Water process piping for RO unit to prevent back pressure 1

[cames]

Hi all,
We currently have an RO unit (~25 years old), the piping setup follows little sketch I drew attached to this post.

Basically, theres city water that goes thru a booster pump then feeds 2 filters then feeds the RO Unit.
The RO unit then passes DI water thru two DI beds and thru a plant feed valve which then feeds the Water to 3 holding tanks. The plant feed valve is roughly 100 feet away from the RO unit.

The tank has a high and low level sensor, when the tanks low sensor turns on the plant feed valve opens which fills the tanks until the high level sensor is activated, then the plant feed valve closes.
Increased pressure in the line causes the RO unit to stop. There is a pressure gauge on the outlet of the RO unit that turns the RO unit off when pressure is greater than 90 psi (meaning the feed water valve has been closed). When the tanks low level switch is activated the plant feed valve is open lowering the pressure in the pipe below 20 which then triggers the RO unit to turn on. Rinse and repeat.

This RO unit has been running this way for the past 25 years, its only being now replaced since the PLC system is on the brink of death.

Anywho, we wanted to install the new RO unit right where this old unit is and keep the communication the same..however the RO manufacturer states that back pressure can cause damage to the RO unit and recomennds using a breaker tank and pump after the RO unit.
What is everyone thoughts? is the back pressure really that much of an iessue? is there another instrument that can be used as a trigger other than back pressure? since the plant water feed valve is 100 ft away from the unit will the water hammer from the back pressure be too far away to not have any effect on the unit?

Thank you

 

[LittleInch]

Why don't you use the signal from the tank to start / stop the RO unit?

Or use the position of the valve as a plant on/ off

Only needs 100 ft of cable.

If the vendor tells you high pressure isn't good for your new unit you have to believe them.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

I think the key to solving the problem is described by, "Will the water hammer from the back pressure be far enough away". Water hammer is not caused by backpressure. It is the result of rapid changes of fluid velocity. A high line pressure when a valve closes only allows the water hammer pressure fluctuations to go that much higher. So I think that the vendor probably has more concerns over the hammering than the high backpressure itself. 100ft is not a long enough distance to cushion water hammer. Changing the control signal will not solve water hammer. That will only ignore it. And you need a couple thousand feet of pipe, or a surge tank, OR ...

The other possibility is slowing down the valve closing time, or using a "surge valve". The problem might just disappear entirely. And, if you can, replace the feed pipeline to the new unit with a larger diameter pipe.




--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Don't know where the surge / water hammer is coming from.

100ft is so close you don't really have time to get a pressure surge so long as your valve doesn't close in 0.1 seconds....

We have no data such as flow rate, water velocity etc

Looking at this again, what "back pressure" does the new vendor want?
Is 20psi or 40 psi to much for him?
You have given uis so little data we can't form a judegement.

Here

's the sketch for thos who don't want to download it.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ashtree]

RO membranes do not like being unpressurised on the feed side and having pressure on the permeate side of the membrane . Typically this is recommended to not exceed about 30kpa (4.5 psi approximately). Its not about water hammer or surge , but about protecting the membranes from back pressure damage. If you are careful you can do it with non return , or power actuated shut off valves to isolate the membranes from the back pressure but you will need high pressure shutdown systems and valve positioners linked to the control circuit to make sure that the valves are open as the RO starts up.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[LittleInch]

So on that system if the towns water pressure is 40 psi then it would actually be ok if there was <40 psi d/s of the /RO units?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

He did mention water hammer.

Water hammer pressures are not one way. They act forward and backward to direction of flow. In short pipes, pressure rise is the same as in long pipes, but nearly instantaneous from beginning to end of pipe, as a pressure spike; without a noticeable wave. The short pipe length makes the waves appear to be one quick spike, which dissipates its energy rapidly, and as long as it is limited in amplitude, may not last long enough to do damage, such as from sustained differential pressure, which might occur if the time between spikes was longer due to longer travel time down to the end of pipe and back. Valve closure time affects the maximum pressure amplitude, but not the reflection time. Fast reflections make the waves appear more as just one pressure spike. A faster valve close time only increases the maximum possible pressure amplitude. You still get all the waves. A short pipe just makes it feel like one.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[TiCl4]

Water hammer is not good for membranes, as it can crush the laminated membrane onto the support resin if the spike is high enough. However, the OEM's statement that backpressure is not good for RO systems is specific to the permeate side of the membrane. A reverse pressure differential (permeate pressure higher than retentate pressure) of only 5 psig or so is all this is needed to delaminate the RO membrane from the support resin and cause bypass (good water bleeding into the permeate water).

RO systems often drive up to 200+ psig on the retentate side. OP, you do not need to worry about backpressure from the storage tank because this is on your retentate side. You DO need to worry about the piping configuration of the retentate. Normally, this is just dumped into a nearby trench or sump. Retentate piping is normally not raised above ~10-11' above the RO system bottom tubes to limit the possible dP available in case of reverse flow during startup or shutdown.

 

[georgeverghese]

There are 2-3 issues here, all of which, in my opinion, can be resolved by process controls and some automated pressure dump valves:
a)Keep dP between RO feed pressure and product RO water +ve but below a high set limit
b)Keep dP between RO product and reject water streams +ve

No need for break tank and new booster pump.

Surge pressure can be resolved by slow closing of DI water tank feed valve.

 

[georgeverghese]

Error at (b) in my last post. Should be
(b) (Feed to reject stream dp) - (feed to product stream dp) must be +ve.