Chemical Engineering - Fluid Mechanics - Discussion

A centrifugal pump cannot “suck” or ‘lift” the liquid into itself. Suction lift in a pump refers to the pressure (negative pressure) on the suction side of the pump. For open systems, atmospheric pressure is the force pushing the liquid into the pump. Think of it like sucking liquid in a straw.

A straw works because the negative pressure created on the mouth end allows the atmospheric pressure to push the fluid into the straw. In space, straws don't work because there is no atmospheric pressure to push the fluid into it. It's the same with pumps.

Throttling the discharge decreases the pressure on the discharge side, increasing the pressure difference between the suction side and the discharge side, therefore increasing the suction lift.

If the discharge of a centrifugal pump is throttled then its suction lift increases. Because of the throttling operation, the flow rate of the centrifugal pump will decrease.

As it is assumed the liquid level inside the pump is the same so hydrostatic pressure is also the same.

Due to a decrease in flow rate, velocity head loss, as well as frictional loss, decreases, by applying Bernoulli\'s equation, pump static suction pressure will increase.

P = head * density * specific gravity.

If discharge throttle, pressure will increase, so the head will increase.

So total head suction + discharge will increase.

Suction lift means I really not get but if we check the hydraulic calculation it remains unchanged.

Throttling decreases pressure on the discharge side. Its effect will also be felt near the impeller. Means pressure near impeller will also decrease by a small amount. Since pressure is decreased at the suction side, more driving force exists for fluid to suck in. Hence suction lift will increase.

Please explain the effect of discharge head on the suction lift?

If the discharge is throttled, the flow decreasing causes the decreasing NPSH require.

So the suction lift should be decreased right ?

How?

It's is because manometric head required for pump decreases when discharge end is throttle.

I think that no any effect of throttling of discharge on the suction lift. Because according to suction life formula no any term exist related to this. But yes output head i.e. manometric head will increase.