DA21F-40P Cryogenic Safety Valves

| Product Overview

The DA21F-40P is a spring-loaded direct-acting safety relief valve purpose-built for cryogenic service, representing the small-bore (DN10 equivalent, M16×1 swivel thread) branch of the DA family alongside the larger DA08F-40P (8 mm orifice, full-lift). The "21F-40P" designation follows the manufacturer's naming convention: DA for safety relief valve (from the Chinese ), 21 denoting the product series/family, F indicating the swivel thread connection type, and 40P signifying the 40 bar (~580 PSI) pressure class. The sub-models—DA-10, DA-10A1, and DA-10A2—differ only in factory-set relief pressure ranges, sharing an identical 96 mm tall × Φ23 mm body envelope. This single-size, multi-range design strategy minimizes inventory SKUs while covering the full spectrum of cryogenic vessel and piping thermal relief requirements from 45 PSIG (low-pressure liquid nitrogen dewars) to 450 PSIG (high-pressure LNG fuel tanks).

The DA21F-40P introduces three design features not commonly found in entry-level cryogenic safety valves: tamper resistance, a quantified reseat specification, and swivel thread connections. The tamper-resistant design is implemented through the stainless steel cap (component 7), which fully encloses the brass adjusting nut (component 4) and spring retainer (component 5). Unlike conventional safety valves where the adjusting screw is accessible with a standard wrench, the DA21F-40P's cap must be physically removed—potentially requiring a special tool or leaving visible disassembly marks—before the set pressure can be altered. This satisfies the operational requirement for unattended cryogenic installations where unauthorized set-point tampering could silently disable overpressure protection.

The reseat specification—"reseat above 85% of set pressure"—is a quantified performance guarantee that distinguishes the DA21F-40P from generic safety valves that may only specify opening pressure. A safety valve that opens at, say, 300 PSIG but reseats at 180 PSIG (60% of set point) will vent far more process gas than one that reseats at 260 PSIG (87% of set point) before closing. Over hundreds of thermal cycles in a cryogenic tank's daily pressure build-up and relief cycle, the cumulative gas loss difference between an 85% reseatvalve and a 60% reseat valve can amount to thousands of cubic feet of vented product per year—a direct operating cost that many specifiers overlook. The DA21F-40P's 85% minimum reseat is achieved through careful spring rate selection, low-friction brass-on-stainless sliding pairs in the disc-guide interface, and the swivel thread connection that eliminates pipe-stress-induced body distortion as a source of disc misalignment.

The "bubble-tight within 3% of set pressure" specification is equally significant in cryogenic service. At cryogenic temperatures, PTFE's coefficient of thermal expansion is roughly 10× that of the stainless steel body it seals against, meaning a PTFE-to-stainless seal that is perfectly lapped at room temperature will develop a micro-gap at −196°C as the PTFE shrinks more than the steel seat. Achieving bubble-tight performance to within 3%—meaning the valve does not exhibit a detectable leak at pressures up to 97% of the set point—requires compensating for this differential contraction in the seal geometry. The DA21F-40P accomplishes this through the brass disc (component 3), which has a CTE intermediate between PTFE and stainless steel, acting as a thermal buffer that moderates the differential strain at the sealing interface.

The swivel thread connection (M16×1, both inlet and outlet) is a practical distinction from the fixed-thread-body designs more common in larger safety valves. In a fixed-thread design, the valve body must be rotated during installation, requiring either a pipe union downstream or sufficient pipe flexibility to accommodate the rotation. The swivel thread—where the threaded body rotates independently of the inlet/outlet sealing faces—allows the valve to be installed into a rigid pipe manifold without rotating the entire assembly. The M16×1 fine metric thread provides 1 mm of axial advance per full rotation, enabling ~40% finer set-point adjustment than a standard M16×1.5 coarse thread would offer (1.5 mm/rev). This matters for a safety valve where the spring preload setting directly determines the relief pressure.

| Working Principle

The DA21F-40P is a spring-loaded direct-acting safety relief valve—the simplest and most reliable overpressure protection architecture in existence, with no pilot stages, no external power, and only one moving assembly. Its function is to open automatically when the inlet pressure exceeds the factory-set relief pressure, vent the excess gas to a safe discharge location, and close automatically when the pressure falls below the reseat pressure. The mechanism is straightforward, but the precision with which it executes these three actions under cryogenic conditions is what distinguishes a certified safety device from a commodity spring-loaded check valve.

Opening Sequence—Force Balance at the Disc:

In the closed state, three forces act on the disc assembly (components 3, 5, and the spring 6):

Opening force: P_inlet × A_orifice, where A_orifice is the effective pressure area of the Φ10 mm orifice (~78.5 mm²). At 450 PSIG set point (DA-10A2 maximum), this is approximately 243 N.

Closing force: F_spring, the preload force of the stainless steel spring, set by the position of the adjusting nut (4) and locked by the cap (7).

Seal adhesion force: A small additional force from PTFE-to-stainless molecular adhesion at the sealing interface, typically 2–5% of the spring force at cryogenic temperature.

