Couplings are short lengths of pipe used to connect 2 joints of tubing or casing.,They come with painted colour codes on their external surface to indicate their grades.Couplings are manufactured according to API 5CT and 5B specifications.

1. API Tubing Coupling:
Specification  
Size (Pipe OD) (in) :1.900, 2-3/8, 2-7/8, 3-1/2, 4, 4-1/2
Upset:Non Upset;External Upset
Grade:J-55, C-75, L-80, N-80, C-95, P-110
Option:Electro-zinc plated;Phosphorised

2. API Casing Coupling:
Specification  
Size (Pipe OD) (in):4-1/2, 5, 5-1/2, 6-5/8, 7, 7-5/8, 8-5/8, 9-5/8, 10-3/4, 11-3/4, 13-3/8, 16, 18-5/8, 20
Threading:Short Round-Thread Casing (STC);Long Round-Thread Casing (LC);Buttress Thread Casing (BC)
Grade:H-40, J-55, K-55, C-75, L-80, N-80, C-95, AS-95, P-110, L80-13CR

API 5B, Specification for Threading, Gauging and Thread Inspection of Casing, Tubing, and Line Pipe Threads.This Specification covers dimensions, tolerances and marking requirements for API threads and the gauges that control the acceptance criteria for the threads. Thread element gauges, instruments and requirements for the inspection of threads for line pipe, round thread casing, and buttress casing connections are included.  

API Spec 5CT, Specification for Casing and Tubing, Eighth Edition, Petroleum and natural gas industries-Steel pipes for use as casing or tubing for wells.This Standard specifies the technical delivery conditions for steel pipes (casing, tubing and pup joints), coupling stock, coupling material and accessory material and establishes requirements for three Product Specification Levels (PSL-1, PSL-2, PSL-3).

Application: casing coupling is a kind of drilling tool in oil field, which is mainly used for casing connection. The casing coupling mainly solves the problem that the existing coupling is prone to fatigue fracture due to stress concentration.

Structure:The casing end is connected with the inner wall of the coupling by tapered thread, and the end of the coupling body is connected with the oil pipe by the same thread and the same pitch flat thread. The utility model has the characteristics of relieving the stress concentration at the root of the external thread of the single taper thread connecting sleeve, not easy to produce fatigue fracture, and the connection effect is good.

The thread type of casing couplngs main include: Long Round-thread Casing Coupling(LC), Short Round-thread Casing coupling(SC), Buttress Thread Casing Coupling(BC), and steel grade mainly include: J55, K55, N80, L80, P110 and so on.

Casing Couplings Short Thread Specifications:

Casing Couplings Long Thread Specifications:

Casing Coupling - Buttress Thread

Tubing Couplings - External Upset End (EUE):

Tubing Couplings - Non Upset End (NUE)

Coupling Pipe Grade Identification By Paint Color