Supporting the Vacuum Tank

Shopping for a new pump truck, a poster wonders about the best way to support the weight of the tank barrel.

Trough or no trough?

Question:

I need some help. I am having a tank fabricated and the company does not think I need a full-length trough on the tank. They say all I need is a 4- or 5-foot piece up front and the same in the back. Also, the builder doesn’t recommend using any membrane (rubber, wood) between the trough and frame rail of the truck. This is the only manufacturer I have come across that deletes these items. Is this a big deal or should I spend my money on something else?

Answers:

Dropping the membrane between the tank and frame is like leaving the rubber out of motor mounts; it should have rubber (lasts longer than wood) between the tank and frame or you will hear clunking every time the tank moves. The tank should not be mounted solid. It should have at least one spring mount in front or back for flex. The hose trays should be full length. It is much easier on the hose. Every time the hose flexes, it loses some life. On the back of my truck I have three hose hooks, giving the hose less bounce, less flex and more life.

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We have always manufactured full-length skids made of 1/4-inch material. I don’t think anyone could argue that it gives greater strength to the barrel. Tanks always collapse from the top down, never from the bottom up! A full-length skid would act as a rib. Also consider equal, even weight distribution as your tank and truck frame twist and contort.

I have seen skids that had been stitch-welded with cracked welds from twisting, contorting and varying pressure points. Skids that are 100-percent welded prevent moisture entrapment. This entrapment will cause extreme corrosion problems down the road. I have also seen skids made of lighter material buckling.

Wood, plastic or rubber act as an isolator between skids and truck frames. The idea is you don’t want to lock down the tank to the frame, but allow the movement it needs. This becomes more important with larger diameter and longer tanks.

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I strongly recommend full-length hose trays and rubber skids with a stainless-steel channel between the rubber and tank. Wood holds moisture and shortens your barrel life. The stainless-steel channel is a barrier you should have. Wood will not last the life of the tank; it will either compress or rot and increase stress on your tank over time. You should get the best quality features to ensure longer tank life.

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I have seen a lot of other trucks with just 3-foot pads for resting the tank on the truck frame. All of these trucks are for oilfield work, so they have to be certified. Keep in mind that these tanks have two or more stiffeners around the diameter of the can, and they have six pads, two in back, two in the middle and two at the front of the tank. All have hoists, so they are not mounted to the truck except by hinges at the back of the tank. For what it’s worth, my new tank has six 4-foot rails for resting on the truck. I installed two stiffeners around the diameter of the can as well. It also has a hoist.

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With hoist units, you have your rear hinges as a solid point and the front lift cylinder is also a capture point. When the hydraulic cylinder is collapsed down, it has not bottomed out steel on steel, but nested on a bed of oil. So hoist units will float and have some movement. The full-length skid may not play the role as they do on a non-hoist tank. In any event, full-length skids give greater strength and integrity to the barrel.

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It’s true the tank is not tied down, and there is movement. But when the hoist is down all the way, there is not enough oil left in the cylinder to act as a cushion. I have pads under the front corners of the tank and on the truck frame showing wear to prove that.

I agree that when a tank collapses, it collapses from the top and not the bottom (I’ve had two do that to me), but the top has no more support than the metal of the can itself (and from the baffles if you have them). The bottom does have support though.

How much more strength does a full skid give a can as opposed to shorter pieces? There is room for argument that shorter pieces of rail have enough support, depending on where they sit (I think the middle of the tank has to be supported as well), and it would weigh less. Not very much less, but as they say, “look after the ounces and the pounds will take care of themselves.”

PTO or separate gas-powered pump?

Question:

I was asked which is more economical, a small gas-engine driven vacuum pump or a power takeoff-driven vacuum pump running off the transmission? I didn’t have an answer and figured maybe you guys could help. I figured since most trucks run with a PTO, it is probably cheaper. I’m sure the numbers have already been crunched.

Answers:

Any diesel engine is going to be far more efficient than a small gas engine, and less polluting as well. I also think of the cost, space, and weight issues of carrying around and maintaining an extra pump. The PTO is definitely more convenient.

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The real issue is “how many CFMs do you need?” Even a small pump will produce vacuum, but it may take forever. Large vacuum pumps require big horsepower, making PTOs more efficient.

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Most small gas engines are not designed for long life or heavy use, and will need to be repaired/replaced more often than the PTO. Then there is the hassle of having more maintenance records, separate fuel sources to manage and maybe different oils and filters.

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The best thing about the PTO is if I can drive to the job site, I am sure the PTO-driven pump will work all the time, unlike a gas-powered auxiliary engine that will refuse to start when most needed.



Discussion

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