Why does this matter? The Public Safety Regulations set out by the US Department of Transportation (DOT) – specifically regulation 192.625 (a) – require the addition of odorants to natural gas to assure that it can be detected readily in the case of leaks. It must contain odorant so that at a concentration in air of one-fifth of lower explosive limit (LEL), the gas is detectable by a person with a normal sense of smell.
Typical odorants are sulfur compounds like mercaptans such as tert-butyl mercaptan (TBM), organic sulfides such as dimethyl sulfide (DMS) or mixtures.
Gas chromatography (GC) technology provides the quantitative speciation of these odorants in natural gas to check the effectiveness of the odorisation and, therefore, public safety in the event of a leak situation.
New gas supply and delivery strategies
With the levels of odorisation capable of significant change in a short period of time as a result of shifting pipeline procedures, the emphasis is on the adequate odorisation of the gas so that the level is constant – and safety upheld.
Variations can be explained by several different factors such as the variation of pipeline diameters or the quality of the injection, depending on odorisation skid capabilities as shown in Figure 1.
Figure 1. Fluctuation of Level of Odorant Composed of TBM, MES and H2S in Natural Gas
LDC’s must therefore measure the level of odorisation of the gas and odorise it properly so that the level is constant; otherwise, customers’ natural gas could potentially arrive non-odorised or under-odorised, causing a public safety hazard, or even over-odorised – which could increase false alarm calls.
Manual testing sensors are being used to detect leaks because of their low cost and ease of use. However, their lack of accuracy and limited list of substances detected, often only TBM, dictated their replacement by gas distributors to provide reliable, certified results. Several different analysers have been – and continue to be – explored.
Based on its global performance, the energyMEDOR auto-GC analyser from Chromatotec was selected and installed at 11 major gates.
Equipment for real-time odorant level monitoring
The energyMEDOR is compliant with the ASTM D7493-14 standard and allows for the rapid, 15-minute speciation of all sulfur compounds generally present in natural gas as part of classical odorant additive formulation: tetrahydrothiophene (THT), hydrogen sulfide, DMS and mercaptans such as methyl-SH, ethyl-SH, isopropyl-SH and TBM.
This instrument is the only solution capable of analysing all previously mentioned sulfur compounds with direct measurement and accurate speciation. Where required, or at the customers’ request, it can be installed in safe or classified areas. Each instrument can be manufactured with purge systems or as an Exd version to meet IECEX, ATEX CSA or CSA International certifications, depending on each country’s specification requirements.
This designation has been achieved thanks to its flameless detector and the use of non-flammable nitrogen as the carrier gas; no hydrogen is required. The Electrochemical Detector (ED) included in the MEDOR® analyser only reacts with sulfur compounds offering remarkable sensitivity in the parts-per-billion (ppb) or parts-per-million (ppm) range.
Figure 2. Rack (left) and ATEX (right) version of energyMEDOR
Fully autonomous thanks to integrated nitrogen gas generators that eliminate the need for gas cylinders, and also an internal calibration system that uses a DMS permeation tube for automatic data validation, the turnkey solution offers a large range of applications without signal saturation thanks to the linearity provided by the MEDOR® detector; it can detect sulfur concentrations as low as 1 ppb and up to low-percent concentrations.
Moreover, the electrolyte solution used is valid for more than 10 years even with routine use, so maintenance requirements are very low.
Automatic odorant injection adjustment methodology
Elaborating on the challenges involved and how these were overcome, Chromatotec explained that to homogenise and adjust the level of odorant according to the regulation, dual pumps were set at different injection rates.
Pumps switched, depending on the analyser reading: when the original odorisation level was under 0.25 lbs/mmcf, the high injection pump (at 0.75 lbs/mmcf) was selected. On the other hand, when inlet odorant was 0.25 lbs/mmcf or above, the low injection pump (at 0.5 lbs/mmcf) was used.
The odorisation level was automatically adjusted without extra work. No low odorant or over-odorisation instances were observed, resulting in reduced maintenance compared to manual testing. A high return on investment (ROI) was achieved, Chromatotec concludes, with approximately 4,000 gallons of odorant saved per year – justifying the major motivation of LDC’s to expand the network of analysers to ensure reliable, accurate gas security monitoring.