Selection of pressure piping materials and piping equipment

Seamless steel pipe

Seamless steel pipe is a kind of steel pipe without weld made by hot working method such as piercing and hot rolling. If necessary, the hot-processed pipe can be further cold processed to the required shape, size and performance. At present, seamless steel pipe (DN 15-600) is the most widely used pipe in petrochemical plant.

Carbon steel seamless pipe

Material grade: 10, 20, 09mnv, 16Mn, 4 kinds in total.

Standard of steel pipe:

• GB 8163 seamless steel pipes for fluid transportation.
• GB/T  9711.1-1997 technical conditions for delivery of steel pipes for petroleum and natural gas industries.
• GB 6479 high pressure seamless steel pipe for chemical fertilizer equipment.
• GB 9948 seamless steel tubes for petroleum cracking.
• GB 3087 seamless steel tubes for low and medium pressure boilers.
• GB 5310 seamless steel tubes for high pressure boilers.

GB /T 8163：
Material grade: 10, 20, 09mnv, 16Mn.
Scope of application: oil, gas and public medium with design temperature less than 350 ℃ and pressure less than 10MPa.
GB 6479：
Material grade: 10, 20g and 16Mn
Scope of application: design temperature – 40 ~ 400 ℃, design pressure 10.0 ~ 32.0mpa oil, oil and gas.
GB 9948：
Material grade: 10, 20, 2 kinds in total.
Scope of application: where GB/T 8163 steel pipe is not suitable.
GB 3087：
Material grade: 10, 20, 2 kinds in total.
Scope of application: superheated steam and boiling water of low and medium pressure boiler.
GB 5310：
Material grade: 1 kind of 20g.
Scope of application: superheated steam medium of high pressure boiler.
Inspection: chemical composition analysis, tensile test, flattening test and hydrostatic test must be carried out for steel pipes for general fluid transportation.
In addition to the tests that must be carried out for the pipes used for fluid transportation, the expanding test and impact test are required for the pipes of GB 5310, GB 6479 and GB 9948; The manufacturing inspection requirements of these three kinds of steel pipes are relatively strict.
GB 6479 also has special requirements for low temperature impact toughness of materials.
In addition to the general test requirements of steel pipes for fluid transportation, cold bending test is also required for steel pipes in GB 3087 standard.
In addition to the general test requirements of the steel pipes for fluid transportation, the steel pipes of GB/T 8163 standard shall be subject to expanding test and cold bending test according to the agreement. The manufacturing requirements of these two kinds of pipes are not as strict as the first three.In addition to the general test requirements for the steel tubes of GB/T8163 standard, flared test and cold bending test shall be carried out according to the agreement requirements. These two kinds of tubes are not as strict as the first three kinds of tubes.
Manufacture of steel pipe:
The steel pipes of GB/T/8163 and GB 3087 are mostly smelted in open hearth or converter, and there are many impurities and internal defects.

GB 9948 is mostly smelted by electric furnace. Most of them add out of furnace refining process, the composition and internal defects are relatively less.
GB 6479 and GB 5310 standards stipulate the requirements for out of furnace refining, with the least impurity composition and internal defects and the highest material quality.
The manufacturing quality grades of the above steel pipe standards are from low to high.

• GB/T 8163<GB 3087<GB 9948<GB 5310<GB 6479

Selection of steel pipe standards

• In general, GB/T 8163 steel pipe is suitable for oil, gas and public medium with design temperature less than 350 ℃ and pressure less than 10.0MPa;
• For oil and gas medium, when the design temperature exceeds 350 ℃ or the pressure exceeds 10.0MPa, the steel pipe of GB 9948 or GB 6479 standard should be selected;
• GB 9948 or GB 6479 should also be used for the pipeline in hydrogen operation or in the environment with stress corrosion tendency.
• GB 6479 standard should be adopted for carbon steel pipe used at low temperature (less than – 20 ℃), only it specifies the requirements for low temperature impact toughness of materials.

