Inspector 1978 Your question if it is related to power plant requirements in variations in code limits for PWHT - yes different codes allow variations in limits for PWHT from thickness and also materials grades from P 1 to P 3. ASA B 31.1 Power Piping code gives exemptions based on hardenability criteria for materials (See Table 132 in B 31.1 2001 edition) ASA B 31.3 code for Process Piping requires PWHT for P1 to P 3 grade materials if thickness > 0.75". B 31.3 also specifies the minimum T.S criteria for PWHT . See Table 333.1.1- B 31.3 code. As additional criteria for P 3 material this code specifies a limit of 225 BHN for PWHT. Pressure vessel code exemptions from PWHT are also based (in part) on the hardenability of the materials. P Nos. (established by ASME Section IX for the general purpose of weldability) are used, although in this case they are not necessarily good criteria. A review of the strengths of the materials in Section IX reveals that P No. 1 materials may have minimum specification tensile strengths within the range of 45 to 95 ksi. Carbon equivalence is a better and more reliable measure of the hardenability of materials than the P Nos. Preheat is also a consideration when determining the resultant hardness of weldments. As the temperature of the material prior to welding increases, the cooling rate (and therefore the hardness) will decrease. The Code exemptions for thicker materials usually require a minimum preheat. Generally variation in limits from Piping vs Pressure Vessel Plate PWHT is based on lower hardenability, particular material grade, Tensile Strength, Pre heat operations before welding and choice of heat input from welding processes (variations from TIG/SMAW/Submerged Arc etc). Hope this helps C.V.Srinivasan Nishi Engineers Pvt Ltd India April 27, 2010 E-mail: nishi@vsnl.com >Dear EXperts > >I have a query regarding PWHT requirement. > >PWHT requirment of pipe (Based on thickness) 19mm > plate : 38 mm > >Why there is a different in PWHT requirement based on >thickness for pipe and plate ? > >Regards

Thank very much Mr Srinivasan and Mr Ashfaq for your replies Regards