Train Horn Safety Relief Valve: PSI Rating, Placement and Testing

A train horn safety relief valve (also called a pop-off or blow-off valve) is a small brass part that keeps your air tank from turning into a pressure bomb if the pressure switch ever fails. It’s cheap and simple, and people overlook it all the time. Here’s how to pick the right PSI rating, where to thread it into the tank, and how to test it so you know it will actually open when it needs to.

What a safety relief valve actually does

A train horn air system has two layers of pressure control. The everyday control is the pressure switch: it starts the compressor when tank pressure drops to the cut-in point and stops it at the cut-out point. The relief valve is the backup. It’s a spring-loaded valve that stays sealed until tank pressure pushes past its set point, then pops open and dumps air until the pressure falls back down.

HornBlasters spells out the order of events in its air pressure guide: if the pressure switch fails to shut off the compressor and the compressor keeps running, the safety blow-off valve opens and releases the excess pressure. If you don’t have a gauge, the company notes, hearing the blow-off go is also a sign your pressure is too high. Our guide to how a train horn pressure switch works covers the switch side of this in detail. The short version: the switch does the regulating, and the valve is only there for when the switch fails.

Most aftermarket relief valves share a few traits:

• A 1/4” male NPT thread. VIAIR states that all of its safety valves use 1/4” NPT.
• A factory-set pressure that you can’t adjust.
• A manual pull ring for venting and testing.
• Brass construction.

What PSI rating do you need?

The rule is simple: the valve’s rating must sit above your pressure switch’s cut-out and no higher than your tank’s rated maximum working pressure. If a kit’s valve is stamped higher than the tank’s own label, ask the manufacturer before you trust it. Set it too low and it vents during normal operation. HornBlasters warns that a blow-off rated too low for your switch will activate prematurely and you won’t be able to build pressure. Set it too high and it can’t protect the tank.

Here’s how a few real kits and parts space those numbers:

Setup	Switch (on/off)	Safety valve	Tank rating
VIAIR 2.0-gallon onboard air kit	Packaged with a 120 PSI switch	145 PSI	150 PSI max working pressure
Kleinn Model 52145 valve	Designed for Model 2121 switch, 90 on / 120 off	145 PSI	Check your tank label
Typical HornBlasters train horn kit	110-150 PSI	175 PSI	Check your tank label

The VIAIR 2.0-gallon kit is the cleanest example because the manual lists all three numbers: the 120 PSI cut-out sits below the 145 PSI valve, and the valve sits below the 150 PSI tank rating. That’s the order you want.

The industrial rule backs this up. OSHA’s standard for compressed air receivers (29 CFR 1910.169) requires safety valves with enough relieving capacity to keep tank pressure from exceeding the receiver’s maximum allowable working pressure by more than 10 percent. That standard specifically doesn’t cover automotive equipment, so it isn’t a legal requirement for your truck. It’s still a sensible benchmark for a pressure vessel bolted under your bed.

Before you buy, read the label on your tank, not just the box your kit came in. HornBlasters’ installation manual for its 540-series kits warns not to fill the included tank above 150 PSI. VIAIR’s 1.0- and 2.0-gallon tank manuals list a 150 PSI maximum working pressure. If you raise your switch settings (see our train horn PSI guide for why more pressure isn’t automatically louder), the tank and the relief valve both need to be rated for it.

Set-point tolerance matters too. One brass 165 PSI pop-off valve’s spec sheet lists its accuracy as plus or minus 3 percent of the set pressure. That’s about 5 PSI either way. Leave some gap between your cut-out and the valve rating so normal variation doesn’t make it seep.

Where to install the relief valve

Location matters as much as the rating. Here’s what the manufacturer manuals and the OSHA receiver standard agree on:

• Top of the tank. VIAIR’s 2.0-gallon manual says to install the safety valve in the topmost position on the tank, with the drain cock in the lowest position if the tank isn’t mounted upright on its legs.
• Threaded directly into a tank port. OSHA’s receiver rule says no valve of any type may be placed between the tank and its safety valve. A shutoff or check valve in between would cut it off from the pressure it’s supposed to relieve.
• Accessible. You need to reach the pull ring for testing and for venting the tank before service.
• Outlet aimed away from people. VIAIR tells users to point the relief valve away from the body when releasing air.
• Protected from the elements. OSHA requires safety valves to be located so they can’t easily be rendered inoperative, including by the elements. Under a truck, that means packed mud, road salt and spray-on undercoating.

• Never on the bottom of the tank. HornBlasters’ 540 manual says not to mount the safety valve on the bottom and not to use it as a drain. The bottom is where water and sludge collect.
• Never downstream of a check valve or shutoff. If something can isolate the valve from the tank, it isn’t protecting the tank.
• Never plugged or capped to stop a leak. A seeping valve gets replaced, not silenced.

Port sizing depends on your tank. VIAIR’s 1.0-gallon tank has four 1/4” NPT ports and its 2.0-gallon tank has five, so the valve threads straight in. HornBlasters’ 5-gallon tank in the 540 kits has seven 1/2” ports and one 3/4” port, so the manual uses a 1/2” to 1/4” reducer bushing for the safety valve.

Installing it: sealant and torque

The install itself is quick:

1. Drain the tank completely to 0 PSI before removing any plug or fitting.
2. Check the valve threads. HornBlasters’ manual says that if the safety valve already has sealant on its threads, Teflon tape isn’t necessary. Otherwise, use tape or pipe sealant, and keep it off the first thread so no bits of tape get into the tank. Our thread sealant guide covers when to use tape and when to use paste.
3. Thread the valve in by hand, then snug it with a wrench on the hex. One 1/4” valve’s spec sheet lists a 9/16” wrench size. Never use the pull ring or the cap as a handle.
4. Orient the outlet so a blast of air won’t hit a fuel line, wiring, or anyone standing next to the truck.
5. Fill the tank and spray soapy water on the threads and the valve outlet to check for bubbles.

