Process Control

Process control is the technology of regulating processes to provide stable and consistent output. Parker strengths include process instrumentation, semiconductor, power generation, medical and analytical applications.
Latest Process Control Blog Posts

Three Reasons Why You Should Include Bolts in Material Specification, Parker Hannifin, Instrumentation Products Division Europe, Petrochemical platformThere are many reasons why bolts should be included in material specification, particularly in the oil & gas and petrochemical industry. In this latest blog from the Process Control team, we present an overview on items for consideration that may not always be considered for a commodity part.

1. High chloride environments are extremely corrosive.

Because the high chloride environments where the instrumentation is used, including seawater and bleach plants, are extremely corrosive, therefore it is a good idea if Corrosion Resistant Alloys (CRA) are specified by engineers for all parts in these applications. Super austenitic stainless steel 6Mo is a high-performance alloy renowned for its corrosion resistant properties. 

2. The wrong material specification can lead to the whole system’s downfall.

It can be a false economy if inferior materials are chosen for the bolts, rather than corrosion resistant alloys. In highly exposed environments, 6Mo is often specified by engineers for instrumentation systems, including tubing, manifold, gauge and fittings. However, bolts may seem like the last piece of the jigsaw - a tiny part of an overall system - but if the wrong material is specified, this could lead to the whole system’s downfall. Corrosion of the bolts in a hook up could lead to stress corrosion and cracking, causing them to snap, which results in catastrophic failure and the potential for accident and injury. 

3. An inferior material specification can cause maintenance issues.

The third reason is that inferior material specification can cause maintenance issues too. Corrosion to the bolts can be so rapid that by the time it is noticed, the bolts simply cannot be replaced.  Instead, they have to be sawn off, potentially causing expensive damage to the instrument tubing system.

Similarly, if tubing clamps are not manufactured from corrosion resistant alloys, this could lead to stress cracking and fatigue, resulting in equipment failure.

Specifying a Corrosion Resistant Alloy, such as 6Mo, throughout the instrumentation tubing system, including all component parts, gives complete peace of mind. Specifiers and customers can, therefore, be reassured that all components, including bolts, will ensure durability and long-term performance of their instrumentation system.

Learn more about corrosion and materials selection for corrosion control.

Three Reasons Why You Should Include Bolts in Material Specification, Parker Hannifin, Instrumentation Products Division, Europe, Deborah PollardDeborah Pollard is business development leader capital projects, Instrumentation Products Division, Europe of Parker Hannifin.






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10 Tips to Ensure Best Practice in the Installation of Condensate Pots, Refinery, Instrumentation Products DivisionCondensate pots play a key role in maximising the accuracy of differential pressure flow measurement on steam or vapour applications. When installed correctly, these simple devices can significantly improve flow measurement accuracy in differential pressure measurement systems by providing an interface between the vapour and liquid phases.

Condensate pots also prevent flashing of liquid in the impulse line, which can occur if there is a sudden change in the temperature of the steam.

As a result, condensate pots are widely used in applications such as refineries, power plants, chemical and petrochemical, steel plants and other process industries as they provide an interface between the vapour phase and the condensed phase in the impulse lines. They also facilitate the minimisation of gauge line error caused by pressure differences in pairs of impulse lines.

Parker’s condensate pots are suitable for use either on vertical or horizontal lines, between the primary (Flow Meter) and the secondary (transmitter/gauge) to act as a barrier to the line fluid, allowing direct sensing of the flow conditions.

The correct installation of condensate pots, however is really important to ensure long service life and maximum efficiency.

Here are 10 tips which will help ensure best practice in the installation of condensate pots.   10 Tips to Ensure Best Practice in the Installation of Condensate Pots, A Condensate Pot, Instrumentation Products Division
  1. Make sure you evaluate the number of connections required on the condensate pot before ordering (for example, inlets, outlets, fill port, drain port, gas vent port.)  This ensures that the Condensate pot meets your specific application requirements.

