Bulkhead Inversion: Strange But True

Proper venting prevents substantial damage from popping a dished bulkhead between water and waste tanks

Question:

One of my drivers brought in our portable restroom service truck the other day and I couldn’t believe what I saw. The tank is a 1,500-gallon two-compartment unit split 1,000 gallons waste and 500 gallons for freshwater. I told my driver on his way out to stop at a hydrant and fill up the water compartment. He said he did, and that the interior bulkhead — which is a dished head — totally inverted. I mean it bulged one way before he put in the water, and in minutes it bulged out the other way. The driver told me he did nothing but hook up to the hydrant and then to a 2-inch adapter to fill the tank. What happened?

Barry Minton

Jacksonville, Fla.

Answer:

We agree. This doesn’t seem possible, but it happens. We’ve seen it a couple times in the past year. We’ve seen it in a steel tank and an aluminum tank. The interior bulkhead is a flanged and dished head. It is used inside the tank to handle the vacuum. The water compartment does not need dished heads if it is vented properly. Venting is the key to what we’ll call bulkhead inversion. Here’s what happened to your tank:

The driver attached the 2-inch adapter to the 2 1/2-inch line coming from the hydrant. He attached the 2-inch adapter to the bottom fill of the water compartment and turned on the water. The pressure coming out of a water hydrant usually runs 65-70 psi. When you reduce the fitting one-half inch, the pressure increases as the water goes into the tank.

As the tank began filling without ventilation, the pressure inside the tank continued to build higher and higher. The driver pushed in more water and it occupied more space. The air inside the tank wanted out and had nowhere to go. The pressure actually builds to the point that it pushes the dished head inside out, if you will, and it inverts. Most water compartments in vacuum truck tanks are designed for no more than 5 psi and even that needs to be vented.

By rifling 65 psi of water into the tank without the properly sized vent — or worse, no vent — your driver was asking for trouble. The tanks we’ve seen in our shop also had damage done to that interior head in the form of leaks. So the air did finally escape by breaking welds or by finding weak spots to create small holes. The damage needs to be repaired. Overall, this weakens the steel or aluminum head inside the tank, but it can usually be repaired.

EXPENSIVE REPAIR

Before anyone asks, let me add that you don’t put pressure on the other side and “blow” it back into its original position. As mentioned, the material in the bulkhead is already somewhat weakened. We simply repair the cracks and the leaks and put the truck back in service. So far that has been enough.

If the bulkhead is damaged beyond repair, and requires replacement, you are looking at a major fix. The trick becomes how to get a bulkhead inside a tank without an opening big enough to accommodate the size of the bulkhead. Either one end of the tank must be removed or the bulkhead must be cut into pieces, inserted through a manhole, and then reassembled inside the tank. Either way, it’s an expensive repair.

So take a look at your tank. Does your water compartment have adequate venting? Does it have any venting? It’s a simple and inexpensive process to install. A simple 2-inch screened mushroom vent will do the job in most cases.

Just as vacuum can literally suck down a tank and cause it to collapse, pressure can have the same power to either blow heads out or “invert” the interior bulkhead. Vacuum and pressure are incredibly strong forces and paying attention to them — knowing how they work — and dealing with them properly will keep you in business and save you a lot of money.



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