API 526

Covers — a purchase specification for flanged steel direct spring-loaded and pilot-operated pressure-relief valves: standard orifice designations (letters D–T) and areas, valve size/rating combinations, materials, pressure-temperature limits, and center-to-face dimensions.

Does NOT cover (exclusions) — the following are governed elsewhere:

• The sizing calculation that selects the orifice → API 520.
• Relief loads and disposal system → API 521.
• Seat-tightness testing → API 527; in-service inspection → API 576.

API 526 standardizes the physical valve envelope; API 520 tells you which orifice letter to buy.

API 526 sets minimum requirements for the standardized PRV:

• Orifice Designations — the D-through-T letter orifices with defined effective areas.
• Dimensions — center-to-face dimensions and inlet/outlet size and rating combinations for interchangeability.
• Materials & Limits — body/bonnet material options and pressure-temperature limits for each configuration.

API 526 is the dimensional purchase specification for flanged steel pressure-relief valves. By fixing the standard lettered orifices (D–T) with defined effective areas, plus inlet/outlet sizes and center-to-face dimensions, it makes PRVs interchangeable between manufacturers.

The workflow is: API 520 computes the required orifice area, you round up to the next API 526 letter orifice, and the resulting valve has a known physical envelope for the piping designer. API 527 then governs seat-tightness acceptance.

Engineering Note: Because orifice areas jump in discrete steps, the installed valve is usually oversized versus the calculated area — check that the chosen orifice does not overprotect or cause instability at low flow.