Specifying a check valve on a compressed-air header is a line-by-line engineering job, not a generic "non-return valve" request: every omitted field on the RFQ tends to come back as either a price uplift or a requote cycle. The compressed-air market has settled on two dominant construction families — disc/spring and piston/spring — with published pressure ratings from 10 bar to 16 bar and temperature ceilings from 60 °C to 150 °C depending on body and seal [S1][S2].
The first concrete answer: a buyer who needs more than 10 bar working pressure, more than 60 °C, or pharmaceutical/chemical compatibility should write the line for a stainless disc check valve (AISI 316 or 304 body, female-female BSP, DN 1/4" to 4", PN 16); a buyer who only needs shop-air duty up to 10 bar and 60 °C in brass or stainless can write the line for a piston check valve (BSP female, DN 1/8" to 1/2", PN 10) [S1][S2].
Header duty profile: pressure, temperature, and what they kill
A factory compressed-air header typically runs 7 bar to 10 bar working pressure with surge spikes from receiver tanks and reciprocating compressors; pharmaceutical and chemical plants push the same valves onto headers carrying dry, oil-free air at 8 bar to 12 bar with line temperatures that can reach 80 °C near the compressor outlet and drop to 20 °C at the far end [S1]. The SFERACO 326-series disc check valve is rated -10 °C to +150 °C and 16 bar (232.1 psi), which covers almost every air-header envelope a buyer is likely to write on an RFQ [S1].
The SAF S.R.L. VU/VUXFF piston check valve is rated -10 °C to +60 °C and 2 bar to 10 bar (29 psi to 145 psi), with DN 0.125 in, 0.25 in, 0.375 in, 0.5 in as the standard port set [S2]. A buyer who copies those four DN sizes onto an RFQ without checking his own header is likely to miss the size: most headers larger than DN 15 (1/2") need a disc-pattern body, not a piston body, because piston poppets lose tight shut-off above DN 1/2" at the lower cracking pressures typical of air service.
RFQ line, field by field: what to write and why
Line 1 — Service: write "compressed air, dry (ISO 8573-1 [-:-:-] class to be specified)" rather than "air"; oil-laden or wet air is the most common reason a disc check valve seat fails within months, and the RFQ should force the supplier to confirm seal compatibility [S1]. Line 2 — Body material: AISI 316 for chemical/pharma headers, AISI 304 for general plant air, brass only for non-corrosive shop air under 10 bar [S1][S2]. Line 3 — End connection: "BSP female-female, cylindrical thread (ISO 228)" — this matches the 326-series spec sheet exactly and removes a thread-standard ambiguity that otherwise becomes a quote exception [S1].
Line 4 — Size: write the DN in both inches and millimetres (e.g. DN 1/2" / DN 15) and the bore; the 326-series is offered from DN 1/4" to DN 4" in AISI 316 and AISI 304 [S1]. Line 5 — Pressure rating: state the required PN and the design pressure, and ask the supplier to confirm minimum backpressure for tightness — the 326-series is 0.5 bar up to DN 1-1/2" and 1 bar from DN 2" to DN 4", which is a non-trivial crack-to-seal step the buyer has to size his system against [S1]. Line 6 — Temperature: minimum and maximum operating temperature with the fluid, not the ambient. Line 7 — Mounting orientation: write "all positions, respect flow arrow" if the disc pattern is selected — the 326-series is specified to mount in any orientation [S1].
Disc vs piston vs swing: a three-way comparison
For compressed-air header duty the practical shortlist is disc, piston, and occasionally swing check valve on large mains; the selection pivots on four numbers. (1) Pressure ceiling: disc 16 bar, piston 10 bar, swing typically PN 16 but at air-flow cracking pressures the disc closes faster and prevents slam [S1][S2]. (2) Temperature ceiling: disc 150 °C, piston 60 °C, swing 200 °C+ but oversized for small air headers. (3) Minimum DN: disc starts at DN 1/4", piston starts at DN 1/8", swing rarely offered below DN 2" [S1][S2]. (4) Minimum backpressure for tightness: disc 0.5 bar to 1 bar (size-dependent), piston typically higher and more sensitive to wear.
SFERACO's published spec captures the disc pattern exactly: "Stamless Steel stamped check valve, all positions (respect the flow direction indicated by the arrow), short length, minimum backpressure for tightness 0.5 bar up to DN 1-1/2, 1 bar from DN 2" to DN 4"" [S1]. The SAF S.R.L. piston line is "uni-directional valves from brass hexagonal bar, female threaded, in-line mounting, allow the compressed air to flow in one direction only" [S2]. For the buyer, the rule of thumb is: DN ≤ 1/2" and PN ≤ 10 bar → piston (brass or stainless); DN ≥ 3/4" or PN > 10 bar or T > 60 °C → disc (AISI 304/316).
