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on topicReplied byon topic Re: union and coupling
1, "What is the basic difference between coupling and unions?"
Both of these fittings are normally only found or used in 2" (DN50) and smaller screwed or socket-weld piping. The basic difference between a coupling and a union is that a coupling is for joining pipe to pipe or pipe to a swedge. A union is made for joining and disassemble of a part of a piping system (a steam trap or control valve, etc) for maintenance or replacement.
2, "What are the advantages of ones over others?"
The union is a higher cost item than the coupling. When you are just joining pipe you would want to use the lowest cost item. When you need the ability to disassemble the piping for removal of an item then you must use the higher cost union.
3, "Where we use coupling and where we use unions ?"
Use couplings to join pipe in pipe racks. Use unions at steam trap or control valve manifolds for removal of items for repair.
4, "Is there any guidelines or code that specifies where should we use what?"
The use of couplings or unions is not a code issue.
Do it once and Do it Right
Coupling
Union
Nipple
Coupling is a short tube, also called an outer joint; it is a commonly used accessory for industrial pipe connections. The forged coupling is formed by heating and forging with a steel ingot or a round bar, and then machined on the lathe to make the thread. The main manufacturing standards are ASME B16.11, BS 3799, and GB/T14383.
According to the connection type, coupling can be divided into socket coupling and threaded coupling generally. Specifically, it can be divided into single socket coupling, double socket coupling, single thread coupling and double threaded coupling. The socket coupling adopts the socket structure to reduce the cost, and is extremely convenient to install, use and maintain, meanwhile it embodies excellent anti–leakage performance. Thanks to its user-friendliness, it is widely used in civil construction, industry, agriculture and other fields.
Union is a common pipe connection that can be easily installed and removed. Compared with coupling, it is easy to disassemble. As the name suggests, the union is active at both ends with a seal ring in the middle. It mainly consists of a nut, a cloud head, and a flat joint. It is a pipe joint machined by round bar or steel ingots.
There are two connection forms for unions, i.e. socket welding and threaded connection. The socket welding is to insert the steel pipe into the socket to get welded. Therefore, it is called “plug-in union”. The threaded connection is to screw the steel pipe into the screw hole for connection. Besides, there are also external thread connections, called "threaded joint" and its main manufacturing standard is MSS SP-83.
The union is widely used to connect pipes of different calibers, and is also widely used for connection of instruments, valves and pipes. It is mainly used in those sectors and fields that require high pressure and precision, such as petrochemical, medical and health, electric power, aerospace, military, fire, metallurgy, shipbuilding, gas, nuclear power, and environmental protection.
Nipple can be divided into single head and double head; it can also be divided into ordinary and hex nipples, both generally of equal diameter. It adopts either external thread or plug-in welding to reach the male/female head of the coupling/union, matching the seal.
Nipples can also be divided into short tube, reducing nipple and threaded short nipple. Threaded short nipple is either single head threaded or double head threaded, commonly used in industrial pipe connections. The main thread standards are the American standard ASME B16.11 and the national standard GB/T14626. Among all, NPT (National-American Pipe Thread) RC threads (Conical Threaded, Female) enjoy great popularity in the market.
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