Self-Priming Pumps

What is a self-priming pump?

A unique type of centrifugal pump which eliminates the need for external priming devices. If a centrifugal pump is located above the suction vessel or sump (the pump is said to have a “suction lift”), it must be primed before it can be started, to remove air from the suction line and casing, and to fill the suction line and casing with liquid. This must be done before starting the pump, otherwise the pump will not work, and the air can damage the bearings.

The priming operation on a regular centrifugal pump that is operating with a suction lift is done by pulling a vacuum at the top of the pump casing, using a vacuum pump or the plant vacuum system. A self-priming pump eliminates the need for this external priming mechanism.

How do they work?

When the pump is shut down, the liquid drains out of the suction line and back into the sump. However, due to the special design of a self-priming pump casing, a quantity of liquid remains in the bottom of the casing. When the pump is started again, the equipment goes through a priming cycle, which typically takes several minutes. During the priming cycle, the liquid that had remained in the pump casing is pushed by the impeller out the discharge, along with some of the air that is in the casing. This creates a vacuum at the impeller inlet, which draws liquid up the suction line. The priming cycle continues until all of the air is pushed out of the discharge and the water has worked its way up the suction line to the impeller. Then, the pump is fully primed and begins working like any primed centrifugal pump.

Where are they used?

Self-priming pumps may be used anywhere that a pump is running with a suction lift. The pump may be more expensive than a regular centrifugal pump, but it eliminates the external priming mechanisms, and eliminates any bearings or motors down in the sump that other pump types might require. Normally, suction lifts are limited to about 20 feet with self-priming pumps.

Centrifugal Pumps in the use of Misunderstanding

1. High-lift pumps for low pump

A lot of hand-pumping lift that the lower electrical load smaller. In this misleading erroneous understanding, buy pumps, the regular election will pump head very high. In fact, for the centrifugal pump, when the pump models identified, its power consumption is the size of the pump and flow in direct proportion to the actual. The pumps lift the flow will increase with the decrease; thereby lift higher, the smaller the flow, the less power consumption. Conversely, the lower the head, the greater the flow, the greater the power consumption. Therefore, in order to prevent electrical overload, generally require the use of pumps pumping lift the actual calibration lift shall not be less than 60 percent. Therefore, when a high-lift pump for the low, easy to overload the electrical and fever, a serious motor can be burned. If the emergency use, it must be mounted in the outlet pipe for a water valve to regulate (or block of wood, and other small outlets), to reduce traffic flow and prevent electrical overload. Attention to the motor temperature, if motor overheating, timely clearance of small outlet flow or shutdown. It is also easy to misunderstand some of hand that plug outlet, the mandatory reduce traffic, which will increase the electrical load. In fact, just the opposite, Centrifugal Pump Irrigation and Drainage formal power units are equipped with the outlet pipe on the valve, in order to reduce the commencement of the motor unit load, should be closed valve, the motor started to be gradually opened after the valve is the reason.

2.Pump large diameter pipes with small pumping
　　Many think that this machine can improve hand lift actual fact, the actual lift pump lift ~ = total loss of lift. When the pump models identified, the head is certain; mainly from the loss of lift line resistance, the smaller the diameter is greater resistance, thus the greater the loss of lift, reduced diameter, the pumps could not lift the actual increase , but will be reduced, resulting decline in pump efficiency. Similarly, when small diameter pipes pumping large pumps, the pumps would not reduce the actual lift, it will decrease due to the resistance line losses decreased lift, the actual head has improved. Organic also believe that the hand pump large diameter pipes pumping, it will greatly increase the electrical load, they think diameter increased, the water outlet pipe in the pump impeller on the pressure, which will greatly increase the electrical load. As everyone knows, only the size of liquid pressure and lift level, and the cross-sectional area and the size of water mains unrelated. As long as a certain lift, pump impeller sizes remain unchanged, regardless of what diameter, in the role of the pressure on the impeller is certain. Diameter is increased, the flow will reduce resistance due to an increase in volume, and power consumption also increase as appropriate. But as long as the nominal head of both diameter how to increase pump can be normal work, and can also reduce the pipe losses, improve pump efficiency.

3.Installation of water pipes, or to the level of the level of Bending Upwards
　　This will gather in the inlet air, reduce water and vacuum pumps, suction pumps to reduce lift, the water reduction. The correct method is to: its level of the water should be slightly tilted direction, and should not be level, but also not upward.

4.Inlet on the elbow by more than
　　If used in the inlet on the elbow, would increase local water resistance. And bend in the vertical direction should be turning, turning allowed in the horizontal direction so that gathered air.

5.Pump inlet directly connected with the elbow
　　 It would flow into the impeller at the elbow uneven distribution. When the pump inlet diameter greater than the intake, should be installed eccentric adjustable pipe. Eccentric variable part of the plane should be installed in the top part installed in the slope below. Otherwise gathered air, outflow water will reduce or hardly pumping water, and impact, such as voice. If the inlet and pump inlet diameter equal, should be in the pump inlet between the elbow and has been the straight tube length shall not be less than the diameter of pipes 2 ~ 3 times.

6.With the inlet valve at the end of the next section is not vertical
　　 If this installation, the valve can not be closed, resulting in leakage. The correct installation method is: with the end of the inlet valve, the next section is the best vertical. As a result of vertical terrain conditions can not be installed, with the horizontal axis angle pipes should be above 60 °.

7.Inlet intake wrong location
　　（1）inlet intake inlet from the wall and the bottom is less than the diameter inlet. If the bottom sediment and other pollutants have, from the bottom of the intake is less than 1.5 times the diameter, when the influent pumping would cause miscommunication or sucked into mud debris to plug the inlet.
　　（2）inlet intake enough water depth, it would cause the water around the inlet have a whirlpool, affecting water and reduce water production. The correct installation methods are: small and medium-sized pump into the water depth of not less than 300 to 600 mm, large pumps shall be not less than 600 to 1000 mm.

8.Outlet pipe of a water tank in the mouth above the normal water level
　　If the outlet in a pool above the normal water level, although an increase of the pump head, but to reduce the traffic flow. As a result of terrain conditions constraints, the outlet must be higher than the water level, I should be in the elbow and short of the installation, plumbing become Siphon, lower outlet height.