How to eliminate water hammer when pumping at high velocities
The Importance of Check Valves
Check valves are common throughout industries where fluids are pumped through piping systems. These are passive, one-way valves, ensuring the flow of fluids before closing to prevent backflow conditions. Check valves are available in a wide range of sizes, materials and styles including swing, double-door, ball, diaphragm and duckbill types. They are simply designed, using the flow velocity and pressure to open and close. When properly designed and operated, they ensure fluid flows in the right direction while protecting the integrity of the system.
Check Valve Problems
A variety of issues can occur with check valves depending on their application, construction and style. Steel flap, swing and double-door valves can rust, stick or become jammed with debris. When these valves fail, they allow backflow. Abrasive sludges or corrosive slurries can create wear and damage to valves, making them inoperable.
A major problem caused by poorly designed or operating check valves is valve slam and water hammer.
Valve Slam and Water Hammer
Valve slam, also called water hammer, occurs after a pump stops, when the flow reverses before the valve is completely closed. Once the valve closes, the sudden change in flow direction and velocity results in water hammer.
Water hammer occurs when a fluid in motion is suddenly forced to stop. The flow of fluid at the leading edge stops, but fluid behind continues to move and compresses. The kinetic energy of the fluid converts to pressure energy that creates a hydraulic shock wave that travels at nearly the speed of sound through the pipeline.
Water hammer is a destructive force, causing major damage to pumps, equipment and piping systems. The problem is exacerbated for systems pumping at high pressures and velocities, especially those pumping abrasive or thick materials. Consequences of water hammer include ruptured pipes, valves or fittings and destroyed foundations, pumps, and instruments. The results of water hammer also pose a safety hazard to personnel.
Related problems include environmental ad property damage caused by resulting leaks and spills, as well as potential regulatory fines and bad press. Using the right check valve for a specific application can eliminate this dangerous and expensive problem.
Abrasive Materials and High Velocities Effect on Check Valves
Abrasive materials like raw sewage and sludge pose special challenges for check valves. Even greater problems are caused in systems with materials that may be both abrasive and corrosive, such as lime slurries, mine tailings and tar sands.
Swing, double-door, and inline steel-bodied check valves may not close properly due to solids in the material. They also experience worn elastomers and seat seals, wearing out quickly due to abrasive or corrosive exposure. Corrosion may cause the valve to stick in an open position.
In addition, due to the high velocities and pressures often used for pumping these materials, destructive water hammer is a common occurrence with these valves.
Elastomeric Check Valves Provide Solutions
Proco’s ProFlex™ 750 jacketed in-line flanged rubber check valve is perfect for heavy-duty wastewater, sludge and slurry applications. The enclosed body check valve has no mechanical parts to wear, corrode or stick. They require no external power sources, so operation costs are non-existent.
Elastomeric check valves are cost-effective and flexible, allowing abrasive materials to easily flow through the valve without significant head losses. The valve seals around any solids trapped inside and prevents sewage, sludge, mine tailings, or tar sands from coming back through the pipeline.
The ProFlex™ 750 can be installed horizontally or vertically and is designed within industry standards for flange size and drilling.
Eliminating Water Hammer
Most importantly, the rubber duckbill design of the ProFlex™ 750 prevents valve slam and water hammer, even in systems with velocities of over eight feet per second.
A duckbill check valve is a one-piece elastomeric sleeve shaped like a duck’s beak. It allows the forward flow of water with positive differential pressure which progressively opens the valve as flow increases. Reverse differential pressure closes the valve. The ProFlex™ 750 features a full port design that opens with minimal head pressure and closes with any back pressures on the valve.
Rubber absorbs a certain amount of energy, and even with high velocities the two rubber faces coming together prevent the hydraulic shock of water hammer.
As an example, the Jimmy Smith Wastewater Treatment Plant in Boone, North Carolina experienced severe and destructive water hammer with the 16-inch swing check valves on their three influent pumps. The treatment plant sits at an elevation of 3,300 feet and the influent station pumps uphill 80 feet to the plant. The three pumps are connected to a common header. Reverse flow from a high elevation and a pump coming on at the same time created violent hydraulic shock waves.
At one point, a severe water hammer ruptured the pipe, filling the pump room with raw sewage. After a second violent water hammer destroyed the check valve on a newly rebuilt pump, the plant’s sales representative recommended the PROCO ProFlex™ 750. Since the ProFlex™ 750 was installed on May 13, 2013 there have been no more water hammer issues with the pump.
In another instance, water hammer on a 30-inch valve was so destructive that it broke the concrete wall where the pipe entered the building. Replacing the valve with a ProFlex™ 750 resolved the problem.