical elbow, there are moments that dry-out occurs, and a large portion of the elbow is dry and not protected by the liquid film. Second, in the
•
observed with the maximum erosion occurring at 40–50° along the outer bend. Under vertical gas-liquid-sand conditions, two different behaviors were discerned when superficial liquid velocity increased. At lower liquid flow rates, an increase in superficial liquid velocity increased erosion. On the other hand, this was reversed for the higher liquid
Fig. 20. Effects of gas velocity on liquid film behavior in the horizontal orientation. Shown are pictures from top and side views. 10
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Fig. 21. Flow and erosion pattern under horizontal annular flow condition; Vsg = 31 m/s, Vsl = 0.05 m/s, and 300-μm sand.
flow rates.
[5] A.T. Bourgoyne, Experimental Study of Erosion in Diverter Systems Due to Sand Production, (1989) New Orleans, Louisiana. [6] N.R. Kesana, S.A. Grubb, B.S. McLaury, S.A. Shirazi, Ultrasonic measurement of multiphase flow erosion patterns in a standard elbow, ASME J. Energy Resour. Technol. 135 (3) (2013). [7] R.E. Vieira, Sand Erosion Model Improvement for Elbows in Gas Production Multiphase Annular and Low-Liquid Flow, PhD Dissertation The University of Tulsa, Oklahoma, 2014. [8] R.E. Vieira, M. Parsi, P. Zahedi, B.S. McLaury, S.A. Shirazi, Ultrasonic measurements of sand particle erosion under upward multiphase annular flow conditions in a vertical-horizontal bend, Int. J. Multiph. Flow 93 (2017) 48–62. [9] R. Vieira, M. Parsi, P. Zahedi, B. McLaury, S. Shirazi, Electrical resistance probe measurements of solid particle erosion in multiphase annular flow, Wear 382 (383) (2017) 15–28. [10] M. Parsi, R. Vieira, N. Kesana, B. McLaury, S. Shirazi, Ultrasonic measurements of sand particle erosion in gas dominant multiphase flow, Wear 328 (329) (2015) 401–413. [11] M. Parsi, R. Vieira, C.F. Torres, N.R. Kesana, B.S. McLaury, S.A. Shirazi, E. Schleicher, U. Hampel, Experimental investigation of interfacial structures within churn flow using a dual wire-mesh sensor, Int. J. Multiph. Flow 73 (2015) 155–170. [12] M. Parsi, M. Agrawal, V. Srinivasan, R. Vieira, C.F. Torres, B.S. McLaury, S.A. Shirazi, CFD simulation of sand particle erosion in gas-dominant multiphase flow, J. Nat. Gas Sci. Eng. 27 (2) (2015) 706–718. [13] C.B. Solnordal, C.Y. Wong, J. Boulanger, An experimental and numerical analysis of erosion caused by sand pneumatically conveyed through a standard pipe elbow, Wear 336–337 (2015) 43–57. [14] M. Parsi, A. Al-Sarkhi, M. Kara, P. Sharma, B.S. McLaury, S.A. Shirazi, A new dimensionless number for solid particle erosion in natural gas elbows, Wear 390–391 (2017) 80–83.
• Liquid film thickness in the elbow, particle velocity in the gas core, and particle entrainment were recognized to be the governing factors for the two behaviors mentioned.
Acknowledgements This work was conducted, when Dr. Peyman Zahedi was with E/ CRC. He would like to acknowledge the member companies of E/CRC for their financial support. References [1] B.S. McLaury, S.A. Shirazi, V. Viswanathan, Q.H. Mazumder, G. Santos, Distribution of sand particles in horizontal and vertical annular multiphase flow in pipes and the effects on sand erosion, J. Energy Resour. Technol. 133 (2011) 0230011–023001-10. [2] M. Pyboyina, Experimental Investigation and Computational Fluid Dynamics Simulations of Erosion on Electrical Resistance Probes, M.S. Thesis Department of Mechanical Engineering, The University of Tulsa, Tulsa, Oklahoma, USA., 2006. [3] R. Dosila, Effects of Low Liquid Loading on Solid Particle Erosion for Gas Dominant Multiphase Flows, M.S. Thesis Department of Mechanical Engineering, The University of Tulsa, Tulsa, Oklahoma, USA., 2008. [4] C. Fan, Evaluation of Solid Particle Erosion in Gas Dominant Flows Using Electrical Resistance Probes, M.S. Thesis Department of Mechanical Engineering, The University of Tulsa, Tulsa, Oklahoma, USA., 2010.
