Because the second affinity law says I can only slow the RPM of this pump by 4% and still produce the head required, the third affinity law says that 8.1 HP should be the lowest power possible at any flow rate. However, I plugged this minimum speed into the curve and can see that the actual power required will drop even lower than the third affinity law states, as flow continues to decrease.
W/o checking the numbers, I'll try to answer as much as I can.
8.1 HP I think is at your min speed to still deliver flow, so anything under that is wasted energy in any case. You should turn the pump/VFD off.
I have read several places where the affinity law assumes two points of a curve that have the same efficiency. How can this be if the efficiency is different at different flow rates? Does that mean that the affinity law only works when reducing the head, not the flow? Otherwise why does the affinity law calculations not match the horse power from the curve?
Reducing speed from 100% to a lower speed is considered to carry the same efficiency at 100% speed down along with it. Thus the two points referred to are any two points along that curve.
If you can say that using a VSD to reduce the horse power from 9.2 to 6.1 is saving energy, then using a valve to reduce the horse power from 9.2 to 6.5 is also saving energy. Then if you take into account that at full speed the losses from a VSD adds 5% to the energy consumption, and a fully open valve does not, it seems there is little if any difference between using a valve and a VSD throughout the entire flow range. So I guess it is also a myth that valves burn energy?
Yes. But the supposition is that in that situation the VSD would (maybe) still save a TINY little bit more than using a valve. There is no doubt that "Valves burn energy", it isn't a "myth". VFDs also use 5% or so.
I was under the impression, as BigInch stated, that using a control valve would cause the pump to run at 100% BEP and 100% head inside the pump. However calculations and the curves show that using a control valve reduces the power required almost exactly the same as using a VSD to vary the RPM.
Yes, the pump brings up the flowrate carried in the impeller to full internal discharge pressure, but since flow cannot exit due to a valve restriction, it is recirculated and the efficiency drops according to what is shown on the pump chart.
In many situations the amount of energy saved by using valve or VFD is very small and essentially immaterial. In those cases I believe a valve is far superior due to the lesser complexity of the system required to do so. In other cases it is significant and a proper choice should be made.
The only thing I can't see from the curve is the loss of motor efficiency at partial load. Is this efficiency the same for a motor that is slowed down to partial load, as compared to a motor that is still running at full RPM and just drawing a partial load? If not, then maybe this is where my problem lies. Otherwise I would say that replacing a valve with a VSD to save energy is another "myth-application".
The loss of efficiency of the motor will never appear in the pump curve. That curve is reserved for hydraulic efficiency and shaft power to the pump only. You must adjust the power rating requirement for all additional electrical equipment yourself using the tables I provided for the motor and VFD above, whether that is at full or partial loads too.
Still nothing produces as many gallons per horse power as letting the pump run at maximum flow and filling a big tank. Second to this a two pump system is still more efficient than using a VSD on a single pump. Now I also believe that using a control valve on one pump, or to round the edges between a two pump system, may be just as good as using a VSD.
With two pumps operating within recommended ranges it may not be possible to reach all flowrates between operating scenarios of running one pump and two pumps, if your flow range is high. For example, lets say your normal allowable range is 50% to 110% for each pump. With a BEP for each pump of 100 gpm your ranges would be 50-110 and 100-220, but operating at 130 gpm would be a poor choice as that would involve running two pumps at 65 gpm each, both with poor efficiencies. A VFD and 1x200gpm pump might be a far superior choice as long as you could still make the required head at lower flows. It might be tough going down to 50 gpm. Not a MYTH. IT DEPENDS.
I wonder how many more "myths have morphed into legends to be considered facts by the un-initiated"?
As you have found out, making a proper evaluation of these types of problems is often not a simple task for experienced engineers.
Only the uneducated perpetuate and believe in myths.
I also wonder how much energy the world is wasting by falling for these myths?
Conservation and proper application is the best method to save energy.
I wish I could go to Sydney to hear you Stanier. Is there anyway you could put your paper here after you have presented it?
I'll let Stanier handle that one.
Knowledge is power^3
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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that
99% for pipeline companies)
