The pump is surging
Some thoughts on pump surging
- Random surging is caused by an air pocket getting loose in the
- Dissolved gases in the system
- The pump suction is below atmospheric pressure (vacuum) and
air is coming into the packing, Stop it by converting to a
- The fluid is vortexing at the pump suction. Use a vortex
- An air pocket at the suction caused by a reducer that has
been installed upside down
- Constant surging is usually a result of a low NPSHA
- Internal recirculation is heating the liquid to its vapor
- A stuffing box suction recirculation line is being used
with fluid at or near its vapor point
- The well is not keeping up with the pump.
- If you have a pump with a flat curve (low specific speed), a
small variation in pressure (head) can cause a significant change
in flow rate.
- If the suction side of the pump is under vacuum conditions,
a liquid at elevated temperature, close to its vapor point is
always a problem
- Is there anywhere in the system that vapor can accumulate
and cause random surging as it eventually finds its way to the
- You could be seeing a very high frequency "water hammer,"
except instead of one or two "hammers" that is usually associated
with a water hammer, it is appearing as a high frequency vibration
- At any restriction in the pipe, a valve, or a change of
pipe area, a certain portion of any pressure wave is reflected,
while the balance continues down the pipe.
- When a restriction is located at a point where the
pressures are reflected in "tune," or at a particular harmonic
being generated by the pump, vibrations can increase in
- It is much more commonly known in reciprocating pumps and
compressors, Centrifugal pump pulsation is less commonly known,
apparently because it is much less of a problem for most users.
- One method used to avoid flow surging problems is proper
location of the discharge throttle valve.
- When a centrifugal pump is operated at a very low flow
rate, recirculation occurs within the impeller, and it surges
at the natural frequency of the system. As the control valve is
moved away from the pump, there is a decrease in the frequency
and an increase in the amplitude of the pressure waves
- The energy imparted to the system by the pump is similar to
the strumming of a guitar string. The frequency is a function
of the length of the string, and is analogous to the distance
from the pump to its control valve. The greater the distance
between the valve and pump, the lower the frequency of this
oscillation and the greater the magnitude of the pressure
- Placement of the valve close to the pump discharge flange
minimizes the amplitude, and thus the effects of the flow
oscillations. The mass of the oscillating fluid is reduced in
volume, and the turbulent flow through the valve destroys the
frequency of the excitation force.
- If the throttle valve is remotely located, flow surging
will be of low frequency and high amplitude since the fluid
mass is large. This situation must be avoided, as it may induce
violent mechanical vibrations in the piping system.
- The preferred throttle valve location will always be close
to the pump discharge flange in order to minimize the potential
and the effects of flow surging.
- Flow surging problems can also be resolved by installing a
bypass line. Bypassing a portion of the pump capacity back to
suction maintains pump operation closer to its design flow
where the amplitude of the hydraulic excitation forces is
small. The bypass line also protects the pump from overheating
and damage if system flow rate is reduced below minimum
- The stiffness of the pumping system affects both the frequency
and amplitude of pump surging. A "soft" system can result in low
frequency high amplitude surging that is detrimental to pump
performance and life.
- A "soft" system can be caused by a number of factors:
- There can be air or gas entrained in the pumping fluid, air
can be trapped in an elbow, or a system can have open tanks or
- A closed loop boiler with a remotely located control valve
that takes only a minimal pressure drop at low flow rates, or a
system in which there were flexible lines attached to the
suction and discharge of the pump, would be typical of a soft
- The following recommendations should be applied to minimize
the effects of soft systems:
- Place the control valve at or within five feet of the
discharge of the pump.
- Use flexible discharge lines only downstream of the control
valve and use a minimum of 10 pipe diameters of straight piping
into the pump suction.
- Size pump control valves so that 5-8% of the differential
pump head is taken across the valve.
- Install a low flow bypass system if operation below minimum
flow is anticipated
- Eliminate entrained air and gasses, or anticipate a higher
minimum flow limit if they cannot be avoided.
- Avoid air traps in pumping systems.
- Consult your pump supplier if operation is anticipated
below 40% of the BEP for any sustained period and the NPSH is
less than 30 feet.
- In hydraulically soft processes where a capacity control
valve cannot be located near the pump, and maximum turndown is
needed, a discharge orifice is often recommended. This isolates
the pump from potential system resonances, and permits stable
operation at flow rates down to 15-20% of BEP (the pump's best
efficiency point). The orifice is normally sized to drop 5% of
the pump head at BEP.
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