Relief Valves and How They Can Save the Day!

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Rotary positive displacement pumps are flow generating machines. With each revolution of the shaft, a fixed volume of liquid is moved from the inlet to the outlet. But what if the discharge line becomes blocked? In this Pump Report, Chad Wunderlich discusses the importance of overpressure protection using relief valves. To learn more, visit our website at vikingpump.com.

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Unlike centrifugal pumps, flow is basically independent of pressure. This is a big advantage, keeping flow consistent regardless of changes in pressure. But this also means that should an upset condition occur like a closed valve or downstream blockage, then the pump may generate pressure beyond its limits or the limits of the downstream system. This is when bad things can happen: breakers tripping, couplings breaking, or hoses bursting are just some of the potential failure modes should PD pumps not be fitted with properly installed and set overpressure protection.

Rotary PD pumps require some form of overpressure protection. And most commonly used is an internal relief valve. These mount directly to the pump and bypass flow internally at an adjustable set pressure.

Let's take a look inside. Here we see the internal workings of an internal relief valve. A movable poppet is held in place by a compressed spring. An adjustable screw can be used to change this compression and the valve's pressure setting. When pressure builds to a point where it overcomes that spring pressure, the valve opens to bypass flow. If the discharge line is completely blocked then 100% of the pump's capacity is bypassed through the relief valve.

Now normally for this pump, the relief valve would have been installed on the head. But for this display, because we wanted to see the pump internals, we used a clear acrylic head instead. So in this case, the relief valve is an in-line valve and it's installed on the discharge pipe inside the unit. Now normally, when operating like this, the pressure is low, but if a blockage starts to occur - pressure is going to start to rise. In this case we're going to close the discharge valve. As I gradually close the discharge valve, pressure starts to rise. Eventually I reach a point where the inline valve opens and flow starts to exit through this bypass line. With the valve completely closed, no liquid is coming out of the discharge line and now 100% is exiting through the bypass line. If I open the valve, the flow returns.

To learn more about relief valves or to view other Pump Reports, please visit our website at Vikingpump.com.