Basic structure and working principles
Discuss the centrifugal pump basic structure and working principle must tightly take the theme of kinetic energy efficient convert to static energy and unfold it.
（一） basic structure
Centrifugal pump basic components are high speed rotating impeller and fixed snail shell shaped pump casing. With several (usually are 4 ~ 12) back curved blades fastening on pump impeller axis, and with the pump shaft driven to do high speed rotation. The pump impeller is directly liquid in the work parts, are the centrifugal pump energy device. The pump casing central suction inlet and suction piping connections, suction pipe bottom with one-way bottom valve. Outlet beside pump shell connect wih outlet piping equipped with regulating valve.
（二） working principles
After the centrifugal pump started, the pump shaft drive impeller together to do high speed rotation, forced beforehand filling between the blades liquid rotation, under the function of inertia centrifugal, liquid does a radial motion from the impeller center to peripheral. Liquid won energy in the process of flow to impeller movement, static pressure energy increases, velocity increased. After liquid depart from impeller to enter into pump casing, because the crust port gradually expands and deceleration, part of kinetic energy changes to static pressure energy, along the tangential inflow discharge piping. So the pump volute shell is not only gathers by impeller outflow liquid components, and also is a convert energy device. When the liquid jilt to peripheral from impeller center, at the same time, the impeller center forms low pressure area, under the action of tank liquid level and impeller center total potential, lead to liquid absorbed into the impeller center. Relying on the impeller's continuous operation, liquid continuous inhaling and exhaling. Liquid in centrifugal pump acquired during the mechanical energy final expression as static pressure increase.
Need stressed is, if before centrifugal pump start-up, do not fill conveyed liquid in pump shell, as the low air density, the centrifugal is small formed after impeller rotation, impeller central area is not enough to form inhaled tank liquid low pressure, so although start centrifugal pump also can not convey liquids. This suggests that centrifugal pump has no self-priming, this phenomenon is called ability gas tied. Suction piping installing one-way bottom valve is to prevent before start, initiating into the pump housing liquid from casing to outflow. Air from suction piping into pump casing will also cause gas tied.
（三） impeller and other components
1． impeller
Impeller is the key part of centrifugal pump.
(1) According to mechanical structure can be divided into closed, semi-enclosed and open 3 kinds. Closed impeller is suitable for conveying cleaning liquid. Semi-enclosed and open impeller are suitable for conveying suspending liquid containing solid particles, this kind of pump efficiency is low.
When closed and semi-enclosed impeller in operation, leave impeller part of high pressure fluid leak into the empty cavity between pump impeller and shell, because the impeller anterior liquid suction entrance pressure is low, so the liquid acts on the front and behind impeller pressure is different, so produced directing impeller suction inlet side axial thrust. This force promotes the impeller moves to suction side, cause the impeller and pump case contact point friction, if serious, it will cause pump vibration, destroy pump normal operation. Drilling several holes drilled in the impeller back cover, can reduce the pressure difference on both sides, and to lessen the axial thrust adverse effect, but also reduce the pump efficiency. These holes called balance holes.
(2) according to different absorb fluid, impeller can be divided into single suction type and double suction type two kinds, single suction type impeller structure is simple, liquid only can be inhaled from one side. Double suction type impeller can simultaneously from impeller symmetry to inhaled liquid, it not only has a greater ability to absorb fluid, and basically
eliminated the axial thrust.
(3) according to the impeller leaf geometric shape, can be divided into sweepback, radial and procurvation three kinds, because sweepback blade is benefit for liquid kinetic energy transfer to static pressure energy, so it is used extensively.
2． guide wheel
In order to reduce leave of impeller liquid directly flow into the pump casing caused by impact energy loss, between impeller and pump shell sometimes installing a stationary and contain blades guide wheel. The guide wheel blade enters into the pump shell, and has gradually turned into the liquid flow continuous expansion, makes some of the kinetic energy effectively converted to static pressure energy. Multistage centrifugal pump usually installs guide wheel.
The snail form pump casing, impeller sweepback blade and guide wheel all can improve kinetic energy to the static pressure energy conversion rate, so all can be regarded as convert energy device.
3． shaft sealing device
Because of pump shaft rotates and pump casing is stationary, and there is a certain gap in the shaft and pump shell contact point. To avoid pump internal high-pressure liquid leakage along the clearance, or prevent outside air from the opposite direction into pump inside, must set shaft seal device. Centrifugal pump shaft sealing device has the stuffing box and machinery (face) seal. Stuffing box is put the pump shaft through shell gap to form sealing ring, in which loaded soft packing (such as oil or coating graphite leaching asbestos rope, etc.). Mechanical seal is composed by an moving ring on the shafy and static ring on the shell. Two ring ends borrow spring force to relatively rotate, plays a sealing function. Mechanical seal applies to higher seal occasions, such as conveying acid, alkali, inflammable, explosive and poisonous liquid.
