A Dangerous Mix - How to Avoid and Handle Mixed-Pressure Connections

When designing a leak-free instrumentation system, one of the first steps to ensuring safety and reliability is to select the right tubing for the intended application. No system integrity is complete without this critical link and its compatibility with the rest of the components. In this post we list four key parameters to consider when selecting quality instrument tube for use with Parker A-LOK® and CPITM tube fittings.

Parker’s instrument tube fittings have been designed to work in a wide variety of applications that demand the utmost in product performance. Their compatibility with selected tubing is critical for providing consistently high-level of reliability.

1. Material compatibility

The most important consideration in the selection of suitable tubing for any application is the compatibility of the tubing material with the media to be contained. Table 1 lists common materials and their associated general applications. It also lists the minimum and maximum operating temperatures for the various tubing materials.

In addition, Parker instrument fittings are designed to work on like materials. Stainless steel fittings should be used only with stainless steel tubing, 6MO fittings with 6MO tubing, etc. The practice of mixing materials is strongly discouraged.

Read about Parker’s stance on using 316 stainless steel tube fittings on 6MO tube.

Table 1. Common tubing materials with their applications and operating temperatures.

2. Tubing hardness

The key is to select tube material which is softer than the tube fitting material. For example, Stainless Steel tubing should be specified as Rb 80 or less hardness value. Parker A-LOK® / CPI™ Tube Fittings have, however, been tested on tubing up to Rb 90 hardness level with excellent performance.

3. Wall thickness

Proper wall thickness is necessary to accommodate accepted safety factors relative to desired working pressures. Tube tables published in Parker’s literature, list the tube Outside Diameter (OD) sizes and Wall Thickness combinations per material for safe use with Parker A-LOK® and CPITM tube fittings. Do not use tube with wall thickness values which fall outside of the table ranges.

All working pressures are calculated following the recommendations contained within the ASME B31.3 Chemical Plant and Petroleum Refinery Piping Code and ASME B31.1, Power Piping. All calculations are confirmed via rigorous and extensive testing procedures at our Parker R&D Laboratories. Each calculation utilises an allowable stress figure that incorporates a 4:1 factor of safety.

All testing is carried out to replicate actual working conditions where possible. Parker does not ‘support’ tube to facilitate failure at certain points as this does not truly represent the forces that our products would see in ‘real-time’ applications.

Not all manufacturers support their assemblies’ pressure rating claims with extensive testing. In these cases it is important to understand the implications associated with using unverified recommendations.

Note: Gas Service

Special care must be taken when selecting tubing for gas service. In order to achieve a gas-tight seal, ferrules in instrument fittings must seal any surface imperfections. This is accomplished by the ferrules penetrating the surface of the tubing.

Penetration can only be achieved if the tubing provides radial resistance and if the tubing material is softer than the ferrules. Thick walled tubing helps to provide resistance and the table below provides a range of wall thickness options available for safe use with Parker tube fittings. The ratings in blue indicate the combinations of diameter and wall thickness which are not suitable for gas service.

Table 2. Maximum working pressures (in psig) for imperial tube in 316/316L Stainless Steel material based on tube O.D. size and wall thickness combinations.

4. Elevated temperatures

At elevated temperatures, a de-rating factor should be applied to the working pressure listed in the Parker tube tables. Please see the table below for the de-rating factors for the common materials Parker offers in our A-LOK® tube fittings range.

In all cases, tube fitting assemblies should never be pressurised beyond the recommended working pressure.

Table 3. This table lists the de-rating factors which should be applied to the working pressures for elevated temperature conditions. Simply locate the correct factor in this table and multiply this by the appropriate value in the pressure tables e.g. Table 2 (above) for 316/316L SS.

* Dual-certified grades such as 316/316L, meet the minimum chemistry and the mechanical properties of both alloy grades.

 

Register today for our industry-leading SBEx (Small Bore Expert) training and learn how to specify and install efficient, safe and leak-free small bore tubing systems.

Remember,

The Right Tubing + The Right Fitting + SBEx Training = High Integrity Solution.

Article contributed by Dave Edwards, product manager - tube fittings, Instrumentation Products Division Europe.

 

 

 

 

Related content:

10 Steps to Cost Effective & Safe Installation of Instrumentation Tube and Fittings Systems. Part 1

Medium Pressure Safety - All Tubing is Not Created Equal

Faster Tubing Connections for Medium Pressure Applications

A Dangerous Mix - How to Avoid and Handle Mixed-Pressure Connections

Why Choose Single Ferrule Tube Fittings for Vibration Protection and Temperature Cycling

Metallurgy Makes or Breaks Tube Fittings

Mixing high- and medium-pressure fittings or incompatible parts makes systems unstable, inefficient and potentially dangerous. At worst, it can cause a potentially fatal explosion.

