Should Instrument Air Storage Tank Safety Valves Be Low-Lift or Full-Lift Type?

Source: Equipment Horizon (WeChat: Equipment Horizon) | Translated & Polished

Figure 1: Comparison of low-lift safety valve (left) and full-lift safety valve (right)

New Technical Regulation: Full-Lift Safety Valves Now Required for Gas Media

A topic that has been widely discussed online recently is: “Low-lift safety valves can no longer be used for gaseous media.” The basis for this is the latest technical regulation that took effect on July 1, 2026 —

“Technical Safety Regulation for Safety Accessories of Pressure-Bearing Special Equipment” (TSG 92-2026), Section D4.1, which states:

“When the working medium is gas, steam, liquefied gas, or liquid with a maximum working temperature equal to or higher than its standard boiling point, full-lift safety valves shall be selected; when the working medium is liquid, low-lift safety valves may be selected.”

For reference, the previous technical regulation TSG ZF001-2006 “Safety Valve Safety Technical Supervision Regulation” described the requirements as follows:

[The previous regulation had less restrictive language regarding safety valve type selection for gas media, allowing more flexibility in choosing between low-lift and full-lift types.]

The Dilemma: Instrument Air Storage Tanks and Safety Valve Selection

This raises an important question: If a full-lift safety valve is selected for the instrument air storage tank of pneumatic control valves or shut-off valves, would it conflict with the original design philosophy?

The role of an instrument air storage tank is to provide stable, dry compressed air to ensure precise actuation of downstream pneumatic control valves and shut-off valves. Excessive pressure fluctuations can lead to inaccurate valve positioning or unintended actions. The smooth and steady discharge characteristics of low-lift safety valves happen to meet this requirement, avoiding violent pressure fluctuations in the system caused by safety valve actuation.

Figure 2: Instrument air storage tank system with safety valve, pressure gauge, and downstream pneumatic control valves

Furthermore, the valve disc of a low-lift safety valve has a smaller opening height, which makes it easier to maintain sealing surface contact pressure in the normal closed state, thereby providing better sealing performance. This is critically important for instrument air systems, as any leakage results in energy waste and system pressure drop.

Technical Reference: Characteristics of Low-Lift Safety Valves

“Practical Manual for Safety Relief Devices of Boilers, Pressure Vessels, and Pressure Piping — Safety Valves” (Edited by Zhou Zhen):

Proportional-action low-lift safety valves, also known as low-lift safety valves, are valves where the disc lifts correspondingly as the medium pressure within the vessel increases. Since there is no specialized mechanism to increase the opening height, the disc opening height is generally only 1/20 to 1/40 of the valve seat diameter. The discharge area is the annular gap formed between the disc and valve seat. Low-lift safety valves are mainly used for liquid media because their opening and closing processes are relatively smooth, preventing significant pressure fluctuations in the protected system, which is suitable for pressure vessels filled with liquid.

Another characteristic of low-lift safety valves is their small discharge capacity, which also makes them suitable for equipment requiring small gas exhaust volumes, such as small air compressors, evaporators, and small boilers.

“Safety Valves” (2nd Edition) (Edited by Zhou Zhen):

Low-lift safety valves mainly come in two types: those with an opening height greater than or equal to 1/40 of the flow diameter d, and those with an opening height greater than or equal to 1/20 of the flow diameter d0. Low-lift safety valves operate on a proportional action principle and are primarily used for liquid applications, though sometimes also used for gas applications requiring very small discharge volumes.

Open Question: Is a Dedicated Safety Valve on the Storage Tank Necessary?

There is another question worth discussing: If the design pressure of the pneumatic valve storage tank is higher than the maximum working pressure of the instrument air, and the upstream instrument air system already has a safety valve installed, is it still necessary to install a separate safety valve on the storage tank?

Discussions and insights from industry professionals are welcome!

Source: Equipment Horizon (WeChat Official Account)