Heat Exchanger Cleaning Requires A Rethink

Readers point out all sorts of purging procedure problems

This Month's Puzzler

We have a simple system for purging heat exchangers at our refinery shop before cleaning them: an upstream high-flow pressure regulator and a pressure gauge, as well as a downstream purge valve and flow meter. We don’t bother to vent the exhaust to a flare stack because it’s such a small amount. We usually operate the purge at about 1 IWC. Last week, a small fire broke out as one of our welders was cutting away an old lug from a purging exchanger. We reckon a spark set off the fire. It was a hot day, about 100°F. Now, the safety group investigating the incident is calling it a near-miss. Did we do anything wrong? Is there a better way to store these exchangers until we can clean them properly?

Do The Job Right

There seem to be serious safety lapses in your shop. It appears that ASME Code Section VIII, Div. 1, and Section III referring to the “N” stamp weren’t followed because the shell was cut into. It doesn’t sound like a burning permit was issued — normally the safety department issues these. You didn’t follow the buddy system and there was no fire watch. In addition, the purge should have gone to a flare stack. I assume N2 was used for the purge although CO2, and even water could have been used. And why weren’t the inlet and outlet blanked off?

Here’s how you should do the purge. Use a calibrated purge flow meter. Test the pressure reading for the gauges and pressure regulator before use. Install an orifice — not a valve — at the outlet; valves can be accidentally shut off. Route a gas purge to the nearest flare and make sure there’s a check valve in the line to prevent backflow. If you use water, route it to the correct wastewater-treatment sewer. Tag the purged equipment so that nobody works on it without a permit.

As for the welding or cutting, follow a written procedure. The safety department should review all isometrics and a marked-up process and instrumentation drawing (P&ID) before issuing a hot work permit. Walk through a job plan, ideally on site, before beginning the work — this will improve situational awareness. When you prepare the job plan, make sure you specify the type of heat exchanger and its material of construction. Include the gaskets, the working and design pressures, the planned purge rate, the purge composition, e.g., for N2, ppm-mass air, the cleaning procedure, and the material safety data sheets and other data on the process fluids going through the exchanger. Lastly, review the work with your mechanical integrity group before the walkdown and review with safety.
William Bohach, president
MP Technologies, LCC, Savannah, Ga.

Comply With Regulations

I’m surprised you got away with this for so long. The first problem is with your field cleaning. The exchanger should have been capped, with all connections sealed and airtight, before being moved to the shop for further cleaning and repairs. Purging was unnecessary and risky. However, you’ve got worse problems.

By U.S. Environmental Protection Agency (EPA) rules, you should have been venting the exchanger to a flare — even a portable flare will do. Some localities require special permits but these can become routine if you fill them out enough. For EPA’s recommendation for a vapor recovery system for vessels, see: http://goo.gl/ymrpq4. A heat exchanger is probably too small for this system. In addition, many states require monitoring the concentration of volatile organic compounds. With your current setup, you should expect a visit from the refinery environmental officer.

So, how should you properly blanket a heat exchanger? Keep in mind that the heat exchanger is a vessel. Change the current inerting process to include a sampler inside the exchanger. You don’t want to sample too close to the exhaust because the sample there may not be representative. Use an O2 analyzer; it should be more accurate than a hydrocarbon monitor. Continue purging until the vapor in the exchanger is below 25% of the lower explosive limit. If you’re working on an exchanger while it’s being purged, monitor the area where the welding is being done. The hydrocarbon monitoring should be continuous and recorded.

I have a few more thoughts. How much time elapsed before the welder started cutting? You may want to run a simulation to estimate the “drying” time. Nitrogen will absorb a hydrocarbon. This can take several minutes or many hours depending upon the temperature and process pipe configuration. Because most hydrocarbons are heavier than air, vapors, unless heated, will hug the ground. Avoid welding or cutting near the ground; if you must cut from above, use a fire blanket to isolate the sparks in a canopy.

Here’re some interesting websites germane to this puzzle: http://goo.gl/wiKB9E and http://goo.gl/Uy55AW.

Dirk Willard, process engineer
A&B Process, Stratford, Wis.

April's Puzzler

My company has received an order to build a skid to make a mixed gel, an intermediate for a personal care product. The skid will include an agitated batch reactor where a powder is added to produce the viscous gel. This is pumped through a heat exchanger and then blended with a fragrance in a static mixer before going to an agitated storage tank. The skid boundaries were to end with feed to the storage tank but have expanded to include clean-in-place (CIP) and utilities skids; the customer insists this shouldn’t affect delivery We’re having trouble defining boundaries between the skids and have received nothing more than some product properties, i.e., viscosity, density and heat capacity at a single temperature. What can we do to ensure this equipment works as desired?

Send us your comments, suggestions or solutions for this question by March 13, 2015. We’ll include as many of them as possible in the April 2015 issue and all on ChemicalProcessing.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to Process Puzzler, Chemical Processing, 1501 E. Woodfield Rd., Suite 400N, Schaumburg, IL 60173. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication.

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