Pump System Questions and Answers > Frequently Asked Questions
Written Expalnation of How a CSV Works
Cary Austin:
The CSV is just a valve that opens and closes like a ball valve. It just has a spring to push it open and a diaphragm to push it closed.
With a 40/60 pressure switch the tank is drained when a tap is opened and the pressure drops to 40 PSI starting the pump. The adjustment bolt sets the spring tension in the CSV at 50 PSI. So once the pump is started the CSV tries to maintain 50 PSI. If you open more taps and the pressure drops to 49 PSI, the CSV opens delivering more water to get the pressure back up to 50. If you close some taps and the pressure increases to 51 PSI, the diaphragm pushes the valve closed enough to bring the pressure back down to 50 PSI.
So no matter how many or how few taps you have open the CSV just keeps the pressure at 50 PSI, and doesn't let it build to 60 PSI which would make the pressure switch shut off the pump. This entire time the water is just flowing right past the pressure tank, straight to the taps. So it doesn't matter if you have a 1 gallon or a million gallon pressure tank.
When you turn off all the taps the CSV closes as much as it can at 51 PSI. However, there is a 1 GPM bypass made on the valve seat so the valve just can't close less than 1 GPM. And since all the taps are closed there is no place left for this 1 GPM to go except for the tank. So now the water makes a right turn into the pressure tank and the pressure slowly increases to 60 PSI where the pressure switch is set to turn the pump off.
Now the tank is again full and the pump is off, waiting for someone to open a tap and start the process all over again.
The CSV has no control over the motor speed. It simply opens or closes a valve to allow the correct amount of water through to match the amount being used.
The counter intuitive part of the CSV is that the motor amps drop when the flow from the pump is restricted with a valve, the same way the amps drop when the motor is slowed down with a complicated, computerized, expensive, short lived Variable Speed Drive (VFD or VSD).
Many people think that choking a pump back with a valve makes the amps go up and the pump to work harder, while slowing the motor speed would make the pumps work easier. In fact just the opposite is true. Choking the flow from a pump reduces the amp draw and makes the pumps work easier, while reducing the speed with a VFD is very hard on the pump/motor.
If you find a pump man or engineer who understands this, you have found a good one. If they think a CSV is hard on a pump and a VFD saves energy and makes pumps last longer, they don't know their butt from a hole in the ground, so find a new pump man or engineer.
ByteMe:
The counter intuitive part of the CSV is that the motor amps drop when the flow from the pump is restricted with a valve, the same way the amps drop when the motor is slowed down with a complicated, computerized, expensive, short lived Variable Speed Drive (VFD or VSD).
Many people think that choking a pump back with a valve makes the amps go up and the pump to work harder, while slowing the motor speed would make the pumps work easier. In fact just the opposite is true. Choking the flow from a pump reduces the amp draw and makes the pumps work easier, while reducing the speed with a VFD is very hard on the pump/motor.
If you find a pump man or engineer who understands this, you have found a good one. If they think a CSV is hard on a pump and a VFD saves energy and makes pumps last longer, they don't know their butt from a hole in the ground, so find a new pump man or engineer.
At first I was going to call your BS. Then, having a touch of OCD.... I researched the power draw of centrifugal pumps. You are correct in your statements about power being less with lower flow.
Then I was wondering why I was thinking that restrictioning the flow would make the pump use more power. The only reason I came up with is that I was thinking about a positive displacement pump that will draw more power as you restrict the flow (is. Pressure goes up to maintain constant flow requiring more power).
So I am guessing that others are thinking about how a positive displacement pump works verses how the centrifugal pump works.
With that said, reducing flow by half on a centrifugal pump does not reduce power in half. I am thinking about figuring out the energy effectiveness of the CSV verses no CSV with a huge pressure storage tank with large drawdown (in the hundreds of gallon range). Granted, you lose the constant pressure of CSV... But then a pressure swing between 60-80 shouldn't be that annoying. I know the huge storage tank would have to be more energy efficient (just a little). But then, how long would the little energy savings take to pay for the huge tank. Am guessing about one persons lifetime. Then add that the tank won't last that long , makes having a huge tank pointless (system lifetime costwise).
You have any thoughts on the direction of my thinking?
Cary Austin:
I am glad you figured it out. But no, others are not thinking it is a positive displacement pump, they just have no idea how a centrifugal pump works. The head engineer for one of the major pump companies once told me "he had no idea the amps on their pumps would drop that much when simply restricted with a valve".
Of course a pump is more efficient when pumping into a pressure tank as it is always running at BEP or Best Efficiency Point. But here is an article explaining it will take 169 years to pay off a 7200 gallon pressure tank with the energy saved using the pressure tank. Of course the larger the pressure tank, the longer it will take to pay off. But the smaller the pressure tank, the smaller the pump and there will be even less savings in energy.
And there are lots of other benefits for using a CSV that should be considered as well. Not only can you use a much smaller pressure tank when using a CSV, but the pump will last longer, the pressure will be much better in the house, and the lack of water hammer will make the check valve, toilet valves, faucets, and all other parts of the piping system last longer as well.
http://www.cyclestopvalves.com/csvtechinfo_24.html
ByteMe:
Thank you for the quick reply and the link, there is some information/ideas in that link I was looking for.
To try and eliminate misunderstandings with my thinking, a couple of points I'd like to make. These are my current opinions/guesses and can/will change as I learn more.
1) Your statements on this website are 95%+ spot on (that my research confirms). The remaining few percent are technically accurate but misleading. Honestly, this is annoying. OK, this is highly subjective and can mostly be just me.
2) I respect that you have an ingenious device that can work very well for many people. In my familly, the way we learn is by questioning and sometimes challenging the knowledgeable person. In no way is this meant to be personally insulting, please don't take it as such.
Now some questions.
1) A CSV will very effectivly stop "water hammer". Won't any pressure tank in-line also do this? Is it that the CSV in combination with a pressure tank works better? How would you quantify this?
2) How reliable has the CSV been? Is there a suggested maintenance schedule? Does the CSV have a 10,20,30 year expected lifespan?
3) A major advantage to a CSV is reducing water pump start/stop cycles. If you have a larger pressure tank, won't this accomplish the same thing.? As an expamle, if a house uses an average of 300 gallons a day and you have a pressure tank with a 30 gallon draw-down, the maximum start/stop cycles will be 10 per day. Does a CSV have any start/stop cycle advantages over a large (10%+ of daily use) tank?
My current thinking;
1) A residential CSV with small pressure tank is only slightly cheaper than a large pressure tank (119/30 gallon). Both being $400-$600 with the large tank on average about $100 more than the CSV setup.
2) Energy efficiency is better with the large pressure tank system. This will vary between 5% and 20%. This one is complex and I don't ever see it making a $100 difference per year on the electrical bills (single household)
3) A slow pressure swing because of a large pressure tank between 50-70 or 60-80 seems a non-issue. No experience here, just a guess. Anyone else care to comment?
4) Given the space for a larger pressure tank and slightly higher initial cost with slightly lower electrical costs. A large pressure tank seems to be the overall better way to go. This is given that a correctly sized pump is used, which is true in either case.
I am now looking at the cost effectiveness of a dual pump large pressure tank residential system. A ~6 GPM pump to handle normal useage and a 12gpm pump to handle hi usage. Maybe it would be better to use two 6gpm pumps at the same time verses turning off smaller pump when demand turns on large pump. Hmmmm, interesting.
Thanks for reading my rambling, and please (anyone) post your thoughts.
ByteMe:
Brain fart. Scratch the idea of multiple pumps with a large pressure tank. That would never be more effective than a single pump on a large pressure tank.
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