: Cone and Thread or FK Series Compression Tube Fittings
Optimizing Medium-Pressure Oil and Gas Assets With Compression Fittings
November 6, 2019 | Chuck Erml, Product Manager, Swagelok Company
Cone and thread fittings have been the standard choice for a variety of important oil and gas applications for years. When used with special antivibration glands and installed by knowledgeable and experienced technicians, these fittings may deliver acceptable performance in medium-pressure applications.
On the downside, cone and thread fitting installations are labor-intensive and time-consuming. If installed without antivibration glands and by a technician who is not intimately familiar with the preparation and assembly process, the cone and thread fitting can leak far earlier than anticipated by the operator.
What is at stake if medium-pressure fittings leak or fail? Offshore oil and gas owners and operators are under intense pressure to ensure safety and environmental compliance, while containing costs and optimizing efficiencies. Leaking or faulty oil and gas medium-pressure fittings can cause significant issues, resulting in unplanned maintenance and environmental and safety concerns. Additionally, upfront installation time of cone and thread fittings can be lengthy compared to installations performed with newer, innovative compression fittings, which are optimal for many critical oil and gas applications.
For example, many medium-pressure applications can utilize compression fittings, like FK series fittings from Swagelok, nearly anywhere cone and thread fittings are traditionally specified. Assemblers can complete FK series fitting installations approximately five times faster than cone and thread fittings, eliminating rework needs after asset delivery and significantly reducing the overall cost of maintenance. Further, these compression fittings follow a simpler installation process with less opportunity for errors by technicians, leading to more consistent, reliable performance over the lifetime of the asset.
These efficiencies can add up to significant labor savings for a reduced total cost of ownership for topside systems, including chemical injection skids, wellhead control panels, umbilical termination units, and hydraulic power units. Throughout this post, we will explore how FK series fittings can lead to major savings for owners and operators.
Optimized Installation
FK series fittings can be installed faster than cone and thread fittings. Here is why:
Fewer required tools. Cone and thread fittings require specialized tools and materials to successfully complete an installation, including coning and threading tools, along with cutting lubricant to reduce friction during the cutting process. By contrast, a compression fitting can be successfully installed with an everyday wrench and vise.
Quicker fitting installation. Both FK series and cone-and-thread-style fittings require the installer to square-cut and deburr a length of tubing, but that is where the similarities end. Using the coning and threading tools, the tube must be properly coned and threaded before being joined with the fitting. Proper care must be taken to ensure no burrs, gouges, or scratches have occurred during the process. Once the tubing prep is complete, a collar is threaded onto the tubing and a gland nut is inserted into the fitting body for final tightening.
By contrast, an FK series fitting is installed by threading a preassembled cartridge (which includes a nut, two ferrules, and a plastic arbor) onto the fitting body. The tubing is inserted and marked to establish alignment, and one full turn is applied to complete the connection. After checking with a gap inspection gauge, the fitting is complete and ready for use.
Watch a side-by-side video comparing a compression fitting installation versus cone and thread fitting installation.
By removing several required tools and tedious steps to complete the connection, compression fittings can help operators save significant time during installation—and that can add up to major savings.
For a comparison example, imagine fabricating a medium-pressure skid package for an offshore oil and gas topside application. Say the skid will use 316 stainless steel tubing and fittings, and a single trained installer will make up each of the skid’s 500 connections.
The time to complete either fitting type will depend on the tubing diameter, material type, and skill level of the installer. Assume the FK series compression fitting takes four minutes from start to finish. The cone and thread fitting takes 20 minutes (or five times longer). Therefore, the total time to complete the skid's 500 fitting connections with FK series fittings is approximately 2,000 minutes (33.3 hours) compared to 10,000 minutes (166.7 hours) for cone and thread fittings.
When installing cone and thread fittings, there is also contamination to consider. Between the resulting metallic shavings from the coning and threading process and the use of cutting lubricants, cone and thread assembly can be a messy process. Any excess residue on the shop floor can lead to a potential safety risk, while contamination from metal shavings risk harming system performance. By eliminating the need for coning and threading, the use of innovative FK series fittings alleviates these concerns.
Long-Term Performance Reliability
Cone and thread fittings installed with special antivibration glands, nuts, and collars can deliver improved performance when installed by a skilled, experienced technician. However, these types of fittings are not always specified, and system performance often suffers as a result. This is driving owners, integrators, and contractors to increasingly seek a more reliable, higher-performing fitting that can provide strong performance.
The answer is FK series fittings, which are less prone to manual installation errors due to their preassembled cartridge design and are more likely to maintain leak-tight performance with no maintenance throughout their lifetimes. Cone and thread fittings can loosen over time due to vibration, whether during transport to the job site or during operation. Manual tightening can usually restore a leak-tight connection, but the additional maintenance costs can add up over time. By comparison, FK series fittings’ double ferrule design creates a twin mechanical grip on the tubing, dramatically reducing the likelihood of fittings backing off when vibration occurs. It's estimated that a typical topside installation will require about 20% of cone and thread connections needing rework during initial assembly and pressure test. By contrast, an FK series fitting requires almost no rework due to its simple installation procedure and the ability to verify installation using a gap gauge tool.
Finally, while both types of fittings can be disassembled and reassembled for system maintenance, reassembling a cone and thread fitting is not always repeatable or reliable. FK series fittings, however, are easily remade and have been validated for many rebuild cycles.
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Overall, FK series compression-style fittings can deliver significant benefits to topside asset owners and fabricators in the oil and gas space. They reduce installation time, lower assembly and maintenance costs, while providing a more reliable and robust connection resulting in increased uptime. This can help save thousands of dollars on each project.
Interested in making the switch? Swagelok FK series fittings deliver quicker installation and thousands of dollars in lifecycle cost savings for your topside assets and equipment.