How to avoid mistakes in the use and selection of flanges

Standard system for flanges

There are mainly two international flange standard systems. One is the European flange standard system represented by the European Union EN standard (nominal pressure level expressed in PN), and the other is the American flange standard system represented by the American ANSI/ASME B16.5 (nominal pressure level expressed in Class). In addition, there is the Japanese JIS flange standard system (nominal pressure class expressed as K). The latter is not commonly used, we mainly talk about the European and American flange standard system today.
The European Union EN flange standard system consists of two systems, one based on the original DIN (German standards), NF (French standards), BS (British Standards) for the development of EN1092-1, the other is based on ASME B16.5 EN1759-1. domestic talk about the European standard flange system often refers to the flange standard system EN1092-1, this paper is also So.
EN1092-1 contains a nominal pressure of PN2.5, PN6, PN10, PN16, PN25, PN40, PN100, a total of 7 files, nominal size DN10-DN4000.
American Standard ASME flange standard system mainly consists of ASME B16.5 and ASME B16.47 two standards, ASME B16.5 caliber range for DN600 and below, ASME B16.47 contains A, B two series, used in DN600 above the occasion. ASME B16.47A is developed on the basis of MSS SP-44. ASME B16.47B is developed on the basis of API605. It should be noted that ASMEB16.47A (corresponding to HG20623A/SH3406-2013 large diameter A), ASME B16.47B (corresponding to HG20623B/SH3406-2013 large diameter B) of the A and B series is a distinction between the pair size and other external dimensions, and the flange receiver end of the imperial and metric outside diameter are two concepts.

Matching size of flange and connecting pipe size

The matching size of the flange refers to the number of bolts, bolt diameter, sealing surface width, sealing surface height and other dimensions of the flange. Only when these dimensions of the two flanges are completely consistent can the accurate connection and reliable sealing of the flange be maintained. Obviously, flanges with different standard series and different pressure grades can not be matched basically.

The European and American standards that we often say to distinguish flanges mainly refer to the flanges of different standard systems. The matching dimensions are inconsistent and cannot be connected. For butt welding flanges, the nozzle size is shown in the figure above φ value. For flat welding flange and socket welding flange, it is the flange opening size of the inserted nozzle.

Piping	European system (PN Series)	American system (Class Series)
Imperial pipe	GB/T 9112-9124-2000 HG 20592-20605-1997	GB/T 9112-9124-2000 HG 20592-20605-1997 SH3406-1996
Metric pipe	HG 20592-20605-1997 JB/T 74-86-1994	—

For different types of flanges with the same nominal diameter, the same standard system and the same pressure grade, the matching size is the same. Otherwise, it is generally not allowed to connect. Are there any exceptions?

There are exceptions. There are two exceptions.
Exception 1.
European standard flange

• PN10/PN16/PN25/PN40: DN10~DN80.
• PN10/PN16: DN100~DN150.
• PN63/PN100/PN160: DN10~DN40.
• PN100/PN160: DN50~DN200.

American Standard Flange

• CL300/CL600: DN15~DN80;
• CL900/CL1500: DN15-DN65.

If you are at the peak of project construction and lack of low pressure rating flanges while high rating flanges are available, flanges in the above range can be paired. However, the temperature pressure rating can only be according to the low pressure rating when used. You can make a big achievement for the project.
Exception 2: HG20623 large diameter pipe flange, there are two series of A,B, the receiver end outside diameter are imperial outside diameter, respectively, from ASME B16.47A/B (and respectively from the API and MSS flange standards), the same pressure rating, pairing size is not the same, can not be paired. And because the B series flange quality than the A series of light, we often choose the B series (unless the B series can not choose).
What issues need to be noted when selecting flanges by temperature pressure rating?
There are many issues that need to be noted. I am here to give an example: HG20615-2009 flange standard, for example, assuming that our material is A105, material group 1.1, pipe design temperature 100 ℃, design pressure 1.70Mpa, we should choose what pressure rating of the flange?

