Published: 31 July, 2014
Achieving process safety and reliability in upstream and downstream oil, gas and chemical processing and power stations has tested the resources of system designers over many years.
Redundant valve systems are widely recognised as the optimum means of achieving this but until recent years these systems have only been available in ‘piped’ versions, involving multiple components and many potential problems down the line. However, significant investment by leading players in the valve sector has created a highly innovative new system bringing unprecedented benefits in performance and whole life costs. PWE reports.
Working with high-value but potentially dangerous materials such as crude oil, natural gas, chemicals and petrochemicals presents a unique set of challenges. Firstly there is the need to minimise unplanned interruptions to production, with costs of £1m per day in lost production alone not untypical. Meanwhile, the health and safety implications – and therefore the legal and reputational considerations – associated with mechanical problems or systems failure are considerable, involving operational and commercial costs which for most companies cannot be countenanced.
The solution where unscheduled interruptions are not allowable is a “two out of two” (2oo2) system containing two valves which allow media to flow even if one valve fails.
In safety-critical situations, where the need to immediately shut down processing is paramount, a “one out of two” (1oo2) system is normally fitted. Again, even if one valve sticks in the open position or fails, the other can be used to shut off production.
While most processing plants use one of these two methods depending on their own specific requirements, there are still some facilities which use a single valve for process control, introducing several issues. Not only is there no redundancy to shut off production if the valve fails, but there is no guarantee that production can be rapidly restored either.
At the other end of the scale, customers are now demanding systems which offer both safety and availability, driving the development of the first “two out of three” (2oo3) valve systems.
Current system options
Traditional redundant systems – whether 1oo2 or 2oo2 – usually have the valves bolted to a backplate and connected together using either stainless steel or hard tubes. However, once a configuration has been chosen, it cannot easily be altered (for example to a 2oo3) without the costly and cumbersome addition of another backplate.
Meanwhile, these systems present both safety and operational issues. Firstly, even if a system uses only SIL (Safety Integrity Level) approved components, the final system itself will not be SIL-approved. Poor design, incorrect specification and sub-optimal installation and configuration can all contribute to creating a system which may not meet local and international safety standards – very worrying given the potential safety issues associated with the system contents.
Meanwhile, the sheer volume of components used can itself create problems. Every additional component included creates further potential leak paths – even more so as distance between those components increases. Not only is this wasteful, but again potentially very dangerous. And of course each system will require considerable time to assemble, with testing required at each stage.
However, the issue of downtime becomes even more pronounced when unscheduled maintenance or replacement is considered. It is inevitable that the entire process will have to be shut down, even for minor maintenance to the valve system. In most instances, this will entail a minimum of a day of downtime when the time for restarting and flushing out the system is factored in. Daily costs for downtime can range from £100,000 to upwards of £1 million in lost production alone in ‘cracker’ plants where oil is being distilled into different products – to say nothing of, for example, damage to heating elements which can burn out if not being ‘fed’. Of course, even these costs can pale into insignificance when compared with the costs of a missed order deadline and the associated reputational damage.
Finally, with most piped 1oo2 or 2oo2 systems, there is no indication that one of the two valves has failed until or unless both valves fail.
Other options currently available include systems which allow individual valves to be bolted together – but which are far from simple to disassemble and are generally only suited to valve and filter regulator combinations. Meanwhile, ‘redundant control systems’ are frequently little more than a piped system in a cabinet. Not only does their large footprint create mounting issues, their operating temperature range is lower – typically only between 0°C and 50°C – while online servicing is not an option. Perhaps most importantly, though, they are only available in 1oo1 or 2oo2 configurations, making them unsuitable for safety-critical applications.
The redundant valve manifold solution
A system which addresses all of these issues is the redundant valve manifold system (RVM) from Norgren. Working in conjunction with lead customers, the development of the system drew on the expertise of Norgren engineers in both the UK Germany. It is based around proven technology in the form of Herion valves, with Maxseal valves redesigned to have the same footprint to allow for simpler integration. Phil Keating, business development director - Energy Sector, Norgren says this is the only system which can accommodate combinations of valves in this way, offering unrivalled diversity. He explains: “RVM is the first true manifold system commercially available for redundant applications in the energy sector. Available in both aluminium and stainless steel, RVM products are compact modular units which negate the need for pipework by incorporating all functions governing both safety and availability in a compact, low-maintenance, integrated valve control unit. The range offers all possible configurations – 1oo2, 2oo2 and 2oo3 – making it suitable for virtually all applications. It has just one inlet and one outlet, making installation rapid and simple, while minimising potential leak paths.”
The complete unit is SIL approved, and can operate at temperatures between -60°C and 90°C, making it suitable for the extremes of temperature commonly found in the global oil, gas and chemical sectors. RVM products use direct acting valves due to their higher force friction ratio (FFR) which increases the likelihood that the valve will operate when required, unlike some systems which use indirect valves. Given the extremes of temperature under which extraction occurs – friction necessarily increases at very low temperatures, while seals can become more ‘adhesive’ at higher temperatures, again increasing friction – this is a key attribute.
Each unit features visual valve status indicators and/ or electrical sensors to provide information on when a fault has occurred, enabling quicker maintenance and replacement. There are also options of a manual or electrical by-pass system to facilitate online servicing and testing of solenoid valves, without the need for a complete process system shutdown.
The range incorporates both standard flow and high flow models, meaning no requirement for a booster which may not itself be SIL approved, and again reducing component count. All RVM products have both TÜV and SIRA approval. Exhaust guards prevent particle ingress from the local environment, while cable terminations are inside the coil, meaning no additional Ex terminations are needed.
For further information please visit: www.norgren.com