The strength of the pipe determines how fast the water hammer shock wave travels. Softer pipe like poly, bounce the shock wave off at about 3,000 feet per second. Stronger wall pipe like steel, bounce off the shock wave at about 8,000 feet per second. I agree that the softer wall poly should even absorb some of the pulse but, canít cause water hammer. Water in motion wants to stay in motion. Water at rest wants to stay at rest. Water hammer happens when you start moving water that was at rest, or when you stop water that was moving. The slower the velocity of water when the pump shuts off, the less the check valve will slam and the less water hammer you have. Same thing is true of starting a pump. When you have tried everything and you still have a hydraulic phenomenon that causes a water hammer, try restricting the flow to 1 GPM before shutting on or off the pump. I guarantee you will not see the pipe move or hear the tink or thud when the pump is only pumping 1 GPM at time of shut off.