API 526 lists fourteen standard safety relief valve orifice letters, D through T, with effective areas stepping from 0.110 in² (0.71 cm²) at the D end to 26.000 in² (167.74 cm²) at the T end [S2][S3].
Engineers use API 520 Part I to calculate the required orifice area in square inches for a given relieving scenario, then pick the next-larger API 526 letter on the purchase order so the certified valve capacity meets or exceeds the required relief load [S1][S3].
What the fourteen letters actually cover
The D-through-T series skips the letter I to avoid visual confusion with the numeral 1, and letters A, B, C are reserved for non-reclosing rupture disc devices, not spring-loaded PRVs [S3]. Each letter pairs with a standard inlet/outlet flange combination defined in API 526: D is 1" × 2", H is 2" × 3", L/M/N share 4" × 6", P is 6" × 8", Q is 6" × 10", R is 8" × 10", and T is 10" × 16" [S3].
The full sequence runs D 0.110, E 0.196, F 0.307, G 0.503, H 0.785, J 1.287, K 1.838, L 2.853, M 3.600, N 4.340, P 6.380, Q 11.050, R 16.000, T 26.000 in² [S2][S3]. The effective area is approximately 90% of the actual curtain (geometric) area, reflecting the real-world discharge coefficient built into the API 520 sizing equations [S3].
Manufacturers are not required to drill a bore equal to the effective area; they are only obliged to certify that the finished valve delivers a flow rate equal to or greater than the value produced by the API formula at that letter [S2]. That distinction is where most spec surprises begin.
API 520 sizing vs API 526 letter vs ASME bore
API 520 Part I treats the D-through-T areas as nominal sizing anchors for early project selection, not as the actual machined flow area inside a delivered valve [S5]. API 526 is the purchasing document that lets a buyer order a standardised, interchangeable SRV by letter without custom engineering [S3].
When relief load calculations land between two letters, the standard practice is to round up to the next-larger API letter, which already biases the selected valve toward overcapacity [S5]. Layered on top, the certified ASME discharge coefficient is derated by a 0.9 factor, and the actual ASME bore is typically about 16% larger than the API nominal area for every letter except T [S5]. The combined effect: a Dresser liquid relief valve in one published comparison flowed roughly 20% above the API-rated capacity at every letter except T, and a Farris design was higher by as much as 64% at the D orifice [S5].
Process engineers who only use API 520 numbers for downstream pipe sizing risk undersizing both inlet and outlet piping, the flare header, scrubbers, and catch tanks; acoustic and flow-induced vibration can also go undetected until commissioning [S5]. Running the front-end engineering against the ASME bore area, not the API letter, is the practical mitigation [S5].
Which relief scenario drives each orifice
Fire case, blocked outlet, heat exchanger tube failure, and runaway reaction are the four sizing cases API 520 Part I requires the engineer to evaluate; the worst case sets the required area and therefore the letter [S3]. Small D and E orifices (0.110-0.196 in²) typically protect instrument skid accumulators and small liquid service vessels; F through K (0.307-1.838 in²) handle chemical reactor relief and small refinery columns; L through N (2.853-4.340 in²) cover mid-size reflux drums and hot oil systems; P and Q (6.380-11.050 in²) appear on hydrocracker reactors and amine contactors; R and T (16.000-26.000 in²) are reserved for large fractionator overhead relief and steam drum safety valves on utility boilers [S3].
For a steam-driven fire case on a 30,000-barrel atmospheric storage tank, the required area routinely lands in the Q-to-R range, while a 2,000-gallon amine surge drum on tube failure typically lands at K or L. Matching the letter to the controlling scenario is the engineer-side decision; the manufacturer-side decision is whether to certify a conventional spring-loaded, balanced-bellows, or pilot-operated design at that letter [S3].
