Priming Tank Sizing Theory

[MSurgeon]

Can anyone provide some guidance on the theory on sizing a priming tank.

The priming tank I am considering is in the suction line of the pump. On pump start-up as the pump draws the water level down in the priming tank the reducing air pressure in the top of the tank causes water to be drawn up the suction line in to the top of the tank. The tank is sized so that the water level in the tank remains above the pump suction when a flow into the top of the tank is achieved.

Assuming a perfect gas under at a constant temperature so that P1xV1=P2xV2 I have carried out some calculations that appear to give reasonable results but I can't find any alternative sources of information to confirm if I'm on the right track or not.

Any feedback would be appreciated

 

[tstead]

How much volume is in your suction line?

That line will initially be empty but will need to be filled.

That is how much water you need to displace out of the suction tank...plus a little extra for good measure.

Tim

 

[MSurgeon]

The priming tank is located in the suction line of the pump with the tank having an initial prime of water. The suction line from the top of the priming tank to the pond is empty but obviously the end of the pipe is under the surface of the pond. Therefore as the water level in the tank drops the air pressure drops and draws water up the suction line in to the tank.

The "Pumping Handbook" gives a very general discussion of priming tanks and gives a rule-of-thumb that the tank volume should be at least 3 times the air volume of the suction line at startup. The problem is that this is not supported by any theroy or calculations so I am not happy using this.

Got any comment on the basic theory behind this?

 

[ChasBean1]

MSurgeon, unless I interpreted incorrectly, you're on the wrong track. An elevated priming tank should be vented to atmosphere or equipped with a pressurized bladder. If you don't vent the tank and rely on makeup water from the original suction source, you're offering your pump suction a useless alternative pathway in an attempt to prime. Say your pump is on Fifth Avenue, you suck from a reservoir at the same height a block down Fifth Avenue, and you have this priming tank on top of the Empire State Building to prime the pump. You start the pump and the tank level drops. Now, to make up this loss, the tank on top of the Empire State Building has to suck water from 105 floors below to replace its depleted water, the pressure loss from that process will equal the pressure gained at the pump suction. The net result is not having the priming tank at all, or worse, further restricting the water at the suction by diverting it through a crazy path. Re-think this strategy. Best bet might be a small recirc line or a priming tank under gravity feed with a float city water makeup line.

 

[MSurgeon]

No, your definately on a different track. I am talking about a chamber (or priming tank) installed in the suction line.

If you email me I can send you a picture of the arrangement I am talking about.

mark.surgeon@mamic.com.au