gabionex,
Perhaps I mis-understand the concerns of your client. The loads of thermal expansion of piping were what I had thought were the issue. It appears that there is an over-pressure concern with the gland flush.
If the pump is shutdown and blocked in with suction and discharge valves, then the gland flush water could continue to be pressurized and allow pressure into the pump casing from any small leak of the gland seal. If the gland flush water pressure is greater than the pump casing pressure limit, then pump casing could certainly rupture.
If the pump flange connections are subjected to a pressure where the flange joint and gasket might leak, then that is another scenario of possible less concern. I think the gland seal flush water pressure would normally be regulated to a small 20 to 30 psi pressure greater than the normal inlet pressure of pump. There would then be little concern for flange leakage (of gland flush water) beyond normal service conditions. A relief valve for the gland seal flush could be an extra measure of protection along with a pressure regulator for the gland flush water.
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R Yee