Pump is operating at normal flow or rated? 1

[roteng518]

I have a pump with rated flow is 338m3/hr, normal flow is 270m3/hr, BEP is 342m3/hr. Normal flow is 80% of BEP flow, which is clos to the manufacture recommended operating region lower limit but with a lower efficiency than BEP. Rated flow is 98.7% of BEP.

I am wondering in the field at most times pump is operating at normal flow or rated flow. Pump is designed by manufacture at rated flow which is very close to BEP. If in the practical field pump is always running at normal flow condition, why don't design the pump at normal flow?

Thanks.

 

[BigInch]

I'd say it's probably the pump motor that was selected to provide continuous operation duty at the "pump's" rated flowrate. Its possible that the pump alone could easily do the BEP flowrate and more, if a different higher horsepower rated motor was supplied, but this equipment package might have a limit rating based on the lowest rating of all components in the package.

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[JohnGP]

roteng518,

It is common practice to have more than one operating point specified for a pump duty. A "Normal" condition and a "Design" point. The "Normal" operating point is just that - the conditions under which the pump is expected to operate at most of the time, but with the capability to move to the "Design" point at a later time when process conditions require, without doing anything more than maybe changing impeller size. "Design" is synonymous with "Rated".

I know I am generalising here, but without knowing the details of your particular application, that's about all I can offer.

Cheers,
John

 

[BigInch]

John, you refer to operating points, to which I would also agree, however "operating" points are only indications of where the end user would prefer to run a pump and do not necessarily directly infer correspondence to a pump's "rated" point.

roteng,
"I am wondering in the field at most times pump is operating at normal flow or rated flow."
The pump will usually be running at normal flow

"Pump is designed by manufacture at rated flow which is very close to BEP. If in the practical field pump is always running at normal flow condition, why don't design the pump at normal flow?"

Its common to select a pump with the nearest BEP to the normal flowrate to avoid special designs and high costs associated with custom designs for an exact BEP to normal flowrate match.


**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

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[cvg]

depending on the application, there is not one "normal" flow. there is a range of flows that is considered to be "normal". This range may be narrow or wide. Hopefully the designer picked a pump that had a BEP which falls somewhere in the range of normal flows.

 

[roteng518]

API-610 10ED/sect 5.1.15 indicates that the pump BEP should be preferably between rated point and normal point. This implies an ideal scenario that the efficiency at normal point is not far away from the rated point on the H/Q performance curve. However the pump vendors are not always doing the way API recommended. Their designed rated point may be left or right of the BEP and the normal point is always a little bit far left of the BEP. However, if in the field the operating points(normal point) are always staying in the vendor recommended operating region, the pump will be OK with not too much effiency loss, excessive vibration/noise and other problems.

 

[roteng518]

cvg,

you are correct. The normal/rated flow is from the process engineer during the design stage for size the pump and other purpose only. Pump in the field is actually running within a range. This range shall be in the preferred operating region that pump manufacture recommended on the data sheet.

 

[Artisi]

"why don't design the pump at normal flow?"
Mainly due to the fact that the normal flow is not known until such times as the system comes online.
That is why you have design or rated flow as a staring point for all the friction loss calculations etc to be based on as well as the initial pump selection.

The problem of normal flow being a lot less than the rated or design flow usually comes from all the fudge factors built in by the various people involved, ie the process guy wants "x" flow plus a little just in case, the boss adds a bit just in case, the pipeline guys adds a factor just in case, the pump guy selects the pump output a little higher just in case. This results in the pump being oversized for the true requirement.

 

[cvg]

even when it comes on line, there still may be a range of flows. that's why you design with a range not a single point. high flow, low flow, high head, low head combine to give you a range of flows and heads that the pump should be able to handle. And when specifying a pump, I usually give 3 points on the design operation curve that need to be met by the supplied pump. As well as that, specify minimum efficiency for each point on the curve. The most likely operating point is the middle one and it should have the highest efficiency.

 

[Artisi]

You would normaly select the pump to be most efficient at the point where you would expect it to run the majority of time in operation.