GB 3087 and GB 5310 standards are specially set for boiler steel pipe 《The boiler safety supervision regulation》 emphasizes that all pipes connected with the boiler belong to the supervision scope, and the application of materials and standards should comply with 《the boiler safety supervision regulation》. Therefore, the public steam pipes (supplied by the system) used in boilers, power stations, heating and petrochemical production units should adopt GB 3087 or GB 5310 standards.
**It is worth noting that the price of steel pipe with good quality is relatively high. For example, the price of GB 9948 is nearly 1/5 higher than that of GB 8163. Therefore, the selection of steel pipe material standard should be based on the comprehensive consideration of service conditions, which is both reliable and economical.

Cr Mo steel and Cr Mo V steel seamless pipe

In petrochemical plant, the standard of Cr Mo steel and Cr Mo V steel seamless steel pipe are as follows:

• GB9948 seamless steel tubes for petroleum cracking.
• GB6479 high pressure seamless steel pipe for chemical fertilizer equipment.
• GB5310 seamless steel tubes for high pressure boilers.

There are four kinds of Cr Mo steel grades in GB9948: 12CrMo, 15CrMo, 1cr2mo and 1Cr5Mo.
There are three kinds of Cr Mo steel grades in GB6479: 12CrMo, 15CrMo and 1Cr5Mo.
There are 6 kinds of Cr Mo steel and Cr Mo V steel in GB5310: 15mog, 20mog, 12crmog, 15CrMoG, 12CR2MOG, 12Cr1MoVG.

Stainless steel seamless pipe

The commonly used standards of stainless steel seamless pipe are: GB/T 14976, GB13296, GB9948, GB6479, GB5310, a total of five standards. Among them, only two or three stainless steel material grades are listed in the latter three standards, and they are not commonly used. Therefore, when the standard of stainless steel seamless steel pipe is selected in engineering, the standard of GB/T 14976 and GB 13296 are basically selected.
GB/T 14976 seamless stainless steel tubes for fluid transport.
Material grade: 0Cr18Ni9 (304), 00Cr19Ni10 (304L), 0Cr17Ni12Mo2 (316), 00Cr17Ni14Mo2 (316L), 0cr18nil0ti (321), 0cr18ni11nb (347), 0Cr25Ni20 (310), etc.
GB 13296 seamless stainless steel tubes for boilers and heat exchangers.
Material grade: 0Cr18Ni9 (304), 00Cr19Ni10 (304L), 0Cr17Ni12Mo2 (316), 00Cr17Ni14Mo2 (316L), 0cr18nil0ti (321), 0cr18ni11nb (347), 0Cr25Ni20 (310), etc.
The code “304” is the corresponding brand in ASTM standard.

• The ultra-low carbon stainless steel (00Cr17Ni14Mo2, 00Cr19Ni10) has excellent corrosion resistance. Under certain conditions, it can replace stable stainless steel (0018ni10ti, 0cr18ni11nb) for medium corrosion resistance;
• The mechanical properties of ultra-low carbon stainless steel are low at high temperature, which is generally used only when the temperature is below 525 ℃;
• Stable austenitic stainless steel has good corrosion resistance and high temperature mechanical properties, but the Ti in 0Cr18Ni10Ti is easily oxidized and lost in the welding process, which reduces its corrosion resistance and its price is high. This kind of material is generally used in more important occasions.

Because 0Cr18Ni9 and 0Cr17Ni12Mo2 have general corrosion resistance and low price, they are widely used.

Pipe fittings

Common pipe fittings include: elbow, tee, reducer (big and small head), pipe cap, reinforcing nozzle, reinforced pipe joint, thread short joint, movable joint, plug, instrument nozzle, hose station quick joint, funnel, water nozzle, pipe hoop, etc.
The bending of the pipe was made by the on-site simmer before. The method has the advantages of high labor intensity and low efficiency, and the material is not in good organization and performance, and the bend is often the weak link of the whole pipeline due to the thinning of the pipe wall thickness; The branch of the pipe is usually connected by directly opening holes on the pipe. Sometimes, although reinforcement is made here, the weld is generally fillet weld, and the stress condition is not good and the quality of the weld is not easy to control. Therefore, it is also the weak link of the pipeline. The above problems are solved well after the pipe fittings are used. Therefore, a large number of pipe fittings have been used in the pressure pipeline, and the investment accounts for about 1/5 of the total pipeline investment.