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Experimental investigation of sand particle erosion in a 90° elbow in annular two-phase flows
T
Peyman Zahedia,∗, Mazdak Parsib, Alireza Asgharpoura, Brenton S. McLaurya, Siamack A. Shirazia a b
Erosion/Corrosion Research Center, The University of Tulsa, Tulsa, OK, United States National Oilwell Varco, Houston, TX, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Solid particle erosion Multiphase flow Annular flow Gas-liquid flow Elbow
Upward gas-liquid-solid flows are often observed in oil and gas transmission pipelines. The presence of solids in such flows is a concern, as it can cause serious damage to pipe fittings, which are prone to erosion. These are normally components such as elbows and tees that can change the flow direction. The current work presents the results of an experimental erosion campaign including 56 tests in a 101.6-mm standard elbow (radius of curvature of 1.5) under gas-sand and gas-liquid-sand flow conditions. The latter included 38 tests (30 distinct conditions) mostly in the range of annular and unstable annular flows. Thickness loss measurements were made using a non-invasive ultrasonic technology instrument at 8 different points along the outer bend of the elbow to investigate effects of various factors on erosion. Additionally, flow visualization and paint erosion studies were performed to examine liquid distribution as well as the particulate phase behavior. Generally, when superficial liquid velocity increased, erosion ratio decreased for low liquid rate conditions. This trend, however, reversed for higher liquid rates. Also, the paint erosion experiments revealed that the location of the maximum erosion was around 40°–50° on the middle outer bend, for all cases examined.
1. Introduction
can acquire a high velocity close to that of the gas in the core and cause erosion, when impacting a wall. The complexity of the erosion phenomenon in gas-liquid-solid flow has led researchers to conduct experiments and collect data on the erosion distribution and magnitude in 90° elbows. These data can be ultimately used to develop erosion prediction models and validate Computational Fluid Dynamics simulations. Next, a review of previous studies on erosion in multiphase flow is provided.
When gas and liquid are present in a pipe, they distribute themselves in a specific configuration called the flow pattern. In vertical upward gas-liquid flow, the flow patterns are generally considered as dispersed bubble, bubbly, slug, churn, annular and transition between some of these patterns. Annular flow is normally observed in gas-condensate production pipelines and characterized by the presence of high-velocity gas flow in the pipe core and a thin liquid film around the pipe circumference. The high-velocity gas exerts a high interfacial shear on the liquid film close to the pipe wall, which in turn causes the formation of the so-called “disturbance waves” at the interface of the gas and liquid. The high shear also causes the detachment of some droplets from the disturbance waves and these droplets become entrained into the gas core. When solid particles are present, it is believed that the same mechanism that entrains liquid droplets into the gas core can also cause the entrainment of the solid particles. This is based on the experimental observation of McLaury et al. [1]. They demonstrated that the liquid and solid particle entrainment ratios were similar, when particle and droplet sampling along the radial axis of a pipe under annular flow conditions was employed. Understandably, the entrained solid particles
∗
2. Background 2.1. Electrical resistance (ER) probe An ER probe is an intrusive instrument, which measures the thickness loss using two elements namely sample and reference. The sample element is in contact with the flow, whereas the reference element is protected inside the probe from impact of solid particles. When impacted, the sample element experiences metal loss, which changes its electrical resistance. Thickness loss of the sample element is calculated by comparing its electrical resistance to that of the reference element. Pyboyina [2] utilized an ER probe flush mounted at 45° in a 2-inch standard elbow to measure thickness losses. Experiments were
Corresponding author. Currently with Bechtel Oil, Gas & Chemicals, Inc., Houston, TX, United States. E-mail address: [email protected] (P. Zahedi).
https://doi.org/10.1016/j.wear.2019.203048 Received 25 June 2019; Received in revised form 8 September 2019; Accepted 9 September 2019 Available online 11 September 2019 0043-1648/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 2. Maximum erosion under gas-liquid-sand annular flow conditions: 300μm sand, 76.2-mm vertical elbow; data extracted from Refs. [7,8].
Fig. 1. Effect of pipe diameter and superficial liquid velocity on erosion: 300μm sand, Vsg = 30 m/s, vertical elbow; data extracted from Ref. [3].
found that the location of the maximum erosion was around 45° for both gas-sand and gas-liquid-sand tests. Moreover, erosion rates were observed to be about 7 times greater in some cases in the vertical elbow compared to those in the horizontal elbow. Samples of Vieira et al. erosion data are shown in Fig. 2. Flow velocities cover superficial gas velocities of 27, 41, and 49 m/s and superficial liquid velocities between 0.005 and 0.04 m/s. As liquid rate increases, the erosion ratio decreases, which can be attributed to the increase in the liquid film thickness that can reduce the particle impact velocity. However, this trend reverses, when the erosion ratio reaches a minimum. It is also observed that the higher the superficial gas velocity, the larger the erosion magnitude. Parsi et al. [10] carried out experiments to investigate sand particle erosion in a 76.2-mm standard elbow under vertical churn flow conditions. They showed that unlike in annular flow, where a localized erosion pattern could be observed, in churn flow, a larger portion of the elbow experienced high values of erosion and erosion hot spots were on the sides of the elbow. This was further investigated through characterizing gas-liquid flow with a Wire-Mesh Sensor [11] and Computational Fluid Dynamics (CFD)-based erosion modeling [12]. It was shown that in churn flow, the so-called huge wave structures demonstrated a chaotic behavior and were larger than the disturbance waves of annular flow, and consequently, particles were distributed over a larger portion of the pipe cross-section before impacting the elbow.
