Some recent emails from readers have come in asking whether or not they should leave their pump running in a ‘dead head’ state. Or put another way: what is a reasonable amount of time that a pump should be left under pressure when it is not being used to process material?
So as not to bore you, I’ll get to the answer – 5 minutes for a direct drive rotary pump and 10 minutes for an intensifier linear pump are good rules of thumb. However, always shut the pump off if you are going to work on the machine or personnel will be within 3 feet, or 1 meter, of the machine.
By the way, ‘dead head’ is a term used to imply that the pump is running, the lines are fully pressurized, and the cutting head is not throwing water. Advanced control systems can be set to shut the pump off at the end of a part program or nest. In addition, programmers and operators often have the machine move the head to a far corner to facilitate access to the work area after completing the part program.
The main factors in consideration when determining whether you should shut your pump off or not are: operator safety, delivery line life, power, pump wear and efficiency. Safe operation of a waterjet machine, as you would expect, dictates that you don’t work on a machine that is under pressure. As for the high pressure delivery lines (they are usually stainless steel lines, fittings and swivels that run from the pump up to and including the on/off valve), they see the greatest fatigue impact going from ambient (zero) pressure to full pressure. Holding the highest pressure has a far lower impact on life than going from zero to full pressure. The cost of power factors into this decision as well. If power is expensive then you might want to cut my suggested times in half.
Pump wear varies depending on whether you have a direct drive rotary pump or an intensifier linear pump. For an intensifier linear pump, the intensifier stops shifting back and forth and no water is consumed other than cooling water, so there isn’t much wear taking place within the pump system when dead heading. For a direct drive rotary pump the high pressure water is always being generated whether it is being used to cut or not. When the water is not going out of the cutting head it is sent down a bypass valve of some type. For my equipment, the bypass is a pressure control valve called the Pac-V. Therefore, this pump style exhibits greater pump wear than the intensifier when in a dead head condition. That is why I recommended only 5 minutes of dead head for direct drive versus 10 minutes for intensifier.
Today’s waterjet machines can handle 5 to 10 minutes of dead heading without issue. Some of my customers even leave the pump up all day, only shutting down for lunch breaks, machine access, or maintenance. That’s their choice, and they know their equipment well enough to know it makes sense for their facility.
Hope this was useful. Let me know in the comments box below if you have any questions or if you have suggestions for future topics.