Using Pressure Reducing Valve for filter bypass

[edwards1800]

I have a small community water system (approx 8,000 gpd). The water source is from VFD well pumps which maintain 70-80 psi throughout the community. The demand varies greatly, from 5 gpm to (unknown – the max capability of the well pumps is 350 gpm, although domestic peak demand is probably near 100 gpm).

We are adding a treatment system (for taste-not safety) which will have pressure drop proportional to flow. The filter documentation lists the following pressure drops:
7 psid @ 30 gpm
15 psid @ 45 gpm
25 psid @ 60 gpm.

In cases where demand is 60 gpm, I guess the community water pressure would decrease to about 50 psi. If the demand increased above 60 gpm, the pressure would drop further. I don’t think this will happen often, but it should be designed for.

I’d like to limit the amount of pressure drop when demand is greater than 60 gpm by installing a bypass line around the filter and including a pressure reducing valve (PRV) set at 50 psi. I figure that when demand is greater than 50 psi, the PRV will open and allow unfiltered water to be blended with the filter water and meet the demands of the community while keeping the pressure at 50 psi.

Is this the right application for a PRV? Should I use a direct-acting PRV or pilot-actuated PRV?

 

[jonr12]

That would work but if it is not used very often, the PRV might be stuck shut when it is needed. You could put a PRV on the mainline after the filter, and set it for 70 PSI, then set your PID control to 100 PSI. It won't be as efficient at the lower flow rates but, you are already using more energy anytime you vary the pump speed. Might also be able to put the pressure transducer after the filter and not use a valve at all.

 

[edwards1800]

Ok, I guess I didn’t even think of those options. Let me get this right…If I put a PRV after the filters, set the downstream pressure (with the PRV) to 70 psi, and set the water supply pressure to 100 psi, then the pressure drop would still vary across the filters as flow increased. With low flow, the PRV would be reducing pressure from 100 to 70 psi. With 60 gpm, the filters would drop 25 psi, then the PRV would drop pressure from 75 to 70 psi. That has the added benefit of keeping the community at 70 psi instead of varying their pressure by 25 psi.

The PRV keeps 70 psi regardless of what the pressure is upstream (100 or 75 psi), right? Does it matter if it is a direct-acting or pilot-operated PRV?

 

[bimr]

In order to measure and control the flow, you should install flow meters on the combined inlet and also through the filter so that you know what flow split is occuring.

The filter should have a pressure drop of 10 psig normal and 20 psig max.

You need to take a pressure drop on the bypass side. Otherwise, most of the flow would go through the bypass.

So, yes. You need a pilot operated prv or control valve on the bypass in order to control and modulate the flow.

If you install a flow controller, the flow controller will modulate the valve.

 

[jonr12]

I was talking about putting a pilot operated PRV on the main line after the filter, and not using a bypass. I think Edwards1800 is describing it correctly. No need for other controllers.

 

[bimr]

Sorry, I misread the post. I was focusing on the bypass and that does not seem to be the issue. The bypass seems to be more for occasional use than for blending.

Seems like you are wasting pumping energy. Have you investigated the use of a larger filter? The pressure drop at peak flow seems to be on the high side. It may be a less expensive and more reliable system if you went with a larger filter.

Bumping up the pump pressure also seems like an energy waste. You are wasting the pumping energy by using the PRV most of the operating time except for the instances of peak flow. You would be better served if you installed a VFD on the pump than a PRV. The pump may also be operating out of its range of efficiency.

Use of a VFD can also assist in maintaining control of the pressure in your system. The VFD would probably be paid for with the energy savings.

 

[jonr12]

He is apparently not concerned about energy savings, as he already has VFD's on the pumps. Anytime you vary the pump speed, you are using more energy per gallon, not less.

http://www.eng-tips.com/viewthread.cfm?qid=260203

If you want energy efficiency, you would want to pump into a big pressure tank or water tower, so that the pump is always operating at BEP. A VFD does not save energy, and in the cases I have dealt with, VFD actually increased the energy consumption considerably.

 

[edwards1800]

Thanks for your comments; you are both correct. Here are a bit more details:

The system is already in use. The wells are VFD driven. The reason for the VFD control is another subject – I can give you details if you want. They can provide way more water than is required for the community.

The filter system is already in use, but for only a portion of the system. Now, the owner would like to connect the rest of the community to the same filter (without increasing the size of the filter). So, I think the best ideas would be the following:

1. Adding a PRV on the bypass, metering both flows. The water would be “blended” at high flows only.
2. Adding a PRV after the filter and increasing the wells’ set-pressure.