Operating temperature (℃)	Maximum allowable working pressure (bar)
	Class150（PN20）	Class300（PN50）	Class600（PN110）	Class900（PN150）	Class1500（PN260）
≤38	19.6	51.1	102.1	153.2	255.3
50	19.2	50.1	100.2	150.4	250.6
100	17.7	46.6	93.2	139.8	233

Our advice is that if used in severe cyclic conditions or flammable and explosive toxic media, you’d better choose CL300 flanges. The reason is that the pressure rating of the flange at design temperature is the maximum allowable working pressure, and this working pressure does not include external load. In fact, flanges installed in the pipeline, inevitably subjected to thermal expansion and contraction, earthquake / wind and other loads caused by external loads, rather than simple media pressure load, these external loads are too large, the flange is bound to leak or failure.
What are the methods to determine whether the flange will leak? How to determine?
There are equivalent pressure method, ASME NC 3658 bolt area method and ASME BPVcode VIII volume 1 sub-part of the flange strength calculation method. And we mainly talk about the equivalent pressure method today: the principle of the equivalent pressure method is to take the design pressure of a particular flange + the equivalent pressure due to external load transformation over the flange leakage sum (equivalent total pressure) and the flange design temperature pressure rating, if not exceeded, the seal is safe.
Among them, the equivalent pressure leading to flange leakage is mainly caused by axial pressure and bending moment, both of which can be obtained through pressure piping stress analysis.
Equivalent pressure method to determine no problem flange will not leak?
It is not that simple. Flange leakage is a comprehensive problem, the equivalent pressure method is only from a side to ensure that the flange does not leak method. Flange final leakage, and the size of the external load, the number of bolts and whether the strength is sufficient, the choice of gaskets is appropriate, the flange stiffness is sufficient, the bolt preload is properly sealed related.
I’ve heard a joke: a design institute, everything is about using good, absolutely safe, so, CL150 flange, people use winding gasket; CL150 flange, stiffness is not large, and winding gasket has a certain hardness, in the tightening, in order to achieve sealing effect, a strong increase in torque, and then the flange deformation, leakage. Not metal gaskets are necessarily better than non-metallic gaskets.

Nominal pressure class of flange

Nominal pressure rating is a grouping of flange strength. Flanges of different materials, different nominal pressure rating, its specific temperature permitted maximum non-shock working pressure is not the same, which is the concept of temperature pressure rating.
For the European standard flange, the domestic commonly used is the HG20592 standard, in its 97 standard, the pressure level were N0.25, PN0.6, PN1.0, PN1.6, PN2.5, PN4.0, PN10.0, because the pressure level is dimensionless, but it is indeed related to the MPa. By the 09 standard, the pressure level becomes related to Bar, respectively, PN2.5, PN6, PN10, PN16, PN25, PN40, PN100, a total of 7 files. For the 09 standard, the European standard flange of a specific material, the maximum working pressure allowed at different working temperatures can be calculated by the following formula: p (bar) = (PN value X design strength) / 225. design strength is obtained at the following values:

• a. When the working temperature is lower than the creep temperature of the material, take the yield strength of the material.
• b. Creep temperature is taken as 1.5 times the allowable stress of the material.
• c. The design strength of the material is higher than 225MPa, take 225MPa.

For the American Standard flange, one of the commonly used domestic standards is HG20615/HG20623A/B standard, in its 97 standard, the pressure level were PN2.0, PN5.0, PN11.0, PN15.0, PN26.0, PN42.0. CLass300, CLass600, CLass900, CLass1500, CLass2500. For the 09 standard, the maximum working pressure allowed at different working temperatures for American Standard flanges of specific materials can be calculated by the following formula:

• p (bar) = (Class value X 10 X design allowable stress)/8750.

For the formula For CLass150, substitute 115. Design allowable stress (MPa) is obtained from the following values:

• a. 60% (carbon and low alloy steel) or 70% (stainless steel) of the yield strength of the material at room temperature or set temperature.
• b. 1.25 times 25% of the tensile strength at room temperature or set temperature.
• c. The allowable stress value determined by the long-term strength at high temperature. And p can not exceed the maximum working pressure limit value determined in accordance with the use of experience.

