API Standard 520, Part I is the primary engineering reference for sizing pressure-relieving devices on fired heaters, pressure vessels, and heat exchangers, and the resulting orifice area is rounded up to the next API 526 standard letter size (D through T) before the installation is checked against API 520, Part II [S1][S5].
A correct API 520 sizing pass produces five deliverables: a documented required relief rate W or Q, a set pressure fixed against the vessel MAWP, an effective discharge area rounded up to a standard orifice, a verified inlet line pressure drop at 3% of set pressure, and a backpressure check that protects the valve from capacity loss or chatter [S1][S2].
Step 1, Build the Overpressure Case and Fix the Set Pressure
API 520 sizing begins with the design overpressure case, because the relief rate is the input to every downstream calculation, and the set pressure is fixed at the maximum allowable working pressure (MAWP) of the protected equipment plus the code-allowed accumulation [S3].
For a single PRV on a vessel exposed to a single contingency, set pressure is MAWP x 1.10; for multiple devices protecting the same vessel the figure rises to MAWP x 1.16; and for the fire case the accumulation climbs to MAWP x 1.21, which is the worst-case relief rate the valve must pass [S2]. Inlet line pressure drop from the vessel nozzle to the valve body must be held at or below 3% of set pressure, otherwise the valve senses a pressure lower than the vessel and fails to open at the design set point [S1].
Step 2, Gas and Steam Sizing: the Compressible Flow Equation
For vapour, gas, and steam service the required effective discharge area is calculated with the API 520 compressible flow equation, which uses the rated coefficient of discharge Kd, a backpressure correction Kb or Ksh, a superheat correction Kn for steam, and a cap factor Kc when a rupture disk is installed upstream [S2].
The general form of the equation for vapours is A = W / (Kd x Kb x Kc x Ksh x P1) x sqrt(T1 / M), where W is the required mass flow, P1 is the upstream relieving pressure, T1 is the inlet temperature, and M is the molecular weight, and selecting the right Kb requires knowing whether the valve is conventional, balanced-bellows, or pilot-operated because each reacts differently to superimposed backpressure [S2][S5]. For a conventional PRV, the standard cautions that superimposed backpressure above about 10% of set pressure will pop the valve early and the capacity is derated; balanced-bellows valves tolerate higher superimposed backpressure up to roughly 50% of set pressure, and pilot-operated valves are the only type that can handle backpressure approaching the set pressure itself [S5].
Step 3, Liquid Sizing: Viscosity, Backpressure, and Capacity Certification
For liquid service the ASME Code requires capacity certification, and API 520 supplies the certified liquid equation A = Q / (38 x Kd x Kw x Kc x Kv) x sqrt(G1 / (P1 - P2)), where A is the required effective discharge area in square inches, Q is flow in gallons per minute, G1 is the specific gravity at flowing temperature, P1 is the upstream relieving pressure, and P2 is the total backpressure, all in psig [S4].
The correction factors do real work and should not be defaulted: Kw handles backpressure, with 1.0 for atmospheric discharge and a value read from Figure 31 of API 520 for balanced-bellows valves in backpressure service; Kc is 1.0 with no rupture disk and 0.9 when a rupture disk is installed upstream without a certified combination; and Kv is a viscosity correction that drops below 1.0 as the liquid thickens and is calculated from the iterative Reynolds number formula Kv = (0.9935 + 2.878/Re^0.5 + 342.75/Re^1.5)^-1.0 [S4]. For viscous service the standard instructs the engineer to first size the valve as if Kv = 1.0, then take the next larger API 526 standard orifice, recompute the Reynolds number, and only then apply the actual Kv factor, otherwise the iteration can be unstable and undersize the device [S4].
Step 4, Select the API 526 Standard Orifice
Once the effective discharge area is known, the engineer rounds up to the next larger API 526 standard orifice, and the standard letter designations run D, E, F, G, H, J, K, L, M, N, P, Q, R, and T, with the larger letters corresponding to larger bore areas and higher rated capacities [S5]. API 526 is a procurement and dimensional document, not a sizing document; it is what tells the purchaser that an "F" orifice on a 2-inch Class 300 flanged steel body has a specific effective area, a specific face-to-face dimension, and a specific pressure-temperature rating, and without it the calculated area cannot be matched to a real, certified valve [S5].
