In ideal cases, you place the pump below the liquid level before you begin pumping. In this case, air pressure and gravity ensure that the pump continually fills to keep air from entering the pump’s suction line.
In most everyday applications, however, such as emptying underground storage tanks, you need to have the pump above the liquid, which is a practical approach. Nonetheless, when starting up, you will have to evacuate or discharge the air in the pump’s suction line before you begin pumping out the liquid.
Since manufacturers build most pumps to move liquids, discharging the air becomes a complicated issue to address. To solve this, you have two options: secondary pumps and self-priming pumps.
One of the most common solutions for evacuating suction lines is using secondary pumps. These use non-return valves and evacuation tanks to keep fluids from draining back into the pump’s suction line once you stop pumping.
However, since this solution requires you use extra piping and accessory equipment, it turns out to be expensive to use, especially for short-term projects.
These are a more economical option than using secondary pumps to evacuate air in suction lines. With these pumps, discharging air begins automatically, and without accessory equipment, right when you begin pumping.
Typical examples of these pumps are the positive displacement pump types, including the diaphragm, rotary gear and vane pumps, which use close tolerance to keep fluids from returning to suction lines.
Manufacturers design and build pumps for different applications, most of which are to discharge liquids. However, in most cases, pumps cannot perform this function, at least not effectively, if there is the presence of air in the pump’s suction line.
Discharging this air every time you are using the pump can be not only time-consuming but also labour-intensive. Thanks to self-priming pumps, however, you can handle this problem without having to involve the tedious processes of evacuating the air in a typical pump’s suction line.