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Self-Priming Pumps or Suction Line Check Valves

by PSG

When a pump is located beneath the fluid level (for example, downhill of a storage tank), gravity will easily feed the fluid into the centrifugal pump, thus completing the priming process. However, in some applications, the challenge begins when the pump is located above the liquid level (for example, in an underground storage tank). In this type of application, the air would have to be displaced within the suction line and replaced with liquid during pump start-up. The question here is, why not simply install a check valve in the suction line and remember to fill it with fluid before start-up to prime the pump?

Suction Line

This potential solution does seem reasonable at first. However, two issues must be considered: ease of start-up and intermitted pump use. When it comes to start-up issues, in the case of a foot valve, the user would need to add a significant amount of fluid to fill the entire suction pipe and pump before starting the pump. For a typical self-priming centrifugal pump, the initial charge of liquid to the pump is only 1–10 gallons of liquid, depending on the size of the pump.

When looking at the second issue pertaining to intermitted pump use, a self-priming pump will retain the priming charge of fluid even if the fluid level drops in the suction line. The user then can simply turn the pump on, and the pump will prime the suction line again. In the case of a foot valve, the user will need to verify that the liquid level in the suction line did not drop since the last time the pump was run and would need to add liquid to the suction line as needed before starting the pump again.

The Griswold® 811SP Series Self-Priming Centrifugal Pump is the optimal solution for these applications. With the Griswold 811SP Series, each case is designed with an integral priming chamber that supplies fluid to the impeller during pump priming. This allows the pump to draw a vacuum in the suction line, drawing fluid up the suction pipe and, ultimately, into the pump resulting in full prime. Since the Griswold 811SP Series utilizes the complete back end of the Griswold 811 ANSI Series Pump, the only components unique to the Griswold 811SP Series are the case and, if applicable, spacers to adjust for a larger pump centerline height. 

Figure 1 811SP Series Case Part Features

Because the Griswold 811SP Series is self-priming, it does not require a check valve or a manual priming procedure before startup, aside from the initial charge of liquid for the case priming chamber. During the priming stage, the self-priming pump transfers air from the suction side to the discharge side, drawing a vacuum on the suction side and lifting the liquid up the suction pipe. An accumulation of air in the discharge piping hinders this process and can stall the priming progress. Therefore, in some installations, an air vent or permanent bypass line from the discharge piping is required to vent the accumulating air pressure.

By-pass Line Drawing

Overall, installing a Griswold 811SP Series self-priming centrifugal pump is a more robust and reliable solution than using a traditional end suction pump with a check valve when it comes to pumping fluid from an underground storage tank.