A bolt tensioner is an annular jack that fits over the fastener to be tightened. Followed by the definition and components mentioned in the previous article on components involved in bolt tensioners, we will provide you an insight into the design of the tool that will also explain how do bolt tensioners operate. We will also be talking about applied and residual load. We highly recommend you to read information about the components from our previous article to understand the operation better.
How do Hydraulic Bolt Tensioners operate?
As mentioned earlier, the tensioner fits over the bolt and nut that requires tightening. The tensioner pushes against the bolted joint and pulls on the end of the bolt, to ensure proper engagement of the puller, a minimum of one bolt dia length of thread above the nut should be maintained. As the force produced by the bolt tensioner is directly applied to the end of the bolt, load equivalent tension generated by the jack is to shank of the bolt. With the jack in the tensioner applying tension, it makes turning the nut with zero torque easy.
Flexible hoses with self-sealing quick connect couplings are connected to multiple bolt tensioners to form a hydraulic ring main. The bolt tensioners and ring man are pressurized by three ways namely air driven, electric driven or hand pump.
These off-shelf bolt tensioners are designed to work on standard flanges like ANSI, API, etc.
To offer convenience, they are available in sets. This allows the operator to tension nearby sized bolts by using the same load cell by only change of puller.
Bolt tensioners work with a max working pressure of 1500 bar to deliver the desired load required to seal a standard flange with RTJ or spiral wound gaskets.
The load calculation of the tensioners is calculated using the simple formula:
L = P x A
L: the Load required on the bolt,
P: the pump pressure
A: the hydraulic pressure area of the tool.
Design of a Bolt Tensioner
All bolt tensioners are designed to tension ANSI type flanges. In some cases, bolt tensioners are customized and designed to cater non-standard flanges; these customized bolt tensioners can fit into restricted spaces or work at a higher pressure to offer more load, since they have a smaller dia in comparison.
To offer the same or more load where radical space is limited, bolt tensioners are built with puller threads that are integral with the load cell to reduce the dia of the tensioners. The only disadvantage of these tensioners is that it is limited to work on a particular bolt dia and thread pitch.
Special requirement for such customized tensioners can be found in industries like infrastructure and wind, where the requirement of bolt load is higher and require a special multistage tensioners to achieve the desired output.
As these tensioners have a small dia and are taller in height, the bolt protrusion above the nut needs to be controlled.
As mentioned in our previous article, multiple bolt tensioners can be fit together on multiple bolts to be tightened simultaneously with the same accuracy. This helps speed up operations on applications like gaskets in pipeline or pressure vessel flanged connections. The high load developed by the multiple bolt tensioners is proportionately distributed around the joint allowing the gasket to flow into the surface irregularities of the flange offering a much better seal.
For subsea, all the bolts are tensioned together i.e. 100% tensioning is achieved. Since, subsea requirements have less space to fit tensioners. All bolts are deployed with one sided tensioners on both sides of the flanges on alternate bolts. This eliminates the need of multiple passages to achieve the desired bolt load.
In the industry, either 25% (one fourth of total no of bolts) or 50% tensioning (every alternate bolts) is very common.
To speed up the operation for large bolts, the bolt tensioners are provided with spring return for the piston and positive load stop.
The positive stop eliminates human errors like operators making mistakes over pressurising and the damages caused due to the piston coming out of the cylinder.
Applied and Residual Load
While tensioning, the load applied will never be the same that will reside on the bolts after operation, the load lost in between is called the residual load.
The load loss depends on the length of the grip and the bolt dia of the bolt. Loss of load is caused due to the nut embedment into the flange, deflections on the bolt thread, etc. A Load Transfer factor helps compensate this loss.
A formula is used to calculate the load transfer factor that determines the desired bolt load on the bolt based on the residual load and the grip length of the joint.
Tensioning short bolts is not advisable as it leads to high load loss. To generate these loads, the tensioner will have to be oversized.
We hope this follow up article on bolt tensioners has provided you a complete insight. Understanding the significance of the design, operation and other factors, hydraulic tools like bolt tensioners have proved to be a better form of elongating a bolt than other methods. Its operation delivers faster, safe and accurate bolting. Visit us next time for more knowledge sharing articles.