Survey the Sources of Electrical Risks

June 14, 2011
A walk-down helps determine the need for purged electrical boxes.

My team was ready. We only had a week to define potential sources of hazards and complete a battery-limit Electrical Area Classification (EAC) drawing. We spent an hour walking down the unit. I completed the source spreadsheet in two hours. We reviewed the spreadsheet in another hour and then sent it to the designers. They finished the EAC drawing in two days. It took several months for us to become this proficient.


An EAC drawing is crucial for reducing the risk of electricity igniting flammable, even explosive, chemicals. As my last two columns have pointed out, the effort starts with defining chemical risk ("Don't Zone Out On Area Classifications"), followed by selecting zones around sources ("Don't Push the Envelope"). Here, I'll describe how sources demarcate the areas requiring purged electrical boxes.

The walk-down team, at a minimum, should include a drawing designer, a unit operator and an engineer. Before venturing into the unit, collect current versions of plot plans, an elevation drawing if available, Process and Instrumentation Drawings (P&IDs), Process Flow Diagrams (PFDs), the equipment index, material safety data sheets and material balances.

The purpose of this battery-limit survey is to identify "clouds" based on the worst-case sources. The walk-down may identify layout errors such as a source near a road or a walkway frequently used by operators.

Developing an uncluttered EAC drawing requires care in source selection. You want a point source posing the greatest risk defined by NFPA 497 and API 500. But it's not as simple as picking a Group B chemical over a Group D one. There are other factors -- e.g., the auto-ignition temperature (AIT), which define electrical risk. The most common Group B chemical is hydrogen, which is lighter than air. Lighter-than-air (LTA) materials have a smaller envelope. And hydrogen has a higher AIT than all Group D compounds; pure hydrogen's AIT is 1,040°F while, for example, naphtha's is 450°F. When both B and D chemicals are present, as in a knockout drum, define points by choosing a nozzle at the bottom for B (LTA) and an upper one for D. Use normal conditions -- if B wouldn't be at the drum bottom then, find a nozzle where it would appear.

Consider high risk factors such as temperature, quantity and pressure. Pick points based on the greatest risk: temperatures, e.g., within 30°F of flash point, sufficient to make a Class II or Class III compound a hazard (OSHA 1910.106(a)(18)(iii):Class II); flows greater than 500 gpm; and pressures exceeding 500 psig. It's best to select a number of points within 50 ft of the battery limit or 100 ft for high-risk sources.

Other factors -- for example, tall (i.e., over 25 ft) or long vessels (including heat exchangers), distillation columns and reactors where composition varies, and banks of equipment -- can complicate selecting source points. With long or tall equipment choose a point every 50 ft for heavier-than-air (HTA) compounds and every 15 ft for LTA ones. For shorter vessels, use one nozzle near the battery limit. Pick an upper flange for an HTA compound and a low nozzle for an LTA one. For vessels with diameters larger than 50 ft, refer to API 500. You can consider a flange or nozzle as part of a vessel only if it's within 10 ft. To measure the height of stacks and nozzles, I suggest a clinometer (see "Strive for a Surveyor's Eye").

Source designations include: pumps -- measured from centerline or seal connection; compressors -- same as for pumps; control valves -- pipe centerline; condensers -- bottom of tubesheet or shell flange; vaporizers -- top of tubesheet or shell flange; miscellaneous heat exchangers -- top for liquids, bottom for vapors; vessels containing liquids -- top-most flange; vessels containing liquids and vapors -- top-most flange and bottom flange; and vessels containing vapor -- bottom-most flange. Also, of course, account for furnaces, diked areas, etc.

Some surveys include flanges and manual valves but many restrict sources to automatic valves and control valves. The thought behind this is that staff will detect leaking manual valves.

After completing the walk-down, prepare the source table based on the points collected and eliminate any redundant ones. Then, run material balances to evaluate compositions. Review the source table at least twice before handing it over to the designers.

DIRK WILLARD is a Contributing Editor to Chemical Processing. You can e-mail him at [email protected]

About the Author

Dirk Willard | Contributing Editor

DIRK WILLARD is a Chemical Processing Contributing Editor.

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