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How to Build More Productive Critical Fluid Systems With FK Series Fittings


Swagelok FK series fittings

Build Productive Fluid Systems

Today’s oil and gas professionals face new and evolving challenges. Deeper wells and longer subsea tiebacks are being used with innovative drilling/ production techniques to maximize returns on investment. All onboard equipment must be carefully specified to contribute to safe and reliable operation in harsh offshore environments throughout the life of the asset. Distinct from the standard Swagelok® tube fitting, the Swagelok® FK series fitting is designed to meet these needs and is an ideal option for many oil and gas applications up to 22 500 psi (1551 bar). This white paper will detail how to achieve high reliability and reduce maintenance costs with FK series by following the proper assembly process, how FK series fittings can help mitigate environmental impact via leak reduction with its unique design, and how to improve overall safety by protecting your systems against corrosion.

Operating More Efficiently in Today’s Upstream Oil and Gas Market

As new wells get deeper and deeper around the world, numerous critical production systems must provide uncompromising safety and reliability in some of the world’s most demanding applications. Even when working in highly corrosive environments and at increasingly high operating temperatures, owners and operators expect a 20- to 30-year service life from their equipment.

Meanwhile, volatile market conditions and increasingly stringent environmental regulations have driven a greater need for operators to reduce their maintenance spend, increase speed of production, lessen their environmental impact, and enhance overall operational safety. Achieving such reliable performance is not always easy, but it is possible. Critical components’ ability to provide reliable, leak-tight service can help oil and gas operators with these challenges, especially in some of the following critical applications.

Oil and gas offshore platform

Chemical Injection (CI) Systems

CI systems must maintain highly accurate and repeatable chemical dosage into the production well. Leak-tight performance is absolutely essential to protect personnel and the environment, and any form of downtime can result in significant production losses. A typical CI system may require hundreds of individual connections. Every one of them is a potential leak point.

Hydraulic Power Units (HPUs)

HPUs provide power to topside, turret, and subsea production systems. HPU downtime can shut down these systems and cause significant financial losses each day. HPUs are complex pieces of equipment, and tube fittings are an essential component that helps the system work reliably.

Topside Umbilical Termination Units (TUTUs)

TUTUs, the associated umbilical connection, and isolation panels must provide a leak-tight connection as well as an isolation point between topside and subsea equipment while ensuring safe isolation for maintenance activities. These systems require high-quality and reliable components to perform their crucial functions. Failure can cause loss of containment or safety incidents, resulting in potential shutdowns and loss of production.

In each of these applications, Swagelok FK series fittings represent an ideal solution to achieve reliable performance, minimal maintenance, and outstanding safety.

Throughout this white paper, you’ll learn several reasons how and why FK series fittings can deliver this on your application’s needs, including:

  • Why following installation best practices leads to efficient, repeatable assembly and reliable long-term performance
  • The engineering and design characteristics that enable high levels of performance compared with traditional fitting technologies common in the upstream space
  • FK series fittings’ material composition considerations for reliable corrosion resistance in extreme environments
Chemical injection system
Chemical injection (CI) systems


FK Series Fitting Assembly Techniques

FK series fittings can be relied upon for long-term, leak-tight performance in medium-pressure applications. Minimal maintenance is required over their lifetimes compared with other fittings, contributing to minimal maintenance spend and greater operational efficiency.

There are several reliable and repeatable methods for installing an FK series fitting, depending on the application or the installation environment (for example, in a workshop versus in the field). Performed correctly, each method will result in a reliable connection.

Tube Preparation

Proper installation of FK series fittings begins with proper tube preparation.

Importantly, FK series fittings require heavy walled tubing. Cutting heavy walled tubing requires either a hacksaw or a powered saw— powered options provide greater consistency and smoothness.

Next, tubes must be deburred for proper fitting function and leak-tightness. Burrs can break off and damage components in other parts of the system by lodging in small holes or vents, or by scratching valve seats and soft seals such as O rings. Manual deburring tools work well, but again, available powered options can create even smoother edges.

Required Tools

Once tubing has been prepared, FK series fitting assembly requires the following tools:

  • A bench vise to hold the fitting body
  • An appropriately sized wrench and marker (for assembly by turns)
  • A torque wrench with the appropriately sized crowfoot wrench head (for assembly by torque)
  • A pre-swaging tool (for assembly via pre-swaging)
  • FK gap inspection gauge
  • FK depth-marking tool—a depth mark helps ensure the tubing is fully bottomed into the fitting

Installation by Torque

Installing FK series fittings by torque requires the use of a properly sized torque wrench. These instructions apply to FK series tube fitting sizes from 1/4-in. to 9/16-in. only.

Insert the tube into the fitting

1. First, insert the hardware into the body and remove the plastic arbor. Then insert the tube into the fitting. Make sure that the tube rests firmly on the shoulder of the fitting body.

Tighten the nut to the specified torque


2. Next, hold the fitting body steady and tighten the nut to the specified torque using the appropriately sized torque wrench.

