Learn what causes this phenomena in pipes and some actions to take to reduce the risk.
by Yasser Amer Ahmed, CPD, LEED AP BD+C, CFPS, MIFireE, PMSFPE
Water hammer is a pressure change caused by a variation of the flow rate in a pipe or tube. It is created by sudden starts or stops of liquid flow. Since water flow is restricted inside a pipe, shock waves of incompressible water will travel back down the pipe, deflecting everything in its path.
Main Reasons for the Water Hammer Phenomena
It is a basic fact that liquid moving through a pipe has two different kinds of energy: kinetic and potential. Its velocity is known as its kinetic energy, and its pressure is known as its potential energy. When friction is disregarded, the total amount of kinetic and potential energy is constant along the whole pipe.
The pressure inside the pipe changes when the liquid velocity is altered, which alters the kinetic energy. Liquid pressure rises when the liquid’s velocity falls (a decrease in kinetic energy/an increase in potential energy).
The most frequent cause of water hammer is when a valve is abruptly closed, stopping the flow of liquid through a pipeline. This causes shock waves to move through the piping system at a pace equal to the sound speed in the liquid (more than 4,800 feet per second for water at 70°F). These waves move forward, backward, and then back again until they reach the next solid obstruction, which is usually a pump or check valve, and the pressure equalizes.
Furthermore, the momentum of the liquid column acts on the shutoff device (disc, gate, or ball valve) of a pipeline when a valve suddenly closes. There is a simultaneous increase in pressure on the upstream side of the valve and a drop in pressure on the downstream side due to this abrupt separation of the water column. The liquid will try to flow past the valve downstream, producing a vacuum that could lead to the pipe collapsing or imploding. If the pipe is situated on a downward slope, the issue may become more severe.
Air and vacuum relief valves, also known as air vents, are positioned immediately downstream of the valve to enable air to enter the line and prevent a vacuum from forming, preventing an abrupt change in pressure close to the shutoff element of the valve.
As a result, safe pipeline operation depends on valves opening and closing appropriately. A pressure wave that travels upstream is produced when a valve at a pipeline’s downstream end is closed. Sudden valve closure is the process of closing a valve faster than the shock wave travels to and from the end of the pipeline. A sudden valve closure can cause a pressure surge and a rapid change in velocity.
Other Reasons for Water Hammer
Additional sources of water hammer include the following:
- Rapid pump initiation may cause a void space downstream to collapse quickly.
- A sudden shift in flow brought on by a fast pump shutdown can result in an increase in pressure on the suction side and a decrease in pressure on the discharge side. The downslope is typically the more problematic of the two. Vapor column separation occurs when the discharge side pressure reaches vapor pressure.
- A check valve may slam shut quickly as a result of abrupt deceleration (check valve slam).
- Air pocket movement within a pipe: Because air is compressible, it can behave like a spring when it travels through a pipeline, expanding at high points and contracting at low points. Pressure changes caused by compression and expansion have the potential to result in significant water hammer pressures if they are large enough.
- When a water column separates and rejoins at a rapid speed, it can also cause significant water hammer pressure values.
Selecting Appropriate Protection
The actions listed below can help lessen or get rid of water hammer.
- Appropriate personnel education and training regarding the risks associated with water hammer and how to reduce them by appropriately opening and closing valves.
- When starting and stopping pumps, follow methods that lessen the chance of producing water hammer circumstances.
- Lower the liquid’s velocity inside the pipe. Certain texts advise maintaining the flow velocity at or below 5 feet per second to keep the risk of water hammer low.
- Slow-closing valves should be used. A gate valve or anything else with a wheel is typically considered slow closing. Fast-closing valves are those having handles, such as ball or butterfly valves.
- Make use of a pipe rated for more pressure. SDR 11 HDPE pipe, for instance, is rated for 160 pounds per square inch (psi), while SDR 26 HDPE pipe is rated for 65 psi.
- To minimize the risk of a partial vacuum and potential pipe collapse, air valves are frequently employed to correct low pressures at high places in the pipeline.
- To stop the pipe from experiencing excessive pressure, install pressure-relief valves.
- To mitigate potential shock, use expansion tanks, surge vessels, accumulators, or air chambers that are only partially filled with gas or air.
About the Author
Yasser Amer Ahmed is an accomplished Egyptian Fire Protection and Plumbing Specialist with 30+ years of international experience at Dar Al-Handasah Consultants, an internationally ranked consulting firm providing planning, design, management, and consultancy. As a Fire Protection & Plumbing Specialist, he excels in managing diverse projects and teams, with expertise in strategic leadership, project management, and engineering, and has strong and diversified experience in the design and coordination of plumbing and fire protection systems for mixed-use complexes, airports, educational, healthcare, residential, commercial, high-rise, and governmental buildings. Yasser graduated from Ain Shams University with a bachelor’s degree in Civil Engineering, Public Works Dept., in 1991, and he received a post-graduate diploma in Sanitary Engineering from Ain Shams University in 1996 and an MBA from Brookline Business School (New York) in August 2020. Yasser is a Certified Fire Protection Specialist (CFPS) since December 2016, Certified in Plumbing Design (CPD) since April 2016, and a LEED AP BD+C since April 2014. He is a member of the National Fire Protection Association (NFPA), ASPE, the Society of Fire Protection Engineers (SFPE), the Institution of Fire Engineers (MIFireE), the Egyptian Engineers Syndicate, and the Egyptian Society of Civil Engineers, and he is authorized as a Fire Protection Consultant Engineer from Egyptian Engineers Syndicate.
The opinions expressed in this article are those of the author and not the American Society of Plumbing Engineers.