Pressure Relief Valve Sizing & Selection (ASME/API)

Explore how back pressure affects spring loaded pressure relief valves, contrast conventional and balanced valves, and learn how back pressure concepts like superimposed and built up pressure influence valve performance.

Compare conventional and balanced pressure relief valves, highlighting how bellows in balanced PRVs reduce back pressure effects on the disc, improving performance in high back pressure systems.

Define key concepts for pressure relief devices, including set pressure, opening pressure, maximum operating pressure, design pressure, and maximum allowable working pressure, plus overpressure and blowdown concepts.

Explore conventional spring-loaded prvs, including open bonnet designs, inlet connections, and test levers. Compare rapid pop openings for gases with gradual lifts for liquids, and note back pressure effects.

Open bonnet valves expose the spring to ambient heat, preventing set-pressure relaxation in high-temperature applications like steam; closed bonnet valves contain fluid and protect internals but may alter set pressure.

Explore the elements of a spring relief valve, including the adjusting screw, bonnet, test lever, disc with lip skirt, spring, stem, and nozzle rings that govern blowdown and set pressure.

Compare gas and liquid pressure relief valves: opening mechanisms, simmering versus reactive and momentum forces, bore control, and blowdown.

Pilot operated relief valves use a pilot valve to sense inlet pressure and lift the main valve, enabling precise set-pressure control for high capacity relief and back pressure independence.

Explore rupture disks, non reclosable relief devices that burst at a set pressure with no moving parts, including graphite and metal disks in forward and reverse acting configurations.

Explore the difference between standards and codes, and how API and ASME publications set the sizing, safety, and quality requirements for pressure relief valves.

Understand ASME code sections 1, 4, and 8 for sizing pressure relief valves on boilers and pressure vessels, setting pressures at the maximum allowable working pressure with 10 percent overpressure.

ASME certification for pressure relief valves requires a QA program, on-site joint reviews, and National Board certification, plus registration, independent testing, and periodic reverification to maintain the stamp.

Explore API standards as voluntary guidelines for sizing, installation, and maintenance of pressure relief valves in the petroleum industry, including API 520, 521, 526, 527, and 2000.

Assess reasonable overpressure by examining single jeopardy scenarios, control system failures, valve malfunctions, anticipated deviations, historical hazards, and regulatory requirements, while distinguishing from double jeopardy and unlikely events.

Double jeopardy means two unrelated causes of overpressure occur simultaneously, with no process, mechanical, or electrical link, as fire exposure with heat exchanger rupture or operator error with power failure.

Instrumentation cannot replace a physical pressure relief device in most cases; API standards allow it only when relief devices are impractical, and such safeguards must meet safety integrity level 3.

Assess closed outlet overpressure scenarios for pumps and vessels; size safety relief valves or restrictive orifices to keep pressure below the maximum allowable working pressure and prevent damage.

Explore a closed-outlet tank heat exchanger scenario where absent venting causes overpressure, and learn how a safety relief valve protects MAWP with correct set pressure and capacity.

Analyze inadvertent valve opening that can overpress steam systems and heat exchangers. Size safety relief valves for worst-case scenarios, including tube ruptures and closed-outlet vapor generation.

Check valve failure can cause reverse flow and tank overpressure; apply overpressure protection and size the safety relief valve using the valve’s CV, or assume zero resistance if unavailable.

Analyze utility failure scenarios causing overpressure and demonstrate sizing safety relief valves for set pressure and vent capacity to handle all vapor loads under complete or partial failures.

Plugged vent filters in vent lines raise pressure; maintain filters to prevent exceeding the maximum allowable working pressure, and size a safety relief valve to relieve the vent.

Assess chemical reactions, including exothermic and runaway events, by estimating vapor generation and two-phase effects to size relief valves for worst-case overpressure, and account for hydraulic expansion in heated liquids.

Explore how a plant fire causes overpressure by vaporization or gas expansion in tanks, and how to size safety relief valves using vaporization rates per API guidelines.

