Keep plant layout from laying you low

May 2, 2008
Spot mistakes before they literally become set in stone on the site

At the very first site at which I worked, an operator could see the guts of the plant from the safety of his control room. Everything was perfectly laid out. Since then, though, I’ve too often seen operators in dark cellars staring at equipment through hazy TV screens. Outside the control room plant pipes and cables were laid out in a disorderly fashion; equipment teardown frequently disturbed the normal flow of the plant.

So, how can plant layout be improved? Here’re a few thoughts on making plant operations smoother — from the beginning. First, identify equipment with poor reliability or likely requiring frequent attention. Next, decide if spares will be inline or from storage; pumps handling a sticky slurry might be an application where inline won’t work. Then, determine the wind direction, the best escape routes and supply requirements such as where to run a railroad spur. Look at parking, etc. Build from the inside out and then the outside in. If a pipe specification exists, establish the line sizes from the P&ID before attempting a layout; choose the smallest diameter and lightest pipe that will do the job but be wary of waterhammer and pressure drop.

Once you’ve rated the equipment for reliability and sized the pipe, it’s time to develop the plot plan — keep it simple. Here’re some ideas for cutting costs:

  1. anchor small equipment to the structure or larger equipment
  2. place equipment on the ground, if possible
  3. install high-maintenance operator-intensive equipment away from high traffic areas
  4. put sampling stations and instruments where operators will be safe
  5. locate the larger equipment and larger pipes and ducts first, then smaller piping and finally conduit, marshalling panels and electrical utilities
  6. create a run for conduit and pipes, usually in the middle of the plant, keeping it above and away from hazardous locations
  7. where possible, construct pipe manifolds and cluster instruments together to maximize serviceability and reduce the number of hazardous (Div. 1) areas
  8. keep pipe and conduit runs as short as possible to avoid problems such as low NPSHA, heat losses and signal interference
  9. for a fire-safe design, put pumps and compressors in areas of good drainage and ventilation, away from critical structures, pipe racks and equipment, and minimize flanges and other leak points
  10. where two-phase flow may exist, avoid rises or allow for traps
  11. put motor control centers as close as possible to control rooms
  12. provide for material storage, for turnarounds, quality assurance, deliveries and routine batch preparation
  13. install air coolers, cooling towers and other equipment affected by heat so as to take advantage of shade and natural cooling, e.g., the wind normally should flow across the packing in a cooling tower
  14. allow for growth in the facility — size the utilities, i.e., electricity, compressed air, steam, heating and ventilation, purge gas, fuel and even sewers and rail spurs, for future expansions.

After the first pass through the design, review the layout with operations and maintenance, together, in the same room. If you regularly use contractors such as riggers and crane companies, ask them to check the drawings for lifting requirements and scaffolding, removal and replacement of major equipment.

Several common mistakes often haunt plant designers. Foremost are those related to pipe racks. Too often, delicate instrument cables are run close above steam piping, parallel to power lines and in places where they are unshielded from the sun. Then, there’re plant expansions. A premium on downtime means that new wires seldom are added to old conduit. This means new wires and new conduit. Good sense dictates filling conduit to a maximum and allowing for future removal and replacement of rotting conduit, terminal strips and pipe. Plan for future tie-ins; one plant had a single shutoff valve in the steam system.

I’ve been burned more than once by designs that are blind to insulation needs. Design engineers ordering tanks frequently forget to allow for insulation lagging of flanges, manways and instruments — sometimes because these requirements aren’t known when the tank was ordered. However, lack of insulation can pose real risks unless the company enforces a safety standard on surface temperature. Operators have been burned to death attempting to squeeze between uninsulated pipe.

Another mistake is not considering the rigging of equipment for installation, now and in the future. Plan for trolleys and anchor points in the layout. I remember one project where lugs hadn’t been installed on some spools of a large ceramic-lined pipe. This oversight helped turn a four-day outage into one lasting 10 days. Another example: a tank to be welded to a structure came with legs.

The goal is to design a layout that improves the operability, safety and production capability of the plant. This requires an understanding of the needs of mechanics, riggers and operators.

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