No matter how many check valves you have, water hammer can happen on pump shut off. When the pump is shut off while pumping maximum flow, the check valve(s) slam shut from a wide-open position. The direction of water flow reverses and the momentum of the water bounces off the closed check valve(s) causing water hammer. The spring in a spring loaded check valve is suppose to close the valve before the direction of water reverses, which helps eliminate water hammer on pump shut down. But if the spring is not strong enough or the check valve drags a bit, the direction of the water will reverse before the check is closed. This causes the water to hit a wall the same as if you dropped a huge boulder on the tracks in front of a moving train.
The best way to eliminate water hammer on pump stop is to reduce the flow from the pump to a small amount before shutting off the pump. As when using a CSV, the water flow will be reduced to 1 GPM before the pump is stopped. At 1 GPM the check valve(s) are only open the width of a piece of paper, and there is no water hammer when the check valve(s) fully close after the pump shuts off.
Now an extra check valve can cause water hammer on pump start, no matter how the pump is controlled. When not if the bottom check closes just a bit slower than the upper check(s), or leaks even a thimble full of water, the water in the pipe will be held in by a negative pressure. The upper check is working the same as holding your finger on top of a straw full of ice tea. So when the pump is started, the momentum of the water flying up the pipe hits the upper check valve with the force of a Boxer hitting you squarely in the nose. This causes a tremendous water hammer, and in some cases will make the pump and pipe jump up out of the well. I have seen a 100HP pump and 300’ of 4” steel pipe jump a foot straight up and slam back down when the pump is started.
When the only check valve in the system is the one down on the submersible pump, the water in the pipe is held in at a positive pressure. This causes the pump to start against a positive pressure instead of a negative pressure, and there is no sudden rush or momentum of water to hit anything and cause water hammer.
Even better yet is having only one check valve AND starting the pump with a CSV attached, as it will start at a very low flow rate instead of the maximum pump capacity. However, even starting the pump with a CSV attached will not help water hammer if there is more than 1 check valve. This is because the CSV cannot maintain its almost closed position when it is slammed with tremendous momentum from the column of water hitting it at full flow rate.
The negative pressure caused by multiple check valves is why they are illegal in states that know what they are doing. Not only do multiple check valves cause water hammer on pump start, but when the pump is off and the negative pressure exist, contamination can be drawn into the pipe line and fed into your drinking water.
This is completely different if you are using an air over water plain galvanized type tank. With this type tank a check valve above ground is needed to help with the bleeder type air charge. But a Schrader (snifter) valve on this extra check valve, which lets air into the system, also keeps any negative pressure from happening.
