FOLLOW US >>

April 2015, Vol. 242, No. 4

Figure 1. Isometric View – Station 1

THE PORTABLE CLAMP­ON ULTRASONIC METER: A TOOLFOR GAS PIPELINE OPERATORSBy Carlos J. Castaño and Jorge L. Navarro, Promigas, Barranquilla, Colombia | July 2013, Vol. 240 No. 7 (/july­2013­vol­240­no­7)

The increasing rate of natural gas users andhigher gas prices in Colombia has createdthe need of considering better, moreaccurate measurement alternatives toreduce the uncertainty of calculated volumesin customer natural gas stations where afiscal custody transfer system is notavailable.

As known, commonly these technologies areused for transmission measurement:

• DP Flow Meters (AGA 3)• Turbine Flow Meters (AGA 7)• Rotary­Type Gas Displacement (ANSIB109.3)• Diaphragm Flow Meters (NTC 2728)• Ultrasonic Flow Meters (AGA 9)• Coriolis Flow Meters (AGA 11)

Promigas is a Colombian natural gas transmission company that delivers gas at 330 points on a daily basis. During thelast year, 85% was consumed by industries and gas­fired power plants. To estimate and calculate gas volumes,Promigas follows best practices such as the American Gas Association reports, Normas Técnicas Colombianas (NTC)and ANSI Standards Recommendations. This wide variety of reports and flow meters implies the use of state­of­the­arttechnologies and application of statistical techniques, not only for gas volumes calculations but the Lost andUnaccounted for Gas (LAUF).

Most of the efforts to supervise and control imbalances in gas pipeline systems ­ during a certain period ­ are focusedon the reliability of measurement systems, including SCADA availability; in fact, the time of unavailability plays animportant role in gas calculations.

Although most customer gas stations have meter by­pass systems or sometimes spools are installed to avoidinterrupting service continuity, they do not have in most cases backup flow meters to quantify the gas volumes. It isimportant to note that in Colombia it is not mandatory to use backup meters on delivery stations. It’s a commonpractice and a fiscal requirement in some European countries to have two custody transfer meters of different types onthe same line. The cost of having backup flow meters can be considerable if they are going to be used as a fiscalcustody transfer meter. At this point, as mentioned earlier, the use of another measurement alternative (flow meter orstatistical techniques) becomes necessary.

Contracts between Promigas and its customers include a clause in which there are four different ways of calculatinggas volumes when fiscal custody transfer system is not reliable:

Despite having these gas calculation alternatives, occasionally mathematical procedures do not produce suitableresults, mostly related to unscheduled deviations from historical consumption reported by clients, and one of the directconsecuences is the noticeable differences in gas balance results at month´s­end closing process.

A market research identified Clamp­On ultrasonic technology, which has been widely used in the water, liquid and gasindustries, as a tool for addresing the abovementioned contingencies. This articler provides a brief description of howPromigas has been using the GE Clamp­On ultrasonic meter to calculate volumes effectively delivered at gas stations.

Test Cases

(/sites/pipelineandgasjournal.com/files/Table1.png)

Two customer gas stations with the following operating conditions were selected to illustrate performance of theClamp­On:

Station 1The turbine used for custody transfer was scheduled to be calibrated at the Promigas Calibration Facility, which is partof the Promigas Metrology Lab and is a unique facility in South America capable of performing metrological tests toflow meters with natural gas at high pressure.

An alternate measurement system was necessary during the calibration test (four days); Clamp­On USM was set upand installed at this location. The first challenge was finding a site with the minimum requirements of straight­run pipe.According to the manual, the location of the transducer measurement point must be located with 20D (nominal pipediameter) of upstream straight­run pipe and 10D downstream straight­run pipe. In addition, the point should be at least10D from any butt welds or flanges. The reason why we called it a challenge is because usually gas stations are builtstrictly with the resources (pipeline, instruments, filters, etc.) in order to be more economically feasible. Figure 1 and 2illustrates an isometric view of the station and the place where the Clamp­On USM was installed.

(/sites/pipelineandgasjournal.com/files/pic1.jpg)

In this case, the turbine meter was going to be calibrated and two blind flanges installed so the natural gas could flowthough the bypass system. The Clamp­On was not located at the bypass system because there were someinstruments, like the axial flow regulator valves, which could produce noise signal in a frequency that affected theportable USM.

In fact, the Clamp­On was first installed in the bypass but some sort of interference was identified by the USM as E4(signal quality problem). The Clamp­On USM was installed on a vertical pipe with a 3­inch nominal diameter. The usermanual recommends installing the transducers at 180° on opposite sides of the pipe in a horizontal plane; however, italso states that additional pipe surveys may provide information that can lead to modifications for optimal installation.

