Pump Affinity Laws for Efficiency

[Kadongo]

Hello Guys,

Does the Efficeincy of the pump has an affinity law to be calculated by in case of speed change? I have a KSB pump that produce a 20000 m3/day with a VFD, I am estimating speed change effect in power consumption and I need to know if there are a law such as affinity laws to know the efficiency.

 

[pierreick]

Hi,
No!
Underneath an equation to get the efficiency of a centrifugal pump taken from GPSA Data book 8th edition /carl Branan the process engineer's pocket handbook .
Range of application
F =50-300 ft
G=100 -1000 GPM

Eff= 80-0.2855*F+3.78E-04 *F*G-2.38E-07*F*G^2+5.39E-04*F^2-6.39E-07*F^2*G+4E-10*F^2*G^2
Eff= pump percentage efficiency
F= developed head ft
G= flow GPM

Good luck
Pierre

 

[FluidPowerUser]

Centrifugal pumps have a BEP (best efficiency point). They are designed to run at set speed to get the best efficiency at a particular speed.

It is possible to run centrifugal pumps at higher or lower speeds, but depending on the impeller and volute design, the efficiency will drop off rapidly.

Fans have affinity laws, and likewise, as turbo devices, the impeller geometry is scalable too. Albeit to somewhat limited level as there are features on the impeller that are critical to the performance. Also, the volute that collects the fluid and converts kinetic energy to head, will be designed for a particular volumetric flow rate.

One aspect of a centrifugal pump is the specific speed. The specific speed equation includes the fluid density, the head, the NPSHA, the speed and the shape of the impeller.

If you get any pump curve and look at the efficiency curve, it will tell you how much the efficiency changes with speed.

 

[Artisi]

As stated by others, there is no affinity rule covering efficiency for reduced /increased speed, your best bet is to contact KSB as they will have a good feel for likely efficiency at another speed, you will need to give them full operating conditions, ie, the anticipated Q / H and speed you have selected etc - from his they can probably give you a considered opinion and likely eff.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[LittleInch]

What sort of speed change range are you looking at?

Power is prop to speed^3 so evena 10% reduction is a 73% reduction in power. Efficiency won't change hugely, but remember the BEP point goes left as you reduce speed.

How accurate do you need to be?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Eff at all flow rates on other RPM curves roughly scales with the leftward movement of the BEP.
It's not entirely accurate, but it should get past the sniff test.


--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[EdStainless]

Measure the actual power of the motor.
With modern monitoring devices it isn't difficult.
Real power tells you a lot about how the system is operating.
In many cases with a VFD you can find that slowing the pump does not change the total system power consumption.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[SB1976]

Yes, it is not direct but rather indirectly related.

1. As part of Pump Affinity Laws, we have one for Power as well.

P1/P2= n1/n2)[sup]3[/sup](d1/d2)[sup]5[/sup]

Hence, if your ultimate aim is to get to know how much power power is consuming with change in impeller diameter / speed, you can apply above formula to know this.

2. Also, the locus of the efficiency at BEP for all the different impellers at same speed is a straight line parallel to the efficiency scale.

Trust above helps.

 

[Artisi]

This is a typical speed change curve it will give you some idea of what to expect.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[1503-44]

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Kadongo]

Many thanks for all of your replies

 

[moideen]

the system's wire-to-water efficiency can be calculated using this formula and can be easily monitored, especially when the speed varies, with an automation system. Install a DP sensor across the header of the pumping system to measure the differential head, along with a flow meter and a PMU (power input to the pumping system)
WWE(%)=Q(gal/min)*H(ft)/53.08(factor)*input kw
Ex: flow rate=1850 gpm, input kw=22, head 40fthd, effciency1850*40/53.08*22=63%


The problem with the world is that intelligent people are full of doubts, while stupid ones are full of confidence.
-Charles Bukowski-

 

[georgeverghese]

KSB pump running at 3600gpm at what head? What type pump - high capacity axial flow low head ? Got a model number ?
What speed values S1 and S2 are you looking at ?