- Email: [email protected]
Clarifying nozzle load and piping
32
Feature
WORLD PUMPS
November 2012
Operating
Clarifying nozzle load and piping Piping designers always want higher allowable nozzle loads to simplify pump piping designs, while pump manufacturers want smaller such loads to assure good alignment, higher reliability and fewer problems in operation. With long-term reliability a key factor, Amin Almasi recommends that users side with the manufacturer.
T
he use of some common practices for general plant piping could lead to real problems in pump piping arrangements. In a pump system under ambient conditions, the dead weight of the piping should be entirely absorbed by suitable supports. Only nozzle loads induced by temperature, pressure and the transported medium (or similar operational loads) at the operating conditions can be compensated for by pre-stressing (or springing) the pipes. In this article, pump nozzle load limits, proper load optimization and correct pump piping are discussed.
Pump nozzle load Nozzle loads are defined as the net forces and moments exerted on equipment nozzles from the weight and thermal expansion of connected piping and other equipment. For pumps, nozzle loads are specified in API 610 (or other pump codes and standards such as ANSI, etc.). The API 610 standard covers horizontal pumps, vertical in-line pumps and vertically suspended pumps for nozzle sizes up to 16 inch (400 mm). For pump nozzles larger than 16 in, nozzle loads should be agreed with the vendor before the pump order is finalized. Generally, small pumps that are not anchored to the foundation are able to tolerate higher nozzle loads. Allowable loads for vertical in-line pumps with supports that are not anchored to the foundation may be twice the values for in-line pumps that are anchored. www.worldpumps.com
Figure 1. An example of a floating production, storage and offloading (FPSO) vessel. An FPSO vessel requires many pumps. The nozzle loads and piping of these pumps present unique challenges.
0262 1762/12 © 2012 Elsevier Ltd. All rights reserved
WORLD PUMPS
Feature November 2012
and shafts at various locations, such as at the drive-end of the shaft and at the register fit of the coupling hub. Thermal growth, piping fabrication errors and different alignment errors all contribute to the actual deflection values and final nozzle load experienced in the field. The pump should operate without any leakage, without internal contact between the rotating and stationary components, and without losing alignment, while subjected simultaneously to the maximum operating conditions (temperature, pressure, speed and power) and the worst-case combination of allowable nozzle loads.
Figure 2. An example of pump piping arrangements.
Some purchasers ask for higher allowable loads (sometimes as much as 1.5 times or two times the API loads) to make the piping design easier. API nozzle load values may be considered optimal by pump engineers and pump manufacturers, but in some situations these values cannot be achieved by the piping engineers and stress analysis specialists. Some pump vendors typically design their pump details based on the API code limits. For higher nozzle load values, all necessary checking, verification, modification and adjustment (such as bearing design check, casing design verification and clearances) should be carried out to ensure long-term component life expectations. For special applications, with a very large difference between the operating and ambient temperatures and very large nozzle sizes, a nozzle load as high as 2.5 times the API values may be specified. Steam turbine drivers (for pumps) should be designed to withstand nozzle loads in accordance with NEMA SM23. For special pumps primarily designed for low pressures (such as axial pumps, lowpressure overhung pumps and pumps designed with open impellers), which rely on close radial and axial clearances between the rotating components (impeller or rotor assembly) and the casing, special care should be taken concerning the allowable forces and moments on the nozzles.
To achieve maximum pump reliability, the pump engineer’s goal is always to keep
Excessive piping strains on the pump can cause both internal and external alignment problems. If the internal components become misaligned, accelerated wear, rubbing and early failure can occur. The effects of external misalignment may not be as obvious as those from internal problems, but these effects will in time reduce the pump life. The vibration levels increase as the couplings become misaligned and a high-vibration trip may cause an
"To achieve maximum pump reliability, the pump engineer’s goal is always to keep nozzle loads as low as practically possible." nozzle loads (imposed by piping) as low as practically possible. However, piping designers and stress analysis engineers usually design piping systems based on the allowable nozzle loads of the pumps (or even sometimes slightly higher) in order to reduce the complexity and expense. Optimum flexibility should be designed into the piping to prevent distortion of the pump alignment or damage to a component.
Alignment issues For pumps, two effects of nozzle loads should be carefully considered: 1. Distortion of the pump casing. 2. Misalignment of the pump shaft and the driver shaft. To minimize misalignment because of piping load effects, the pump casing and pump package base-plate should be constructed with sufficient structural stiffness to limit the displacement of casings
unscheduled outage. Extended operation at high levels of misalignment may cause coupling failure, possibly bearing damage or even catastrophic failure.