conducted in two orientations of vertical upward-horizontal and horizontal-horizontal, with the sand size being 150 μm . For annular flow experiments, he found that erosion ratio decreased by increasing the liquid flow rate. Dosila [3] also used ER probes flush with the elbow outer radius and conducted experiments in 50.8-mm and 76.2-mm vertical upwardhorizontal elbows under annular and slug/churn flow conditions and investigated effects of pipe diameter and superficial liquid velocity on erosion. As illustrated in Fig. 1, erosion rates are higher in the smaller elbow. Moreover, the injection of a small amount of liquid to gas-sand flow causes a significant decrease in erosion ratio. There is also a critical superficial liquid velocity (≈ 0.008 m/s) above which erosion shows an increase with the increase in liquid rate. Fan [4] extended Dosila's [3] experiments to a 101.6-mm elbow under gas-low liquid flow conditions. Experiments were performed in vertical upward-horizontal and horizontal-horizontal orientations with superficial gas velocities of 15 m/s and 23 m/s and two sand sizes of 150 μm and 300 μm . Higher erosion rates were observed in the vertical orientation compared to the horizontal configuration. Although this previous pioneering research provided useful information for multiphase erosion magnitudes, the ER probes are intrusive and can affect the interaction of particles with the thin liquid film. 2.2. Ultrasonic technique
3. Experimental program
Highly accurate and non-intrusive, this testing method includes ultrasonic transducers that utilize high frequency sound waves traveling through the target wall material. Thickness of a wall at the location of an installed sensor can be obtained by computing the sound wave velocity in the material and the travel time of the wave through the material. Bourgoyne [5] used this technique and studied effects of various factors on erosion. He concluded that generally the erosion rate was about two orders of magnitude higher in gas-sand mixtures in comparison to liquid-sand mixtures. Kesana et al. [6] studied erosion in a 76.2-mm elbow under horizontal pseudo-slug flow conditions. Effect of sand size on erosion was investigated using two sand sizes of 150 μm and 300 μm . Results indicated that increasing the particle size increased the metal loss. Effects of particle size, orientation, and gas and liquid velocities were further investigated by Vieira et al. [7–9] in a 76.2-mm elbow under annular and low-liquid flow conditions. Erosion experiments were performed with ER probe and ultrasonic instruments, and it was
3.1. Flow loop The E/CRC flow loop at the North Campus research complex of The University of Tulsa was used for conducting erosion experiments in a 101.6-mm standard elbow (r/D = 1.5). A schematic of the flow loop including associated components is shown in Fig. 3. The major components of the loop were air compressors, pneumatic pumps, slurry tank, test section, and separator tank. The loop was equipped with two Ingersol Rand air compressors (shown by 1 and 2) each with a capacity of 400 standard cubic feet per minute. To reach a greater gas flow rate, the compressors were connected in parallel. The loop also had pumps P2b and P3a, which were larger capacity pumps compared to P1a, P2a, and P1b. The latter three pumps were used when lower rates of liquid were required. Temperature and pressure gauges were installed right after the outlet of compressors 1 and 2. High pressure gas provided from the 2
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Fig. 3. Schematic of the experimental facility.
steady-state condition, before each flow measurement. Afterwards, the liquid flow rate was measured. This was executed using a stopwatch and accounting for the liquid volume loss inside the slurry tank over a certain period. In all cases, air and water were used as the carrier fluids. After the required gas and liquid flow rates were obtained, sand was added to the liquid in the slurry tank. Once gas was mixed with the slurry, the mixture of gas-liquid-sand was sent to the test section and the elbow became eroded due to sand particle impacts. Experiments were performed for a sufficient amount of time to attain a measurable thickness loss in each test. Thickness losses were measured using a temperature compensated ultrasonic technique. Details about this technique can be found in our previous studies [6–8,10]. Eight ultrasonic transducers were mounted at different locations on a 101.6-mm elbow (Fig. 4). Two of these transducers were mounted between 40 and 50° on the outer bend of the elbow, where usually the maximum wear takes place. Two transducers were mounted at 60°, since a V-shaped erosion pattern is usually seen in vertical tests and four other transducers were distributed at the locations, where they could be hot spots in horizontal tests. Before and after each experiment, wall thickness data were gathered from the ultrasonic data acquisition. Besides the erosion experiments, flow visualization tests were conducted using an acrylic 90°standard elbow encompassing two pieces bolted to each other (Fig. 5). To record the flow pattern under various flow conditions and observe the phase distributions inside the elbow, three cameras were mounted on the acrylic elbow. The same elbow was further used for paint erosion studies. In this case, inside the elbow was painted and exposed to sand impacts which caused the paint to be removed, and one could easily observe the erosion pattern. Note that the same testing procedure was used for gas-sand experiments except that 1- sand and air were not recirculated and 2- sand was directly injected to the gas through a sand feeder and nozzle mounted at the inlet of the test section.
