Running on Empty?

With a sight glass installed at the lowest point on your tank, you can make sure all contents are evacuated, or remove the glass to flush out frustrating clogs

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QUESTION: I recently purchased a used vacuum truck. One thing I noticed is it has three sight bubbles on the rear head. One of those glasses sits right dead center at the bottom of the head. Am I missing something, or does that sight glass have a purpose? Why would I ever want to know if I have 1 or 2 inches of material in my tank? Did somebody put it in the wrong place?

Nick Wendeborn

Little Rock, Ark.

ANSWER: For the most part, you are right. The bottom-level sight glass is not going to serve any real purpose as you fill your tank on the job. But as I’ve thought about it, there are some good reasons to install a sight glass near the bottom of the tank.

First, the position of the sight glass indicates that your dump valve comes out the bottom of the tank. This allows for the tank to be fully emptied. Many tanks have the dump valve on the rear head where your sight glass is located, and if it isn’t flush with the bottom of the tank, the tank will not completely empty. Sometimes this can be a liability as the waste builds up over time.

Your sight glass on the rear head does allow you to see if the tank is empty. You can talk with a lot of pumpers and you’ll find many who thought their tank was empty, only to find they couldn’t finish a job because there’s no more room in the tank. So the sight glass really doesn’t serve the purpose of watching as you fill the tank, but it serves an excellent purpose when you are dumping.

Just because your dump valve is located out the bottom of the tank, there could still be reasons your tank wouldn’t empty properly. Something could block the outlet, or the materials could be too heavy or chunky to move during unloading. Consequently, materials back up and rather than unloading, the process is stalled. Time is lost. Money is lost. Worse yet, the tank is not empty.

One other benefit to your sight glass configuration should be mentioned. If they are 5-inch diameter glass bowls, they are most likely removable. If you see the tank is not empty, you have an easy way to remove debris. Wherever you see waste getting clogged, simply remove the glass bowl above that area and stick a hose in.

Now, because you have that bottom sight glass, you will be able to see the removal of the remaining waste. Hence, the guesswork is gone and your tank is ready for the next job.

Over the years, we never paid much attention to putting a sight glass down at the bottom. Because, like you, we thought it made little sense. If your dump valve is a fitting coming out of the manway on the rear head, you always run the risk – when the tank is plugged – of opening up the rear manway and getting a super-sized surprise.

Over the years we have generally installed three sight glasses on the rear head. One near the top, one near the middle and one about a foot up from the bottom. In most of those tanks, the outlet was coming straight out the rear head. Even when the rear dump came out the bottom, we still had that bottom sight glass a foot up on the head.

The truth is your sight glass located down at the bottom of the rear head can serve as a nice insurance policy for making sure you get maximum use of your tank. It ensures that each time out the gate, the tank starts out empty.


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