Pressure Relief Devices

Bhavik Kasundra · March 25, 2024

Engineering

All pressure vessels shall be protected by Pressure relieving device that shall prevent pressure from rising more than 10% OR 3 PSI Whichever is greater above MAWP or Design Pressure.

PRV Characteristics

Set Pressure
- Change of Set Pressure is allowable in +- 5% only. Above which Spring redesign required.
Overpressure
- Allowable Overpressure : 110 % MAWP
Blowdown
- Difference between closing pressure and set pressure

System Characteristics

Back pressure

LIFT

Cold Differential Set Pressure (CDTP)
- Pressure at which PRV Opens in test bench
- Includes correction factors for service conditions of back pressure and temperature.
Closing Pressure
- Pressure at which disc establishes contact with seat

Relief System Design Methodology

Relief Pressure should not exceed MAWP (Accumulation)

Pressure Safety Valve (PSV)
- Actuated by static press
- Rapid Opening (Pop action)
- Compressible media (Gas, Vapor, Steam)
- Refer LIFT vs PRESSURE Graph of PSV
Pressure Relief Valve (PRV)
- Static Pressure
- Proportional opening with overpressure
- Non Compressible Media (Liquids)
Thermal Relief Valve (TRV)
- Similar to PRV
- Due to thermal expansions, pipelines blocked or isolation conditions
Rupture Disc (RD)
- Flat sheet + Disc holder
- Designed based on Coeff. of Discharge Method (Valves are designed on this way?), Flow resistance method (Pressure drops due to entrance, exit sudden expansion)
- Predetermined Weak point in Pressurized condition
- Opens at burst pressure
- Does not reclose
Pilot Operated Relief Valve (PORV)
- PRV with main valve is controlled by Auxiliary relief valve
- Clean services and Operating pressure is near to set pressure
- Area Top > Area seat so seal is tight.
- Opening is Proportional to Differential Pressure
- Low Pressure, Vacuum Protection
  - Series 90, Series 9000 POPRV
Pressure and Vacuum Relief Valve (PVRV)
Weight Loaded Breather Valve

Conventional PRV have no back pressure (equals to atm pressure) and vents directly to atm.

Back pressure increases set pressure

Pv An = Fs + Pb An

Set pressure adjusting screw
Spring: Spring Steel (Cadmium Plated)
Bonnet
Guides: Steel Cadmium Plated
Trim SS316 or …
- Disc Holder
- Seat Disc
- Blowdown adjustment Ring
- Bellows (in Balanced Piston RV)
Body: Casting (CS)
Gaskets: SW or Metallic
Fasteners: High Tensile Material

1 Year after commissioning
Frequency : 2-4 years based on condition
Static and Mobile Pressure Vessels (SMPV) : LPG Bullets, Hydrogen Bullets once per year
PSV, PVSV : Steam Drum inspected by IBR once in year
Waste Heat Boilers once in 2 years

Corrosion: Incompatible metallurgy, change in service
Damaged Seating Surfaces: Corrosion, Ingress of solid particles, chattering, erosion
Failure of Springs: Weakening, Corrosion, crack, incorrect setting (overload)
Failure of Bellows
Improper Setting: Incorrect Test stand, PGs, Blowdown ring setting, test medium
Fouling: corrosive inside valve
Chattering: Spring Relief devices require 25% min flow for seat to be in open condition else it will chatter and causes valve to frequently open and close.