Posted by Process Control Team on 25 Jul 2017
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:
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.
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:
Article contributed by Franck Grignola, product manager EMEA - high pressure, Instrumentation Products Division Europe.
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.
Industrial settings with strenuous applications, such as thermal cycling and vibration, are recognised as some of the toughest environments for tube fittings. In all cases such applications demand highly precise, leak-free technology to ensure protection from vibration and thermal cycling. For example, in power generation, nuclear and chemical plants, worker and atmospheric safety is business-critical.
Engineers face numerous options when choosing tube fittings for instrumentation, process and control systems and equipment. In most cases, buyers need a solution that will minimise risk, prevent leakage and provide robust value for money.
In instrumentation installations such as impulse pipework, typically a three piece (single ferrule) or four piece (double ferrule) design is preferred. While both types of fittings have their own merits, the four piece fittings are slightly better known in the market. But in some cases, there are definite advantages for choosing a three piece, single ferrule design.Benefits of single ferrule designs
With just three pieces required - a nut, body and single ferrule - this type of fitting is easy to install. The ferrule design also provides a strong anti-vibration hold on the tube.
Single ferrule fittings typically offer six core benefits:
Many engineers constructing fluid or gas handling systems aim to eliminate potential leak paths. This approach also applies to the design of tube fittings.
With single ferrule fittings such as CPI™, only two potential leak paths need sealing. But double ferrule fittings have three or more paths to seal, depending on the fitting design and composition.
2. Sealing mechanism
With double ferrule designs, the front ferrule contacts the body seat over the entire surface. This causes the end load to be distributed over a wide area.
With Parker’s CPI™ design, there is less body seat contact; the end load is concentrated over a smaller area. Higher contact pressure enhances the ability of the single ferrule solution to seal low density gases, which means it can out-perform its double ferrule counterpart in some applications.
The CPI™ fitting also has the benefit of the one-piece ferrule, utilising Parker’s unique Suparcase™ system. This offers better corrosion resistance and is less likely to be damaged in service; but it’s not available on the front ferrule in twin ferrule tube fitting designs.
3. Dampening vibration
Tubing vibration can affect the ferrule seal. However, with single ferrule designs, a light compression grip at the rear of the ferrule isolates seal points from system vibration. This creates a cushioning effect, providing excellent vibration resistance – again, a potential advantage over double ferrule designs.
4. Better performance in temperature cycling
During temperature cycling, metals naturally expand and contract; this causes dimensional changes in fitting connections. With single ferrule fittings, the ferrule features a ‘spring loaded’ effect; this creates a constant tension between the fitting body and nut.
The spring action compensates for cycling changes. By storing excessive force in the bowing action of the ferrule, it maintains effective sealing points. There is no corresponding spring action with double ferrule designs.
5. Ease of installation
The CPI™ single ferrule tube fitting uses a Molybdenum Disulfide coated nut, which reduces the torque required to make up the assembly by as much as 40%. This coating also gives the added advantage of precise and consistent re-makes. As a result, the fitting can last longer in service - reducing potentially costly maintenance work.
6. Precision assembly
Both single and twin ferrule designs can potentially seal all leak paths, as long as they are assembled precisely. With double ferrule fittings, there’s scope for incorrect alignment, lining fittings up backwards, or leaving a ferrule out completely.
Ultimately, with fewer components, there is less that can go wrong; single ferrule solutions are quicker and easier to assemble, so the scope for error is reduced. With Parker’s CPI™, fittings are sold completely assembled and ready for immediate use.
Offering superior corrosion resistance, the CPI™ product series is manufactured to the highest quality standards and available in a range of sizes, materials and configurations. There is documented heat code traceability on stainless steel fittings for nuclear and other critical applications.
Article contributed by Dave Edwards, fittings product manager, Instrumentation Products Division Europe.