Self-priming pumps offer industrial users many advantages
Despite the fact that self-priming, centrifugal pumps offer significant advantages in initial cost, maintenance and operation, few of the pumps are used in industrial plants. This is most likely due to a lack of understanding of the pumps’ operation and design principles.
What is a self-priming pump?
A self priming pump is a pump that will free itself of air when it becomes air-bound, and will resume pumping without attention. The key point here is air – though this applies to any vapor.
How self-priming pumps work
Certainly self-priming pumps are worth considering when you realize the high number of air-entrained liquids in a paper mill or the volatile liquids used in chemical processes. Self-priming pumps work to transfer vapor from the suction side of the impeller to the discharge. When this is done continuously, a vacuum is created in the suction line and atmospheric pressure on the surface of the liquid supply pushes the liquid into the pump. Two schemes are typically used by pump designers to achieve this vapor transfer: recirculation priming and diffuser priming.
This method uses a check valve between the suction passageway and a liquid reservoir on the pump discharge. When air is in the suction passageway, the check valve opens and liquid from the reservoir recirculates back through the impeller, taking a few air bubbles along on each trip. This soon exhausts the air in the suction line, which fills with liquid and closes the check valve.
Diffuser priming was developed to avoid the check valve and some other problems of recirculation priming. These types of self-priming pumps are considered more efficient and are the most widely used today. In this method the liquid from the reservoir does not return to the suction, but combines with air at the circumference of the impeller. The air is transferred -- but unlike recirculation priming, diffuser priming uses diffuser vanes and a reservoir, which constantly keeps the entire circumference of the impeller submerged.
Liquid ring pumps
When it comes to priming ability, this design is the best. As of now, one American manufacturer and several European manufacturers are producing liquid ring pumps. They are the only ones capable of regularly self-priming on foam. The principle is similar to a Nash Hytor Vacuum pump.
Self-priming pump tips
- Self-priming pumps require an initial fill of liquid – they need something to recirculate.
- Prevent excessive back pressure on the discharge of the pump. Typically more than 10 or 15 pounds of discharge back pressure gives the air from the suction no place to go. You can eliminate this problem by placing an air release valve in your discharge line between the pump and the check valve. This lets the air escape.
- Buy self-priming pumps with a mechanical shaft seal. Air leaking in through the stuffing box makes the priming difficult. Most self-priming pump manufacturers offer a grease lubricated, double type mechanical seal made specifically for contractor use -- If a road contractor can’t ruin them, they must be good!
- Many self-priming pumps have a check valve just inside the suction flange. Its main purpose is to prevent a back siphon effect from emptying all of the liquid out of the pump when it’s shut down.
- There’s a limit to how far you can lift a liquid, even with a perfect vacuum. Consult the pump’s NPSH curve when you put the pump on a suction lift.
- Air leaks in the suction line will reduce or eliminate the pumps self-priming capability. In addition, the more air in the suction line, the slower the pump primes. Know that a non-reinforced suction line will collapse.