End of Curves
End of Curves
(OP)
It is being seen as a trend in the Middle East Oil & Gas companies specification that the drivers shall be designed for the ënd of curve operation. Also I understand that it is common practice to size the driver for end of curve also pumps operating in parallel.
What is the definition of end of curve and how is it achieved in practice? I am sure it is not 120% BEP as defined by the max. allowable flow in API 610 . During a test I witnessed I observed that the OEM described the end of curve even with the discharge valve not fully open which for me is not really the "ënd of curve"
By the way, if the driver is oversized, is there any way the pump operation is limited other than high vibration concerns?
Kindly share your experience
What is the definition of end of curve and how is it achieved in practice? I am sure it is not 120% BEP as defined by the max. allowable flow in API 610 . During a test I witnessed I observed that the OEM described the end of curve even with the discharge valve not fully open which for me is not really the "ënd of curve"
By the way, if the driver is oversized, is there any way the pump operation is limited other than high vibration concerns?
Kindly share your experience





RE: End of Curves
Specifically, where the the pump's head is degraded by 3% (may be 1% now) due to insufficient NPSHA is the end of the curve. Note this will change with the NPSH available, so when defining end of curve, it is really application specific. There is also nothing wrong with a spec that asks for a driver coverage up to 120%BEP as long as it is clear that pump must have sufficient NPSH.
In general, the manufacturer's head capacity curve ends on the right side where the pump started to cavitate during original pump performance test. NPSH will typically be the limiting factor on the right side of the curve.
RE: End of Curves
For vertical pumps, most will be tested in an open sump, even if the final configuration is in a barrel. NPSHA is head due to elevation (32ft at sea level) plus the submergence, so not very likely to limit the flowrate.
Horizontal pumps being tested for performance, test loop capability should be able to provide ample NPSH for "end of curve" operation.
You can argue all you want, but "end of curve" is either a defined flowrate (120% of BEP, or the flowrate with no NPSH margin) or in the absence of an agreed upon flowrate, it is where the line shown on the submitted proposal curve ends.
Some ways that pump flow rate can or should be limited: NPSH, vibration, thrust, fluid velocity, noise, common sense, "recommended operating region" and "allowable operating region"
RE: End of Curves
Johnny Pellin
RE: End of Curves
1. can the pump achieve end of curve operation in any abnormal operation, a pump with high static head may not be able to achieve end of curve.
2. What is the pump / system doing at end of curve, NPSH may well limit its operation.
3. selecting a non over-loading motor may well put you into a vastly over sized motor, needing bigger cables and start gear,heavier base plate, bigger coupling, more space etc
The motor should be selected for normal operation plus some margin for upsets, in many cases the power requiremnents might fall between 2 motor sizes with the larger motor having sufficient margin - any increased power requirement due to abnormal operation could be catered for by overload protection in the control panel.
Oversizing the motor doesn't cure any hydraulic operating problems.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: End of Curves
DubMac, I checked the NPSHr curve for the pump. Even at very high flow rates the NPSH difference was very high. So I don't think NPSH was the problem
1gibson, the estimated curve was only upto 630 m3/hr but the OEM tested it at 810 m3/hr. You are very right in your opinion but I cannot interpret why was that test point so very different from predicted performance curve.
By the way, is there any way to know when the stonewall starts fior pumps. This phenomenon is known to me only for cent. or axial compressors and not for pumps.
Artisi, I can mention one hydraulic case where the end of curve is probable i.e. when thwo pumps are operating in parallel and one of them trips. In this scenario I believe we have to size the driver considering the end of curve criteria.
RE: End of Curves
IMO its a pretty mindless practice with little thought put into the process. I've been asked to revise curves to reduce the end of curve point so as to limit motor power.
If you want to size for "end of curve" then that should be defined as 120% of rated or 120% of BEP whichever is greater.
For most pumps in API 610 applications you should not be operating past 120% of BEP from a vibration and hydraulic thrust standpoint.
As to the "stonewall" point, that depends greatly on the specific speed of the pump and the impeller design. What will happen is either the flow incidence angle will cause pressure side vane cavitation in the impeller, or the volute/diffuser throat will cavitate (so called discharge side cavitation). In both cases this will choke the flow.
A low specific speed could choke at 110% of BEP, a high specific speed pump with impeller vane incidence angles set for an excessively high flow, could run out to 150% of BEP.
RE: End of Curves
RE: End of Curves
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: End of Curves
This is a common need within our refinery. Perhaps an example might help. In most of our services, the flow is controlled by a control valve attempting to achieve set-point. For one of our crude units, we have three large crude booster pumps running in parallel. If one of them were to trip off, the other two would take up the load as the control valve opened to hold the total flow constant. If the motors on the pumps were not sized for end of curve, then the two remaining pumps would overload and trip off and all crude flow to the unit would be lost. This would result in a disastrous unit shut down costing many millions of dollars.
In my example, there would also be another economic driver to size the motors for end of curve. When one of the three pumps is down for maintenance, we would want to maximize the flow from the remaining two pumps. Even if this put the pumps at a flow rate that was not within the preferred operating range, we might decide to operate there for a few hours in order to capture a high unit incentive.