Fig. 4. Locations of the transducers mounted on the 101.6-mm elbow.
compressors was sent to the test section through one of the two pipes with 25.4-mm and 50.8-mm diameters at the gas entrance section. Depending on the required gas flow rate, either Valve-1 or Valve-2 was opened and adjusted to reach the desired flow rate. Gas flow rate was measured using one of the two TRIO-WIRL V gas vortex flow meters indicated as CFM-1 and CFM-2. The former was mounted on the 25.4mm pipe and could measure a flow rate as low as 7 CFM, while the latter was installed on the 50.8-mm pipe and was used when the flow rate was higher than 20 CFM. Another component of the loop was a 250-gallon slurry tank, which was equipped with a propeller mixer. The mixer prevented sand particles from depositing at the bottom of the tank and created a homogenous slurry. Liquid from the slurry tank and gas from the compressors were mixed and sent to the test section. The test-section housed two pipes of 76.2-mm and 101.6-mm diameters both of which were made from Stainless Steel-316. There were also some clear PVC observation sections along the two pipes. The testsection was 18 m long promoting fully-developed flow before the elbow. All experiments in this study were conducted using the 101.6mm pipe set. Another important component of the loop was the gas separator. Gas was exhausted from the top of this separator, while the liquid-sand mixture was pumped back from the bottom of the separator to the slurry tank to be recirculated through the loop.
3.3. Test matrix Fig. 6 plots the experimental test conditions on a flow pattern map for the gas-liquid-sand erosion tests using FLOPATN. Superficial liquid velocity ranged from 0.01 m/s to 0.28 m/s and superficial gas velocity varied from 15.1 m/s to 35.7 m/s. The test matrix was designed to cover annular flow and transition between annular flow and unstable annular flow with falling films. It should be noted that for velocities greater than 27 m/s, annular flow pattern was observed in the experiments.
3.2. Testing procedure The first step of performing gas-liquid-sand erosion experiments was to reach the desired gas flow rate by adjusting Valve-1 or Valve-2. It was ensured that both temperature and pressure of the gas reached a 3
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liquid on erosion magnitude and pattern, some erosion experiments were performed under gas-sand conditions. In this case, gas velocities in the range of 10 to m/s 37 m/s were employed. 4. Results and discussion 4.1. Gas-sand experiments 4.1.1. Effect of gas velocity on erosion Eighteen experiments were conducted in the vertical elbow under gas-sand conditions with two sand sizes of 75 and 300 μm and gas velocities of 15, 23, 31 and 37 m/s. Results of these experiments are demonstrated in Fig. 8. Generally, erosion increases with gas velocity and there is no significant difference between erosion caused by the two different sand sizes. These erosion data were further examined to find a parametric trend (Fig. 9). The recent work by Parsi et al. [14] shows that erosion rate (ER ) with a unit of m/s nondimensionalized by gas velocity (Vg ) has the following relationship with dimensionless penetration ratio, Pnr:
ER = 2.2855 × 10−12 (Pnr )1.0612 Vg Fig. 5. Acrylic elbow and cameras used for flow visualization and paint erosion studies.
Dimensionless penetration ratio, Pnr, is a power-law combination of four dimensionless groups: Reynolds number (Re ), the particle diameter (dp ) to the pipe diameter (D ) ratio, the particle density ( ρp ) to the fluid density ( ρf ) ratio, and a particle diameter nondimensionalized by a reference particle diameter (dPref ).
