Analysis of the standard specification for the design and selection of pipeline materials for chemical plants in China

In the design of a chemical plant, the design and selection of pipeline materials are very important, which is related to the reliability, safety, and economy of the whole chemical plant. Based on the relevant work experience, the author briefly summarizes the relevant standards and precautions for the design and selection of pipeline materials in chemical plants.

0. Why should we scientifically design and select pipeline materials for chemical plants?

Because the medium of pipeline transportation in chemical plants has certain particularity: complex and dangerous, the design and selection of pipeline materials are related to the safety of the whole plant. If the design is unreasonable, it is easy to cause pipeline rupture danger and chemical safety accidents, causing serious economic losses and causing huge social problems. Therefore, it is necessary to scientifically design and select pipeline materials for chemical plants. The design should conform to the relevant standards and specifications and pay attention to matching them.

1. The basis of pipeline material selection

Reasonable selection of pipeline and its component materials and determination of pipeline pressure level is mainly based on medium characteristics, design pressure, design temperature, toxicity, fire hazard, material processing performance, welding performance, and economy.

2. Design principles

The piping materials of the chemical plant should be designed according to TSGD 0001-2009, GB/T 20801-2006, GB 50316-2000 (2008 edition), SH/T 3059-2012, and other standards.

3. Classification of piping systems

Piping systems include pipes, fittings, flanges, gaskets, fasteners, valves, etc.
The allowable stress of materials in different standards is different, and we should use the value of Table A.1 of 20801.2-2006.

3.1 Pipe

The pipe is the most widely used and has the largest components. Pipes should be marked with manufacturing standards and size standards. Common manufacturing standards include GB/T 3091-2008, GB/T 8163-2008, and GB/T 14976-2012. The selection of manufacturing standards should conform to the provisions of Table 1 of TSGD 0001-2009, especially GB/T 8163 cannot be used for the GC1 pipeline; the size standard of the chemical pipeline is HG/T 20553-2011, and the commonly used outer diameter series are divided into Inch series (Ia series) and Metric series (II series). Wall thickness series: metric pipe is represented by wall thickness, and inch pipe is represented by pipe gauge number. II series pipe can’t be connected with screw thread. When Ia series pipe is used, the screw thread type of screw thread pipe fitting and valve must be indicated.
Selection of seamless steel pipe: extreme, high hazard, flammable medium or high pressure temperature or withstand mechanical vibration, pressure pulsation, and temperature change of the pipeline should choose seamless steel pipe; seamless steel pipe should be selected for any medium pipeline with pressure grade PN100, Class600 or above. Under the condition of selecting seamless steel pipe, when the size of the pipe is large, it is necessary to select the welded steel pipe, and the weld of the steel pipe must be 100 % X-ray flaw detection; GC1 fluid pipe, the welded pipe should have the same mechanical properties as seamless pipe.
Notices for pipe wall thickness calculation:

• (1) The value of the negative deviation of the material thickness should be selected according to the pipeline manufacturing standard, and the negative deviation of the thickness of different manufacturing standards is different.
• (2) Thickness addendum is the sum of corrosion, erosion allowance, and machining depth. The thread connection pipe thickness addendum should add thread depth.

3.2 Pipe fittings

Pipe fittings are mechanical components of pipeline systems that connect or assemble into pressure seals, including elbows, tees, reducers, pipe caps, flanging joints, and flexible joints.
The commonly used pipe standards are GB/T 14383-2008, GB/T 12459-2005, and GB/T 13401-2005. The commonly used pipe connection forms are divided into butt welding connection, socket welding connection, thread connection, and flange connection, and welding is preferred. Generally, DN ≥ 50 pipes and fittings are connected by butt welding. If the tubes and pipe fittings with DN ≤ 40 are connected by butt welding, the misalignment has a great influence, it is easy to burn through, and the welding quality is not guaranteed. Generally, butt welding is not used, and socket welding or thread connection should be adopted.
The socket welded pipe fittings material is forgings, commonly used in tubes ≤ DN40. The socket welded pipe fittings are reliable and easy to install and construct without filming. Still, they are unsuitable for the following pipes: pipes with wall thickness ≥ Sch.160, pipes with crevice corrosion medium, pipes with clean requirements such as lubricating oil, pipes with 100 % flaw detection requirements for socket welds, composite pipes, and lining pipes. A threaded connection is often used on occasions that are not easy to weld and need to be disassembled. For example, galvanized steel pipes for instrument air ≤ DN80 are connected by a thread. Application limitations of threaded connections: highly toxic medium pipelines, crevice or stress corrosion, and where alternating loads may be generated.
Cautions:

• (1) The outer diameter of pipe fitting I series can be omitted, and the outer diameter or special size of pipe fitting II series should be marked;
• (2) Butt welded seamless pipe fittings to indicate the material grade and steel pipe standard, such as 20 (GB 9948);
• (3) All the butt-welding ends of the welded pipe fittings should be marked with wall thickness. When the wall thickness of the two ends of the welded reducing pipe is different, the wall thickness of both ends should be marked;
• (4) Thread connection should adopt taper pipe thread; taper pipe thread is divided into 55 ° taper pipe thread and 60 ° taper pipe thread (selection should be noted).

3.3 Flanges

Flange is the benchmark for determining the nominal pressure level of the pipeline. The allowable working pressure of the flange should be determined according to the pressure-temperature rating parameters listed in the flange standard. Flanges are generally divided into the European system and the American system.
Notes:

• (1) The European system flange is suitable for steel flanges with nominal pressure ≤ PN160. When the pressure > PN160, only the American system flange can be selected;
• (2) The outer diameter of steel pipe applicable to the flange of the European system includes two series of A (British pipe) and B (metric pipe). The flange of B series steel pipe must be marked with B;
• (3) When selecting the weld neck flange, the wall thickness of the connecting pipe at the butt-welding end should be indicated; 
• (4) If the flange sealing surface adopts a male and female (M & F) or tongue and groove (T&G) face, the downward nozzle on the equipment comes with a raised face surface flange or tongue surface flange, which is conducive to gasket alignment and installation.

3.4 Gaskets

Gaskets are generally divided into non-metallic, semi-metallic, and metal gaskets.
Notes:

• (1) Asbestos gaskets or gaskets containing asbestos are not allowed to use;
• (2) Standard pipe flanges with nominal pressure ≤ PN20, such as half-metal gaskets such as wound gaskets, metal-coated gaskets, or metal ring joint gaskets, should adopt rigid structures such as neck-to-neck welding;
• (3) When using spiral wound gaskets, in addition to the basic type, the inner ring should be used for spiral wound gaskets with nominal pressure ≥ PN63, nominal pressure ≥ Class600, and PTFE as filling material. In addition to the outer ring, the inner ring can be added to the winding pad used on the RF flange > DN600 to increase the stiffness to prevent damage.

3.5 Fasteners

Different fasteners are selected for the flanges of the European system and the American system.
Notes:

• (1) Pay attention to the use requirements of commodity-grade fasteners;
• (2) The bolts of the spectacle blind flange and the clamping valve must be lengthened, and the lengthening length should be calculated according to the actual situation.

3.6 Valves

Commonly used valves are gate valves, cut-off valves, check valves, ball valves, butterfly valves, drain valves, safety valves, regulating valves, etc. The selection of valve type is one of the key tasks in chemical design, and it is also one of the difficulties.
Notes:

• (1) When the pipeline needs to use the ball valve and plug valve of a non-metallic seat, the valve should have a fire-proof and anti-static structure, and the pressure and temperature level should be checked according to the design conditions;
• (2) Valves with threaded valve caps should not be used in extreme and highly hazardous media and liquefied hydrocarbon pipelines;
• (3) For the valve whose temperature is higher than 600 °C (except the check valve), it is better to adopt the structure of lengthened valve cover with a heat sink;
• (4) For cryogenic valves with a temperature lower than-46 °C (except for check valves), the structure of an extended valve cover should be adopted;
• (5) The end of the welding valve, when the welding and heat treatment process will be deformed valve seat, should choose a long valve body or end with a short pipe valve;
• (6) GC1 grade pipeline should choose a special petrochemical valve.

4. Conclusion

In the design of a chemical plant, the design and selection of pipeline materials are very important. It is very rigorous work, which requires the operation of special pipeline material designers. We should not only meet the relevant standards and regulations but also pay attention to many details to ensure the safe and reliable operation of chemical plants and save investment.
Author: Huang Li