When choosing a jetter’s output, first determine if you need 'cutting' power or 'flushing' power
I designed and built our first jetter 20 years ago; a little 3.5 gpm/4,000 psi portable unit. Back then I didn’t really know what a jetter was, I just had a set of specs.
I’ve learned a lot since then, and the most common question I get asked is: “When choosing a jetter, what is more important: gpm or psi?”
When choosing a jetter’s output in psi and gpm, remember this hydrojetting rule: Pressure (psi) is “cutting” power, while gallons per minute is “flushing” power.
Pressure cuts into blockages
“Thinner,” high-pressure waterjet streams from a good jetting nozzle will have the most “cutting” power if the psi is adequate. Experience has shown that a 3,500 to 4,000 psi jetter with 6 or more gpm and the right nozzle or tool will cut out root masses, chop up hard grease, and carve out hard settled dirt. We’ve shipped dozens of jetters that do 9 gpm/4,000 psi for cutting out this kind of crud from 3- to 8-inch drains and sewers. Pressure cuts are also why 4,000 psi trailer-mounted jetters at 18 gpm are so common with contractors who get paid to cut out blockages. This is also why the little 1,500 psi electric-powered units don’t cut out much at all.
Volume flushes out debris
“Wider” waterjet streams shooting out of a jetting nozzle will have the most “flushing” action, and more volume also means more impact in the cleaning. If the gpm is adequate, the sheer volume of water will push and carry debris. If you want to move larger rock, bricks, grease-logs, etc., you’ll need more gpm. This is why city and county crews clean 8- to 12-inch pipe with 40 to 60 gpm, and larger pipelines with 80+ gpm trucks. This is also why little 2 gpm electric jetters don’t flush out much debris. The gpm you will need depends greatly on the pipe diameter being cleaned and whether or not you really are required to flush/scour the line clean, or if you just need to penetrate a blockage to restore flow.
Example from the field
The following is a story from a contractor who was hired by a city to video inspect an 8-inch pipe: When he got there, the city crew had been working for two hours trying to clear roots from the line with their big 65 gpm/2,000 psi jetting truck. The contractor had a 6 gpm/4,000 psi jetter in his van along with his TV system, and the city let him take a crack at the roots. Using a Warthog slow-rotating cutter nozzle, the contractor cleared the roots and then finished the video inspection. Notice that he got done with a 20 hp unit what the city could not do with a 100 hp unit. Why? Because a) he had the right tools; and b) pressure cuts. The contractor said, “You should have seen their faces!”
In contrast, it is important to also realize that if we were talking about clearing rocks and debris (instead of cutting roots) then the story would be the opposite. The 65 gpm truck would run circles around the 6 gpm jetter, because volume flushes. Note: Using the right nozzle or tool also makes a huge impact. Choose the right tool for the job.
Choosing minimum jetter gpm
The gpm-to-pipe-diameter ratio is another good rule of thumb in choosing minimum jetter gpm. The gpm of the jetter and nozzle should be at least equal to the diameter in inches of the largest pipe you clean, and the best cleaning is a 2-1 ratio (i.e. 8 gpm/4-inch pipe). Therefore, before choosing a jetter, it is wise to consider: a) what are the most common pipe sizes that your jetter will need to clean, and b) what will the jetter’s primary job be (flushing debris/rocks/dirt, etc., or cutting out grease/roots, etc.)?
Most contractors are in the job of re-establishing flow in drains and side sewers. You can break through blockage in 4-inch and smaller pipe with less than 6 gpm, while the 8 gpm to 18 gpm machines will do a better job of cleaning and are much better in 6- to 18-inch pipe. Again, choose gpm based on what size pipe you most commonly deal with.
Also, understand that a jetter’s gpm is always listed at maximum throttle, so if the engine speed is reduced then the gpm reduces. Therefore, if you can afford a higher gpm jetter then you can also “slow it down” for use in smaller-I.D. pipe. The pressure will be maintained if you also have nozzles sized for the lower gpm in addition to your standard full gpm nozzles. It also works great to do this with a smaller-diameter “mini” jet hose, a very common and profitable practice among high-flow jetter operators.
Hope this was helpful — get jetting!
For more information on jetting equipment, visit www.jettersnorthwest.com.