Pump operating outside the characteristic curve 3

[whycurious]

We are looking at a situation where the pump will be selected for higher head, ~9-10 bar but at times it is required to operate at lower head ~2-3 bar. This is well outside the region of pump curve. What could be the potential problems with this design and operation. Are there any guidelines for extrapolating the charateristic curve.

 

[Artisi]

Initial advice is to get yourself a good pump book and do some research on pumping systems and pump applications.
Nothing like some self-education if for no other reason than to understand the likely replies to this posting.

As for your question, it is too brief and lacking any real information to give any specific advice as much will depend on how the system is to be configured.

Additional information might result in meaningful advice from the guys in this forum.

 

[BigInch]

Centrifugal pump characteristic curves are a plot of two variables, head and flow. You have only given the head requirements at (supposedly) two points of the curve. If you plan to or can operate at the two flowrates corresponding to the two head values, you will be, "on the curve".

The problems come when you plan to operate at only one flowrate for each of the given discharge heads. You could experience differential pressures greatly different from those at which the pump was designed to operate, which besides having a very low operating efficiency, would no doubt drastically unbalance the usual loads on the impeller to create maintenance headaches. (search this forumn for "operating outside the curve"). A VSD might lower the energy costs associated with the poor efficiencies, but will not solve the unbalanced pressure issues.

You may do well to investigate the possibility of operating methods that are more evenly balanced, by pumping at higher heads and flows for shorter times, then turning the pump off, or pumping at lower heads and/or flows for longer periods and filling an elevated tank for times when high heads or flows are needed. Short of those, investigate the possibility of using two pumps.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[JJPellin]

This does not sound like a pump problem as much as a control valve problem. If you plan to operate with a much lower system pressure but at the same flow rate, then the pump will not be affected. By definition, the pump can only operate on its curve. If you need a lower pressure, you will have to take the pressure drop across a control valve. An excellent alternative is mentioned by BigInch. If you need comparable flow (you did not specify this), but lower pressure, simply slowing the driver down with a VFD could be the answer. But since you offered so little information, all of my reply is based on speculation. A few facts would give us a better opportunity to give you more useful replies.

 

[Sterilecuckoo58]

Don't extrapolate a curve... there's a reason the pump manufacturers stop drawing the curve where they do. If you have to pump at similar flows under both conditions, then install two different pumps - yes, thats means you spend nearly twice as much right off the bat, but if you don't you'll be servicing and replacing the one-size for all conditions pump frequently....

I don't think use of VS pumping will allow similar flow capacities at different head except to the extent that you don't run off the right end of the curve to begin with, so if you are not on the curve at design speed, then no speed adjustment will move you to the left on the new curve.

If your low head pumping needs are occasional, and energy costs are calculated to be inexpensive enough, then induce head on the low head side so you get back on the curve... throttle a valve (which will likely cause cavitation at the valve and slowly disintegrate the valve, but thats cheaper than a pump) or reduce pipe diameter (how much and how far is a calc you can do), or add discharge restrictions....

Have fun.

 

[JJPellin]

I am sorry. I missed a key word in the original post; extrapolate. If you truly intend to extrapolate beyond the end of the curve then the other replies say it well. This would be a bad idea. A larger pump, two different size pumps for the different conditions or even (I hate to say it) parallel operation of two identical pumps at the higher rate would be preferable. Operation at excessively high flow, beyond the end of the curve can result in a number of problems such as cavitation, shaft deflection, reverse thrust, etc.

 

[BigInch]

Looking at a flow histogram (required flowrate over time) in combination with the piping system curve will make it possible to decide what configuration of pumps are best suited to your application. The best configuration will yield the lowest energy cost, while still permitting flow at any required rate. As such you should not hate to say something like, "parallel operation of two identical pumps", since if half flow was required for 50% of the time and full flow for the other 50% of the time, that pump configuration could well be perfect. And, that configuration IMO would generally be much prefered over two different parallel pumps, not to say that two different pumps might sometimes actually be the most efficient combination, but that's usually not the best configuration for operators, spare parts and maintenance.

Pump curves can be extrapolated within reasonable accuracy out to the 0 flow point. That is not to say that it is a good idea to operate at any point on the curve you are able to plot on a piece of graph paper. Operation near BEP is the key and, since BEP represents the optimum design point for all pump design variables, operating away from BEP for extended time periods should not be taken lightly.

On the other hane, pump curves should not be "affinitized" (adjusted for rpm variations, change in impeller diameter, etc.) by anything reaching a difference of more than 10 to a maximum of 20% from the original curve.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[BigInch]

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[lilliput1]

Check if using variable frequency drive to index pump at set lower speed will work. Motor would have to be inverter duty type though. Check also if putting a recirculating bypass piping & valve would help put the pump back in the curve.