A relief valve is a very simple spring loaded device which has the sole purpose of providing overpressure protection.  A relief valve is not sophisticated enough allow for control or pressure regulation.  Pressure Regulators on the other hand are not regulated by ASME Code because they do protect against catastrophic failure.
In my experience, many users attempt pressure regulation with relief valves, but it is not a good option because the result is usually "chatter" which destroys the relief valve and leads to other problems. My advice is, Pay up front for the right tool to do the job.

J. Alton Cox
President
DeLuca Test Equipment
www.delucatest.com

A back pressure regulator is an operational device. A relief valve is a safety device.

The bulk of my experience is with steam. Manufacturers like Spirax Sarco make back pressure pilots for their control valve bodies, and they operate to maintain a set pressure upstream of the valve. Their literature, in large, bold print indicates very clearly that this device is NOT a safety or relief valve, and must not be used in place of one. You can certainly use it in parallel with a properly sized and selected safety valve. You can set it for some pressure below the safety valve setting so the safety wouldn't necessarily lift under some operational conditions where the pressure may drift up.

JAlton is right - doing it right the first time is ALWAYS the cheapest option in the long run.

Perhaps I should explain a little more.  I am an admitted scrounger.   For the past year I have been scrounging parts for an engine driven 40 gph watermaker.  So far I have a stainless Cat 241 pump, electric clutch, over/underspeed switch, two 40" membranes and pressure vessles, supply and product flow meters, diverter solenoids, pressure switch, digital pressure guage and all the hoses, prefilters and fittings.  All new surplus with a total investment to date of $1,256.40.  Not bad considering the pump alone list for $1,700.

There is just one snag.  The last part I need is the stainless steel back pressure regulator to maintain a constant 800 psi in the membranes.    These things must be pretty rare because I have not been able to scrounge one and the cheapest one I can find is the Circle Seal BP3 at close to $600.  That would be 30% of the total cost in one valve.  Surely there is another solution.

You might want to look at Fulflo valves.  We used them on the discharge of the lube oil pumps where they typically recycled part of the stream back to the storage tank to maintain lube oil inlet pressure.  

Operations and maintenance were very happy with their performance, they were used on ALL the compressors at this site and we took credit for them as relief valves if the piping was accidentally blocked in.  I don't know about materials of construction for seawater but if they have suitable material, it would likely do the trick for you.  Can't say what the costs are, don't think they are really expensive but might be more than you want.

The 5% set pressure variation might be another problem, need to ask the vendor if you went this route.

Thanks.  I checked the Fullflo site and it looks like they may be able to help.  I will try their tech people Monday.  Now that Circle seal has bought out almost eveyone else I find it hard to get to a tech person who can help.  

Fulflo is a good option.  I had forgotten about them.  They make a valve which works similar to a relief valve, but it is not positive shut-off.  Good suggestion TD2K.

J. Alton Cox
President
DeLuca Test Equipment
www.delucatest.com

If the pressure vessel is rated by ASME , it generally requires the use of an ASME recognized safety or relief valve for prevention of overpressure.

In the cases where it permits a control valve for part of the relief requirements, the control valve needs to be stamped for its rated capacity and automatic control system runback or trips are additionally required to protect the unit ( ie, once thru steam generators)

Other cases, such as fuel gas lines ( B31.1) can avoid usde of code relief valves but require a series of other measures to protect the piping, and the pressure vedssels still need a token code relief valve. For the case of a fule gas pressure regulator, 2 independent regulators are arranged in series, with a pressure switch hardwired to a inlet stop valve that will fail closed in 1 sec.