Oil and gas production handbook An introduction to oil and gas production

part of a normal procedure 
•  Planned depressurization of subsea production flowlines and export 
pipelines 
•  Venting from equipment operating close to atmospheric pressure 
(e.g., tanks) 
 
The systems are typically divided into a 
high pressure (HP) flare and a low pressure 
(LP) flare system. The LP system is 
operated slightly above atmospheric 
pressure to prevent atmospheric gases 
such as oxygen flowing back into the vent 
and flare system and generating a 
combustible mixture. With low gas flow, 
inert gas is injected at the flare nozzle to 
prevent air ingress. 
 
Traditionally, considerable amounts of 
hydrocarbons have been more or less 
continuously flared. In these cases, a 
continuously burning pilot is used to ensure 
ignition of hydrocarbons in the flare. 
 
Stronger environmental focus has eliminated continuous flaring and the pilot 
in many areas. Vapors and flare gas are normally recovered, and only in 
exceptional situations does flaring occur. To avoid the pilot flame, an ignition 

---

120 
 
system is used to ensure safe ignition, even when large volumes are 
discharged. One patented solution is a "ballistic ignition" system which fires 
burning pellets into the flare gas flow. 
8.5 Instrument air 
A large volume of compressed air is required for control of pneumatic valves 
and actuators, tools and purging of cabinets. It is produced by electrically 
driven screw compressors and further treated to be free of particles, oil and 
water. 
8.6 HVAC 
The heat, ventilation and air conditioning system (HVAC) feeds conditioned 
air to the equipment and accommodation rooms, etc. Cooling and heating is 
achieved by water-cooled or water/steam-heated heat exchangers. Heat 
may also be taken from gas turbine exhaust. In tropical and sub-tropical 
areas, cooling is achieved by compressor refrigeration units. In tropical 
areas, gas turbine inlet air must be cooled to achieve sufficient efficiency and 
performance. The HVAC system is usually delivered as one package, and 
may also include air emissions cleaning. Some HVAC subsystems include: 
 
•  Cool: cooling medium, refrigeration system, freezing system 
•  Heat: heat medium system, hot oil system 
 
One function is to provide air to equipment rooms that are secured by 
positive pressure. This prevents potential influx of explosive gases in case of 
a leak. 
8.7 Water systems 
8.7.1 Potable water 
For smaller installations, 
potable water can be 
brought in by supply 
vessels or tank trucks. 
 
Photo: Lenntech Water 
treatment and air purification 
Holding B.V. 
 
For larger facilities, it is 
provided on site by 

---

121 
 
desalination of seawater though distillation or reverse filtering. Onshore 
potable water is provided by purification of water from above or below 
ground reservoirs.  
 
Reverse filtering or osmosis requires a membrane driving pressure of about 
7000 kPa/1 PSI of pressure per 100 ppm of solids dissolved in the water. For 
seawater with 3.5% salt, 2.5 MPa, 350 PSI is required. 
 
8.7.2 Seawater 
Seawater is used extensively for cooling purposes. Cold water is provided to 
air compressor coolers, gas coolers, main generators and HVAC. In addition, 
seawater is used for the production of hypochlorite (see chemicals) and for 
fire water. Seawater is treated with hypochlorite to prevent microbiological 
growth in process equipment and piping. 
 
Seawater is sometimes used for reservoir water injection. In this case, a