MechChem Africa January-February 2025

Water hammer analysis for complex piping Ben Keyser from Applied Flow Technologies (AFT) talks about water hammer in complex piping systems and describes how AFT Impulse surge analysis software can be used to minimise its impact.

W ater 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. 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 den sity, 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 bal ance equations in a characteristic grid approach. For multi-branched or looped piping systems,

however, very large spreadsheet grids are re quired, 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 de signs or operational changes. A water hammer analysis example A Liquified Natural Gas (LNG) plant piping mod el is shown in Figure 1. The plant, initially with three pumps operating in parallel, was undergo ing 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, lead ing 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 caus

ing 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 signifi cantly 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 significant ly 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 so lution 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 engi neering more efficient. Important parameters

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.

Figure 2: A comparison of a linear two second valve closure (top case), with a Swaffield & Boldy's recommended 80/20 valve closure rate over two seconds (bottom case), which shows significantly reduced surge pressures.

18 ¦ MechChem Africa • January-February 2025