I am leaning toward the first choice, since it minimizes excessive pressure drop across the filters. I am thinking that if I set the PRV to 25 psi less than the VFD, then the flow through the filters would be limited to 60 gpm, and the PRV/bypass would deliver the rest of the demand. With the second option, if the community demand is 100 gpm, then the pressure across the filters would be something greater than 25 psi, which may not be very good.

As far as PRV’s go, I can understand the difference in the operation of direct-acting and pilot-actuated PRV’s – but why would I chose one over the other? It seems like a direct-acting 2” PRV could provide 100gpm. The cost for the direct-acting is much less than a pilot-actuated PRV, so why choose a pilot-actuated PRV? I might not have the terms correct:
2" Direct-Acting PRV example
Pilot-actuated PRV example

 

[jonr12]

Direct acting PRV's are cheaper for a reason. For one thing they have a lot more friction loss than a pilot operated valve. The reduced pressure fall off with a direct acting valve is going to be your main problem. Check the spec sheets for reduced pressure fall off. A pilot operated valve will stay at 50 PSI regardless of the flow. A direct acting valve will give less and less pressure as the flow increases.

 

[edwards1800]

Ok, so say for example I am using the Watts 2" that I link to in my previous post (you can see the spec sheet on that page)...

I have my water source maintaining 75 psi..if I adjust the spring on the top to "set" the PRV to 50 psi...then the downstream pressure will be about 50 psi at low flows through the PRV, but at about 95 gpm, then the pressure after the PRV would be 25 psi, right? Whereas if I used the pilot-actuated PRV, it would not have 25 psi drop when flow throught the valve is 100gpm, right?

Thanks, I am trying to figure out why i should not use the simpler valve. On paper, it is preferred since it is cheaper and smaller (space is a concern here).

 

[bimr]

The pressure drop through either valve will be the same.

However, the direct acting valve is subject to reduced pressure falloff at higher flows. Unlike pilot controlled pressure reducing valves, direct acting valves are subject to "reduced pressure falloff" (RPF). Reduced pressure falloff is the decrease in downstream regulated pressure that occurs when the flow increases. When the demand for flow increases, the valve must open wider and wider to permit the flow. The only way the valve can open is for the spring force to be greater than the hydraulic force under the diaphragm (the force trying to close the valve). The downstream pressure therefore, must "fall off" or decrease before the spring can open the valve. All spring actuated direct acting valves have similar operating characteristics.

http://www.cla-val.com/pdfs/E-990.pdf

The pilot operated valve will tend to function more like a control valve.

At low flows, the direct acting valve will be fully open. I don't understand why jonr12 would say that the flow will be reduced at low pressures. That is incorrect. The problem will occur at high flows.

The pilot operated valve has more operating features than a a direct acting valve.

 

[jonr12]

Again, cheaper and smaller for a reason. Yes the reduced pressure is about 25 PSI at 100 GPM, or 15 PSI at 60 GPM. What they don't show is a friction loss chart. If you add about 25 PSI for friction loss to what the pump can do, to the 25 PSI or 15 PSI pressure fall off from the valve, there won't be much left. A lot of friction loss in direct acting valves. If you are just reducing line pressure, friction loss is not important. When attached to a pump, friction loss is very important. And you are pricing a Cla-Val pilot operated valve against a Watts direct acting. Cla-Val is good but, it is going to be the most expensive. Might look at different brands of pilot operated valves, there are a bunch of them.

 

[bimr]

The "friction loss" (in your words) for a PRV is the difference between the inlet pressure and the set pressure (discharge side pressure).

What you are calling the "friction loss" is simply the pressure reduction across the valve. The pressure reduction will be the same for a pilot actuated valve or a direct operating valve.

You can't list the pressure reduction in a catalog because it is an operating parameter. The catalog just lists operating ranges.

The problem with the direct operating valves is that the direct operating valves can not pass a large flow. The pilot operated valves can open up to a much larger flow orifice than a direct operating valve.

The direct operated valve is not recommended for this application for that reason.