Piping support design There are numerous rules for piping and supports that may be valid for general plant piping, but are invalid for pump systems. For example, consider the following: • Selecting the support types based on the thermal expansion displacement. • Setting the spring supports using the so-called ‘dead weight balance at operating conditions approach’. With respect to the support practices for general plant piping, the types of supports are usually selected based on the vertical thermal displacement that is expected at the support location. Rigid supports are usually used when the displacement is very small. Variable www.worldpumps.com
33
34
Feature
WORLD PUMPS
November 2012
springs are employed for medium displacement. Constant-effort supports are used when displacements are great. This practice seems logical at first. However, a problem can arise for pump piping systems. The use of spring and constant-effort supports (non-rigid supports) can create more problems than rigid supports. The key point is that the misapplication of a rigid support will be detected as soon as the stress analysis is performed. On the other hand, stress analysis on non-rigid supports (such as spring or constant-effort supports) may not readily indicate any misapplication. The end result can be a piping-support system with unnecessary flexible supports (such as spring supports) that is overly expensive, totally unreliable, inherently unstable and very vulnerable to various dynamic and vibrational excitations. Figure 3. Typical expansion joints with anchors.
Providing the right support There are some pump piping systems designed with several spring supports for various piping spools near the equipment nozzles (because of large thermal movements), mainly because free thermal displacement was used as a criterion for support selection. For example, some engineers perform free thermal movement analysis and use flexible supports (spring supports or similar) for all support points with a displacement above a certain level (say 75 mm). Aside from a very high cost, at first glance this design may appear to have no problems. The computer stress analysis shows perfect results. However, it is unstable and vulnerable to various dynamic issues and operational problems. This design will present problems on site at commissioning and start-up (when the travel stops are removed). The actual piping system weight is different (usually heavier) than the theoretical assumed weight. This difference can result in the failure of the system. Whether the actual weight is heavier or lighter compared to the theoretical values, field adjustment cannot be effective (or is not possible) for numerous flexible supports. This flexible support system design is unable to absorb the uncertainties resulting from the manufacturing tolerances, any inaccuracies in analytical calculations and other possible errors. This unnecessarily over-flexible support system is also unstable. It always vibrates and will certainly fail sooner or later. Flexible supports should only be used where really necessary. Proper rigid www.worldpumps.com
supports should always be provided to absorb any load uncertainties (fabrication tolerances, analysis inaccuracies or similar) and to control the various instabilities.
Extreme temperatures In a pump that works at a very high (or very low) operating temperature, the spring support(s) is sometimes set in such a way that the spring force and piping
supports have to be locked in place during piping installation. This makes the piping installation, spool fit-up, piping adjustment and assembly inspection very difficult. Practically it is almost impossible to properly adjust the piping-support system. In addition, locks (or stops) must be removed when the system is ready for operation. In many cases a severe twisting and jerking occurs when the stops are removed. This approach is very
"Excessive piping strains on the pump can cause both internal and external alignment problems." system dead weight will balance each other out under the operating conditions. Engineers who use this method argue that it is important to minimize operating stresses (stresses at operating temperature and pressure). Another reason that leads a designer to use this method to decrease the nozzle load at the operating conditions could be insufficient flexibility of the pump piping system. In addition, a low stress at a high operating temperature can be important to control the creep. For low-temperature applications, low stresses still offer some advantages. The ‘dead weight balance at operating conditions approach’ may be a wellknown practice for general piping design, but it should not be used for pump piping systems. By adopting this dead weight balance approach, the spring
risky and can impose high loads on the pump nozzles. This can damage the pump’s sensitive components. Because of the practical adjustment issues, the deviation of theoretical loads from actual loads and transient effects, the dead weight balancing at operating conditions approach nearly always results in applying unpredictable and very high loads on pumps. Pump alignment problems have often been reported as a result of this approach. Damage has also occurred in some cases. The theoretical minimum operating load is in reality only promising on paper. This kind of uncertainty is simply too much of a risk to be taken with pumps. A more reliable approach is therefore needed. Certainly, the dead weight of the piping system should be entirely
WORLD PUMPS
Feature November 2012
absorbed by suitable supports at the ambient condition. This is known as the ‘ambient condition dead weight balance approach’. In this approach, the fitting up of the piping to the pump is normally carried out with the spring supports unlocked. The spring load should then be adjusted to bring the nozzle loads (loads on the rotating machine nozzles) to a minimum. In this way, it is certain that the piping load at ambient conditions is almost zero, although some load is expected under the operating conditions. This load at operating conditions is highly predictable. The ambient condition dead weight balance approach will make field adjustment much easier.
