1、 1 中文 7762 字 The pump in pumping station selection problem Rotary pumps These are built in many different designs and are extremely popular in modern fluid-power system. The most common rotary-pump designs used today are spur-gear, generated-rotary , sliding-vane ,and screw pump ,each type has advan
2、tages that make it the most suitable for a given application . Spur-gear pumps. these pumps have two mating gears are turned in a closely fitted casing. Rotation of one gear ,the driver causes the second ,or follower gear, to turn . the driving shaft is usually connected to the upper gear of the pum
3、p . When the pump is first started ,rotation of gears forces air out the casing and into the discharge pipe. this removal of air from the pump casing produces a partial vacuum on the pump inlet ,here the fluid is trapped between the teeth of the upper and lower gears and the pump casing .continued r
4、otation of the gears forces the fluid out of the pump discharge . Pressure rise in a spur-gear pump is produced by the squeezing action on the fluid ad it is expelled from between the meshing gear teeth and casing ,.a vacuum is formed in the cavity between the teeth ad unmesh, causing more fluid to
5、be drawn into the pump ,a spur-gear pump is a constant-displacement unit ,its discharge is constant at a given shaft speed. the only way the quantity of fluid discharge by a spur-gear pump of type in figure can be regulated is by varying the shaft speed .modern gear pumps used in fluid-power systems
6、 develop pressures up to about 3000psi. Figure shows the typical characteristic curves of a spur-gear rotary pump. These curves show the capacity and power input for a spur-gear pump at various speeds. At any given speed the capacity characteristic is nearly a flat line the slight decrease in capaci
7、ty with rise in discharge pressure is caused by increased leakage across the gears from the discharge to the suction side of the pump. leakage in gear pumps is sometimes termed slip. Slip also increase with arise pump discharge pressure .the curve showing the relation between pump discharge pressure
8、 and pump capacity is often termed the head-capacity or HQ curve .the relation between power input and pump capacity is the power-capacity or PQ curve . Power input to a squr-gear pump increases with both the operating speed and discharge pressure .as the speed of a gear pump is increased. Its disch
9、arge rate in gallons per minute also rise . thus the 2 horsepower input at a discharge pressure of 120psi is 5hp at 200rpm and about 13hp at 600rpm.the corresponding capacities at these speed and pressure are 40 and 95gpm respectively, read on the 120psi ordinate where it crosses the 200-and 600-rpm
10、 HQ curves . Figure is based on spur-gear handing a fluid of constant viscosity , as the viscosity of the fluid handle increases (i.e. ,the fluid becomes thicker and has more resistance to flow ),the capacity of a gear pump decreases , thick ,viscous fluids may limit pump capacity t higher speeds be
11、cause the fluid cannot into the casing rapidly enough fill it completely .figure shows the effect lf increased fluid biscosity on the performance of rotary pump in fluid-power system .at 80-psi discharge pressure the pp has a capacity lf 220gpm when handling fluid of 100SSU viscosity lf 500SSU . the
12、 power input to the pump also rises ,as shown by the power characteristics. Capacity lf rotary pump is often expressed in gallons per revolution of the gear or other internal element .if the outlet of a positive-displacement rotary pump is completely closed, the discharge pressure will increase to t
13、he point where the pump driving motor stalls or some part of the pump casing or discharge pipe ruptures .because this danger of rupture exists systems are filled with a pressure relief valve. This relief valve may be built as of the pump or it may be mounted in the discharge piping. Sliding-Vane Pum
14、ps,These pumps have a number of vanes which are free to slide into or out of slots in the pup rotor . when the rotor is turned by the pump driver , centrifugal force , springs , or pressurized fluid causes the vanes to move outward in their slots and bear against the inner bore of the pump casing or
15、 against a cam ring . as the rotor revolves , fluid flows in between the vanes when they pass the suction port. This fluid is carried around the pump casing until the discharge port is reached. Here the fluid is forced out of the casing and into the discharge pipe. In the sliding-vane pump in Figure
16、 the vanes in an oval-shaped bore. Centrifugal force starts the vanes out of their slots when the rotor begins turning. The vanes are held out by pressure which is bled into the cavities behind the vanes from a distributing ring at the end of the vane slots. Suction is through two ports A and AI, pl
17、aced diametrically opposite each other. Two discharge ports are similarly placed. This arrangement of ports keeps the rotor in hydraulic balance, reliving the bearing of heavy loads. When the rotor turns counterclockwise, fluid from the suction pipe comes into ports A and AI is trapped between the v
18、anes, and is carried around and discharged through ports B and BI. Pumps of this design are built for pressures up to 2500 psi. earlier models required staging to attain pressures approximating 3 those currently available in one stage. Valving , uses to equalize flow and pressure loads as rotor sets
19、 are operated in series to attain high pressures. Speed of rotation is usually limited to less than 2500rpm because of centrifugal forces and subsequent wear at the contact point of vanes against the cam-ring surface. Two vanes may be used in each slot to control the force against the interior of th
20、e casing or the cam ring. Dual vanes also provide a tighter seal , reducing the leakage from the discharge side to the suction side of the pump . the opposed inlet and discharge port in this design provide hydraulic balance in the same way as the pump, both these pumps are constant-displacement unit
21、s. The delivery or capacity of a vane-type pump in gallons per minute cannot be changed without changing the speed of rotation unless a special design is used. Figure shows a variable-capacity sliding-vane pump. It dose not use dual suction and discharge ports. The rotor rums in the pressure-chamber
22、 ring, which can be adjusted so that it is off-center to the rotor. As the degree of off-center or eccentricity is changed, a variable volume of fluid is discharged. Figure shows that the vanes create a vacuum so that oil enters through 180 of shaft rotation. Discharge also takes place through 180 o
23、f rotation. There is a slight overlapping of the beginning of the fluid intake function and the beginning of the fluid discharge. Figure shows how maximum flow is available at minimum working pressure. As the pressure rises, flow diminishes in a predetermined pattern. As the flow decreases to a mini
24、mum valve, the pressure increases to the maximum. The pump delivers only that fluid needed to replace clearance floes resulting from the usual slide fit in circuit components. A relief valve is not essential with a variable-displacement-type pump of this design to protect pumping mechanism. Other co
25、nditions within the circuit may dictate the use of a safety or relief valve to prevent localized pressure buildup beyond the usual working levels. For automatic control of the discharge , an adjustable spring-loaded governor is used . this governor is arranged so that the pump discharge acts on a pi
26、ston or inner surface of the ring whose movement is opposed by the spring . if the pump discharge pressure rises above that for which the by governor spring is set , the spring is compressed. This allows the pressure-chamber ring to move and take a position that is less off center with respect to the rotor. The pump theb delivers less fluid, and the pressure is established at the desired level. The discharge pressure for units of this design varies between 100 and 2500psi.