See the link for further information:

However, the direct acting valve is subject to reduced pressure falloff at higher flows. Unlike pilot controlled pressure reducing valves, direct acting valves are subject to "reduced pressure falloff" (RPF). Reduced pressure falloff is the decrease in downstream regulated pressure that occurs when the flow increases. When the demand for flow increases, the valve must open wider and wider to permit the flow. The only way the valve can open is for the spring force to be greater than the hydraulic force under the diaphragm (the force trying to close the valve). The downstream pressure therefore, must "fall off" or decrease before the spring can open the valve. All spring actuated direct acting valves have similar operating characteristics.

http://www.cla-val.com/pdfs/E-990.pdf

 

[jonr12]

"The pressure reduction will be the same for a pilot actuated valve or a direct operating valve."

This is not correct and conflicts with the following quote, which is correct.


"The problem with the direct operating valves is that the direct operating valves can not pass a large flow. The pilot operated valves can open up to a much larger flow orifice than a direct operating valve."

This is why direct acting valves have a lot more friction loss than pilot operated valves.


"I don't understand why jonr12 would say that the flow will be reduced at low pressures. That is incorrect. The problem will occur at high flows." bimr

That is not what I said, I said....
"A direct acting valve will give less and less pressure as the flow increases."

When the pressure is low enough that the valves should be in their fully open position, there is a lot more friction loss across a direct acting valve than a pilot operated valve. Plus you have reduced pressure falloff with the direct acting valve, and not with the pilot operated valve.

 

[bimr]

The pressure reduction will be the essentially be same for a pilot actuated valve or a direct operating valve until the direct operating valve reaches the point where "reduced pressure falloff" (RPF) begins.

If you install a PRV on the main line after the filter (which I do not recommend), the pressure reduction is going to be whatever is set on the PRV (assuming you operate within the flow range of the PRV).

It is somewhat ludicrous to put a PRV on when you already have a VFD that will allow you to vary the pressure. The PRV will have an energy loss associated with the pressure reduction.

All the poster has to do is to move the pressure sensing point for the VFD to the discharge side of the filter.

I disagree with "A VFD does not save energy, and in the cases I have dealt with, VFD actually increased the energy consumption considerably."

It is somewhat simple to understand that if you operate a pump at a lower flow and pressure (which a VFD will do), the energy costs for pumping should be lower.

If you use mechanical device such as a PRV, you have energy loss from such devices.

Cla-Val makes direct acting and pilot type valves. You can run the application past Cla-Val:

http://www.cla-val.com/pdfs/E-990.pdf

 

[jonr12]

I said earlier that all he needed to do was move the pressure transducer after the filter.

If you use an electronic device such as a VFD, you have energy loss from such devices.

"I disagree with "A VFD does not save energy and in the cases I have dealt with, VFD actually increased the energy consumption considerably.""

Well it doesn’t, and it has increased energy consumption in several of my applications.

"It is somewhat simple to understand that if you operate a pump at a lower flow and pressure (which a VFD will do), the energy costs for pumping should be lower."

Nobody said anything about lowering the pressure, (which you usually cannot do). And lowering the pressure is the only way a VFD can reduce energy consumption. Even then it would be more efficient to have a correctly sized pump than to slow down a larger one. So a VFD is using more energy. It is not as simple as you think. Anytime you slow down a pump, the amps drop but, you are still using more energy per gallon.

 

[bimr]

Yes, there is some energy consumption associated with using a VFD. If you desire to understand how a VFD will save energy, please review this paper:

http://www.pumped101.com/comparing energy consumption.pdf

 

[jonr12]

Looks like he has made a few changes since the last time I read "pumped101". It was actually one of the articles that got me to believe the myth. It is all about the example you choose. In his figure #2, use 140' as the minimum head and everything changes. I just don't see those kinds of results in my little world. Here is the link where this has been discussed before.

http://www.eng-tips.com/viewthread.cfm?qid=260203

The last example in the previous link is another one of those bad examples for comparison.

The following link is an example curve that shows little to no benefit in power consumption using a VFD. This is more the kind of pump systems I am familiar with, which has caused me problems because the VFD increased the electric bill. Sorry, no more off topic comments.

 

[bimr]

Maybe you will be an control expert:

http://www.controlglobal.com/articles/2010/Optimization1003.html

 

[jonr12]

I agree with him that multiple pumps or surge vessels (water towers) are the most efficient way to control a pump system.

I don't see how anybody could look at a curve like the following and say that it saves 60% to remove a control valve and replace it with a VFD. There is barely a nickels worth of difference anywhere on this power curve. Maybe it's just that people no longer know how to read a curve and pick a pump that has good brake horse power characteristics, so the myth continues. I do know for a fact that if you remove a pressure tank and add a VFD, the energy use will increase.