Addressing Low Pressure Transients: Part 2

Addressing Low Pressure Transients FEATURED STORY

Negative pressures often do not get the focus they deserve when evaluating a piping system. It is more natural to be concerned with the maximum pressure ratings of a pipe. In fact, when determining the root cause of a system failure, it is all too common to seek out and settle upon what circumstance could have caused such catastrophic high pressures. In reality, however, negative pressures are just as likely to occur in nearly every system, and quite often they are the unknown root cause of piping failures. With the right tools, engineers and operators can not only uncover such root causes, they can also design effective mitigation solutions adding years of life to a pipeline.

Part 2 will look at the types of valves used to mitigate the risk of pipe failure cause by negative pressures. Part 1 of this article can be found in the June 2022 issue of Valve World Americas.

By Amy Marroquin, Senior Hydraulic Engineer – BLACOH, and Scott Lang, Engineering Software Developer – Applied Flow Technology


Transient monitoring, also known as high frequency pressure monitoring, is a manner of measuring pressure multiple times a second. Transient monitors can measure, and typically record, pressures 20 to 500 times a second. Normal methods for pressure measurement range from a dial gauge to SCADA systems, where the pressure might be recorded at 1Hz at best. At worst, the pressure is recorded dependent on an operator’s rounds. Because transient pressure waves travel near the speed of sound, a record of pressures recorded at 1Hz or slower will only convey bits and pieces of most transient events. This concept is conveyed through the following figure where transient data was recorded at 100hz. The same data was copied and scrubbed of the extra points to mimic a data set recorded at only 1hz. The lower frequency data indicates a very moderate pressure envelope, when in reality there are potentially severe problems only revealed at the higher frequency. In addition, many more pressure fluctuations were captured for the higher frequency dataset.

The minimum recommended frequency is dependent on the system’s wavespeed and communication time. Generally speaking, the higher the frequency, the more accurate and all-encompassing the data will be. Historically the biggest challenge with higher frequency pressure detection devices has been data management; however, the technology continues to advance and in this day even includes cloud based database capabilities. The minimum recommended frequency is dependent on the system’s wavespeed and communication time. Generally speaking, the higher the frequency, the more accurate and all-encompassing the data will be. Historically the biggest challenge with higher frequency pressure detection devices has been data management; however, the technology continues to advance and in this day even includes cloud based database capabilities.

Figure 10: High versus low frequency pressure monitoring.
Figure 11: Air vacuum valve place downstream of transient inducer.
Figure 12: Computer simulation used to show air valve opening, allowing air into the system, then closing roughly 100 seconds later.

Mitigation Equipment

Relief valves are often the go to for protecting piping systems. However, relief valves will only relieve a system from high pressure. They will not alleviate vacuum pressures. Further, relief valves have a delay in opening and are sized based on a worst case condition. Lesser cases of high pressure events can cause a relief valve to lift or chatter which will induce additional transient pressure waves.

Discharge control valves can assist in shutting the pressure in, however the operation and control sequence should be carefully determined through computer surge modeling, so as not to cause a high pressure transient event.

Mitigating low pressures requires a means for adding energy to the system, often as quickly as possible. Even a one second delay can be greatly problematic for some systems. Two of the most appropriate equipment types capable of quickly adding energy to a piping system are air vacuum valves and surge vessels.

Figure 13: Pressure comparison downstream of valve: single stage (solid) versus multi-stage (dotted) air vacuum valves.

Air Valves

One of the first thought of solutions for a negative pressure condition is to include an air vacuum valve into the design. Depending on the application and sizing, air valves can be an adequate solution. They are commonly found in raw water systems but for many other systems they are used only as a last resort measure due to pump loading concerns, corrosion, contamination, etc. For hydrocarbon applications, they are prohibited due to the risk of combustion.

