API Coupling is a specialized connector that meets the standards of the American Petroleum Institute (API), used to join oil or casing tubing in oil wells, ensuring the structural integrity and sealing of downhole tubular strings. It connects to the pipe body via precision threads and is widely used in oil and natural gas drilling and production operations.
Based on application, API couplings are mainly divided into two types:
Tubing Coupling: Used to connect tubing, with common thread types being Round Thread (NU/EU), compatible with steel grades such as J55, N80, and L80.
Casing Coupling: Used to connect casing, with primary thread types including STC (Short Round Thread), LTC (Long Round Thread), and BTC (Buttress Thread), manufactured in accordance with API Spec 5B, ensuring reliable performance under high pressure, high temperature, and complex downhole conditions.
Coupling material is typically carbon steel, heat-treated to enhance strength and corrosion resistance, and must undergo quality inspection processes such as hydrostatic testing and non-destructive testing.
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API Casing Coupling Dimensions and Tolerances Table
| Size Designation | Outside Diameter D (mm) | Coupling OD W (mm) | Minimum Length NL (mm) | Counterbore Diameter Q (mm) | Bearing Face Width b (mm) | Mass (kg) | OD Tolerance | Counterbore Tolerance |
|---|---|---|---|---|---|---|---|---|
| 4-1/2 | 114.30 | 127.00 | 158.75 (Short) | 116.68 | 3.97 | 4.15 | ±1% or ±3.18mm (whichever is smaller) | +0.79 / 0 mm |
| 177.80 (Long) | 5.75 | |||||||
| 5 | 127.00 | 141.30 | 165.10 | 129.38 | 4.76 | 5.23 | ||
| 6-5/8 | 139.70 | 153.67 | 171.45 | 142.08 | 3.18 | 6.42 | ||
| 7 | 168.28 | 187.71 | 184.15 | 170.66 | 6.35 | 9.12 | ||
| 7-5/8 | 177.80 | 194.46 | 184.15 | 180.18 | 4.76 | 8.39 | ||
| 9-5/8 | 244.48 | 269.88 | 196.85 (Short) | 248.44 | 6.35 | 25.66 | ||
| 266.70 (Long) | 57.04 | |||||||
| 13-3/8 | 339.72 | 365.12 | 203.20 | 343.69 | 5.56 | 34.91 |
API Tubing Coupling Dimensions and Tolerances Table
| Size Designation | Tube OD D (mm) | Coupling OD W (mm) | Minimum Length NL (mm) | Counterbore Diameter Q (mm) | Bearing Face Width b (mm) | Mass (kg) | OD Tolerance | Counterbore Tolerance |
|---|---|---|---|---|---|---|---|---|
| 2-3/8 | 60.32 | 73.02 | 104.78 | 62.50 | 3.18 | 1.81 | ±1% or ±3.18mm (whichever is smaller) | +0.79 / 0 mm |
| 2-7/8 | 73.02 | 88.90 | 114.30 | 75.59 | 3.97 | 2.84 | ||
| 3-1/2 | 88.90 | 104.78 | 127.00 | 91.44 | 4.76 | 4.36 | ||
| 4 | 101.60 | 117.48 | 139.70 | 104.14 | 5.56 | 6.12 | ||
| 4-1/2 | 114.30 | 130.18 | 152.40 | 116.84 | 6.35 | 8.03 |
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FAQ
What is a 3/4 coupling?
A "3/4 coupling" typically refers to a pipe coupling with a nominal size of 3/4 inches. Widely utilized in plumbing, gas, and industrial piping systems, it serves to connect two sections of pipe with matching outer diameters, thereby facilitating both a leak-proof seal and a structural connection.
How long is a 4 in coupling?
The length of a 4-inch coupling varies depending on its specific type and applicable standards; such couplings are commonly utilized in the connection of oilfield casing or industrial piping. The following serves as a reference for the lengths associated with the primary classifications:
API Casing Couplings:
For 4-1/2-inch (approximately 4-inch) API round-thread casing couplings, the minimum length for a standard "Short" coupling is 158.75 mm, while that for a "Long" coupling is 177.80 mm.
In the case of Buttress Thread Casing (BTC) couplings, the standard length is approximately 196.85 mm.
Industrial Pipe Couplings:
In civil or industrial piping systems, the standard length for 4-inch NPT or BSPT threaded couplings is typically 88.9 mm (3.5 inches), in accordance with the ASME B16.11 standard.
