The performance of a $300,000 combination truck actually depends on the choice of the much less expensive piece of metal at the end of the hose
Contractor Duane Johnson recalls accompanying a city crew that was proud of its brand-new combination truck. They were trying without success to clean grout from a line. Johnson suggested switching to a high-speed rotating nozzle.
Upon deploying the nozzle, the operator looked at the pressure gauge, went into a panic, and hit the truck’s kill switch. He said the truck should not be operating at such a high pressure at the engine rpm he was using.
Upon questioning, it turned out that the truck had never developed adequate pressure because the crew had been using for the past eight years a single basic cleaning nozzle that had a useful life expectancy of six months. “I guarantee that crew never really cleaned a stick of pipe,” Johnson says. “They had been driving that truck all over town and never really cleaned a thing.”
This case, while extreme, illustrates the importance of nozzles in the pipe cleaning process, says Johnson, vice president of Affordable Pipeline Services in San Diego, Calif. The kind, quality and condition of nozzles ultimately determines the effectiveness of a cleaning truck in any given application, Johnson told attendees at an Education Day seminar at the 2010 Pumper & Cleaner Expo in Louisville.
Why it matters
A combination truck water pump delivers a specific amount of energy. The amount of that energy applied to the cleaning task depends on what happens to the water between the pump and the pipe.
“The first thing that affects energy is the hose,” says Johnson. “Every time you put a fitting on a hose, you lose seven pounds of pressure. For every foot of 1-inch hose, you lose one pound of pressure. The pressure loss for a 1-inch inside-diameter hose 600 feet long is 600 psi. So for a combination truck delivering 2,000 psi, the pressure at the nozzle is 1,400 psi.”
The number of fittings on the nozzle, the number and size of the jets, and the angles of the jets also affect performance. Therefore, it’s important to apply the appropriate nozzle to any given task. “Often, contractors pay a lot for a truck and then are reluctant to spend money on nozzles,” says Johnson. “But why would you spend $350,000 on a truck and then use a $60 nozzle?”
Flow versus pressure
At the most basic level, Johnson says, contractors need to understand the functions of flow and pressure. In general, pressure cleans pipe and flow volume provides thrust and moves material. “Going from 60 gpm to 80 gpm enables you to move more heavy solids,” says Johnson. “You’ll have more thrust to propel the nozzle down the line. But you’ll have less ability to cut material off the pipe wall.”
Conversely, while a 4,000 psi trailer-mounted waterjetter may be more effective at cutting roots than a 2,000 psi combination truck, the trailer jetter will be less effective in moving the debris out of the line because it delivers only 12 to 18 gpm, versus the truck’s 60 to 80 gpm.
Also fundamental to jetting, in Johnson’s view, is the use of skids. That’s first and foremost for safety reasons – to help keep the nozzle from shooting out of the manhole with lethal force if, for example, the cleaning truck operator is inattentive and fails to notice the leader hose emerging from the manhole. He prescribes a nozzle-skid combination 1.5 times the diameter of the pipe being cleaned.
A skid also improves cleaning: By keeping the nozzle off the bottom of the pipe, the skid enables more of the jets to do their work. While most nozzles are designed to be positioned at or near the center of the pipe, the application dictates the type of skid to use.
Johnson notes that operators may refrain from using a skid because it makes it harder to extract the nozzle from the line at the end of a cleaning run. Safety implications aside, “We spend twice as long cleaning the pipe because we don’t want to spend five minutes kicking the nozzle out of the line.”
Advances in nozzles
Johnson notes that nozzles have improved greatly in recent years – a huge variety of kinds and sizes enable contractors to fit a nozzle to almost any application. The question is which nozzles to select in the face of specific needs and a limited budget.
“The biggest change in the past several years is in nozzle technology,” Johnson says. “Nozzles used to be made by guys at lathes. Now they’re designed by engineers.” Nozzles are classified in three tiers.
Tier 1 nozzles consist of a steel housing with orifices drilled out in different locations and sizes and at different angles. “The issue with these nozzles is that they have a flat inner surface,” says Johnson. “So all energy we haven’t lost in the hose comes into the nozzle, hits the flat surface, and has to redirect and come out.
“There are circumstances where a Tier 1 nozzle will do the job, and circumstances where it might take a week to do what you could have done in four hours with a nozzle better suited for the task. These nozzles are less expensive, but they also deliver less performance, and they don’t last as long.”
Tier 2 nozzles are designed for longer life and better performance. Durable inserts can be removed and replaced while the main body of the nozzle remains intact. Some of these nozzles use long-lasting titanium or ceramic inserts and include flow straighteners that improve fluid mechanics by reducing turbulence.
Tier 3 nozzles are the next evolution. They include features such as internal ceramic discs and controlled rotation, but most important, they direct flow more efficiently so that energy loss is greatly reduced. The nozzles therefore operate at higher capacity and clean more effectively. In general, these nozzles cost significantly more than Tier 1 and Tier 2 nozzles, but they can make a drastic difference in performance.
Johnson advises contractors to select nozzles with an eye toward the application, be it general sewer or storm drain cleaning, clearing blockages, cutting roots, or removing tuberculation. Rarely will a single nozzle suffice.
“If you’re going to clean a neighborhood with 7,000 feet of pipe in one day, you can’t do it with one nozzle,” he says. “You’ll never find 7,000 feet of pipe in which every section has the same problems. Some areas will have roots, some will have grease, some will have sags in the pipe. Each condition requires a different nozzle.”
In shopping for nozzles, Johnson recommends asking manufacturers exactly what each nozzle is designed to do. By shopping with basic knowledge of nozzles and their characteristics, contractors can make choices that enable them to select an assortment that will meet everyday needs cost-effectively.
Finally, Johnson recommends training all waterjet equipment operators thoroughly, and for that he suggests starting with the Jetter Code of Practice Manual, published by NASSCO.
“Many cities have training programs for jetter operators,” he says, “but in the contractor world, training often passes from Bill, to Tom, to Fred, to Sam, to George, and something is left out in each of those exchanges. I would suggest that anyone who owns a cleaning truck get this manual. It is an excellent source of information, even for highly experienced operators.”