Connection form

The common connection types among pipe fittings and pipes are butt welding connection, socket welding connection, threaded connection and flange connection.

Butt welding connection

It is a common connection type for DN ≥ 50 pipes and their components. For DN ≤ 40 pipes and their components, because its wall thickness is generally thin, the butt welding joint has a great influence on the wrong joint, easy to burn through, and the welding quality is not easy to guarantee. Therefore, butt welding connection is not generally used. The following exceptions are:

• 1) For pipes and components with DN ≤ 40 and wall thickness greater than or equal to sch160, the wall thickness of them is relatively thick. When butt welding is adopted, the problems mentioned above are no longer present, so butt welding connection is also commonly used;
• 2) In the case of crevice corrosion medium (such as hydrofluoric acid medium), even if DN ≤ 40 and wall thickness is less than sch160, butt welding connection is also adopted to avoid crevice corrosion. At this time, argon arc welding with small welding wire diameter and small welding current is often used in welding construction instead of general arc welding;
• 3) For lubricating oil pipeline, when socket welding is used, impurities are easily accumulated in the joint gap and have adverse effect on mechanical equipment. At this time, butt welding connection should also be adopted.

Socket weld connection

It is mainly used for the connection between the pipe and the pipe with DN ≤ 40 and thin pipe wall.
Socket welding connection must be one is socket, the other is socket pipe fittings.
Generally, the reducer and thread are spigot fittings;
Elbow, tee, pipe cap, reinforcing nozzle, union, pipe hoop, etc. are socket fittings. The combination of these fittings and the required structural space should be considered in the application.

Threaded connection

Threaded connection is also used for connection between DN ≤ 40 pipes and their components. It is often used in places where welding is not suitable or detachable.
The threaded connection is divided into male thread and female thread.
In common pipe fittings, the short thread is male thread, while elbow, tee, pipe cap, union and so on are mostly female threads. When using, the collocation and combination of them should be noted. Compared with welding, the screw connection has low joint strength and poor sealing performance. Therefore, it is often limited by the following conditions when it is used:

• 1) The pipe fittings connected with thread shall be tapered pipe thread;
• 2) Screw connection is not recommended for temperatures greater than 200 ℃ and below – 45 ℃;
• 3) Threaded connection shall not be used on highly toxic medium pipeline;
• 4) Threaded connections are not recommended for pipes where corrosion, crevice corrosion or vibration, pressure fluctuation and temperature change may produce alternating loads;
• 5) When used on combustible gas pipelines, sealing welding should be used for sealing.

The common taper pipe threads can be divided into:

• 550 taper pipe thread (mostly for Europe).
• 600 taper pipe thread (mostly used in the United States).
• (ISO 7/1) is 550 taper pipe thread. Japanese standard includes 550 taper pipe thread and 600 taper pipe thread.

GB7306 is the same as 550 taper pipe thread (ISO 7/1).
The taper of thread is: 1:16
Tooth angle: 550
Dimension range: 1/16 “~6”
Thread mark code: R (tapered external thread)
                               RC (tapered internal thread)
GB 12716 is equivalent to 600 taper pipe thread (ANSI/ASME B1.20.1).
The taper of thread is: 1:16
Tooth angle: 600
Dimension range: 1/16 “~12”
Identification: NPT
The two taper pipe threads cannot be interchanged.

Butt welded pipe fitting

The common butt welded pipe parts include elbow, tee, reducer (big and small head) and pipe cap. Most of the first three items are made of seamless steel pipe or welded steel pipe by pushing, drawing and extrusion, and the latter is mostly made of steel plate stamping.
They can achieve the same strength as the pipe by nominal wall thickness grade (pipe table number or wall thickness value). As for the reinforcement of local stress concentration, it is the matter that the manufacturer should solve. The manufacturer shall design the strength of welded pipe fittings and verify them by verification test method.

