轮机英语课件lesson 15.ppt

Lesson 15 Marine pumps The pumps employed on board ship can be divided into two main categories(种类): (i) positive displacement pumps(正排量式泵、容积式泵) and (ii) centrifugal pumps(离心泵)。 They can be broken down(细分) into three classes(种类): (1) reciprocating(往复式), (2) rotary(回转式) and (3) centrifugal(离心式). Under reciprocating are the direct acting(直动式), power, and crank-fly-wheel types. Under rotary are the gear, vane(叶片式), cam-and-piston and screw types(螺杆式). Under centrifugal are the volute(旋涡式), diffuser(扩散式), mixed-flow, axial-flow and turbine or regenerative(再生的) types. Displacement pumps are those where the volume of the pump chamber is alternately(交替地) increased to draw the liquid in from the suction pipe and and then decreased to force the liquid out into the delivery pipe. Centrifugal pumps are those wherein an impeller(叶轮) rotating at high speed throws(甩、扔) the liquid by by centrifugal force from the center to the periphery(园周) of the impeller where the liquid is discharged through the delivery outlet. Gear pumps consist of two or more meshing(啮合) gears [spur(直齿轮), single or double helical teeth (螺旋齿)] enclosed in a close-fitted housing. These are used extensively(广泛地)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(空间), 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偏心环) by a drive shaft coupled to a prime mover. A cylindrical rotor with sliding vanes, generally in radial slots, rotates within the ring. In a simple vane pump(单作用叶片泵), the centre of the rotor is offset(偏移) from the centre of the ri