Explosion enhancement through a 90° curved bend

Explosion enhancement 90° curved bend H. Phylaktou, Department Received

through

a

M. Foley and G. E. Andrews

of Fuel and Energy, 18 March

The University

of Leeds,

Leeds,

LS2 9JT,

UK

I992

Junctions and bends are commonly incorporated in pipeline systems conveying potentially explosive mixtures. Very little data exists on how these affect the development and transmission of an accidental explosion. There is therefore great uncertainty when designing explosion protection measures for such systems. A 3 m long, 162 mm diameter tube, closed at both ends and incorporating a 90” curved bend was used to investigate the influence of the bend on the development of gaseous explosions. Methane/air mixtures of 10% and 5.7% by volume were used. The mixture was ignited at one end of the tube at 5 mm or 53 mm from the flange. It was found that for all explosions, the flame moved faster around the inner wall of the bend than the outer and hence it was elongated. This gave rise to an overall acceleration of the flame and a significant increase in the rate of pressure rise. The enhancement factor due to the bend ranged from 4 to 6 for the 10% mixture and it was equivalent to the effect of an orifice plate with a 20% blockage in the path of the flame. These findings highlight the need for more work on bends of different shapes and in different layouts. (Keywords:

explosion

enhancement;

bend;

flame

Process plants contain large pipeline systems connecting various reaction vessels. In many situations flammable gases or liquids are handled and there is therefore a risk of an accidental explosion occurring. Such piping systems usually incorporate bends, junctions and changes in cross-section. Flow through such geometries changes the velocity and creates turbulence. Both of these effects will accelerate a flame, the former by flame surface area distortion, and the latter by turbulence enhancement of the burning velocity. Therefore, an explosion in a piping system containing these flow disturbances would be expected to be more violent than one in a straight pipe, and thus different protective measures would be required. This fact is acknowledged in current explosion vent design guidelines1,2 where it is pointed out that there is insufficient information to give detailed venting relationships for obstructions of this kind. The present project was directed at this problem with the aim of investigating how an explosion propagates round a 90” bend and of quantifying the effect of this common flow device on important venting parameters such as the rate of pressure rise and the speed of flame propagation. In the deflagration of a flowing gaseous mixture the flow characteristics are an integral part of the combustion process. Even if the combustible gas is initially stagnant, the expansion of the hot burnt gases This paper was presented at the 13th ICDERS, 1991. 0950-4230/92/010021-09 @ 1993 Buttewnrth-Heinemann

Japan,

July

speeds;

rates of pressure

rise)

will set up a flow which, in turn, may stretch and fold the flame, produce turbulence, and initiate combustion instabilities. All these phenomena contribute to the enhancement of the combustion rate. This means that the combustion sets up a gas flow, which acts as a positive feedback loop on the combustion itself. This coupling mechanism between flame acceleration and gas-flow dynamics is the key problem in gas explosions, whether confined or unconfined. Fluid flow in bends is fairly well understood. Besides being an important practical problem, it has stimulated much interest by virtue of the fascinating features of the flow. Figure 1, taken from Ito3, shows the static pressure distribution, plotted in a nondimensional form, for incompressible turbulent flow through a bend of circular cross-section with upstream and downstream tangents (lengths of straight pipe attached to the bend) sufficiently long for fully developed flow to exist away from the bend. This diagram demonstrates that the influence of the bend on the axial pressure gradient exists a few diameters upstream of the bend and a large distance (fifty or so diameters) downstream. In Figure 1 a common way of defining the bend loss coefficient is illustrated: it is the pressure drop that takes place within the bend in excess of the pressure drop that would occur in a straight pipe of the same length. A comprehensive analysis of published data on the flow and pressure losses in a bend has been made by Ward Smith4. Figure 2, presented by this author, shows typical measured static pressure distribution at

Ltd

J. Loss Prev. Process

Ind., 1993, Vol 6, No 1

21

Explosion

enhancement

through

a bend:

H. Phylaktou

et al.