  2. Carefully define the condensate pot volume in litres, system pressure and temperature requirements.  This is important as the size of the pot needs to relate to volume of steam passing through the steam pipeline. 

  3. It may be necessary to trace heat and insulate all impulse lines.  This ensures that the vapour phase is maintained in the tube lines between the pipeline and the condensate pot. It may also be required to prevent freezing in the liquid lines between the condensate pot and the transmitter. 

  4. Consider adding an anti-freeze media, such a glycol, to the water lines.  This may be essential in climates where below freezing temperatures are reached.

  5. Keep vapour impulse lines as short as practicably possible.  This ensures that the steam can remain in this state and requires minimal or no heating.

  6. Ensure both condensate pots are mounted at the same level, minimising possible error that could arise due to unequal head of fluid in the connecting pressure lines. This should take into account both vertical and horizontal steam pipelines. The higher connection point should be the reference. 

  7. The differential pressure measuring device (DP) should be mounted below both the condensate pots and the steam pipe line.

  8. It is recommended that both impulse lines from the condensate pot to the DP include the facility for ‘blow down’. Blowing down these lines periodically prevents the collection of debris, which could impact on measurement accuracy.

  9. Ensure both the high pressure (HP) and low pressure (LP) impulse lines are the same length, which should eliminate pressure head errors.  The theory of operation for condensate pots is that between the process taping and the pot is steam vapour. Between the pot and the differential pressure transmitter is water (liquid) thus eliminating any measurement errors due a liquid / vapour mix at the measurement device. 

  10. It is advisable to select condensate pots as part of a complete Parker instrumentation solution.  We can supply all associated valves, manifolds, tubing and fittings alongside condensate pots, ensuring that all components work together and providing added reassurance about accuracy and safety.   This also includes providing tubeline heating and insulation to ensure performance is maximised.

Parker condensate pot pressure ratings are for temperatures up to 100°C. We can also supply condensate pots to meet other pressures and temperatures. The most commonly used materials to manufacture condensate pots are steel, 316, 304, 6MO stainless steel and monel.”

For more details visit our website or view our condensate pot brochure

10 Tips to Ensure Best Practice in the Installation of Condensate PotsArticle contributed by Graham Johnson, Small Bore Product Marketing Manager - EMEA, Instrumentation Products Division Europe.





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Making Ash Analysis Simpler in the Field - White Paper, Parker Hannifin, Instrumentation Products Division Europe, Coal conveyor Traditional coal ash sampling and analysis using a laboratory facility can take a few hours if there is an on site lab or days if the samples are sent away for analysis. Using the Parker Bretby Gammatech portable Ash Probe, the results are available in a few minutes by probing the coal pile with no special training required by the operator. All the data is collected during the shift and can be downloaded onto a memory stick or direct USB cable to a PC in CSV format for analysis and reporting. The unit comes with the Parker Bretby Gammatech utility software for ease of download. This white paper describes the journey the Parker team made in producing the new AshGraffix controller for the Ash Probe.

Download white paper as pdf.

During the latter part of 2012, we were advised by our supplier that the micro processor used in the Ash Probe system was being made obsolete; this gave us two choices:

1. Incorporate the new processor and modify the software for the existing display unit.
2. Develop a new design display unit based on our customers feedback using the latest processor.

Old display unit - new software code 

Making Ash Analysis Simpler in the Field - White Paper, Parker Hannifin, Instrumentation Products Division Europe, AshGraffix display unitFig 1. Old QWERTY keyboard display unit with LCD display in English only.

To support sales of the Ash Probe while we developed the completely new unit, we chose to write the new software code for the existing display unit as the old processors were getting harder to obtain; this was essential so we did not leave customers waiting for the upgraded version of the display unit and gave us the opportunity to upgrade older units coming back for repair.