Sometimes it is tempting to put an anchor system on an expansion joint next to a pump to resist the end force. The problem with this anchor arrangement can be explained from the start-up sequence. When the pipe is heated up, both ends of the expansion joint expand inwards, leaving slack at the tie-rods. As soon as the tie-rods become loose, the pressure and force acting on the pump are not balanced. In other words, the expansion joint anchor system can present an unpredictable transient load to the pump. By using tie-rods, instead of the natural and smooth flexible behaviour
of the expansion joint, the shift of the loosened anchor can present a transient force to the pump. This force can be sufficient to push the pump off alignment, causing operational problems.
Contact Amin Almasi Lead rotating equipment engineer WorleyParsons Services Pty Ltd Level 10, 151 Roma Street (East Tower) Brisbane, QLD 4000, Australia Tel: +61 7 3319 3902 Email: [email protected]
Thermal movement To accommodate thermal growth, expansion loops or bends are usually added to the pump piping. An expansion joint is sometimes required to limit the nozzle loads, particularly for large piping sizes. However, the use of an expansion joint (which is expensive and maintenanceprone) should be avoided to the maximum possible extent; it should be considered only as the last resort. One modern approach is for the pump manufacturer to model the entire system (including the piping and the pump) at the same time. This concurrent modelling can reduce the inherent conservatism and may allow the thermal movements to be accommodated correctly by both systems. This may result in a more-flexible combined system and thus allow a better optimization. The engineering time needed to re-model, re-evaluate and re-design the entire system is often paid for by the elimination of the expansion joint. This modern optimization simulation should be included in the pump specification before the pump is ordered. All possible operational scenarios (as well as ambient temperature, installation situations, start-up, all shutdown situations and others) should be considered. One of the most important requirements in designing an expansion joint system in general plant piping is to install a sufficient anchor system (Figure 3) to resist the pressure end forces (the sum of the forces). However, in those pumping systems where the use of an expansion joint cannot be avoided, anchors should not be used on an expansion joint near a pump. There is potential for unexpected movement and high stress in a system using an expansion joint with an anchor. www.worldpumps.com
35
Feature
WORLD PUMPS
November 2012
Operating
Clarifying nozzle load and piping Piping designers always want higher allowable nozzle loads to simplify pump piping designs, while pump manufacturers want smaller such loads to assure good alignment, higher reliability and fewer problems in operation. With long-term reliability a key factor, Amin Almasi recommends that users side with the manufacturer.
T
he use of some common practices for general plant piping could lead to real problems in pump piping arrangements. In a pump system under ambient conditions, the dead weight of the piping should be entirely absorbed by suitable supports. Only nozzle loads induced by temperature, pressure and the transported medium (or similar operational loads) at the operating conditions can be compensated for by pre-stressing (or springing) the pipes. In this article, pump nozzle load limits, proper load optimization and correct pump piping are discussed.
Pump nozzle load Nozzle loads are defined as the net forces and moments exerted on equipment nozzles from the weight and thermal expansion of connected piping and other equipment. For pumps, nozzle loads are specified in API 610 (or other pump codes and standards such as ANSI, etc.). The API 610 standard covers horizontal pumps, vertical in-line pumps and vertically suspended pumps for nozzle sizes up to 16 inch (400 mm). For pump nozzles larger than 16 in, nozzle loads should be agreed with the vendor before the pump order is finalized. Generally, small pumps that are not anchored to the foundation are able to tolerate higher nozzle loads. Allowable loads for vertical in-line pumps with supports that are not anchored to the foundation may be twice the values for in-line pumps that are anchored. www.worldpumps.com
Figure 1. An example of a floating production, storage and offloading (FPSO) vessel. An FPSO vessel requires many pumps. The nozzle loads and piping of these pumps present unique challenges.
0262 1762/12 © 2012 Elsevier Ltd. All rights reserved
WORLD PUMPS
Feature November 2012
and shafts at various locations, such as at the drive-end of the shaft and at the register fit of the coupling hub. Thermal growth, piping fabrication errors and different alignment errors all contribute to the actual deflection values and final nozzle load experienced in the field. The pump should operate without any leakage, without internal contact between the rotating and stationary components, and without losing alignment, while subjected simultaneously to the maximum operating conditions (temperature, pressure, speed and power) and the worst-case combination of allowable nozzle loads.