At Parker, we’re all about making our products safe as possible and helping customers to minimise risk. Safety doesn’t happen by accident, so you need to know how to avoid the risk and what to do if you come across a mixed-pressure connection that could be hazardous.

Dangerous connections

Our engineers have recently come across situations where customers have created system connections using components with different pressure ratings. These instances are rare, but they can and do happen; creating unnecessary risks for everyone using the system.

If a Medium pressure tube is machined with High pressure tooling, this reduces the wall thickness below safe limits, and can lead to catastrophic failure. Similarly, combining a High pressure fitting with a Medium pressure tube means the male and female connections will not fit correctly. It may seem like the threads match, but the gland nuts will not engage to the required depth for the system to remain safe. The mechanical properties of the metals used in High and Medium pressure fittings are very different, meaning medium pressure units cannot withstand the application of high pressure.

Different pressure-rated fittings just don’t mix, and there’s no safe way to combine Medium and High pressure elements. Mixing different types of connections, product lines or parts from different manufacturers is also dangerous and removes any warranties.

Remember, just because something "can be done", doesn't mean that it "should be done"!

How it happens

The diagram below shows how the positions of collars and glands are different, depending on the pressure of the connection.

Fig. 1. The difference in positions of collars and glands in medium and high pressure connections.

Avoiding and reducing risk

We recommend that you never mix together:

• High pressure and Medium pressure Parker Autoclave Engineers parts
• Parker parts with those from other manufacturers.

All Parker Autoclave Engineers fittings carry our manufacturer’s name, part number, material, heat code and maximum pressure rating. We also make it easy to distinguish between different pressure ranges by clear notch markings on High pressure nuts.

Parker Instrumentation Products Division provides specialist SBEx (Small Bore Expert) safety training on correct and safe assembly of instrument connections to improve safety and skills across the engineering sector. For further support you can contact your local Parker Instrumentation Products distributor or visit our website.

Fig. 2. Notch marking on Parker Autoclave Engineer's High pressure fitting nuts provides differentiation from Parker's other pressure fittings.

Making a safe connection

If you need to assemble Medium pressure with High pressure in one system, we recommend the use of Parker adapters. For safety reasons, the Maximum Allowable Working Pressure (MAWP) rating is based on the lowest rating of any component (Maximum 20,000 psi).

Fig 3. Male-to-male medium-pressure to high-pressure adapter.

The second adapter (Fig. 4) overcomes the problem of the final rotational position not being predictable, making it safe and practical to use when positioning elbow/tee fittings and valves.

Fig. 4. Male-to-male adapter.

 

Download Parker Autoclave Engineers adapters technical catalogue.

If you find a mixed-build connection

If you come across a mixed-pressure arrangement, you should immediately isolate, remove and replace it with a proper connection using matching and compatible Parker parts. Otherwise, there is a real risk of failure or burst that could lead to catastrophic failure, resulting in injury or death.

Learn more about Parker Autoclave Engineers range of fittings and tubing suitable for low, medium and high pressure applications:

Parker Autoclave Engineers cone and thread fittings - up to 150,000 psi

Parker Autoclave Engineers Tubing - from 15,000 psi to 150,000 psi

 

Article contributed by Franck Grignola, product manager EMEA - high pressure, Instrumentation Products Division Europe.

 

 

 

 

Related Content:

New Faster Tube Connection Technology For Medium Pressure Applications

Medium Pressure Safety - All Tubing is Not Created Equal

A Wealth of Worldwide Opportunities For High Pressure Technology

Small Bore EXpert (SBEX) Training | Offshore Europe

NORSOK M650 Compliance – An Essential Prerequisite for the Offshore Industry

 

Securing leak-free connection of impulse lines to manifolds for applications that use differential pressure flowmeters is a subject that has taxed instrumentation engineers for more than a century. Back in 1910, when the very first orifice plate installations made an appearance, they involved 33 connections and 16 lengths of tubing! Thanks to the development of highly integrated manifolds, today’s installations often only require just two tube connections. However, ensuring the long-term integrity of these connections remains a contentious issue.

The first manifolds on the market used NPT taper threads for their tube connections. Despite being the bane of installers’ lives, this type of technology continues to enjoy widespread use today, with most manifold manufacturers still offering it as an option. But much better tube connector technologies are now available.

What’s wrong with taper threads?

Unlike compression type tube fittings with one or more ferrules, taper thread fittings rely on the threads themselves to provide the seal. During make-up, progressively larger diameter threads on the fitting are compressed into progressively small diameter threads on the manifold, until eventually there is no clearance left between the crests and roots of the threads and they effectively form a metal-to-metal seal.