---+--_ 0----o---

Downstream

Figure 1 Typicat static pressure distribution in the neighbourhood and x,, are the upstream and downstream distanoas reapactively

Upstream

1.0

Bend Outer

0.5

tangent

of a bend with long tangents

(after

3)). x,,

wafl

along duct

axis,

I

t Inner

wall

I

I

id/h

l

55.0

X

3.45

0

1.15

A

0

L, = Length of downstream tangent

L

Figure 2 Static pressure distribution Smith, 1971 (Reference 4))

xd Ih

I

Symbol

h = Height of duct f-M on duct wall in centreline

the duct wall in the centreline plane for bends of square cross-section. Variations in the static pressure due to the presence of the bend start to occur at between one and two duct diameters in the upstream tangent. An adverse pressure gradient develops on the outer surface of the bend and a favourable gradient is formed on the inner surface. At some stage the pressure

22

Ito, 1SW (Reference

Downstraam

Distance

-2.0

Inner side Top Bottom

J. Loss Prev. Process Ind.,

1993, Vol6,

No 1

of bend R/0=1.15,

where

R is the curvature

of the bend (after Ward

distribution on the outer wall becomes favourable, whereas an adverse pressure gradient is formed on the inner wall. Providing the downstream tangent is sufficiently long, the variations in static pressure at a cross-section persist into the downstream tangent up to about 1.5 duct diameters4. Very few experiments have been carried out in

Explosion order to investigate the effect of a bend on an explosion. Burgess5 investigated the effect of bends of various angles on the projection of the flame of an explosion beyond the original confines of the explosive mixture and compared it to the projection of the flame from an equivalent straight gallery. He found that there was no marked effect with bends of 30” and 60” but with bends of 90” and 120” the projections in both directions were increased. Fearnley and Nettleton used single shot Schlieren photography to freeze the shapes of initially planar shocks in either air or argon as they propagated round bends in a channel 22 X 47.4 mm. The influence of the shock strength and radius of curvature of the bend on the shape of the resultant shock was investigated. For all shock velocities and for all bends the overall effect of the bend was to reduce the shock velocity at 6 15 diameters downstream of the bend. This was apparently a result of the non-symmetrical interactions of the wave systems produced at the inner and outer walls. In sharp bends enhancement of shock velocity at the outer wall and its attenuation at the inner wall led to highly asymmetric stressing of the bend. The authors concluded that considerable caution is required in applying the concept of a safe containment pressure to a bend which is potentially subject to the effects of an internal explosion. So, although fluid flow in bends is fairly well documented, very little data is available concerning the effect a bend would have in an explosion. In this paper we present some data on the explosion-induced flow round a bend, its interaction with the propagating deflagration front and the overall influence on the pressure rise in the system.

Experimental A 3 m long (L), 162 mm diameter (D) horizontal stainless steel tube was used, closed at both ends (LID=18.7). It was constructed out of six sections, five equal length straight sections and one 90” curved bend section 565 mm in length. The radius of the bend curvature was 222 mm. The sections were connected together with bolted flanges, as shown in Figure 3. The bend section was the third one from the ignition end. Methane/air mixtures were used and were formed by partial pressures to 1 atm mixture pressure. An external recirculation pump was used to ensure good mixing within the vessel. Ignition was implemented by means of a spark discharge at one end of the vessel. One set of explosions was carried out using a spark eiectrode whose tip was 53 mm from the flange and another set using a spark plug 5 mm from the flange. The flame travel was recorded by an axial array of mineral-insulated, exposed junction, type K thermocouples. The time of flame arrival was detected as a distinct change in the gradient of the analogue output of the thermocouple and from this the average flame speed between any two thermocouples could be calculated. This method of measuring the flame speed has been compared with high speed photographic

enhancement

through

a bend:

H. Phylaktou Pressure

et al.

transducer

Thermocouples

Spark PM igniter

Figure 3

Experimental set-up

measurements in spherical flame explosions and has been shown to give excellent agreement’. Kumar et a/.* recently used this technique to detect flame arrival in hydrogen explosions. Thermocouples were also placed along the inner and outer radius of the bend in order to characterise the flame travel around the bend. The pressure variation was recorded using a SENSYM pressure transducer mounted at the centre of the flange at the far end from the ignition. Pressure tappings were also located across the bend at the inner and outer walls to measure the pressure drop due to the explosion-induced gas flow through the bend, ahead of the propagating flame. This measurement was made with a SENSYM ? 1 bar differential pressure transducer. From this, the velocity of the unburnt gas ahead of the flame could be calculated using steadystate concepts of flow in bends. The practice of using steady-state equations for a flow induced by a highly transient event has been shown by the authors9 to give acceptable results. A 16-channel (1 MHz) transient data recorder (AIMS, Computerscope) was used to record and analyse all data. Each explosion was repeated at least three times and averaged readings were used.