Conventional, bellows, and pilot-operated: the letter is not the design
Conventional spring-loaded PRVs are the default: a direct spring holds the disc on the seat, the valve pops when inlet pressure overcomes the spring set force, and the popping point is sensitive to superimposed back pressure [S3]. Balanced-bellows designs isolate the spring chamber from the outlet, so built-up back pressure does not change the setpoint; this is the spec for systems with variable header back pressure, such as a manifolded flare with multiple PRVs sharing a common outlet [S3].
Pilot-operated PRVs use a smaller pilot valve to load and unload the main piston, allowing the D-through-T letter to be delivered in a much smaller face-to-face envelope and at set pressures closer to 90% of MAWP without chatter [S3]. The orifice letter is the same on the nameplate, but the geometry and the manufacturer's certified Kd differ; mixing API Kd with a vendor's ASME area, or vice versa, is the most common mistake in retrofit specs [S5].
Conventional valves are for clean service and stable back pressure; bellows where back pressure varies more than about 5-10% of set; pilot-operated when set pressure must exceed 90% of MAWP or when drip-tight shutoff is required on light hydrocarbon or toxic service. The letter you write on the PO is identical across all three; the Kd used in the capacity check is not.
Common spec traps and how to avoid them
Trap 1: ordering by letter without confirming the certified Kd. The API Kd is a sizing default, not a guarantee; the manufacturer's ASME-certified coefficient can be higher, which inflates both flow and pressure drop downstream [S5]. Trap 2: rounding up twice. If the API 520 area is calculated at 0.93 in² and the engineer jumps from G (0.503) past H (0.785) to J (1.287), the valve is now 1.4× the required area; piping and the safety relief valve outlet header should be rechecked at the actual flow, not the API letter flow [S3][S5].
Trap 3: treating T as a universal answer. T is 26.000 in², but it is not a license to skip the fire case calculation; on a large storage sphere, even T can be insufficient and a 16" × 20" or larger valve outside the standard letter set is required, with reduced certified capacity. Trap 4: ignoring the rupture disc interaction. When a rupture disc is installed upstream of a PRV, the certified capacity is multiplied by 0.9 (or per the manufacturer's combination rating) and the letter selection must absorb that derate from the start [S3].
Best practice: state the API 520 calculated area, the next-larger API 526 letter, the ASME certified Kd from the named vendor, and the resulting certified capacity on the same datasheet row, so the downstream pipe specifier is not working from the API number alone [S5]. Front-end engineering that runs on the ASME bore, not the API letter, avoids the 2.7× pressure-drop overshoot seen in the Farris comparison and keeps the fire safety and disposal train correctly sized [S5].
Sourcing and standards landscape
API 526 is the purchasing specification for flanged steel pressure relief valves and is the source of the D-through-T letter list, face-to-face dimensions, flange ratings, and inspection requirements [S3][S7]. API 520 Part I is the sizing and selection companion, and Part II covers installation; together they are the two API documents engineers cite on every PRV datasheet [S3].
API 526 was last published in its 7th edition, with the orifice letter table (Table 1) identical to the area list above; the API-published 526 PDF still references "D" and "T" effective orifice areas as the bookends of the spring-loaded PRV range [S7]. Third-party engineering guides reproducing the table show D at 0.110 in² and T at 26.000 in² without variation, confirming the D-through-T set is stable across recent editions [S2][S3][S6][S7].
ASME Section VIII (UG-127 through UG-137) governs the overpressure protection requirement on pressure vessels that drives the letter selection in the first place; the orifice letter is the procurement answer to the ASME protection mandate. Machine safety and safety certification frameworks for the broader plant rely on these same valves as the last line of defence after ESD and operator intervention, which is why the letter and certified Kd audit trail is non-negotiable at handover [S1].
Trackable signals to watch through 2026: any API 526 revision that adds or retires a letter, any change to the certified Kd derate factor, and any update to the API 520 fire-case wetted area method that shifts the D-through-T distribution on storage tank specs.
See also our earlier report, Deck-to-wall membrane termination: heights, laps, and edge protection.