Cracking pressure, backpressure, and the seat-tightness trap
The single most-often-missed line on a check-valve RFQ is minimum backpressure for tightness, because the figure is not a vendor preference — it is a system-design constraint. The 326-series disc valve needs at least 0.5 bar of reverse differential to seal DN 1/4" to DN 1-1/2", and that figure rises to 1 bar for DN 2" to DN 4" [S1]. On a receiver-downstream header where the receiver is at atmospheric relief during a compressor stop, the valve may pass air backwards for the first 0.5 bar to 1 bar of decay — a buyer who has not written this line ends up debugging a "leak" that is actually the check valve doing what its data sheet says it will do.
Spring material is the second trap: most disc-pattern air check valves use a stainless spring, but at sustained temperatures above 120 °C the buyer should request spring material on the RFQ rather than assume it. Elastomer seat is the third trap — FKM (Viton-class) is the default for hot pharmaceutical air, NBR is the default for general plant air, and EPDM is wrong for hydrocarbon-contaminated air. The 326-series spec sheet does not publish the seat elastomer, so a buyer who needs a specific compound must add a line: "seat elastomer: ____ (FKM/NBR/EPDM) — confirm in quote" [S1].
Standards, end connections, and what to pin on the RFQ
For an air-header check valve the standards discipline is shorter than for a process check valve, but the buyer still has to pin four things. End connection: ISO 228 (BSP cylindrical) is what the 326-series and the VU series both use, so writing "BSP female to ISO 228" removes the NPT-versus-BSP ambiguity that drives half the cross-border requotes [S1][S2]. Thread standard: do not assume the supplier will switch to NPT for a US shipment — write "BSP unless otherwise agreed". Body material traceability: for pharmaceutical headers, write "EN 10204 3.1 certificate required" so the quote includes mill traceability for the AISI 316 body.
Air quality: write "compatible with ISO 8573-1 Class [__] air" and fill in the class — this drives the seat and lubricant choice. Pressure test: the SFERACO spec sheet does not publish a hydrostatic test pressure, so the buyer should add "shell test 1.5 × PN, seat test 1.1 × PN, hold 60 s" on the RFQ line. The piston check valve range at 2 bar minimum working pressure is itself a flag — the VU/VUXFF will not open below 2 bar (29 psi), and on a low-pressure branch header that is a real flow restriction, not a footnote [S2].
RFQ mistakes that force requote cycles
Three patterns account for most of the "we need a new quote" emails on check-valve RFQs. First, missing fluid code: writing "check valve" without naming compressed air lets the supplier default to a water-service body (often cast iron, wrong elastomer) that has to be re-quoted in stainless [S1][S2]. Second, missing temperature: 60 °C-rated piston valves are the default if the RFQ is silent on temperature, and they will be undersized for any header near a compressor outlet [S2]. Third, missing cracking/backpressure: a buyer who does not write "minimum backpressure for tightness ≤ 0.5 bar at DN ≤ 1-1/2"" will receive quotes on swing-pattern valves that need gravity plus 1 bar to 2 bar to reseat, and a disc-pattern requote will follow [S1].
For a tighter procurement cycle, copy the 326-series spec line shape directly: "Disc check valve, AISI 316 body, BSP female-female to ISO 228, DN [__], PN 16, -10 °C to +150 °C, minimum backpressure for tightness 0.5 bar (DN ≤ 1-1/2") or 1 bar (DN ≥ 2"), all-position mounting, EN 10204 3.1, suitable for ISO 8573-1 Class [__] compressed air" [S1]. For the smaller DN 1/8" to DN 1/2" branch duty, the equivalent line shape is: "Piston check valve, brass or AISI 316 body, BSP female to ISO 228, DN [__], PN 10, -10 °C to +60 °C, cracking pressure [__] bar, suitable for ISO 8573-1 Class [__] compressed air" [S2]. A flow sensor spec on the same header line is a separate procurement track and is covered in flow sensor selection criteria.
Trackable signals for the next buying window: watch the published 326-series minimum backpressure figures (0.5 bar up to DN 1-1/2", 1 bar from DN 2" to DN 4") in any 2026/2027 catalogue revision, and confirm whether the 326/327 AISI 316 versus AISI 304 price gap has narrowed for European buyers under the 2026 stainless surcharge cycle [S1].
For component-level specifications, see air solenoid valve.