Safety Valve vs Pressure Relief Valve: Differences, Standards & Selection

Pressure relief devices are the last line of defence against vessel overpressure — when all other control and safety systems fail, the relief valve must open and prevent catastrophic rupture. Understanding the distinction between safety valves, relief valves, and safety relief valves is not merely academic: specifying the wrong type or undersizing the orifice creates a compliance gap that leaves equipment unprotected and personnel at risk. This guide resolves the terminology confusion and provides a systematic selection framework.

Definitions — Safety Valve vs Relief Valve vs Safety Relief Valve

Per ASME Section VIII and API 520, these terms have precise meanings. A Safety Valve (SV) opens rapidly (pops fully open) when set pressure is reached — it is used for compressible fluids (steam, gas, vapour). The opening action is rapid and 'snap-acting': it goes from zero lift to full lift within 3% of set pressure overpressure. A Pressure Relief Valve (PRV) opens proportionally to overpressure above set point — it is used for incompressible fluids (liquids). The opening action is 'modulating': lift increases with overpressure, and the valve recloses as pressure returns to set. A Safety Relief Valve (SRV) can function as either a safety valve (pop action) or a relief valve (modulating), depending on construction and application — used for applications where the phase may change from liquid to vapour.

ASME Code Jurisdiction — Section I vs Section VIII

ASME Boiler and Pressure Vessel Code Section I (Power Boilers) governs safety valves on power boilers. Key requirements: accumulation limit of 3% above MAWP (maximum allowable working pressure); full lift must be achieved within 3% overpressure accumulation; all Section I safety valves must carry the ASME 'V' stamp (drum) or 'UV' stamp is NOT applicable — boilers specifically require 'V'. ASME Section VIII (Pressure Vessels) governs safety/relief valves on unfired pressure vessels: accumulation limit of 10% (single relief device); 'UV' stamp required (or 'UD' for rupture discs). Important: 'V' stamped safety valves may be used on Section VIII vessels, but 'UV' stamped valves cannot be installed on Section I boilers without additional certification.

API 520 and API 526 — Refinery and Process Industry Standards

API 520 Part I (Sizing, Selection, and Installation of Pressure-Relieving Devices — Sizing and Selection) provides the methodology for calculating required relieving capacity in vapour, liquid, and two-phase service for refinery and process industry vessels. API 520 Part II covers installation requirements. API 526 (Flanged Steel Pressure-Relief Valves) is the procurement specification — it defines standard orifice areas (designated A through T), standard inlet and outlet flange ratings and sizes, face-to-face dimensions, and required materials and testing. Using API 526 orifice designations ensures interchangeability between manufacturers — an 'F' orifice (0.307 in²) from any API 526 qualified supplier is dimensionally identical.

API 526 Orifice Designations

Sizing Methodology — API 520 Part I

For vapour/gas service, API 520 sizing uses: A = W / (C × Kd × P1 × Kb × Kc) × √(TZ/M), where A is effective orifice area (in²), W is required relieving capacity (lb/hr), C is a function of specific heat ratio k, Kd is discharge coefficient (typically 0.975 for gas, 0.65 for liquid), P1 is relieving pressure in psia (set pressure + accumulation), Kb is back pressure correction, Kc is combination correction factor (0.9 if rupture disc upstream). For liquid service: A = Q / (38 × Kd × Kw × Kc × Kv) × √(SG/ΔP), where Q is in USGPM. Always size at 10% accumulation for fire case (ASME allows 21% accumulation for external fire).

Pressure Setting, Blowdown, and Simmer

Set pressure is the inlet pressure at which the safety valve starts to open. Blowdown is the pressure differential between opening (set) and reclosing — it is set by adjusting the blowdown ring in the valve. API 526 requires blowdown of 7–10% of set pressure for steam service, and ≤ 10% for gas service. Simmer or simmering describes partial lift before full pop — if a safety valve simmers frequently, the set pressure is too close to normal operating pressure. The ASME-recommended margin between normal operating pressure and set pressure is minimum 10% above operating pressure, or 25 psi, whichever is greater. Excessive simmering causes seat damage and premature wear.

Balanced Bellows vs Conventional Safety Valves

Conventional safety valves are sensitive to back pressure — elevated back pressure reduces effective set pressure and relieving capacity. When variable or constant back pressure exceeds 10% of set pressure, a balanced bellows valve is required. The bellows (metallic expansion capsule) connects the bonnet to the back pressure source and equalises the pressure forces on the disc, making the opening pressure independent of back pressure variations. Balanced bellows valves are required when: built-up back pressure exceeds 10% of set pressure in conventional valves; multiple safety valves discharge to a common header; or when the relief system design results in variable back pressure depending on which valves are open.

Safety Valve Selection Checklist

• Identify fluid phase at relieving conditions: vapour → safety valve (pop action); liquid → pressure relief valve (modulating); two-phase → safety relief valve
• Identify ASME code jurisdiction: Section I boiler → 'V' stamp; Section VIII vessel → 'UV' stamp
• Calculate required relieving area per API 520 for all applicable cases: blocked outlet, fire case, control valve failure, utility failure
• Select API 526 orifice designator (D through T) that meets or exceeds required area
• Check back pressure: if > 10% of set pressure, specify balanced bellows design
• Specify body material per service: WCB for steam/gas, CF8M for corrosive service, carbon steel for hydrocarbons
• Specify nozzle and disc material: SS 316, Hastelloy, or PTFE-coated for corrosive service
• For IBR service (India): specify IBR Form III-A certification and EN 10204 3.2 MTCs