I was building skids for a process in Canada that involved a combustible dust and a flammable liquid; however, my customer wasn’t concerned that we were using general-purpose equipment. Nor was the company owner, the project manager or even the process lead — who should have known better. As an expert in electrical area classification (EAC), I was wondering where to begin with less than 90 hours available for the project. I endured questions like: “Can’t I just add a high temperature cut-off?” Nope, the solution has to be intrinsic. And “Why can’t we move the equipment away from the explosive hazard so we don’t have to buy that expensive intrinsically safe stuff?” “Sure, just move it to Cleveland,” I replied in exasperation. As I emphatically made clear, we needed to re-design the powder feed system and the skids with equipment and instruments suitable for the fire hazard. When we were building the skids, there were two piles in the shop: one for NFPA-rated Class I, Div. 1 and 2 equipment, and another for general-purpose items destined to be returned to the vendors.
There’s still some confusion out there. So, let’s consider three important questions I didn’t cover in my first series on EAC that appeared five years ago (“Don’t Zone Out on Area Classifications,” “Don’t Push the Envelope,” and “Survey the Sources of Electrical Risks”).
1. While NFPA Class II liquids pose a hazard when heated near their flash points, do Class III liquids pose the same hazard? You might argue for leniency with Class III — see: http://goo.gl/t650Ti. However, until the U.S. Occupational Safety and Health Administration makes a decision, it’s best to err on the safe side and treat Class III liquids when they are heated within 30°F of their flash point as a risk like Class II ones.
2. Can I use general-purpose equipment and instruments by putting them in a purged box? Well, you can — so long as you meet three conditions for the box: 1) it must be purged with fresh air, i.e., air not contaminated with something flammable; 2) there must be a sensor to monitor if a flammable vapor is present; and 3) that sensor must trigger an alarm when the concentration of such a vapor exceeds a limit.
For an enclosure to be considered safe, it must satisfy two criteria: contain < 25% of the lower explosive limit (LEL) for all combustible vapor components, and undergo twelve air changes per hour for the volume of the enclosure. Refer to API RP-500, sections 6.3.2, specifically 22.214.171.124 and 126.96.36.199.5, respectively.
Some will argue that all you need to do is put an instrument in an explosion-proof box. However, that doesn’t mean the instrument will survive an explosion inside the box. It won’t be there when you need it. And, the safety of the instrument relies on the integrity of a seal that someone invariably will forget to check.
3. Is it important to consider temperature and pressure in classifying an area for electrical equipment and instruments? Yes, it’s crucial that you understand the nature of safety and environmental standards. Remember, though, they are guides. Don’t treat them as templates that absolve you of the need to think.
Suppose you have a Class III flammable liquid that is heated well above its flash point and flows at 300 psig in a pipe 8-ft above the ground; it’s heavier than air. Do you think the rating of a flow meter fifteen feet above should be general purpose or Class 1, Div. 1? By a liberal reading of API RP-500, general purpose is okay. However, my understanding of physics leads me to say no. Why? Flash points are above boiling points and hot gas rises. Use your imagination! Too often, engineers just walk-through hazard and operability checklists to evaluate safety risks. Safety managers developed the checklists to spur thinking, not to cover all contingencies. Ponder what else could happen. That’s why you have a brain. Use it wisely.
DIRK WILLARD is a Chemical Processing contributing editor. He recently won recognition for his Field Notes column from the ASBPE. Chemical Processing is proud to have him on board. You can e-mail him at email@example.com