Elbow

• Long radius elbow (r = 1.5dn): generally, it should be preferred;
• Short radius elbow (r = 1.0dn): it is used in the case of size limitation. The maximum working pressure should not exceed 0.8 times of the elbow with the same specification and long radius.

Bend (r = nDN): used to alleviate the scour and kinetic energy of medium at the turning point, and can be used to r=3dn, 6dn, 10dn, 20dn.
According to the different manufacturing methods, it can be divided into push elbow, extrusion elbow and welding elbow.
Push and squeeze elbows: commonly used in medium and small-sized pipes with more stringent medium conditions.
Welding inclined elbow: it is commonly used on large-scale pipes with mild medium conditions, and the bending radius shall not be less than 1.5 times of its nominal diameter. When the angle of diagonal elbow is greater than 450, it should not be used on highly toxic and combustible medium pipelines, or pipelines that bear mechanical vibration, pressure fluctuation and alternating load due to temperature change.

Tee

Equal tee
Reducer tee
Y-type tee (lateral tee) is often used to replace general tee, which is used to transport solid particles or pipeline with serious erosion and corrosion.

Reducer (large and small head)

Concentric reducer is usually used.
Eccentric reducer

Cap (head)

With flat cover head
Ellipsoidal head 
Flat head is easy to manufacture and low in price, but its pressure bearing capacity is not as good as that of standard elliptical head, so it is usually used in the condition of DN ≤ 100 and medium pressure less than 1.0MPa. The standard elliptical head is the elliptical head with folded edge, and the ratio of the inner diameter to the long axis of the ellipse is 2:1, which is the most widely used head.
In many cases, such as the end of the pipe on the pipe gallery, the pipe cap is replaced by flange to facilitate the purging and cleaning of the pipe.

Socket welding and threaded connection pipe fittings

It generally refers to the pipe components with DN ≤ 40, including elbow, tee, reinforcing nozzle, reinforced pipe joint, pipe cap, pipe hoop, reducer short joint, movable joint, plug, instrument nozzle, hose station quick joint, water nozzle, etc.

Standard of common pipe fittings

National standard:

• GB12459-90 steel butt welded seamless pipe fittings DN = 10-500MM Series A and B.
• GB/T 13401-92 steel plate butt welded seamless pipe fittings DN = 350-1200mm Series A and B.
• GB/T 14383-93 forged steel socket welded pipe DN = 15-80mm Series A and B.
• Forged steel threaded pipe fittings  DN=8－100.
• GB/T 17185-1997 steel flange & pipe fittings DN = 25-600mm PN 2.0, 5.0, 11.0, 15.0, 26.0mpa.

Sinopec standard:

• SH 3408-1996 steel butt welded seamless pipe DN = 15-500mm.
• SH 3409-1996 steel plate butt welded pipe DN = 200-1200mm.
• SH 3410-1996 forged steel socket welding pipe DN = 10-80mm.

Ministry of chemical industry standards:

• HG/T 21634-1990 forged steel socket welded pipe 1/2 “- 11/2”, 3000 and 6000 lbs.
• HG/T 21635-1990 seamless butt welded pipe of carbon steel and low alloy steel DN = 50-600mm.
• HG/T 21631-1990 steel butt welded pipe DN = 300-1000mm.

Ministry of petroleum standards:
SY/t0510-1998 steel butt welded pipe DN = 15-600mm.
All kinds of pipe fittings standard, butt welding seamless and steel plate made equal and effective use of ANSI b16.9 and ansib16.28. The equal and effective use of ANSI B16.11 is adopted for socket welding and threaded pipe fittings made of forged steel. But the structural dimensions of the same type of pipe fittings are different.

Flange and fastener

Flange, gasket and bolt constitute the detachable connection structure in the pipeline, which is widely used in the pressure pipeline and is also a very important connection form. Usually, flange, bolt and gasket constitute a sealing pair, and they work together to ensure the good sealing of the joint.
Blind plate, 8-shaped blind plate, flow limiting orifice plate and mixed orifice plate are closely related to flange, gasket and bolt. They are often used together to introduce.

Flange

Flange is the benchmark to determine the nominal pressure class of pipeline. It can be seen from the previous introduction that there are many kinds of flange. Different types of flange have different sealing performance and application occasions. This paper only discusses how to select different types of flange for reference only.