Our customers gave us some important clues as to what features they would find useful in a new unit:

  • A smaller, lighter more ergonomic unit for daily use in the stockpile areas and trains
  • Increased number of calibrations that can be stored
  • Multi language support
  • Must be IP65 rated
  • Colour touch screen
  • Improved download capability from RS232 and Hyperterminal utility
  • Easy to navigate through menu options
  • Customised settings through password protection levels
New software and new display unit

During 2014 we set out to design the new product and software called AshGraffix with the customer needs in mind. The project time frame was 12 months from concept so we would be able to demonstrate the first unit at mining shows in 2015. The design for hardware, electronics and software was all done in house by the Parker team. Support from our local suppliers was critical as we went through several iterations of the printed circuit board design as this was the first time we had used surface mount components and touch screens. Finding low power components was key to the end result as the unit needed to work a full shift in some very harsh conditions from the heat and humidity of India and Vietnam to the low temperatures and dry conditions in Mongolia and Siberia.

Component list

Neil Jenkinson, our mechanical Engineer, selected an aluminium extrusion for the outer case that was rugged and could hold the printed circuit board and touchscreen. He found a superb membrane protector product that would sit on top of the touchscreen preventing scratching and potential failure if sharp items hit the screen by accident or in duality use, e.g a pen tip to tap the screen. The industrial cable connectors were retained as they had given good service over 20 years and readily available for after market sales. Different types of of battery packs were tested for durability and full function duration with industry standard Metal Halide rechargeable cells coupled with an intelligent charger selected as the best all round option for the AshGraffix.

As Human Machine Interfaces (HMI) have become more prevalent in the industrial world and availability for colour touchscreens has increased, we had a good choice of suppliers knocking on our door to show us their ranges. In the end a choice was made and the quality has remained high during the last two years of sales.

Circuit board Making Ash Analysis Simpler in the Field - White Paper, Parker Hannifin, Instrumentation Products Division Europe, New AshGraffix circuit board

The multi-layered printed circuit board was the most difficult item to develop as we wanted all the electronic components and connections to the outside world on a single board only slightly larger than the touchscreen. This task did not phase Chris Knight, our electronics engineer, who put in many hours working on the pcb design software to get the SMT components and tracks right on the multi layered design before it went out to prototype manufacture. The end result was a triumph for the design team.

Fig 2. New AshGraffix multi-layered single printed circuit board with new processor and using many surface mount components (SMT).


The next task that faced Kevin Corcoran, our intrepid software engineer, was to incorporate all our ideas into code. After many discussions and updates during 2014 (usually accompanied with tea and some form of cake - this was before the Great British Bake Off was aired to the nation, as we found this was the best way to inspire creativity), we designed the home screen, menus and navigation through the system with the question to the operator:

What do you want to do today?

The concept was to give the operator access to commence ash sampling with two taps from power up screen and standard icons used for configuration and saving files to the unit itself or to USB output.

Making Ash Analysis Simpler in the Field - White Paper, Parker Hannifin, Instrumentation Products Division Europe, Bretby Gammatech Ash ProbeWith additional processing power, came new features:
  • Ability to store up to 100 calibrations (from 9 in the old system)
  • Graphically represent the days ash analysis on screen
  • Multi language support which is so important for our overseas customers
  • USB connection for transferring data
  • Supports transfer of data across multiple versions (Windows XP/7/8/8.1 compatible).

Fig 3. Ash Probe and AshGraffix complete system.

During the first half of 2015 we were happy with the final design; software bugs had been eliminated and some test users had put the unit through its paces; only then did we go to the mining shows and offer the unit to the market.

In conclusion, the key take aways for updating the display unit were:
  • Do it for the right reasons, not just because technology allows it
  • Listen to the customer and put the key features into the product
  • Be open to new ideas and allow staff to be creative
  • Make something as a team you can be proud of and can sell!



