Tech Topics: Check Valves Key for Submersible Pump Installations
Check valves are an important component of every submersible pump water system. In this article, we will talk about the reasons for using check and the different types of check valves, and show which ones are best for submersible applications. And, finally, we will go over the proper placement of check valves in these systems.
Why are check valves important? First of all, they hold pressure in the system when the pump shuts off. Additionally, they prevent back spin and up thrust, and help to minimize water hammer. Before taking a closer look at each one of these factors, which type is best for submersible applications?
Types of Check Valves
If you run down to your local hardware store looking for a check valve, what you’ll likely find is what’s called a swing check. See Figure 1.
Swing check valves are not suitable for pump applications because they do not close quickly enough when the pump shuts off, resulting in the potential for water hammer. Water hammer can occur in this situation because the water starts to flow backward in the drop pipe because the check valve flapper (No. 5 in the drawing) doesn’t swing closed the instant the pump shuts off. This can cause water hammer, described in detail below. Spring checks on the other hand close quickly enough when the pump stops to prevent back flow and the resulting water hammer. See Figure 2.
Materials of Construction
This was a much more important subject 10 years ago when the groundwater industry was making the shift from materials containing lead to no-lead materials. Since that time, the industry has transitioned to lead-free materials like no-lead brass, stainless steel, ductile iron and plastic. For health and code reasons, do not use leaded materials in potable water systems.
Why Use a Check Valve?
Here are the principal reasons we use check valves in submersible water systems.
Holding pressure: Most submersible pump systems use a pressure tank to maintain pressure when the pump is off. Imagine having a system without a pressure tank where the pump would have to come on every time someone flushed a toilet or brushed their teeth. With a check valve installed between the pump and pressure tank, pressure is maintained at all times on the demand side of the check valve.
Preventing backspin: With no check valve or if the check valve fails, the water in the drop pipe and the water in the system will flow back down the drop pipe and through the pump when the pump stops. This will cause the pump to rotate in a reverse direction. If the pump were to be started while this is happening, a heavy strain would be placed on the pump/motor assembly, possibly causing the pump shaft to break or the motor thrust bearing to be damaged.
Preventing upthrust: Without a check valve, water in the drop pipe could drain back into the well and the pump would start under a reduced-head condition. This can cause an upthrust on the impeller/shaft assembly. This upward movement carries across the pump/motor coupling and creates an upthrust condition in the motor. Repeated upthrust at each start can cause premature wear and failure of the pump or the motor or both.
Preventing water hammer: When water flows through a piping system, it has kinetic energy (weight and velocity). When the pump stops, the water continues to move. Its energy must be absorbed in some way. The pressure tank normally absorbs this energy. However, in a system with leaking, slow acting or too few check valves, the water will reverse direction momentarily. When it stops, the rapid dissipation of energy can cause a shockwave to travel through the piping. This is water hammer. It can split pipes, break joints and damage the pump. Water hammer varies in intensity depending on the velocity with which the water is traveling and how suddenly it stops. For more information on water hammer, see our November 2009 article in this publication.
Placement of Check Valves
Figure 3 shows a typical submersible pump water system and the location of check valves. Starting at the pump, the first check valve should be installed on or directly above the pump unless you have a slow-producing well that sucks a little air from time to time. In this case, the first check should be installed at least 5 feet and no more than 20 feet above the pump to allow the pump to purge itself of air more easily on startup. The second check valve should never be installed more than 25 feet above the lowest pumping level in the well. For deeper settings, it is recommended that a check valve be installed every 200 feet. Another check valve should be installed in the horizontal piping at the surface or just below the well seal or pitless adapter to hold pressure in the pressure tank when the pump is off.
Finally, pay attention to the load and pressure rating of your check valves when you are working on deep-set applications. Adhere to the manufacturer’s recommendations and you’ll have a long-lasting trouble-free installation.