Underground gas transmission pipelines are externally wrapped in a protective plastic type coating. Gas temperatures in excess of 130°F to 140″F can cause embrittlement and eventual failure of this coating. For this reason, the usual industry practice is to specify that natural gas discharging into a transmission pipeline be cooled to less than 120°F. Also gas entering a pipeline is cooled to promote efficient glycol dehydration. For example, with an ordinary triethylene glycol dehydration unit, operating at a 900 PSIG contactor temperature, an inlet gas temperature of not more than 125°F is necessary to meet pipeline moisture specifications.
Natural gas effluent from a compressor is typically 150°F to 200°F. Wellhead gas from high pressure wells is also in this temperature range. Most often, gas is cooled in a fin-fan air cooler as shown in figure 5-1. The fan is rotated by a belt drive powered by a compressor’s engine. Alternately, the fan may be powered by circulating high pressure oil.
WHAT CAN GO WRONG
- Air cooling is deceptively simple. For instance, I have encountered
- the following problems while troubleshooting air coolers:
- Air leakage around the tube bundle.
- Fan speed too low.
- Belts loose.
- Fan blade pitch wrong.
- External tube fouling.
- Internal tube fouling.
- Maldistribution of gas in parallel tube passes.
- Excessive number of tubes plugged.
- Pass-partition baffle leaking.
- Excessive gas inlet temperature.
