Article #5: Relief Valves for PD Pumps - Internal or External?
Positive Displacement pumps are knows as "flow generators", while centrifugal pumps can be thought as pressure (or head) generators.
It is easy and convenient to control the centrifugal pump with a discharge valve: it changes a system curve, resulting in pump operating at different flows (see Article "How does pump suction limit the flow?"). Not so for the positive displacement pumps.
All types of a PD pump (piston, rotary gear, lobe, vane, etc.) have a major similarity, which distinguishes them from the centrifugal pumps: PD pump curves show that they produce almost (depending on viscosity and internal clearances) the same flow, regardless of the differential pressure:
Figure 5-1: Hydraulic performance pump curves - centrifugal (head versus flow) and positive displacement (flow versus differential pressure)
A PD pump "does not know and does not care" what happens on the system side - with each rotation of a shaft, a rotary gear pump, for example, moves the same amount of fluid from its inlet, to discharge - regardless of the differential pressure (second order dependency actually does exist, as function of viscosity and clearances, but this is covered in another article). The pump does, however, require more torque, more power, as pressure increases, but as far as flow is concerned - it stays constant.
As a result, a PD pump will try to move the same amount of fluid, regardless of the position of a discharge valve (or other system-related conditions downstream) - resulting in pump discharge pressure building up very rapidly if valve begins to close - sometimes with almost negligible incremental turn of the valve. Eventually, something is bound to fail - a burst hose, a sheared off shaft, a seal, and so forth. Such failures are catastrophic and dangerous.
To prevent them, a relief valve should always be employed with a PD pump. Some pump designs incorporate the internal in-built relief valves, and others do not and require the external relief valve. The advantage of the internal relief valve is obvious - it is an integral pump of the design, and comes with a pump. However, internal valves are usually limited to application of relatively "benign" substances, such as, for example, oils.
The reason for that is as follows. Say a gear pump is pumping against a 60 psi differential pressure, and its internal relief valve spring is set to open the valve to by-pass at a 100 psi (actually it would crack somewhat earlier, say at 90 psi, and reach a full by-pass at 100 psi). When the valve opens, 100 psi drives the liquid from the discharge port immediately to a suction port, in a very close, "short-circuited" manner. The energy delivered to the liquid at 100 psi differential has to go somewhere - and it goes to heat, with vaporization (flashing) of the liquid possible. Depending on the specific heat of the fluid, and the rate of the heat rejection through the casing walls, flashing may be more, or less, likely.
The nature of the pumped fluids is also important: overheating of oils, as bad as it is, may not be as dangerous as overheating of a sulfuric acid, caustic, or other similar chemical.
It is better in such cases to use the external relief valve, where fluid would bypass from the discharge back to the tank, eliminating pump short-circuiting altogether. For this reason, pumps designed for chemicals and other similar substances should not use internal relief valves, but the external ones.
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