DA21F-40P（DA-15,DA-15A1,DA-15A2,DA-15A3) Cryogenic Safety Valves

| Product Overview

The DA21F-40P is a spring-loaded cryogenic safety valve engineered for overpressure protection in deep-cryogenic systems operating down to −196°C. It is a precision-machined, all-metal pressure-relief device whose function is to open automatically when the system pressure exceeds a preset threshold, vent the excess gas, and reseat once normal pressure is restored—protecting cryogenic storage tanks, dewars, piping, and process vessels from the pressure build-up caused by boil-off, thermal expansion, or upstream over-supply.

The valve body is manufactured in stainless steel, selected for its deep-cryogenic toughness and corrosion resistance across the full industrial gas spectrum—LOX (liquid oxygen), LIN (liquid nitrogen), LAr (liquid argon), and LNG (liquefied natural gas). The sealing element is a PTFE soft seat, which provides a tight, leak-free closure at cryogenic temperatures where metal-to-metal seats are prone to leakage due to differential thermal contraction. The valve is rated for a maximum set pressure of 35 bar (508 psig), with the inlet/outlet port sizes determined by the specification table (typically Φ12–Φ42 mm), allowing the same valve family to be matched to different flow capacities.

Three design details distinguish this valve in field service. First, the adjustable set-pressure cap on top allows the opening pressure to be tuned to the specific protection requirement of the installation. Second, the manual pull-ring provides a means to open the valve by hand—used both for periodic functional testing and for emergency manual pressure release without waiting for the set point to be reached. Third, the threaded inlet and side-mounted threaded outlet enable compact, direct installation into a cryogenic piping system, with the outlet oriented to direct the vented gas safely away from personnel and equipment.

The valve is manufactured in compliance with ISO 4126 (safety devices for protection against excessive pressure) and the Chinese national standard GB/T 29026, giving users a documented basis for acceptance in both domestic and export installations. It supports both vertical and horizontal mounting, and its spring-loaded, self-reseating design means it can be reused across repeated overpressure events without replacement—unlike frangible devices such as rupture discs.

| Working Principle

A spring-loaded cryogenic safety valve is a mechanical, self-actuating pressure-relief device. It requires no electrical power, no pneumatic signal, and no operator action to perform its primary function—it is held closed by a calibrated spring and opened by the very pressure it is designed to protect against.

The Closed State — Spring Preload and Sealing:

Inside the valve, a compression spring is preloaded against the valve disc (the moving closure element) through the adjustable cap on top. The spring force presses the disc down onto the PTFE seat, forming a gas-tight seal between the process side and the discharge side. The preload is set so that the downward spring force balances the upward force exerted by the process pressure acting on the disc area. At normal operating pressure—below the set point—the spring force exceeds the pressure force, and the valve remains closed with zero leakage. The PTFE soft seat is critical here: at cryogenic temperatures, metal components contract at different rates, and a rigid metal-to-metal seat can develop micro-gaps that leak. PTFE's elasticity and low-temperature compliance maintain a continuous sealing line even under differential thermal contraction.

The Opening Event — Set Point Reached:

As the system pressure rises—from boil-off accumulation, thermal expansion of trapped liquid, or upstream over-supply—the upward pressure force on the disc increases proportionally. When the process pressure reaches the set point, the upward force just equals the spring preload. Any further increase overcomes the spring, lifting the disc off the seat. The valve opens and the excess gas is discharged through the side-mounted outlet, which is oriented to vent safely away from personnel. Because the disc lifts progressively with pressure, the valve provides a controlled, proportional release rather than an all-or-nothing burst.

The Reseating Event — Pressure Restored:

Once the overpressure condition is relieved and the system pressure falls back below the set point, the spring force again exceeds the pressure force, and the disc is pushed back onto the PTFE seat. The valve closes automatically and re-establishes the gas-tight seal. This self-reseating behavior is the defining advantage of a spring-loaded safety valve: it can protect against repeated overpressure events over its service life without replacement, and it vents only the minimum amount of gas required to restore safe pressure.

The Manual Pull-Ring — Test and Emergency Release:

The pull-ring on top provides a manual override. Pulling it lifts the disc off the seat by hand, opening the valve regardless of the current system pressure. This serves two purposes: (1) functional testing—verifying that the disc moves freely and the seat seals properly during routine maintenance; and (2) emergency manual release—venting the system immediately in a hazardous situation without waiting for the pressure to climb to the set point. The pull-ring is a mechanical linkage to the stem, so it operates even if the spring preload is at its maximum setting.

Set-Point Adjustment:

The opening pressure is determined by the spring preload, which is adjusted via the cap on top. Turning the cap compresses or relaxes the spring, raising or lowering the set point within the valve's rated range (up to 35 bar). This allows a single valve model to be field-tuned to different protection requirements, though in practice the set point is typically locked and tagged after factory calibration to prevent unauthorized tampering.

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 Factory Shoot

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 Application Video

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 Product parameters

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Comment & FAQ

1: What is a cryogenic safety valve and where is it used? 

A: A cryogenic safety valve is an overpressure protection device designed specifically for deep-cryogenic systems—that is, systems handling cryogenic liquids such as liquid nitrogen (−196°C), liquid oxygen (−183°C), liquid argon (−186°C), or LNG (−162°C). Its function is to open automatically when the system pressure exceeds a safe threshold and vent the excess gas, preventing storage tanks, dewars, piping, or process vessels from being damaged or even ruptured by pressure build-up. Typical applications include: vapor-space protection of cryogenic storage tanks, liquid nitrogen dewar pressure relief, oxygen pipeline overpressure protection, LNG storage and transport facility safety, and any cryogenic process step that requires automatic pressure release when pressure gets too high.