Calculate the vaporization rate for a fire in a liquid-filled vessel by using wetted surface area and heat absorption per API guidelines to size the pressure safety valve.

Size and select pressure relief valves for gas-filled vessels under fire exposure using ASME/API methods, noting choked flow and gas-vs-liquid distinctions.

Api 2000 and Api 521 guidelines for fire relief sizing are covered, noting open area and isolated vessel assumptions and differences in heat absorption for liquids and gases.

A pressure safety valve is not a one-size-fits-all solution; water hammer, steam hammer, and condensate-induced hammer show slow response and require pulsation dampeners or surge valves, proper piping, and training.

Examine underpressure in tanks caused by outflow, evaporation, and condensation, and apply protection strategies like vents, vacuum relief devices, and nitrogen purge to maintain safe pressure.

Develop a comprehensive relief system package that defines the plant relief strategy, identifies protected equipment, and analyzes overpressure scenarios to guide valve sizing and documentation per API/ASME standards.

Learn preliminary sizing of pressure relief devices using ASME/API guidelines, including effective versus actual area, differential test pressure, and basic relief calculations for gas, steam, and liquids.

Explore API and ASME sizing guidelines for pressure relief valves. Read the sizing figure to determine set pressure, MAWP, and overpressure for single, multiple, and fire scenarios.

Maintain PRV piping losses at <=3% of set pressure to prevent chattering and valve damage, and reduce losses by shortening or enlarging the pipe; include rupture disk losses if present.

Determine cold differential test pressure to ensure the relief valve opens at the correct service pressure under real conditions, accounting for back pressure and temperature effects.

Compare the effective area and effective coefficient of discharge with the actual area and rated coefficient of discharge, using this distinction for preliminary sizing before verifying flow with manufacturer data.

Explore sizing equations for gas flow through a pressure relief valve, determining critical versus subcritical flow, and applying area calculations using the ideal gas specific heat ratio k.

Apply pressure relief valve sizing equations in a critical flow example to determine relieving pressure, compute 11.8 mm^2 area, and select standard size (71 mm^2, designation d) per apa 5-6.

Determine subcritical PRV sizing by computing the initial effective orifice area with V, T, Z, SG, P1, P2, and f2, then apply CD and KC corrections.

Explore subcritical flow in PRV sizing, analyzing vent stack back pressure, API sizing equations, and selecting standard orifice sizes for a safety valve in overpressure scenarios.

Size steam pressure relief valves using the PRV sizing equation, incorporating upstream pressure, discharge coefficient, back-pressure, rupture disc, and superheat corrections.

Demonstrates sizing a spring-loaded pressure safety valve for steam in a reactor vessel, from mass flow to the optimal orifice size ensuring pressure stays under 110% of MAWP.

Explore liquid pressure relief valve sizing using the preliminary orifice area equation, including corrections for back pressure, ruptured disks, viscosity, and Reynolds effects, with non-viscous and viscous examples.

Apply PRV sizing equations for a high-viscosity liquid at 100 m³/h to determine the effective orifice area, including viscosity correction factor and designation chart iterations, finalizing 506.5 mm² (designation H).

Explore rupture disc sizing guidelines per ASME/API, including marked burst pressure, MAWP, and accumulation limits for single or two rupture discs, with fire-exposure considerations and limits up to 121%.

Calculate the rupture disc burst pressure range from a 100 psi g marked value with +6%/-4% manufacturing tolerance and ±5% burst tolerance to assess suitability against the maximum operating pressure.

Calculate the rupture disk burst pressures, determining upper and lower limits from tolerance. Use these values to evaluate the vessel's maximum allowable working pressure and disk selection.

Match rupture disc selection to system mawp, fluid state, and temperature sensitivity. Assess cycling, relieving capacity, placement, and configuration to specify burst pressure, material, and tolerances.

Size rupture disks using coefficient of discharge method (CD ≈ 0.62), ensuring discharge to atmosphere within eight pipe diameters, and that rupture disk's net flow area meets calculated orifice area.