Intrinsic safety versus explosion proof

Economics
Joe Kaulfersch, market analyst for Pepperl+Fuchs Americas says, “Most users believe the cost is higher and it is more complicated to have intrinsic safe circuitry.  However, the cost of ownership is much lower considering customers want to have higher system availability and be able to produce product without shutting down in addition to running the process safely. With Explosion Proof equipment the electronics must be locked out and turned off.” Unfortunately, with fieldbus systems this is not possible as they must be worked on live and connected to the network.

In part to help dispel these misunderstandings, MTL Instruments have prepared a publication “Cost comparison of methods of explosion protection” TP1110-3 that is accessible from their web site. Based on their analysis, the installed cost of an IS analog loop is 16% less than Explosion Proof while for a discrete signal this is 17% less expensive.

Despite claims that Intrinsic Safe installations allow Live Maintenance, in practice they, like Explosion Proof require the use of hot work permits because many of the maintenance tools used are not IS and the work itself has the potential of creating a spark of sufficient energy to initiate a conflagration. The MTL report investigated the potential impact of maintenance on ownership over a 10 year period by taking into account the effort required in permitting associated with maintenance, finding that for a discrete signal (38% cheaper for I.S.) and analog signal (34% lower for I.S) the savings continue.
Where are we today?

Herman Storey from a major oil and gas company states “We use neither explosion proof nor IS. Our plants are classified as Division 2 and neither EX or IS classification is required. I think plants in Europe will move to Zone 2 as well because of ATEX and the impossibility of complying with Zone 1 for mechanical equipment.”

Storey continues, “The reason we use Non-Incendive rather than IS is that IS simply doesn’t make any logical sense if you follow it through. You get into arbitrary safety factors that are compounded many times, and it gets expensive. Non-Incendive was designed with Zone 2 in mind and it makes good risk management sense.”

United Kingdom-based Chris Towle, states, “The only reason for sticking to explosion proof for instrumentation is inertia.” He then goes on to say “I believe the major change in the use of IS will come with the acceptance of the “ic” concept in Zone2/ Division 2 to replace the “non-incendive/ energy limited” concept. This should clarify the requirements for instrumentation in Zone 2 and lead to uniform practice applicable to all Zones.”

Dermot Coady, group business manager at MTL Instruments says, “One of the perceived limitations of IS is the unreliability of low powered solenoid valves. This was true in the early days of IS installations however the design and reliability of these devices have significantly improved to such an extent that's no longer considered an issue.”

When asked why IS is not being used in the US, Joe Kaulfersch replied, “In  North America most hazardous locations are considered Class I, Div. 2  which  very closely resembles the Zone 2 specification of Europe. The fact is many of the European factories are located very close to large populations. Even though Zone 2 classifications are relatively safe, there has never been an accident on record with an intrinsically safe installation. Insurance companies like to eliminate risks rather than mitigate risks. North American engineers are reluctant to use ‘new’ technology even though it is 47 years old.”

Storey summarizes the current state of affairs best. “I don’t think the USA will ever move toward IS,” he says;“It could go the other way if anyone ever tried to make sense of the problem.”