2. How should the model designation DA21F-40P be understood? 

A: The model designation is the manufacturer's code for the valve's structure, sealing form, and pressure rating. Here, "DA" typically denotes the cryogenic safety valve series, "21" denotes the spring-loaded construction type, "F" denotes soft sealing (PTFE seat), "40" denotes the nominal pressure rating (corresponding to the PN40 system, i.e. a nominal pressure of up to about 40 bar; this valve is rated for a maximum set pressure of 35 bar), and "P" denotes a stainless steel valve body. It should be noted that model coding rules may vary slightly between manufacturers; when purchasing, always refer to the manufacturer's model description and specification sheet.

3. Can the set pressure be adjusted on site? How? 

A: Yes. The adjusting cap on top of the valve is used to change the spring preload and thereby the opening pressure. Turning the cap clockwise compresses the spring and raises the set pressure; turning it counter-clockwise relaxes the spring and lowers the set pressure. However, note that the set pressure must remain within the valve's rated range (35 bar maximum for this valve), and the actual opening pressure should be verified with calibration equipment after adjustment. In practice, the set point is usually locked and lead-sealed/tagged after factory calibration to prevent unauthorized on-site adjustment. If your service condition requires a different set pressure, it is recommended to specify it at the time of purchase and have it calibrated by the manufacturer.

4.  Why is PTFE used for sealing? Isn't metal-to-metal sealing good enough? 

A: At cryogenic temperatures, metal components contract at different rates, and a rigid metal-to-metal seat can develop micro-gaps due to differential thermal contraction, causing leakage. A PTFE soft seat has elasticity and low-temperature compliance, maintaining a continuous sealing line even at −196°C and achieving a tight, leak-free closure. This is the key to cryogenic safety valve reliability—leakage in the closed state means continuous medium loss and a potential safety hazard. The trade-off is that PTFE is less wear-resistant than metal, so this type of valve is better suited to "normally closed, occasionally opened" overpressure protection duty rather than frequent modulating service.

5. What is the pull-ring on top for? 

A: The pull-ring is a manual override device used in two scenarios. First, functional testing—periodically pulling the ring verifies that the disc moves freely and the seat seals correctly, ensuring the valve will not stick when it is actually needed. Second, emergency manual release—in a hazardous situation (such as abnormal pressure rise before the set point is reached, or when the system must be vented immediately), the valve can be opened by hand without waiting for the pressure to climb to the set value. The pull-ring is a mechanical linkage to the stem, so it operates even when the spring preload is at its maximum setting.

6. What is the difference between a safety valve and a rupture disc? Why do some systems install both? 

A: A safety valve is a reseatable device: it opens on overpressure, vents, and closes automatically once pressure is restored; it can be reused repeatedly and vents only the minimum amount of gas required to restore safe pressure. A rupture disc is a non-reseatable device: a metal membrane that bursts at a set differential pressure and then remains open, discharging the full contents and requiring replacement. The advantages of a safety valve are reusability and controlled discharge; its drawbacks are possible failure due to spring fatigue, seat freezing/sticking, or contamination. The advantages of a rupture disc are its simple construction, non-clogging nature, and absence of moving parts; its drawbacks are single-use and full-volume discharge. Therefore, for high-hazard media or strict safety-code applications, a "safety valve + rupture disc" dual configuration is often used to provide defense in depth—normal overpressure is handled by the safety valve, and the rupture disc acts as a fail-safe final protection if the safety valve fails.

7. What should be noted during installation? 

A: Key installation points include: (1) connect into the piping system via the bottom threaded port, ensuring the threaded connection is tight and using sealing materials suitable for cryogenic service; (2) the side outlet should face a safe area, directing vented gas away from personnel, equipment, and heat sources, with a discharge line added if necessary; (3) vertical or horizontal mounting is supported, but the valve body should not be subjected to additional piping stress; (4) the installation position should allow easy access to the top adjusting cap and pull-ring for testing and maintenance; (5) no isolation valve that could impede pressure transmission should be installed upstream of the valve (or it must be interlocked per code), otherwise the overpressure protection function is weakened; (6) under deep-cryogenic service, the valve body and connections should be protected against low temperature to prevent icing from affecting operation.

8. Is regular maintenance and calibration required? How often?

A: Yes. A safety valve is a safety-critical device, and the following maintenance cycle is recommended: (1) Functional testing—periodically (e.g. quarterly or per plant safety procedures) pull the pull-ring to confirm the disc moves freely and reseats with good sealing; (2) Set-pressure calibration—annually or per local regulations, verify on a calibration bench that the actual opening pressure remains within the permitted tolerance of the set value, and recalibrate if out of tolerance; (3) Visual and seal inspection—check the valve body for corrosion, icing, and external damage, and the inlet/outlet for signs of leakage; (4) Seal replacement—the PTFE seat may wear after long-term use or repeated operation; replace it per the manufacturer's recommended interval. After maintenance, recalibrate, re-seal, and keep records. The specific intervals should follow local pressure-vessel/pressure-piping safety regulations and the manufacturer's maintenance manual.