As everyone is mentioning... you need to design for the churn/static condition on the pump.
At pump churn, what is the differential pressure between the static inlet and the "shut off head" (i.e. what is the differential pressure that the pump will produce at a no flow condition).
AS AN EXAMPLE:
Say you have specified a pump to provide 150 psi of differential pressure at 400 GPM. Look at the pump curve and read what the shut off head (i.e. far left hand side - when flow is zero). Let's say that this is 200 psi. What also is the inlet pressure at the pump suction under no flow conditions. Let's say the pipeline has a static pressure of 100 psi, but will be 80 psi when there is load on the line.
The maximum discharge pressure, and therefore, the design condition that you should be using - will be when the pipeline is at maximum pressure (100 psi) and the pump first starts (i.e. at the shut off head).
In this case, the maximum working pressure at the pump discharge is 200 psi (shut off head of pump) + 100 psi (static inlet pressure) = 300 psi. That is the condition that you need to design the discharge flange for. That is also possibly what you need to design everything downstream for, unless there is a relief valve, etc. to prevent overpressurization. Everything between the discharge flanges and the relief valve will need to be rated for this maximum pressure scenario (300 psi in our example).
A quick search on the web will show you what the maximum working pressures for flanges (and then you pick the one you need based upon the operating conditions).
gentelem 
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