Pump cavitation due to discharge pressure

[nak12]

Hello, I was curious on centrifugal pump cavitation. If there is a discharge valve on the discharge section of the pump that is fully open, why does discharge pressure decrease and why can that cause cavitation in the pump. I understand that if the discharge valve was shut then discharge pressure increases but it stumps me as to why it can decrease when the discharge valve is open.

 

[gempump]

Hi Nak12,
Please see the pump performance curve,then you will know the relationship between the Flow and Head.

Heavy-Duty Centrifugal Slurry Pump Manufacturer

http://www.gempump.com

 

[TD2K]

It's a matter of NPSHA versus NPSHR. NPSHR for most centrigal pumps increase with flow and past the BEP, significantly. A fully open discharge valve gives you the maximum flow if there no type of flow control device on the pump such as variable speed or a discharge control valve. So max flow can result in cavitation. It might not, it depends on the system.

 

[Artisi]

You need to describe the pump and the operating system more fully to arrive at a meaningful answer and somewhere near correct assumption.
What makes you think the pump is cavitating?


It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[thkurian]

Initially pump head for a particular flow is decided after calculating the system head (combination of static lift and frictional losses). When the pipes are new or an error occured in calculation, actual system head will be lower than calculated system head. So if no additional frictional loss is created by discharge valve, the head will be lower and flow and NPSHR will be higher than specified and purchased for the pump. If NPSHR happens to be higher than NPSHA, pump will cavitate. Higher noise and vibration is expected. If high flow and cavitation occurs, either discharge valve to be in partially closed position or impeller diameter to be reduced.

 

[bimr]

When a pump suction is at low pressure (high vacuum condition), suction cavitation occurs. The pump is being starved or is not receiving enough flow. When this situation occurs, bubbles or cavities form at the eye of the impeller. As the bubbles carry over to the discharge side of the pump, the fluid conditions change, compressing the bubble into liquid and causing it to implode against the face of the impeller.

An impeller that has fallen victim to suction cavitation will lose chunks of material, causing it to look like a sponge.

Typical causes of suction cavitation:
plugged inlet strainer;
blockage in suction pipe (ie. air pocket);
over-running pump on the right side of the pump curve;
incorrect piping design;
inadequate NPSH for the selected pump.

 

[TenPenny]

"I understand that if the discharge valve was shut then discharge pressure increases but it stumps me as to why it can decrease when the discharge valve is open."

Think about that logically. If the pressure increases when the valve is shut, then logically, it will decrease when the valve is opened. One is the reverse of the other.

Otherwise, you could close the valve, and get an increase in pressure, then open the valve, the pressure would stay the same, and then you close the valve again, and magically you'd have a higher pressure than the first time you closed it.