Hydraulic bolt tensioners: picking the right Atlas Copco range for your flange
Written by the Middle East ABC technical team · authorised supply of Atlas Copco industrial toolsA torque wrench tells you how hard you pulled. A bolt tensioner tells you how much load the stud actually carries. On a flange that has to hold pressure for years, that difference is the whole job. Atlas Copco builds four families of tensioning tools, and on paper they look similar. In the field they behave very differently, so here is how we sort them for customers.
Why tension a bolt instead of torquing it
When you torque a nut, most of the effort goes into friction under the nut face and in the threads. Only a fraction ends up as stretch in the stud, and that fraction moves around with lubricant, surface finish and thread condition. Two people tightening the same flange with the same wrench can leave studs with noticeably different loads.
A hydraulic tensioner works the other way round. It grips the stud above the nut, pulls it straight with hydraulic pressure, and lets you run the nut down by hand against a load you can read on a gauge. Because the tool pulls the stud in line, there is no twisting load on it. And because several tensioners can be fitted to a flange and fed from one pump, every bolt in the pattern can be brought up together instead of one at a time.
The four ranges side by side
The figures below come straight from the Atlas Copco catalogue entries for each family. Load capacity is the rated load of the tool at its working pressure, not the load you will normally use on a given bolt.
| Range | Where it is used | Stroke | Working pressure | Load classes |
|---|---|---|---|---|
| CTST topside tensioner | Offshore and onshore flanges: ANSI B16.5, B16.47 Series 1, MSS-SP44, API-6A, API-17D | 10 mm (No.1), 15 mm (No.2-6) | 1,500 bar | 22.9 / 44.5 / 81.4 / 127.8 / 183.6 / 265.4 t |
| Airbac auto-retract | ANSI, ASME and API flanges where cycle time matters | 10 mm | 1,500 bar | AB-01 to AB-06, same six load classes as CTST |
| COMPACT-8 subsea | Standard subsea flanges, some Vector SPO compact flanges | 30 mm | 1,500 bar | C8-02: 51.7 t · C8-07: 407.4 t |
| Hydraulic nut | Joints where the nut itself becomes the tensioner | 5-10 mm | 2,275 bar | M48x3: 56.0 t · M72x6: 125.7 t |
CTST: the workhorse for topside flanges
CTST insert-type tensionerCTST is an insert-type tensioner. Each base tool covers one load class, and a conversion kit adapts it to the stud thread on your flange, so one body can serve both metric and imperial bolting. Our catalogue lists 117 CTST items, and most of them are exactly that: conversion kits, bridges and reaction nut sets for the six base sizes.
It is the sensible default when you have mixed flange sizes, a maintenance crew that wants one tool family, and no special pressure on cycle time. The tool is simple, serviceable in the workshop, and well known in the field.
Airbac: when time on the flange is the cost
Airbac auto-retract tensionerAirbac covers the same six load classes as CTST (AB-01 to AB-06, from 22.9 to 265.4 t), so the choice is not about capacity. The difference is that the piston retracts on air instead of being pushed back by hand. Between pulls you do not touch the tool, which is where crews lose minutes on every bolt of a large flange.
Atlas Copco positions Airbac as the lowest total cost of ownership option, and the reasoning holds up: faster cycles and less handling mean fewer hours on the job and less wear on the tool. The trade-off is that you need a compressed air supply and the tool is a little heavier in the same class. A size 2 Airbac weighs about 5.6 kg against roughly 5.1-5.3 kg for a CTST No.2.
COMPACT-8: eight tools for subsea work
COMPACT-8 subsea tensionerThe name is literal: the range is eight tools, and together they cover most standard subsea flanges. The tools are made from high-strength stainless steel, which cuts down the washing, stripping and corrosion work after a subsea job. The reaction nut is a split design and the bridge carries an integrated load cell, so a diver can fit the tool and read the load without extra parts.
Stroke is 30 mm, three times that of Airbac, which helps on long studs. Sizes run from C8-02 (M36x4, 51.7 t) up to C8-07 (3.1/2"-8UN, 407.4 t).
Hydraulic nuts and wind turbine tensioners
Hydraulic nut (bottom collar)A hydraulic nut replaces the ordinary hex nut and stays on the joint. You pressurise the nut, it stretches the stud, and you lock it with the collar. With a working pressure of 2,275 bar and a low profile, it suits joints where there is no room above the nut for a pulling tool, or where you want the tensioning mechanism to remain part of the assembly. Because it is built into the joint it also resists loosening from vibration and dynamic loads.
Wind turbine work is its own corner. The WTB and WTF families are built for tower and blade bolting; for example the M64x6 WTB tensioner is rated at 222.9 t at 1,350 bar. We keep the catalogue for these separate from the oil-and-gas ranges for that reason.
How we pick: six questions
- Which flange standard? ANSI, API and MSS-SP44 flanges point to CTST or Airbac. Subsea and compact flanges point to COMPACT-8.
- Which stud thread and how much sticks out? Thread size decides the conversion kit, and the protrusion above the nut must clear the tool.
- What load class? Match the rated load to the bolt load with margin. Both CTST and Airbac come in six classes up to 265.4 t.
- How many bolts, how often? A few jobs a year suits CTST. Dozens of flanges per shutdown is where Airbac pays back.
- Is there air on site? Airbac needs it; CTST does not.
- Is the joint a permanent installation? If there is no room for a tool above the nut, look at hydraulic nuts.
Whichever range you choose, remember that a tensioner is a system. It needs a suitable pump, hoses and couplings, and load cells or bridges if you want to log the result. Our team can put the full set together with you, down to the thread of every stud.
Not sure which range fits your flange?
Send us the flange standard, stud size and bolt count. We will come back with a tool and accessory list.
Figures: Atlas Copco product catalogue entries for CTST, Airbac, COMPACT-8, hydraulic nuts and WTB tensioners (Atlas Copco UAE product pages). Ratings are catalogue values and should be confirmed for your joint before ordering.