System Curve of a Throttled Valve 2

[ericdayo26]

Hi there fellow engineers,

I have a question with regards to a system with a throttled valve.
First let me explain the system,

A centrifugal pump supplies stock (3.5% consistency) to the top of Chest B, from Chest A (both chest are open to the atmosphere).
The Levels on both chest are maintained on a certain level, so we can assume that NPSHa does not really change.
The Pump's nameplate reads; 36.6m, 209m3/hr, 1780rpm. The motor is rated 50Hp, 575volts.
A control valve, 41% open, restricts the flow to chest B, so that it is maintained at 190m3/hr during stable operations, but is sometimes fully opened to during certain situations when Chest A's level are near maximum. The flow when the Control Valve is fully open is 270m3/hr.
I have computed for the Friction head of the piping with the Elevation head, and got 31 meters, which is very near when plotted against the Pumps curve, so i guess my system curve is correct. I have attached my plotted curve.
The pump was evaluated because we saw potential power savings on the pump, because it is normally open at 41% only. And also the pump is an old model, so the boss wanted to replace it with a new one that will have available spare parts.
But as I evaluate the system, i think that the pump is not really wasting energy on the throttled valve, because it is still operating near BEP at a 41% open valve. Perhaps it was designed that way to adjust with varying flow requirement. Plus the fact that it not really a giving problems to the maintenance people.
Anyway, what i dont get is how the system curve works? Because normally, i would suggest an option to replace the pump with a smaller one that is rated at 190m3/hr and 32 meters of head. But based on what i've read on articles about the system curve, it is suggesting that I only need 20 meters of head. Am I right with 190m3/hr x 32 meters? or do i only need 20m?
I'm also suggesting to replace the pump with a variable speed drive, which i think is the best option for this system.
Can anyone confirm help me out? thanks

 

[rotw]

You could think to reduce the capacity of the pump, i.e shifting the BEP to the left, so indeed you will run your most onerous duty even closer to the BEP so more efficient and marginal service (270 m3/h) will be at lower performance, power savings will not be huge anyway.
However if you look to your BEP x 1.1 (staying conservative as some would go 20% to the right off design) you get about 264 m3/h flow.

If this pump has been sized ignoring which duty is the most onerous (throttle valve 41% open and full open), you could reduce a bit the design capacity of the pump provided you will have enough room to the right of the curve.

It looks to me the system (assuming throttling solution is maintained) is already pretty well designed because the marginal service is already reasonably at the right of the curve. I would not squeeze the pump further apart if these minors gains (opex and pump cost) are worth to you.