Ugh!! Water Well contractors...

[npalen]

It depends on the particular pump and where you look on the curve. At low flow most pumps are 7% tp 10% different between variable speed and a simple valve. At high flow the control valve is usually 3% to 5% better. Overall the difference is probably only "a couple percent". I was using a 10HP just to make the numbers easy. It would certainly not be much difference with a 1HP pump.

I was referring to this portion of the graph where the bph for 6.5 GPM flow is nearly double that of the VFD. (approx 3.5 bph vs 2.0 bph)
Maybe I'm reading incorrectly.

 

[Valveman]

I was referring to this portion of the graph where the bph for 6.5 GPM flow is nearly double that of the VFD. (approx 3.5 bph vs 2.0 bph)
Maybe I'm reading incorrectly.

It's a 10HP motor. So, 1.96HP is 19.6% of max power. 3.25HP is 32.5% of max power. That would be 12.9% difference between valve control or VFD control on this pump at this flow rate. The VFD also has some losses that are not shown on the pump curve.

Motors have lower efficiency at reduced speeds. There are charts for this.

Motors are also less efficient running on the non-sinusoidal power from a VFD.

The VFD itself uses about 5% power. But they say it as a VFD is 95% efficient. Lol!

Add these things back into the equation and the differences are almost zero.

The losses from a VFD also add about 5% energy cost at full speed and full flow.

When running at max flow, controlled by a simple contactor, powered by smooth sinusoidal electricity straight from the grid, a fully open pump control valve can be 5% more efficient than a VFD.

 

[Egon]

You have to check the pump curve. This curve shows a pump drawing 10 HP load and producing 100 GPM. But when you slow the pump down with a VFD until it only requires 2HP energy, it is only pumping 8 GPM. The pump is much less efficient when the speed is reduced.

However, you should notice this same pump drops from 10HP to 3.25HP by simply restricting the flow with a valve like the CSV. The point being that a VFD is only a couple percent more than a control valve, and the control valve will cost much less and make the motor last longer instead of shorter like a VFD.

I think that I did not provide sufficient information. I was thinking of a constant rpm centrifugal pump with variable valve control.

 

[ponytug]

I was referring to this portion of the graph where the bph for 6.5 GPM flow is nearly double that of the VFD. (approx 3.5 bph vs 2.0 bph)
Maybe I'm reading incorrectly.

No, you have it. You are reading the chart right. About 43% less energy to run the VFD pump.

There are reasons why VFD are used so frequently in industrial use; energy and cost savings.

All the best,

Peter

 

[Valveman]

I think that I did not provide sufficient information. I was thinking of a constant rpm centrifugal pump with variable valve control.

The 3,450 line is showing the constant speed pump with a variable CSV valve. The horsepower just drops so much you would think the pump speed is reduced, but it is not. That is just the way all centrifugal pumps work. But it is counter intuitive so it is hard for people to understand.

 

[Valveman]

No, you have it. You are reading the chart right. About 43% less energy to run the VFD pump.

There are reasons why VFD are used so frequently in industrial use; energy and cost savings.

All the best,

Peter

A VFD can burn 43% less energy than a control valve at minimum speed. But 43% of 3.25HP doesn't add up to much. A VFD can burn 12.9% less energy from this 10HP pump than a control valve, which isn't much different either. A VFD burns 5% more energy than a control valve at full pump speed and flow. A 5% increase added to the 10HP load can add up quickly, making it more expensive to run a VFD controlled pump through it's overall range.

But it is funny that we are even comparing a VFD to a control valve. Everybody knows a valve burns energy, right? So if there is only a few percent difference from a VFD to a control valve, the VFD is "burning" almost as much energy as a control valve, not saving energy.

Looking at the pump curve again, you will see the VFD is using 1.96HP to pump 6.5 GPM from 231' of head. 6.5 GPM from 231' only requires a 1/2HP pump. So comparing the VFD to an efficient way to deliver 6.5 GPM at 231' the VFD is wasting almost 400% energy. It would also only take 1/2HP energy for the 10HP to supply 6.5 GPM at 231' when controlled with a pressure tank instead of a VFD. With a pressure tank the 100 GPM, 10 HP pump would only be on 1/20th of the time, which is only using 1/2HP energy and saving nearly 400% over the VFD.

VFD's are not used on centrifugal pumps in industry because of energy or cost savings. Industry always does what makes them the most money, and costs savings to you is not conducive to making them the most money.

A VFD can only save energy when controlling a compressor or some other positive displacement pump, not a centrifugal pump. Letting you believe a VFD is saving energy on regular pumps is just icing on the cake for the manufacturers. That gets you to purchase the most expensive and shortest lived pump control, from which they make the most money.

Once someone has drank the VFD Kool Aid it is like they are in a cult. They will not listen to any facts. Just because everyone in the industry is doing it, they are adamant they are right. It is called "groupthink" when people are convinced they are right because that is what everyone else thinks, even though there is no proof to back it up. These are the people who just get loud instead of posting any facts on the subject. Good conversation though as most engineers can't make it this far with me.

 

[Egon]

The 3,450 line is showing the constant speed pump with a variable CSV valve. The horsepower just drops so much you would think the pump speed is reduced, but it is not. That is just the way all centrifugal pumps work. But it is counter intuitive so it is hard for people to understand.

Still did not get my statement clarified properly.

Constant RPM ( no on off of the motor ) and flow regulated by only a discharge valve. This is a common arrangement for industrial applications that need flow regulation.

 

[Valveman]

Still did not get my statement clarified properly.

Constant RPM ( no on off of the motor ) and flow regulated by only a discharge valve. This is a common arrangement for industrial applications that need flow regulation.

Sorry maybe I don't understand the question. But the attached curve shows a constant speed 3,450 RPM pump and how the horsepower is affected by a variable control valve. The pump drops from 10HP load to 3.25HP load without ever changing speed.

 

[Doofy]

I have a Grundfos book from 2000 and it doesn't show the MQ. I guess they could have been around for 20 years, as time seems to fly by as I get older. But if you have an MQ that old it is the only one that has lasted that long. Most are short lived. Just search for MQ and you will find countless sad stories from people who have had a lot of problems.

Thanks for the insight Valveman. I suppose I should have a spare pump of some sort on hand just in case the Grundfos craps out.

 

[kenmac]

I would love to have a well with good water.
These little rural water companies charge an arm and leg for their water.
Sad part is, the well drilling is $25.00 per foot with no guarantee of good water.

There are many around me that have wells that were drilled before private company water came to the area, but the water is either Iron, or sulfur. So they are on the private company water system.