A safety relief valve protects a vessel from too much pressure inside it. That is the failure everyone plans for.
The opposite failure gets planned for far less often, and it is usually the cheaper one to cause: pressure inside the vessel drops below atmospheric, and the atmosphere pushes in. A tank that will happily hold 15 psi of internal pressure can be crushed by a small fraction of that from outside, because it fails a different way — a pressure vessel under internal pressure is pulled tight, while a vessel under vacuum buckles. Thin-wall and light-gauge tanks are especially unforgiving.
A pressure relief valve will not help you. It is built to open outward when pressure rises. When pressure falls, it seals harder.
How a vessel ends up under vacuum
Almost always by accident, and almost always during normal operation:
- Draining or pumping out a closed vessel faster than air can get back in
- Steam or vapour condensing as a hot vessel cools — this one catches people out, because nothing is being removed; the contents simply shrink
- Thermal contraction overnight, or after a washdown
- A blocked or undersized vent during outflow
- A vacuum pump or blower on the suction side doing exactly what it was installed to do
The last one is the most common in the equipment we quote: conveying systems, vacuum pumps, and bulk hauling trailers, where the vacuum is deliberate and the only question is how far it goes.
What a vacuum relief valve actually does
It works backwards from a safety valve. It sits closed while the vessel is at or above atmospheric pressure, and opens inward when internal pressure drops far enough below atmospheric — letting outside air in to break the vacuum before the vessel deforms.
Because it is admitting air rather than containing it, it is set in inches of mercury (in. Hg) rather than psig. A Kunkle 215V, for example, is set anywhere from 2 to 29 in. Hg.
Why you cannot substitute one for the other
The two valves can look almost identical. The Kunkle 215V and 337 share a body casting, the same 2″, 2½” and 3″ sizes, the same nozzle and the same flow coefficient. Side by side on a shelf, the practical difference is that the 337 has a lift lever on top and the 215V has a plain sealed cap.
They do opposite jobs, and the datasheet is blunt about it: Model 215V valves are not suitable for positive pressure. Fit one where pressure relief was meant and the vessel has no overpressure protection at all.
| Vacuum relief | Pressure relief | |
|---|---|---|
| Protects against | Vessel collapse | Vessel rupture |
| Opens | Inward, admitting air | Outward, venting contents |
| Triggered by | Internal pressure falling | Internal pressure rising |
| Set in | Inches of mercury | psig |
| Example | Kunkle 215V | Kunkle 337 |
How to tell which one you have
Read the nameplate, not the shape.
A code-stamped safety valve carries an ASME stamp — ‘V’ for Section I, ‘UV’ for Section VIII — and a National Board (‘NB’) number. A vacuum relief valve generally carries neither, because vacuum relief is not covered by an ASME code stamp the way pressure relief is. The Kunkle 215V is a non-code valve for exactly this reason.
So: no ASME stamp and no NB number is a clue, not a defect — but it is also how a vacuum valve and a low-set non-code pressure valve can be confused for one another. If the nameplate gives a setting in inches of mercury, it is a vacuum valve. If it gives psig, it is not.
Many systems need both
These are not alternatives. A vessel that can be over-pressurised and pulled into vacuum needs protection in both directions — often a pressure relief valve and a vacuum relief valve on the same tank, or a combination pressure/vacuum vent where one is available for the service.
If you already have a safety valve on a vessel and have never thought about the vacuum case, that is the question worth asking this week: what happens to this tank when it cools down, or when we drain it with the vent closed?
Not sure what you’re looking at?
Send us the part number off the nameplate and we’ll tell you what it is, what it’s set at, and which direction it protects you in. Requests received before 3 PM ET are typically quoted the same business day.
For information only. This article is general technical guidance, not a specification or a recommendation for any particular installation. Valve selection, sizing, set pressure and inspection intervals depend on the equipment’s design data and the code it was built to. Always confirm with a qualified engineer and your jurisdiction’s boiler or pressure vessel inspector before specifying, changing or relying on a relief device.