Pressure Drop
Pressure Drop
(OP)
If a pump requires 7' NPSHr, and has a pressure gauge on the suction would I expect to see the pressure drop on the gauge when the pump is started or would the pressure hold constant if the suction pressure was well above that of the NPSHr? A debate has insued on where we would expect to see the pressure drop whether it is at the eye of the impeller only or over the full length of the suction piping.





RE: Pressure Drop
RE: Pressure Drop
RE: Pressure Drop
The energy to accelerate the fluid in the suction line can't come from the pump, it comes from the energy available in the suction line.
The pressure wave moves vary fast and you would see the pressure drop quite a way back.
RE: Pressure Drop
NPSHR is a value describing a minimum suction head needed during steady-state operation.
Your questions discuss startup of a pump. This is a transient event. As bimr says, the pressure wave will travel quickly up the pipe as the pump starts. Over short distances, you might not notice the time it takes. Over long distance (not typical practice for a suction line) it's much more noticeable. This transient pressure variation is typically non-linear with position along the pipe.
Linear or non-linear pressure variation in steady state is related to piping configuration and fluid properties. It's possible that you would get a linear pressure variation along the pipe length if the pipe is straight, has no size changes, and has no pressure reducing features.
RE: Pressure Drop
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Pressure Drop
I am talking about a power plant BFP that is fed by an overhead deaerator.
The slamming and pressure surges have broken pipe supports and have caused pump seizure and shaft failure.
Unlike any other type of pump, a detailed analysis of the "hot-water slug" that goes flying through the pump is necessary.
A large margin on NPSHa is strongly advised....
Superb info here:
http://www.power-eng.com/articles/print/volume-114...
MJCronin
Sr. Process Engineer
RE: Pressure Drop
When a pump is started, the pressure at the discharge side of the pump rises sending a positive pressure wave (which increases pressure) down the pipeline toward the downstream reservoir. The resulting peak pressure can cause the pipe to collapse if the pressure rating of the pipe is less than the maximum surge pressure. When the initial positive pressure wave reaches the downstream reservoir, it is converted into a negative pressure wave which propagates back to the pump and may induce cavitation. On the suction side of the pump, the solid sloping line represents the initial hydraulic grade and the straight dashed line depicts the final hydraulic grade, while shutdown transients are not shown.
Speed of wave is shown in this video:
https://www.youtube.com/watch?v=ZuIsIe2kKq8
Speed of pressure wave~ 1400 m/s
RE: Pressure Drop