In the end, most of the comments above are correct. In some services, there is a very great need to size the motor for end of curve. In other services, there might be no need at all. Each service should be analyzed. But, for our plant, the default answer is to size the driver for end of curve operation. We can decide to deviate from that based on the results of our analysis. But, in case a project slipped through without being fully analyzed, we would prefer to have the driver slightly oversized rather than slightly undersized.
Johnny Pellin
RE: End of Curves
RE: End of Curves
I'd refer to that as a "proposal curve" which is slightly more accurate description than "predicted performance curve" in the following regard: what you were quoted and you purchased is a pump with an "end of curve" at 630 m^3/hr. If you operate beyond that flow, the pump warranty might not cover anything that could *potentially* be caused by that operating scenario. Meaning, just about any pump problem you can think of.
Out of curiosity, what % of BEP is the 630 m^3/hr? And 810 m^3/hr?
RE: End of Curves
The below statement from bradshsi should give you the answer.
RE: End of Curves
Fluid friction backs the performance up the curve.
The motor is sized at end-of-curve to avoid overload when pipe is empty.
The practice also provides a motor that matches the entire operating range of the pump.
RE: End of Curves
We know it happens particularly when operators make trade offs between pump longevity vs maximizing production.
The question though is what is the "end of curve". It certainly isn't the exact point the manufacturer tested to. Since end of curve will often be limited by NPSHa and since that NPSHa (at higher than rated flowrates), is seldom provided when the pump is specified, the EPC is never really sure where that is.
The EPC fallback of course is to size the motor for whatever the end of the published curve shows. However as discussed above this is not where the pump could potentially runout to... This does not seem a satisfactory way to size motors since your motor could still be undersized.
Only proper system analysis can determine true end of curve and in my experience it is seldom performed. More often the operators use the smoke method. That is they run the pumpset until something starts smoking. That's your EOC :)
Sizing your motor to fill empty discharge pipes is bad practice IMO since this condition is seldom present and can be better regulated by using a discharge valve.
RE: End of Curves
Even though a pump may runout beyond the end of published curve, the manufacturer is in no way bound to any extrapolations of the curve by varying operating points. There are a lot of commercial considerations to opening yourself up to that kind of vague wording.
I'm not sure if the same is true today, but I was trained at Worthington years ago by I. Karassik and I do remember one of the immutable rules for pump manufacturers was that we only will guarantee ONE rated point for each pump sold. Although the pump may operate all over the place on the curve, ONLY ONE POINT was guaranteed for pump performance. Any "full coverage" driver language should be tied to that ONE rated operating point rather than the vague "end of curve" terminology.
You can really open yourself up to all sorts of liability claims if you allow such open-ended language as the "end of curve" only approach. As stated earlier, if there is not sufficient NPSHA to operate at end of the published curve, what good is it to provide driver coverage there??
API 610 uses a very sensible approach that requires drivers to handle X% of Rated design capacity with X varying according to driver HP (this is a bullet point and needs clarification by specifier).
O&G end-users specs vary all over the place but have generally accepted the "% over rated capacity" approach to size driver along with other variants thrown in from time to time. Another approach used in specs is to size the baseplate to accept the largest driver possible even if that size motor is not supplied. It sure saves money not to have to dig up foundation and replace baseplate if larger driver is req'd later.
At the end of the day, you will provide driver for whatever the customer determines is "end of curve" coverage; but contract should be carefully worded to protect the supplier from unintended liabilities from "rogue" operation.
RE: End of Curves
RE: End of Curves
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RE: End of Curves
Perhaps now engineers will specify the pump for the duty without rules of thumb being applied by using more science. We have the tools to correctly size pumps.
What is the point of blindly specifying a motor for a duty point that cannot be achieved? Or a VFD when a control valve would be far more efficient in a system?
Oversize motors have higher embodied energy and are less efficient.
"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.
http://waterhammer.hopout.com.au/
RE: End of Curves
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: End of Curves
RE: End of Curves
RE: End of Curves
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: End of Curves
Anyway I get a good feed redoing the engineering and pointing out how to reduce the carbon footprint. Who would have thought an old red neck like myself would start using greenie speak? Now the green movement have given me and my bank balance a new lease of life! Revenge is sweet.
"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.
http://waterhammer.hopout.com.au/
RE: End of Curves
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: End of Curves
If every pump was only ever going to operate at one point, the world would be a simple place.
RE: End of Curves
RE: End of Curves
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: End of Curves
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: End of Curves
1.) BEP flowrate somewhere between avg and max, tank probably needed
2.) On/Off Operations, tank probably needed
3.) n Multiple pumps sized at BEP/n, most common ranges maybe no tanks req'd.
4.) Multiple pumps with BEPs for each different flowrate, probably no tanks req'd.
5.) A combination of the above.
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: End of Curves
If the future is going to hold even more operating conditions check that you can fit a larger/smaller impeller add additional of different pumps.
Whole of life costs involve energy and maintenance costs alongwith prime and future Capex.
"In physical science the first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of Science, whatever the matter may be."
Lord Kelvin [PLA, 1883-05-03]
"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.
http://waterhammer.hopout.com.au/