MSI QI-1000, Technical Manual, Page: 52 / 56

8.00 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.