Selecting PUF-insulated bends
A bend, also called an elbow or a knee, is a pipeline connecting piece used to change the line’s direction at the angle required. It allows to turn or reverse, raise or lower the trunk line, avoid obstructions in the path, compensate for pipeline movements, arrange branch lines and implement other engineering concepts.
In terms of the manufacturing method, bends are divided into sharply-curved bends, bent branches and sector bends. Let us discuss in detail.
2D and 3D sharply-curved bends
Sharply-curved bends are used for pipelines of up to 820 mm in diameter, operating under the load of up to 16 MPa and the temperature of -70 to +450 °C.
They are manufactured using three methods:1) Hot drawing for bends DN 50—800 mm;
2) Stamping and welding for bends of over 800 mm in diameter;
3) Welding of sectors, mostly, for diameters above 500.
Sharply-curved bends manufactured using the hot drawing method are the most common in our application.Issues may arise when you work with bends 530 mm in diameter or more, and in this case there are a couple of things you should know.
Differences between sharply-curved bends with various bending radii
Sharply-curved bends, including large diameter bends, may be of the following two types:
1) 3D, i.e. according to GOST 17375 — R = 1,5Dn;
2) 2D, i.e. according to GOST 30753 —R = 1Dn.
Where R is the curve radius of the bend center line or bending radius, and Dn is the pipe size or nominal diameter.These main parameters determine the connecting sizes, i.e. the length of straight sections and overall dimensions of products.
Detailed explanation
The radius of a 90° bend 530х10.0 in diameter according to GOST 17375 is 1.5xdiameter, hence the length of connection pipes is 750 mm. Based on these parameters, the weight of a product is +/- 153 kg.
A similar bend designed according to GOST 30753, with the radius of 1xdiameter, is fitted with connection pipes +/- 500 mm in length and weighs around 102 kg.The length of a 530 mm bend arm specified in GOST 30732-2020 is 1200 mm. Therefore, to insulate a bend according to GOST 17375, 450 mm long connection pipes need to be welded, while for a bend according to GOST 30753 the length of connection pipes will be as high as 700 mm.
Even more details
Bends under GOST 30753-2001 (2D) are shorter than their counterparts according to GOST 17375-2001 (3D) and less expensive. However, their insulation requires much longer connection pipes. Connections are made of pipe that also has a cost, so the initial savings in price are canceled out by the expenses incurred when the parts are subsequently prepared for insulation.
Overall dimensions of bends affect not just the cost of the product itself. As a result, a different approach to preparation and applying the PUF insulation has to be used.
Why are bent branches rarely insulated with PUF?
Some background information first. Let us go back to the parameters and standard specifications of sharply-curved bends, bending radius R between 1 and 1.5DN, turning angle 30°, 45°, 60°, 90°.
Sharply-curved bends can be used to handle 99.9% of tasks that arise in the process of pipeline installation. However, occasionally projects include sections with 2°, 7°, 11°, 14°, 55° turns which cannot be built using either standard sharply-curved bends, or elastic bending of the pipeline. In this case, bent branches come to the rescue.Cutting to the chase
Bent branches are manufactured using two methods, cold and hot bending. In terms of construction, bent branches are essentially a curved pipe. They are produced using seamless or electric-welded pipe workpieces of the appropriate length that are bent in special pipe-bending machines by one of the following methods.Cold working, when pipe workpieces are not heated before deformation, ensures a large bending radius of up to 15 meters or 20DN. However, in this case the bending angle is limited to 27° maximum.
Hot working, when high-frequency currents are used to heat the whole workpiece or the part of workpiece to be deformed. This allows to obtain a bending radius of 1.5DN to 6DN with angles of 2° to 90°.Unfortunately, due to wide variations in design, bent branches, in contrast with sharply-curved bends, are not produced in standard sizes. They are custom-made only. This creates a number of issues, such as limited selection of suppliers, long lead times and restrictions in terms of minimum order batch. This is exactly why they are rarely used for PUF insulation.
What about sector bends?
Sector bends are manufactured by means of cutting the pipe into segments that are subsequently joined by welding. Typically, they are used in low-pressure trunk lines.
We use these bends in exceptional cases and only with the customer’s prior approval. Why? Because they are produced according to TCs, i.e. to manufacturer Technical Specifications, instead of the State Standard.Most often, they are needed for large-diameter pipelines, 630 mm or higher.
Please keep in mind that sector bends are rarely in stock and are custom-made.
Conclusions
When pipelines in PUF insulation are designed, most often sharply-curved bends with 30°, 45°, 60° and 90°angles manufactured according to GOST 17375-2001 or GOST 30753-2001, are provided.
