Rated Duty & Nominal Duty

From rainman

Member of Forum at www.lightmypump.com

March 14th, 2008,

Pump Rated Conditions

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For centrifugal pumps, what do the term “rated” parameters (flow, NPSHr, etc.) mean? How do the pump manufacturers determine these values? Are these values specified by the customer or the pump manufacturer? My questions arise because I wonder if my rated flow for any particular pump should change when the normal flow for this pump is changed, as a result of a new condition being implemented in the plant where it is used (say a revamp). To illustrate my point further, I have a pump originally designed to handle a normal flow of 50 m3/h and for which a “rated” flow of 56 m3/h, was defined, according to the information provided in the pumps data sheets for the original project. After some years, a revamp of the plant is under way, which requires a new normal flow of 53 m3/h. Under these circumstances, will I have a “new” rated flow also, if the pump and the hydraulic (curve) system remain the same? What if I change my system (by increasing, for instance, some pipe diameters)?

Thank you all in advance for your feedback,

Rainman

From S. L. Abhyankar

Senior Member

March 17th, 2008,

Dear "Rainman",

By my understanding of the terms "Rated Duty" and "Normal Duty", Rated Duty would be the duty on the nameplate of the pump, printed by the manufacturer, declaring the pump's compliance with the duty mentioned in the order from the buyer. "Nominal Duty" would either be

i) the duty at the point of best efficiency (BEP) or

ii) If it is a pump supplied as per a standard, such as ISO-2858, there are "Nominal" duties or ratings specified in the standard

Coming to your point about changes that may happen in the performance of the pump, or change needed in the performance of a pump during revamp, the duty obtainable from a pump is the point of intersection of the system curve with the pump's curve. For example, if you change the system, using larger pipe sizes, as mentioned by you, the system curve would become one with less friction. So, one would get more discharge from the pump. Whereas less friction and higher flow would mean better performance of the pump, at higher flow the driver may get overloaded. So, one needs to also check the power requirement of the pump at the changed duty.

I hope, I have touched most of the points raised by you.

S. L. Abhyankar