Principles of Operation

A self-priming pump is one in which the impeller, operating in the priming liquid, removes the air from the pump suction line. This results in a vertical rise of the process fluid in the suction line from a source below the pump. Once sufficient air is thereby removed, the process fluid flows into the pump and the pump primes. The vertical rise of the liquid is called “lift”. The removal of air from the suction piping permits pressure, acting on the liquid surface of the source tank, to push the process fluid up into the pump. Usually the source is at atmospheric pressure, which provides a theoretical lift of 34 feet at sea level; however, the practical lift limit is 20 feet.

If the liquid in the source tank is at such an elevated temperature that vacuum pulled in the suction line causes the liquid to flash to vapor in the suction line, the pump cannot prime.

A self-priming pump requires liquid in the casing to achieve the priming action. An initial prime is accomplished by filling the pump casing with liquid through the priming hole in the top of the casing. The pump is then started and allowed to prime. The pump may be turned off and reprimed thereafter with the liquid retained in the special pump casing.

The action inside the pump during priming is one of air entrainment and larger bubbles passing with the priming liquid through the upper volute educer passage. The air separates out of this
liquid in the upper chamber of the casing. This liquid returns to the entry port in the lower volute section and is recycled back into the impeller.

This action continues until sufficient air has been expelled to allow the pump to prime. Once prime is established, the pump operates just like a standard centrifugal pump.

During the priming cycle, there must be no air leaks in the suction piping or the shaft sealing device. Leakage of air into the system may prevent the pump from achieving prime.

When the pump prime is broken, the liquid in the suction pipe flows back into the liquid suction source. There may be a siphon effect, greater or lesser, depending on height, liquid and velocity. Although this pulls more liquid from the casing, there is always sufficient liquid retained in the pHP casing to start the priming cycle again. Thus, repriming can occur.

A self-priming pump is not a cure for an NPSH problem. The NPSH required by the pump is necessary for the pump to continue operating. A self-priming pump will handle occasional periods of vapor or air and will recover, but the NPSH required by the pump must be provided. This will depend upon the amount of lift required and the vapor pressure of the fluid being pumped.



Priming Time Calculation

• Determine the effective suction lift. (Multiply vertical suction lift by specific gravity).
  
• Refer to the priming time curves. Using the effective suction lift,
  enter the priming time curve, read across to the impeller
  diameter selected and down to the priming time.

This is the priming time if the suction line is the same size as the suction flange and is all vertical lift without horizontal component.

To correct priming time for a different size of suction line and/or horizontal piping, use the formula below:

tpc = corrected priming time in seconds
tp = priming time from curve
Lt = total length of suction pipe above the liquid surface in feet
Lv = vertical height from surface of liquid to centerline of pump
D = diameter of suction pipe, nominal, in inches
d = pump suction size, nominal, in inches
tpc = tp (Lt/Lv) (D/d)2

Example

Assume:
Vertical lift (Lv) = 20 feet
Total length of suction pipe (Lt) = 40 feet
Priming time (tp) = 42 seconds
Suction line D = 4” nominal diameter
Suction flange d = 3” nominal diameter
Then:
tpc = 42 (40/20) (4/3)2 = 149.3 seconds

Heating of priming liquid can reduce the maximum lift, but
usually the NPSH above the vapor pressure requirement for the operating point has long been exceeded. The heating of the priming liquid is of the magnitude of 10°F per minute.

To illustrate:

3 x 3 x 10, pHP2140, 3550 rpm, full diameter
Lift of 20 feet in 3” pipe
Water @ 80°F
From the priming time curve for the 3 x 3 x 10, pHP2140, the priming time is 32 seconds.

Priming at 20 feet requires an absolute pressure at the pump eye to be (34 feet – 20 feet), or 6.06 psia. From the steam tables, water boils at this pressure at about 170°F. The allowable maximum temperature rise in the priming liquid is (170°F - 80°F), or 90°F. At a temperature rise of 10°F per minute (60 seconds) the pump could run for nine minutes (540 seconds). Putting this time back into the priming time formula gives:

tpc = tp (Lt/Lv) (D/d)2
540 sec. = 32 sec. (Lt/20) (3/3)2
Max. Lt = 540 sec. (20) (1)
32
Max. Lt = 337 ft.
Insure that max. Lt Ž actual Lt

In applying a self-priming pump, consideration of the discharge piping is necessary. The air from the pump casing must be expelled without being compressed substantially above atmospheric pressure. Although a self-priming pump will prime against a small discharge pressure, the priming time will increase. However, if the pressure in the discharge line is too high, the pump will not achieve prime.

If the pump expels air into an empty discharge system, there is more than adequate volume available to contain the discharged air. If the discharge line contains a check valve, the check valve should be placed a minimum of one half times Lt away from the pump
discharge to provide adequate volume, provided that the suction and discharge lines are the same size as the flanges on the pump. Caution should be taken to insure that the available volume in the discharge line is at least 50% of the suction line volume, provided that the suction and discharge lines are the same size as the flanges on the pump. Caution should be taken to insure that the available volume in the discharge line is at least 50% of the suction line volume to be
evacuated.

Another priming solution would be to install a valved line from the pump discharge system back to the suction source. This is left open during the priming cycle and closed when priming is accomplished and during pumping. With this arrangement, a check valve must be installed in the pump discharge line downstream of the bypass
connection.