Flange type

◆  According to the connection mode with the pipe, the pipe flange can be divided into the following six basic types: flat welding, butt welding, socket welding, loose sleeve, threaded flange and integral flange, as shown in Fig. 6-1.

Figure 6-1 connection mode of pipe flange and pipe
There are five kinds of flange sealing surfaces: full plane, convex table, concave convex surface, mortise surface and annular groove surface. See Figure 6-2.

Figure 6-2 flange sealing surface type

Flange code

Different standards have slightly different names and codes of sealing surfaces and types of flange. See table 6-1 and table 6-2.
Table 6-1 comparison of names and codes of flange sealing surfaces

Serial number	SH 3406－96	HG 20592～20635－97	JB/T 74～90－94	GB 9112－2000
1	Flat Face (FF)	Flat Face (FF)		Flat Face (FF)
2	Raised Face (RF)	Raised Face (RF)	Raised Face (RF)	Raised Face (RF)
3	Male-and-Female (M&F)	Male and female face (MFM)	Male and female face (MFM)	Male-and-Female (M&F)
4	Tongue-and-Groove (T&G)	Tongue-and-Groove (T&G)	Tongue-and-Groove (T&G)	Tongue-and-Groove (T&G)
5	Tongue-and-Groove (T&G)	Ring-Type Joint (RTJ)	Ring-Type Joint (RTJ)	Ring-Type Joint (RTJ)

Serial number	SH 3406－96	HG 20592～20635－97	JB/T 74～90－94	GB 9112－2000
1	Welding Neck Flange (WN)	Welding Neck Flange (WN)	Welding Neck Flange（－）	Welding Neck Flange（－）
2	Socket Weld Flange (SW)	Socket Weld Flange (SW)		Socket Weld Flange（－）
3	Slip on flange (SO)	Slip on flange (SO)	Slip on flange（－）	Slip on flange（－）
4	Thread Flange (TH)	Thread Flange (TH)		Thread Flange（－）
5	Lap joint Flange (LJ)	Lap joint Flange (LJ)	Lap joint Flange（－）	Lap joint Flange（－）
6	Blind flange（－）	Blind flange (BL)	Blind flange（－）	Blind flange（－）
7		Integral flange（IF）	Integral flange（－）	Integral flange（－）

Selection of structure type

Selection of sealing surface type

Bolts / nuts

Gasket

Non metallic gasket

Semi metallic gasket

Form	Suitable for sealing surface type	Gasket type code
		SH 3407-96	HG20610-97	JB/T 90-94	GB4622.1-93
Basic type	Male-and-Female (M&F)	A	A	A	A
With inner ring	Male-and-Female (M&F)	C	B	B	C
With outer ring	Raised Face (RF)	B	C	C	B
Inner and outer rings	Raised Face (RF) PN≥5.0MPa、T≥350℃	D	D	D	D

Gasket material	Nominal pressure of flange（MPa）	Temperature range ℃
Austenitic stainless steel / special asbestos	≤25.0	－50~500
Austenitic stainless steel / flexible graphite	≤25.0	－196~600
Austenitic stainless steel / polytetrafluoroethylene	≤10.0	－196~200

Metal gasket

Spectacle blind flange, 8-type blind plate, flow limiting orifice plate and mixed orifice plate

Valves and other pipeline equipment

Quality requirements of valves

Internal leakage

Leakage problem

Material quality of valve

Test requirements before valve delivery

Contents of Valve Specification

Selection of valve type

Valve standard:

API and ANSI B16.34, the former is dedicated to petrochemical plant, while the latter is widely used.

Application of valve:

• 1) Compared with gate valve, stop valve has certain regulating function, so it is often used for regulating valve group bypass.
• 2) The stop valve needs to overcome the resistance of the medium when it is closed, so its maximum diameter is only DN200.

• 3) Throttle valve is especially suitable for throttling, which is used to change the cross-section area of the channel and regulate the flow or pressure.

Source: Piping System Supplier: www.epowermetals.com

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at [email protected]