Making Ash Analysis Simpler in the Field - White Paper, Parker Hannifin, Instrumentation Products Division Europe, Gary Wain, Product Manager, Bretby GammatechGary Wain is Product Manager, Parker Hannifin Manufacturing Ltd, Bretby Gammatech, Instrumentation Products Division, Europe







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Smarter Instrument Mounting Using Close-Coupling Techniques, Application Image, Instrumentation Products DivisionInnovations in the design of primary isolation valves and manifolds for mounting pressure instrumentation can deliver enormous pressure control advantages to both instrument and piping engineers, ranging from significantly enhanced measurement accuracy, to simpler installation and reduced maintenance. Parker Hannifin has created a comprehensive range of instrument manifold mounting solutions for the main types of pressure instrumentation, employing close-coupling techniques which eliminate impulse lines and tube fittings to improve overall instrument performance and reliability.

What is close-coupling?

There is no formal definition for close-coupling, but it has come to mean any instrument mounting system that enables a user to connect an instrument directly on to the process line, and primary flow control isolation valve. The overriding objective of this is to optimise the accuracy of measurement, by eliminating the long runs of tubing, tube fittings and bends and joints between process pipe and instrument that can cause pressure drops, and gauge/ impulse line errors.

Transmitter ‘hook-ups’ are often configured individually for each application, and can be large, heavy and difficult to install. By replacing such arrangements with purpose-designed close-coupled manifold/mounting solutions, users are able to optimise accuracy and reap a whole range of additional benefits such as...

  • Increased transmitter accuracy and repeatability by eliminating impulse lines
  • Reduced size and weight, less stress on the process pipework
  • Easy winterisation, no heat traced impulse lines to power up or freeze
  • Faster installation, and the option of transmitter assembly and testing
  • Reduced maintenance, and higher instrument up time
  • Elimination of threaded connections
  • Simpler design with less components and less leak paths
  • No risk of impulse lines blocking, as impulse lines are eliminated
  • Pre engineered compact design eliminates the need for impulse line field design
Faster process measurement

‘Hook-ups’ for pressure transmitters often involve the custom configuration of complex arrangements of tubing, with multiple connections and valves. Measurement errors can be introduced as a result of long length impulse lines. These errors are frequently compounded by the use of different tube, fitting and valve components whose diameters may vary throughout an instrument installation.

Inaccuracies can distort the pressure impulse signal, causing errors of up to 15% (on flow measurements).

Smarter Instrument Mounting Using Close-Coupling Techniques, Traditional Hook Up, Instrumentation Products DivisionTraditional ‘hook-up’ for a differential pressure transmitter

This traditional solution uses two sets of valve assemblies to create the double block and bleed valves, which are connected with impulse lines and connectors to the instrument manifold. It involves numerous discrete components, with all the associated costs and assembly time, and introduces bends that cause attenuation and turbulence that can affect measurement accuracy. If not carefully specified, other measurement accuracy problems can arise from differences in bore diameters of the various components, and unequal lengths of tubing.


The close-coupled alternative Smarter Instrument Mounting Using Close-Coupling Techniques, Close-Couples Technique, Instrumentation Products Division
  • Short and straight flow path to instrument from the process, increases transmitter accuracy.
  • Integration of the manifold in to double block and bleed valve, eliminates numerous leak paths.
  • No hook-up drawings or Bill of Materials are required to create an instrument hook-up.
  • Assembly takes only minutes compared to hours with a conventional impulse line system.
  • Low maintenance system, which is very easy to maintain if required.
  • Smaller and lighter assembly placing much less stress on the pipework.

View the Parker Close-Coupled Instrument Mounting System here.

Smarter Instrument Mounting Using Close-Coupling Techniques, Jim Breeze, Instrumentation Products DivisionJim Breeze is Product Manager, Instrumentation Connections and Process Valves, Parker Hannifin, Instrumentation Products Division, Europe.