 

[dabluffrat]

Centrifugal Pumps operate against a System Head Curve. Depending on how long it takes your system to come online, the pump's operating flow will vary. Pumps are designed to run at BEP flow and full diameter head, and a good pump selection is made when the Rated head/flow your system requires corresponds to the pump's BEP design head/flow, and typically within 10% of full diameter impeller trim.

Rated flow always exceeds Normal flow, and both should always be to the left of BEP flow for a good pump selection. As wear clearances open in the pump, BEP will shift to the left and drop slightly. Net efficiency will remain the same if you're to the left of BEP.

There's a distinction in API-610 as to the definition of Rated and Normal operating points (Sec 3.31). Normal operating point is the condition the pump is expected to operate under normal process conditions. Rated (3.45) is the point at which the pump vendor certifies that pump performance is going to be within the tolerances specified.

Did you know that 76.4% of all statistics are made up...

 

[cvg]

again, I think we are comparing apples to bananas. My experience has been with large vertical turbines pumping into municipal water systems. Potentially with one, two or even three pumps running at any one time into a discharge header. System head varies from day to day and minute to minute. Depends on amount of water demand, location of the demand, status of well pumps water level in the suction tank and water levels in the tanks in the pressure zone. Sometimes additional transmission mains are constructed. Sometimes valves may be left partially open or closed. Sometimes PRV's are feeding other zones. Pump selection has to accomodate a very large range of possible flows and pressures. These are not process pumps where the operating point is easily calculated and the pump is designed to run at the BEP.

 

[BigInch]

Specifying several points for one pump doesn't appear to be the best method for reaching an optimum pump configuration for a multiple flowrate system, if those required flowrates are over a wide range. If you had a such a wide range of flows, your method would require a pump with a broad relatively flat curve through all the ranges which presumedly would also need rather high efficiency lines for all flowrates within the range. I think it would result in pumps with maximum capacities well and way above what you would need for your normal flows and that would still have rather poor efficiencies at your min and max flowrates, no matter what you specified as the minimum and maximum efficiency at those points. You'd more than likely just get vendors taking exception to your efficiency specs. Likewise, specifying 3 points for a one-pump design where flowrates fall within the preferred operating ranges of 80-110% of BEP would not typically be necessary, since not all that much variation is found inside that preferred range and specifying 3 common flowrates where some fall outside of the preferred range could result in overly high power costs for those flowrates. So while specing many wide ranging operating points for a single pump may solve the pump selection problem, it doesn't appear like it would result in a good pump configuration for a system that considers lifetime costs. Pumps will of course operate out of that 80-110% range, but not without increased maintenance expense and energy cost penalties. If operation at flowrates outside the preferred range are not possible with one pump operating within the preferred range, a multiple (N) number of pumps should be chosen such that the most commonly required system flowrates fall within the preferred ranges, ie. number of pumps N * (BEP-20% to BEP+10%) should include as many of those flowrates as possible, perhaps with one or more pumps operated with VSDs to catch the odd flowrate or two that fall outside the preferred N * 80-110% BEP ranges. That design principle would allow efficient operation at many different flowrates and, since energy consumption is the single largest expense for a pumped system frequently several orders of magnitude above capital cost, maintaining high efficiency with an appropriate configuration is quite important.

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[roteng518]

Good discussions!

Dabluffrat, you mentioned that "Rated flow always exceeds Normal flow, and both should always be to the left of BEP flow for a good pump selection". But API-610 5.1.15 says BEP point should be preferably between the rated point and normal point. Can you explain why you think both rated and normal point should always be the left of BEP?

Thanks.

 

[sheiko]

Rated flow = design flow = normal flow + margin in order to allow for wear, flow variance, and calculation errors.

Rated flow is the actual capacity realized by the actual, supplied centrifugal pump when running at the design speed and TDH.

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.
The manufactuer selects the closest model pump that fits the conditions.

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

 

[TenPenny]

What I find interesting is process engineers who will specify a 'design' flow and head, and a 'normal' flow and head (both lower figures), while specifying a fixed speed centrifugal pump. Which means that the pump picked for the design conditions cannot actually operate at the normal condition.