API 520/521 & API 526 | Surge Tank, Accumulator & Relief Valve Sizing
Understand surge tank sizing, accumulator pre-charge, and relief valve orifice calculations
This calculator sizes three types of surge control equipment: surge tanks (volume sizing), bladder accumulators (pre-charge pressure and volume), and relief valves (orifice sizing per API 520/521 and API 526).
Pressure surges (water hammer) are caused by rapid valve closure, pump trip or startup, and flow rate changes. The resulting pressure waves can exceed the pipeline MAOP and damage equipment if not properly mitigated.
Surge control equipment sizing follows API 520 and API 521 for relief valve sizing methodology, API 526 for standard orifice designations, and ASME Section VIII for pressure vessel design of surge tanks and accumulators.
Pre-charge pressure is typically set to 90% of the minimum system operating pressure using dry nitrogen. This keeps the bladder in contact with the fluid during normal operation while maximizing usable volume for surge absorption.
API 526 defines standard orifice letter designations (D through T) with effective areas from 0.110 to 26.0 square inches. After calculating the required orifice area per API 520, select the next larger standard orifice from API 526.