Long sweep bends vs targeted tees in choke and kill lines
Every change of direction in a choke or kill line is a place where sand and cuttings hit the wall. There are two ways to handle it: let a fluid cushion take the blow, or turn gently enough that the blow is small.
API Standard 53 asks for choke and kill lines to run as straight as possible. Where they must turn, 90° block ells and tees need a target or fluid cushion on the flow side, and so do bends whose radius is less than 10 times the pipe diameter. Bends at R/d of 10 or more are large-radius and may not need targets. A long sweep elbow below R/d 10 wears less than a standard elbow, but in these lines it still counts as a short-radius bend.
Why direction changes wear out
When a well is circulated out through the choke line, the fluid can carry sand, cuttings and gas at high velocity. On a straight run, the solids travel with the flow. At a turn, they keep going and strike the outer wall. The sharper the turn and the faster the flow, the harder and more concentrated the impact. Line size is chosen partly to keep velocity, and with it erosion, within limits; turns are where the remaining wear concentrates.
What API Standard 53 asks
| Element | Requirement |
|---|---|
| Line routing | Choke and kill lines as straight as possible |
| 90° block ells and tees | Targets or fluid cushions on the flow side |
| Short-radius bends, R/d less than 10 | Targets or cushions facing the expected flow; in both directions where flow can reverse |
| Large-radius bends, R/d 10 or more | May not need targets or cushions |
| Flexible lines | Installed within the maker's minimum bend radius |
| Supports | Restrain the line at changes of direction against flow forces |
From the conformance checklist for the fourth edition of API Standard 53. Check the edition your project is built to.
Targeted tees and fluid cushions
A targeted tee turns the flow 90° into a tee whose opposite run end is blanked. Fluid trapped in the blind end forms a cushion, and the incoming stream strikes that cushion, or a hardened, replaceable target, instead of the line wall. The wear is concentrated in one known place that can be inspected and renewed. Heavy-wall 4130 tees suit this use because the extra wall is erosion allowance, and in flanged manifolds the same job is done by studded target blocks.
Long sweeps and large-radius bends
A bend with a larger radius spreads the turn over a longer path, so solids strike the wall at a shallower angle and wear it more slowly. API Standard 53 sets the line at R/d of 10: a bend that gentle may run without a target. Most long sweep elbows are well below that. Our standard 90° long sweeps run about 3 to 5 times the bore, for example 8 in center-to-end on a 2 in XXS bore of 1.5 in. They are a clear improvement on a 1.5D long radius elbow, and in an API Standard 53 choke or kill line they still need a target or cushion. A larger radius can be made to drawing.
| Option | R/d | Target or cushion under API Std 53 | Space | Where the wear goes |
|---|---|---|---|---|
| Block ell or tee | Abrupt | Yes | Smallest | Into the cushion or target |
| Long radius elbow | About 1.5 | Yes | Small | Outer wall of the bend |
| Long sweep elbow, our standard | About 3 to 5 on the bore | Yes | Moderate | Outer wall, spread over a longer arc |
| Large-radius bend | 10 or more | May not be needed | Largest | Outer wall, lightly |
Whichever way a line turns, the outer wall of the turn is where to put extra metal. Our long sweeps carry a heavier body than their weld ends for that reason, and 3D double-back elbows a heavier back; see 4130 long sweep fittings.
Choosing
- Where space allows and the design wants no targets, use large-radius bends at R/d 10 or more.
- Where space is tight, use a targeted tee or block ell with a cushion, and plan to inspect it.
- Where a turn must be compact but gentler than a standard elbow, use a long sweep, with the target or cushion the standard asks for.
- In every case, support the line at the turn against the reaction force.
Frequently asked questions
What is a targeted tee?
A tee used to turn the flow 90°, with the run end opposite the inlet blanked. Flow strikes a pocket of fluid, or a replaceable target, in the blind end instead of the pipe wall, so the wear lands somewhere that can be inspected and renewed.
What does R/d mean?
The bend radius divided by the pipe diameter. A long radius elbow is about 1.5, our standard long sweeps about 3 to 5 on the bore, and API Standard 53 treats a bend as large-radius at 10 or more.
Which is better, a long sweep or a targeted tee?
Neither in every case. A large-radius bend avoids a target but takes space; a targeted tee is compact and puts the wear in one place you can inspect. Long sweeps below R/d 10 sit between the two and still need a target or cushion in an API Standard 53 choke or kill line.
Sources
- IADC, API Standard 53 fourth edition conformance assessment tool (choke and kill line clauses 6.2.3 and 6.2.4)
- API Standard 53, blowout prevention equipment systems for drilling wells