PetroStreet

Can somebody provide some details on the subject factor which is used in calculation of pipe and pipe bends thickness as per Process Piping Code ASME B31.3.

Weld joint strength factor is used to accommodate the long term creep strength of the welds that may be lower than the base material. In the absence of more reliable / applicable data, W shall be taken as 1 for all materials 950 deg F and below and it shall be taken as 0.5 for all materials at 1500 deg F. It can be linearly interpolated for intermediate temperatures. W above 1500 deg F shall be taken after testings and getting reliable data.

What is to be done for design temperatures above 1500 deg F? Does actual testing need to be made before assigning the right value of W in thickness calculation formula?

See if you are designing against process piping code ASME B31.3 and the design temperature is above 1500 deg F, for sure you have to undertake practical testing and examination to determine the creep behavior of weld joint compared to the base metal and then to decide for W.
Normally, in process plants, these higher temperatures are encountered in reformers and there API 530 governs the design of heater tubes and takes care of all the properties.

Thanks.