When P_inlet × A_orifice exceeds F_spring, the disc lifts off the body seat. Because the spring is a linear elastic element, the force required to continue lifting the disc increases linearly with displacement (F = k × x, where k is the spring rate and x is the lift height). The valve reaches full lift—the point where further increases in inlet pressure no longer increase flow capacity because the orifice is fully uncovered—at a lift of approximately D0/4 = 2.5 mm for a Φ10 mm orifice.

Bubble-Tight to 3%—The Differential Contraction Problem:

The 3% bubble-tight specification means that at 97% of the set pressure, the PTFE seal must form a leak-tight barrier against the stainless steel body seat—no detectable bubble leakage. Achieving this at −196°C is a materials engineering challenge because PTFE contracts roughly 10× more than stainless steel over the 221°C cooldown span:

     PTFE CTE: ~120 × 10⁻⁶/°C (varies with temperature range, higher below glass transition)

     304SS CTE: ~16 × 10⁻⁶/°C

     Differential contraction over 221°C: ΔL/L ≈ (120 − 16) × 10⁻⁶ × 221 ≈ 2.3%

For a disc with a sealing face width of ~2 mm, this 2.3% differential contraction produces a gap of ~46 μm—far larger than the ~5–10 μm threshold at which helium leak detection becomes positive. The brass disc (CTE ~20 × 10⁻⁶/°C) mitigates this by acting as a thermal compliance buffer: the brass disc shrinks more than the stainless body (by about 0.09% relative), partially compensating for the PTFE's 2.3% shrinkage. The remaining gap is closed by the spring preload, which compresses the PTFE into the seat with sufficient force to cold-flow the polymer into the microscopic surface asperities of the stainless seat. This is why the DA21F-40P specifies bubble-tight performance—the PTFE cold-flow compensation requires a specific spring preload range, and if the set pressure is adjusted too low (below the design minimum for that spring), the preload becomes insufficient to close the differential contraction gap.

Reseat Above 85%—Hysteresis and the Blowdown Ratio:

"Blowdown" is the difference between the opening (set) pressure and the closing (reseat) pressure, expressed as a percentage of the set pressure. The DA21F-40P's reseat above 85% corresponds to a blowdown below 15%. This is an exceptionally tight specification for a small-bore cryogenic safety valve.

The physics determining blowdown involves two competing effects during the closing stroke. As the inlet pressure falls after venting, the spring pushes the disc back toward the seat. The first effect is aerodynamic: the high-velocity gas flowing through the partially open orifice creates a low-pressure region (Bernoulli effect) on the downstream face of the disc, reducing the net opening force and accelerating closure. The second effect is mechanical: the brass disc sliding within the stainless body bore experiences static friction (stiction) that opposes movement, and the PTFE seal, once lifted, presents a slightly different contact geometry to the seat on the return stroke than it did during the opening stroke—a hysteresis effect.

The DA21F-40P achieves a sub-15% blowdown through three mechanical refinements. First, the brass disc sliding in a stainless bore provides a naturally low-friction tribological pair at cryogenic temperatures—brass's lower hardness (~80 HRB vs ~90 HRB for 304SS) means it burnishes the stainless bore rather than galling it. Second, the spring retainer (component 5, also brass) provides a self-aligning upper spring perch that prevents coil buckling, which would introduce non-linear friction as the spring compresses and extends. Third, and most critically, the swivel thread connection eliminates pipe-stress-induced body bore ovalization. In a fixed-thread safety valve, the installation torque on the inlet thread can distort the valve body bore from circular to slightly elliptical, creating tight spots that increase disc sliding friction and blowdown. The DA21F-40P's swivel thread decouples installation stress from the precision bore, preserving the circularity that enables consistent low-friction disc motion.

Tamper Resistance—The Cap as a Safety Interlock:

The stainless steel cap (7) fully encloses the adjusting nut (4) and spring retainer (5). In standard practice, the cap is factory-sealed with a lead wire and crimped seal after the set pressure is verified on the test stand. The cap must be physically removed—which requires cutting the seal wire or, in some configurations, a special spanner tool—to access the adjusting nut. This satisfies two regulatory requirements: TSG ZF001 (China's safety valve technical supervision regulation) requires that safety valves be protected against unauthorized adjustment, and ISO 4126 requires that the set pressure adjustment mechanism be designed to prevent inadvertent changes. The cap design also serves a secondary function as a weather shield, preventing ice accumulation on the adjusting threads in outdoor cryogenic installations where atmospheric moisture can freeze on exposed metal surfaces.

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 Factory Shoot

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 Application Video

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 Product parameters

| Data Sheet

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Comment & FAQ

1: How do I choose between DA-10, DA-10A1, and DA-10A2 ranges?

