Filter Pressure Variation

Pool pumps, pool filters and the plumbing of
swimming pools. Sand filters, cartridge filters,
fabric filters and alternative filter media.
szn9rg
Pool Newbie
Pool Newbie
Posts: 3
Joined: Thu 16 Jul, 2009 22:14
My Pool: Hayward S-200 filter
Location: Defiance, OH

Filter Pressure Variation

Postby szn9rg » Thu 16 Jul, 2009 23:48

I have a Hayward S-200 sand filter with a 1 hp motor. I have a valve between the pump and filter so I can regulate the pressure to filter. Set at 10psi after backwash and don't touch. This year I've been having problems with the pressure increasing during the day. It may start at 10 psi and by late afternoon it may jump 5-10 psi. If I turn the pump off and let it sit awhile and then turn back on the pressure reads 10 psi again but will kreep back up slowly as before. The problem seems to come and go, but this condition occurs most of the time. I tried a new gauge but that wasn't the problem. When the pressure has jumped the discharge to the pool is still strong as when it read 10 psi. Typically as the pressure goes up the discharge decreases. I know the pressure is actually increasing because as the pressure increases, the filter shell starts to leak due to age and stress cracks. I have replaced the same last summer. I can't understand how the pressure can be high and the discharge is still as strong? This doesn't seem possible. I've run out of ideas so hopefully someone has experienced the same problem and can help me out. THANKS IN ADVANCE


Me...
Swimming Pool Superstar
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Posts: 302
Joined: Thu 26 Feb, 2009 11:11

Filter Pressure Variation

Postby Me... » Tue 21 Jul, 2009 09:29

If I was to put a valve in between the pump and filter it would be for flow, not pressure.

Your filter will be rated for a maximum flow and the installer may have put a big pump on to give that flow against the high head of plumbing size or distance. Or maybe a cleaning system or something else that might cause high losses in the system. That same pump would probably put way to high a flow through the filter on backwash and a valve placed there is a perfect place to restrict the flow down to a proper backwash flowrate.

As for your problem it sounds like the filter media may be plugging up too fast now. Try a series of backwashes. Backwash for 2-3 minutes then rinse and filter for another few. Repeat that a few times to see if it loosens the sand bed and releases some more debris. Do not simply backwash for 10 minutes as that can cause damage.

Next would be that running the pump on backwash with that valve wise open could have caused the sand bed to become so mixed that a lot of debris is now trapped in the bottom and wrapped around the laterals or just simply plugged the entire sand bed instead of the top several inches. This is caused by extreme flow in backwash mode which will basically cause a huge sand storm inside the filter. If this happened you will need to change all the sand.
szn9rg
Pool Newbie
Pool Newbie
Posts: 3
Joined: Thu 16 Jul, 2009 22:14
My Pool: Hayward S-200 filter
Location: Defiance, OH

Filter Pressure Variation

Postby szn9rg » Sun 26 Jul, 2009 22:20

Thanks for the info. I have a couple of questions. You spoke of the proper pressure or flow when backwashing. What should this pressure be? Currently with the valve fully open the backwash pressure is 18-20 psi. If this is to high than that could have mixed things up on the inside of the filter as you said. The thing that I can't understand is why it would cause this problem now as I've done it this way for the last 10 years. Do you agree with closing the valve so I get 10 psi in filter mode right after a backwash? Thanks for the feedback. I appreciate it very much.
Me...
Swimming Pool Superstar
Swimming Pool Superstar
Posts: 302
Joined: Thu 26 Feb, 2009 11:11

Filter Pressure Variation

Postby Me... » Mon 27 Jul, 2009 10:08

Actually pressure has little to do with flow, at least in this instance. Your filter is rated at 2.2 sq.ft. and the manual states a max flow of 44gpm which is at 20gpm. per sq.ft. As in ..

2.2sq.ft. x 20gpm per sq.ft. = 44gpm

If you were to run that flow on the filter cycle when there is equipment, plumbing and returns to fight against it would not damage the filter. However when you go to backwash with no restriction the flow can take a huge jump and destroy the sand bed by totally fluidizing it and mixing it all up INCLUDING AND ENTRAPPING the debris you were trying to get rid of. A few rounds of this type of backwashing and your filter media bed will be in very bad shape.

A good backwash flow rate is about 15gpm per sq.ft., in your case about 33gpm. And if I was designing one for you I would filter at maybe 12gpm per sq.ft which is about 26gpm. When you then go to backwash the flow will naturally increase to a good backwash flowrate. The lower filtration flow rate will also filter better as you are not driving the water through the media bed.

So yes, I constantly harp at people to spend an extra couple hundred dollars on a bigger filter when the spend the small fortunes they do on the pools they build and that is why.

O yes, your filter will be tested to I think 75psi and rated to operate at 50psi. The pressure gauge is there to help you decide when to backwash not to judge how much water if flowing through your filter.
szn9rg
Pool Newbie
Pool Newbie
Posts: 3
Joined: Thu 16 Jul, 2009 22:14
My Pool: Hayward S-200 filter
Location: Defiance, OH

Filter Pressure Variation

Postby szn9rg » Tue 28 Jul, 2009 11:54

Thanks for the info. I see what you are saying about flow verses pressure. Since I use the valve to reduce flow to the filter in filter mode (10 psi), how much further should I open this valve when backwashing or should I leave it just on it was in filter mode? I realize the tank is rated at 75 psi but due to the age of the shell, I need to run at a lower psi to prevent it from destroying itself. Has small stress cracks that leak as pressure increases. Even though I run at lower psi the force of the flow back into the pool is very very strong. Its hard to hold your hand in front of it espically after a backwash. The other question I have is isn't it the pressure that causes the flow. If there was no pressure, what would make the water move? How does one measure flow as the only measuring device is the pressure gage? Sorry for all the questions but most of this is new to me.
Me...
Swimming Pool Superstar
Swimming Pool Superstar
Posts: 302
Joined: Thu 26 Feb, 2009 11:11

Filter Pressure Variation

Postby Me... » Thu 30 Jul, 2009 11:54

It does take pressure to move water through the pipes yes. Up to about 45gpm is ok for 1.5" pipe. This might be common pool plumbing and you might see 10-15psi on a gauge. In order to move that 30gpm through 1" pipe the gauge might jump to 25psi or so. But move 30gpm through a 2" pipe and the pressure might drop to 5psi. And this will depend on if the pipe is the only resisting factor, which it will not be. There are pool returns, heaters, chlorinators, filters, strainer baskets, suction fittings and of course the type and number of fittings in the plumbing system and the length of pipe. Not too mention suction lift if the pump is above water level.

You measure flow with a flow meter and it is another great tool to have if you really want to know what your pool is doing. Most any pool store should have them, just make sure it is sized for your pipe. Easy to install also.

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