Air Compressor Safety Valve: Function and Replacement
An air compressor safety valve opens when tank pressure passes its preset level and closes again when pressure falls, and this guide covers how it works, how to spot a worn one, and four brass valves to match to a home compressor.
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A Saturday brake job stops cold when the compressor tank hisses at a fitting and the pressure gauge will not hold still. The small brass valve on the tank is the part that guards against overpressure, and knowing how it works keeps a routine shop day from turning into a tank problem. An air compressor safety valve is a preset relief valve that opens when tank pressure passes its set level, vents the extra air, and closes again when pressure falls back. This guide covers what the valve does, how to spot a worn one, and four brass valves that fit a home garage compressor.
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Below: how the valve works, how to check one, and four brass relief valves preset at 150, 175 and 200 PSI.
Milton S-1090-200 1/4" MNPT ASME Safety Valve
A brass ASME safety valve with a 200 PSI pop off pressure, 170 cfm flow and 1/4 inch male pipe threads, made in the USA.
Pros
- 200 PSI pop off pressure, the highest here
- 170 cfm flow stated on the listing
- ASME listing stated in the title
- Bubble tight to 10% of pop off pressure
Cons
- No warranty term is stated
- Single valve, no spare in the box
- No thread seal treatment stated
- 200 PSI preset fits only matched compressors
1/4'' NPT Male 150 PSI Air Compressor Pressure
A brass 150 PSI relief valve with 1/4 inch NPT male threads, a 58 scfm flow and seal thread locks, sold as a 2-piece set.
Pros
- 2 pieces, so a spare is on hand
- Seal thread locks stated on the threads
- Brass body with corrosion resistance stated
- Opens at 150 PSI and closes as pressure falls
Cons
- 58 scfm flow, the lowest of the scfm figures here
- No warranty term is stated
- Title calls it a check valve, though the function is relief
- 150 PSI preset fits only matched compressors
HONSNKS 2 Pcs 1/4" NPT(M)
A brass 175 PSI relief valve with 1/4 inch NPT male threads and a 67 scfm flow, sold as a 2-piece set for air compressors.
Pros
- 175 PSI preset, between the 150 and 200 PSI valves here
- 67 scfm flow stated
- 2 pieces, so a spare is on hand
- Brass body with corrosion resistance stated
Cons
- Part number reads 165 PSI while the title states 175 PSI
- No warranty term is stated
- No stated temperature range
- Package bullet calls it a check valve, though the function is relief
DALQUIS 1/4" Male NPT Brass Safety Valve, 1 Pack
A brass 150 PSI relief valve with 1/4 inch NPT male threads, a 70 scfm flow and a stated temperature range of 32 to 176 degrees Fahrenheit, sold as a single valve.
Pros
- 70 scfm flow, the highest of the 150 PSI valves here
- Temperature range of 32 to 176 degrees Fahrenheit stated
- Brass body with corrosion resistance stated
- Threads treated for sealing, per the listing
Cons
- Single valve, no spare in the box
- No warranty term is stated
- 150 PSI preset fits only matched compressors
- No stated length or width
Air Compressor Safety Valve: What It Does and When It Opens
The valve sits on the tank and guards the tank whenever it holds pressure. When pressure climbs past the preset level, the valve opens and vents air until pressure falls back, then it seats again. That venting is the guard against a tank that keeps building pressure after the pressure switch fails to stop it.
| Valve | Preset | Flow | Thread |
|---|---|---|---|
| Milton S-1090-200 | 200 PSI | 170 cfm | 1/4 in MNPT |
| HONSNKS 175 PSI 2-Pack | 175 PSI | 67 scfm | 1/4 in NPT male |
| LSTOPGO 150 PSI 2-Pack | 150 PSI | 58 scfm | 1/4 in NPT male |
| DALQUIS 150 PSI Valve | 150 PSI | 70 scfm | 1/4 in NPT male |
Preset is the number that matters, and it is fixed on each valve here: 150, 175 and 200 PSI across the four picks. Match the preset to the tank’s maximum pressure in the manual, and match the thread too: every valve here uses a 1/4 inch NPT male thread. A preset above the tank’s maximum gives no guard, while a preset below the compressor’s normal working pressure vents air the shop still needs. Flow states how fast the valve vents: the Milton lists 170 cfm, the DALQUIS 70 scfm, the HONSNKS pair 67 scfm and the LSTOPGO pair 58 scfm. The Milton figure is stated in cfm while the others are stated in scfm, so the figures are not on the same basis and cannot be ranked against each other.
How to Check a Safety Valve and When to Replace It
Lift the pull ring briefly with the tank holding pressure: a short hiss, then the valve seats again. If it keeps hissing, it is not seating. If the ring will not lift, the valve is stuck. Replace a valve that leaks at normal pressure, vents early, or fails to open when the ring is lifted. Follow the compressor manual for the check interval and drain the tank before a swap.
Never plug a safety valve or fit a valve with a higher preset to stop nuisance venting. A plugged or capped relief valve leaves the tank with no guard. The DALQUIS and LSTOPGO valves state a temperature range of 32 to 176 degrees Fahrenheit, so a valve left in freezing air sits outside what its maker states.
Air Compressor Safety Valve Picks for Home Compressors
Matching the preset and the thread is the step that needs the gear below: every valve here threads into a 1/4 inch NPT fitting, and the choice comes down to the preset and the stated flow for the compressor it guards.
Drain the tank after use so water does not rust it from the inside, never exceed the tool's rated pressure, wear eye protection, and run 240V units on the circuit the manual calls for. Do not run a gas engine compressor in a closed garage. Read our Safety Disclaimer.
FAQ: garage tools
What PSI should an air compressor safety valve be?
Match the valve to the compressor: the four valves here are preset at 150, 175 and 200 PSI, and the right one is the preset the compressor manual lists for the tank. A preset above the tank’s maximum gives no guard, and a preset below the compressor’s normal working pressure vents air the shop still needs.
How do I know if my air compressor safety valve is bad?
A worn valve leaks or hisses at normal pressure, keeps venting after the tank falls back, or fails to open when the pull ring is lifted. A valve that will not seat, or a ring that will not lift, needs a replacement before the compressor runs again.
Can I replace an air compressor safety valve myself?
The swap is a thread-in job: every valve here uses a 1/4 inch NPT male thread, so it threads into the same fitting the old valve came out of. Drain the tank first, match the preset and the thread to the old valve, and follow the compressor manual.
What size thread is an air compressor safety valve?
Every valve in this guide uses a 1/4 inch NPT male thread, and the listings state that thread for each one. Measure the tank fitting before ordering, since other compressors and other valve listings can use different thread sizes.
Does a safety valve need a warranty?
None of the four listings here states a warranty term, so buy on the stated preset, flow and listing marks instead. The Milton valve carries an ASME listing in its title, while the other three state a preset relief pressure and a brass body without a stated listing.
Related: the air compressors hub, the picks for the best air compressor for home garage, the guide to what size air compressor you need for air tools and how an air compressor dryer works.
