PRESSURE CONTROL VALVES (PCVs) To be delivered by MUHAMMAD ARIF NASIM E & I (Instrument) Apprentice Engineer, 13th Batch July 09, 2008

WHAT IS A PCV Matches the flow of a fluid. Maintains the system Pressure within acceptable limits.

BENEFITS OF PCV OVER CONTROL VALVE PCVs are simpler devices than control valves. PCVs are self operated control devices. PCV requires energy from the controlled system to operate whereas control valve requires external power sources and control instruments.

SELF-OPERATED PCVs Also called Direct-operated PCVs Used for low flow rates. Simpler in construction. Three components Restricting element Loading element Measuring element

1. RESTRICTING ELEMENT Restricts/regulates the flow of a fluid. Usually a PLUG that can be positioned fully open, fully closed, or in throttling mode. Restricting Element

2. LOADING ELEMENT Provides force and moves the restricting element. Connects directly with restricting element. Usually a SPRING or WEIGHT. Loading Element

3. MEASURING ELEMENT Senses the downstream pressure. Connects directly with loading element. Usually a diaphragm that can sense the pressure and move accordingly. Measuring Element

EXAMPLE OF SELF-OPERATED PCV Model No 95BH Self-operated PCV Measuring Element (Diaphragm) Restricting Element (Plug) Loading Element (Spring)

WORKING PRINCIPLE OF PILOT-OPERATED PCVs Basically a self-operated PCV along with a pilot. Used for higher flow rates. Fast response. Four components Restricting element Loading element Measuring element Pilot } These three components are same as for self-operated pcv

PILOT-OPERATED PCV USE OF PILOT TWO-PATH CONTROL METHOD

EXAMPLE OF PILOT-OPERATED PCV Model No 92B Pilot-operated PCV

SELECTION CRITERIA OF A PCV Outlet pressure to be maintained. Inlet pressure to the regulator. Capacity required. Shutoff capability required. Process fluid. Process fluid temperature. Pipe size required. End connection style. Material requirements.

OVERPRESSURE PROTECTION METHODS RELIEF VALVES SERIES REGULATOR WIDE-OPEN MONITOR

RELIEF VALVES Similar to Self-operated PCV except that it senses upstream pressure rather then downstream pressure. Vents process fluid to atmosphere (Usually)

SERIES REGULATOR Two PCVs are connected in cascading (head to tail) If any PCV fails, the downstream pressure will be maintained by the other PCV within safe maximum pressure.

WIDE-OPEN MONITOR Under normal conditions Monitor is fully open and Worker is in regulating mode. Both PCVs senses the same downstream pressure. Monitor has slightly high Set Pressure than Worker. Monitor Worker

APPLICATIONS OF PCV Air Steam Liquids Process gases Natural gas Water etc

THE PCV DOES NOT MATCHES THE REQUIRED FLOW CAPACITY ? WHY DAP HGG PCV FAILED Downstream Pressure was within range. Inlet pressure was perfectly suitable for the selected PCV. End connections were OK. Process fluid matches with the specifications of PCV. The pipe size were perfectly OK. Overprotection was provided by means of slam-shut. THE PCV DOES NOT MATCHES THE REQUIRED FLOW CAPACITY WHAT IS REMAINING

QUESTIONS