Results and discussion General features In two recent papers Phylaktou et af.‘O and Phylaktou and Andrews” presented detailed studies of explosion development in straight large LID enclosures. Figure 4 shows the pressure and flame position against time for a 10% methane/air explosion in a straight tube with the same diameter and similar length to the present arrangement. The influence of the bend was assessed by comparison to this equivalent straight tube data. In a straight tube of large L/D there was an initial phase of rapid flame acceleration along the axis of the tube, associated with a high rate of pressure rise, (dP/dt),.

J. Loss Prev. Process Ind., 7993, Vol6,

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23

Explosion

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, spark

at

through

a bend:

H. Phylaktoo

53mm

6 _--

--

16 ---

0 0

100

200 Time

300

400

(ms)

Figure 4 Typical pressure-time and flame position-time history of a 10% methane/air explosion in a straight pipe with no blockages (ignition at 53 mm)

This was attributed to the flame front moving faster axially than radially, resulting in an elongated flame with a large area, and therefore high combustion rate. At about four to six tube diameters from the spark a large part of the flame area was quenched on the vessel walls and this resulted in lower flame speed and lower rate of pressure rise, (dP/dt),. The initial phase of combustion was identified by the authors as the most critical for explosion protection of these vessels because: 1. it results in destructive overpressures (in excess of normal venting pressures); 2. it occurs very early in the explosion, compared to the long delay period in spherical deflagrations; 3. the fast combustion rate results in a high burnt gas expansion rate which, combined with the tubular geometry, induces a fast velocity flow of the unburnt gas ahead of the flame. This fast flow may result in turbulence which would further enhance the combustion. The last point is particularly true when obstacles are present in the path of the induced flow. Andrews and Herath12 and Andrews et at.13 demonstrated the explosion enhancing effect of such obstructions. Phylaktou and Andrews9 reported over a lOO-fold increase in the combustion rate of a stoichiometric methane/air mixture with a single orifice plate in a tube with L/D = 21 (D=76 mm). The two most influential parameters were determined to be the blockage ratio of the obstacle and the velocity of the flow just ahead of the flame. The latter influence was demonstrated by varying the position of the obstacle.

24

J. Loss Prev. Process ind., 7993, Vol 6, No I

et al. The effect of the obstacle was eight times higher when it was positioned 7 D from the spark (i.e. just after the initial fast flame speed region) than when it was at 14 D from the spark (i.e. in the slower flame speed region). This again underlines the significance of the fast initial phase of combustion in large LID enclosures. The position of the bend section in the present experiments was equivalent to the 7 D position of the orifice plate that had the greatest effect on the explosion. Figure 5 shows typical pressure-time and flame position-time histories of a 10% methane/air explosion with the bend in position. Compared to Figure 4, the initial phase of rapid flame movement and fast rise in pressure remained the same despite the presence of the bend. At the bend region however, a re-acceleration of the flame took place and it was accompanied by an almost step increase in pressure. Downstream of the bend, the flame speed and rate of change in pressure returned to the ‘no bend’ levels. So, in qualitative terms, the bend accelerates the flame movement and increases the rate of pressure rise. In this work, these influences were quantified and compared to the effects of an orifice type blockage. Flame movement Bend flow theory states that the pressure at the outer wall of a bend will be greater than that at the inner wall as shown in Figures 1 and 2. This pressure difference was measured for the 10% methane/air explosions. The maximum values obtained were 2 and 4 mbar for the short and long spark plug respectively. Using standard equations for bend flow metering, the

I

7

6

With

bend

10%.

spark

I at

I

20

53 mm

-

1

0

I

I

I

100

200

300

Time

16

Id 400

(ms)

Figure 5 Typical pressure-time and flame position-time history of a 10% methane/air explosion in the present experimental set-up (ignition at 53 mm)

