QUESTION:
I bought a truck recently and it has a good-sized pump with a four-way valve. I have always relied on gravity to off-load waste at the dump station. People tell me that if I use the four-way valve to unload my tank, I could get it done much quicker and be back on the road. Is this true? And if it is, what do I need to do to make it work? Nick Scznik
St. Louis, Mo.
ANSWER:
Just as your pump builds vacuum, it can also build pressure. By cranking your four-way valve handle 180 degrees and starting the pump, the tank will begin to build pressure. The “in” port is now the “out” port, and the “out” port is now the “in” port. This is what creates the pressure. By pressurizing the tank while off-loading, dumping can be done much quicker.
But there is a catch. Care and safety must be taken when building pressure and opening the dump valve. Just as vacuum is a great force for filling the tank, pressure will push out the contents of the tank incredibly fast. Stand to the side and don’t get in the way.
And one other point of information: You can shift the pump from vacuum to pressure while the pump is running. But don’t make the shift all at once. First, shift the four-way valve lever to neutral (a 90-degree turn of the handle) and wait until the existing vacuum bleeds off. This might take a few minutes. Take a look at your vacuum/pressure gauge and make sure that the vacuum returns to 0. Then shift the lever to the pressure position (another 90 degrees). This pressure should immediately begin to register on the gauge.
This process, done correctly, will save you time and get you back to pumping more quickly than waiting for gravity to work its magic. And in this competitive environment and challenging economy, every little bit of time-saving helps raise your business’s bottom line.
QUESTION:
I know it’s been discussed in your column before, but I still don’t understand why it takes longer to evacuate my tank in the mountains than it does for people who are lower in elevation. It just baffles me that this thin air would take longer to evacuate a tank than normal air. Ron Williams
Denver, Colo.
ANSWER:
You’re exactly right. It’s about the “thin” air. Since there is less air, one has to wonder why it would take longer to vacuum it out of the tank. For lack of a better word, we’ll call it “differential.” At lower elevations, when a technician evacuates the tank, there is a large “differential” between the inner tank and the outer air because there are more molecules.
At higher elevations like yours, the air inside the tank and the air around you is “thin.” In order to create that differential, the pump has to work longer to build the vacuum.
To give a couple of examples, at 5,000 feet elevation, the maximum vacuum you’re going to generate is 23.06 inches Hg, or about 23 percent less than at sea level. Jump up to 10,000 feet in elevation and your maximum vacuum is 20.58 inches Hg, or about 31 percent less than at sea level.
The key to remembering how this works is how much differential is being created when the vacuum pump is on. The higher the elevation, the more difficult it is to create this differential. You still get a pretty strong vacuum and your pump works longer and harder, but it is not going to be the same as sea level.
