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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.


  • 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.


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 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.

20210707033607 78184 - Selection of pressure piping materials and piping equipment

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.
20210707033705 45075 - Selection of pressure piping materials and piping equipment

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


Flat Face (FF)

Flat Face (FF)


Flat Face (FF)


Raised Face (RF)

Raised Face (RF)

Raised Face (RF)

Raised Face (RF)


Male-and-Female (M&F)

Male and female face (MFM)

Male and female face (MFM)

Male-and-Female (M&F)


Tongue-and-Groove (T&G)

Tongue-and-Groove (T&G)

Tongue-and-Groove (T&G)

Tongue-and-Groove (T&G)


Tongue-and-Groove (T&G)

Ring-Type Joint (RTJ)

Ring-Type Joint (RTJ)

Ring-Type Joint (RTJ)

Table 6-2 Comparison table of structural type code

Serial number

SH 3406-96

HG 20592~20635-97

JB/T 74~90-94

GB 9112-2000


Welding Neck Flange (WN)

Welding Neck Flange (WN)

Welding Neck Flange(-)

Welding Neck Flange(-)


Socket Weld Flange (SW)

Socket Weld Flange (SW)


Socket Weld Flange(-)


Slip on flange (SO)

Slip on flange (SO)

Slip on flange(-)

Slip on flange(-)


Thread Flange (TH)

Thread Flange (TH)


Thread Flange(-)


Lap joint Flange (LJ)

Lap joint Flange (LJ)

Lap joint Flange(-)

Lap joint Flange(-)


Blind flange(-)

Blind flange (BL)

Blind flange(-)

Blind flange(-)



Integral flange(IF)

Integral flange(-)

Integral flange(-

Selection of structure type

  • Flat welding flange: it is mostly used under mild medium conditions, such as low-pressure non purified compressed air and low-pressure circulating water. Its advantage is that the price is relatively cheap;
  • Butt welding flange: the most commonly used one, it is butt welded with the pipe, the quality of the welding joint is better, and the neck of the flange uses the taper transition, which can withstand more harsh conditions;

  • Socket welding flange: commonly used in PN ≤ 10.0MPa, DN ≤ 40 pipeline;
  • Loose flange: commonly used in the medium temperature and pressure are not high and corrosive medium. When the medium is highly corrosive, the flange contacting the medium (flanging nipple) is made of high-grade corrosion-resistant materials such as stainless steel, while the outer flange ring of low-grade materials such as carbon steel is used to clamp it to achieve sealing;

  • Integral flange: flange and equipment, pipes, pipe fittings, valves are often made into one, this type is commonly used in equipment and valves.

Selection of sealing surface type

  • Full plane sealing surface: it is often matched with flat welding type to be suitable for the working condition (png1.0) with mild operating conditions; It is commonly used for cast iron flange or steel flange connected with cast iron;
  • Convex sealing surface: it is the most widely used type. It is often used with butt welding and socket welding. In American flange, it is commonly used in pn2.0, pn5.0 and partial pn10.0mpa pressure rating; In “European flange”, it is commonly used in pn1.6 and PN2.5MPa pressure class;

  • Concave convex sealing surface: it is often used with butt welding and socket type. It is not often used in American flange, but commonly used in pn4.0 and pn6.4mpa grade in European flange. But it is not convenient to replace the gasket;
  • Tenon surface sealing surface: the use of concave convex flange;

  • Ring groove sealing surface: often used with butt welding connection type (not with socket welding), mainly used in high temperature, high pressure or both conditions. In “American flange”, it is commonly used in PN10.0 (partial), PN15.0, PN25.0, PN42.0mpa pressure rating. In “European flange”, it is commonly used in PNl0.0, PN16.0, PN25.0, PN32.0 and PN42.0.

Bolts / nuts

When selecting the fastener materials for flange connection, the factors such as pipe operating pressure, operating temperature, medium type and gasket type should be considered at the same time.

Gasket type and operating pressure, operating temperature, are directly on the fastener material strength requirements. For example, the flange connection of low-pressure highly toxic medium pipeline sealed by spiral wound gasket, although the operating pressure and temperature of the pipeline are not high, but because the specific pressure required for the spiral wound gasket to form the initial seal is larger, the load of the fastener is also required to be large. Therefore, in this case, the fastener is required to use high-strength alloy steel material.

The alloy steel stud shall be made of high quality steel, that is, the letter A shall be added after the material grade, such as 35CrMoA and 25crmova.

