Dry lift pumping applications often require lifting a fluid from below the level of the pump. This is generally not a problem for a positive displacement pump that is primed with fluid. However, getting the pump to self-prime depends not only on the characteristics of the pump, but also on the design of the system and the operating conditions. To understand better the application of a pump required to self-prime, let's discuss some pump fundamentals and good design practices.
Self-priming with a liquid level below the pump requires lifting the fluid. This means that the pump has to develop enough negative pressure (or vacuum) to pull the fluid up the inlet line into the pump. It is important to note that Micropump® products operate very well under high vacuum fluid conditions. However, when a pump is attempting to prime without fluid inside the pump, it is essentially pumping air. Although Micropump gear pumps are intended for use as fluid pumps, they are able to move air because the internal clearances are relatively small.
The amount of air that a pump is able to move depends on the system design, the type of pump, the amount of internal clearance, how well the internal clearances are sealed, the speed of operation, and the characteristics of the fluid to be pumped.
The first thing to consider in a self-priming application is whether or not it is absolutely necessary to make the pump perform this function. It is always good design practice to avoid making the pump dry lift. A flooded inlet is best. However, if it is necessary for the pump to dry lift, it is recommended that the lift elevation be kept to a minimum.
In some cases a flooded inlet is not enough. Pumping out of a condenser or evacuated reservoir may require additional energy to get the fluid into the pump. It may be necessary to raise the reservoir up 10 meters (30 feet), or more, to provide the necessary inlet pressure needed to fill the pump.
The outlet condition of the pump will also affect the ability of the pump to self-prime. If the pump outlet is blocked or restricted, the air being evacuated from the inlet side has nowhere to go except to leak back through the internal clearances of the pump, and the vacuum created will be considerably reduced.
The type of pump is the next consideration in a self-prime application. Micropump manufactures two types of positive displacement gear pumps: a conventional cavity style and a suction shoe design. In general, the conventional cavity style, such as the Series GJ, is preferred. This series has a fixed cavity with small internal clearances, which make its sealing capabilities very good.
There are three gear materials available in the cavity design: PTFE, Ryton and PEEK. PTFE gears have a relatively high coefficient of thermal expansion compared with stainless steel. This means that if the pump is run dry for an extended period, the gears will heat up and try to expand beyond their internal clearances. This will generally decouple the pump and wear the tips of the gears in the process. This will reduce the volumetric efficiency of the pump, as well as its ability to self-prime.
If PTFE gears are not required, consider Ryton or PEEK gears. These materials have better wear characteristics than PTFE and can be run dry for a longer period of time without damage. Micropump recommends that our pumps never be run dry; however, we have heard claims of pumps with Ryton gears accidentally being run dry for extended periods, and then restarted with fluid, showing no significant loss in performance.
The second type of positive displacement gear pump manufactured by Micropump is the “Suction Shoe” design. In this design, the gears are sealed by a floating “shoe” that is pressure loaded by the differential pressure developed by the pump. In addition, there are two types of suction shoe pumps available: one with graphite components and the other with Ryton or PEEK.
The Series GA pumps use carbon graphite gears (X21, V21, and V23) and suction shoes that are machined to very close tolerances. This low displacement, high precision pump has very small internal clearances and uses a bias spring to physically hold the suction shoe against the gears when self-priming. In this case, these features prevail over the inherent self-priming problems in this design. The design gives the Series GA pumps have very good self-priming characteristics.
In most cases, moisture will improve the sealing of the internal clearances. All Micropump pumps are tested before shipment so they will generally have some fluid left inside. However, if it is possible to wet the gears before starting, the self-priming capability of the pump will significantly improve.
Speed of operation is also important in self-priming. In general, a small displacement pump running at a higher speed will lift better than a larger displacement pump at a lower speed. However, this relationship is not proportional.
There are several types of fluid that are difficult to lift regardless of the pump and system. These include volatile, or low-viscosity fluids, or fluids at high temperature, which can reach their vapor pressure in the inlet line before reaching the pump. Fluids with specific gravities higher than water require higher vacuum to lift the same distance. Also, fluids with high viscosities need more vacuum to overcome viscous drag in the inlet line.
It must be noted that when pumping volatile fluids, such as from a barrel, the fluid path should always be grounded. It is possible for static discharge to occur and cause a spark or fire.
In conclusion, applications requiring a pump to self-prime can be successful if the proper pump design and drive are selected, the properties of the fluid are considered, and good design practices are followed. The pump selection should consider materials and self-priming capability, as well as operating performance. The system should be designed so that lift elevation is minimized and the outlet of the pump is not restricted.