PD vers Centrifugal pumps 4

[sheiko]

Dear pros,

I used to believe that either Positive-Displacement or Centrifugal type pumps (and compressors) create flow and not pressure. That is, they conform their discharge pressure to the process' total back-pressure at their discharge flange.

Now I have come across a serious article from IMO pumps that claims the above statement is true only for PD pumps, and that centrifugal pumps create PRESSURE (and not flow).
Here is the article (see § "system response"):

http://www.warrenpumps.com/resources/pdpumps.pdf

It is mentionned that:

"In a PD pump, pressure is created by the system's response to flow.
If there was no connection at the discharge flange, the flow would exit the pump at atmospheric pressure.

Centrifugal pumps create pressure by first imparting velocity to the fluid with the impeller, then converting the velocity to pressure with the volute.
If there was no discharge flange connection, the flow would exit the pump with that developed pressure."

So my questions are:
- What is your opinion? do centrifugal pumps create FLOW or PRESSURE or BOTH? and could you please explain how?
- Do you confirm that the same reasoning is applicable to compressors?

Many thanks in advance.

"We don't believe things because they are true, things are true because we believe them."

 

[SNORGY]

Except for the "happense" part...

Regards,

SNORGY.

 

[CaracasEC]

Hi Sheiko,
The main difference between these two type of pumps is that their characteristic curves. A centrifugal pump has variable volume, constant differential head, sensitive to liquid properties, sensitive to system changes and self limiting. A positive displacement pump has constant volume, variable differential head, relatively insensitive to liquid properties, relatively insensitive to system changes and not self limiting. The main objective of a pump system is to move liquid and not compressed it. Just take an ethylene for a example. If the ethylene is in its liquid state, what you need is a pump to move the ethylene from one its destination. If the ethylene is in its gaseous state then what you need is a compressor.Does pump moves liquid and compressor for gas. One thing for sure is they move money. Stop the pump and plant revenue will stop.

 

[sheiko]

Even if pumps transfer liquids and compressors compress, I believe same reasoning can be applied to compressors. Centrifugal compressors create head and positive-displacement create flow. Do you agree?

"We don't believe things because they are true, things are true because we believe them."

 

[CaracasEC]

Hi Sheiko,
Both types create head and flow. However, for a centrifugal type flow is variable. You can always have one flow with its corresponding head. For a positive displacement type, you can have a variable head and a fixed flow depending on the available power of the driver. On pumps we are dealing with incompressible fluid which is in a liquid form and on compressor, we are dealing with compressible fluid in its gaseous form. For both pump and compressor the head is develop by the impeller and the speed. Thats why a centrifugal type is a self limiting that a relief valve downstream is not mandatory. However for a Positive Displacement type a relief valve is mandatory as this type is not self limiting and will always have enough head depending on its driver power capacity..

 

[gurmeet2003]

Very interesting discussion. A centrifugal pump/compressor has a shut off condition; impeller is turning yet there is no flow. In a positive displacement pump/compressor you can not move the piston and not have flow (an external control mechanism; unloader etc. is needed to change this).

Gurmeet
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