Figure 2. An example of pump piping arrangements.
Some purchasers ask for higher allowable loads (sometimes as much as 1.5 times or two times the API loads) to make the piping design easier. API nozzle load values may be considered optimal by pump engineers and pump manufacturers, but in some situations these values cannot be achieved by the piping engineers and stress analysis specialists. Some pump vendors typically design their pump details based on the API code limits. For higher nozzle load values, all necessary checking, verification, modification and adjustment (such as bearing design check, casing design verification and clearances) should be carried out to ensure long-term component life expectations. For special applications, with a very large difference between the operating and ambient temperatures and very large nozzle sizes, a nozzle load as high as 2.5 times the API values may be specified. Steam turbine drivers (for pumps) should be designed to withstand nozzle loads in accordance with NEMA SM23. For special pumps primarily designed for low pressures (such as axial pumps, lowpressure overhung pumps and pumps designed with open impellers), which rely on close radial and axial clearances between the rotating components (impeller or rotor assembly) and the casing, special care should be taken concerning the allowable forces and moments on the nozzles.
To achieve maximum pump reliability, the pump engineer’s goal is always to keep
Excessive piping strains on the pump can cause both internal and external alignment problems. If the internal components become misaligned, accelerated wear, rubbing and early failure can occur. The effects of external misalignment may not be as obvious as those from internal problems, but these effects will in time reduce the pump life. The vibration levels increase as the couplings become misaligned and a high-vibration trip may cause an
"To achieve maximum pump reliability, the pump engineer’s goal is always to keep nozzle loads as low as practically possible." nozzle loads (imposed by piping) as low as practically possible. However, piping designers and stress analysis engineers usually design piping systems based on the allowable nozzle loads of the pumps (or even sometimes slightly higher) in order to reduce the complexity and expense. Optimum flexibility should be designed into the piping to prevent distortion of the pump alignment or damage to a component.
Alignment issues For pumps, two effects of nozzle loads should be carefully considered: 1. Distortion of the pump casing. 2. Misalignment of the pump shaft and the driver shaft. To minimize misalignment because of piping load effects, the pump casing and pump package base-plate should be constructed with sufficient structural stiffness to limit the displacement of casings
unscheduled outage. Extended operation at high levels of misalignment may cause coupling failure, possibly bearing damage or even catastrophic failure.
Piping support design There are numerous rules for piping and supports that may be valid for general plant piping, but are invalid for pump systems. For example, consider the following: • Selecting the support types based on the thermal expansion displacement. • Setting the spring supports using the so-called ‘dead weight balance at operating conditions approach’. With respect to the support practices for general plant piping, the types of supports are usually selected based on the vertical thermal displacement that is expected at the support location. Rigid supports are usually used when the displacement is very small. Variable www.worldpumps.com
33
34
Feature
WORLD PUMPS
November 2012
springs are employed for medium displacement. Constant-effort supports are used when displacements are great. This practice seems logical at first. However, a problem can arise for pump piping systems. The use of spring and constant-effort supports (non-rigid supports) can create more problems than rigid supports. The key point is that the misapplication of a rigid support will be detected as soon as the stress analysis is performed. On the other hand, stress analysis on non-rigid supports (such as spring or constant-effort supports) may not readily indicate any misapplication. The end result can be a piping-support system with unnecessary flexible supports (such as spring supports) that is overly expensive, totally unreliable, inherently unstable and very vulnerable to various dynamic and vibrational excitations. Figure 3. Typical expansion joints with anchors.
Providing the right support There are some pump piping systems designed with several spring supports for various piping spools near the equipment nozzles (because of large thermal movements), mainly because free thermal displacement was used as a criterion for support selection. For example, some engineers perform free thermal movement analysis and use flexible supports (spring supports or similar) for all support points with a displacement above a certain level (say 75 mm). Aside from a very high cost, at first glance this design may appear to have no problems. The computer stress analysis shows perfect results. However, it is unstable and vulnerable to various dynamic issues and operational problems. This design will present problems on site at commissioning and start-up (when the travel stops are removed). The actual piping system weight is different (usually heavier) than the theoretical assumed weight. This difference can result in the failure of the system. Whether the actual weight is heavier or lighter compared to the theoretical values, field adjustment cannot be effective (or is not possible) for numerous flexible supports. This flexible support system design is unable to absorb the uncertainties resulting from the manufacturing tolerances, any inaccuracies in analytical calculations and other possible errors. This unnecessarily over-flexible support system is also unstable. It always vibrates and will certainly fail sooner or later. Flexible supports should only be used where really necessary. Proper rigid www.worldpumps.com
supports should always be provided to absorb any load uncertainties (fabrication tolerances, analysis inaccuracies or similar) and to control the various instabilities.
