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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