7E-Why would an RO tank lose some capacity?

[Anonymous]

I have a 4 gallon storage tank thay typically held about 2-1/2 gallons of water. I understand that the rest of the tanks capacity is for air. The air creates presure thus allowing the water to be dispensed. There is an air valve on the tank where air could be added/removed.

I don't really know if the above disertation is even close, it's just my theory.

The tank now suddenly only dispenses about 1 - 1 1/2 gallons of water when it is full.

So what do I need to do to get the old capacity back. There appears to be only two options. 1. Add air. 2. Remove air. Which is correct?

Louey

 

[Anonymous]

I would check a couple of things

1. Water pressure from RO to tank.
If it is too low then it wouldn't be able to fill the tank as much.

2. The Air pressure in the tank.
If it is too high then the air would take up too much space in the tank.
If it was too low then the tank would fill up with water but not have the back pressure to pump it out of the faucet.

 

[Anonymous]

I don't know the answer- but one other thing to consider: pressure tanks sometimes keep the air in a bladder- the one in my house main system does. It broke. Then the pump in the well had to continually shut off and on because the tank had no pressure.

I would try adding air and see what happens.

 

[O P Ing]

hi.
The reason that you only get 1.5 gallon out of a 4 gallon pressure tank is because:
1, there is only 1.5 gallon in there when it is full. Or
2, there is more than 1.5 gallon in there, but the air pressure only can push 1.5 gallon of it out.

You can tell if you have problem #1 if you lift up the emptied tank, it should weight the same as it used to when empty (back then when the capacity was 2.5 gal.). This rarely happens, but you need to make sure it is not the case.

For problem #2, if you lift up a supposingly "empty" tank, and it weights many pounds more than it used to when empty. There are two main reasons for this:

A, the pressure outside the water bladder (within the tank) is too low, and can't push all the water out as it should.

B, the water bladder had ruptured.

For reason A, you need to use a bike pump (please don't bring the pressure tank to air station in you local garage! You may end up with the plastic bladder being pushed out of the tank!) to increase the pressure to the factory's specification.

For reason B, you need to replace the pressure tank.

Rarely do people need to decrease the pressure by bleeding it. Air does not get it there by itself, unless you have a mischievous spouse.

 

[Anonymous]

Rarely do people need to decrease the pressure by bleeding it. Air does not get it there by itself, unless you have a mischievous spouse.

:lol: :lol: :lol:

Good one!

Thanks for the detailed explanation. I'll check it out when I get home tonite!


Louey

 

[Anonymous]

It turned out to be problem A. It took allot of "pumps" from the bycycle pump to get it working properly. After about 30 pumps or so, the air gauge still read zero. Within the next 10 - 15 pumps I got it up to 7 PSI and the tank emptied out completely.

I wonder how all that air got out? Weird!

Thanks again!

Louey

 

[SPC]

Great job on the pumping there Louey
Steve

 

[O P Ing]

hi.
If you have problem keep the pressure on the tank, you either have a ruptured bladder, or a faulty nipple. Sorry for the body terms :wink:

 

[M.E.Milz]

I have a 10 gallon bladder tank that continued to hold/store less and less water until it would only store 1-2 gallons. I figure that the problem was either:

1) too much or too little air pressure in the bladder, or

2) a decrease in my water pressure.

I was wrong. The problem was with my filter membranes. In particular, my sediment pre-filter had gotten too dirty, thereby decreasing the water pressure applied to the RO membrane. I replaced my filter cartridges and the problem disappeared instantly.

 

[O P Ing]

hi.
Very good point. However, it should be notice that no matter how dirty your prefilters are, as long as each of the water molecules can passes thru one by one, your water pressure applied to the membrane will be the same as the line pressure. The appearant pressure drop you implied is when the ASO valve opens, not when there is no flow (static) thru the RO system.

This may sound trite to some people, but misunderstand of static and dynamic pressure can be a problem. For example, some people mistakenly use a needle valve to "regulate" pressure with bad consequences.

 

[M.E.Milz]

hi.
Very good point. However, it should be notice that no matter how dirty your prefilters are, as long as each of the water molecules can passes thru one by one, your water pressure applied to the membrane will be the same as the line pressure. The appearant pressure drop you implied is when the ASO valve opens, not when there is no flow (static) thru the RO system.

This may sound trite to some people, but misunderstand of static and dynamic pressure can be a problem. For example, some people mistakenly use a needle valve to "regulate" pressure with bad consequences.

Well it has been a few years since my hydrodynamics classes, but I understand your point. This is precisely why I had assumed that the filters were NOT the problem, and that it had to be some other reason (either a drop in water pressure or a problem with the bladder).

