Priming Centrifugal Pumps
Hale Fire Pump Company
DESPITE COMMON fire service terminology concerning pumping at draft it is not quite true to say that a pump is lifting water from the source. Actually the pump is creating a partial vacuum within its suction chambers, that is, a pressure lower than atmospheric. The atmospheric pressure on the surface of the water being pumped then forces the water up through the suction hose and into the pump.
When the pump is full of water, it will create quite a high vacuum, but when the pump has been drained, containing only air, it is a very poor vacuum pump. In order to obtain water we then need to create a vacuum within the pump by some other means, so we use a priming pump, which should develop a vacuum of at least 22 inches of mercury. The priming pump withdraws air from the fire pump, reducing the pressure within its casing substantially below atmospheric pressure.
The question is often asked, “Should a pump be primed from the top of the pump or from the suction of the pump?” The answer depends upon whether the pump is running or stationary during the priming operation. It is the purpose of this discussion to illustrate why, for consistently reliable priming, the pump should be primed from the suction side near the impeller eye when the pump is running and why it should be primed from the top of the pump when it is stationary.
The sketches illustrate the reason for this. For simplicity, no discharge valves, relief valves, priming pump or other accessories are shown. A single-stage pump is sketched, the principle being the same with two or more stages. The source of vacuum may be a pump, ejector, or the engine intake manifold.
When priming from the top with the pump stationary, the primer is operated until a steady stream of water flows from the primer discharge. It is then shut off and the main pump is put in gear. When priming from the suction of the pump, pump running, the primer is not shut off until a steady stream is flowing through the discharge hose. Any unavoidable air trap in the suction hose is more easily removed by priming from the suction of the pump.
Periodically the priming system system should be used to create a vacuum of at least 22 inches of mercury within the pump and attached capped suction hose. With the priming pump shut off, the vacuum should drop no faster than 10 inches in 10 minutes. For such a test a mercury manometer should be used rather than the compound gage, because it is easier to read and more accurate. The operating manual furnished with the pumper should be carefully followed.
Air leaks in a pump make priming uncertain. If the pump is primed in spite of air leaks, operation will be rough and efficiency will be lowered. The maximum capacity of the pump will be substantially reduced, especially on lifts higher than normal and at high altitudes. While it is hard to define the size of an air leak, any air leak that drops vacuum faster than 10 inches in 10 minutes will cause reduced performance, rough operation, hose pulsation and a ragged stream.
When pumping from draft, or the booster tank, through a ¾-inch or 1-inch booster hose, the flow through the pump is so small compared to the capacity of the pump that it is difficult to always scavenge all the air from the pump, resulting in unexpectedly low pressure. It is a good practice when this occurs to momentarily open the 2½-inch discharge valve. The larger flow for a few seconds while the priming pump is still running completes removal of air.
Figure 1 shows a typical air trap in the suction. The unit shown is a portable centrifugal primed from the suction inlet with an exhaust ejector while the pump is running. The effect of an air trap on priming is the same, however, regardless of the size of the pump or method of producing vacuum.
With an inverted air trap in the suction hose, water rising in the suction flows over the “hump” and runs into the pump, leaving air trapped at the top of the “hump.” The pump, full of water, shows pressure. The discharge valve is opened, flow through the suction hose increases sweeping air down into the pump, resulting in temporary loss of prime.
As under priming procedure (3), the priming is continued until all or nearly all of the air swept over from the hap is scavenged from the suction of the pump. The discharge valve need not be closed while scavenging, but, depending on the lift, it may speed priming to do so. If the air trap volume is large, scavenging will take longer than if the air trap is small.
When the pump is primed stationary from the top of the pump (Procedure 1), each time the primer is shut off and the pump is started some of the air is swept into the eye of the impeller, breaking the prime, and the pump is running as in Procedure 2. It is now’ necessary to stop the pump. Start priming again until water runs from the priming pump. Shut off primer, start pump, etc., until a complete prime is reached. This may take two, three or more trials to finally prime depending on the size of the air trap. It is for this reason that air traps are more easily handled by priming from the eye with the pump running than by priming from the top of the pump with the pump stationary.
PROCEDURE 1—PRIMING FROM DISCHARGE, PUMP NOT RUNNING WHEN PRIMING
This method primes consistently.
- Pump fills with water.
- Priming line shut off.
- Pump started.
Water flows into suction and out discharge.
PROCEDURE 2—PRIMING FROM DISCHARGE, PUMP RUNNING WHILE PRIMING
This method does not prime consistently.
- Pump suction fills with water and enters pump.
- Water flows by centrifugal force to outside of impeller and into discharge.
- Air goes to center of pump, being tighter than water.
- Priming system keeps on working but pulls only water, which slips through the air in the center of the impeller. Air remains in center.
- Impeller cannot start moving water through the suction, pump and discharge line until it is full or nearly full of water.
- When pump is running, remove the air from where the air is—the center of the pump.
PROCEDURE 3—PRIMING FROM SUCTION—PUMP RUNS WHILE PRIMING
This method primes consistently.
- Water rises in suction tube and fills pump.
- Air goes to center of pump, being lighter than water.
- Primer removes air from center of pump, near impeller eye.
- Water will start to flow out discharge before all the air is removed, but full stream will not flow until all or nearly all air is removed from center of pump.
- Primer to be kept in operation after opening discharge valves until steady stream flows.
PROCEDURE 4—PRIMING FROM SUCTION—PUMP NOT RUNNING WHILE PRIMING
This method does not prime consistently.
2.Primer shut off.
Fig. B: 1. Pump started, throwing water to outside of impeller and discharge. Air comes into center.
- Primer raises water to top of suction, continues sucking water only, leaving air in top of pump and discharge.
- Pump not yet primed, because running impeller cannot move water through suction, pump and discharge until it is full or nearly full of water, just as in Procedure 2. This method does not, therefore, prime consistently.
- To complete the prime, restart the primer to exhaust the air from the center of the pump, while the pump is running, as done in Procedure 3. Therefore, for consistent priming a pump should be primed at the top when not running, and at the suction, close to the impeller, when running.