2 replies to this topic

[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.warrenpum...ces/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.

Edited by sheiko, 11 August 2011 - 06:28 PM.

[Qalander (Chem)]

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.
See the following website for example: http://www.engineeri...pumps-d_54.html,
where it is said that "a pump does not create pressure, it only creates flow. Pressure is a measurement of the resistance to flow."

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.warrenpum...ces/pdpumps.pdf

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

Many thanks.

Dear Hello,

• On the face of it one thing is certain that the case of compressible fluids(i.e. gases) can never be similar to (almost) In-compressible fluids(i.e. liquids)
• Further comments after reviewing attachments.

[kkala]

1. Objective of any type of pump, either positive displacement or centrifugal, is to transfer liquid at a specified rate from location A to location B. So it is practical (and probably simple) to say that a pump creates flow . This is what generally understood as pump function, irrespectively of any consequential head (produced or required) to realize this. Said “produced” head characterizes the positive displacement pumps, “required” head the centrifugal pumps.
2. Prerequisite for mentioned liquid transfer is power, supplied to liquid by pump drive to overcome friction, hydrostatic head, destination pressure; this has been mainly transformed into pressure power (pressure energy in unit time) at pump discharge.
3. Performance curve Q (m3/s) – H (m of liquid) of a positive displacement pump is represented by a line vertical to Q axis; flow Q is practically constant (specified by cylinder dimensions and piston frequency in the most simplified case). Discharge pressure comes from pressure gradient in the discharge line as a consequence of mentioned flow Q. So one can say that this sort of pump primarily creates flow (head is by consequence).
4. Performance curve Q – H of a centrifugal pump is of interest between point of design flow (being close to best efficiency point) and shutoff. Shutoff head for most process pumps is only 20% higher than head of design flow. So head has little variation over all practical flow range, represented by a line almost vertical to H axis (but not quite). This head results in flow, as a consequence of pressure gradient created in the discharge line. So one can say that this sort of pump primarily creates head, i.e. pressure (flow is by consequence). This is not so clear, as in the case of positive displacement pumps above.
5. To control feed flow from a positive displacement pump, you have to return excess flow to the source.
To control feed flow from a centrifugal pump, it is enough to install a variable resistance (e.g. a manual or control valve at the discharge) to vary head. Actual range will be more narrow than H – 1.2*H as above.
6. Arguments of para 3 & 4 assume that handled liquid receives enough power, or else required flow Q will be reduced.
Considering an electrical analogy, a pump can be similar to a generator, creating almost constant voltage (V) at its poles. Amperage is a consequence of it, being inversely proportional to external circuit resistance. In this sense a generator is similar to a centrifugal pump. If Amperage (A) had been created and resulted in Voltage at the ends of circuit (i^2*R, A^2*Ohm), a generator would have been similar to a positive displacement pump.
We say that the generator supplies (electrical) power V*A (cosφ is not met in fluid mechanics) to the circuit, just as the pump supplies power H*Q*ρ*g to the fluid (ρ=density, g=gravity acceleration). It might be more precise to say that a pump supplies power, but this is abstract compared to flow. Even for generators amperes are of primary interest to operators (voltage is more or less known).
7. Above tries to interpret statements of attached Warren article by D. Parkev, written for positive displacement pumps. Statement “if there is no discharge flange connection, the flow would exit the pump with that developed pressure” may need further explanations. To my understanding the pump will get “end of curve operation” in this case, with actual flow higher than its design flow Q; discharge pressure will be close to zero, it will not correspond to head H at the design point.
8. In fact query of “flow or pressure” creation by a pump may not be clear enough for centrifugal pumps, even though established concept is supposed to be “flow creation”. Well designed pumps of any type supply the liquid with adequate power to realize the transfer required. Being aware of performance curves (different between the two types of pumps) probably makes us able to bypass mentioned query.
I have read in several sources that pumps create flow rather than pressure, apparently in order to explain control schemes, etc. Having “lost” these explanations, any other opinion on the topic (or comment on the above) would be highly appreciated.
Note: Booster pumps create pressure.

Edited by kkala, 17 August 2011 - 10:48 AM.