Pressure pipeline materials

Commonly used pipe fittings include: elbows, tees, reducers (large and small heads), pipe caps, reinforced nozzles, reinforced pipe joints, reducer nipples, threaded nipples, live joints, thread plugs, instrument nozzles, hose station quick connectors, funnels, water nozzles, pipe clamps, etc.

The bends of pipelines were previously made on site. This method has high labor intensity and low efficiency. The organizational state and performance of the materials are not good. Moreover, the bends often become the weak links of the entire pipeline due to the thinning of the pipe wall thickness. The branches of the pipeline are generally connected by opening holes directly on the pipe. Although reinforcement is sometimes carried out here, the welds are generally fillet welds, the stress conditions are not good, and the weld quality is not easy to control. Therefore, this place often becomes the weak link of the pipeline. After the use of pipe fittings, the above problems are better solved.

1. Connection form

The commonly used connection types between pipe fittings and between pipe fittings and pipes are butt welding, socket welding, threaded connection and flange connection.

a. Butt welding

It is a commonly used connection type for pipes and components with DN≥50. For pipes and components with DN≤40, because their wall thickness is generally thin, the misalignment effect is greater when using butt welding connection, it is easy to burn through, and the welding quality is not easy to guarantee, so butt welding connection is generally not used at this time. However, there are several exceptions:

① For pipes and components with DN≤40 and wall thickness greater than or equal to SCH160, the wall thickness is relatively thick, and the above-mentioned problems no longer exist when using butt welding connection, so butt welding connection is also commonly used;

② In the presence of crevice corrosion media (such as hydrofluoric acid media), even if DN≤40 and wall thickness is less than SCH160, butt welding connection is also used to avoid crevice corrosion. At this time, argon arc welding with small wire diameter and small welding current is often used instead of general arc welding during welding construction;

③ For lubricating oil pipelines, when socket welding is used, impurities are easily accumulated in the joint gap and have an adverse effect on mechanical equipment. At this time, butt welding connection should also be used;

b. Socket welding connection

It is mostly used for the connection between pipes and pipe fittings with DN≤40 and thinner pipe walls.

Socket welding connection must have one socket and the other socket pipe fitting.

Generally, reducing nipples and threaded nipples are socket fittings;

Elbows, tees, pipe caps, reinforced nozzles, unions, pipe clamps, etc. are socket fittings. The combination of these pipe fittings and the required structural space should be considered in the application.

c. Threaded connection

Threaded connection is also mostly used for the connection between pipes with DN≤40 and their components. It is often used in occasions where welding is not suitable or detachable is required.

Threaded connectors are divided into male and female threads.

Among the commonly used pipe fittings, threaded nipples are male threads, while elbows, tees, pipe caps, unions, etc. are mostly female threads. When using them, attention should be paid to their matching and combination.

Compared with welding, threaded connection has low joint strength and poor sealing performance. Therefore, its use is often restricted by the following conditions:

① Threaded pipe fittings should use tapered pipe threads;

② Threaded connection is not recommended for temperatures above 200℃ and below -45℃;

③ Threaded connection shall not be used on pipelines with highly toxic media;

④ Threaded connection is not recommended for pipelines where corrosion, crevice corrosion or vibration, pressure pulsation and temperature changes may produce alternating loads;

⑤ When used on combustible gas pipelines, sealing welding should be used for sealing.

Commonly used tapered pipe threads can be divided into

55⁰ tapered pipe thread (mostly used in Europe)

60⁰ tapered pipe thread (mostly used in the United States)

(ISO7/1) is 55⁰ tapered pipe thread. The Japanese standard includes both 55⁰ tapered pipe thread and 60⁰ tapered pipe thread.

GB7306 is equivalent to 55⁰ tapered pipe thread (ISO7/1)

Thread taper: 1:16; tooth angle: 55⁰;

Size range: 1/16"~6";

Thread mark code: R (conical external thread); Rc (conical internal thread)

GB12716 is equivalent to 60⁰ tapered pipe thread (ANSI/ASME

B1.20.1)

Thread taper: 1:16; tooth angle: 60⁰;

Size range: 1/16"~12"; Marking: NPT

The two types of tapered pipe threads are not interchangeable.

2. Butt-weld pipe fittings

Commonly used butt-weld pipe fittings include elbows, tees, reducers (large and small heads) and pipe caps. The first three items are mostly made of seamless steel pipes or welded steel pipes through pushing, drawing and extrusion, and the latter are mostly made of steel plates by stamping.

They achieve the same strength as the pipe through the nominal wall thickness grade (pipe number or wall thickness value). As for the reinforcement of local stress concentration, it is something that the manufacturer should solve. The manufacturer should design the strength of the welded pipe fittings and verify it through the verification test method.

a. Elbow

① Long radius elbow (R=1.5DN): Generally, it should be used first;

② Short radius elbow (R=1.0DN): It is mostly used in occasions with limited size. Its maximum working pressure should not exceed 0.8 times that of the long radius elbow of the same specification.

③ Elbow (R=nDN): used to alleviate the scouring and kinetic energy of the medium at the bend, and can be used for R=3DN, 6DN, 10DN, 20DN

According to different manufacturing methods, it is divided into push elbows, extruded elbows and welded bevel elbows

① Push elbows and extruded elbows: commonly used on small and medium-sized pipelines with harsh medium conditions

pipe-fitting (2).webp

② Welded bevel elbows: commonly used on large-sized pipelines with relatively mild medium conditions, and the bending radius should not be less than 1.5 times its nominal diameter. When the bevel angle of the bevel elbow is greater than 45⁰, it should not be used on pipelines with highly toxic or flammable media, or on pipelines that are subject to mechanical vibration, pressure pulsation and alternating loads due to temperature changes.

pipe-fitting (3).webp

b. Tees and crosses

Same-diameter tees, different-diameter tees, y-type tees, and crosses

Y-type tees are often used instead of general tees and are used to transport pipelines with solid particles or severe scouring and corrosion.

c. Reducers (large and small heads)

Usually there are concentric reducers and eccentric reducers

d. Pipe caps (ends)

There are flat caps and standard elliptical heads

Flat heads are easier to manufacture and cheaper, but their pressure bearing capacity is not as good as standard elliptical heads, so they are often used under conditions where DN≤100 and medium pressure is less than 1.0MPa. The standard elliptical head is an elliptical head with a folded edge. The ratio of the inner diameter of the ellipse to the major and minor axes is 2:1. It is the most widely used end.

In many cases, such as the ends of pipes on the pipe gallery, the pipe caps are replaced by flanges to facilitate the purging and cleaning of the pipes.

3. Socket welding and threaded pipe fittings

It generally refers to pipeline components with DN≤40, including elbows, tees, reinforced nozzles, reinforced pipe joints, pipe caps, pipe clamps, reducers, unions, thread plugs, instrument nozzles, hose station quick connectors, water sprinklers, etc.

4. Common pipe fittings standards

(1) National standards:

①GB/T12459-2017 Steel butt-welded seamless pipe fittings

DN=10-500mm A, B series

②GB/T13401-2017 Steel plate butt-welded seamless pipe fittings

DN=350-1200mm A, B series

③GB/T14383-2021 Steel socket welding and threaded pipe fittings

DN=15-80mm A, B series

④GB/T14626-93 Forged steel threaded pipe fittings

DN=8-100mm

⑤GB/T17185-2012 Steel flange pipe fittings

DN=25-600mm PN 2.0, 5.0, 11.0, 15.0, 26.0MPa