Back pressure regulating valve

Hi,

In a NH3 process refrigeration plant, I need to regulate a BPRV that is not doing it's job. It is intended to maintain the evaporating pressure constant but it is variating according to the compressor suction pressure. The valve is 6" and the photo is:
Attachment 14188
Any one recognizes this kind of valve? I need to know the reference or manufacturer so that I can search for the manual, but in the body or the documentation available nothing exists.

Thank you in advance for any information.

Best regards, Carlos

cduque,
I am not sure what brand valve is, only to doubt it's still made, as looks very old.
In Parker link below might give some idea of what's in it, also see some small Parker solenoids in your pictures.
Valve could be worn out, or simple as diaphragm plate cracked or seat worn.
Have had success with other brands of valve by using Parker diaphragm & trimming to suit with tin snips (page 4 items 16,17,19).
Otherwise suggest new valve, Parker or Danfoss.
Most valves these days are weld in, no flanges.




http://www.parker.com/parkerimages/rs/50-23.PDF

Thank you Ranger1 for your ideas. I understand the valve is very old, Parker and Danfoss I know well but this one I have never seen before.