However, the flow pattern model predicts churn flow for those tests. In all gas-liquid-sand erosion experiments, the concertation of sand added to the slurry tank was kept at a constant value of 1% by mass of water. Therefore, the sand flow rate for each test is calculated as
3.41
dp Pnr = Re1.372 ∗ ⎛ ⎞ ⎝D⎠ ⎜
m˙ sand = C × ρl × Vsl × Apipe
(2)
(1)
where m˙ sand is the sand flow rate, C is the sand concentration fraction (i.e. 0.01), ρl is the liquid density, Vsl is the superficial liquid velocity, and Apipe is the pipe cross-sectional area. This means that sand flow rate changes with superficial liquid velocity. Therefore, to be able to compare erosion magnitudes for different superficial liquid velocities, thickness loss measurements are reported in mm/kg, which denotes millimeter of wall thickness loss per kilogram of sand. Microscopic images of sands used in the experiments are shown in Fig. 7. These were 75 μm sand and 300 μm sand (California #60) with sharp edges. Sharpness plays an important role in erosion damage, where small sands with sharp edges can cause severe erosion damage [10]. According to the E/CRC database for 75 μm sand size distribution, about 70% (by mass) of this sand is in the range of 44 μm to 105 μm . Also, about 60% (by mass) of the California sand is in the range of 250 μm to 425 μm . To reach a good understanding of the influence of the presence of
⎟
1.45
⎛ ρp ⎞ ∗ ⎜ ⎟ ρ ⎝ f⎠
3.1
dPref ⎞ ∗ ⎜⎛ ⎟ ⎝ dp ⎠
(3)
Fig. 9 shows that the erosion data for different particle diameters and gas velocities are similar. Here, however, a factor of 3.8854 has been used in Eqn. (2) (instead of 2.2855). In general, the dimensionless erosion rate data ( ER ) is proportional to Pnr to the power of 1.0612. Vg
4.1.2. Paint erosion study Three cameras were mounted on the elbow to record paint removal sequences and identify the location of maximum erosion. Fig. 10 demonstrates such sequences for a test with the sand size of 300 μm and gas velocity of 31 m/s. Two observations can be made here. First, the location that the paint is initially removed is approximately at 40–50° outer bend. This can be explained considering the gas velocity profile in the straight section of the pipe. Understandably, particles traveling at the pipe center acquire high velocities comparable to the gas velocity. These particles can directly hit the elbow at 40–50° outer bend and cause the
Fig. 6. Test conditions for a) vertical and b) horizontal erosion experiments. 4
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Fig. 7. Microscopic images of sand particle used in the tests a) 75 μm and b) 300 μm Sand.
“inadequate for predicting erosion” and rather particle–wall collision models accounting for the wall roughness were required. The discrepancy between the current work and the work mentioned might be because of the very high concentrations of sand used in their experiments, as they injected 200 kg and 300 kg of sand over the course of 3hr. To examine effect of sand size on erosion pattern, another experiment was conducted with 75 μm sand and gas velocity of 31 m/s. A pattern like that caused by 300 μm sand was observed (not shown here); however, the rebounds of 75 μm particles did not cause a paint removal as significant as that by 300 μm sand. This could be due to the lower Stokes number of the smaller sand causing it to follow the flow streamlines after the first impact. Fig. 8. Effect of sand size and gas velocity on erosion magnitude under gas-sand flow conditions in the vertical configuration.
4.2. Gas-liquid-sand experiments 4.2.1. Effect of flow rate on erosion Effects of superficial gas and liquid velocities on maximum erosion in unstable annular and stable annular flow conditions are presented in this section. Fig. 11a and b show samples of maximum erosion results for superficial gas velocities of 23 m/s, and 31 m/s, respectively. The superficial liquid velocity of zero refers to gas-sand experiments. The sand size here is 300 μm and the elbow orientation is vertical. Addition of liquid to gas-sand flow significantly reduces erosion. With further increase in the superficial liquid velocity, the reduction in erosion persists up to the superficial liquid velocity of 0.02 m/s. Then, there is a range of superficial liquid velocities, where erosion may increase as superficial liquid velocity increases. To understand this, three events must be considered. 1- as superficial liquid velocity increases, the liquid film thickness in the straight pipe before the elbow increases. This results in a decrease in the gas passage area, which in turn increases the gas core velocity (conservation of mass). Of course, the higher the gas core velocity becomes, the entrained sand particles can impact with higher velocities. 2- as superficial liquid velocity increases, droplet entrainment and consequently sand entrainment rate increases. Therefore, more sand particles can impact the elbow. 3- as superficial liquid velocity increases, the liquid film around the elbow, which can act as a protection, becomes thicker. Therefore, when liquid flow rate increases, whether erosion magnitude decreases or increases is determined by the prevailing factor among the followings: increase in particle velocity in the gas core, increase in particle entrainment, and increase in thickness of the protective liquid film around the elbow. Effect of flow rate on erosion can also be looked upon from flow regime point of view, as illustrated in Fig. 12. Here, variation of erosion
Fig. 9. Parametric trend: erosion rate data nondimensionalized by gas velocity plotted against dimensionless penetration ratio, Pnr.