Common standard sizes of bends are similar to pipe sizes, namely, 32, 38, 45, 57, 76, 89, 108, 133, 159, 219, 273, 325, 377, 426, 530, 630, 720, 820, 1020 mm.In terms of wall thickness , it is preferable to use the seamless pipe schedule, excluding the intermediate sizes of 7.0, 9.0, 11.0 and so on:
32 — 3.0 or higher, preferably 3.5 mm, even better 4.0 mm, 5.0 mm, 6.0 mm;38 — 3.0 or higher, preferably 3.5 mm, 4.0 mm, 5.0 mm, 6.0 mm;
45 — 3.0 or higher, preferably 3.5 mm, 4.0 mm, 5.0 mm, 6.0 mm;
57 — 3.5 mm or higher, 4.0 mm, 5.0 mm, 6.0 mm;
76 — 3.5 mm or higher, 4.0 mm, 5.0 mm, 6.0 mm;
89 — 3.5 mm ot higher, 4.0 mm, 5.0 mm, 6.0 mm;
108 — 4.0 mm or higher, 4.5 mm, 5.0 mm, 6.0 mm;
133 — 4.0 mm or higher, preferably 4.5 mm, even better 5.0 mm or 6.0 mm;
159 — 5.0 mm or higher, preferably 6.0 mm, possibly 7.0 mm, 8.0 mm and more;
219 — 8.0 mm or higher, subsequently 10.0 mm;
273 — 8.0 mm or higher, 10.0 mm, 12.0 mm and more;
325 — 8.0 mm or higher, 10.0 mm, 12.0 mm, 14.0 mm and more;
377 — 8.0 mm doubtful, preferably 10.0 mm and more;
426 — exclusively 10.0 mm and more;
530, 630, 720 — 10 mm or higher;
820, 920, 1020 — 12 mm or higher.
In most cases, PUF insulation cannot be applied on a bend without adding connection pipes, because the bend’s straight sections are not long enough. To apply the insulation, we weld steel connection pipes of suitable diameter to the bend in order to increase the arm length. Standard sizes of PUF insulated bends are given in GOST 30732-2020, Appendix E, Table E1.
Table E.1 — Bends
Steel bend outside diameter, d |
Shell outside diameter, D | L, mm, depending on angle α ** | ||||||
---|---|---|---|---|---|---|---|---|
For PE shell | For galvanized shell | 90° | 60° | 45° | 30° | |||
Type 1 | Type 2 | Type 1 | Type 2 | |||||
32 | 125 | - | - | 125 | 1000 | 1000 | 1000 | 1000 |
38 | 125 | - | - | 125 | 1000 | 1000 | 1000 | 1000 |
45 | 125 | - | - | 125 | 1000 | 1000 | 1000 | 1000 |
57 | 125 | 140 | - | 140 | 1000 | 1000 | 1000 | 1000 |
76 | 140 | 160 | - | 160 | 1000 | 1000 | 1000 | 1000 |
89 | 160 | 180 | - | 180 | 1000 | 1000 | 1000 | 1000 |
108 | 180 | 200 | - | 200 | 1000 | 1000 | 1000 | 1000 |
114 | 180 | 200 | 200 | 225 | 1000 | 1000 | 1000 | 1000 |
133 | 225 | 250 | 225 | 250 | 1000 | 1000 | 1000 | 1000 |
159 | 250 | 280 | 250 | 315 | 1000 | 1000 | 1000 | 1000 |
219 | 315 | 355 | 315 | 355 | 1000 | 1000 | 1000 | 1000 |
273 | 400 | 450 | 400 | 450 | 1000 | 1000 | 1000 | 1000 |
325 | 450 | 500 | 450 | 500 | 1050 | 860 | 786 | 720 |
377 | 500 | 560 | 500 | - | 1100 | 883 | 786 | 720 |
426 | 560 | 630 |
560 |
600 | 1100 | 889 | 807 | 734 |
530 | 710 | 800 | 675; 710 | 775 | 1200 | 946 | 848 | 761 |
630*** | 800 | 900 | 775; 800 | 875 | 1300 | 945 | 848 | 761 |
630* | 800 | 900 | 775; 800 | 875 | 1280* | 1014* | 911* | 819* |
720*** | 900 | 1000 | 875; 900 | 1075 | 1400 | 1066 | 948 | 819 |
720* | 900 | 1000 | 875; 900 | 1075 | 1370* | 1066* | 948* | 843* |
820*** | 1000 | 1100 | 975; 1000 | 1175 | 1600 | 1073 | 990 | 820 |
820* | 1000 | 1100 | 975; 1000 | 1175 | 1470* | 1073* | 990* | 820 |
920*** | 1100 | 1200 | 1075; 1100 | - | 1772 | 1132 | 1032 | 846 |
920* | 1100 | 1200 | 1075 | - | 1570* | 1132* | 1032* | 846* |
1020*** | 1200 | - | 1175; 1200 | - | 1924 | 1189 | 1022 | 874 |
1020* | 1200 | - | 1175; 1200 | - | 1620* | 1189* | 1022* | 874* |
1220* | 1425 | - | 1375; 1425 | - | 1820* | 1304* | 1105* | 927* |
1420* | 1600 | - | 1575; 1600 | - | 2020* | 1420* | 1188* | 980* |
* Сварные отводы. ** Допускается изготовление отводов с другими углами. *** Допускается изготовление крутоизогнутых отводов по ГОСТ 17375. Отводы допускается изготовлять с другими значениями L по технологической документации предприятий изготовителей. |
Often, wall thickness of a bend exceeds the thickness of connection pipes welded to the bend by 1 or 2 mm. A change of route direction is the point of increased concentration of stresses, hence a thicker bend should be selected.
Из этого вытекают 2 правила
Rule 1 — the difference between connecting pipes and bends to be joined must not exceed 2 mm. Otherwise, connection pipes may only be welded after additional edge preparation and back-welding of special rings.
Rule 2 — if the bend and welded connection pipes are different in thickness, approval is required.
Final considerations
Conventions used in the text: BEND (turning angle), (GOST on welded connection pipes) BEND DIAMETER x WALL THICKNESS OF CONNECTION PIPES (WALL THICKNESS OF BEND) — INSULATION TYPE — INSULATION DESIGN.
Now, you should be able to select PUF-insulated bends without any issues.