Generally the lining thickness of sea water line(cooling water line) is very small and the effect of cement-mortar will be minor and negligible in a piping stress evaluation. However, it will affect to the support and equipment nozzle loads and the maximum deflection of piping system due to combined weight must be checked not to bring the damage of lining material during operation.
There are two ways to evaluate stress of lined pipe. Equivalent moment of inertia(i.e. equivalent thickness) and equivalent elastic modulus. It seems that equivalent thickness method is more commonly used.
The equivalent thickness is calculated by first calculating the combined stiffness of the cement-lined pipe, and the equivalent moment of inertia; the equivalent thickness can then be obtained in terms of the pipe diameter and the equivalent moment of inertia.
For more detailed equations, please e-mail to me.
For D/t ratio larger than 100 pipe, the SIF and pressure calculation in the ASME B31.3 may not be valid. For code compliance, see para.304.7.2 in the B31.3. The valid SIF may get from a fitting vendor or FEA(FE/Pipe, as mentioned above).
The simplest way is to use thicker pipe only for elbows and branches for main process lines. However, for utility system like cooling water line, it is no big deal….it’s up to your decision.
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Sun Wee