maximum paint removal. The second observation is the paint removal between 60° and 90°, which is due to the particle rebounds. Overall, the results indicate the presence of a bunny-like paint removal pattern; a circular region at 40–50° plus a V-shape region between 60 and 90°. Unlike the current work, the experimental-numerical work by Solnordal et al [13] does not report the V-shape region under gas-sand condition; gas velocity of 80 m/s and sand median diameter of 184 μm. They used a surface profiler to provide erosion data on a 40 × 20-point grid for a 101.6-mm elbow. Their experimental data indicated the presence of a circular erosion pattern on the elbow. They further showed that smooth wall assumption in CFD was 5
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Fig. 10. Sequences of paint erosion patterns in vertical gas-sand experiments; Vg = 31 m/s and 300 μm sand. The last two figures show the erosion pattern from two different views. The red arrow shows the flow direction. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
is more pronounced under annular flow conditions (Fig. 13). Erosion caused by 300 μm sand is about 3–4 times higher than that caused by 75 μm sand in lower gas velocities of 22 m/s and 23 m/s. At higher superficial gas velocities of 27 m/s and 35.5 m/s, however, erosion caused by 300 μm sand and 75 μm sand is comparable.
with superficial gas velocity is demonstrated for two superficial liquid velocities of 0.02, and 0.04 m/s. Surprisingly, transition from co-current upward annular flow to unstable annular flow with liquid flow reversal appears to have an impact on maximum erosion magnitude. At a gas superficial velocity of 23 m/s, where unstable annular flow was the flow regime observed, erosion is slightly greater compared to that measured in annular flow condition with a superficial gas velocity of 27 m/s. This behavior has also been reported by Vieira et al. [8] whose experiments were conducted in a 76.2-mm elbow. Note, however, that under annular flow conditions, erosion increases, as superficial gas velocity increases from 27 m/s.
4.2.3. Location of maximum erosion Samples of erosion values and locations of maximum erosion in gassand and gas-liquid-sand experiments in the vertical orientation and for various superficial gas and liquid velocities are shown in Fig. 14. The values on each transducer indicates erosion in mm/kg. Vieira et al. [8] and Parsi et al. [10] observed that as flow pattern changes from annular flow to slug/churn flow, location of maximum erosion moves towards the sides of the elbow. This location transmission was attributed to the periodic liquid structures existing in churn
4.2.2. Effect of sand size on erosion Compared to gas-sand flow conditions, effect of sand size on erosion
Fig. 11. Effect of gas and liquid flow rates on maximum erosion magnitude in vertical annular flow; a) Vsg = 23 m/s, and b) Vsg = 31 m/s. Sand size is 300 μm. The superficial velocity of zero indicates gas-sand flow. 6
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Fig. 12. Effect of flow regime on erosion; variation of erosion magnitude with superficial gas velocity at constant superficial liquid velocities of (a) 0.02 m/s and (b) 0.04 m/s.
orientation to understand the liquid film behavior inside the elbow. In the experiments, water dyed blue and tests were performed with a constant superficial liquid velocity of 0.06 m/s for different superficial gas velocities of 26 m/s, 34 m/s, and 42 m/s (Fig. 15). Of course, the liquid film on the elbow is originated from the liquid film around the circumference of the straight pipe before the elbow. The latter film moves towards the center of the outer bend (as shown by small triple-arrows) and generates a relatively thick liquid layer at the center of the outer bend. As gas velocity increases, liquid entrainment rate from the liquid film into the gas core increases and consequently, the thickness of the film at the center decreases. Additionally, effect of superficial liquid velocity on liquid film behavior at a constant gas velocity was investigated (Fig. 16). It is seen that as superficial liquid velocity increases, the liquid film becomes thicker. Secondary flow inside the elbow drains the liquid from the outer bend towards the inner bend causing streaks of water to appear on the sides of the elbow. Further exploration of the videos revealed that liquid droplets moved in the gas core, impacted the outer bend of the elbow, and coalesced with the liquid present at the outer bend. Under gas-liquidsand flow conditions, these droplets may contain sand; therefore, the sand must penetrate through the liquid film to impact the bend. Fig. 17 examines the flow visualization results in the context of erosion. As indicated earlier, there is a range of superficial liquid velocities, where erosion may increase with an increase in liquid velocity, and then erosion decreases afterwards. It is seen that for experiments with Vsl ≤ 0.06 m/s, the liquid films on the outer bend are not significantly different. Yet, in the straight pipe
Fig. 13. Effect of sand size on erosion magnitude under vertical gas-liquid-sand flow conditions.
flow. However, in the current work, it was observed that regardless of gas and liquid velocities, location of maximum erosion was approximately between 40° and 50° on the outer bend of the elbow. For the location of the maximum erosion (not the pattern) under gas-sand flow conditions, the results of the current work are consistent with the observations of Solnordal et al [13]. 4.2.4. Liquid film behavior Flow visualization studies were performed in the vertical
Fig. 14. Samples of erosion values in gas-sand and gas-liquid-sand experiments in the vertical orientation. 7
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Fig. 15. Effects of gas velocity on liquid film behavior in the vertical orientation. Shown are pictures from top and side views.
