A tracer dilution method based on steady-state tracer concentration at constant injection rate has been designed and implemented successfully at Imperial Oil's Quirk Creek gas plant to measure flare gas flow rate which, together with the H2S content, is required for renewing the clean air operating licence. The gas flow rate was determined to be 2.06 ± 0.05 Sm3/min or 2.48 ± 0.05 m3/min at the stack conditions, and the H2S concentration was averaged at 13.7 ± 2.9% over a period of six hours. The results agree with those obtained by simulation and mass balance. This is the first known reliable method for measuring flare gas flow rate in a low pressure, low flow rate and extremely sour environment. The method is well suited to accurate measurement of gas, oil, water flow rates in refining as well as production facilities. Complications due to tracer partitioning into the discharged liquid and dissimilar gas and tracer allocations in a complex flow field are discussed. A method for simultaneous measurement of flare gas and liquid discharge rates in a liquid knock-out tank using partitioning tracers is also presented.
In the Quirk Creek gas plant, sour glycol and produced water constitute the two major sources contributing H2S to the flare gas stream. In the gas sweetening unit, sour gas is dehydrated by glycol before entering the amine absorber. The dissolved sour gas and water vapour are removed from the spent glycol in a subsequent glycol re-generation cycle. The separated gas is combined with the sour produced water overhead gas and gases from various other sources and then piped to a liquid knock out tank. This mixed gas stream which has a H2S content as high as 40% is flared with a valid clean air operating licence. The current licence was to expire in May 1993 and, in accordance with the government regulations, the daily sulphur emission rate has to be included in the licence renewal application. Sulphur emission calculation requires the knowledge of H2S concentration and the flare gas flow rate. While the H2S content can be accurately measured by gas chromatograph, there is no suitable gas flow meter which can be used in a low pressure, low flow rate and highly hazardous environment. A tracing method was thus developed by for measuring gas flow rate at such conditions.
Gas flow rate can be determined by a steady-state tracer dilution method. A gas tracer with high vapour pressure which can completely flash into the gas phase is injected at a constant rate into the gas stream and the tracer concentration is monitored downstream of the injection point, preferably near the gas outlet. The gas flow rate can be derived from the steady-state tracer concentration and the tracer injection rate. The tracer dilution method has been used successfully in measuring gas flow rate through compressor(1). The tracer results agreed with those measured by orifice flow meters within an accuracy of ± 1%.
