“What is Live Loading and why would I want to use it on a flange?” Adding Belleville springs to a bolted gasketed assembly, or “live loading” it, helps to hold the joint together and maintain the necessary sealing pressure.
If your piping system is low temperature and / or low pressure and it doesn’t cycle very often, you probably don’t have many problems with leaking flanges. If you do, the problem is likely to be loss of bolt load. If you have a high temperature, high pressure cyclic piping system, like what’s found in a petrochemical facility, you probably have areas in that facility where a leaking flange can be a serious problem. These flange leaks can be a significant risk to workers safety, the environment or to operating efficiency and profitability. Live loading these applications using Belleville spring flange washers can be of great benefit.
Some flange joints leak simply as a result of human error. Proper training and procedures will help reduce leaks caused by humans. Hopefully, the general accepted good practices spelled out in ASME PCC-1-2013 will become more of the norm, helping to reduce, or better yet, eliminate human errors. The larger problems lie in the understanding why other types of leaks occur and how to implement the appropriate solution.
Bolted gasketed flange joints leak when the stress on the gasket falls below the stress required for it to seal properly. There are several factors working on the joint that can cause that to happen. They include:
Embedment Relazaon Gasket Creep Bolt Creep / Stress Relaxaon Vibration Differenal Thermal Expansion Elasctic Interaction / Cross Talk
Embedment Relaxation happens when a new bolt and nut is tightened for the first time. New bolts and nuts have microscopic high and low points on their threads. On initial tightening, the threads load on these high points. This causes them to be stressed above their yield point and deform or relax. It has been reported that 50% or more of the initial bolt preload can be lost in just the first 24 hours after the bolt or stud has been tightened. That is one reason some procedures require bolts to be retightened after 24 hours or even hot tightened.
Gasket Creep happens after the initial tensioning. Gaskets creep or flow to obtain a seal. They will connue to creep or flow, even after the load has been applied. This can decrease the amount of bolt preload on the gasket.
Bolt Creep / Stress Relaxation happens at high temperatures causing the bolt to relax over time. The amount of loss is a factor of the bolt material, temperature and the time at temperature.
Vibration, is the hardest of all the factors working on a bolted flange joint to detect. Vibration takes a long time to reduce bolt load, but when it does, the preload loss is rapid. With vibration, joints that remained tight for a very long time suddenly become loose.
Differenal Thermal Expansion can happen when a flange heats up from the process media. The parts of the flange joint are made from different material and heat and cool at different rates. The parts closest to the media or heat, expand faster than the parts further away from the heat. When the flange cools, the parts of the flange joint also cool at different rates. This can cause an addional loss in preload.
Elastic Interaction / Cross Talk, occurs anytime you have multiple bolts in a bolted connection. This can occur on any bolted joint, not just flanges. When you tighten one of the bolts on a flange, the bolt is stretched and the joint is compressed. When the bolt beside it is tightened, the joint is further compressed allowing the first bolt to relax. The greater the number of bolts in the joint, the greater the effects of elastic interaction or cross talk. Tightening all the bolts in the joint at the same time, would eliminate the problem, but this can be difficult or even impossible to do in some cases.
One method to try to reduce the effects of elastic interaction or cross talk it to tighten the bolts in a star pattern and to tighten them in increments. The most common procedure is to tighten the bolts in at least 3 passes. The first being 30% of the final bolt load. The second pass to 60% of the final bolt load and the final pass at 100% of the final desired bolt load. Some procedures require on more tightening pass in the reverse order of the other 3 passes.
Now that we know why bolted gasketed joints sometimes leak, how do we minimize the problem? By the addion of Belleville Spring Flange Washers. Why not use some other method to keep the bolts tight? There are a lot of methods and products that are designed to get to a desired bolt stress, load or torque. They are not designed to maintain that bolt stress, load or torque. A Belleville spring will maintain the desired bolt stress, load of torque over a much longer period of time. Remember, a bolted gasketed joint tends to leak over time, not immediately.
So what is a Belleville Spring Flange Washer? It is a conical shaped spring that provides a high spring force in a small space. This is the simple definition of a Belleville Spring. Belleville Spring Flange Washer is just a Belleville Spring that has been specifically designed to fit on a standard pipe flange and be able to carry the heavy loads required to maintain the sealing stresses required by the gasket.
The addion of Belleville Spring Flange Washers to the bolts or studs on a bolted gasketed joint, changes the spring rate of that joint. This allows the joint to relax at a much slower rate and maintain a much higher bolt load over a much longer period of time. This reduces the effects of gasket/bolt creep, vibration, differential thermal expansion and elastic interactions.
Typically a Belleville Spring Flange Washer is installed under the head of the nut or bolt, with the OD of the spring contacting the flange and the ID of the spring contacting the nut. In some cases more than one spring is used below the bolt. Below are several examples of different stacking arrangements.
When the use of Belleville Spring Flange Washers was first introduced, it was common practice to tighten flange bolts to 30,000, 45,000 and 60,000 psi bolt stress. You will see this reflected in the part numbers. An H13-1-60 is a Belleville Spring Flange Washer that is made from H13 Tool Steel, for a 1/2” bolt. It was designed to produce approximately the same load that would be required to ghten a 1/2” B7 or B16 bolt or stud to 60,000 psi bolt stress. An H13-1-45 would be for the same size bolt of stud, but was designed to produce the load required when tightening a B7 of B16 bolt or stud to 45,000psi bolt stress.
Recently, there has been a change in the way the bolt load or torque is calculated for a bolted gasketed joint. As previously stated, bolt stress was the defining criteria for the calculaons. Now gasket stress is the defining criteria. In other words, what is the load needed to keep the gasket tight, not the bolts.
Belleville Spring Flange Washers are available in several different materials. The material used is determined by the environment of the application.
H13 Tool Steel is the most commonly used material for Belleville Spring Flange Washers due to it’s excellent strength and working temperature range. Ambient to 1100 F.
17-7 Stainless Steel is typically used where the bolt loads are much lighter and for cryogenic applications. It has a working temperature range of -400 F to 550 F. 17-7PH Stainless Steel should not be used in chloride or fluoride applications.
Inconel 718 and X750 are also available in Belleville Spring Flange Washers as well as several other exotic materials. Patriot Bolt and Supply's engineering department can assist in selecting the correct material for your application.
Please note that when discussing operating temperatures, the temperature of the spring can be significantly lower than the actual process temperatures. The spring temperature can be as little as 50% of the process temperature on an uninsulated flange.
Credit for content in this article goes to Belleville International; Butler, PA, USA. Information provided by Fran Pugliese