Why a watermaker by-pass valve?

[Hoghead]

Looking at this schematic, the watermaker by-pass valve, simply by-passes the regulating valve, to pressurize the membrane.

Given that I have a needle valve for a regulator, why use a ball valve for the by-pass and simply open and close the needle valve to either regulate or by-pass as required

No shock loading of the membrane, and you still need to unload a the end of each watermaking session anyway

[Albro359]

I don't see a "bypass" valve in your schematic...I do see a "relief" valve....what is it you're getting at here ?
Cheers
Alan

[Hoghead]

Originally Posted by Albro359

I don't see a "bypass" valve in your schematic...I do see a "relief" valve....what is it you're getting at here ?
Cheers
Alan

See to the right of the 2 blue membranes. There are 2 valves shown and the by-pass, bypasses or simply goes around the regulator

It is not my drawing

[Albro359]

OK...got it
No don't see the point of that at all
You can decrease the pressure on the membranes with the needle valve, donlt see why you need a bypass valve
My Dessalator watermaker is pretty simple and the manual says gradually reduce the pressure with the needle valve and then rinse at no pressure for a few minutes before turning off...
Cheers
Alan

[Captain Bill]

I don't see much point in the bypass valve. I have seen some needle valves that are a bit too restricted even when wide open and keep a little pressure on the membrane. Then again most HP pumps want a minimum of 100 psi on the output. The thing that does not make sense to me is that they have the cleaning/layup water coming off of the brine side and the fresh water bypass going overboard. All the units I have seen have the brine going overboard and the fresh water bypass going to the cleaning/layup reservoir. Mine simply goes to a long hose which I can put in a bucket or overboard at my convenience. I always clean/layup with fresh R/O water, never brine. Why would you want to store expensive metal parts in salt water, unless you were someone hoping to sell lots of replacement parts.

[a64pilot]

I was going ot guess that you would want to bypass completly to help with start up loads on an electric motor, but then opened the .pdf and say it was for an engine driven pump, maybe it's actually schematic for an electric system?

Pure guess though

[ebs001]

The bypass valve bypasses the pressure valve thus allowing a flushing of the membrane. This is done when pickling , cleaning or winterizing so that these fluids are not forced through the membrane. By opening this valve there is not sufficient pressure to affect reverse osmosis. I also use this as a salt water flush before recommissioning each year.

[Dymaxion]

guesswork here, but if you've dialed in the operating pressure with the needle valve, then opening the bypass valve when rinsing/pickling saves recalibrating the needle each time. Also keeps any rinse crud out of the needle orifice?

[Hoghead]

I do not think that it is needed other than a convenience in that the regulated pressure is set using the regulator/needle valve, and then a ball valve bypasses this for flush or start up.

I note that it is recommended to open the by-pass ball valve slowly so that one does not shock the membrane. This occurs naturally when using the needle valve as the regulator/by-pass. Dial it down at the end of the watermaking cycle to flush, and leave in that mode for start up.

Seems to me that all this can be accomplished with a needle valve as a regulator, and the by-pass valve is not needed. Input is filtered to 5 microns so crud clogging should not be an issue.
Easy, simple, and cheaper - decision made unless convinced otherwise

Note that my system is engine driven at a constant RPM so a needle valve will work fine. This may not be the case for all systems.

[44'cruisingcat]

I like having the by-pass valve because it means I can "set and forget" the pressure regulator valve, instead of having to get it to the right pressure on every start up. The pressure regulator can be a bit fiddly to get right.


On start up I simply open the by-pass valve, run for a couple of minutes, then slowly close the by-pass. With the pressure regulator pre-set, there's little risk of over pressuring the membrane.


The by-pass valve also allows full flow for circulating pickling or flushing.


But yeah you could probably get away with not having one.

[SV THIRD DAY]

It's simply not needed.
You can fresh water flush or pickle your water maker easily without it, just by opening up your pressure regulating needle valve. Why have extra SS ($) parts you don't need.

KISS....get rid of **** you don't need.

[Hoghead]

Originally Posted by SV THIRD DAY

It's simply not needed.
You can fresh water flush or pickle your water maker easily with it, just by opening up your pressure regulating needle valve.

KISS....get rid of **** you don't need.

Agreed

[Tellie]

Perhaps it is needed for purpose or convenience. There is important information left out in this design that is, I'm sure, available but not shown. The information not provided is the needle valves make, design, and it's open flow capacity, and the size and flow capabilities of the HP pump. Not all restrictive needle valves are created the same. When a system of this size, using a high pressure pump, is going to be stored with a pickling agent you want to make sure that the system flows freely from intake to output to assure there is not an over pressurization of the membranes during the pickling process which can damage the membranes by driving the storage solution into the membrane surface. This may have been the mind set of the person designing this particular system considering the parts he intended to use. The free flow through the needle valve may be, for example, 100psi. Not enough to start the RO process but certainly enough to over pressurize the membranes with a storage solution and cause damage to the membranes.
Just a thought.

Halden Marine Services | Marine Watermakers, Solar Panels, Wind Generators

[Hoghead]

Originally Posted by Tellie

Perhaps it is needed for purpose or convenience. There is important information left out in this design that is, I'm sure, available but not shown. The information not provided is the needle valves make, design, and it's open flow capacity, and the size and flow capabilities of the HP pump. Not all restrictive needle valves are created the same. When a system of this size, using a high pressure pump, is going to be stored with a pickling agent you want to make sure that the system flows freely from intake to output to assure there is not an over pressurization of the membranes during the pickling process which can damage the membranes by driving the storage solution into the membrane surface. This may have been the mind set of the person designing this particular system considering the parts he intended to use. The free flow through the needle valve may be, for example, 100psi. Not enough to start the RO process but certainly enough to over pressurize the membranes with a storage solution and cause damage to the membranes.
Just a thought.

Halden Marine Services | Marine Watermakers, Solar Panels, Wind Generators

Excellent point

I am using a Nishok 400 series 1/2" valve. Flow coefficient is 0.44
I believe it goes to full open