Let's Take A Look At The Tricks For Connecting Valves And Pipes

Whether the connection method between Lined Valve and pipelines or equipment is correct or appropriate will directly affect the probability of pipeline valves running, bubbling, dripping, and leaking. Common valve connection methods include: flange connection, butt clamp connection, butt welding connection, threaded connection, ferrule connection, clamp connection, self-sealing connection and other connection forms.

1. Flange connection

Flange connection is a valve body with flanges at both ends, corresponding to the flanges on the pipeline, and the flanges are fixed in the pipeline by bolts. Flange connection is the most commonly used connection form in valves. Flanges are divided into convex surface (RF), flat surface (FF), convex and concave surface (MF), etc. According to the shape of the mating surface, it can be divided into the following types:

(1) Smooth type: used for valves with low pressure. It is relatively easy to process;

(2) Concave-convex type: the working pressure is relatively high, and medium-hard gaskets can be used;

(3) Tongue-and-groove type: gaskets with large plastic deformation can be used, which are widely used in corrosive media and have better sealing effect;

(4) Trapezoidal groove type: oval metal rings are used as gaskets, which are used for valves with working pressure ≥64 kg/cm2, or high-temperature valves;

(5) Lens type: the gasket is lens-shaped and made of metal. It is used for high-pressure valves with working pressure ≥100 kg/cm2, or high-temperature valves;

(6) O-ring type: This is a newer flange connection form. It has developed with the emergence of various rubber O-rings. It is more reliable than ordinary flat gaskets in terms of sealing effect.

2. Clamp connection

The valve is installed between two flanges. There are usually positioning holes on the valve body to facilitate installation and positioning. The valve and the two pipes are directly clamped together with bolts.

3. Welding connection

(1) Butt welding connection: The two ends of the valve body are processed into butt welding grooves according to the requirements of butt welding, corresponding to the welding grooves of the pipeline, and fixed to the pipeline by welding.

(2) Socket welding connection: The two ends of the valve body are processed according to the requirements of socket welding, and connected to the pipeline by socket welding.

4. Threaded connection

Threaded connection is a simple connection method, commonly used for small valves. The valve body is processed according to various thread standards, with internal and external threads. Corresponding to the threads on the pipeline. Threaded connection is divided into two situations:

(1) Direct sealing: The internal and external threads directly play a sealing role. In order to ensure that the connection is leak-proof, lead oil, hemp and polytetrafluoroethylene raw tape are often used for filling; among them, polytetrafluoroethylene raw tape is increasingly widely used; this material has good corrosion resistance and sealing effect, is easy to use and store, and can be removed intact when disassembling because it is a non-sticky film, which is much better than lead oil and hemp.

(2) Indirect sealing: The force of tightening the thread is transmitted to the gasket between the two planes, allowing the gasket to perform a sealing function.

There are five common types of threads:

(1) Metric common thread;

(2) Imperial common thread;

(3) Threaded sealing pipe thread;

(4) Non-threaded sealing pipe thread;

(5) American standard pipe thread.

The summary is as follows:

① International standards ISO228/1 and DIN259 are parallel internal and external threads, code G or PF (BSP.F);

② German standards ISO7/1, DIN2999, and BS21 are external tapered and internal parallel threads, code BSP.P or RP/PS;

③ British standards ISO7/1 and BS21 are internal and external tapered threads, code PT or BSP.Tr or Rc;

④ American standard ANSI B21, internal and external tapered threads, code NPT G (PF), RP (PS), Rc (PT) tooth angles are all 55°, NPT tooth angle is 60° BSP.F, BSP.P and BSP.Tr are collectively referred to as BSP teeth.

There are five types of American standard pipe threads: general-purpose tapered pipe thread NPT, straight pipe internal thread NPSC for pipe joints, tapered pipe thread NPTR for guide rod connections, straight pipe thread NPSM for mechanical connections (free-fitting mechanical connection) and NPSL (loose-fitting mechanical connection with locking nut). It belongs to non-threaded sealing pipe threads (N: American National Standard; P: pipe; T: cone).

5. Ferrule connection

Ferrule connection has only developed in my country in recent years. Its connection and sealing principle is that when the nut is tightened, the ferrule is subjected to pressure, so that its blade bites into the outer wall of the pipe, and the outer cone surface of the ferrule is tightly fitted with the cone surface of the joint body under pressure, so it can reliably prevent leakage. Such as instrument valves. The advantages of this connection form are:

(1) Small size, light weight, simple structure, easy to disassemble and assemble;

(2) Strong connection force, wide range of use, and can withstand high pressure (1000 kg/cm2), high temperature (650℃) and impact vibration;

(3) A variety of materials can be selected, suitable for corrosion protection;

(4) Low processing accuracy requirements;

(5) Convenient for high-altitude installation.

At present, the ferrule connection form has been adopted in some small-diameter valve products in my country.

6. Clamp connection

This is a quick connection method that only requires two bolts and is suitable for low-pressure valves that are frequently disassembled. Such as sanitary valves.

7. Internal self-tightening connection

The above various connection forms all use external force to offset the medium pressure and achieve sealing. The following introduces the connection form that uses medium pressure for self-tightening. Its sealing ring is installed on the inner cone, which forms a certain angle with the side facing the medium. The medium pressure is transmitted to the inner cone and then to the sealing ring. On the cone surface at a certain angle, two components are generated, one is parallel to the center line of the valve body and the other is pressed against the inner wall of the valve body. The latter component is the self-tightening force. The greater the medium pressure, the greater the self-tightening force. Therefore, this connection form is suitable for high-pressure valves. It saves a lot of materials and manpower compared to flange connection, but it also requires a certain pre-tightening force so that it can be used reliably when the pressure in the valve is not high. Valves made using the self-tightening sealing principle are generally high-pressure valves. There are many forms of valve connection. For example, some small valves that do not need to be removed are welded to the pipe; some non-metallic valves use socket connections, etc. Valve users should treat them according to specific circumstances.

Note:

(1) All connection methods must refer to the corresponding standards. It is necessary to clearly understand the standards required by the user to avoid the selected valve being unable to be installed.

(2) Usually, large-diameter pipes and valves are connected by flange connection, while small-diameter pipes and valves are connected by threaded connection.