before the elbow, as superficial liquid velocity increases, the liquid film becomes thicker compressing the gas path. Consequently, a small increase in the superficial liquid velocity also increases the gas core velocity. This in turn increases the particle velocities in the gas core and thus erosion increases. Further increase in the superficial liquid velocity (e.g. Vsl = 0.10 m/s), however, creates a thicker liquid film on the elbow, which can protect the wall from the particle impacts and thus erosion decreases.
vertical orientation is higher than that in the horizontal inclination. The difference is very significant for the pair with superficial gas velocity of 23 m/s and superficial liquid velocity of 0.15 m/s, where a difference of a factor of 40 can be noticed. Although 8 transducers were mounted on the elbow to have the maximum coverage on the outer bend, it is possible that in horizontal tests transducers are not in appropriate location to measure the maximum thickness loss. To compare the erosion results in the vertical and horizontal elbows, there are two main factors to be considered; liquid film and entrainment rate distributions due to gravity. The effects of these two factors on erosion were investigated through flow visualization and paint erosion tests in the horizontal elbow. Let us first explore the liquid film behavior in the horizontal elbow. Fig. 20 displays flow visualization results for different superficial gas and liquid velocities. Considering Fig. 20a, in stratified wavy flow (Vsg = 15 m/s, Vsl = 0.04 m/s), the liquid film merely flows in the lower half of the elbow, where it slides over the outer bend. However, as superficial gas velocity increases to 35 m/s and annular flow regime is generated (Fig. 20b), a uniform film forms on the outer bend in both the upper and lower halves of the elbow. Fig. 20c and d reveal the effect of superficial liquid velocity on the film behavior. For the same superficial gas velocity of 20 m/s, as superficial liquid velocity increases from 0.04 m/s to 0.08 m/s, liquid on the bottom half of the outer bend slides towards the upper half. As a result, a wider area of the outer bend is covered by liquid protecting the elbow from entrained particles that are mostly concentrated near the liquid gas interface due to gravity.
4.2.5. Paint erosion study Fig. 18 demonstrates the (top view) sequences of paint removal for two experiments with superficial gas velocities of 23 m/s and 31 m/s. The superficial liquid velocities were 0.08 m/s and 0.05 m/s, for the former and latter, respectively. Both experiments were performed in the vertical orientation with 300 μm sand. There are two eroded regions. The first region is where the elbow experiences the maximum erosion; the circular area about 40–50° at the outer bend center. The second eroded region is the single stripe-like area about 60–90°. The erosion here occurs due to impingements of rebounded sand as well as abrasion by sand present in the liquid film sliding on the outer bend. 4.2.6. Effect of flow orientation on erosion Some erosion experiments were performed in the horizontal orientation to investigate the influence of flow orientation on erosion. Fig. 19 compares the results of those experiments with erosion data obtained in the vertical elbow. In all cases examined, erosion in the
Fig. 16. Effects of superficial liquid velocity on liquid film behavior in the vertical orientation. Shown are pictures from top and side views. 8
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Fig. 17. Effects of liquid velocity on erosion magnitude at a constant gas velocity equal to 31 m/s (vertical elbow).
Besides the liquid film behavior, the effect of gravity must be considered. This effect can be spotted in Fig. 21, where an asymmetrical erosion pattern is observed; on the upper-half of the outer bend, more paint has been removed. Because of the gravity force, most of the particles are carried by the liquid film at the bottom of the straight pipe,
When covering the elbow, the liquid film can play a role of protection, as it can decelerate particles impacting. This protection can be seen in Fig. 19, where erosion is much lower for a high superficial liquid velocity of 0.15 m/s compared to the case with Vsl of 0.02 m/s (horizontal elbow).
Fig. 18. Paint removal sequences in vertical annular tests with 300 μm sand; left column: Vsg = 31 m/s and Vsl = 0.05 m/s, right column: Vsg = 23 m/s and Vsl = 0.08 m/s. 9
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horizontal orientation, particles mostly flow with the low-velocity liquid film before entering the elbow, while in the vertical orientation, particles can entrain into the high-velocity gas core and impact the elbow. Note that at the lowest superficial velocities (Fig. 19), the difference between erosion magnitudes in the two orientations is the lowest. This is expected, since as the liquid amount decreases, the gas-liquid flow becomes more like gas-only flow, where flow orientation has no impact on erosion. 5. Conclusions Different experiments were performed to study solid-particle erosion under gas-sand and gas-liquid-sand conditions in a 101.6-mm elbow. Ultrasonic measurements were conducted to evaluate wallthickness loss during each experiment. In summary:
Fig. 19. Comparison of maximum erosion in vertical and horizontal experiments under annular flow conditions; 300 μm sand.