Data Analysis Station 1As mentioned before, the measurement system was dismounted and there was no telemetry (SCADA) information onpressure, temperature, and other variables at the station. However, there were other nearby locations to monitor thevariables. Gas volume information was recorded by the Clamp­On in uncorrected volume conditions (ft3). In order toconvert volumes to base conditions, it is necessary to at least have the pressure and temperature data for a roughcalculation. For pressure, information for five days prior to the calibration was taken for forecasting. Figure 3 illustratesthe behavior of pressure in the system before being regulated at STATION 1 for a period of five days and a forecast ofone day.

As shown in Figure 3, the behavior of the pressure is seasonal and it can be forecasted with statistical methods. Thereare many Time­Series Forecasting Methods for seasonal data behavior. In this case, the data are seasonal but withno trend and the Additive Seasonality Forecasting Technique can be used (parameters α=0.8294, γ=0.9990). Theforecast was done using Crystal Ball Predictor software.

(/sites/pipelineandgasjournal.com/files/fig2_2.png)

The average deviation between real values and forecasted values was 1.2%, giving a suitable approach to the realvalues. An average for the same time period was used for the temperature to be used. Figure 4 shows the trendbetween the Clamp­On USM and the turbine meter after the calibration, and the results were satisfactory overall(average hourly differences: 1.3 ft3/h). A correlation factor that represents the difference between both measurementssystems was applied to the volumes recorded by the Clamp­On meter.

(/sites/pipelineandgasjournal.com/files/fig2b_0.png)

Station 2The following case is a performance comparison between the DP­Flow meter device and the Clamp­On. At thislocation, the minimum requirements of straight­run pipe were available. Figure 5 illustrates the place where the devicewas installed and the direction of gas flow.

(/sites/pipelineandgasjournal.com/files/fig5_1.png)

A black plastic covering was used due to the changing climate conditions (too much rainfall at night). It is important tonotethat a regulated pressure and temperature values were fixed at this station in the Clamp­On settings for flow rateestimations and in situ comparisons.

After four days of data recording, Figure 6 shows the gas flow trend between both gas measurement systems. Thereis a short interruption in the SCADA continuity and a straight red line shows this event.

(/sites/pipelineandgasjournal.com/files/fig6_1.png)

• Recording time: four days• Data recording interval: 60 seconds• Units: MMSCFH• Average pressure: 350 psig

• Average Temperature: 84 °F

ConclusionsUsing the portable Clamp­On ultrasonic meter in different applications corroborated its versatility, recognizing thelimitations that establish the minimum requirements for installation and metrological performance.

In Promigas this equipment has been used successfully to determine the consumption of customers in the absence ofcustody transfer system during maintenance, calibration or failures. Additionally, this technique has been applied toverify measuring flow patterns in gas delivery stations in order to identify performance problems or disturbances onsite promptly.

Our customers have accepted the results of these measurements, reaffirming its validity as a useful tool for gaspipeline operators globally.

AuthorsCarlos J. Castaño has worked for Promigas since 2002 and holds degrees in Mechanical Engineering and ProjectsManagement from Universidad del Norte. Among his responsibilities are control of lost­and­unaccounted­for gas,operation of measurement devices and controller training programs. He is certified as an Operations and MaintenanceInstructor by the National Center for Construction, Education and Research (NCCER). As a Promigas employee hehas been leading projects of OQ Rule, Online Simulation, Controllers’ Training and BCM.

Jorge L. Navarro is a gas controller who graduated as an industrial engineer from Universidad del Norte and hasworked for Promigas since 2009. His experience includes gas balancing and among his responsibilities are estimatingcustomer’s gas volumes using best statistical methods and the administration of the integrated gas platform softwareused by internal and external users (NTGAS 2.0).

RELATED ARTICLES

What's New: April 2015(/whats­new­april­2015)

(/whats­new­april­2015)

Gas Gathering, GrowthProjects... (/gas­gathering­growth­projects­driving­oneok­

investments)

(/gas­gathering­growth­

projects­driving­oneok­

Alaska: Gas ProjectCould Ensure... (/alaska­gas­project­could­ensure­

energyeconomic­future)

(/alaska­gas­project­could­

ensure­energyeconomic­

Welspun Changes, ButKeeps Focus... (/welspun­changes­keeps­focus­line­pipe)

(/welspun­changes­keeps­

focus­line­pipe)