MechChem Africa January-February 2025

⎪ Water, wastewater and pumping solutions ⎪

Figure 3: Maximum and minimum pressure profiles for the flow path highlighted in green from Figure 1 showing the transient pressure along the flow paths at the exact time of 100% valve closure after 3 seconds. Left: pre-expansion scenario. Right: post-expansion scenario.

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 mini mum 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

est 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 sig nificantly 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 calcula tions, many other issues could have been missed. Water hammer software enables multiple scenarios to be evaluated, such as pump trip sce narios 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. www.chempute.com/aft-2025.html

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 hydro statics that impact the transient force loads and simply multiplying pressure times area at a loca tion 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 larg

Figure 4: Transient pres sure at the valve inlet for pre expansion and post-expansion scenarios. Results are very similar for both scenarios.

January-February 2025 • MechChem Africa ¦ 19