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Pressures in Sewer Cleaning
Tech Tips by NASSCO is a bi-monthly article on trends, best practices and industry advice from NASSCO’s trenchless technology membership professionals.
Without a doubt, the most misunderstood facet of sewer cleaning is the relationship between flows and pressures. Generally, pressure is mistakenly viewed as the wildcard or the reason for concern, and flow is usually ignored. Operators often misunderstand the operation of their pump, the relationship to hose diameter and length to performance and other variables of jetting.
A knowledgeable sewer equipment operator should know several things about the equipment being used and the lines to be cleaned:
GPM (gallons per minute) of the pump What is the total output capacity of the pump on board? How long can a machine jet before being out of water? If a pump operates at a max of sixty (60) gallons per minute, and the truck being used has a fifteen hundred (1,500) gallon water tank, then the answer is twenty-five (25) minutes, i.e. 1500/60 = 25.
PSI (pounds per square inch) pressure rating of the pump How is the pump on board rated to operate in a pressure range up to a maximum pounds of pressure (there are safety factors built in, but it is not wise to exceed the manufacturer’s designed operating pressure)? It should be noted that a pump does not create pressure (this is done by the nozzle and how it restricts flow at the end of the hose), it only creates flow.
Hose diameter, hose length, and pressure rating the jetter hose What is the diameter hose being used? Half inch (1/2”) systems have a greater pressure loss per hundred feet than a three quarter inch (3/4”) system, and so on. Further, how much hose does the jetter have on board? A jetter with a four hundred foot (400’) hose experiences far less pressure loss per hundred feet than a jetter with an eight hundred foot (800’) hose, etc. Finally, what is the pressure rating of the hose being used? Sewer hoses are color-coded to indicate operating pressures.