Integrated safety, control and hazardous area capability

08 November 2011

Addressing functional safety and regulatory control in a single system has been a challenge for many years – even more so when systems are destined for use in a hazardous area.

A technical solution from Siemens is said to offer these combined capabilities delivering benefits such as a simplified safety verification process, tangible cost savings and scalability Ian Curtis, safety consultant for Siemens Industry Automation, explains.

The functional safety and hazardous area protection world’s are often closely associated. However, when it comes to meeting the requirements of these two complementary yet distinct disciplines in a distributed I/O system, there are many technical challenges to overcome.

A range of SIL 3 capable failsafe I/O modules for the Siemens ET200iSP hazardous area remote I/O station gives users the potential for new safety system architectures which boast simplified engineering and a reduction in the total lifecycle cost for automation and safety.

Early process automation systems were typically distributed but, ironically, with the advent of the Distributed Control System (DCS), system architectures actually became much more centralised. In recent years, there has been a shift back toward a more distributed approach. This same trend has been reflected, albeit to a lesser extent, in distributed safety within the process industry but the recent addition of capability for integrated failsafe I/O in the hazardous area looks set to accelerate this trend.

Given the conservative nature of the industry there are still many users who prefer to stick to a centralised approach, particularly when intrinsic safety requirements are involved. The traditional practice of putting the controllers and I/O in the safe area and using IS barriers, is well understood and still in common use. However the tough economic climate of the last few years has increasingly prompted end users and OEMs to assess and adopt new concepts such as distributed failsafe systems which can actually solve many problems.

The scalability of these distributed systems, particularly those that combine control and safety in the same infrastructure, means they can also be used cost effectively for small process units, OEM skids or rotating equipment with smaller I/O counts.

The distributed approach reduces the need for multi-core cables carrying I/O signals; this means reduced installation effort; reduced risk of wiring errors and simplified bus connection of I/O stations.

SIL3 capability in a Zone 1 hazardous area is a step change that will strengthen the success of distributed I/O systems and really open up new possibilities. Users from the oil and gas industry, chemical industry, and other major hazard industries will look to benefit from the ability to combine configurations that include non-fail-safe modules, such as standard inputs/outputs and relay modules, alongside failsafe modules. Another key benefit is the potential for cost saving through the elimination the ex-barriers, less wiring and space optimisation.

Many OEM suppliers are also exploiting the possibilities of distributed automation in hazardous areas, particularly when they market their products to target emerging markets. If the end customer's employees lack expertise, the use of a centralised configuration often leads to wiring errors - and a lengthy commissioning phase.

When the ET200 iSP remote I/O station is located directly at the machine, or process skid, commissioning is straightforward and the space savings are considerable. As complicated and space-consuming as the earlier approach was – with remote I/O cables, terminals, and ex-barriers – this marshalling effort can now be completely eliminated. It is also easier to achieve the redundancy required in many applications: The ET 200iSP is connected via RS485-iS in hazardous areas. The path from the CPU in the control room to the field can also be redundant. Digital and high availability requirements are covered thanks to bus use.

Because of an increasing popularity with the OEM market, if an end user doesn't start out with distributed safety as a strategy for their plant they often “inherit” it as process skids and OEM type equipment come equipped with their own safety systems.

The first large customers for the ET200 iSP F-modules have been from the oil and gas industry. They have used the fail-safe modules in water-oil separating equipment and tank farms. Other early adopters have come from the chemical industry. Offshore projects generally also lend themselves to distributed safety and the combination of failsafe and hazardous area capability afforded by these new modules will be attractive for such applications.

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