Friday, December 20, 2013

What is back pressure in centrifugal pump


Back pressure in centrifugal pump

- area of volute casing is greater at discharge than at suction side
- velocity of fluid at the discharge is lesser than at the suction side
- pressure at discharge side is greater than at the suction side
- volume of fluid at discharge is larger than at the suction side


Analysis of Volute Casing of the Centrifugal Pump:


Similar to the Venturi principle & Bernoulli's principle,

SUCTION SIDE

- smaller area
- greater fluid velocity
- lesser pressure
- smaller fluid volume

DISCHARGE SIDE

- larger area
- lesser fluid velocity
- greater pressure
- larger fluid volume

Important:

1.  At the discharge side of the pump, there is more space for the fluid, thereby causing the fluid to slow down in speed and lose kinetic energy.

2.  First Law of Thermodynamics states that energy is neither created nor destroyed but is transformed or converted to another form.

3.  The fluid at the pump discharge decreased in kinetic energy (speed is reduced) but the reduction in kinetic energy is converted into pressure energy, therefore, the discharge pressure is higher than the suction pressure.


Back pressure of a valve at the discharge side of centrifugal pump:

- back pressure is the pressure behind the flow (at the back of the flow)
- backpressure is the pressure at the inlet side of the discharge valve (which is mounted at the discharge side of the pump)
- backpressure is created by restricting the flow (closing the valve that is mounted at the pump discharge)


Analysis of Valve Fitted at Discharge side of Centrifugal Pump:

When the valve is restricted (slightly closed down),

VALVE INLET

- higher back pressure
- higher volume
- slower fluid speed

VALVE OUTLET

- lower pressure
- lower volume
- faster fluid speed


Benefit of back pressure in centrifugal pumps:

- higher back pressure at the inlet of the discharge valve will decrease the power requirement of the driving motor of the pump


Effects of higher back pressure in centrifugal pumps:

- flow at the discharge valve outlet is reduced
- pressure of fluid at the discharge valve outlet is reduced
- longer time to fill up (tank, container, etc.)
- longer time to empty (tank, container, etc.)


Reasons Why are Centrifugal Pumps widely used in the industries?

- large flowrates handling capacity
- versatile
- simple in construction
- cheaper price
- easier and less expensive to operate, maintain and repair


1 comment:

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