Emergency priming of centrifugal pumps
Immediate action methods for drafting water in the event of primer failure
MODERN CENTRIFUGAL fire pumps are very efficient and will pump great quantities of water on demand. There are times, however, when a pumper must draft from a pond, river, lake, watering trough or whatever supply is available. Before drafting, a priming operation is involved due to the fact that a centrifugal pump is not a positive-displacement type and therefore cannot obtain water initially by itself. An auxiliary means or secondary pump is required for priming. In the event of failure of the priming method, it is still possible to prime the pump by the application of logic and the use of a second pumper.
It is standard practice in most fire departments to periodically test the primer vacuum of each pumper and make any necessary repairs if the test discloses deficiencies. In addition, fire engine operators are trained and capable, but preventive maintenance, training and planning do not eliminate the possibility of a pumper failing to draft for some reason.
To call another pumper at this time would mean that the fire might gain headway or progress beyond the capability of the equipment at the scene. In smaller cities where the number of pieces of apparatus is limited, it might become of vital importance to have all the pumpers in operation and not have any unavailable for service because they were unable to draft water.
When a fire occurs in Detroit, two or more pumpers are usually dispatched. The situation on arrival may call for one or more pumpers to draft. The first-arriving pumper company may stretch lines from the fire to the water supply and then find they are unable to draft. The fire incident could rapidly become serious if a supply of water were not provided to confine it. The second pumper could connect to the first pumper’s lines, but this would duplicate efforts if only one pumper’s lines are necessary, and could possibly increase the spread of the fire if lines and water from both pumpers are needed immediately and are not available to confine it.
Simple action required
Centrifugal pumpers are fitted with a priming device which exhausts the air from the fire pump and the suction. The partial vacuum created by this auxiliary pump or ejector permits the atmospheric pressure on the surface of the water to exert sufficient force to cause the water to rise in the suction. When the water has completely filled the suction and pump, the centrifugal pump is then capable of pumping the water and maintaining the necessary vacuum to continue the operation. The priming auxiliary’ is no longer necessary.
Emergency priming methods merely require the filling of the fire pump and suction with water in some manner. Then the pumps are engaged and the natural flow maintained.
The following methods devised by the training division were tested with each Detroit pumper and proved to be rapid and effective in all cases. The staff deliberately tried to add to the difficulties by opening drain cocks, play pipes, etc., but the fire engine operator, following directions, was able to draft and provide the necessary water for fire extinguishment. The methods vary in applicability and speed of connection.
Method No. 1
This is probably the easiest emergency method to employ. It permits two pumpers to stretch the necessary lines and work on the fire. Using some of the excess water available from Pumper No. 2, tire first pumper is primed so that it may supply its lines.
Both pumper companies connect stiff suction and strainers and drop the hard suction assemblies into the water source. One 2 ½-inch discharge line is connected from Pumper No. 2 to the suction of Pumper No. 1. If a 2 ½-inch gated suction connection is provided on Pumper No. 1, no further adaption is necessary. If not, the connection is accomplished by dividing a 5-inch to 2 1/2-inch suction reducer into its two parts and placing a 4-inch hydrant gate between the suctions.
When the connections are made, the operator of Pumper No. 2 discharges water into the suction opening of Pumper No. 1. The operator of Pumper No. 1 engages the pumps and when a solid pressure is created, he opens the discharge gates and supplies his lines. When he is sure that his pumper is pumping on its own, he may then close the suction gate to eliminate the necessity of Pumper No. 2 continuing to pump into this line. The pumpers may remain connected together and may be used in case Pumper No. 1 has its water flow interrupted for any reason and again cannot prime.
Method No. 2
This method may be used by two pumpers, when both are needed to extinguish the fire without shutting down or moving the second pumper. The first pumper company which is unable to draft, connects the stiff suetions and strainer and drops it into the source of water. The second pumper also connects hard suctions and strainer and drops into the source. A 2 1/2-ineh line is connected from Pumper No. 2’s discharge to Pumper No. 1’s discharge, using a double female coupling. Pumper No. 2 then proceeds to draft and when a supply is gained, proceeds to discharge into the connecting 2 1/2-inch line at a pressure of at least 75 psi.
The operator of Pumper No. 1 engages his pumps, then opens the gate to which the connecting 2 1/2-inch line is attached. As the pressure indicated on the discharge gage increases, the operator slowly opens the discharge gates to which his own line is connected and increases the speed of the pump. When a constant, solid discharge pressure is achieved, the gate connecting Pumper No. 1 to Pumper No. 2 may be closed.
Again the centrifugal pump is primed by filling with water discharged by the second pumper. The operator must remember that he is at draft and will lose his prime and not be able to pump if he disengages.
Method No. 3
This method is perhaps the most time-consuming and least applicable. Pumper No. 1 lays a line or lines from the fire to the water supply and proceeds to connect stiff suctions and strainer to one of the suction openings and lowers the assembly into the water. The second pumper places stiff suctions on one of its suction openings, then reduces the 5-inch suction to a 2 1/2-inch female opening with the equipment earned on the apparatus, making sure all connections are tight. This 2 1/2-inch female opening is then connected to any one of the discharge gates of the first pumper and the gate opened. The second pumper proceeds to draft in the normal manner and when it receives a supply of water, the operator of the first pumper proceeds to engage his pumps and start pumping. If he is able to produce pressure, which shows his pump is primed, he then shuts the gate to which the suction is connected, opens his regular discharge gates, and disconnects the suction. This will allow the second engine to return to quarters or stretch to the fire.
If the priming has been successful and the second pumper has disconnected, the operator of the first engine does not disengage his pumps until the fire is extinguished or he will lose his prime and again be unable to draft and pump a water supply.
Good drafting practices
Emergency priming methods are not meant to eliminate or supplant regular maintenance and vacuum tests but they are an important addition to the knowledge of fire engine operators. In Detroit we teach pump operators the methods described herein and in addition, we insist that operators observe the following rules at all times:
- When at draft do not disengage the pumps when the lines are shut down. If necessary, discharge water out of unused outlet to keep pumps from heating.
- Open and close discharge gates slowly. If these gates are opened and closed rapidly the pumper may lose its prime.
- Do not try to gain prime by using excessive speed. Excessive speed may cause auxiliary pump to bum out.
- All suction connections must be tight and leakproof. If air leaks into the suction side it will reduce the partial vacuum and reduce or eliminate the possibility of priming the pump.
- Do not leave the auxiliary pump engaged for too long. To eliminate necessary wear, the auxiliary pump should be disengaged when the discharge gage shows pressure.