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A wealth of worldwide opportunities, Oil & Gas Industry, Instrumentation Products DivisionThere are many different industries that work with pressures over 10,000 psi, ranging from the oil and gas industry to laboratories and waterjet applications. With such high pressures and often elevated temperatures involved, safety is critical and choosing robust, tested and certified tubing and cone and thread fittings is essential. In fact, engineers and specifiers should always start by identifying compatible tubing and cone and thread fittings that can withstand these severe conditions. Taking this approach not only ensures the safety and reliability of the process but ensures that buyers have a clear specification that enables like-for-like comparison.

Parker Autoclave Engineers

When our company was established some 70 years ago, one of our major markets was laboratories. Today our pressure vessels can be found in laboratories all over the world and are renowned for their ability to withstand extremely high pressures and temperatures. The parameters we work within are between -423oF to 1200oF and full vacuum to 150,000 psi using over 40 different metals to-date.

Over the past seven decades, Parker Autoclave Engineers technology has been used in an extensive range of applications from providing tubing and cone and thread fittings for fueling rockets for the space industry to supplying the same for use on oil & gas wells in the deepest oceans.

Emerging Markets

Over the years, new markets have emerged that Parker Autoclave Engineers has been able to service, for example, the plastics industry has grown significantly in the last 30 years manufacturing producing LDPE (low density polythene) plastic being produced by the petrochemical industry to make food wrap, grocery and refuse bags. The raw LDPE plastic is made at very high pressures which we help to control.


The waterjet industry is anoA Wealth of Worldwide Opportunities, Waterblasting, Instrumentation Products Divisionther growing market where our expertise in producing high pressure tubing and fittings is used. Waterjets are employed for a huge variety of applications today from cutting lettuce in a farmers field, to slicing cake, chicken, meat or fish, to cutting gaskets, carpet, marble and even steel up to 10” thick. Often the waterjet industry requires pressures of over 60,000 psi and our team at Parker Autoclave Engineers are continually developing new technologies that can address these ever more challenging requirements.

A closely related industry to waterjet is waterblast. This involves wide high pressure or rotating streams of water, which are supported by our tubing and fitting systems. Waterblast products are used in a variety of applications from removing paint from the hull of a ship and immediately ready for repainting, to cleaning rubber off aviation runways. And in even higher pressure applications it can be used to break up concrete on a highway, without damaging the rebar, ready for repair.

There are many good environmental reasons to use this technology, which has resulted in its ongoing growth and importance in the marine, aviation, construction, civil engineering and quarrying industries. New uses are continually being identified, opening it up for a number of other sectors.

In most cases, we supply directly to the OEMs (Original Equipment Manufacturers) who recognise the fact that we have extensive capabilities and offer high quality products, with more connection choices available than any other manufacturer globally. This has led to us becoming a market leader in all our key markets. As part of our service, we provide a consultative and technical supporting role for our customers, ensuring their staff are fully trained in the use of high and medium pressure tubing and cone and thread fittings when operating at high temperatures and high pressures for a wide variety of applications.

The variety of uses for high and medium pressure tubing and cone and thread fittings is just about endless and you will find Parker Autoclave Engineers components in an extensive range of applications globally, where safety and performance are essential. View the Parker Autoclave product range here.


Michael OA wealth of worldwide opportunities, Michael O'Keane, Instrumentation Products Division’Keane – Product Marketing Manager for Parker Autoclave Engineers.







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Metallurgy Makes or Breaks Tube Fittings - For over forty years, Parker has the lead the development of the chemical hardening process ideal for ferrules designed to grip and seal stainless steel tubing. The process is called Suparcase. This article reviews the importance of metallurgy and how Parker has utilized Suparcase technology. The best compression tube fittings balance hardness, strength, and corrosion resistance. Parker's Suparcase ferrule-hardening process does not require the high temperatures and long duration of more-conventional case-hardening procedures that, in turn, lower stainless steel's corrosion resistance.

Stainless-steel compression tube fittings make it easy to install and maintain measurement and control instruments used in chemical processing, petrochemical plants, and many other industrial settings. They seal a broad range of aggressive fluids and chemicals, and resist internal and external corrosion. The fittings grip and seal by compressing the nose of a ferrule into the tubing OD. High-quality compression fittings hold internal pressure without leaks or failure until the tubing fractures. And users can repeatedly disassemble and reassemble them with no loss of sealing integrity. 