Sizing errors almost always come from the wrong direction: an engineer matches the calculated area to the existing vessel nozzle or pipe size, which routinely forces the wrong orifice letter and silently derates the valve, and PDH Online's course material flags this as one of the most common field mistakes [S3]. A 4.0 in^2 calculation should pick an "L" or larger orifice, not whatever 2-inch line is already welded to the vessel [S3].
Step 5, Inlet Piping, Backpressure, and the Part II Installation Check
API 520 Part II is a 55-page installation document, and the two clauses that catch most field problems are the inlet piping diameter check and the outlet backpressure check, both of which use the actual pipe geometry rather than assumed pressures [S1]. The inlet pipe run from the vessel to the valve must be sized so that the pressure drop at the rated relief flow is at or below 3% of set pressure; if the line is too long, too small, or full of elbows, the valve will not see the vessel's true pressure and will lift late or chatter [S1][S2].
Outlet backpressure is split into superimposed (constant, present at all times) and built-up (the variable increase that occurs while the valve is open), and the capacity correction Kb or Ksh is read from figures in API 520 Part I as a function of the ratio of total backpressure to set pressure; balanced-bellows and pilot-operated geometries are required once the ratio exceeds the conventional valve's threshold [S2][S5]. For a clean review the engineer pairs these checks with the reference baseline on safety relief valve sizing and the broader machine safety envelope that includes the upstream process isolation.
Comparison: Which API 520 Sizing Branch Applies
Three service branches, three equation sets, and three different correction factors drive the calculation, and the table below is the decision shortcut most engineers actually use on a sizing sheet [S1][S2][S4][S5].
Gas/vapour service uses the compressible flow equation with Kd, Kb, Ksh, Kn, Kc; liquid service uses the certified liquid equation with Kd, Kw, Kc, and Kv, and a viscosity iteration for Re under 10^4; two-phase and steam high-quality flows are flagged as out of scope for the standard gas or liquid equation and are typically handed off to API 521 methods or a depressuring tool [S2][S5]. A pilot-operated PRV is the only geometry that can be specified when total backpressure will approach set pressure during the relief event, and that decision is made before the orifice is selected, not after [S5].
Common Failure Modes and Limits of the API 520 Method
API 520 assumes a single-phase fluid, a subsonic relief event, a stable inlet pressure, and a properly installed rupture disk if one is used, and the standard is explicit that it does not cover special applications that require unusual installation considerations [S1]. Two-phase flashing flow, subsonic-to-sonic transitions across the valve, and reactive chemistry in the discharge pipe are all outside the Part I equations, and the engineer is expected to escalate those cases to API 521 or to a dynamic simulation tool [S5].
Three quantitative failure modes recur in field audits: an undersized orifice caused by matching the existing nozzle size instead of rounding up to the next API 526 letter [S3]; a choked inlet that drops more than 3% of set pressure before the valve body, which the valve reads as "not yet at set pressure" and refuses to open [S1][S2]; and a conventional PRV installed on a header where built-up backpressure at the worst-case relief rate exceeds the manufacturer's published Kb curve, derating the valve to a fraction of its nameplate capacity [S2][S5]. Each of these is detectable in the API 520 Part II review before the valve is welded into the line.
Sourcing, Standards, and What a Sizing Sheet Has to Cite
An API 520 sizing sheet is a legal-safety document, and the references that must appear on the calculation page are API Standard 520 Part I (sizing and selection) and Part II (installation) [S1], API Standard 526 for the selected orifice letter and rated area [S5], the ASME Section VIII Division 1 basis for the vessel MAWP and the allowed overpressure [S3], and, for liquid service, the rated Kd from the manufacturer's capacity certification per the ASME Code [S4]. When the same design passes through a downstream flare, the API 520 deliverable is paired with the API 521 hydraulic check on the flare header, and a copy of the relief load case is filed with the PHA package for the unit [S5].
For engineers moving between adjacent mechanical disciplines, the same rounding-up discipline that drives an API 526 standard orifice selection shows up in how to read a gasket material selection chart for chemical compatibility and in plunger pump flow rate from stroke and diameter, where the next standard size up is the conservative answer the safety case is built on. Track the next two signals on any new project: whether the project specifies a pilot-operated PRV for any case where total backpressure at relief rate exceeds 50% of set pressure, and whether the inlet line is mechanically rechecked at the rated relief rate, not at normal flow, before the line is approved for hydrotest [S1][S2][S5].
For component-level specifications, see fire safety.