The Other Half

Knowledge of good vacuuming procedures can help operators get more performance for the investment in a combination truck
The Other Half
Distance matters when it comes to sizing a combination jet/vac truck. The farther the debris has to travel, the more friction the vacuum must overcome.

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Getting the most out of a combination jetter/vacuum truck requires homework and knowledge. After 25 years of selling equipment, sales manager Rick Lewis of MAX-LIFE Manufacturing in Stanton, Calif., says, “I’m amazed when I come upon people who invest in a product who never took the time to learn about it. You have to know your equipment, your sewer system, and the material you’re working with.”

Speaking at the Education Day seminar at the 2011 Pumper & Cleaner Environmental Expo in Louisville, Lewis shared a story of working with a contractor that had problems with a combo truck. The company had used a fan machine, which vacuums only on the surface, for years before getting a positive displacement (PD) unit. “They’d had the PD for four years, but nobody taught them their PD could be used underwater. That’s a waste of money and time.”

 

PD vs. fan

The first decision is selecting a PD or fan vacuum. Fan machines dominated the combo market for a long time, but PD units now make up about 70 percent of the trucks on the road today, according to Lewis, who is also a member of the Southern Sections Collection Systems Committee.

PD vacuums are designed to work over long distances, work underwater, and most run off the truck engine. “A PD with 27 inches of vacuum moves 4,500 cfms and can vacuum 200 feet vertically and 500 feet horizontally,” says Lewis. “It can vacuum underwater all day long with a fluidizer tube.”

Fluidizers should be placed no closer than six inches and no greater than 72 inches from the surface of the liquid. “You need to get the vacuum tubes right to the bottom and get them working; you’ll get less water and more debris that way. Some trucks have the ability to shut off the vacuum with a flapper valve so you can submerge the tubes all the way to the bottom before turning it on. That lets you get debris from the bottom and put less water on the truck.”

 

Horsepower

Determining horsepower needs will take some homework. “Cities need to know their systems and contractors have to know their customers. Will they need large-line cleaning or will they be doing storm drains? You need to consider all those things and find out what you’re going to be vacuuming the most. The weight of the debris will drive your decision on the sizing and getting the right horsepower.”

You must also consider the other equipment drawing from the engine, as well. An average combo truck will have a water pump that draws around 175 horsepower, the vacuum (170 hp), hydraulics (25 hp), and you need about 15 hp for reserve power.

In his days as a contractor in southern California, Lewis says he went for the biggest. “I wanted the most horsepower I could get, the biggest PD and the biggest water pump, because there was such a variety of work. You may not need to buy a monstrous truck to handle everything.”

 

Sizing a PD

PD vacuums come in sizes from 15 to 27 inches of vacuum. “If we used four trucks with different sized PDs to vacuum the same slurry, you’re talking about 30 to 35 seconds difference in fill time,” says Lewis. The smaller vacuums, though, may not be able to handle heavier material. “What are you vacuuming? What is the distance and how much does the material weigh?”

Proper sizing also allows you to run at lower rpm. “PDs are very efficient at lower rpm if you have adequate horsepower,” explains Lewis. “Once you start vacuuming the material, you can bring down the rpm. The vacuum will drop, but the material is still being conveyed and you’re not running the heck out of your truck. The same can be done with jetting.”

One key difference between a PD and a fan unit is that PDs don’t start working until needed. “Until a PD comes in contact with the material being pumped, it is at rest,” says Lewis. “Even though you have the truck running, the gauge will show only about 5 inches of vacuum. Once you put the tubes in contact with the material, the gauge is going to jump up to full vacuum pressure.”

The configuration of the truck also matters because every bend in the vacuum system reduces vacuum pressure. “It’s a very important factor that many people forget,” he says, noting that a vacuum’s actual performance may not equal its rated capacity. “A rough guideline is a 5 percent drop in vacuum pressure for every bend.”

 

Use the right tubes and use them right

“I’ve had people with an 8-inch tube say they’re not fitting into the trough the same way they did with a 6-inch tube,” Lewis says. “Get a reducer and fit a 6-inch tube on there; people don’t know they can do that.”

He’s also noticed that tubes with holes at the crown end, which are needed for fan vacuums, are being used with PDs. “The holes create turbulence and interfere with the PD vacuuming,” he says. “Put some duct tape over those holes and you’ll see a difference.”

Tubes also have to be kept in good condition. “You’re asking tubes to convey material; they need to be in good working order,” says Lewis, who has seen plenty of tubes with dents and dings, missing gaskets, and patched with duct tape.

Like the tubes, gaskets and flanges are also an integral part of the vacuum system. Gaskets are among the first thing Lewis checks when he goes out on a trouble call. “The tighter the system, the more true vacuum you’re going to get. It never hurts to carry extra gaskets on your truck. It’s the same with flanges; they get abused and wear out.”

 

Protecting the system

Jet/vac trucks are a big investment, but Lewis says they generally don’t get the attention they deserve. “You need to understand how to protect your unit. It will run for years and years if you take care of it properly.”

He says the most common causes of serious problems are operator error and lack of maintenance. “The primary filter is supposed to get cleaned daily, yet nobody does it. Then they wonder why it’s not picking up like it used to.” The primary filter may be enough to protect a PD truck. Vacuuming dry material requires a cyclone separator, while a secondary filter is needed for hydroexcavation.

Relief valves must be maintained to prevent the debris tank from collapsing. “That’s why your debris tank is rated at 50,000 psi,” says Lewis. “You need the air coming in through the relief valves or your vacuum can collapse the tank; that’s the scream you hear when you vacuum.”

Relief valves are sized to the unit based on the size of the vacuum, so there could be two, six, or even eight valves. They do wear out and springs can get rusted, so they have to be checked and maintained. “Do not try to rebuild one,” he warns. “Take the top off the valve and that spring is going to come out right at you and cause injury.”

PD lubricant levels must be maintained by regularly checking the oil level sight glasses. It also must be the proper kind of oil, nondetergent and nonfoaming. “Put regular motor oil in there and you’re going to have a real problem. So read the manufacturer’s recommendation. The sight glasses are supposed to be half-full when the PD is not running. They will go full when it is running.”

The oil breather is another maintenance item that is often ignored. “Vapor comes out and dirt will build up on the breather,” he says. “Make sure it’s clean, especially when working in dusty environments or driving on unpaved roads.”

For a fan unit, the critical component is the edge of the blade. If they get chipped, the fan will lose its ability to move air and vacuum pressure can quickly diminish to unacceptable levels.

Know what the customers need, understand the truck and its equipment, and follow proper maintenance guidelines; Lewis says those actions will ensure that an investment in a jet/vac combo truck pays off for years for come.



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