Maintenance Best Practice
My father used to tell me that the right tool exists for every job. I tend to agree with that statement, especially considering the risk of what can happen when we don’t use proper equipment available to accomplish a specific task.
As a Mechanical Engineer, I stand behind that claim. In the Turbomachinery Industries (Oil & Gas, Power Generation, Petrochemical, etc.) the strict maintenance schedules and tooling best practices are not only required but are essential to providing the safest and most efficient repair cycle possible.
When a scheduled shutdown takes place, Field Technicians and Engineers use this downtime to perform preventative maintenance or upgrades within a specific window of time. It is worth noting, that during this downtime, the output of the powertrain being worked on comes to a stop, which makes quite an impact on the overall operation or efficiency of the plant. It also goes without saying, the potential cost of any additional downtime not planned for can be detrimental.
Therefore, it is critical that technicians use only the best practices and have the knowledge of proper tooling and methods available to help prevent the unexpected extension of planned shutdowns. For most Turbomachinery applications, Hydraulic Tensioning should be the preferred method of bolt loading for specific reasons that will be highlighted in this article.
Torque Versus Tensioning
At a first glance, the method of bolt loading may not seem like a huge consideration, but there are a few significant advantages that tensioning provides over torque, especially in Turbomachinery applications. To better understand these advantages, lets first break down the method of bolt loading using torque.
The primary function of bolting is to clamp two or more joints together to create a ‘preload’ which will put the bolt in tension. Examples within Turbomachinery that require joint clamping include Casing Closures found on Gas or Steam Turbines, Reciprocating Compressors, and Gear Boxes just to name a few.
While many bolted joints are loaded using torque, there are difficulties associated with loading a fastener in this way. Friction is one of the biggest downfalls to using torque due to the issue between the sliding surfaces. The sliding surface is not only from the nut biting into the surface it is being seated against, but also between the threads of the nuts and bolts themselves. The higher the torque, the greater the friction, which results in Torsional Wind Up. This friction must be dealt with, otherwise it can become excessive, and the result will lead to torsional loading or commonly seen as “bolt twist”. The real issue with this is the loss in the final preload in the fastener and the desired/needed clamping force that will not be achieved as a result. Along with friction there are other areas that can cause significant loss in the final preload of the fastener. Issues such as tool accuracy, thread and flange geometry and even operator error are all common issues users face when using torque as a bolting method.
Due to these issues, a better method of achieving a more accurate bolt loading is to use a Hydraulic Tensioner. Tensioning reduces or eliminates many variables in the bolting process which are present when torque is involved. The results from tensioning will provide improved accuracy and repeatability in addition to reduced installation and removal time.
Below are five reasons why using Hydraulic Tensioners should be the preferred method of bolting for Turbomachinery applications.
There are many situations that are not ideal for bolt loading using wrenches or torque tools, but limited space is a common occurrence. If the space is narrow or has poor/limited access to the fastener, a Hydraulic Tensioner can provide the proper loading and fit into these restricted spaces. For example, the Casing Bolts found on some Split Line Compressors do not provide enough hex nut spacing for any tool to fit around it. Using Hydraulic Tensioning, a special design known as the Hydraulic Rod Tensioning System can be used to tension the stud within the same foot print the original Hex Nut had.
Repeatability & Precision
The uncertainty of friction can be eliminated with Hydraulic Tensioning and can reduce the amount of error in most applications to better than ±5%. When coupled with stretch control measurements and simultaneous tensioning of multiple fasteners, the accuracy of the bolt load can be increased even further. Experimental data has shown, the average preload scatter for torqued fasteners can be more than ±25%.
Even Loading with Simultaneous Tightening
When improvements of load distribution over multiple threaded fasteners are required, such as split line flanges, gasketed joints or any joint that needs precision sealing, tensioning is the method of choice. This is true because multiple fasteners can be loaded at the same time using multiple tensioners and one common hydraulic power unit, that will evenly load the joint. When using torque, achieving uniformity is an exceedingly difficult task to do.
When applications require the use of High Strength, High Temperature or Corrosion Resistant materials, they have the tendency to stick or gall under high torque loads. As the preload levels increase, the chance of running into galling issues increases significantly. Hydraulic Tensioning introduces a pure tension load into the stud and the nut is threaded down without the frictional resistance generated in a stud using torque. This significantly reduces the likelihood of galling in the threads or between the nut and flange surfaces.
All of the previously discussed advantages of Tensioning lead to one of the biggest reasons Hydraulic Tensioning is a preferred method of bolt loading, and that is Time Savings. Due to advancements in technology, it is now just as easy to tension 4” diameter fastener as it is a 1” diameter fastener when using the power of Hydraulics to do the work. Improved preload accuracy and repeatability eliminate the time spent re-tightening leaking joints. With thread galling no longer a concern, the fear of cutting off stuck fasteners and damaged hardware/flanges is eliminated. These key factors reduced installation times and allow for more predictable outage and maintenance schedules.
Jon Williams is a Mechanical Engineer and first came to Riverhawk Company in June of 2012. Jon specializes in the hydraulic tensioning product lines and assists some of the most well-known turbomachinery OEMs with standard and custom design tensioner configurations. He holds a Bachelors of Applied Science in Mechanical Engineering from SUNY Polytechnic Institute.