Flared gas

Product quality is not limited to oil and gas quality certain effluent streams will also have to meet a legal specification. For example, in disposal of oil in water, the legislation in many offshore areas demands less than 40 ppm (parts per million) of oil in water for disposal into the sea. In the UK, oil production platforms are allowed to flare gas up to a legal limit. 

In some undersaturated reservoirs with non commercial quantities of gas but too much to flare, gas has be used to fuel gas turbines and generate electricity for local use. 

Gas flaring in offshore installations and oil refineries represents a source of loss of energy making it important to operators and authorities to monitor the amounts of flared gas. In some countries the flare gas is subject to CO2 tax. Flow metering systems are installed on some but not all flare systems. 

The gaseous tracer method yields the equivalent piston flow linear velocity of the gas flow in the pipe without any constraints regarding flow regime under the conditions prevailing for flare gas flow. 

The method is based on the international standard ISO 4053/IV. A small amount of the radioactive tracer is injected instantaneously into the flare gas flow through e.g. a valve, representing the only physical interference with the process. Radiation detectors are mounted outside the pipe and the variation of tracer concentration with time is recorded as the tracer moves with the gas stream and passes by the detectors. A control, supply and data registration unit including PC is used for on site data treatment... 

The main sources of error which define the accuracy are counting statistics in tracer concentration measurements, the dispersion of the tracer cloud in the flare gas stream, and the stationarity of the flow during measurements. 

Vapor recovery systems floating roof tanks pressure tanks vapor balance painting tanks white Cyclones-precipitator-CO boiler cyclones-water scrubber multiple cyclones Vapor recovery vapor incineration Smokeless flares-gas recovery... 

Recommended nominal steam rates at 60 m/s exit velocity for a typical flare tip are shown in Figure 2. At lower velocities, higher steam ratios are required. Typical steam control consists of a flow ratio controller with adjustable ratio set point, related to flare gas flow. The ratio adjustment, located in the control house, provides for the higher steam ratios necessary at low flaring rates. 

The topic of flare gas measurement is treated below. If necessary, continuously vented surplus low pressure steam can be used for smoke control at low flaring rates, with high pressure steam cutting in through a flare ratio controller designed to handle large releases. 

Smokeless Center Steam Cheapest steam-injection flare tip. Steam jet emerges at high velocity and penetrates to the exit plane of the flare without mixing completely with flare gas. Results are intense steam noise (much greater than with steam ring for the same steam rate) and higher steam consumption than the steam ring. 

Ts = Average stack gas temperature, °K. This temperature depends on the heating value of the flare gas and the percent excess air. It may be assumed that Tj = 1538 C = 1811 K, which is a reasonable approximation and is further justified by the fact that Equation 4 is relatively insensitive to changes in T. ... 

The primary air flow rate per jet necessary for smokeless combustion depends on the molecular weight and degree of unsaturation of the flare gas. Experience indicates that it varies linearly with percent unsaturates, from a minimum of 20 % excess air for a flare gas containing 0 % unsaturates to 35 % excess air for a gas containing 67 mol % unsaturates. Based on this relationship and a gas flow rate of 72.2 mVh per jet, the required primary air flow rate can be computed directly from the gas composition, or approximated conservatively from the following equation ... 

The vapor space is sized to avoid water entraimnent in the flare gas. As a rule, vapor velocities in the drum should not exceed 150 % of critical. This however can be increased to 230 % critical velocity) when considering... 

If HjS is never present in the flare gas, seal water effluent should be routed through an open fuimel (to permit checking of seal water flow) to a manhole in a vented section of the oily water sewer. The water inlet to the manhole must be sealed. 

If HjS is continuously present in the flare gas or if the flare seal drum also functions as a sour water disengaging drum, then the effluent seal water must be routed to a sour water stripper, desalter, or other safe means of disposal. Withdrawal from the drum is by pump in place of the normal loop seal arrangement. Two pumps are provided one motor driven for normal use, and the other having a steam turbine drive with low pressure cut-in. The seal drum level is controlled by LIC with high and low alarm lights plus an independent high level alarm. 

Flow surges in the seal drum are likely generated by the cyclic formation of large bubbles as the flare gas is discharged into the drum. These pulsations can be virtually eliminated by the use of a horizontal sparger ineorporating many small diameter holes arranged specifically to allow the open area to increase as flow increases. These holes must be spaced sufficiently far apart to avoid interference between bubbles. 

Metering flare gas is important for loss accounting and for control of steam injection. A special requirement for flare gas meters is low pressure drop and the ability to continue functioning in fouling conditions. The flare gas metering methods listed below have been used with varying degrees of success. 

Anemotherm - The Anemotherm, a hot wire anemometer, has been extensively applied to flare gas measurement and has been successfully used after modifications to reduce probe fouling. A technique used to reduce fouling is the installation of a steam coil around the tip of the probe, to prevent gases from condensing on the probe. 

Pitot Probe - A simple pitot probe with "bouncing ball flow detector gives a qualitative indication of flare gas flow and can be used in the identification of leakage sources. 

Other types of flowmeters that are applicable to flare gas metering include... 

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