Pump and Fan Flashcards
Describe the variation of liquid water pressure in centrifugal pump
in centrifugal pump, the pressure of liquid water first decreases, then it increases, and there must be a minimum pressure and lowest pressure liquid water is always in the specific location
describe the position of the lowest pressure liquid in the centrifugal pump
according to the test study, the lowest pressure point of liquid water in the impeller is at K, later than vane entrance, where pressure is denoted as Pk
describe on whether the liquid water in the centrifugal pump produces bubbles in several cases
1) when Pk is greater than the corresponding saturation pressure Psat, at the liquid water temperature, the liquid water will not vaporize and will not produce bubbles
2) when Pk is equal to the corresponding saturation pressure Psat, at the liquid temperature, the liquid water has just started to vaporize, resulting in bubbles
3) when Pk is less than the saturation pressure Psat corresponding to the liquid temperature, the liquid water will violently vaporize and a large number of bubbles will be generated.
describe the exact cavitation phenomenon in centrifugal pump
liquid vaporization and water vapour condensation form a high frequency impact load, resulting in noise and vibration, resulting in mechanical peeling and electrochemical corrosion of metal materials. and this comprehensive phenomenon is collectively known as cavitation
explain the reason of cavitation in centrifugal pump
the reason of the cavitation in centrifugal pump is that minimum liquid pressure Pk near the vane inlet is equal to or less than the saturation pressure Psat at the liquid temperature
describe the conditions to avoid cavitation in centrifugal pump
there is only ONE condition to avoid cavitation in centrifugal pump, and that is Pk > Psat
describe the damage of cavitation to noise and vibration of centrifugal pump
- bubble collapse
- liquid impact each other but also impact the metal surfaces
- provide variety of frequency noise
- serious can be heard in centrifugal pump “crackling” explosion
at the same time, vibration of the unit is caused. if the impact frequency is equal to the vibration (natural) frequency of the unit, a stronger cavitation resonance will be generated, resulting in the unit to force stop.
pitting effect on flow through components
most of the components damaged by cavitation are near the vane inlet. at the beginning, pitting appears on metal surfaces. then, groove and honey comb-like traces appear. serious can cause vane, front and rear cover plate perforation, or even impeller rupture. this can cause serious accident, seriously affect the life of the pump
effect of cavitation on centrifugal pump
- the existance of large number of bubbles blocked the flow passage, destroy the continuous flow of liquid in the pump so that the pump flow, head and effieciency decreased significantly. the performance curve of the pump is steep drop and the pump is unable to continue to work
the total pressure … backward impeller
for the backward impeller, the general trend of the performance curve is that the total pressure is gradually decrease with the increase of flow rate. because of the different structural parameters, there are 3 basic types of performance curve.
- steep drop type : suitable for the situation of when the flow rate change is not large and the total pressure change is large
- flat type : suitable for large change in flow rate and small change in total pressure requirements
- hump shaped type : on the left side of the critical point, the working area is unstable. the unstable area shoulc be reduced as soon as possible during design. and the critical point should be passed quickly during use
the harm of surge of centrifugal fan
- the air flow pulsate and produce strong noise
- strong vibration of centrifugal fan device and piping system results in the damage of seal and bearing
- the moving components collide, cause a serious accident
the measure to prevent the surge of centrifugal pump
- hump shape full pressure flow performance curve should be avoided in large volume capacity in pipeline system
- make the flow in any condition less critical, set the regulating pipe/open the vent valve
- the unstable section on performance curve is reduced by appropriate regulation
