Flow Control, Flow Control April 2017

Cover Series Upstream Midstream Operations Dont blink Fugitive CO2 low Btu delivery in a shale gas pipeline Analyzer evolution for a changing industry S hale gas remains a phenomenon in the U S turning one of the most energy hungry countries on the planet into a natural gas exporter in a few short years November 2016 for instance saw the second ever shipment from the U S heading to China as the country worked toward a record level of exports The changing patterns of energy supply and demand are not only raising issues of quantity however but also quality Composition and parameters such as the calorific value that determine values can change daily Variation is the new normal For larger pipelines fed from multiple wells significant variations in the composition can be present Without accurate metering at entry and exit points operators cannot be confident supplies meet agreed energy British thermal unit Btu values and risk either contractual breaches or costly giveaways to end customers In some cases gas chromatographs the traditional equipment used to determine the composition of natural gas streams and identify the proportions of methane propane butane and oxygen are simply not up to the task Missing energy This can be illustrated by the case of a large U S pipeline company The pipeline has multiple points of gas entry with about a dozen inlet stations drawing from a range of suppliers in a regional shale production zone To ensure the natural gas meets minimum energy values for its customers the pipeline company requires suppliers to provide gas with no more than 2 percent of carbon dioxide CO2 To meet this each inlet has CO2 membrane style separators CO2 and hydrogen sulfide H2S are sent through a compressor to a second membrane unit from which the permeates are sent to flare while the residual hydrocarbons and nitrogen gas N2 flow into the pipeline see Figure 1 The company had an ultrasonic custody grade meter and gas chromatograph GC in place at each inlet These measured composition ratios and calculated energy content for Btu billing purposes The composition readings also fed a flow computer that was used to solve AGA 8 compressibility and yield a standard volume rate and total This in turn was used to calculate an overall energy rate and total Despite this the pipeline found customer deliveries consistently recorded lower total BTUs than the sum metered at the points of entry The quality of gas at the inlets was evidently falling below the required levels with greater than 2 percent CO2 yet this was not being detected by the GC In fact what was happening was that suppliers were randomly but regularly shipping gas with a CO2 content higher than the separators maximum capacity This was detected by the membrane protection system which monitors the pounds per square inch differential psid or differential pressure across the membrane It would then activate a valve to bypass the separation membrane section dumping the high CO2 content gas straight through to the pipeline until the psid fell back within the units capacity These bypass periods were brief usually no more than 10 seconds During this time however the CO2 levels were regularly up to 14 percent The result of the fugitive CO2 dumping was enough to significantly alter the composition of the entire pipeline flow By Charles Derr Honeywell Process Solutions Gas quality analyzer All graphics courtesy of Honeywell Process Solutions 14 April 2017 Flow Control