Then, in fact, it is quite clear that:
For European standard flanges, we commonly use PNXX to indicate its nominal pressure level. Since PNXX is a dimensionless unit, we generally do not follow the bar (HG20592-09) or MPa (HG20592-97) after PNXX.
For the American Standard flange HG20592-97 had used PN2.0, PN5.0, PN11.0, PN15.0, PN26.0, PN42.0 to express its pressure level, to the 09 version of the HG20615/HG20623 standard, only Class (abbreviated as CL) XX to express its pressure level. Written as 150LB, etc. is also not a standard way to write. Any flange nominal pressure rating selected, must know the premise of the clear flange material, check the flange standard, review its temperature pressure rating. Of course, the temperature and pressure rating of your own calculation in accordance with the above method can also be.

About European and American standard flange correspondence

In fact, this problem is very common and wonderful. First, the mating dimensions are different. In addition, the outer diameter of the nozzle may be the same or different. More importantly, can the flange of CL150 be used in PN2.5, PN6, PN10 and PN16? Check the corresponding flange standards to see whether the specific temperature and pressure rating (the maximum allowable working pressure of the flange of a specific material at the working temperature) can meet the requirements. However, pairing with flanges of PN2.5, PN6, PN10 and PN16 is not possible.

How to label the flange

At least, you should indicate the following contents: pressure grade, material, sealing surface type, outer diameter and wall thickness of connecting pipe. Of course, for flat welding flange / socket welded flange, it is not necessary to indicate the wall thickness of connecting pipe.

How to select GB flange

According to the four existing standards in China, the common flanges basically include: integral flange, butt welding flange, flat welding flange, threaded flange, socket welding flange and so on. So what should we choose and use flanges based on?

• (1) Nature of conveying medium, including general medium, special medium, toxic medium, flammable and explosive medium;
• (2) According to the parameters, working pressure and working temperature of the medium, when the medium is determined, the nominal pressure PN of the flange is determined according to the working temperature and working pressure of the medium.
• (3) The connection mode and sealing surface form of flange and pipe shall be determined according to the place of use and connection conditions.
• (4) Determine the flange specification according to the connection object.

According to the connection mode with the pipe, the pipe flange can be divided into five basic types: flat welding flange, butt welding flange, threaded flange, socket welding flange and loose sleeve flange. There are many types of sealing surfaces of flanges, generally including convex (RF), concave (FM), concave convex (MFM), tenon and groove surface (TG), full plane (FF) and ring joint surface (RJ) butt welding flanges.
Flat welded steel flange: applicable to carbon steel pipe connection with nominal pressure not exceeding 2.5MPa. The sealing surface of flat welding flange can be made into smooth type, concave convex type and tenon groove type. Smooth flat welding flange has the largest application. It is mostly used when the medium conditions are relatively mild, 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: it is used for butt welding of flange and pipe. It has reasonable structure, large strength and stiffness, can withstand high temperature and high pressure, repeated bending and temperature fluctuation, and has reliable sealing performance. The butt welding flange with nominal pressure of 0.25 ~ 2.5MPa adopts concave convex sealing surface.
Socket welding flange: commonly used in pipes with PN ≤ 10.0MPa and DN ≤ 40.
Loose flange: loose flange is commonly known as looper flange, split welding ring looper flange, flanging looper flange and butt welding looper flange. It is often used when the medium temperature and pressure are not high and the medium is highly corrosive. When the medium is highly corrosive, the part of the flange contacting the medium (flanging nipple) is made of corrosion-resistant high-grade materials such as stainless steel, while the outside is clamped by the flange ring of low-grade materials such as carbon steel to achieve sealing.
Integral flange: it is often integrated with equipment, pipes, pipe fittings and valves. This type is commonly used in equipment and valves.

Source: China Flanges 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.)

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