Use the FK gap inspection gauge to ensure that the fitting has been sufficiently tightened

3. Finally, use the FK gap inspection gauge to ensure that the fitting has been sufficiently tightened. Gently push the inspection gauge into the gap between the body and nut. If you cannot insert the gauge, the fitting is sufficiently tightened.

Installation by Turns

Installation by turns uses a simple wrench instead of a torque wrench and eliminates the variable of calculating required torque. This may help installers be more consistent. These instructions apply to FK series tube fitting sizes from 1/4-in. to 3/4-in.

Insert the tube into the fitting

1. First, insert the tube into the fitting. Be sure that the tube rests firmly on the shoulder of the fitting body.

Tighten the nut one full turn

2. Next, mark the nut with your marker. Hold the body steady, then, using the appropriately sized wrench, tighten the nut one full turn.

Use the FK gap inspection gauge to ensure that the fitting has been sufficiently tightened

3. Finally, use the FK gap inspection gauge to ensure that the fitting has been sufficiently tightened. Gently push the inspection gauge into the gap between the body and nut. If you cannot insert the gauge, the fitting is sufficiently tightened.


Assembly via pre-swaging can be advantageous when making many connections in sequence. For example, assembling fittings in a workshop environment via an assembly line process may find the pre-swaging process more efficient. These instructions apply to FK series tube fitting sizes from 1/4-in. to 9/16-in. only.

Thread the preassembled cartridge into the pre-swaging tool until finger-tight.

1. First, thread the preassembled cartridge (nut, ferrules, and plastic arbor) into the pre-swaging tool until finger-tight.

Remove the plastic arbor from the preassembled cartridge

2. Next, remove the plastic arbor from the preassembled cartridge.

Insert the tube into the pre-swaging tool

3. Then, insert the tube into the pre-swaging tool.

Be sure that the tube rests firmly on the shoulder of the pre-swaging tool body

4. Be sure that the tube rests firmly on the shoulder of the pre-swaging tool body; rotate the nut until finger-tight.

Mark the nut and tighten it three-quarters of a full turn

5. Then, hold the pre-swaging tool steady, mark the nut and tighten it three-quarters of a full turn.

Remove the tube with pre-swaged ferrules from the tube

6. Remove the tube with pre-swaged ferrules from the tube.

Insert the tube with pre-swaged ferrules into the fitting until the front ferrule seats against the fitting body


7. Insert the tube with pre-swaged ferrules into the fitting until the front ferrule seats against the fitting body; rotate the nut until finger-tight.

Hold the fitting body steady and tighten the nut to the specified torque


8. Finally, hold the fitting body steady and tighten the nut to the specified torque. Alternatively, rotate the nut to the previously pulled up position. At this point, you will feel a significant increase in resistance. Tighten the nut an additional one-quarter of a turn with a wrench.

The pre-swaging method can also be performed using a multihead hydraulic swaging unit for more efficient production.


When working with FK series fittings, it can be helpful for technicians to go through Swagelok’s comprehensive training program, which dives into further detail about the methods described above. Training helps all staff understand the correct method to install FK series fittings and can help reduce additional maintenance costs. All training is performed by Swagelok certified specialists and is available globally for projects in any location.


Designed and Engineered for Leak-Tight Performance

Assembled correctly, FK series fittings deliver reliable, long-term performance in critical oil and gas applications via their unique design attributes. Their leak-tight performance can help both improve overall system efficiency and lessen your platform’s impact on the environment by reducing potential emissions.

Tube fitting cutaway
This cutaway image shows how optimized seal contact can be achieved with two-ferrule fitting technology.

Grip Strength

FK series fittings feature a colleted mechanical grip using two ferrules—an ideal design for a fitting to create a robust grip. A hardened front ferrule enables the fitting to physically bite into the tubing, creating a very high pressure rating. These case-hardened ferrules deliver outstanding grip strength for uncompromising applications.

Vibration Resistance

The FK series’ hinging-colleting action allows for a slight amount of movement in the fitting (called “spring back”) while maintaining grip and force, creating outstanding vibration resistance. This feature improves reliability in applications that regularly experience operational vibration. Spring back also helps fitting connections withstand thermal changes that can cause metallic alloys to expand and contract, which can compromise performance.

Leak-Tight Gas Seal

FK series fittings incorporate two contact zones across longer sealing surfaces—one along the tube and another along the fitting. These contact surfaces are angled slightly, providing optimized stress levels to maintain an uncompromising seal. The FK series fitting achieves such a seal with its two ferrules—the front ferrule creates a long plane of contact while the back ferrule effectively grips into the tubing itself and creates its own surface seal. Long, precisely machined surfaces on the front ferrule further contribute to a highly reliable gas-tight seal.

Optimized Sizing

FK series fittings are available in sizes up to 1 inch, ideal for the extreme depths of today’s drilling methods. Larger sizes allow platforms to achieve high flow rates at greater depths, creating efficiencies that other fittings cannot.

A Comparison: FK Series Fittings vs. Cone and Thread

Cone and thread fittings have traditionally been chosen in critical oil and gas applications for their high pressure ratings. However, cone and thread technology has a few drawbacks when compared to FK series fittings that may contribute to inefficiency and additional maintenance needs.


