When planning a chemical plant, the design and selection of piping materials are of great importance as they are related to the reliability, safety and economic efficiency of the entire chemical plant. Based on relevant professional experience, the author briefly summarizes the standards and precautions relevant to the design and selection of piping materials in chemical plants.
0. Why should we scientifically design and select piping materials for chemical plants?
Because the means of pipeline transportation in chemical plants has certain peculiarities: complex and dangerous, the design and selection of piping materials are related to the safety of the entire system. If the design is inadequate, it is easy to cause pipeline ruptures and chemical safety accidents, causing serious economic losses and huge social problems. Therefore, it is necessary to scientifically design and select chemical plant piping materials. The design must be compliant and care must be taken to meet relevant standards and specifications.
1. The basis of gas pipeline material selection
Reasonable selection of piping and its component materials and determination of piping pressure level are mainly based on the properties of the medium, design pressure, design temperature, toxicity, fire risk, material processing performance, welding and economy.
2. Design principles
Chemical plant piping materials should be designed in accordance with 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 the material is different in different standards and we should use the value in Table A.1 of 20801.2-2006.
3.1 Tube
Tubes are the most commonly used and have the largest components. Pipes must 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 must comply with the provisions of Table 1 of TSGD 0001-2009. In particular, GB/T 8163 cannot be used for GC1 pipelines. The size standard for chemical piping is HG/T 20553-2011. The commonly used outer diameter series are divided into inch series (Series Ia) and metric series (Series II). Wall Thickness Series: Metric pipes are represented by wall thickness and inch pipes are represented by the pipe measurement number. Series II tubes cannot be connected with screw threads. If Series Ia pipes are used, the thread type of the threaded pipe and valve connection must be specified.
Selection of seamless steel pipes: Extreme, highly hazardous, flammable media or high pressure temperatures, or the resistance to mechanical vibration, pressure pulsation and temperature fluctuations of the pipeline should choose seamless steel pipes; Seamless steel pipes should be chosen for any medium piping with pressure class PN100, Class 600 or higher. Under the condition of selecting seamless steel pipes, for large pipes, it is necessary to select the following: welded steel pipe and the steel pipe weld must pass 100% X-ray flaw detection; For GC1 liquid pipes, the welded pipe must have the same mechanical properties as seamless pipes.
Notes on calculating pipe wall thickness:
- (1) The negative thickness deviation value of the material should be selected according to the manufacturing standard of the pipeline, and the negative thickness deviation is different in different manufacturing standards.
- (2) Thickness tolerance is the sum of corrosion and erosion tolerance as well as machining depth. The allowable thickness of the threaded connection pipe must add the thread depth.
3.2 Pipe fittings
Pipe fittings are mechanical components of piping systems that connect or mount pressure seals, including elbows, tees, reducers, pipe caps, flange connections, and flexible joints .
The most commonly used pipe standards are GB/T 14383-2008, GB/T 12459-2005 and GB/T 13401-2005. Commonly used pipe connection ways are divided into Butt welding Connection , W socket elding Connection wire Connection And Flange connection and welding is preferred. Generally, pipes and fittings with DN ≥ 50 are connected by butt welding. When pipes and pipe fittings with DN ≤ 40 are connected by butt welding, misalignment has a great influence, there is a risk of burning, and the welding quality is not guaranteed. Generally, butt welding is not used, and socket welding or threaded connection should be used.
The material for socket-welded pipe connections are forgings, which are commonly used in pipes ≤ DN40. Socket welded pipe fittings are reliable and easy to install and manufacture without the need for filming. However, they are unsuitable for the following pipes: pipes with wall thickness ≥ Sch.160, pipes with crevice corrosion media, pipes with cleanliness requirements such as lubricating oil, pipes with 100% flaw detection requirements for socket welds, composite tubes and casing tubes. A threaded connection is often used in cases where welding is not easy and parts need to be disassembled. For example, galvanized steel pipes for instrument air ≤ DN80 are connected by a thread. Application limitations of threaded connections: Piping for highly toxic media, crevices or stress corrosion and where alternating loads may occur.
Careful:
- (1) The outer diameter of series I pipe fittings may be omitted, and the outer diameter or special size of series II pipe fittings shall be marked.
- (2) For butt-welded seamless pipe fittings, the material quality and steel pipe standard must be specified, for example 20 (GB 9948);
- (3) All butt weld ends of welded pipe connections shall be marked with the wall thickness. If the wall thickness of the two ends of the welded reducing tube is different, the wall thickness of both ends should be marked.
- (4) The threaded connection must have a tapered pipe thread. The tapered pipe thread is divided into a 55° tapered thread and a 60° tapered thread (the selection must be observed).
3.3 Flanges
The flange is the criterion for determining the nominal pressure level of the pipeline. The allowable operating pressure of the flange must be determined according to the nominal pressure-temperature parameters listed in the flange standard. Flanges are generally divided into European and American systems.
Comments:
- (1) European system flange is suitable for steel flanges with rated pressure ≤ PN160. If the pressure is > PN160, only the American system flange can be selected.
- (2) The outer diameter of steel pipes applicable to the European system flange includes two series: A (British pipe) and B (metric pipe). The flange of B series steel pipes should be marked with B.
- (3) When selecting the weld neck flange, the wall thickness of the connecting pipe at the butt weld end should be taken into consideration;
- (4) If the flange sealing surface has a tongue-and-groove (M&F) or tongue-and-groove (T&G) surface, the downward-facing nozzle of the device has a raised table surface. or Spring Surface Flange which makes seal alignment and installation easier.
3.4 Seals
Seals are generally divided into non-metallic, semi-metallic and metallic seals.
Comments:
- (1) The use of asbestos seals or seals containing asbestos is not permitted.
- (2) Standard pipe flanges with rated pressure ≤ PN20, such as semi-metallic joints, such as rolled joints, metal-clad joints or metal ring joint seals must adopt rigid structures such as neck-to-neck welding;
- (3) When using spiral seals, in addition to the basic type, the inner ring for spiral seals with rated pressure ≥ PN63, rated pressure ≥ Class600 and PTFE must be used as filling material. In addition to the outer ring, the inner ring can be added to the exchanger used in the RF > DN600 flange to increase rigidity and prevent damage.
3.5 Fasteners
Different fasteners are chosen for the flanges of the European and American systems.
Comments:
- (1) Pay attention to the requirements for using commercial-grade fasteners.
- (2) The screws of the eyeglass blind flange and the pinch valve need to be extended, the length of the extension should be calculated according to the actual situation.
3.6 Valves
Commonly used valves are gate valves, globe valves, check valves, ball valves, butterfly valves, drain valves, safety valves, control valves, etc. one of the difficulties.
Comments:
- (1) If the pipeline needs to use ball valves and plug valves with non-metallic seats, the valves must have a fireproof and anti-static structure, and the pressure and temperature must be checked in accordance with the design conditions.
- (2) Valves with screw caps cannot be used in pipelines for extremely hazardous media and highly hazardous liquids, as well as for liquefied hydrocarbons.
- (3) For valves whose temperature is above 600°C (except check valves), it is better to use the extended valve cover structure with heat sink.
- (4) For cryogenic valves with a temperature below -46°C (except check valves), the extended valve cover structure should be selected.
- (5) At the end of welding valve, if the welding and heat treatment process deforms the valve seat, a long valve body or short tube end should be chosen.
- (6) For GC1 class pipelines, a special petrochemical valve must be selected.
