It sounds like a hydraulic problem before a stress problem.
Have you ruled out cavitation?
What's the vapor pressure of the fluid?
Where is the suction pressure measured?
Where is the fluid being redirected once the discharge is blocked? To the pump inlet? Is the pump being shut off when the discharge is blocked? If not, how long are they running it after it's been blocked off?
For preventive the cavitation problem I had already venting the line before operation.
Suction pressure is measured using a pressure gauge.
when the discharge valve is closed, fluid is routed back to the deaerator as in the sketch below and running for three hours after it's been blocked off.
https://s10.postimg.org/gt6icyjeh/bfw_recirculation.jpg
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