How steep should a pump curve be for vsd

[davefao]

How steep should the curve of a centrifugal pump be for smooth control by a vsd (for different types of system curves )? In our installation we have pumps that are equipped with a restriction orifice to obtain a steep curve , they are on discharge pressure control and they are also operating in parallel (makes the combined curve even flatter). One pump is driven by an electric motor and the other one by a steam turbine ; I want to look if there is not a possibility to make the hole in the orifice a little bit larger since there is a problem with the power of the steam turbine (several reasons). At this moment I still have to investigate how the the system curve looks like .

 

[d23]

davefao,

You have asked a good question, but a little more information would help.

1) How critical is the system pressure?
2) Can you maintain a steady state operation with the turbine and control pressure with the VSD?
3) How large of a speed range does your system (VSD) require to maintain the pressure?

In most cases the larger you make the time between speed (frequency) change the more stable the operation will be.

In the event you require an extremely flat, quick changing discharge pressure; have you considered a pressure regulator that will recalculate or bypass some fluid? This may offer a quick response that will maintain discharge pressure until the VSD can respond.

D23

 

[ccfowler]

davefao,

The steepness of the pump curve is not necessarily much of an issue, but it is usually important that the head vs. flow curves rise continuously to shut-off (zero flow). This is significant for stable operation and start-up.

With a system such as you describe, I would want to have good tachometers on both pump shafts to be sure that they are both operating at suitable speeds. (If the pumps are identical, then the pump shaft speeds usually should be kept identical.) It is far too easy for one pump to hog the load if the shaft speeds do not match very closely. The turbine's governor speed droop characteristics vs. the motor slip characteristics can lead to significant differences in shaft speeds between the two pumps.

I would carefully review the system head vs. flow characteristics and the pump curves. I would want to eliminate the orifices if at all possible. Any lack of symmetry in the pump-specifc piping should be carefully considered for the effects that would tend to cause unequal flow through the pumps.

To this point, I've been assuming two identical pumps. If the pumps are not identical, then I would want to include flow rate measurement for each pump in the control system to assure that both pumps remain suitably loaded at all times.

You have an interesting problem with the possibility of introducing significant energy and maintenance savings. Good luck!