Explosion corresponding maximum unburnt gas velocities induced ahead of the propagating flame were 30 and 42 m/s. The pressure difference between the inner and outer wall of the bend was expected to give faster flames around the inner wall. Figures 6 and 7 are contour plots showing the approximate flame shape and position at certain times. They were drawn using time of arrival data and flame speeds between thermocouples. The acceleration of the flame around the inner wall of the bend is clearly evident. This effect becomes very marked when it causes the flame to reach a thermocouple further downstream before the one that precedes it on the bend. For example, in Figure 6 the flame is detected to arrive at thermocouple B5E before it arrives at B4E. This is due to the flame shape and the slower rate of flame travel at the outer wall of the bend. Burning probably then occurs radially towards the outer wall consuming the pocket of unburnt gas. The difference in flame shape suggested in Figures 6 and 7 is due to the difference in the ignition position. As discussed by Phylaktou and Andrews” such small differences in spark position significantly affect the development of the flame in the initial phases. With the short spark plug (almost flush with the flange) the flame begins as a hemisphere attached to the flange and then elongates. With the longer spark electrodes, a spherical flame is initially allowed to develop. This eliminates the initial heat losses and increases the burnt gas expansion rate and hence the flame speed. Also, the extra flame surface area increases the mass burn rate and further accelerates the flame. The work of Ellis and Wheeler14 showed photographically the differences in the explosion progress with different spark positions. A further conse-

enhancement

through

a bend:

et al.

H. Phylaktou

quence of these differences is that the flame originating from the flush spark plug expands and reaches the vessel walls at a shorter distance from the ignition point. A number of authors1&r6 have reported and investigated the phenomenon of ‘tulip flames’. The tulip-shaped flame, as well as the pressure oscillations” evident in Figures 4 and 5, appear to be triggered by the sudden cooling and retardation of the flame as a result of its contact and quenching at the vessel side walls. So, since for the shorter spark plug the flame reaches the side walls earlier, the tulip flame forms earlier as illustrated by the shape of flame seen entering the bend in Figure 7. The flame speeds along the centreline of the rig together with the inner and outer wall flame speeds at the bend region are shown in Figure 8. Figures 8a and 86 were obtained using long spark ignition in 10 and 5.7% methane/air mixtures respectively. Flush ignition at the flange in 10% mixtures gave the results in Figure 8~. In all cases the results clearly demonstrate that the flame moves faster in the inner side of the bend and slower on the outer side. In Figure 9 the centreline flame speeds with the bend in position are compared to those in a straight pipe with no obstruction and to those in a straight pipe with a single hole orifice plate at the bend equivalent position, causing a 20% blockage to the flow. The bend and the orifice plate had a similar effect on the explosion; they did not significantly affect the upstream flame behaviour but the flame was notably accelerated downstream of both. The higher flame speeds persisted for about 6 to

B2E

B3i 821 ’

BlM

BlM

-

t

Dir&ion of propagation

Figure6 Constant time contour plot of a 10% flame position in the bend (ignition at 53 mm)

t

Direction of propagation

methane/air

Figure7 Constant time contour plot of a 10% flame position in the bend (ignition at 5 mm)

methane/air

J. Loss Prev. Process Ind., 1993, Vol 6, No I

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Explosion

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IO%-spark

at

53 mm

Centreline

X

40

et al.

H. Phylaktou

0

Inner

0

outer

30

a

Distance/diameter

(X ID 1

3.00

log-spark

5.7% -spark

2.50

X

at 53mm

ilc Centreline 2.00

0

Inner

0

Outer

at

5mm

Centreline

0

inner

0

Outer

z -z :: $ 1.50 L.7 t S IA 1 .oo

0.00

b

1

,

I

I

I

1

0

4

8

12

16

20

Distanceldlameter

( x/D1

Figure8

0 C

I

t

t

,

4

a

12

Distance/diameter

Centreline flame speeds along the tube and inner and outer wall flame speeds at the bend: at 53 mm: (b) 5.7% methane/air, ignition at 53 mm; (c) 10% methane/air, ignition at 5 mm