How to test a pop-off valve

There are two checks, and both are easy.

1. The pull-ring check. With some pressure in the tank, pull the ring for a second or two and let go. You should hear a sharp burst of air, and the valve should snap shut and stop. One shop compressor operator’s manual tells owners to pull the safety valve ring every day to make sure it’s working and to confirm before startup that the valve moves freely. A train horn tank sees far fewer hours, so a monthly check, plus one whenever you drain the tank, is a reasonable habit. Wear eye protection and keep your face away from the outlet.

2. The cut-out check. Run the system up with the gauge in view. The compressor should shut off at the switch’s cut-out point, and the relief valve should stay quiet. Our first-fill bench test guide walks through this. If the valve pops before the switch shuts off the compressor, either the switch is reading high or the valve is weak.

• Don’t bypass or jumper the pressure switch to force the valve to pop. If you need to confirm the exact set point, replacing the valve is cheaper and safer than overpressurizing a tank on purpose.
• Don’t try to adjust a factory-set valve. If it’s the wrong rating, buy the correct one.

Signs your relief valve needs replacing

Relief valves are cheap. VIAIR lists its 145 PSI safety valve at $12.50 and one retailer lists Kleinn’s 145 PSI valve at $10.50. If a valve looks questionable, just replace it.

• It keeps hissing after a ring pull. HornBlasters’ troubleshooting chart lists a defective safety valve as a cause of tank pressure dropping after the compressor shuts off.
• It pops while the compressor is still running below cut-out. The same chart lists a faulty pressure switch or a defective safety valve when the compressor runs continuously and opens the safety valve. Use the gauge to tell which.
• The ring is stuck or gritty. If it won’t move freely, the valve may not open when it needs to.
• Water spits out when it vents. HornBlasters ties excess moisture in the safety discharge to water in the tank. Drain more often (see how to drain a train horn tank) and check the tank for corrosion.
• Visible corrosion on the valve or the tank around it. VIAIR warns that rusted tanks can fail and should be discarded. Our air tank rust inspection guide shows what to look for.

Choosing a replacement valve

Match these four things and you’ve got the right part:

Thread

Usually 1/4” male NPT (use a bushing for larger ports)

Set pressure

Above the switch cut-out, at or below the tank rating

Certification

ASME code-stamped. VIAIR lists its valves as ASME certified and UV/NB approved

Test feature

A manual pull ring

VIAIR offers 1/4” NPT safety valves in 145, 155, 175, 205 and 250 PSI. Kleinn sells 145 and 155 PSI pop-off valves. Brass 165 PSI valves are also easy to find from compressor parts suppliers. One such valve is rated to relieve 80 SCFM, which is far more than any 12V horn compressor puts out. That’s the point: the valve has to dump air faster than the compressor can make it.

Keep reading

• Train horn pressure switch explained
• Train horn PSI explained
• How to inspect a train horn air tank for rust
• Train horn air leak diagnosis
• Train horn check valve guide

Sources

• HornBlasters — Understanding PSI: What Air Pressure Really Means — blow-off opens when the switch fails; too-low valve activates prematurely; valve must be rated above the switch; 110-150 PSI switch with 175 PSI blow-off in most kits
• HornBlasters 540-series kit installation instructions (PDF) — do not fill included tank above 150 PSI; seven 1/2” + one 3/4” tank ports; 1/2” to 1/4” bushing for safety valve; no tape needed if pre-sealed; do not mount on bottom or use as drain; troubleshooting chart (defective safety valve, moisture in safety discharge)
• VIAIR — Safety Valves — all valves 1/4” NPT; 145/155/175/205/250 PSI options; $12.50 price for 145 PSI part 92145; ASME certified, UV/NB approved; manual pull ring
• VIAIR 10002 Quarter Duty kit user manual (PDF) — 2.0-gallon tank with five 1/4” NPT ports; 145 PSI safety valve; 120 PSI switch; 150 PSI max working pressure; valve in topmost position, drain cock lowest; point valve away from body; rusted tanks can fail
• VIAIR 10000 Ultra-Light Duty kit user manual (PDF) — 1.0-gallon tank with four 1/4” NPT ports; 150 PSI max working pressure; safety valve in top of tank; use pull ring to vent before servicing
• Kleinn Safety Valve Model 52145, 145 PSI (S&P Power store) — 145 PSI, 1/4” male NPT, brass, designed for Model 2121 switch (90 on / 120 off), $10.50
• Kleinn Safety Valve, 155 PSI (S&P Power store) — 155 PSI Kleinn pop-off valve option
• Conrader AS250165 165 PSI brass safety relief valve (Compressor Source) — plus or minus 3% set-pressure accuracy; 80 SCFM relief; 1/4” male NPT; 9/16” wrench size; UV/NB certified
• OSHA 29 CFR 1910.169 — Air receivers — not applicable to automotive equipment; relieving capacity limits overpressure to 10% above MAWP; no valve between receiver and safety valve; safety appliances not to be rendered inoperative by the elements; test frequently
• Air compressor owner’s manual, Canadian Tire product 399-6296 (PDF) — pull the safety valve ring daily; confirm the valve moves freely before startup; do not adjust the factory-set safety valve