18

Constant HorsepowerControlThis control limits power output to a givenvalue for a given pump speed and maintainsit at a nearly constant level within a givenflow range (Figure 18) It has the advantage of keeping pump power source requirementsat a minimum

The purpose of the constant horse-power is to limit the maximum pump power by beginning flow reduction at a given inter-mediate system pressure (point A) and tocause the pump to reach near rated pressureat a given intermediate flow (point B)

Yoke Spring PreloadedRefer to Figure 17 System pressure Ps actsat all times on area ldquoHrdquo The yoke spring is preloaded with a force equal to area ldquoHrdquo multiplied by the pressure at which it is desired to have flow reduction begin (referto point A in Figure 18) As system pressurereaches this value the preload is overcomeand further pressure increase will produce a proportional decrease in flow The rate ofthe spring and the area ldquoHrdquo will determinethe slope from ldquoArdquo to ldquoBrdquo (Figure 18)

Design Flexibility AllowedThis control allows considerable design flexibility As system pressure reaches avalue slightly less than rated the compen-sator valve opens control pressure acts onarea ldquoCrdquo and from point B to zero flow thepump operates as a basic flat or differentialcut-off unit

Provisions For SpecialRequirements1) Integral Boost StageIf your application requires the pump to oper-ate at unusually low inlet pressure a booststage can be added This is a centrifugalpump that permits operation at inlet pressurewell below atmospheric A typical applicationis an aircraft main engine pump with a mini-mum inlet requirement of five psia

The impeller adds to the length andweight of the pump but results in overallsystem weight reduction and increased reliability in applications where inlet pressuremay be quite low

2) Startup Bypass ValveA simple reliable low-cost valve may be incorporated in the pump valve block to bypass flow and minimize torque duringstartup This is sometimes important toreduce cranking torque of an engine or startup current of a motorpump The flow isbypassed to inlet at very low pressure untila given flow is reached the bypass valvethen closes and remains closed until systempressure is reduced to near zero at whichtime it opens and is ready for the next startOperation is completely automatic and self-contained There are no electrical connec-tions and torque during startup is even lessthan that of the EDV feature in the depres-surized mode

3) Pressure PulsationA pulsation damping chamber (attenuator)can be incorporated in the valve block to provide reduction of the outlet pressure ripple This adds to the envelope and weightof the pump but is justified for applicationswhere lower pressure pulsation levels areessential and the required compressibility is not provided by the load circuit

CompensatorValve

PsOutlet

High Pressure

CompensatorValve Spring

PiInlet

Low Pressure

Pressure Adjustment

Pc Area ldquoCrdquoYoke Actuating Piston

Area ldquoHrdquo (annular area)

Yoke Spring

Drive Shaft

Pump Yoke

PcaseCase Drain

(Pi) Inlet Pressure (Ps) Outlet Pressure (Pc) Control Pressure (Pcase) Case Pressure

Figure 17