Ignore Utility Piping at Your Peril

Don’t wait for a failure to draw your attention to such lines

By Dirk Willard, Contributing Editor

The packaging line produced about 450 boxes of cereal per minute. It was designed for about 600 boxes. With the plant down, we still consumed about 800 scfm of compressed air. Fixing the problems with the system and eliminating vented valves saved about $150/minute in lost production. That’s how important utilities like compressed air usually are. Yet, I’d wager the most neglected piping at your site are the lines handling utilities — compressed air, instrument air, nitrogen, steam, fuel gas, caustic, acid, condensate, refrigeration, water and sewer. If you don’t believe me, try walking down your utility process and instrumentation diagrams (P&IDs). I bet you don’t have working isometric drawings for utilities.

I bet you don’t have working isometric drawings for utilities.

So, the first thing you should do is prepare accurate process flow diagrams (PFDs), P&IDs and isometric drawings, if you can. Start with the PFDs, then the pipe rack plans, and finally the P&IDs. Next, perform approximate material balances, one for peak flow and another for normal flow. Be prepared for a shock. Remember the story about the old lady in Cleveland who plugged in her air conditioner and caused the blackout of 2003? Something like that could happen to your air system the next time somebody in your shop blows out a pipe. One way to prevent this is by using loops. Create alternative parallel pathways to feed demand; this works for air, nitrogen, steam, fuel and most other utilities. Another simple improvement is to shut off unneeded demand — that’s how I solved the problem with the packaging line. Lastly, remember some demands likely are over-estimated, perhaps unchanged since design, often with an arbitrary flow rate; cut these back.

Once the survey is completed and you’ve harvested the low-hanging fruit, the real work begins. Consider this riddle: What’s considered a gas but really isn’t? The answer is any gas stream with the exception of pure nitrogen. Walk down your pipe and look for dips. If there’s a dip, it’s collecting liquid and costing you pressure — eliminate the dip or install a trap. Apply the same thinking to vent lines involving gas and solids. I remember a series of fires in a spray dryer caused by collected solids in a particular spot in the vent; no amount of instrumentation can protect against settling — fix the pipe. Even fuel gas isn’t immune: when its liquid trap failed, a boiler operating on propane blew up, throwing debris across an Interstate highway. And while you’re eliminating traps, consider fluid dynamics. The compressed air system I fixed suffered from choked flow in a 60-ft section of 3-in. pipe. Check the Mach number for all sections of gas pipe, including steam, air, vents, etc. M should be less than 0.25. Use a simulation package that can handle two-phase flow and dynamics.

During your walk-down, look for pipe construction errors, i.e., pipe that’s inadequately supported, poorly designed, and expands until it breaks. Support spacing in feet for uncomplicated, uninsulated, black iron schedule-40 pipe should be at least 7 × (nominal pipe diameter)½ for water and 1.80 × (nominal pipe diameter) + 9 for gas (N2, air) for nominal diameters down to ¼ in. Pay particular attention to thermal expansion of tubes and small pipe — this often is ignored and can easily be accommodated by an expansion loop. I discourage hose and expansion joints. (For more on dealing with thermal expansion, see: “Properly Tackle Thermal Expansion Issues.”)

Steam and steam condensates pose an interesting problem. Condensate often includes three phases, not two, with gas-blown rust causing erosion, especially in softer stainless steel components chosen to fight the CO2 in steam.

Fuel gas can cause unexpected fouling, especially if sulfur compounds are present.


dirk.jpgDIRK 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 dwillard@putman.net

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