Design Pressure & Temp

Dear Friends,
Please guide me what is the basis for detemining the Design Pressure & Temperature of a pipe to for Pipe thickness calculation, For example I want to determine the pipe thickness of PZV outlet piping.The PZV set Point is 450 Psig.The outlet of PZV is connected to flare header running at 100 psig and 120F
The Inlet piping thickness can be designed at this preessure taking 3% pressure drop into consideration.
The inlet pipe is class 300 while the outlet of PZV is 150 class rating.If we consider the logic then the oultlet of PZV at the start will be subjected to PZV set pressure + overpressure.Then the pipe thickness will be more. If we go by 150 class and flare header pressure as the design pressure then the thickness will be less.
In both the cases there is a difference in cost.Please guide me the basis for establishing the design pressure and temperature of a pipe for thickness calculation.Please provide me the inetrnet links related to this topic.

Please guide me of any references or internet links related to Design Basis for General Pressure Piping in Plant Normal Operating Condition.Some of the points I am following is:

1. The design pressure for process piping in plants shall be the highest of the design pressures for connected equipment, unless adequate relief is provided at a lower design pressure.
2. For piping systems protected by a pressure relief device, the design pressure shall equal the set pressure of the pressure relief device plus static head.
3. For piping systems unprotected by a pressure relief device, the design pressure shall equal the maximum pressure that can develop due to control valve failure, pump shut-off or any other contingent condition plus static head.
4. All piping systems operating below atmospheric pressure shall be designed for full vacuum.

Utility Piping -The design pressure of utility piping shall not be less than the maximum supply pressure.

Thanks & Reagrds,

Md Danish

[Non-text portions of this message have been removed] Received on Wed Sep 14 09:38:00 2005