According to the different structural types, bolts can be divided into hexagon head bolts and double head bolts (also known as studs), and double head bolts can be divided into threaded and non threaded.

  • Hexagon head bolt: it is often used with flat welding flange and nonmetal gasket for mild operation. The common material of hexagon head bolt is BL3 or Q235B;
  • Stud bolt: it is often used with butt welding flange under harsh operating conditions. Because there is no change of cross section shape on the threaded stud bolt, it has strong bearing capacity. However, the relative bearing capacity of non threaded stud is weak.

The nut material is usually determined according to the bolt material matched with it, and these combinations are specified in general standards. In general, the nut material should be slightly lower than the bolt material, and ensure that the nut hardness is about hb30 lower than the bolt hardness.


The gasket is compressed by the bolt preload through the flange to produce elastic-plastic deformation, fill the micro geometric gap between the flange sealing surface and gasket, increase the flow resistance of the medium, so as to prevent or reduce the leakage of the medium. The performance of gasket and the selection of gasket have great influence on the sealing effect of sealing pair.

Commonly used gaskets can be divided into three categories, namely non-metallic gasket, semi-metallic gasket and metal gasket.

Non metallic gasket

Asbestos rubber gasket is made by adding different additives to asbestos. In the United States, asbestos products are listed as carcinogens and banned in many standards. But in the world, asbestos is still widely used because of its good elasticity, high strength, good oil resistance, high temperature resistance and easy access.

Scope of application: t ≤ 260 ℃, PN ≤ 2.0MPa (sh3401).

                                  t ≤ 400 ℃, PN ≤ 4.0Mpa (national standard).

It is used in water, air, nitrogen, acid, alkali, oil and other medium conditions.

PTFE coated gasket:

Scope of application: T = – 180 ~ 200 ℃, PN ≤ 4.0Mpa.

It is often used in low temperature or clean environment.

Semi metallic gasket

There are three kinds of semi-metallic gaskets: spiral wound gasket, metal clad gasket and flexible graphite spiral wound gasket.

Spiral wound gasket: it is the most ideal and widely used gasket in semi metal gasket.

Features: good compression resilience and high strength, which is conducive to adapt to the changes of pressure and temperature, and can maintain good sealing performance under high temperature, low temperature, impact, vibration and alternating load.

Winding steel strip: 20, 1Cr13, 0Cr19Ni9, 0Cr18Ni10Ti, 0Cr17Ni12Mo2, etc.

Non metallic winding tape: special asbestos, flexible graphite tape and polytetrafluoroethylene tape.

Scope of application: PN = 2.0 ~ 10.0MPa.

Table 6-3 type and code of spiral wound gasket


Suitable for sealing surface type

Gasket type code



SH 3407-96


JB/T 90-94


Basic type

Male-and-Female (M&F)





With inner ring

Male-and-Female (M&F)





With outer ring

Raised Face (RF)





Inner and outer rings

Raised Face (RF)






Table 6-4 Service conditions of common spiral wound gaskets

                   Gasket material

Nominal pressure of flange(MPa)

Temperature range

Austenitic stainless steel / special asbestos



Austenitic stainless steel / flexible graphite



Austenitic stainless steel / polytetrafluoroethylene



Iron clad gasket: the sealing performance is not as good as spiral wound gasket, so it is not used in pressure pipe. It is often used in flange connection sealing pairs such as heat exchanger head and other large diameter flange connection.
The flexible graphite composite pad is composed of the punching teeth or punched metal core plate and expanded graphite particles. It is suitable for flange with raised face, concave convex face and tenon groove surface.

Metal gasket

Metal gasket is often used on high pressure grade flange to bear higher sealing pressure. The common metal gasket includes flat gasket, octagonal gasket and ellipsoid gasket.

Metal flat gasket: often used with convex surface, concave convex surface, tenon groove face flange.

Octagonal metal gasket and elliptical ring metal gasket: often used with ring groove flange. Compared with oval metal gasket, octagonal metal gasket is easy to process, so it is used more.

The material of metal gasket shall be selected in combination with flange material, and the hardness of gasket shall be lower than that of flange sealing surface (no less than hb30).

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

They are often sandwiched between two flanges for different purposes.

They shall have two sealing surfaces identical to the matching flange.

