Instrumentation Fittings for Power Plants
on Wed, 04/01/2020
One area where instrumentation can be improved in some instances is during the process of selecting the most optimal type of fittings. There are a number of different options available, and several factors should be considered before settling on the fitting that best meets the need of a power plant.
Safety, efficiency, and costs are all determining factors during the instrumentation fitting selection process. By having an adequate understanding of how each type of fitting relates to these factors, one will be best equipped to make the right decision. In most cases, the decision-making process will begin by determining whether a single- or double-ferrule fitting makes the most sense for a given application.
A Long HistoryThe single- and double-ferrule fittings have both had a presence in the industry for more than 50 years. However, due to various reasons that are mostly associated with specific market penetration and patent issues, the double-ferrule design has been the one that most dominated the power plant instrumentation industry (Figure 1). However, one cannot definitely say that this design was the one that most plant operators independently desired. The design itself has some issues that the single-ferrule fitting doesn’t share. For example, there is a big difference in the assembly.
One element that would cause a plant operator to lean toward a double-ferrule design as opposed to a single is its assembly reliability. Basic logic would dictate that when a product is more complicated to assemble, the chance is greater that it will be done incorrectly. Most of this complication comes from the fact that there are so many ways to achieve this incorrect assembly. In fact, there are 14 different ways that a double-ferrule instrumentation fitting can be assembled the wrong way. On the other hand, there are merely two possible ways to incorrectly assemble a single-ferrule instrumentation fitting. Of course, when a fitting is assembled incorrectly and placed in service, there are various safety concerns that will arise.
Today, many managers list safety as the most important workforce objective. Thus, it is very typical to see power plants dedicate entire committees solely for the purpose of keeping safety top of mind and reducing unnecessary, preventable accidents from occurring.
As new employees are hired into the power plant environment, they typically go through extended safety training. Additionally, current employees are required to attend safety training on a regular basis (annually at the very minimum). As new personnel continue to learn the safest way to conduct a specific process, such as instrumentation fitting assembly, keeping it simple and eliminating possible points of failure is an essential goal. The next safety element to take into consideration when selecting an instrumentation fitting is the part’s heat code traceability.
Tracing Heat CodesThe ability to track an instrumentation fitting’s heat code is critical to maintaining a safe power plant environment (Figure 2). Considering all the different types of environments that use instrumentation fittings, a power plant is one of the most high-risk, largely due to the nature of the process: power generation. Because of this high-risk status, introducing elements such as instrumentation fittings should be a meticulous exercise, with each part having strong traceable heat code documentation.
An engineer designing the process and/or the operator entrusted in keeping everything running smoothly needs to know the pedigree of the material. This includes the location the material came from and passed through. Anything that is introduced into a process in a power plant environment must have this type of documentation. Any oversight in this area could lead to complete failures of the system, costing exorbitant amounts of downtime and ultimately reduced revenue.
LubricationKeeping each component of a process properly lubricated can be quite challenging at times. This ongoing initiative can be particularly difficult when dealing with many small parts. This is no different when discussing instrumentation fittings.
Both single- and double-ferrule instrumentation fittings require lubrication on the nut’s inside threads to prevent galling. This is due to the stainless steel design of the fittings. However, the two different designs typically approach this challenge from two different directions.
Single-ferrule fittings are known to incorporate a coating called molybdenum disulfide, known as moly for short. There is a general consensus that moly is the superior form of lubrication coatings for this application due to its anti-galling capabilities. Unlike the alternatives, this coating is known to work very well in corrosive and high-temperature environments. Moly provides for superior temperature values and due to its nature will allow for more remakes without adding additional torque through the life of the fitting.
Corrosion ResistanceFinally, a big consideration for decision-makers is the fitting’s corrosion resistance or lack thereof. Frequently, if corrosion is affecting a plant’s instrumentation, the plant will fail to generate power at its most optimum level. More extreme than this, however, corrosion can cause complete failure of equipment in the plant. Mentioned above is the exorbitant amount of revenue that can be potentially lost due to an unplanned shutdown or failure. One of the biggest contributors to such a painful event is equipment failure due to corrosion.
If not addressed or planned for, corrosion has the potential to leave both equipment and adjacent equipment inoperable. The emergency maintenance/equipment replacement that must be conducted in order to return the plant to full operating order can get very pricey, very quickly. Furthermore, this doesn’t account for all the revenue lost due to a plant being down.
All these unintended consequences point to the fact that corrosion needs to be actively protected against inside of a plant’s instrumentation. Therefore, when selecting the single-ferrule design that works best for a specific plant’s instrumentation fittings, consideration of its corrosion resistance must be understood. Typically, with ferrules, the leading edge is hardened (only the back ferrule is hardened in a double-ferrule design) to complete the compression grip on the tubing for optimal sealing, which protects the fitting from corrosion.
Corrosion comes in two forms for this type of application: process and external. Process corrosion affects what is being fed through the tube. For power plants, this applies to steam service and ammonia injection systems. External corrosion, alternatively, concerns the environment surrounding the operation. When reviewing the specification of the hardening process for ferrules used in an operation, special attention must be given to the material the ferrule is manufactured from to avoid this devastating, unintended consequence.
Selecting the most optimum ferrule design for an operation is a critical step for decision-makers at power plants. Taking into consideration source material, heat code traceability, lubrication, and corrosion resistance requirements will help guide the decision. Spending the time and money to get this piece of the puzzle right has the potential to save plant leaders stress, anxiety, and most importantly, revenue. The single-ferrule design provides excellent vibration resistance and temperature compensation, and the design reduces the number of fitting pieces and simplifies assembly.
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