• Under gas-sand conditions, a rabbit ear-like erosion pattern was
which has a low velocity. As a result, the bottom half of the outer bend is exposed to particles with low velocities. Revisiting Fig. 19, in the horizontal elbow, there is always a liquid film protecting the elbow from the impacts, whereas in the vertical elbow, there are moments that dry-out occurs, and a large portion of the elbow is dry and not protected by the liquid film. Second, in the
•
observed with the maximum erosion occurring at 40–50° along the outer bend. Under vertical gas-liquid-sand conditions, two different behaviors were discerned when superficial liquid velocity increased. At lower liquid flow rates, an increase in superficial liquid velocity increased erosion. On the other hand, this was reversed for the higher liquid
Fig. 20. Effects of gas velocity on liquid film behavior in the horizontal orientation. Shown are pictures from top and side views. 10
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Fig. 21. Flow and erosion pattern under horizontal annular flow condition; Vsg = 31 m/s, Vsl = 0.05 m/s, and 300-μm sand.
flow rates.
[5] A.T. Bourgoyne, Experimental Study of Erosion in Diverter Systems Due to Sand Production, (1989) New Orleans, Louisiana. [6] N.R. Kesana, S.A. Grubb, B.S. McLaury, S.A. Shirazi, Ultrasonic measurement of multiphase flow erosion patterns in a standard elbow, ASME J. Energy Resour. Technol. 135 (3) (2013). [7] R.E. Vieira, Sand Erosion Model Improvement for Elbows in Gas Production Multiphase Annular and Low-Liquid Flow, PhD Dissertation The University of Tulsa, Oklahoma, 2014. [8] R.E. Vieira, M. Parsi, P. Zahedi, B.S. McLaury, S.A. Shirazi, Ultrasonic measurements of sand particle erosion under upward multiphase annular flow conditions in a vertical-horizontal bend, Int. J. Multiph. Flow 93 (2017) 48–62. [9] R. Vieira, M. Parsi, P. Zahedi, B. McLaury, S. Shirazi, Electrical resistance probe measurements of solid particle erosion in multiphase annular flow, Wear 382 (383) (2017) 15–28. [10] M. Parsi, R. Vieira, N. Kesana, B. McLaury, S. Shirazi, Ultrasonic measurements of sand particle erosion in gas dominant multiphase flow, Wear 328 (329) (2015) 401–413. [11] M. Parsi, R. Vieira, C.F. Torres, N.R. Kesana, B.S. McLaury, S.A. Shirazi, E. Schleicher, U. Hampel, Experimental investigation of interfacial structures within churn flow using a dual wire-mesh sensor, Int. J. Multiph. Flow 73 (2015) 155–170. [12] M. Parsi, M. Agrawal, V. Srinivasan, R. Vieira, C.F. Torres, B.S. McLaury, S.A. Shirazi, CFD simulation of sand particle erosion in gas-dominant multiphase flow, J. Nat. Gas Sci. Eng. 27 (2) (2015) 706–718. [13] C.B. Solnordal, C.Y. Wong, J. Boulanger, An experimental and numerical analysis of erosion caused by sand pneumatically conveyed through a standard pipe elbow, Wear 336–337 (2015) 43–57. [14] M. Parsi, A. Al-Sarkhi, M. Kara, P. Sharma, B.S. McLaury, S.A. Shirazi, A new dimensionless number for solid particle erosion in natural gas elbows, Wear 390–391 (2017) 80–83.
• Liquid film thickness in the elbow, particle velocity in the gas core, and particle entrainment were recognized to be the governing factors for the two behaviors mentioned.
Acknowledgements This work was conducted, when Dr. Peyman Zahedi was with E/ CRC. He would like to acknowledge the member companies of E/CRC for their financial support. References [1] B.S. McLaury, S.A. Shirazi, V. Viswanathan, Q.H. Mazumder, G. Santos, Distribution of sand particles in horizontal and vertical annular multiphase flow in pipes and the effects on sand erosion, J. Energy Resour. Technol. 133 (2011) 0230011–023001-10. [2] M. Pyboyina, Experimental Investigation and Computational Fluid Dynamics Simulations of Erosion on Electrical Resistance Probes, M.S. Thesis Department of Mechanical Engineering, The University of Tulsa, Tulsa, Oklahoma, USA., 2006. [3] R. Dosila, Effects of Low Liquid Loading on Solid Particle Erosion for Gas Dominant Multiphase Flows, M.S. Thesis Department of Mechanical Engineering, The University of Tulsa, Tulsa, Oklahoma, USA., 2008. [4] C. Fan, Evaluation of Solid Particle Erosion in Gas Dominant Flows Using Electrical Resistance Probes, M.S. Thesis Department of Mechanical Engineering, The University of Tulsa, Tulsa, Oklahoma, USA., 2010.
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