Water hammer in a piping system occurs whenever a pumping system transitions from one steady-state of operation to another. It is present in all piping systems and is not limited to water circuits. Hammer events are caused by any operational changes, sudden, planned or unplanned in all piping system.
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Figure 1: This LNG plant analysis model includes a second riser with additional pumps. The blue lines represent pipe runs with transient force loads. Green line represents a single continuous flow path from pump P-101C to valve LV-1564A2.
It is a potential problem in every piping system. Whenever a pump is started or a valve closed, water hammer is introduced into the system. A classic cause is fast valve closure. But water hammer can also be caused by pump trip and startup events, relief valves opening and closing due to over pressure, control valves failing, check valves slamming, and many more. Anything that causes a sudden change in fluid flow or pressure will result in water hammer.
Fast valve closure events are typically used to explain the phenomenon. These can be analysed using the Joukowsky equation, which determines the maximum theoretical pressure surge for an instantaneous fluid-flow change event. The equation depends on the fluid density, the wave speed of the fluid, and the change in velocity, and it applies to anything that causes an instantaneous change in fluid velocity.
While useful for determining the maximum theoretical surge pressure, there are times when the pressure surges will be larger than the Joukowsky equation predicts, such as when transient cavitation is present in a piping system. Various other methods are available to quantify the pressure response during a transient event, such as the Method of Characteristics, which solves the transient mass and momentum balance equations in a characteristic grid approach. For multi-branched or looped piping systems, however, very large spreadsheet grids are required, which can be impractical. This is where the need for quality water hammer analysis software comes in.
AFT’s water hammer analysis software can be used to conduct water hammer analyses for simple or complicated piping systems, without requiring a doctoral study in water hammer theory. It helps engineers to better understand their plant piping systems, to determine the root cause for existing problems, to analyse water hammer related accidents, or to incorporate preventative approaches into new piping designs or operational changes.
A water hammer analysis example
A Liquified Natural Gas (LNG) plant piping model is shown in Figure 1. The plant, initially with three pumps operating in parallel, was undergoing an expansion to bring two additional pumps online with a third pump acting as a spare.
In this system, two sets of pumps with risers tie into a main header. The flow later splits, leading to two separate discharge valves. The pipe runs highlighted in blue are legs where transient force loads need to be analysed for pipe stress; while the pipe run highlighted in green is a single continuous flow path from pump P-101C to valve LV-1564A2.
As well as the water hammer effects of valves closing, transient pressure waves propagate through piping systems at several thousand feet per second. These wave patterns can cause additive interference in the system that can lead to pipes rupturing at high pressure spikes. But low pressures due to subtractive interference can be just as problematic, with sub-atmospheric pressures potentially causing pipes to collapse. Transient cavitation can also occur, where product pressure reaches its vapour pressure, very large pressure spikes can occur. This is prevalent in LNG facilities, because its vapour pressure is not as low as that of water.
To understand the impacts on the existing system, the classic valve closure scenario in Figure 1 was modelled. The two discharge valves at the outlet of the system were analysed based on closing within three seconds with a linear valve closure profile.
Linear valve closures are often assumed, with longer valve closing times seen as helping to mitigate against surge pressures. However, this is not always the case. Sometimes with specific types of valves, the change in pressures and flowrates in the system may not be seen until the last few percentages of valve closure. So closing valves slowly may not always help.
Swaffield & Boldy recommends that if 80% of the valve closure is accomplished in the first 20% of the time, while the remaining 20% of the closure is done over the longer 80% of remaining time, the resulting pressure surge can be significantly reduced. This is highlighted in Figure 2.
As seen by the pressure at the inlet of the valve in Figure 2, the 80/20 guideline significantly reduces the transient surge pressure upon valve closure compared to the case with the same closure time with a linear closure profile.
AFT’s quality water hammer analysis solution has powerful scenario management capabilities, where the pre-expansion and post-expansion piping and pumping system can easily be directly compared to each other. The two cases can even be evaluated simultaneously, rather than separately, helping to make engineering more efficient. Important parameters to evaluate include the minimum and maximum pressures in the system and how they compare to the maximum allowable operating pressure; the possible presence of vapour formation and cavitation; and the transient performance of components such as how the speed of a pump may change if a pump trip occurs; how the pump’s suction and discharge pressure and flow rate may change through a pump during a transient event; or how a relief valve may cycle during a surge situation.
Figure 3 provides the maximum and minimum pressure profile for the pre- and post-expansion flow path highlighted in green from Figure 1. The green line in each plot is the pressure along the flow path at the exact time the valves fully close after 3 seconds. The transient pressure along the flow path at three seconds for both plots in Figures 3 remain very similar. While the post-expansion scenario does result in higher transient pressures, this is due to line packing and additional flow in the system, because more pumps are operating. This highlights an example where the transient pressures can be higher than the prediction of the Joukowsky equation. Figure 4 shows the transient pressures at the inlet of the closing valves over time. Post-expansion pressures at the valve inlet are higher than pre-expansion scenarios, but also very similar. As noted earlier, transient force loads can be quite large, which can lead to piping system being knocked entirely off their supports. AFT’s water hammer analysis software allows these transient force loads to be easily calculated and compared.
An analysis of the largest transient forces for the pre- and post-expansion scenarios for the LNG plant revealed that the highest transient force loads in each of the scenarios occurs in Force legs 3 and 6.
The largest force load may often occur at the location of the valve that closes, but this is not always the case. There are many hydraulic effects such as friction, momentum and hydrostatics that impact the transient force loads and simply multiplying pressure times area at a location does not provide the correct force values.
Water hammer analysis software must also consider the frictional and momentum effects for inclusion in the force load calculations. It is not easy to identify exactly which Force leg will carry the largest transient forces, and the largest forces may not always occur at the moment a valve closes. Due to the way pressure waves interfere with each other in a complex systems.
For the LNG plant shown in Figure 1, the expansion did not result in transient pressures that were higher than the maximum allowable pressure of the system. Nor did it result in significantly higher transient force loads. The wave speed in this system is about half of the typical wave speed in a water system, but if a different fluid was being used, the results may very well have been more critical. Also, if the system had not been analysed with respect to force calculations, many other issues could have been missed.
Water hammer software enables multiple scenarios to be evaluated, such as pump trip scenarios where either all the pumps trip together or one pump trips by itself, potentially leading to a large check valve slam event, and many more.
AFT’s Impulse advanced water hammer and surge analysis software, which is supported in South Africa by Chempute, can be a great tool to help engineers improve the safety and reliability of piping systems in an effective and efficient manner.
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