Although Waterhammer analysis has been brought to light in the past ten years, many individuals still do not quite understand what waterhammer is, how to analyze it, and how to mitigate these phenomena.
By Trey Walters, P.E., Applied Flow Technology
Waterhammer is a hot topic. Engineers actively attend conferences, watch webinars, join forums, and reach out for any and all self-education to advance their skills.
They are proactively designing systems and expansions with waterhammer prevention in mind. To those engineers; thank you. Thank you for being mindful and for taking the extra step to prevent failure which would cause significant risk to life, property, or the environment. Engineering safe, reliable systems can indeed be a heavy burden.
On the other hand, many systems carry relatively harmless fluids and may not require waterhammer analysis if the failure impact is minimal. Regardless, engineers and their companies need to assess failure impacts for all systems and proceed accordingly.
Start at the Beginning
Waterhammer (also known as surge) occurs when fluid velocity is changed by actions such as valve position changes, or planned/unplanned pump trips. Little guidance currently exists in codes and standards, and accidents are more frequent than most would like to admit. By summarize existing knowledge and practice on waterhammer, discussing the abilities and limitations of commonly used calculation methods, and providing warnings on what may happen when systems experience phenomena such as transient cavitation and liquid column separation, users can gain some high-level guidance on how to solve surge issues in pumping systems.
Waterhammer is fundamentally the same phenomenon across all industries which need to transfer fluids. However, depending on the nature of the fluid (benign, toxic,flammable, biologically active, etc.) and nature
of the application (high pressure, proximity to people, remotely located such as in space) different concerns and strategies are involved. It is essential that engineers take proper precautions in their design and operations to ensure safe operation of pumping systems.
Why Perform Waterhammer Calculations?
Engineers may decide the main reason for calculating waterhammer is to understand the maximum pressures so adequate pipe strength and wall thickness can be selected to avoid bursting the pipe. However, pipe
integrity is only part of the story. Here are some of the many reasons to calculate waterhammer:
• Predict maximum pressures to avoid bursting the pipe.
• Predict minimum pressures to avoid large diameter pipes being crushed by vacuum conditions inside the pipe.
• Ensure proper operation of equipment such as maintaining adequate NPSH to operating pumps during a transient.
• Predict maximum or time varying imbalanced forces in the pipe system so pipe supports can be adequately designed.
• Ensure forces on components, such as closed valves, do not exceed the components’rated maximum value.
• Predict if the fluid reaches the vapor pressure and begins to cavitate – this can generate even greater pressures after the cavities eventually collapse.
• Predict minimum pressures to avoid pulling a vacuum on the pipe and thus potentially contaminating from outside ambient conditions a treated or a specialty fluid (e.g., treated water in a municipal distribution system).
• Size and locate surge suppression devices and systems to minimize high and low waterhammer pressures.
Water Hammer Modeling/Analysis in Piping System.
Codes and Standards
Unfortunately, very little guidance exists from codes and standards on waterhammer. In practice, engineers are expected to use judgement and experience. Here are two excerpts from ASME piping code: ASME B31.4: “Surge calculations shall be made, and adequate controls and protective equipment shall be provided, so that the level of pressure rise due to surges and other variations from normal operations shall not exceed the internal design pressure at any point in the piping system and equipment by more than 10%.”
ASME B31.3: “In no case shall the increased pressure exceed the test pressure used under para. 345 for the piping system.” And, “Occasional variations above design conditions shall remain within one of the following limits for pressure design: Subject to the owner’s approval, it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the increased condition by not more than 33% for no more than 10 hr at any one time and no more than 100 hr/yr or 20% for no more than 50 hr at any one time and no more than 500 hr/yr.”
Waterhammer is a broad term that encompasses fast pressure transients as a result of a rapid change in liquid velocity. The liquid velocity change can be caused by three fundamental mechanisms:
1. Liquid-full system where there is a planned or unplanned change in equipment or component operation. For example, pump trips and starts or
changes in valve position.
2. Liquid or vapor system where there is a rapid phase change which causes a change in volume which then accelerates a liquid slug. For example, condensation of a vapor which creates and/or augments a liquid slug.
3. Liquid/gas two-phase flow where differences in velocity can cause liquid slugs to impact equipment and elbows. For example, oil and natural gas near extraction points that are put into a common pipeline before separation; starting a pump into an evacuated, air-filled line; or air trapped in storm water systems where free surfaces exist and liquid is accelerated due to movement of the air.
The second and third types of waterhammer are important but not as well understood as the first. The first type of waterhammer in liquid-full systems can be disused in terms of:
• Instantaneous Waterhammer: The Joukowsky Equation
• Communication Time in Pipe Systems
• Waterhammer Physics in a Frictionless Single-Pipe System
• Waterhammer During Rotodynamic (Centrifugal) Pump Trips and Starts
• Waterhammer During Positive Displacement (PD) Pump Trips and Starts
• Waterhammer and Throttling Valves, Check Valves, and Other Types of Components
• Transient Cavitation and Liquid Column Separation
At the design stage, engineers have more latitude in their surge suppression. They can differ the design by considering options such as: using larger pipe diameters, using pipes with lower wavespeeds, changing how pumps and components behave over time, or adding surge suppression equipment/systems. When considering surge suppression, it is important to understand the source of the transient. For solving installed system surge problems, high frequency transient monitoring alongside computer surge modeling are two methods for discerning the root cause or potential root causes of a transient. It is also important to understand the likelihood of a surge event that is initially caused by a drop in pressure due to events such as pump trips or in some cases the downstream side of a valve closure.
Some industries that have historically taken waterhammer very seriously include nuclear power, aerospace, and the pipeline industries. Today, many EPC firms incorporate waterhammer and surge analysis into the design of many more industry systems. They have made it a best practice. Additionally, facility owner/operators are proactively adding such analysis to the project scope.
A better understanding of waterhammer and surge suppression options, combined with a well-organized graphical interface for computational analysis, lowers the bar for performing waterhammer analysis. It also lowers barrier to entry and many companies are giving engineers the option to bring waterhammer analysis in house.
The full version of the Technical Paper, “Understanding Waterhammer in Pumping Systems and Surge Suppression Opti ons,” can be read at: htt ps://www.aft .com/documents/TechnicalPapers/Understanding-Waterhammer-in-Pumping-Systems-and-Surge-Suppression-Opti ons.pdf
Authored by Trey Walters, P.E., of Applied Flow Technology, along with Amy Marroquin, EIT and Frank Smith III of BLACOH Surge Control
About the Author
Trey Walters is the Founder and President of Applied Flow Technology Corporati on in Colorado Springs, Colorado, USA. At AFT he has developed soft ware in the areas of incompressible and compressible pipe fl ow, waterhammer, slurry system modeling, and pump system optimization. He performs and manages thermal/fluid system consulting projects for numerous industrial applications including power, oil and gas, chemicals and mining. Mr. Walters holds a BSME (1985) and MSME (1986), both from the University of California, Santa Barbara. He is a registered engineer, an ASME Fellow and sits on multiple standards committees of the Hydraulic Institute.