Pressure Drop

[Cmb78]

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.

 

[miningman]

Don't know if this helps or not, but is there flow in the suction?? If no flow ,there is no pressure drop. Since a pressure differential is required to initiate and maintain flow, there is a pressure drop over the full length of the suction pipe. Just IMHO of course.

 

[Cmb78]

I guess what I am really wanting to understand is what does the pressure drop profile look like. As soon as the pump is turned on the rotation of the impeller creates the low pressure needed to draw the fluid into the pump. How far does that pressure drop carry. Is it all within inches of the eye or does it reduce linearly to the point the product leaves the tower its feeding the pump from?

 

[bimr]

A pressure gauge on the suction of a pump would show a relatively large momentary loss in pressure while the pump was being started. That is why a pump will cavitate on startup. Once the flow has been increased to its steady state value, the suction pressure would come partly back up.

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.

 

[someguy79]

It sounds like there may be some confusion here about steady-state pressure drop versus transient pressure variations.

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.

 

[LittleInch]

It would also vary depending on the relative size of the pump inlet and the size of your inlet header/pipe. If the pipe is quite big compared to the pump, you might not see much of a drop. If this is a multi pump system on a large header again not much of a transient or stay state drop will be seen.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MJCronin]

For comparison purposes, I think that the most extreme case of pump suction-side fireworks is the transient that can occur when a boiler feedpump starts up.

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/issue-2/features/deaerator-pressure.html

MJCronin
Sr. Process Engineer

 

[bimr]

Figure 6-2: Transient caused by pump start-up.
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

 

[bimr]

Hydraulic Transients