For example, a project has the following conditions of the piping system in a pipeline grade, and the pipeline material is 20:

  • Steam: the design pressure is 1.6Mpa and the design temperature is 330 ° C.
  • Steam: design pressure is 2.6 MPa and design temperature is 260 ° C.

If the temperature pressure problem of flange is not considered, please write the minimum pressure level of the pipe grade and write down the combination of two or more flanges and gaskets.


1) Pressure level: 4.0Mpa;

a. HG flange

PN4.0 WN-MFM; Gasket: PN4.0 with inner ring winding gasket.

PN4.0 WN-MFM; Gasket: PN4.0 flexible graphite composite gasket.

b. GB flange

PN4.0 WN-MF; Gasket: PN4.0 with inner ring winding gasket.

PN4.0 SO-RF; Gasket: PN4.0 with outer ring winding gasket.

c. JB flange PN4.0 wn-mfm; Gasket: PN4.0 with inner ring winding gasket.

Valves and other pipeline equipment

There are many kinds of valves used in engineering, and the common valves are as follows:

There are gate valve, stop valve, check valve, ball valve, butterfly valve, drain valve, safety valve, regulating valve, etc.

Other commonly used pipeline equipment: expansion joint, filter, mirror, flame arrester, etc

How to select valves and other pipeline equipment correctly is a complex problem, with more contents. Here is a simple introduction.

Quality requirements of valves

Internal leakage

Whether there is internal leakage or internal leakage is the main technical index to measure the quality of a valve. For pressure pipelines, the media to be treated are mostly combustible, flammable, explosive and toxic media. When the valve is closed, it is hoped that the less leakage (internal leakage) through the valve plate is better, even if the leakage requirements of some media are zero. The commonly used standards for judging valve leakage are API598, ANSI b16.104 and JB/T 9092.

Leakage problem

Leakage refers to the leakage of medium at the valve stem packing and bonnet gasket. It is also an important indicator of the quality of the valve. For some media, the requirements of external leakage are even more strict than that of internal leakage, because it can directly leak into the atmosphere, which will cause personal injury directly. In this case, bellows sealing valve or diaphragm valve is sometimes used to ensure that the valve leakage is zero. Most of the standards for limiting leakage are currently limited by the EPA, i.e. no more than 500ppm.

Material quality of valve

Material quality is an important index to measure the reliability of valve strength and service life. As we all know, most DN ≥ 50 valves are cast valve bodies, if the quality is not good, it will directly affect the reliability and service life of the valve. ASTM and China’s material standards generally require relatively low requirements. In order to ensure that the materials can meet the requirements of operating conditions in harsh conditions, many optional additional inspection items are set in these standards. How to select these additional items according to the use conditions is a technical problem, if the requirements are not correct, It will increase the investment in infrastructure without any meaning.

Test requirements before valve delivery

  • 1) Before the valve leaves the factory, the shell pressure test and sealing test shall be conducted according to JB/T 9092-1999 inspection and acceptance of valves. The sealing test includes upper seal, low pressure seal and high pressure seal test.

  • 2) The sealing test is selected according to the valve type. The upper and lower pressure sealing tests shall be carried out for gate and stop valves.
  • 3) For shell pressure test, generally, water with temperature not exceeding 52 ℃ or non corrosive fluid with viscosity no more than water is generally used, and the nominal pressure of 1.5 times of 38 ℃ is used.

  • 4) The low pressure sealing test is generally conducted with air or inert gas at the pressure of 0.5-0.7mpa.
  • 5) SH 3064 selection, inspection and acceptance of general valves for petrochemical steel has put forward corresponding inspection requirements for different grades of pressure pipes, which is more strict than JB/T 9092.

Contents of Valve Specification

The general valve specification shall include the following contents:

  • 1) Standard code adopted;
  • 2) Name, nominal pressure and nominal diameter of valve;

  • 3) Material of valve body and external connection mode of valve body;
  • 4) Sealing surface material of valve seat;

  • 5) Stem and bonnet structure, stem and other internal parts materials, packing type;
  • 6) Type of flange gasket, fastener structure and material in valve body;

  • 7) The valve code or tag number proposed by the designer;
  • 8) Other special requirements.

The current method of valve model representation in China has no regulations on the material of valve stem and internals, packing type, gasket type of middle flange and type of fastener of Sino French orchid, which can not fully explain the valve properties.

Selection of valve type

The selection of valve type is one of the key work contents and difficulties of material engineer. It requires that material engineer should not only be familiar with the structural type, performance characteristics and relevant standards of common valves, but also be familiar with application environment and engineering requirements.