10 tube diameters downstream of the disturbance and then returned to the straight pipe / no blockage values. For the 10% mixture with ignition at 53 mm from the spark (Figure 9~) the bend accelerated the flame to speeds of about half the magnitude of those of the 20% blockage. When ignition was 5 mm from the flange (Figure SC) the influence of the blockage was reduced. This was due to the lower upstream flame speeds with this spark position, which induced lower unburnt gas velocities through the orifice and hence lower turbulence levels and flame speeds downstream. Similar ‘behaviour

26

J. Loss Prev. Process Ind., 1993, Vol 6, No 1

I

I

16

20

( x/D) (aI 10% methane/air,

ignition

was expected with the bend. Instead, the flame speeds through and downstream of the bend remained very much unchanged for this spark position despite the 30% reduction in the induced unburnt gas velocity through the bend as shown earlier. As a result, the flame speeds induced by the bend were very similar to those induced by the 20% blockage. A possible explanation for this situation is that, in the case of the bend, the reduction in turbulence levels is partly compensated by the increased game area of the tulip flame entering the bend as shown in Figure 7.

Explosion

10%-Spark

enhancement

through

a bend:

H. Phylaktou

et al.

at 53 mm

w( No disturbance 0 20% blockage

4

0

6

16

12

20

Distance/diameter ( x/D1 5.7% -Spark

2.00

c 2

at 53mm

10%-Spark

50

YI No disturbance 0 20% blockage 0 Bend

%

T

0 0

1.60

40

1.20

30

at 5 mm

disturbance 20% blockage Band No

8 zI 0.80 I#.

-

20

d -

0.40

10

I

0.00I”

b

4

8 DismnceIdiameter

16

12 ( xlD

20

1

0

c

I

,

4

8

I

I

12

16

I 20

Distanceldlarneter (x/D)

-9 Comparative centreline flame speeds: (a) 10% methane/air, ignition at 53 mm; (b) 5.7% methane/air, ignition at 53 mm; (c) 10% methane/air. ignition at 5 mm

Ram of pressure rise A better indicator than the flame speed for the severity of an explosion is the rate of pressure rise. This parameter is commonly used in the design of pressure relief vents. The rate of pressure rise is characteristic of the rate of production of burnt gases and therefore determinant of the rate of venting and consequently the vent size needed in order to keep the pressure in the system below a dangerous level. Rate of pressure rise readings were taken from the pressure trace of each explosion as shown in Figures 4 and 5. These are

summarized in Table 1 together with the peak and mean maximum pressures attained, as defined in Figure 4. This table shows that the first rate of pressure rise is more or less independent of the .downstream configuration. In contrast the second rate increases with the presence of either the bend or the obstacle. In order to get a clearer indication of the relative effect of the bend and the obstacle, the rates of pressure rise with these obstructions in position were normalized by dividing by the equivalent straight pipe/ no blockage rates. The findings are shown in Table 2.

.I. Loss Prev. Process Ind., 1993, Vol 6, No 1

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Explosion

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Table 1 Rates of pressure Explosion

through

a bend:

et al.

H. Phylaktou

rise

type

No blockage No blockage No blockage Bend Bend Bend 20% blockage 20% blockage 20% blockage

Table 2 Ratios comparing

Spark position distance from flange (mm)

Fuel percentage

(dfldt), (bar/s)

(dl’ldt), (bar/s)

Peak PMax (bar)

Mean P MDX (bar)