Extreme temperatures In a pump that works at a very high (or very low) operating temperature, the spring support(s) is sometimes set in such a way that the spring force and piping
supports have to be locked in place during piping installation. This makes the piping installation, spool fit-up, piping adjustment and assembly inspection very difficult. Practically it is almost impossible to properly adjust the piping-support system. In addition, locks (or stops) must be removed when the system is ready for operation. In many cases a severe twisting and jerking occurs when the stops are removed. This approach is very
"Excessive piping strains on the pump can cause both internal and external alignment problems." system dead weight will balance each other out under the operating conditions. Engineers who use this method argue that it is important to minimize operating stresses (stresses at operating temperature and pressure). Another reason that leads a designer to use this method to decrease the nozzle load at the operating conditions could be insufficient flexibility of the pump piping system. In addition, a low stress at a high operating temperature can be important to control the creep. For low-temperature applications, low stresses still offer some advantages. The ‘dead weight balance at operating conditions approach’ may be a wellknown practice for general piping design, but it should not be used for pump piping systems. By adopting this dead weight balance approach, the spring
risky and can impose high loads on the pump nozzles. This can damage the pump’s sensitive components. Because of the practical adjustment issues, the deviation of theoretical loads from actual loads and transient effects, the dead weight balancing at operating conditions approach nearly always results in applying unpredictable and very high loads on pumps. Pump alignment problems have often been reported as a result of this approach. Damage has also occurred in some cases. The theoretical minimum operating load is in reality only promising on paper. This kind of uncertainty is simply too much of a risk to be taken with pumps. A more reliable approach is therefore needed. Certainly, the dead weight of the piping system should be entirely
WORLD PUMPS
Feature November 2012
absorbed by suitable supports at the ambient condition. This is known as the ‘ambient condition dead weight balance approach’. In this approach, the fitting up of the piping to the pump is normally carried out with the spring supports unlocked. The spring load should then be adjusted to bring the nozzle loads (loads on the rotating machine nozzles) to a minimum. In this way, it is certain that the piping load at ambient conditions is almost zero, although some load is expected under the operating conditions. This load at operating conditions is highly predictable. The ambient condition dead weight balance approach will make field adjustment much easier.
Sometimes it is tempting to put an anchor system on an expansion joint next to a pump to resist the end force. The problem with this anchor arrangement can be explained from the start-up sequence. When the pipe is heated up, both ends of the expansion joint expand inwards, leaving slack at the tie-rods. As soon as the tie-rods become loose, the pressure and force acting on the pump are not balanced. In other words, the expansion joint anchor system can present an unpredictable transient load to the pump. By using tie-rods, instead of the natural and smooth flexible behaviour
of the expansion joint, the shift of the loosened anchor can present a transient force to the pump. This force can be sufficient to push the pump off alignment, causing operational problems.
Contact Amin Almasi Lead rotating equipment engineer WorleyParsons Services Pty Ltd Level 10, 151 Roma Street (East Tower) Brisbane, QLD 4000, Australia Tel: +61 7 3319 3902 Email: [email protected]
Thermal movement To accommodate thermal growth, expansion loops or bends are usually added to the pump piping. An expansion joint is sometimes required to limit the nozzle loads, particularly for large piping sizes. However, the use of an expansion joint (which is expensive and maintenanceprone) should be avoided to the maximum possible extent; it should be considered only as the last resort. One modern approach is for the pump manufacturer to model the entire system (including the piping and the pump) at the same time. This concurrent modelling can reduce the inherent conservatism and may allow the thermal movements to be accommodated correctly by both systems. This may result in a more-flexible combined system and thus allow a better optimization. The engineering time needed to re-model, re-evaluate and re-design the entire system is often paid for by the elimination of the expansion joint. This modern optimization simulation should be included in the pump specification before the pump is ordered. All possible operational scenarios (as well as ambient temperature, installation situations, start-up, all shutdown situations and others) should be considered. One of the most important requirements in designing an expansion joint system in general plant piping is to install a sufficient anchor system (Figure 3) to resist the pressure end forces (the sum of the forces). However, in those pumping systems where the use of an expansion joint cannot be avoided, anchors should not be used on an expansion joint near a pump. There is potential for unexpected movement and high stress in a system using an expansion joint with an anchor. www.worldpumps.com
35