Conditional Based Monitoring of an Three Column Gas Chromatograph

Abstract

Chromatography is an analytical technique which facilitates the separation of components in a mixture for subsequent use or analysis. On-line Gas Chromatography is frequently used within the oil and gas sector to obtain the composition of natural gas for the purposes of such things as allocation, fiscal reporting and sales/custody transfer. The composition measured by the Gas Chromatograph (GC) is therefore likely to come under the direct scrutiny of the buyer(s), the seller(s) and the regulator. The GC is widely known as a device that conducts its measuring functions recurrently and it is imperative that the instrument remains stable and repeatable. The requirement for high repeatability makes the setting of the GC valve timing fundamentally important to accurate chromatography and hence there is a requirement to detect the presence of potential drift before the measurement becomes compromised. The composition measured by the GC may be used to calculate various parameters, however, the gas calorific value is commonly used as the basis for financial transactions and as such the accuracy of this measurement is critically important. The author has performed a live test, which, in the absence of condition based monitoring, would have resulted in an undetected problem with the GC settings that would have caused an error of up to 1.4% in the calorific value. For the case in question, the monetary value of this error would have been in excess of £300,000 per month. It is very clear that failure to calibrate and operate a GC to its full potential can be very costly and result in significant financial exposure for all the parties involved. This paper presents a condition based monitoring strategy aimed at ensuring that the GC performance is maintained and that any potential problems are identified early to prevent erroneous measurement.