W. Parker Browne
It's a given that suburban and rural fire departments that routinely draft from folding tanks or other static sources must have well-trained pump operators. I have learned during fire pump operations and drafting water from a static source instruction that priming the centrifugal fire pump is generally one of the most challenging and hardest tasks for the student to understand and master. Automatic priming offers ease of operation when priming a centrifugal pump.
Automatic priming was a standard feature and natural benefit for fire pumps installed in most of the early fire trucks built in the United States. These fire pumps were typically positive displacement, either piston or rotary type. Positive displacement fire pumps were truly self-priming because they were able to pump both air and water. The operator just connected a suction line from the pump to the water source, engaged the pump, opened a discharge, and waited for the air to pass through the pump and then for the water to flow. It was that simple.
If the pump swallowed some air during operation because of turbulence at the entrance to the hose, or even a slow leak in the hose itself, the fire pump would move air through and continue to provide water pressure as long as the pump was running and the water supply lasted. With the advent of the centrifugal pump, manufacturers had to find another way to prime because the centrifugal pump cannot move both water and air. When the centrifugal pump came into common use, so did the primer.
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(1) The simplicity of the air-powered primer design is in its
ability to create a high vacuum without the use of moving parts or
high electrical current draw. Passing the pressurized air
through a series of nozzles creates the vacuum. The reliability of a
product that has no moving parts and is powered by a vehicle
system as carefully engineered and maintained as the air brakes
has led to the air-powered primer's growing use on new vehicles as
well as its retrofit popularity on older vehicles equipped with air
brakes.
(Photos and illustrations courtesy of Trident Emergency Products, LLC.) |
Conventional Priming
A conventional primer is a small vacuum-producing pump that can remove the air from the suction hose and pump. Since the primer's inception, the pump operator has been required to activate it manually, which can lead to problems if the operator is not trained in its use. If it is shut off too soon, the prime can be lost-if left running too long, the primer can burn out its vanes and seals.
Across the ocean, European fire truck builders also use the centrifugal-style pump. However for the past several decades, they have been providing many of their pumps with an automatic priming feature. Their approach also uses a separate small positive displacement priming pump, but the primer is automatically engaged whenever the pump shaft is rotating and it senses little or no discharge pressure.
Although this approach is not quite as simple as using a truly self-priming water pump, it does automatically tie the separate primer's operation to the water pump's discharge pressure status. The Godiva, Rosenbauer, and Ziegler brands all have fire pumps available with automatic priming.
As the world grows smaller, many of the more useful ideas for firefighting techniques and equipment are crossing the oceans in all directions. The automatic priming idea for centrifugal fire pumps is one of these and makes perfect sense on any fire truck for several reasons:
• It is simple and makes the pump operator's job easier, allowing time for other tasks.
• It will automatically remove any