Avoid Trouble by Monitoring Flow & Pressure on Vacuum Trucks

Lost vacuum on your route this morning? Follow these steps to clear obstructions and get back to making money.

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QUESTION: Where are the spots throughout the truck’s vacuum system where the normal flow of vacuum can be blocked?

Randy Wilson
Seattle, WA

ANSWER: It’s easily overlooked, but more often than not the first point of obstruction is the end of the suction nozzle. A technician has no idea what has been thrown in a portable restroom holding tank, and therefore sucked up to the hose. Such things as rags, tennis balls, baseballs, shoes, etc. will definitely be tested by the vacuum in your system.  So first thing, check the end of your nozzle for any obstruction.

The second trouble spot is the vacuum hose itself. If these items make it through your nozzle, they can then get stuck in the hose. To check for obstruction, disconnect your hose and run the pump. If it runs normally, you will isolate the problem to the hose or nozzle.

If the pump is still running but the vacuum isn’t building like it should, the next possible spot of obstruction will be the primary shut-off. If the tank is close to full, the ball in the primary will block waste from getting to the pump. If the obstruction is a rag or something similar, you have to figure out how to get the item unstuck. You can open the lid of the primary to remove the obstruction. If it’s easier to reach through from the manhole (if your tank has one) then that works just as well.

If the obstruction hasn’t been cleared at this point, it’s time to check the secondary.  Remove the bottom of the secondary and remove the obstruction.

The worst-case scenario is next. After making it through the hose and nozzle, past the primary and skipping through the secondary, the obstruction gets to the pump. You’ll hear the problem very well at this point. It’s possible to break vanes and ruin a good pump. However it is rare that obstructions make it to the pump.

Monitoring vacuum pressure is serious business

QUESTION:  My employee had the good fortune to implode the tank on one our trucks this week and he can’t believe he did anything wrong. Would you please explain to him what causes a vacuum tank to implode? We’re one truck short now, so I want him, and anybody else out there, to understand how to avoid this costly problem.

Doug Wilshire
Great Barrington, MA

ANSWER: The answer is simple. The force of vacuum inside the tank was greater than the ability of the steel shell to contain it. This is an instance where the vacuum not only wins the battle, but also wins the war. What you really want your employee to know is how to avoid this ever happening again.

Every vacuum truck is equipped with a vacuum relief valve. Generally it is set to open up at around 20 inches of mercury. At 20 inches of vacuum, most everything can be sucked up in no time flat. Now, suppose the vacuum relief valve becomes stuck and won’t bring in outside air at 20 inches, and the vacuum builds further.

At a certain point as the vacuum increases inside the tank, the steel gives up the battle and collapses. To witness it is an incredible sight that reinforces the incredible power of vacuum. So the first cause is possibly a runaway vacuum relief valve that is stuck.

The second cause is often more subtle. The shell of a vacuum tank is usually the shape of a round cylinder. For a cylinder to collapse, molecules in the steel must press inward to the point of squishing each other. I’m sure there is a more scientific description for the compressing of these molecules, but you can understand that steel, which already has great tensile strength, would even be stronger when compressed inward.

Last year, a Pumper reader submitted the same question. In his situation, he had backed his vacuum truck into stout tree branches, leaving a few dimples in the tank. Those depressions caused a tank implosion. A vacuum tank is only is as strong as its weakest point. As the vacuum builds, those weak points are quick to surrender and implosion takes place.

Those are two causes that result in tank implosion. Most implosions happen due to a stuck vacuum relief valve. So check that vacuum relief valve every day. Build up some vacuum and then check to see when you push in the vacuum relief valve that air is rushing into the tank. If not, replace the valve. Do the math: Replace the valve, or replace the tank.



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