It depends on if it is P5A or P5B. It also depends on what code you are working to. Sec. I or B31.1. It sounds like it's P5A or 2.1/4 Cr. ER is TIG wire, ER502 has been replaced with ER90sB3 if my memory is right. If I remember right and it is P5A than PWHT is not required for material 1/2" or less. Anything over 1/2" thick than it's according to what code your working to and the WPS. Normal ranges are 350 deg. pre-heat and PWHT is 1300 deg. to 1400 deg. F for up to 2 inch thick material at a rate of one hour per inch with a 15 min. minimum. You need to get a understanding of A. what code your working to. B. what your WPS says, because you have to qualify your procedures. C. What exactly the material is. It's not good to ask these kind of questions on a open forum where you are going to get the kind of answers like I gave you. Information is the key to all problems solving. At a minimum you need access to CODE BOOKS.
JPH

ER502 was replaced by ER80S-B6, which is 5 Cr, 1 Mo material. Same as the A335 Gr. P5 material. I don't have any code books with me right now to see if there are any situation where PWHT is not required, but I know we PWHT all welds in this alloy, and think it would be wise to do so. Depending on the customer/code, we PWHT anywhere from 1250F 1350F. The top end of the range is usually preferred. We usually PWHT for 1 hour per inch of weld with a 4 hour minimum.

If you don't already know what ER502 is and what PWHT is required for 5%Cr, then you have no business welding on pressurized piping. If you don't know these basic details, how are you going to know how to weld it properly? Where's your engineering department?