A: Select the range whose window fully contains your required set pressure, with the set pressure as close to the center of the range as possible. DA-10A1 covers 45–152 PSIG—suitable for low-pressure liquid nitrogen dewars, small laboratory cryostats, and low-pressure liquid argon tanks. DA-10 covers 217–275 PSIG—suitable for medium-pressure LNG vehicle fuel tanks and industrial gas cylinders. DA-10A2 covers 275–450 PSIG—suitable for high-pressure LNG storage and transport tanks, high-pressure gas delivery trailers. The set pressure is factory-preset; specify your exact required set pressure when ordering, and the factory will calibrate the spring preload and seal the cap. Do not attempt to adjust a DA-10 (217–275 PSIG) down to 150 PSIG—the spring is outside its linear range at that preload level, and reseat performance will degrade.

2. What does "bubble-tight within 3% of set pressure" mean in practical terms?

A: It means that at any pressure up to 97% of the set pressure, the valve seat will not exhibit a detectable gas leak when tested with a bubble leak detector (typically a soap solution or an electronic leak sniffer with a sensitivity of 1 × 10⁻⁴ cm³/s helium equivalent). In operational terms: a DA-10A1 valve set to 150 PSIG will be leak-tight at 145.5 PSIG and below. At 148 PSIG (98.7% of set point), a barely perceptible weep may begin—this is normal and does not constitute a failure. The 3% margin is deliberately tighter than the 5–10% margin common in industrial PRVs, reflecting the value of cryogenic gases (venting LOX or LNG is a direct product loss) and the safety implication of any undetected seat leakage in oxygen service (oxygen enrichment in confined spaces creates fire hazards).

3. How does the DA21F-40P's brass disc compare to a stainless steel disc?

A: Brass is selected here for three engineering reasons, not cost. First, brass has a CTE (~20 × 10⁻⁶/°C) intermediate between PTFE (~120) and stainless steel (~16), making it a thermal compliance buffer that reduces the differential contraction gap at the seal interface by approximately 15% compared to a stainless-on-PTFE direct pair. Second, brass-on-stainless is a naturally low-friction tribological pair at cryogenic temperatures, where stainless-on-stainless can gall, particularly after repeated thermal cycles—the brass disc burnishes the stainless bore rather than micro-welding to it. Third, brass's lower elastic modulus (~105 GPa vs 193 GPa for SS) provides a small amount of mechanical compliance at the disc-seat contact, allowing the PTFE seal to conform to microscopic surface irregularities more effectively than a rigid stainless disc. The trade-off is that the brass disc is a consumable wear item requiring replacement at major overhauls; a stainless disc would have a longer service life but worse seal performance and higher blowdown.

4.  What does "swivel thread" mean and why does it matter?

A: A swivel thread connection is one where the threaded portion of the valve body rotates independently of the sealing faces. In a conventional fixed-thread safety valve, tightening the valve into the pipe manifold applies torque to the valve body, which can distort the precision-machined bore inside. A distorted bore increases disc sliding friction, which increases blowdown, which increases product loss. The DA21F-40P's swivel thread isolates installation torque from the internal bore, preserving the bore's circularity and the consequent low-friction disc motion. Additionally, the M16×1 fine metric thread provides finer rotational-to-axial conversion (1 mm/rev) than a standard M16×1.5 (1.5 mm/rev)—useful during factory calibration where small spring preload adjustments translate to precise set-point changes.

5. Is the DA21F-40P a full-lift or proportional-lift safety valve?

A: The DA21F-40P is a proportional-lift safety valve. In a proportional-lift design, the disc lift height increases gradually with overpressure above the set point—the valve does not "pop" fully open at the exact set pressure. In contrast, a full-lift design (such as the DA08F-40P) uses a huddling chamber or a secondary orifice to create an aerodynamic force that snaps the disc to full lift once the set pressure is exceeded. The DA21F-40P's Φ10 mm orifice and compact body do not incorporate a huddling chamber. This is appropriate for the DA21F-40P's application: thermal expansion relief, where the overpressure transient is gradual (minutes to hours, not milliseconds) and the required discharge capacity is modest. Proportional-lift valves are simpler, cheaper, and more resistant to chatter than full-lift valves in low-capacity thermal relief service.

6. How does the tamper-resistant cap work—and can I still adjust the set pressure in the field?

A: The cap is a physical enclosure, not a locking mechanism. It is factory-sealed after calibration, and removing it requires cutting a lead seal wire (or, in some configurations, using a special pin spanner on the cap's internal features). Field adjustment is possible but not recommended for three reasons. First, breaking the factory seal voids the certification traceability required by TSG ZF001 and ISO 4126—the valve is no longer a "certified" safety device after field adjustment unless re-certified on a calibrated test stand. Second, the spring preload-to-set-pressure relationship depends on the assembled valve geometry, including the exact PTFE cold-flow state at the seal interface; a field adjustment made at ambient temperature will not produce the same set pressure at −196°C as a factory calibration that compensates for known differential contraction. Third, the M16×1 adjusting nut requires a thin-walled socket or pin spanner that most field maintenance crews do not carry. If a set-point change is required, return the valve to the factory or a certified valve repair shop with a cryogenic test capability.