12 Safety


Hazardous Environment – Focus on the Area of Use for Device Selection


CorDEX Instruments Ltd. Unit 1 Owens Road, Skippers Lane Industrial Estate, Middlesbrough,TS6 6HE Email sales@cord-ex.com • Web www.cordexinstruments.com • Tel +44 (0)1642 454373


The phrase “intrinsically safe” is often used generically to describe products destined for hazardous (explosive) areas, when, in fact, the term is actually one of a number of methods for enabling a product to be used within a hazardous (explosive) environment. This kind of term is referred to as a protection concept.


Further examples of protection concepts are: • Explosion-proof, • Purge and Pressurised, • Encapsulated.


Unfortunately, the use of the term intrinsically safe as a catch-all term is used frequently, if incorrectly. For example, when searching for a camera designed for hazardous areas, the norm would be to search for “intrinsically safe camera,” where in fact there are cameras that use intrinsically safe, explosion-proof and purge and pressurised protection concepts.


This generalization causes problems at some levels. Users can be fi xated on obtaining an “intrinsically safe” device, when in fact the means or protection may actually be irrelevant; it is the environment in which the device is required to safely operate within that is important.


For example, the explosion-proof protection concept enables a device to be safely used within a Class I, Division 1 environment. However, intrinsically safe devices have a number of safety levels, meaning a device that is certifi ed as intrinsically safe may not be acceptable in the same areas as one certifi ed as explosion proof.


Basically, the area determines the device selection, not the protection concept. More on this later.


Protection Concepts If we address a few of the more common protection concepts, we can see they differ in the method used to create a device that is deemed safe for use within a potentially hazardous (explosive) area.


Explosion-proof


This was the fi rst method of creating a “safe” device. Explosion-proof, referred to in Europe as fl ameproof, is a means by which a device with higher power ratings is encased within an enclosure. The enclosure itself does not prevent explosive gas from entering, nor does it prevent a spark from occurring within.


The enclosure is designed to: (a) contain the initial overpressure caused by an internal ignition, and


(b) allow the expanding gas to escape in a controlled manner by means of carefully designed and calculated gaps known as fl ame paths.


Flame paths are not proprietary information. They are defi ned in the explosion-proof standards and have been calculated over the years as the minimum safe gap allowed for a specifi ed volume and intended target location.


The purpose of fl ame paths is to allow the expanding hot gas to exit the enclosure via a series of gaps. As the gas expands into the outside world via these gaps, it cools to the extent where it no longer has suffi cient energy to cause an explosion outside of the enclosure, hence rendering the facility safe. The downside is the equipment itself is generally heavy due to the strength required to contain any explosion. As such, the creation of portable explosion-proof equipment is generally not undertaken.


Figure 1: Typical explosion-proof digital camera


Typical examples of fl ameproof devices are motors and switchgear, whose energy is far above that required to ignite the area into which they must operate.


Purge and Pressurised


Purge and pressurised is a protection concept intended to effectively create a safe area within an enclosure into which uncertifi ed equipment is installed within “safe-area” equipment. In this method, an inert gas, such as nitrogen or argon, is fl ooded into an enclosure creating a non- incendiary environment.


Purge and pressurised specifi cs change from location to location, however, in some instances, a control system is required to monitor the gas pressure. If a leak in the inert gas is detected by way of a pressure drop, the control system automatically shuts down power to the enclosure, de-energising


AUGUST / SEPTEMBER • WWW.PETRO-ONLINE.COM Figure 2: Typical intrinsically safe thermal imager internal components rendering them safe until gas is refi lled.


The use of the term “purge” in this protection concept is of particular interest here. Once gas pressure is lost and the enclosure is ready to be refi lled, it must be purged with inert gas for a predetermined period in order to reduce the residual oxygen to safe levels. In many purge and pressurised systems, there are pockets that cannot be adequately purged by a single tapping, therefore a gallery made from a tube or series of tubes may be used to “chase” out oxygen collecting in hard to reach areas.


While purge and pressurised may seem like an obvious route for lighter product design (no need to contain an explosion), the sealing requirements and the need for some local codes for purging make it more prevalent in fi xed equipment.


Typical examples of the purge and pressurised protection concept would be control panels, although some cameras have been manufactured and certifi ed using this method.


The upside of purge and pressurised is a much lighter enclosure design. The downside is the inert gas is a consumable, so portable equipment using this concept requires recharging and repurging.


Encapsulation


Encapsulation effectively attempts to perform a similar function to purge and pressurised in that the explosive atmosphere is prevented from reaching critical components or equipment by means of hard setting encapsulant.


However, this encapsulant must be of a particular type, which itself must be tested to withstand signifi cant environmental stresses intended to simulate a lifetime of fi eld use. These tests are by no means simple and an entire design can be failed based solely on the material used to encapsulate it.


Additionally, accessibility to the circuit (in the context of an electronic product) is sacrifi ced as the protection concept is based totally on coverage by the encapsulant. On the positive side, there are minimal circuit restrictions once the encapsulant has been tested.


Encapsulation is not accepted in North America as a protection concept.


Intrinsically Safe Intrinsically safe as a protection concept is defi ned as limiting the electrical energy available for ignition. It is sometimes assumed, albeit incorrectly, that this refers only to voltage and current limitations.


In reality, electronic circuits are not only severely limited on voltage and current, but also on inductance and capacitance coupled with strict physical requirements to prevent short circuiting of safety components and/or individual components exceeding the intended auto-ignition temperature of the target environment under a fault condition.


With an intrinsically safe circuit, the battery itself must be tested separately. The use of higher voltage Lithium Ion batteries is rare since they can explode under a fault condition. More commonly implemented secondary (rechargeable) cells use NiMH chemistry which is less volatile.


Many intrinsically safe devices use primary (non- rechargeable) cells. This is more of an advantage as in recent years the chargers themselves must be deemed “associated” devices and be certifi ed as part of the entire product. There is a certain logic here; if a standard battery charger is used on a certifi ed


battery and causes damage, how can we be sure the battery remains safe? The answer is we can’t be sure, therefore, the charger itself must now be tested and certifi ed.


More restrictions!


Over the years, I have heard many highly intelligent, educated people refer to intrinsically safe as a “wrapper” for a non-intrinsically safe device. If only this were true!


To illustrate, let’s just say we have jumped through the hoops and now have an electronics design that complies with the standards and is intrinsically safe. It would be simple to assume that because the electronics are now safe, the packaging is irrelevant.


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