To maintain design discharge pressure from a centrifugal pum

Software design is a(46) process. It requires a certain amount of fl air on the part of the(47). Design can not be learned from a book . It must be practiced and learnt by experience and study of existing systems. A well(48) software system is straightforward to implement and maintain , easily(49) and reliable. Badly(48) software systems, although they may work are(50) to be expensive to maintain , difficult to test and unreliable.

A．create

B．creative

C．creating

D．created

If a centrifugal pump is driven by a constant speed electric motor and produces a discharge pressure less than designed, the cause is probably ______.

A.excessive pump speed

B.excessive suction head

C.worn wearing rings

D.gross pump misalignment

Throttling in the _____ line of a centrifugal pump involves the risk of the pump cavitation.

A.suction

B.exhaust

C.pressure

D.discharge

What is the purpose of pressure-vacuum relief valves ________．

A.Regulation of discharge pressure from cargo pumps

B.Maintaining constant velocity in cargo lines

C.Regulation of suction head on cargo pumps

D.To maintain a tank or void at atmospheric pressure

第二组

Displacement pumps are those where the volume of the pump chamber is alternately increased to draw the liquid in from the suction pipe and then decreased to force the liquid out into the delivery pipe. Centrifugal pumps are those wherein an impeller rotating at high speed throws the liquid by centrifugal force from the center to the periphery of the impeller where the liquid is discharged through the delivery outlet.

Gear pump consists of two or more meshing gears (spur, single or double helical teeth) enclosed m a close-fitted housing. These are used extensively for pumping fuel oil, lubricating oil and hydraulic oil. When gears are unmeshed on the inlet side, a pressure drop occurs that helps the atmospheric pressure acting on the surface of the oil in the reservoir to force it up the pipe to fill the void, the oil is then transferred around the periphery of the pump housing. As the gear teeth mesh again on the outlet side, they form. a seal that prevents oil from backing up to the inlet, the oil in the void is then be forced out into the discharge line.

All vane pumps have a rotor driven within a ring (cam ring) by a drive shaft coupled to a prime mover. A cylindrical rotor with sliding vanes, generally in radial slots, rotates within the ring. The center of the rotor is offset from the center of the ring. In a simple vane pump, as the rotor turns, the vanes are forced outward against the inner surface of the cam ring by centrifugal force. This outward radial movement of the vanes and turning of the rotor causes the chamber between the vanes to increase as the vanes pass further away from the inlet port. This increase in volume results in a lowering of pressure until the atmospheric pressure is sufficient to force oil from the reservoir into the inlet chamber. Oil from the inlet is swept away by the vanes toward the outlet port through a decreasing series of chambers until it is forced through the outlet port. A vane pump of balanced design has an elliptical cam ring so that two pumping chambers are formed. This pump is in hydraulically balance, since the two intake and two outlet ports are diametrically opposed to each other. Therefore the side loads exerted on the rotor cancel out, thereby increasing bearing lift and permitting high operating pressure.

According to the last sentence of paragraph 1, from where the liquid is discharged through the delivery outlet?

A.from the eye of the impeller

B.from passage of impeller

C.from the periphery of the impeller

D.from the impeller

What is the function of wearing rings found on some centrifugal pumps ________．

A.Absorb erosion of high velocity discharge stream

B.Seal pump shaft against entry of air

C.Isolate the outlet side from the inlet side

D.Dampen the turbulent discharge flow

Software design is a(71)process .It requires a certain(72)of flair on the part of the designer. Design can not be learned from a book .It must be practiced and learnt by experience and study of existing systems .A well(73)software system is straightforward to implement and maintain ,easily(74)and reliable .Badly(73)software systems ,although they may work are(75)to be expensive to maintain ,difficult to test and unreliable.

A．create

B．created

C．creating

D．creative