WWTP Kills Bad Vibes with Pulsation Dampener

Aug. 23, 2006

At the heart of any fluid system is a pump, which is controlled by valves and connected by pipes. Depending upon the type of pump, it may also be

At the heart of any fluid system is a pump, which is controlled by valves and connected by pipes. Depending upon the type of pump, it may also be the source of some potentially dangerous and damaging failures. Positive-displacement pumps with reciprocating action, for example, produce pulsations that can trigger a leak over time or create a surge suddenly that can rupture pipes and destroy valves and controls.

When the City of Colorado Springs’ Wastewater Treatment Plant (WWTP) encountered a similar situation where it was experiencing broken pipes, damaged valves, failed gaskets, and even pipe supports that were being pulled from foundations, officials at the facility were keen to investigate the cause. The pump system was installed to transfer sludge through the facility. Because of its solids handling capability, officials installed a mechanical double-diaphragm pump to push the sludge. However, its reciprocating action was the source of serious pulsations and surges that were shaking everything on the fluid system violently. Altering the flowrates and discharge pressure of the pump seemed to have little impact on the pulsations, and regardless the plant needed the flow and the specified pressure to move the sludge effectively.

When plant officials consulted with their local pump and fluid system experts, they recommended a device to dampen the pulsations in the system without altering either the flowrate or discharge pressure of the system. A pulsation dampener acts like a shock absorber for the fluid pressure spikes. Blacoh Fluid Control (www.blacoh.com) manufacturers a line of pulsation dampeners that isolate the liquid (sludge in this case) with a diaphragm, which has pressurized air on the other side of it. The charge pressure of the air determines the effectiveness of the dampener. When the reciprocating pump discharges the pressure pulse it gets absorbed by the dampener. The reciprocating pump cycles into a suction stroke, which causes a low-pressure pulse (void). It is at this point the reserve capacity of the dampener fills the void to even out the flow.

In addition to sizing the dampeners correctly, there are a few other elements critical to optimum dampening results. First, the location of the dampener, which according to Blacoh, needs to be less than 10 times the pipe diameter from the source of the pulsations or pressure surge (pump in this case). Secondly, the charge pressure can either maximize the effectiveness or make the dampener redundant. If the charge pressure is excessive it will effectively remove the dampener from the fluid system. If the pressure is too low it will not remove pressure spikes efficiently. Blacoh recommends the charge pressure be 80 percent of the average discharge pressure in the system for most pumps. Last but not least, it is necessary to ensure corrosion resistance of the material of the pulsation dampener against the liquid that is being moved.

In the application under discussion here, Blacoh worked with plant officials through their distributor in Colorado to conduct a thorough analysis of the entire fluid system to determine the root cause and other contributing factors. Blacoh also helped to size and select the right pulsation dampener for the application. A Sentry1 model with 370 cubic-inch capacity was selected. Blacoh also provided guidance for compatible material selection (Neoprene in this case) to work with the liquid being pumped. The result was elimination of vibrations caused by pulsations. The whole system worked smoothly, virtually getting rid of all premature failures and leaks caused by pressure spikes and pulsations. Now Colorado Springs’ WWTP specifies a pulsation dampener whenever it puts together a system driven by a reciprocating positive-displacement pump.

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