QUESTION:
I’ve only been in the pumping business a little over a year and I’ve read these articles. Most likely you’ve covered the difference between the vacuum relief valve and the pressure relief valve. Maybe you could refresh the veterans and inform the rookies about what those valves do and what we need to know about them.
Mike Henderson
Atlanta, Ga.
ANSWER:
We have discussed pressure relief and vacuum relief valves in the past, but you make a good point about a refresher course. PRO is picking up new readers all the time; the veterans need a friendly reminder, and the newbies like you need to know how important these valves are in the operation of all vacuum trucks.
Over the years, we’ve run into a lot of pumpers who didn’t know the difference between a pressure relief valve and a vacuum relief valve. The simplest way to tell them apart is the spring and the ring.
The vacuum relief valve has a spring, while the pressure relief valve has a ring on top. So the next time you’re checking out your truck, locate each of these valves. A plastic cover on the neck of the valve may hide the spring on the vacuum relief valve, but the pressure relief valve does not have a cylindrical neck. It has a flat top with a red ring.
PULLING IT IN
Obviously, the vacuum relief valve relieves vacuum. The valve is calibrated and set to open and relieve the pressure at a safe level, thus saving your tank from damage caused by too much pressure. If there is no vacuum relief valve or the valve happens to get stuck and fails to work, the possibility increases dramatically that the tank will implode. That means the top part of the tank could be crushed until it is nearly touching the bottom of the tank.
As a general rule, vacuum relief valves are set at 20 inches of mercury. That level of pressure is more than enough to handle all portable sanitation or septic service tasks.
The best way to avoid having trouble with your vacuum relief valve is to test it often. Here’s how to do that: Build up the vacuum in your tank and then push the spring on the neck of the valve. If it is working properly, air should rush in, reducing the vacuum. If the spring won’t budge, you’ve got a problem. Most likely the spring is broken or plugged and the valve should be replaced. Because of the need for frequent tests, it’s critical that your pressure relief valve is easy to reach.
PUSHING IT OUT
If your pump has a four-way valve, you are able to pressurize your tank. If you don’t have a four-way valve, you don’t have to worry about a pressure relief valve. To check your pressure relief valve, pressurize the tank with all the tank openings closed. Pull the red ring on the valve and you should hear air rushing out. If you do, then the valve is working properly. If you don’t hear air rushing out, the valve is stuck and not working properly. If, after a few tries, the valve still fails, replace it. Excessive pressure not allowed to escape from your tank can cause it to explode.
The valve can be set to open at any pressure level; find out where the tank manufacturer or the truck builder initially set it to open. To verify the calibration, watch your pressure/vacuum gauge to see if the valve opens at the designated pressure.
These valves are critical to the safe operation of your vacuum system. Get to know them and test them at least once a week. We’ve seen tanks that have imploded and tanks that have exploded (the remnants are hard to find). It’s an unbelievable sight and a financial disaster for the owner of the unit. Know your vacuum and pressure relief valves, test them regularly, and your system will last a long time.