Metallurgy makes or breaks tube fittings, Supercase ferrule-hardening process, Instrumentation Products Division












Today, compression tube fittings are available from many fluid system technology suppliers, and they tend to look the same although they may vary slightly in design details and manufacturing processes - but looks are deceiving.

The ferrule, perhaps the most-critical component in compression tube fittings, appears rather simple. Yet it is highly engineered and, to function properly, requires considerable design, metallurgy, and production expertise. Not all products on the market meet these stringent requirements. For instance, the ferrule must precisely deform elastically and plastically during fitting assembly to properly grip and seal the tubing. Its front edge must be harder than the tubing to grip and seal through surface scratches and defects, but if the entire ferrule is too hard, it may not deform properly. Therefore, only the gripping edge of the ferrule is hardened while the rest has different, tightly controlled mechanical properties. Also, the hardening process must not compromise stainless steel's corrosion resistance. And finally, production processes must consistently turn out defect-free ferrules that hold tight tolerances and maintain metallurgical specifications.

Design Evolution - One Ferrule to Rule them All

This article focuses on single-ferrule compression fittings, but many of the principles also apply to two-ferrule compression fittings. Ferrules were originally machined from cold-drawn stainless-steel bar stock. Cold drawing strain hardens the metal and imparts mechanical strength throughout the ferrule. But the ferrule's front edge was often still not hard enough to seal against tube surface defects such as scratches, weld seams, ovality, and hardness

One solution was to plate ferrules with a soft metal (such as silver) for a better seal when dealing with high-pressure gas. This improved resistance to impulse pressures, temperature swings, and vibration. Many ultra high vacuum and high-pressure seals deform hard edges into soft metal gaskets. Deforming the soft component with a hard one provides intimate metal-to-metal contact over the contact surfaces and overcomes surface irregularities. (A good source of detailed information is Industrial Sealing Technology, H. Hugo Buchter, John Wiley and Sons, 1979.) Manufacturers applied this concept to tube fittings by case hardening ferrules, which substantially increases surface hardness and lets them shear through surface defects and compensate for tubing variations.

Conventional gas nitriding case hardens the inner surface to a depth of approximately 0.004 in. During assembly, the ferrule front edge shears into the tube. If disassembled, the ferrule remains tightly locked to the tubing, allowing remakes with consistent sealing integrity. The fitting handles internal pressures, impulse pressures, temperature changes, and vibrations until the tubing fractures or fails in fatigue. However, gas nitriding (as well as carburization and carbonitriding) substantially lowers stainless steel's corrosion resistance. Process refinements let manufacturers harden only a band approximately 0.050 in. from the ferrule nose — sometimes termed a "limited nitrided" ferrule. This reduces the likelihood of corrosion, as the nitrided band is buried in the tubing surface. But it still poses a potential corrosion problem if, due to improper make up or surface defects, chemicals contact the band. Also, uninstalled fittings stored in corrosive environments, such as salt air, sometimes rust on the nitrided band.

Case Hardening Metallurgy makes or breaks tube fittings, Case-hardening process, Instrumentation Products Division

Conventional nitriding and carburizing require high temperatures for the hardening constituents, nitrogen and carbon, to penetrate the passive oxide layer that makes stainless steel corrosion resistant. The high temperatures permit chromium, an anticorrosion alloying element, to diffuse through the metal and form chemically stable nitrides and carbides. These compounds give the surface layer most of its hardness, but in this chemically combined form chromium no longer resists corrosion, and the nitrided or carburized layer corrodes in many environments, including seawater and even moist air.

In addition, nitriding and carburizing can "sensitize" austenitic stainless steel exposed to high temperatures for an extended time. Carbon, which has low solubility in stainless steel, precipitates as chromium carbides in the grain boundaries, depleting regions adjacent to the grain boundaries of the chromium necessary for corrosion resistance. This process is known as sensitization.

