9 replies to this topic

[sheiko]

Hello,

I would like you to confirm the following statements please:

1/ Normal capacity corresponds to the flowrate given by the PFD/Material balance (nornal case)?

2/ Design capacity corresponds to the flowrate given by the PFD/Material balance (design case) or to the normal capacity + a margin?

3/ Rated capacity is the maximum capacity of the pump (i.e where the line die out on the right side of the pump curve)?

I also would like to know what is the capacity at which the customer wants the pump selected on which to base any guarantee of efficiency and power calculations? is it design or rated capacity?

Thanks in advance

[fallah]

Rated isn't the maximum capacity, and is the capacity at which the customer wants the pump selected on which to base any guarantee of efficiency.

Usually power calculation is performed based on capacity in End of Curve, especially for pums starting in open valve position.

[Art Montemayor]

Sheiko:

Your queries can be answered in several contexts - all based on what the situation is, or what the phase of the project is. For example, in my capacity a consultant to an operating company your queries are answered as follows:

1/ Normal capacity corresponds to the flowrate given by the PFD/Material balance (nornal case)?
Yes, this the the accepted definition of "Normal Capacity".

2/ Design capacity corresponds to the flowrate given by the PFD/Material balance (design case) or to the normal capacity + a margin?
The Design Capacity is the Normal Capacity multiplied by a contingency factor (usually 1.10 to 1.25) in order to allow for wear, flow variance, and calculation errors.

3/ Rated capacity is the maximum capacity of the pump (i.e where the line die out on the right side of the pump curve)?
No. The Rated Capacity is the actual capacity realized by the actual, supplied centrifugal pump when running at the design speed and TDH. You see, manufacturers will sell you an existing design of a pump. No manufacturer is going to initiate a special design and fabrication for your specific pump specifications - not if he/she wants to be competitive in price. The manufactuer selects the closest model pump that fits the conditions. By common sense, that selected pump is not going to give the exact specified conditions - but it will perform up to the specified flow rate and TDH, plus a contingency.

[sheiko]

Thank you

On item 3/

I understand that the rated capacity (on process data sheet) corresponds to the design capacity and that this is the capacity at which the customer wants the pump selected on which to base any guarantee of efficiency and power calculations. Am i correct?

Plus,

4/ Is there a special terminology for the end-of-curve capacity? maximum capacity?

For info. these questions arise from http://www.eng-tips....d=227183&page=4, where i found that rated capacity = end-of-curve capacity (curtis26's post)!

[Art Montemayor]

Sheiko:

Note Artisi's response on the Eng-Tips Forum. Artisi is an experienced, pump engineer. I have found flaws in Curtis26's explanations. Maybe it is his/hers way of writing, but the explanations lack total context. For example, note that Curtis26 makes NO REFERENCE to the TDH when discussing the "rated capacity". ANY cenyrifugal pump capacity, expressed solely as a FLOW RATE is an INCOMPLETE DEFINITION. Logic demands that the TDH of the pump ALSO MUST accompany the flow rate in order to identify WHERE IN THE PERFORMANCE CURVE you are talking about.

In my opinion, no one is interested in the flow rate at the end-of-the-curve - EXCEPT TO IDENTIFY THE MAXIMUM HORSE POWER REQUIRED AT THAT POINT. Your process TDH is usually well over the the end-of-the-curve TDH, so the information is only academic. The term "Rated" from a PROCESS (not mechanical) point of view is meant to mean AT THE DESIGN OPERATING CONDITIONS.

In my opinion, you are correct in stating "the rated capacity (on process data sheet) corresponds to the design capacity and that this is the capacity at which the customer wants the pump selected on which to base any guarantee of efficiency and power calculations."

[fallah]

For info. these questions arise from http://www.eng-tips....d=227183&page=4, where i found that rated capacity = end-of-curve capacity (curtis26's post)!

I don't think curtis26 statement as:"...where the line die out on the right side of the pump curve" means end of curve.He/she probably means where the pump curve begin to fall down that may still its flowrate on the curve has reasonable difference with the value of flowrate in end of curve.

[Art Montemayor]

Fallah:

In American vernacular English, the term "die out" means ENDS. Therefore, that is exactly what curtis26 meant.

Even if what he/she was trying to state was, as you say, "probably means where the pump curve begin to fall down" it is still wrong to attribute the term "rated" to such an irrational and unrelated place on the curve. The outstanding fact still remains: an engineer cannot identify a position or place on the performance curve without stating the full identity of TWO variables - not just one. You must state the TDH AS WELL AS THE FLOW RATE in order to fully identify a position on the Performance Curve.

[sheiko]

I don't think curtis26 statement as:"...where the line die out on the right side of the pump curve" means end of curve.He/she probably means where the pump curve begin to fall down that may still its flowrate on the curve has reasonable difference with the value of flowrate in end of curve.

On top of what Mr Montemayor said, Curtis26 's exact statement is: "the rated flow rate is the maximum capacity of the pump (i.e where the line die out on the right side of the pump curve)"

[Qalander (Chem)]

I don't think curtis26 statement as:"...where the line die out on the right side of the pump curve" means end of curve.He/she probably means where the pump curve begin to fall down that may still its flowrate on the curve has reasonable difference with the value of flowrate in end of curve.

Curtis26 's exact statement is: "the rated flow rate is the maximum capacity of the pump (i.e where the line die out on the right side of the pump curve)"

Dear sheiko Hello,

Although forum's highly learned/ experienced colleagues discussion is already on.

Can we not consider BEP (Best Efficiency Point of Flow Charaterstic Curve) to define
the rated flow in your query's case?
Regards
Qalander

[sheiko]

Qalander,

From my understanding, BEP is indeed characterized by the rated head (ordinate) and the rated capacity (abscissa) on a manufacturer's pump curve. However, this point will hardly perfectly match the design operating conditions (specified by the process engineer on process data sheet) because of what Art Montemayor previously said ("No manufacturer is going to initiate a special design and fabrication for your specific pump specifications...").

Please feel free to correct me if i missed something