Drafting - A two-step process

One of the most difficult things for new (and old) pump operators to grasp is the concept of getting a “draft” from a standing body of water or a portable tank. Many fire service personnel often debate the best RPM at which to run a pump at in order to get a “primed pump” with the least amount of difficulty. The reason these questions probably developed lies in the history of motorized fire apparatus.

Years ago, centrifugal pumps on pumpers were often “primed” (air removed from the pump cavity) by an exhaust primer or a transfer-case driven rotary-gear priming pump. In either case the effectiveness or output of the priming pump was dependent on engine RPM to perform. Generally, the faster the operator ran the engine, the better the exhaust primer or gear-case driven rotary-gear primer would evacuate the air. Pump operators were often taught, “Run the pump at 1500 RPM to get a prime” or some other words of wisdom. There were basically two different pumps operating off the same drive-line. The fact that the impeller was turning at a certain speed had nothing to do with evacuating air from the pump. As we know, a centrifugal fire pump can’t pump air. In fact, higher engine speeds increase the difficulty of gaining a draft!

So along come 12-volt, electric driven primers. These drivers are totally independent from the main drive-train of the truck. Many operators have been taught to rev up the throttle even with though these primer drivers were driven by a separated 12- volt electric motor. About the only benefit to the higher RPM was the headlights wouldn’t dim as much when the high amperage of the primer would put a load on the batteries. Thus the “RPM= drafting success” myth was perpetuated.

Walk up to any modern pumper and pull the primer with the battery on but not with the motor running. If you hear the loud electric primer sound, that proves the priming pump is totally independent of the engine/pump speed. This leads up to an obvious question. Do we need the fire pump turning at all to successfully get a draft? The answer is no. We need it turning only to pump water.

While this may seem radical, let’s think about it another way. Air is compressible and expandable. Water basically is not. Think of a squirt-gun half full with water. Pump and pump the gun and there is more air in it (the air is changing size). The water remains the same “size” and is basically incompressible. The rotary- gear or vane pumps in our primers operate on the principle of expanding air, ultimately removing it from the pump cavity. Once the water has risen up through the suction hose and the primer is starting to pump water (gurgling sound), there is little more the priming pump can do (water is incompressible). A primer actually does a poor job of pumping water at that point.

Now think about the fire pump. If water is incompressible, which it is, and centrifugal force is added by putting the truck in gear, starting the impellers turning, there has to be pressure in the pump. If the primer has evacuated all the air, suction lines and pump are reasonably tight, and the pump is full of water, there is no way you cannot have a solid reading on the pressure gauge.
Try this the next time you are out at a drafting site. Get out of the cab without putting the pump in gear (probably better for the increasingly popular mechanical seals). Set up your suction hose and with a dry pump pull the primer. Prime until you get all the air out of the pump and the primer starts spitting water. Go in the cab, shift from road to pump and put transmission in drive. When you return to the pump control panel you’ll notice 30-50 psi on the gauge, ready for you to advance the throttle.



Northeastern Fire Associates, Inc
128 Blacks Road, Cheshire, CT 06410
Phone 203.217.5809 Fax 203.272.6833

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