NPT taper thread fittings are popular because they are relatively inexpensive, but they also have distinct disadvantages. The fittings cannot easily be installed with a specific torque, which makes it all too easy to crack or distort the female part by applying too much torque, or to apply too little, resulting in potential leak paths due to incorrect thread cling. There is also always some thread clearance due to manufacturing tolerances, which means that if the fitting is not tightened to the point where thread deformation creates a metal-to-metal seal, there is a spiral leak path. Furthermore, the upper and lower machining limits of NPT taper threads mean that there might only be two turns of thread engagement in an assembled connection; the most reliable means of preventing this is to use, if possible, a matched pair of male and female parts produced by the same manufacturer.

Taper thread can suffer from limited thread engagement

Another major disadvantage of NPT fittings is that their radial orientation cannot easily be adjusted without compromising connection integrity.

Most installers of NPT fittings elect to use some form of thread sealant to help prevent leaks. This usually comprises a fluid carrier which transfers a filler compound into the threads and then cures. Unfortunately, not all sealants act as lubricants and they are also very easy to misapply. Too much sealant can cause system contamination, which can result in unseated valves or blocked lines, while too little can allow the threads to gall (cold weld) during installation, requiring replacement of the entire manifold and tubing system. A further problem is that the fitting cannot be adjusted once the sealant has cured.

A popular alternative approach is to use PTFE tape as a sealant. This additionally acts as a lubricant during assembly and facilitates tighter connection of taper thread fittings – although it can lead to over-torquing. Another issue with PTFE tape is that it has a tendency to shred and cause system contamination. For this reason, its use is often prohibited in sensitive instrumentation systems.

Parker now offers two manifold connection solutions – PTFree connect and inverted A-LOK – which completely eliminate the need for taper threads, PTFE tape and thread sealant.

PTFree connect™

Our PTFree connect system provides a simple means of connecting impulse lines to manifolds without involving the use of taper threads, PTFE tape or thread sealant. Available as an option for every type of manifold valve block that Parker produces, PTFree connect offers different versions that accommodate metric tube sizes from 6 to 12 mm and imperial sizes from 1/4 to 1/2 inch.

 

    

Parker’s PTFree connect system is available on all types of manifold

Manifolds fitted with the PTFree connect system are exactly the same as their standard counterparts, except that their inlet/outlet and drain/test ports can be factory fitted with male adapters, supplied complete with a preassembled nut and ferrule(s). The (parallel) thread on the male adaptor is screwed into the manifold and uses the same type of stainless steel sealing washer as the valve heads, to provide a high-pressure leakproof and bubble-tight connection. The adaptor is securely locked by a cam or locking plate mechanism. We have used this connection principle well over a million times, so you can be confident that it’s a tried and trusted system.

Installation of PTFree connect manifolds is simple. A variety of connection bodies can be used – including straights, elbows and tees – and all angled components can be freely swivelled to facilitate secure positioning. And if anything does go wrong during installation, the sacrificial element is the male adapter – not the manifold – so the cost of remedial action is considerably less than with any other type of connection.

Inverted A-LOK fittings

More recently, we developed inverted A-LOK fittings, designed specifically for connecting impulse lines directly to manifolds. Like our PTFree connect system, these also eliminate taper threads and the need for PTFE tape or thread sealant, but they do not involve the use of adapters – the tube is fitted directly to the manifold. Each of the two manifold ports forms the female half of a connector and is machined with a cone-shaped orifice and a standard parallel (non-tapered) thread. Each male part comprises a tube and an inverted nut, with threads on its outside surface, which drives two ferrules forward during assembly; the pressure seal is provided by the front ferrule – not the threads of the connector.

Inverted A-LOK fittings facilitate direct-to-manifold connections

A key advantage of our inverted A-LOK fittings is that the tube is not twisted during installation – all make and remake motion is transmitted axially to the tubing. Since there is no radial movement of the tubing, it is not stressed and its mechanical integrity is not compromised. These fittings are suitable for both thin wall and thick wall tubing, can be used with a wide variety of tubing materials and accommodate repeated disassembly and remake. However, they still require careful installation. If the internal cone becomes damaged, the manifold block will need to be replaced. And due to cross-hole drilling in the block, the technology is not available on all types of manifolds.

 

Spencer Nicholson, product manager, instrument manifolds, Instrumentation Products Division Europe.

 

 

 

 

 

 

Related content:

NORSOK M650 Compliance – An Essential Prerequisite for the Offshore Industry

Ten Steps to Leak-free Connections in Oil and Gas Applications

Smarter Instrument Mounting Using Close-Coupling Techniques