A new hardening process that was introduced by Parker Hannifin in the late 1980s does not reduce the corrosion resistance of stainless steel. More recently, some other fittings manufacturers have introduced ferrule-hardening processes with similar advantages.

These new processes do not require the high temperatures and long durations that permit chromium diffusion. This keeps chromium in solid solution as a corrosion-resistant alloying element. The hardened layer is continuous, free of defects and voids, as the process tends to fill surface inclusions and substantially reduce end-grain corrosion effects.

The new processes also do not affect the bulk metal. There is no sensitization or change in mechanical strength beneath the hardened layer. The ductile layer deforms with the ferrule during assembly without cracking or spalling.

In these processes, carbon supersaturates the hardened layer. Carbon atoms occupy interstitial sites in austenitic stainless steel's face-centered, cubic crystal lattice, strengthening the hardened layer. The hard crystal-lattice structure would like to expand to accommodate the carbon atoms, but is constrained by the unhardened substrate. As a consequence, high compressive stress further enhances hardness. Compressive stress has the added benefits of substantially increasing a ferrule's fatigue and stress-corrosion resistance.

In general terms, the process removes the passive oxide layer from the steel surface, letting carbon atoms diffuse directly into the metal lattice without traversing the passive layer barrier. The carbon atoms diffuse at lower temperatures than other alloying elements, thus avoiding problems caused by formation of carbides and nitrides.

Mechanical Action

A balance of metallurgical properties is critical to a ferrule's mechanical action during fitting assembly. For instance, the front edge of Parker Hannifin's CPI single-ferrule fitting shears down into the tubing, while the body arcs and clasps the tubing at the trailing edge. The front-edge grip prevents blow-out under pressure.

The ferrule must also work equally well across the tubing diameter tolerance range, typically ±0.005 in., and handle surface defects such as scratches that may be several thousandths of an inch deep. The arcing action turns the ferrule into a spring of sorts, letting it maintain tension against the tubing and the proper seat angle to seal despite vibration, mechanical shock, and thermal expansion. The back of the ferrule also loosely grips the tubing, damping vibrations that would otherwise transmit to the sealing interface.

Mechanical properties such as yield strength and hardness must be precisely controlled to effect this action. An extremely hard ferrule will be too stiff during assembly and will not bow and properly grip the tubing. But if it is too soft, the underlying material will not support the case-hardened surface. The result is an eggshell effect: the gripping front edge collapses during assembly and cannot hold the tubing under pressure. It also reduces the arcing spring effect.

Cold working is the only way to increase hardness and strength of Type 316 austenitic stainless steel after annealing. However, work-hardening rates change with the steel's composition, and constituent percentages can vary within an allowable range. Cold working can also reduce corrosion resistance. Thus, manufacturers must precisely control composition to maintain consistent mechanical properties and retain the austenitic structure, and case hardening must not uncontrollably change these.


Stainless-steel parts that rub together under high pressure have a strong tendency to cold weld and seize. And to form high-integrity, leak-free tubing connections, ferrules must only slide forward during assembly and not rotate with the nut. To prevent seizing and ensure only linear ferrule movement, engineers must precisely control surface conditions and lubrication at the nut/ferrule and nut/body interfaces.

All mating surfaces must be smooth and free of defects, which exacerbate seizing. A bonded molybdenum-disulfide coating is the recommended lubricant for many compression fittings. Solid molybdenum disulfide readily adheres to surfaces, is noted for its lubrication and anti-seizing properties, and the solid does not squeeze out like liquid or soft, waxy lubricants under extreme pressure. The result is low assembly torque and consistent performance, even with repeated remakes.


Metallurgy makes or breaks tube fittings, Jim Breeze, Instrumentation Products DivisionArticle contributed by Jim Breeze, product manager, Instrumentation Connections and Process Valves, Instrumentation Products Division Europe. 







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