Selection principle of valve type:

a. Gate valve: gate valve shall be preferred in general switch.

Structural features: gate valve is driven by valve stem, and moves up and down along the sealing surface of valve seat, which can connect or cut off the passage of fluid, which is mainly used for the closing of pipeline.

Scope of application:

  • 1) Compared with the stop valve, gate valve is the most common valve, which has small flow resistance, small opening and closing force and reliable sealing;

  • 2) When part is opened, the medium will produce eddy current on the back of the ram, which will easily cause erosion and vibration of the ram, and the sealing surface of the valve seat is easy to damage, so it is not generally used as throttling;
  • 3) Compared with ball valve and castellated valve, gate valve has a long opening time and large structure size, so it is not suitable for large diameter.

Valve standard:

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

GB12232, it can be connected with pipe by thread, socket welding, flange or butt welding.

b. Stop valve and throttle valve: for the switch occasions (such as bypass of regulating valve, hose station, etc.) with certain regulation function and the occasion of transporting liquefied petroleum gas and liquid hydrocarbon medium, the stop valve should be selected instead of gate valve.

Structural features: all valves are closed downward, valve disc driven by valve stem, along the center line of valve seat to do lifting movement of the valve. The structure of stop valve and throttle valve is basically the same, but the shape of disc is different. The disc of the stop valve is disc shape; The disc of throttle valve is mostly conical streamline type.

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.

c. Check valve: check valve shall be selected for the occasion that the medium reverse flow can be prevented automatically.

Structural features: check valve is also called one-way valve, which only allows the medium to flow in one direction. When the medium flows along, the valve disc will open automatically, and it can be closed automatically when the medium flows in reverse direction. When installing, it should be noted that the flow direction of medium should be consistent with the arrow direction on the check valve.

Lifting check valve: it is to open the valve by medium pressure. When the medium flows in reverse direction, it is closed by self weight (sometimes closed by spring), so the lifting check valve can only be installed on horizontal pipe; Due to the limitation of installation requirements, DN ≤ 40 is usually used in small diameter cases.

Swing check valve: it is to open the valve by medium pressure and close the valve by medium pressure and gravity, so it can be used on horizontal pipe and vertical pipe (at this time, the medium must be bottom-up)( DN≥50).

Clamp check valve: small structure and low cost of manufacture, which is often used to replace lift and swing check valves.

SH uttle check valve: it is a problem that Dn40 lifting check valve can not be used on vertical pipe.

  • When DN ≤ 40, lifting check valve should be used (only allowed to be installed on the horizontal pipe ‘channel);
  • When DN = 50-400, swing check valve should be used (it is not allowed to install on vertical pipeline from top to bottom of medium);

  • When DN is ≥ 450, it is advisable to select the filling disc check valve;
  • DN = 100-400, or clamp type check valve can be used, and its installation position is not limited;

  • d. Butterfly valve: butterfly valve is generally used for the occasions where the design pressure is low and the pipe diameter is large, and the quick opening and closing is required.

Structural features: it has the characteristics of 900 rotation, quick opening and closing, light weight, small structure size (especially for clamp butterfly valve). But the sealing performance is not as reliable as gate valve, and can be replaced by gate valve in some conditions.

e. Ball valve: for the occasions requiring quick opening and closing, ball valve is generally selected.

Structural features: the valve disc is a sphere with channel in the middle, and the ball rotates 900 around its axis to achieve the purpose of opening and closing. The biggest characteristic of ball valve is that the flow resistance is the least and the flow characteristic is the best among many valve types. The sealing performance of the system is reliable. Compared with butterfly valve, it has a large weight and large structure size, so it is not suitable for pipes with too large diameter.

As with butterfly valve, the problem that can not be applied in petrochemical production unit for a long time is that the sealing will be not tight after heat expansion or wear. Although soft sealing ball valve has good sealing performance, it must be subject to fire safety test and anti-static test when it is used on flammable and explosive medium pipes. Therefore, the application of ball valve in petrochemical production plant is not much. In recent years, many ball valve manufacturers have developed some new type of ball valves, such as track ball valve, eccentric ball valve, etc., some ball valves set the valve seat as metal elastic valve seat, so that it still has good sealing under the condition of heat expansion and wear. Therefore, these ball valves are also used in petrochemical production units.

Source: Piping System Supplier:

(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.)

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