4.5　Factory Prefabrication of High Pressure Pipes

4.5.1　Before factory prefabrication,the commissioned company shall organize the examination of the support drawings to confirm the numbering,size,weld position,groove type,stubtube position and its aperture,fixture block,suspension and support numbering as well as its position,adjustment section position of the pipe horizontal and vertical section.The ratio of factory combining welds shall not be less than 50% of the total number of welds.

4.5.2　Pipe cutting shall meet the requirements of DL/T 869 Code of Welding for Power Plant.

4.5.3　After cutting off high pressure steel pipes and alloy steel pipes,the original marks shall be transplanted immediately on their surface.

4.5.4　The thickened pipes or pipes with the positive tolerance of wall thickness shall be adopted for bend manufacturing.

4.5.5　Radius of bends shall meet the design requirements.If there is no design requirements,the minimum radius of bends shall meet the requirements of DL/T 515 Pipe Bends for Power Station.

4.5.6　When intermediate frequency heating is adopted to heat bends,the following requirements shall apply.

1　The heating temperature for low-carbon steel pipes shall be between 850℃and 1000℃.When the wall thickness is not more than 25mm,water cooling or forced air cooling can be adopted and heat treatment is not mandatory after bending.

2　When bending low alloy steel pipes,the heating temperature of the pipe back profile shall not be higher than 900℃.Forced air cooling shall be adopted,and normalizing and tempering treatments shall be carried out after bending.

3　The bending of new steel material pipes shall meet the requirements of the manufacturer.If there is no requirements,the maximum deformation part of this bend back profile must be tested first.The bending process can be determined after confirming no intergranular corrosion cracks and other defects in it.

4.5.7　When heat treatment process of common steels has been changed or after the new steel have undergone heat treatment,the samples from the bending parts shall be inspected to obtain the data information required by Article 4.2.1 of this Part.

4.5.8　After bending,the internal and external surface of the pipe shall be cleaned without defects such as cracks,delamination,overburning.If in doubt,non-destructive test and metallographic examination shall be carried out.

4.5.9　Non-destructive test shall be carried out to the high pressure pipes after bending.If there are defects,the wall thickness after regrinding shall not be less than the minimum wall thickness of the straight section.

4.5.10　The surface of the transition region of forging pipe fittings and accessories shall be rounded.After mechanical processing,there shall be no defects on the surface such as cracks which can influence the strength and tightness.

4.5.11　After the bending and heat treatment of alloy-steel pipes,metallographic examination and hardness test shall be carrid out,and the result shall meet the requirements of DL/T 438 Technical Supervision Code for Metal in Thermal Power Plants.

4.5.12　Product quality inspection certificates shall be submitted for the high pressure bends.

4.5.13　High pressure welded tees shall meet the following requirements.

1　Socket welding shall not be adopted for high pressure welded tees.

2　Weld quality shall meet the requirements of DL/T 869 Code of welding for Power Plant.

3　Heat treatment shall be qualified.

4.5.14　Pipe orifices of each high pressure and high temperature pipe fittings shall adopt mechanical processing.Both inner and outer diameter of the orifice,as well as groove type,shall meet the design requirements.

4.5.15　The factory prefabrication of medium and low pressure pipes can be implemented referring to the provisions of high pressure pipes.