When I finally talked to a rep at Aquatic Reef Systems about the problem, he was convinced that the filters were the problem. He said the problem is quite common. And like I said, I replaced the filters and the problem resolved it self instantly.

I still don't understand why dirty filters caused this problem. Nevertheless, I suspect this is because when flow through the pre-filter is too restricted, it CAN have an impact on the pressure (dynamic and/or static) applied to the RO membrane.

 

[O P Ing]

I still don't understand why dirty filters caused this problem. Nevertheless, I suspect this is because when flow through the pre-filter is too restricted, it CAN have an impact on the pressure (dynamic and/or static) applied to the RO membrane.

hi.
You guess is correct. When the prefilter is clogged badly (nominal orifice diameter is smaller than the flow restrictor diameter on the brine output), on a three-way system (pressurized input from water source, pressurized permeate output to tank, and open brine output to drain), as the ASO valve opens pressure drop will happens. When the pressure inside the bladder tank is equal to the pressure between the flow restrictor and the prefilter, the tank will stop filling up.

Many RO system come with a pressure gauage on the pressure vessel of the RO membrane, and that's precisely what it is useful for. When the pressure drop below normal when the RO turns on, it is a indication that the prefilter is clogged. If you RO pressure vessel have one of these gauages, it is a relatively easy for people to trouble shoot.

 

[M.E.Milz]

No gauge. Thanks for the explanation.

 

[Anonymous]

Dang it. Pressure in the tank has already dropped back down to 2 PSI in only 12 hours time.

Looks like I'm on the market for a new tank. Now I got to figure out where I can put a larger one. Honey, ......... :roll: 8O

Louey

 

[Anonymous]

Steve said:

Great job on the pumping there Louey

Hey, l quite proud anytime time I get 30-40 pumps in!

 

[O P Ing]

back down to 2 PSI in only 12 hours time.

hi.
Before trashing the tank, fill it up with some water (so that it will sink), pump some air in, and dunk it on a large bucket and see if the nipple is the problem. Sometimes, you just need to make sure the metal stem is holding the air back, and put the cap on without ruin it, and it should hold pressure for a long while.

If the bladder is rupture, there is not much you can do. Good luck.

 

[Anonymous]

I water tested the tank today. I didn't notice any air leaking out of the stem or the top where the output valve is. So it must be the blatter.

I'm gettin a bigger tank, I'm gettin a bigger tank......

But where to put it, that is the question?

Thanks 7!

Louey

 

[O P Ing]

hi.
I put the tank under the kitchen sink. I took the drain pipes apart in order to slide it in, and put them back together afterward. Most of the time, you don't need a large storage tank. It is not cheap to get a large one either. Why do you want a larger tank? The pressure tank (same as your broken tank) I used were able to meet my drinking/cooking water need, as well top-offs and occasional water change... last one was a 5 gal water change 1.5 years ago. There are 4 adults and one toddler in my family, and we drink/cook about 4 gal every day, in addition to the fish tanks.

 

[Anonymous]

I would like to be able to refill my top-off bucket (7G) and still have water available for cooking, drinking, etc. I got my eye on a 20G tank. Cost $100 at Lowe's. It'll fit in my laundry room where the RO unit is and won't get in the way to much. I'm hoping that this tank will actually hold about 12 1/2 gallons. I could fill my 32G brute at night with the first 12 1/2 G's, then leave the RO running overnight and it should be close to filled in the AM. Actually I only fill it to about 25-27G's. More than enough to do a large WC on my 75G reef or 37G FOWLR.

Louey

 

[Anonymous]

Hey 7,

I bought a new pressure 42G tank, but the RO unit won't fill it. The tank is the size of a twenty, but because of it's (supposed) design it has the capacity of a standard 42G. The tank is supposed to hold 19G of water. The tank came precharged at 40 PSI. I figured that RO system is designed to operate at 5-7 psi. So I reduced the tank pressure accordingly. The new tank only fills to about 1 gallon, regardless of the PSI. I have let it run at 40, 20, 12, 7, and 5 PSI. Not mcuh change at any value.

It seems that the RO isn't even producing like it should. Normally, you hear the drain line kick in every few seconds. Now it's very inconsistent. Sometimes a few seconds, sometimes 10 or more seconds. I wonder if there is some other problem with my RO. I replaced all of the filters a few months ago except the membrane and the post carbon filter. Both are supposed to last at least 2 years. The unit is about 18 months old. Current TDS reading is 4 ppm.

FWIW- the unit is a 50 GPD 6 stage RO/DI. One sediment filter, two carbon, one membrane, one DI cartride, and one post carbon.


Any ideas?

Louey