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.

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.

@Kishore.

Throttling means accelerator.

Throttling is valve not fully open.

What is throttling?

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

Yes, I think it remain unchanged.

I think remains unchanged because if we throttle the pump delivery flow pump load will reduce but speed of the impeller and flow coming to impeller eye will be the same. Again if we increase the pump delivery valve to get more flow without doing anything flow will come that means suction static pressure will be the same.

If the discharge of a centrifugal pump is throttled, then its suction lift will increase (by considering liquid level in suction pond/sump is same as before). When I say suction lift, I am directly pointing out the Static Suction Pressure of the pump.