Phosphate Rocks Chapter 32: Design

The great commissioning disaster began with the uprate to the ammonium nitrate plant. 

The design of the new ammonia supply was entrusted to Keith, a contract mechanical engineer who could quote British Standards verbatim but avoided eye contact. 

Keith was a tall, lean, distant man. In his early forties, he kept his curly brown hair short and his beard and moustache neatly clipped. He’d worked hard to progress from fitter’s assistant to graduate engineer, proud to have pulled himself up by his bootstraps from humble beginnings, attending night classes then labouring to pay his way through university. He had a low opinion of the men who had once been his team-mates, and an exaggerated respect for design codes, level-five project plans, quality systems, contracts, performance indicators and compliance. 

At the fertiliser factory, ammonia gas could go one of three ways. To be consumed by the granulation or Nitram plants as a small supplement to the main liquid feed, or to the refrigeration plant where it was cooled and returned to the storage spheres as liquid. 

The amount of gas available varied. On a sunny summer day, too much gas formed when a warm rail tanker offloaded into the cold spheres. The refrigeration plant had to be run at full power and the rate of tanker discharge controlled. On winter nights, when the temperature fell below freezing, a compressor was required to push the liquid ammonia from the rail cars into storage and no surplus gas was available. 

Both gas and liquid ammonia travelled in pipelines. The pipelines ran from storage spheres at the far north of the site, west to the Nitram plant or south to granulation. The pipes soared above the site as they crossed roadways and rail tracks, snaking across low utility buildings and around tall chimneys, kept out of harm’s way by a pipe bridge, a set of steel girders on tall concrete supports. 

Gas lines have to be much fatter than liquid lines to carry the same mass flowrate. A gas occupies more than a thousand times the volume of a liquid at the same temperature and pressure. You can reduce that volume by compressing the gas, applying Boyle’s law³⁶, (see Robert Boyle⁽ⁱⁱⁱ⁾) but at Leith the pressure was set by the operating pressure of the spheres. 

Keith’s first design decision was to eliminate the ammonia gas line to the Nitram plant. He wanted to use the existing pipe bridge and there wasn’t room. The old gas line was barely used, and it was occupying the space he needed for his new liquid lines. 

Keith made a simple calculation. The annual saving in electricity gained by sending ammonia gas to the Nitram plant, instead of turning it back into a liquid in the refrigeration plant, was £1,000 a year. The cost of a new pipe bridge was £20,000. A no-brainer. The new pipe bridge was removed from the scope of design and preparations made for the decommissioning and removal of the ammonia gas line. 

Mistake number one, although it wasn’t uncovered until after Keith vanished.