check valve problems

I have 7 humidifiers on a roof of a 2 story building that are supplied from a pump on the 1st floor. I have now had my 4th failure in 2 years of a true union check valve nut manufactured by Colonial Valve. Water hammer breaking these or just bad material. Thanks

There should be absolutely no pressure on the nut, other than from tightening, so it must be bad material.

hj said:

There should be absolutely no pressure on the nut, other than from tightening, so it must be bad material.

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Thanks for the reply HJ. I was thinking the same, but I will put the whole scenario in front of you to see what you think. The pump on the 1st floor is connected to 1 1/2" sch 80 pvc, controlled by a pressure switch set at 50-30. The branch lines going to the humidifiers go down to 1/2". They pass through the roof and connect to the humidifiers with a true union ball shut off valve and a true union check valve. The failures on the union nuts breaking on the check valves have been on 2 of the 7 units. 3 on the #3 unit and 1 on the # 6 unit. All of the valves are made by Colonial Valve. I notice a small jump in the pipe when the pump cycles on the 1st floor. Do you think this could be a water hammer issue? Or am I still faced with junk fittings? I know how to tighten these. Hand tight plus a bit. Thanks.