Get to Know Your Vacuum Excavator – A Better Understanding of Vacuum Excavators Will Serve Contractors Well as the Machine’s Use in Potholing Increases
By Gregg Hennigan
Vacuum excavators were born out of the necessity for increased safety during digging.
There are numerous applications for this piece of equipment, including locating underground utilities, cleaning up drill fluid on horizontal directional drilling (HDD) jobs, cleaning water valve boxes, cleaning sewers, industrial vacuum excavation and more.
In the underground construction industry, vacuum excavators are increasingly being used for utility locates, which is commonly called “potholing.” This is the use of pressurized water or air to loosen soil to be removed via vacuum, allowing an operator to physically see the location of existing utilities. Identifying these lines is an important part of any job, and it’s getting more difficult as the underground infrastructure gets more crowded.
“More contractors and municipalities are turning to vacuum excavators for their potholing work,” says Jeff Wage, vice president of McLaughlin, a manufacturer of vacuum excavators. “They see the advantages over other methods.”
Vacuum excavators can help underground crews verify the exact location of utilities, are less likely to damage a line than an excavator or shovel, and cause less ground disruption than other equipment.
As the use of vacuum excavators spreads, contractors and municipalities may benefit from a better understanding of how the machine works. This could help them be more knowledgeable about its operation and help them find the vacuum excavator that best fits their needs.
Considerations during the purchasing process include:
• the type of work the vacuum excavator will perform
• the spoil and water tank capacities
• air, hydro, or combination unit
• other common specifications like the engine, the blower and size of suction hose
Answering those questions is not too difficult, especially when consulting with a knowledgeable dealer or manufacturer. What can be harder is understanding how the machine actually works.
A simple explanation is that a vacuum excavator uses pressurized air or water to loosen the ground, suck up the debris through a suction hose, and deposit the material into a tank. A lot of science goes into making that happen. Inches of mercury, cfm and cyclonic filtration are all terms you’ll hear associated with vacuum excavators.
It all starts with the vacuum blower, which plays a pivotal role in the creation of the suction, or vacuum, that lifts the spoil. The two primary types of blowers in vacuum excavation are positive displacement blowers and centrifugal units.
In a positive displacement blower, air enters a blower and is trapped against a cylinder until it is forced (displaced) through a discharge pipe. A centrifugal blower, also called a fan, typically uses rotating impellers or blades to increase the pressure of the air before discharging it.
Positive displacement blowers are increasingly the most common type used in potholing applications, according to John Mattick, Vice President at P.D. Blowers Inc., a distributor and service provider for positive displacement blowers, vacuum pumps and related equipment.
“With a centrifugal fan, you can get a lot of airflow, but if you have any change in vacuum pressure and lift, it changes that air flow drastically,” Mattick says. “Positive displacement blowers are, more or less, constant volume machines that maintain velocity and lose very little airflow when in operation.”
A key specification with a blower is cfm, which stands for cubic feet per minute. It is the measure of the volume of air that is being moved. The cfm specification on a vacuum excavator must be in direct correlation to the diameter of the hose on the machine for optimal performance. Optimal hose size is dependent on the type of spoil most commonly being removed. For example, cobble may be easier to remove with a 4-inch or larger suction hose and will require a larger blower than less challenging ground conditions where a 3-inch hose would be optimal.
“If you’re using a 3-inch-diameter hose, you do not need as much cfm to maintain velocity,” Mattick explains. “If you’re doing something that requires a 6-inch hose, then you need a lot more cfm to keep the velocity the same.”
You want enough velocity so that the spoils, once they enter the hose, are suspended all the way to the tank. If the material settles in the hose, it could clog the system.
In the underground industry, there are fairly direct relationships between cfm and hose size. For a 3-inch diameter hose, the standard is 575 cfm. It’s 1,025 cfm for a 4-inch hose and 3,000 cfm for a 6-inch hose. When you get up to the large 8-inch hoses, the lowest is 3,600 cfm, but they generally run 5,500 cfm or higher.
Inches of mercury is a unit of measurement for pressure. It is more or less a function of how much material the vacuum excavator can move, or lift, at a time.
“When you’re creating a vacuum, you’re basically removing some of that atmospheric pressure,” Mattick says. “When you’re at 15 inches of mercury, that is actually the lift involved to be able to pull a column of water 17 feet up into the air.” Wage says that for a potholing application and most drill fluid cleanup, 15 inches of mercury suffices, and that’s the standard for trailer-mounted units in the underground industry. More inches of mercury may be beneficial when dealing with heavy fluid or slurry, or in industrial settings.
IMPORTANCE OF FILTRATION
The filtration system on a vacuum excavator is very important. A vacuum excavator inhales a large amount of material — some of it large, but most of it small. If not properly filtered, that material would progress into the blower. Most vacuum excavators have a filter between the spoils tank and the blower.
Even better are vacuum excavators with cyclones that reduce the amount of material that reaches the filter. An example is a three-stage filtration system, in which a cyclone separator catches much of the material and deposits it into a canister that can be emptied. Any fine material that gets through is captured by a 2-micron poly filter.
“If the filtration system does not stop debris from getting into the blower, it can damage the blower or wear on it over time, which can cause a catastrophic blower failure or cause the blower to lose performance, and the vacuum excavator doesn’t move air as it should,” Mattick says.
Changing filters as scheduled is one of the top maintenance tasks an owner should perform to help keep a vacuum excavator healthy.
Mattick says proper lubrication is a priority with blowers. A blower running at 15 inches of mercury can reach 300 degrees Fahrenheit. He and Wage both recommend changing the oil every 500 hours.
“The oil will break down faster at that high a temperature,” Mattick says. “It’s a good idea to change the oil on a more regular interval than you would normally think is necessary.”
He also recommends, if applicable, greasing the blower every couple of weeks, although he says some manufacturers have developed smaller blowers with a dual oil bath system that require just the oil change rather than greasing every two weeks.
Also make sure the V-belts have the proper tension. If left loose, a V-belt can slip or fray and come off the machine. If too tight, then the blower shafts can fail.
Gregg Hennigan is a technical writer for Two Rivers Marketing in Des Moines, Iowa. He frequently writes about the underground construction industry.
This article originally appeared in the Tennessee 811 November 2015 newsletter.