Striking a Balance Between Pressure and Flow

Jetting a sewer line requires both pressure and flow. How much of each depends on what you’re trying to accomplish.
Striking a Balance Between Pressure and Flow

In the most basic sense, you need pressure and flow to jet a sewer line. The pressure is your cutting ability and the flow is your flushing ability.

“You need both, but generally you take whatever kind of pressure you need to clean the pipeline and you add flow to it and that will speed up the process,” says Andy Whitehead of American Jetter.

But finding the right balance between the two can be a little more nuanced, and several different factors come into play.

HOSE SIZE

Say you have a machine that produces 40 gpm at 2,000 psi. Just because the gauge reads 2,000 psi doesn’t mean that’s the pressure coming out of the end of the hose. A certain amount of pressure loss will occur depending on the size of the hose. In the case of a 40 gpm, 2,000 psi machine, the pressure loss would be about 600 to 800 psi — depending on the length of hose — with a 3/4-inch-diameter hose, says Nick Woodhead, founder and president of US Jetting.

“That is probably the one mistake most operators make is that they look at the gauge and think that is what they are getting at the end of the hose and it is not,” Woodhead says.

Pressure loss is a combination of the hose length and its diameter. The pressure loss will be greater as the hose diameter decreases, and the length increases.

PIPE SIZE

Pipe size also has to be considered, especially in terms of flow. A nozzle is going to lose its velocity more quickly in a larger-diameter pipe, so adequate flow is needed to maintain cleaning power.

“If the nozzle is in the center of the pipeline, a higher-flow nozzle is going to have more impact on the walls of the pipe than a lower flow if it is any distance from the pipe wall,” Woodhead says.

Adds Whitehead, “Contractors judge pressure as the end-all a lot of times and pressure is important but it is also important to have that flow. Once you cut away whatever is in the way with pressure you have to have the flow to push it down the pipeline.”

THE RIGHT MACHINE

According to Woodhead, the secret to ensure you have the right equipment is to think of the goal.

“One of the things that I ask people when they call me is, ‘What is your main objective? What are you trying to do with the machine?’ Their answer will determine what machine we offer them.”

Smaller jetters typically operate at higher pressures and lower flows, and pressure gets the nod over flow for cleaning smaller lines. Flow is more important than pressure for larger pipes.

For example, if a customer is cleaning 24-inch storm drains full of rocks and sand, volume is imperative. Woodhead says he’ll recommend a machine that is in the range of 40 to 80 gpm. If a customer is cleaning sanitary sewer lines 4 to 10 inches in diameter with a lot of grease and root buildup, Woodhead says he’ll suggest a machine with lower flow (18 to 20 gpm), but much higher pressure (4,000 psi). High pressure is what is needed in order to actually cut roots.

“A high-volume system (normally 2,000 psi, 65 gpm) does not have enough pressure at the nozzle to cut the roots so it has to have a mechanical root cutter,” says Woodhead, adding that often the cutting heads that can solve the problem become stuck and require extraction. “A high-pressure, low-volume unit uses the speed of the water coming out of the nozzle to do the cutting. For instance the speed of the water with a 4,000 psi, 18 to 25 gpm unit is approaching 470 mph as it exits the nozzle which is sufficient to cut the roots on contact and also to pulverize grease into small pieces when a rotary nozzle is used.”

According to NASSCO, some pipe condition factors also have to be part of the pressure and flow equation. Water will move through a pipe more slowly on a flat grade, so in those situations it is better to err on the side of less flow since a pipe of any size has a finite capacity. And pressure should be kept in check depending what defects a pipe may have. For example, clay pipe with severe cracking should be cleaned at a lower psi than concrete pipe in relatively good condition.



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