Pressure ratings for 4130 pipe and fittings
A "15K" line can mean several things: a working pressure, a test pressure or a union figure. Here is how each rating is set, and how 4130 pipe and fittings are matched to it.
A high-pressure line has a working pressure, the most it sees in service, and a test pressure, the hydrostatic pressure each part is proven at. API 6A tests 10,000, 15,000 and 20,000 psi equipment at 1.5 times its rating: 15,000, 22,500 and 30,000 psi. A 4130 butt-weld fitting takes the rating of straight pipe of the same wall and strength, as ASME B16.9 sets out, while a union-end fitting takes the cold working pressure of its hammer union figure.
Three pressures, three meanings
| Term | Meaning | Where it is used |
|---|---|---|
| Rated working pressure (RWP) | Maximum pressure in service | API 6A and 16C equipment: 5K, 10K, 15K, 20K |
| Cold working pressure (CWP) | Maximum pressure at ambient temperature | Flowline iron and hammer unions: FIG 1502 is 15,000 psi CWP |
| Hydrostatic test pressure | Pressure each part is proven at before service | Factory and recertification tests |
API 6A ratings and their test pressures
The 22,500 psi that appears on 4130 catalogs and data sheets is usually the test pressure of a 15,000 psi part, not a separate rating. Read the two together. The flange families and ring gaskets for each API 6A rating are on the API 6A flanges page.
Butt-weld fittings are rated like the pipe
ASME B16.9 does not give butt-weld fittings pressure classes. It rates them as straight seamless pipe of equivalent material and wall, calculated to the piping code the line is designed to. For a 4130 line, that means the rating comes from three things: the wall, the outside diameter and the strength level the heat treatment delivers. A 3 in XXS fitting at 75 ksi minimum yield and the same fitting at 85 ksi carry different ratings in the same calculation. That is why we ask for the working pressure and the strength level together, and why our 15,000 psi fittings and long sweeps are quenched and tempered to 85 ksi minimum yield and 110 ksi minimum tensile.
We make parts to the wall, strength and tests the line's design needs. The pressure rating of the line comes from that design, to the code your project uses, for example API 16C for choke and kill equipment or ASME B31.3 for process piping.
Union-end fittings take the figure's rating
| Figure | Standard service | Sour gas service |
|---|---|---|
| FIG 602 | 6,000 psi | - |
| FIG 1002 | 10,000 psi | 7,500 psi |
| FIG 1502 | 15,000 psi | 10,000 psi |
| FIG 2002 | 20,000 psi | - |
| FIG 2202 | - | 15,000 psi |
Ratings as published by union makers, for example TechnipFMC for its Weco unions.
More on figures, and why they must never be mixed, is in hammer union figure numbers explained.
What lowers a rating
- Sour service. Hardness capped at 22 HRC lowers the strength available, and union figures carry lower sour ratings. See 4130 for sour service.
- Temperature. Ratings hold over a temperature class; API 6A classes run from K (-75 to 180 °F) to Y (0 to 650 °F).
- Wall loss. Erosion and corrosion thin the wall in service, which is why flowline iron is inspected and recertified, and why designs carry an erosion allowance.
- Tolerance. Pipe and fittings may be under nominal wall within their standard's tolerance; ASME B16.9 allows down to 87.5% of nominal.
Frequently asked questions
What is the difference between CWP and working pressure?
Cold working pressure (CWP) is the rating of flowline iron and hammer union connections at ambient temperature, the number behind a figure such as FIG 1502 at 15,000 psi. Rated working pressure (RWP) is the term API 6A uses for wellhead and pressure control equipment. Both are the maximum pressure in service; neither is the test pressure.
Can a 4130 butt-weld fitting be rated at 15,000 psi?
Yes, when its wall and strength level support it in the line's design calculation. ASME B16.9 rates a butt-weld fitting as straight seamless pipe of the same material and wall, so a heavy-wall 4130 fitting quenched and tempered to 85 ksi minimum yield is rated with the 15,000 psi line it is made for.
Why is a hydrostatic test pressure higher than the rating?
The shell test proves the part holds well above the pressure it will see in service, with margin for tolerances and wear. API 6A tests 10,000, 15,000 and 20,000 psi equipment at 1.5 times the rated working pressure.
Sources
- API Specification 6A, hydrostatic shell test pressures and temperature classes (as listed in Stream-Flo's API 6A purchasing guide, May 2020)
- ASME B16.9, factory-made wrought butt-welding fittings, design rating
- TechnipFMC, flowline products and services catalog