Monitor Pressure & Vacuum Relief Valves to Avoid Tank Implosions & Explosions

Where you place the vacuum and pressure relief valves is important to promote frequent vital safety monitoring.
Monitor Pressure & Vacuum Relief Valves to Avoid Tank Implosions & Explosions

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QUESTION: The used truck I bought wasn't the jewel I thought it would be. I am having trouble locating my pressure relief and vacuum relief valve. I know you've touched on the subject in the past, but could you explain how they work and the best place to locate them on the truck?

John Reddington
Miami, Fla.

ANSWER: The pressure relief valve and the vacuum relief valve do exactly what their names imply. If your pump is in the pressure mode from the four-way valve, the pressure relief valve can be set to open when the air in the tank reaches a predetermined level of pressure. With the valve open, the air in the tank rushes out through the valve and drops the pressure inside the tank.

A tank that builds up pressure beyond its normal capacity can explode. I saw the results of one of those explosions many years ago. In this case, it was a pressure tank designed for water use at a retirement community in Sun City, Arizona. When the tank exploded, the two heads (ends) of the tank blew out and land about 200 yards away. It was fortunate that no one was hurt and damage was minimal. The pressure tank system was located on a golf course.

The message is clear: Be careful with pressure building in your vacuum tank. The pressure relief valve should take the worry away.

The vacuum relief valve does exactly the opposite job. As vacuum builds in the tank, the preset vacuum relief valve will open and allow incoming air to be released, ensuring the tank will operate safely in the future. If the vacuum relief valve is stuck or broken, it is possible heightened vacuum inside the tank will cause it to collapse, or implode.

The vacuum relief and the pressure relief valves are inexpensive components to help you operate safely. But you can't ignore them and hope they continue to work properly. You need to identify them (see the detailed photo) and check them. For pressure relief, you simply pull the round ring and for vacuum relief, you push the spring in.

Sometimes grit and other debris settle on the valves and make them inoperable. Both valves need to be tested at least once a week to make sure they are in good working order. To simplify the regular testing, have the valves on any new truck located where they are easy to see and access. I am a big fan of locating them as close to the pump as possible. To make the job even easier, place the vacuum/pressure gauge in between the two, as shown in the photo.

In the past, I have seen the valves located in many different spots on vacuum trucks. It's almost as if they were put in last and no one was quite sure what they were or where they belonged. The arrangement in the photo shows you the best way to bring your safety valves into play.

And a final note concerning the pressure/vacuum gauge: I have always likened this valuable tool to a medical thermometer. For humans 98.6 degrees F is a normal temperature. To stray away from that tends to indicate trouble. It's the same with the pressure/vacuum gauge. If your gauge suddenly sees an increase or decrease in normal operation, you have a sick system. Stop, diagnose the situation, and get it fixed.

Ultimately, your pressure/vacuum gauge is the key to determining the health of your vacuum system. The pressure relief and the vacuum relief valves might be the key to saving your system. Pay attention to these items.


Thanks to everyone who answered the questions on the Truck Corner quiz in the January issue. The first winning entry was submitted by Richard Davis, of Davis Plumbing and Mechanical Inc. in Aztec, N.M. Davis received a Truck Corner prize package for answering all quiz questions correctly.


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