A Double Dose of Protection

The primary shutoff and secondary moisture trap work in tandem to keep waste from fouling your vacuum pump
A Double Dose of Protection
Figure 1


I’m looking for an explanation or a description of how the primary shutoff works. Maybe I’m confused, but it seems the secondary should be plenty protection for a vacuum truck. What can you do to help me understand the primary and why it is so important?

Jack Hempstead, Billings, Mont.



The purpose of the primary shutoff as well as the secondary moisture trap is to provide protection for the vacuum pump. This is accomplished by making sure that waste — solid or liquid — does not work its way into the vacuum pump, where it can cause damage and prompt expensive repairs. So how does it work?

Sometimes a picture provides more insight than words. Take a look at the basic primary design as shown in Figure 1.

In the photograph, you can see the area called the cage, where the ball float rises and drops depending on the amount of waste in the tank. As the tank fills, the ball rises, and when the tank is nearly full, the ball seals off the passage to the secondary. If left operating, the pump will attempt to create more vacuum but with the passage to the secondary now sealed shut, the vacuum relief valve will need to open and relieve the increasing amount of vacuum. These two components, the primary shutoff and vacuum relief, work together to save the life of your vacuum tank and pump.

There is no doubt that, over time, random bits of debris get caught in the airflow and head up through the primary shutoff. More and more, especially if the tank fills to capacity, the waste and the water eat away at the primary. As you see in Figure 1, the cage is painted with primer to help prevent rust and corrosion.

As the airflow of vacuum moves at a steady rate, and then rests overnight, it is impossible to stop corrosion entirely. Figure 2 is a photograph showing the internal wear and tear of a primary shutoff. The sludge builds up and moisture attacks the steel. You can clearly see the ball float in the primary.

What can occur is that the bottom bar that keeps the stainless steel ball in the cage corrodes and the ball falls to the floor of the tank, removing a line of protection for the pump.

Nearly all vacuum trucks take advantage of the primary shutoff. It is not necessary, but when it comes to the economics of running a vacuum truck, its nominal cost promotes longevity and profitability for the pumper.


Truck Corner Quiz Winner Announced

Congratulations to our “Test Your Vacuum Truck Knowledge” quiz winner, Todd Boelter from Minden, Nev. Todd was the first entry to come up with all the right answers to the Truck Corner test that ran in the May issue of PRO. For those of you who played and for those who are still wondering, here are the correct answers to last month’s quiz:

1. C

2. B

3. C

4. D

5. D

6. F

7. F

8. A

9. B

10. A.

For being the first reader to answer all the questions correctly, Todd received a copy of the book, Pumper 101 – The Complete Guide Owning and Operating a Vacuum Truck. Thanks to all who entered.


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