53 53 5 53 53 5 53 53 5

IO 5.7 10 10 5.7 10 10 5.7 10

30.2 0.59 17.6 31.4 0.27 23.5 35.8 0.60 21.6

13.4 0.59 6.20 53.8 0.27 37.68 77.7 1 .Ol 35.5

6.00 1.72 4.84 5.58 0.77 5.69 6.97 1.87 5.37

4.45

blockages,

bends and straight

4.34

5 mm from flange 10

5.7

10

Fuel percentage

4.63 5.20

pipes

53 mm from flange

Spark type

3.93 4.62

Ratios

b:st

ZO%:st

b:st

ZO%:st

b:st

20%

(dfldt), (dP/d t), Mean PNlax

1.040 4.015 1.038

1.185 5.799 1.169

0.458 0.458

1.107 1.712

1.335 6.077 1.178

1.227 5.726 1.104

where

b:st = bend:straight

pipe; ZO%:st = 20% blockage:straight

For 10% mixtures and with ignition at 53 mm from the flange, the bend enhanced the combustion rate by a factor of 4 while the 20% blockage caused an enhancement of about 6. With ignition at 5 mm from the flange the enhancement was about 6 for both cases. It must be noted that the rates of pressure rise as well as the flame speeds (Figures 8b and 96) for the 5.7% methane/air mixture followed the trends described for the 10% mixtures in general terms, but appeared to contain discrepancies and inconsistencies and were therefore unreliable to draw general conclusions from. The most likely reason for this behaviour is the fact that this was a very weak mixture, near the lean flammability limit, with a very slow burning rate. This allowed time for buoyancy to take effect lifting the hot gases to the top of the horizontal tube and thus the propagation was continuously under the influence of buoyancy. Despite this, the important message from the 5.7% mixture results is that in a tubular containment a deflagration in this mixture can propagate at significant speeds, higher than stoichiometric mixture flame speeds in spherical explosions, and can be accelerated by obstructions causing significant overpressures.

A 90“ smooth curved bend caused a significant acceleration of the flame propagation in a tube. The combustion rate downstream of the bend was enhanced

J. Loss Prev. Process.lnd.,

pipe with no blockage

by as much as six times. This was comparable to the enhancement factor induced by an orifice plate with a 20% blockage at the same position. It is expected that a sharp 90” bend or a bend with a more acute angle will cause even greater enhancement than the present findings. These results demonstrate that the presence of a bend in a pipeline system would greatly affect the explosion venting. requirements of the system. Of course more work is required before we have a clear picture of the influences of the bend. Work on bends of different shapes and layouts is necessary to assess the problems that could arise from these being in the path of an explosion.

Acknowledgements The authors thank the UK Science and Engineering Research Council for financially supporting this work.

References 1 NFPA 68, National Fire Protection Association, Booklet No 68 ‘Guide for explosion venting’, 1978 2 Lunn, G. A. ‘Venting of Gas and Dust Explosions - A Review’, IChemE Industrial Fellowship Report, The Institution of Chemical Engineers, 1984 3 Ito, H. Tram. Am. Sot. Mech. Engineers (Series D) 1960, 82, 131 4 Ward Smith, A. S. ‘Pressure Losses in Ducted Flows’, London,

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Butterworths, 1971 5 Burgess, M. J. ‘Safety of Mines Research Board’, 1934, Paper No 83 6 Feamley, P. and Nettleton, M. A. IChemE Symposium Series 1983, 82, E34

Explosion 7 Herath, P. ‘Closed vessel explosions: the influence of baffles’, PhD Thesis, University of Leeds, 1986 8 Kumar, R. K., Dewit, W. A. and Greig, D. R. Cornbust. Sci. and Tech. 1989, 66, 251 9 Phylaktou, H. and Andrews, G. E. Cornbust. and Flame 1991, 85. 363 10 Phylaktou, H., Andrews, G. E. and Herath, P. J. Loss Prev. Process Ind. 1990, 3, 355 11 Phylaktou, H. and Andrews, G. E. Cornbust. Sci. and Tech. 1991, 77, 27 12 Andrews, G. E. and Herath, P. ‘Progress in Aeronautics and

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13 14 15 16 17

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a bend:

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et al.

Astronautics, Dynamics of Explosions’, editors A. L. Kuhl er al. AIAA 1988, 114, 512 Andrews, G. E., Herath, P. and Phylaktou, H. J. Loss Prev. Process Ind. 1990, 3, 291 Ellis, 0. C. de C. and Wheeler, R. V. J. Chem. Sot. 1928, 3215 Dun-Rankin, D., Barr, P. K. and Sawyer, R. F. ‘Twentyfirst Symp. (Int.) on Combustion’, The Combustion Institute, Pittsburgh, 1986, 1291 Starke, R. and Roth, P. Cornbust. and Flame 1986, 66, 249 Leyer, I. C. and Manson, N. ‘Thirteenth Symp. (Int.) on Combustion’, The Combustion Institute, Pittsburgh, 1971, 551

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