Cone and thread assembly requires specific knowledge and experience, and the assembly process can be labor-intensive and time consuming. Cone and thread fittings also 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, an FK series fitting can be successfully installed with an everyday wrench and vise. FK series fittings allow assemblers to complete installations approximately five times faster than comparable cone and thread fittings, providing a first-time, leak-tight seal.


By achieving a leak-tight seal upon proper initial installation, FK series fittings minimize the need for rework after asset delivery and significantly reduce the overall cost of maintenance. Over time, cone and thread fittings will eventually and naturally back off, requiring additional attention. It is estimated that a typical topside installation will require about 20% of cone and thread connections needing rework during the initial assembly and pressure test. By contrast, a property installed FK series fitting should require almost no rework due to its simple installation procedure and the ability to verify installation using a gap gauge tool.


Cone and thread fittings require additional antivibration components within systems that experience regular shock or vibration. If installed without antivibration glands or 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.

Time Savings That Add Up

FK series can offer considerable time savings. Consider fabricating a medium-pressure skid package for an offshore 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.

Save installation time with FK series fittings

It is reasonable to 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).

Save installation time with FK series fittings

The total time to complete the skid’s 500 fitting connections with FK series fittings is approximately 2000 minutes (33.3 hours) compared to 10 000 minutes (166.7 hours) for cone and thread fittings.


Material Selection Considerations

Corrosion occurs when a metal atom is oxidized by a fluid, leading to a loss of material in the metal surface. This loss reduces the wall thickness of a component and makes it more prone to mechanical failure. Because oil and gas platforms operate in harsh environments where corrosion is a constant concern, material selection when specifying all manner of critical components is important. To best protect your assets from corrosion-related issues, FK series fittings are available in several different material options, including alloy 2507, alloy 625, and 316 stainless steel.

Alloy 2507 specifically provides our highest levels of corrosion resistance in high-chloride applications, helping to prevent issues on offshore platforms. Composition includes nickel, molybdenum, chromium, nitrogen, and manganese, offering excellent resistance to general corrosion, pitting corrosion, crevice corrosion, and stress corrosion cracking (SCC), while maintaining weldability.

Additionally, 2507 provides higher yield and tensile strength for increased pressure ratings, allowing FK fittings to achieve high-pressure capabilities with lower wall thickness, allowing for increased flow of fluids.

Swagelok material science specialists are available to help you select an optimized combination of materials to best suit your needs in the most cost-effective manner. Our teams have been fighting corrosion since 1947. We can help you simplify selection with our deep understanding of factors that contribute to corrosion, as well as the properties of materials that help fight it.

Know Your Corrosion Types

Several different forms of corrosion can form on offshore platforms. Here is a quick breakdown of each type and how to identify them.

General corrosion

General Corrosion

General corrosion—the most common form and the easiest to identify—occurs relatively uniformly across a metal surface. While it does not typically lead to catastrophic failure, the gradual decrease of the wall thickness of a component must be considered when calculating pressure ratings.

Localized pitting corrosion

Localized Pitting Corrosion

Pitting corrosion causes small cavities, or pits, to form on the surface of a material. Although detectable by thorough visual inspection, these pits can grow deep enough to perforate tubing. Pitting is more often observed in high-chloride environments at elevated temperatures.

Localized crevice corrosion

Localized Crevice Corrosion

In a typical fluid system, crevices exist between tubing and tube supports or tube clamps, between adjacent tubing runs, and underneath dirt and deposits that may have accumulated on surfaces. Crevices are virtually impossible to avoid in tubing installations, and tight crevices pose the greatest danger for corrosion to occur.

Stress corrosion cracking

Stress Corrosion Cracking (SCC)

SCC is dangerous because it can destroy a component at stress levels below the yield strength of an alloy. In the presence of chloride ions, austenitic stainless steels are susceptible to SCC. The ions interact with the material at the tip of a crack where tensile stresses are highest, making it easier for the crack to grow. While in progress, SCC can be difficult to detect, and final failure can occur suddenly.

Swagelok FK Series fittings

A Strong Choice for Today’s Oil and Gas Applications

The challenges faced by today’s upstream oil and gas operators point to the need for high-quality, reliable components that can deliver high levels of performance in unforgiving applications. From initial construction and commissioning through daily operations and maintenance, there is tremendous focus on reducing total life cycle costs.

Swagelok FK series fittings offer a way to reduce maintenance costs, increase speed of production, mitigate environmental impact via leak reduction, and enhance overall operational safety. Their robust design, simplified assembly process, and availability in high-performing alloys make FK series fittings an optimal choice for the demanding nature of today’s upstream oil and gas industry. Plus, FK series fittings are backed by our full suite of service offerings, including specialized training, on-site evaluations, design and assembly services, and much more.

Our oil and gas specialists are happy to help you learn more about how you can benefit from specifying FK series fittings on your next project. Get in touch with our teams to start a conversation about how you can achieve more efficient installations and greater application reliability with FK series fittings.