4.8
Charge Pump Intake
Unless careful attention is put into the design of the suction supply system, the well service pump can
experience cavatations. The resulting erosion of components, and cyclical stresses into the flow lines as a result
of vibrations, can be a serious hindrance to the safety and suitable operation of a well service package design.
MSI therefore recommends the use of a centrifugal charge pump as part of a properly designed supply system.
Well service pump cavitations will occur if the pump suction pressure drops to a level approaching the vapor
pressure of the fluid being pumped. Because of the rapid stopping and starting of fluid at each of the suction
valves, especially at high flow rates, the sudden demand for fluid and resulting rapid pressure drop at the valve
inlet can cause cavitations at the well service pump suction manifold inlet. MSI recommends that the inlet
charge pressure be a minimum of 50 PSI.
A centrifugal charge pump should be sized to run at less than 100% flow capacity due to the cyclical nature of
the inlet flow into a well service pump. The charge pump should be sized so that it can supply these sudden
volumes of fluid that flow at momentary velocities and run 7%* higher than the average pump flow velocity.
For slurry applications, the centrifugal pump should be sized for 1 1/2* times the recommended flow rate of the
well service pump, considering the flow rate at the largest fluid end plunger size and pump RPM expected on
any well service job. Also, the line between the centrifugal and the well service pump should be sized so that
flow velocity changes do not exceed 1.5 feet per second.*
A cavitating pump can shake an entire well serviced truck and can even make offshore decks vibrate
considerably. The resulting annoyance is not the only effect. Excessive vibrations can result in manifold and
flow line fatigue failures, which could result in injury, death, equipment damage, and loss of well control.
Because well service pumps often pump slurries, it is also important that fluid velocities be kept high enough to
prevent solids from settling out of the fluid. The settling solids will accumulate at bends or on the blind side of
the suction manifold and can cement together into a solid mass. Design the system to eliminate any traps which
may collect solids.
Flow velocities of the piping and NPSH leading into the centrifugal charge pump need to be sized according the
the centrifugal pump manufacturer’s recommendation to prevent cavitations. A cavitating centrifugal pump
will introduce vibrations and entrained air into the inlet of the well service pump and can cause it to cavitate.
If a suction supply line is inadequately designed and causes cavitations to occur at high flow rates, the use of a
bladder type suction stabilizer will often eliminate the problem. Consult the stabilizer manufacturer for proper
sizing and installation.
Placing a liquid filled 0-100 PSI pressure gauge with pulsation snubber nearest to the suction inlet of the well
service pump is useful for diagnosing cavitation problems. If the gauge needle vibrates excessively, the pump
may be cavitating. Keeping the charge pressure at the inlet at 50 PSI or above for high pumps RPMs should
resolve this.
* These numbers are taken from a popular centrifugal pump manufacturer’s handbook on sizing a centrifugal pump for charging a
reciprocating pump while pumping slurries. Consult the manufacturer of your selected centrifugal pump for specific
recommendations on your application.
Revision K 03/12/2019
MSI - A Division of Dixie Iron Works, Ltd. All rights reserved.
Page 30