Foot Valves and Well Check Valves: Keeping a Pump Primed

Every application where the liquid level sits below the pump has the same problem: when the pump stops, the water in the suction line runs back down under gravity. The line empties and the pump finds air instead of water at the next start. A foot valve is fitted at the bottom of the line, submerged in the liquid, to prevent that.

What an empty suction line costs

The consequences arrive cumulatively. At every start the pump first has to refill the line; through that period its delivery is erratic and air passes with it. Centrifugal pumps cannot pump air — an impeller full of air keeps turning and does not fill the line.

Meanwhile the liquid volume inside the pump has dropped, so seal and bearing cooling are lost. That is precisely what dry running is, and its consequences are measured in minutes. On an automatically started system it is more insidious still: nobody watches how many seconds the pump turns dry on each start, and only later does someone notice the seal weeping.

What the valve actually does

A foot valve combines two functions in one body. The first is a check valve: fluid passes upward toward the pump only, and the valve closes under its own weight and the column of water above it when flow stops. The line stays full and the pump finds water at the next start. The second is a strainer holding solids back at the source. The pairing is no accident — the bottom of the line is the one point that has to both shut and screen.

That combination has a price, and it is the main thing to watch: fouling of the strainer does not spoil the closing function, but it does lower suction pressure. The pump goes on drawing the same flow, so pressure drops across the restricted area. If it drops far enough the liquid vaporises on the suction side; the bubbles collapse where pressure rises inside the impeller and start hammering the metal. Cavitation is the classic result of a fouled foot valve, and because it shows up inside the pump its cause is usually looked for elsewhere.

Two distances during installation

The position of a foot valve is set by two dimensions that pull in opposite directions:

1. Submergence. The valve has to sit far enough below the surface. A suction close to the surface forms a vortex above itself and draws air down its centre, which ruins the pump whether or not the line is full. The governing condition is the lowest operating level — a well that drops at the end of summer, or a tank being emptied.
2. Bottom clearance. It also has to sit far enough above the bottom. A strainer close to the bottom lifts settled silt and sand with its own flow and then draws it in continuously. It fouls quickly, and fouls again after cleaning, because the cause is the position rather than the strainer.

Where both distances cannot be satisfied, the answer is not to force the valve up or down but to move the suction point.

The brass threaded well check valve

In small-bore well applications — a borehole pump, an irrigation line, an auxiliary feed — the same function is served by a small threaded body. Its advantage is installation simplicity: no flange pair, gasket or bolt set, it screws straight onto the pipe end. On a line going down inside a narrow well casing that difference decides the job.

Its limit comes from the same place. A threaded joint wears as it is made and broken, and replacing the item at the bottom of a line already down a well means pulling the whole line. Even in small sizes, then, two things matter: the compatibility of the body material with the minerals in the water, and whether the strainer can be cleaned. Where well water carries sand, the strainer belongs in the maintenance plan — pulling the line once a year is cheaper than replacing the pump once a year.