FM 1-506 Fundamentals of Aircraft Power Plants ... - Survival Books

WWW.SURVIVALEBOOKS.COMFM 1-506pump or a method for regulating pump output in thevariable displacement (piston) pumps.Constant-Emplacement PumpGear-type pumps have approximately straight-lineflow characteristics, whereas fuel requirements fluctuatewith flight or ambient air conditions. A pump of adequatecapacity at all engine operating conditions will haveexcess capacity over most of the range of operation. Thisis the characteristic which requires the use of a pressurerelief valve for disposing of excess fuel. The impeller,which is driven at a greater speed than the high-pressureelements, increases the fuel pressure from 15 to 45 psi,depending on engine speed.This fuel is discharged from the boost element (impeller)to the two high-pressure gear elements. Each ofthese elements discharges fuel through a check valve to acommon discharge port. High-pressure elements deliverapproximately 51 gallons per minute at a discharge pressureof 850 psig.Shear sections are incorporated in the drive systemsof each element. If one element fails, the other elementcontinues to operate. Check valves prevent circulationthrough the inoperative element. One element can supplyenough fuel to maintain moderate aircraft speeds.A relief valve is incorporated in the discharge portof the pump. This valve opens at approximately 900 psiand is capable of bypassing the total flow at 960 psi.Excess fuel is recirculated. The bypass fuel is routed tothe inlet side of the two high-pressure elements.Variable-Displacement PumpThe variable-displacement pump system differsfrom the constant-displacement pump system. Pumpdisplacement is changed to meet varying fuel flow requirements;that is, the amount of fuel discharged fromthe pump can be made to vary at anyone speed. With apump; of variable flow, the applicable fuel control unitcan automatically and accurately regulate the pump pressureand delivery to the engine.Where variable-displacement pumps are installedtwo similar pumps are provided and connected in parallel.Each pump can carry the bad if the other fails duringnormal parallel operations. At times one pump maybeinsufficient to meet power requirements. Pump duplicationincreases safety in operation, especially duringtakeoff and landingThe positive-displacement, variable-stroke-typepump incorporates a rotor, piston, maximum speedgovernor, and relief valve mechanism.Fuel FiltersAll gas turbine engines have several fuel filters atvarious points along the system. It is common practice toinstall at least one filter before the fuel pump and one onthe high-pressure side of the pump. In most cases thefilter will incorporate a relief valve set to open at aspecified pressure differential to provide a bypass for fuelwhen filter contamination becomes excessive.Paper Cartridge. This filter is usually used on thelow-pressure side of the pump (Figure 4-1). It incorporatesa replaceable paper filter element which iscapable of filtering out particles larger than 50 to 100microns (the size of a human hair). (One micron equals0.000039 inch, or 25,400 microns equal one inch.) Thecartridge protects the fuel pump from damage due to fuelcontamination.Screen Disc. This disc (Figure 4-2) is located on theoutlet side of the pump. This filter is composed of a stackof removable fine wire-mesh screen discs which must bedissassembled and cleaned periodically in an approvedsolvent.Screen. This screen is generally used as a lowpressurefuel filter. Some of these filter screens areconstructed of sinter-bonded stainless steel wire clothand are capable of filtering out particles larger than 40microns (Figure 4-3).In addition to the mainline filters, most fuel systemswill incorporate several other filtering elements. Theymay be located in the fuel tank, fuel control, fuel nozzles,and any other place deemed desirable by the designer.Pressurizing and Drain (Dump) ValvesThe pressurizing and drain valve prevents flow to thefuel nozzles until sufficient pressure is reached in the mainfuel control. Once pressure is attained, the servo assembliescompute the fuel-flow schedules. It also drainsthe fuel manifold at engine shutdown to prevent postshutdownfires. But it keeps the upstream portion of thesystem primed to permit faster starts.Flow Divider. The pressurizing and dump valveused on some engines has a somewhat different function.In addition to the draining or dumping function, this unitalso serves as a flow divider. At the beginning of an enginestart, the fuel control supplies a pressure signal to thepressurizing and dump valve. This causes the valve toclose the manifold drain and open a passage for fuel flowto the engine. On engine shutdown, fuel flow is cut offimmediately by a valve in the fuel control. The pressuresignal drops, the dump valve opens, and fuel drains from4-2