WE ARE DESIGNING A GAS RECOVERY SYSTEM FROM A PROPANE REFRIGERATION LOOP.

PROPANE VAPORS RECOVERED WILL BE REINJECTED IN A BOIL-OFF SYSTEM TO BE COMPRESSED AND LIQUEFIED AGAIN WITH VAPORS COMING FROM STORAGE TANKS.

THE TIE-IN POINT IS LOCATED AT ABOUT 100 M FROM THE RECOVERY POINT IN THE BOIL-OFF COLLECTOR IN WICH PRESSURE MUST NOT EXCEED 0.15 KG/CM2.

THE RECOVERED GASES PRESSSURE VARIES FROM 5 TO 3 KG/CM.

THE OPERATION TAKES ABOUT 30 MINUTES BY MEANS OF A RSTRICTION ORIFICE.

AT THE INJECTION POINT, A PRESSURE CONTROL VALVE MUST DECREASE THE PRESSURE TO ABOUT 0.15 KG/CM2 WHATEVER PRESSURE OR FLOW WILL CHANGE.
THE CONTROL VALVE MUST GIVE 0.15 KG/CM2 AT THE OUTLET EITHER PRESSURE IS 5 - 4 - 3 KG/CM2, AND FOR VAIRABLE FLOWRATE.

Nothing you asked for sounds much different from a regulator, otherwise known as a pressure reducing valve.  Normally regulators have "droop" or proportional band of about 10% of their range, but pilot-operated regulators can be much tighter.  

Or, you could go for a control valve, with a positioner in a full control loop with a pressure transmitter monitoring the downstream poressure for a pressure controller that outputs to the valve.  The control loop can give you better steady-state accuracy, but will probably not be as quick as the regulator on a sharp transient.   I'M ASKING IF SUCH VALVES EXIST, IF YES HELP TO GET SPECIFICATIONS AND DESIGN CALCULATION METHOD AND A SCHEMATIC FOR THE CONTROL LOOP.

How precise a system can be will also be dependent on the amounts (and temperatures) we are talking about. The figures will point to commercial available valves or equipment from different manufacturers at different prices and accuracy, and different sizes.

Only a fully detailed and more precise analysis will give you a full answer.

Additionally: If the pipeline system is exposed to different temperatures (sun heating and / or frost for instance) the pressure inside the system will vary and need to be regulated or safeguarded according to this.

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