A natural gas compressor station has several large compressors used to boost the pressure in a cross-country pipeline. The capacity of the pipeline is 500 million standard cubic feet (scf, with standard conditions at 1atm and 60F ) per day. The compressors are housed in a building 200ft long, 80ft wide, and 30ft high at the eaves. The building has a sloped roof, and if the volume of the compressors and ancillary equipment is considered, the net free air volume of the building is 510,000ft3. The temperature in the building averages about 90F, and the pressure about 1.0atm. The building has a ventilation system designed to operate at two speeds: the low speed provides a ventilation rate equal to 6 air changes per hour and the high speed provides 20 air changes per hour. The ventilation rate is based on entering air, measured at building temperature and pressure. The building is normally ventilated at the low rate, but a gas-sensing system monitors the air in the building. If gas is released in the building and reaches a concentration of 25% of the LFL, the ventilation system automatically increases to the high rate. LowerrlammageLinr (a) Assume one of the compressors develops a seal leak of 10,000 scf per minute of natural gas (considered to be pure methane) in the building. How long will it take for an alarm to sound if the alarm sounds when 25% of the LFL concentration is reached (which will then increase the ventilation system to the higher speed)? LFL manwen: 4.4% (b) What is the maximum concentration reached if the leak continues indefinitely? Ventilation will be at the higher rate. (c) If the gas leak is shut off when the concentration reaches 25% of the LFL, how long will it be until the concentration has dropped to 5% of the LFL if (1) ventilation is at the low rate and (2) ventilation is at the high rate? (d) If the leak detection alarm goes off at 25% of the LFL, how long will it be before the concentration reaches the LFL? Ventilation will be at the higher rate as long as the gas concentration is more than 25% of the LFL. Is that sufficient time for the source of gas to be found and shut down in time to prevent the concentration from reaching the LFL? (e) Calculate the energy released if the natural gas, which has a lower heat of combustion of 1044Btu/scf, is ignited and completely burned when the concentration of gas is at the LFL. Compare that energy to the equivalent mass of TNT, if the explosive energy of TNT is 2000Btu/lb. In your calculations (carry them out to 3 significant figures), you may assume the air and gas in the building to be completely mixed. Also, assume ideal gas behavior, since the pressure is low and the temperature is moderate