It is common practice to place air valves at the high elevation points within a system which is a good measure for alleviating moderate vacuum. However, remedying transient pressures appropriately first begins with a solution at the source of the inducer15. This is because the wave travel time must be respected – the air valve cannot react until the wave reaches it. Before it does, the entire expanse of piping between the pump and the air valve is at a low pressure. Even when located at the source of the low pressure transient, a modeler will incorporate up to a 0.3sec delay in opening.

Second, air vacuum valves must be sized appropriately. As the line pressure drops, the pressure across the air valve increases, resulting in higher air flow. However, at some point this air flow may become sonically choked16 – the flowrate will not increase with dropping line pressure, effectively allowing the line pressure to drop without restraint.

Lastly, there are different designs of air valves, some with multiple stages of air release allowing for a controlled release of air back out of the pipe. In a single-stage design, where the inlet size is the same as the outlet, the air may be expelled extremely rapidly, while the system is still in a transient state, causing valve slam. This propagates a very high pressure wave which starts a similar wave cycle over again. Restricting air release to prevent slam is often critical.15

A single stage air vacuum valve, when installed near the outlet of the inducer can mitigate the initial low pressure transient, however a secondary transient is created after all the air is released. Reference Figures 11-12.

Replacing the single stage air vacuum valve with a multistage type allows for the initial low pressure transient to be remedied without causing any secondary transient challenges, reference Figure 13. Even with extreme care in sizing the air valve, sufficient protection may not be possible from air valves alone.1 In the next section, surge vessels will be discussed as a primary solution for low pressure transients.

Surge Vessels

Surge vessels are unique in that they are used to relieve both high and low transient pressures without breaking containment and without any delay. Surge vessels are typically ASME Sec VIII Div 1 pressure vessels with an internal air or nitrogen gas charge. When placed online and in steady state operations, the fluid enters the vessel and assumes a fixed liquid level according to the system’s steady state pressure and the vessel’s initial gas charge. When the system is in a transient state, the liquid level rises and the gas compresses as the vessel receives a high pressure transient wave. And vice versa, the liquid level falls and the gas expands in a low pressure transient event.

Continuing on with Example 1, where a system operating under a normal flow of 9,000gpm and 50psig pressure at the valve discharge,, a surge vessel placed up stream of the valve will both accept energy from the initial high pressure transient and give energy during the succeeding low pressure transient. As a result the transient pressures are mitigated. Reference Figures 14-16.


Low pressure transients when unmitigated can lead to a multitude of problems. The abrupt and often extreme pressure fluctuations within a pipeline can further fatigue an already aged system. Sensitive systems are exposed to contaminants with negative pressures and pipe breaks. Unpredictable high pressures are generated with the collapse of vapor pockets that have formed with low pressures transients. With the right tools and practices such as transient modeling and monitoring, appropriate and effective solutions can be incorporated into the design of piping systems adding years of design life, reducing safety risks and significantly reducing cost.

Figure 14: Surge vessel placed upstream of transient inducer.
Figure 15: Gas volume within surge vessel during transient event.
Figure 16: Pressure comparison upstream of valve: with (dotted) and without (solid) surge vessel.


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14.Stewart, M., Walters, T.W., Wunderlich, G. and Onat, E.A., 2018, “A Proposed Guideline For Applying Waterhammer Predictions Under Transient Cavitation Conditions”, ASME PVP Conference, July 15-20, 2018, Prague, Czech Republic, ASME PVP2018-84338
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About the Authors

Amy Marroquin is an Engineering Project Manager for BLACOH Surge Control. She is highly experienced in the field of modeling and analysis with almost 20 years of engineering and manage-ment experience specifically in piping systems, pressure vessels and packaged equipment. Designs have encompassed numerous land and offshore applications including water, sewer, chemical, natural gas and sour gas fluid media.


Scott Lang, PE, is a Sr Engineering Software Developer at Applied Flow Technology where he helps lead the software development efforts to research, improve, and create models for fluid analysis within AFT products. Scott holds a Bachelor of Science in Mechanical and Electrical Engineering from Colorado School of Mines.

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Sara Mathov is a feature editor contributing to Valve World Americas, Stainless steel World Americas and other related print & online media.