According to ASME standard,for the P1, PWHT can be done before RT.Is there any other supporting specification or standards to support this idea?

Mr.Reza Khaghani apart from the ASME code, you convince the client that i am ensuring the welding soundness before PWHT by taking RT. Usually the films of RT are mentioned by BSR (Before Stress Relieving) and after PWHT by ASR (After Stress Relieving). You just take the RT review your films by your team. Once it is ok go for PWHT. So client cant object unless some safety concerns RT in the plant. 

As per ASME B31.3 you have to find the PWHT before the RT

yes I know that and i tried to convince my client that way but unfortunately they did not accept.that will be great if you find any other evidence like a project specification prepared by a well known and reputed company,you can send me a PDF copy so i will use it to complete my evidences. 

As per ASME31.3 you will find PWHT before RT