Automatic Custody Transfer System for Unmanned LNG Station

INTRODUCTION

Natural gas (NG) has become an important part of the energy mix, particularly in electric power generation, where it’s prized as an easily handled, clean-burning liquid. Liquefied Natural Gas (LNG) is a more cost-effective way to bring gas to market across oceans and continents. Construction of many overland pipe-lines is underway, but gas companies are also investing in LNG supply chains, building massive LNG tankers to convey gas from port to port, and terminals to convert gas to its form for easy transport, and back to gas for consumption. In this Paper, we’ll analyze the challenges of Flow measurement at LNG service applications.

COMMON LNG FACILITIES POSE METERING CHALLENGES

Flow meters used in LNG custody transfer should have the following as minimum requirements.

• Low Pressure drop to avoid LNG vaporization.
• No moving Parts.
• The ability to perform in the cryogenic applications.
• Proven ability & accuracy in liquid custody transfer applications.
• As in custody transfer applications flow measurements also need to be verifiable with    check meters and provers.

EVALUATING FLOW METERS FOR LNG MEASUREMENT

Two different meters can satisfy the previous said requirements.

• CORIOLIS
• ULTRASONIC

From field tests conducted at an LNG facility in Alaska, users were able to determine how accurate the meters were by comparing flow measurements to level measurements in the LNG dispensing tanks and onboard a receiving LNG tanker. Under test conditions both meters offered similar accuracy and good repeatability.

WHAT IF A CORIOLIS FLOW METER IS INSTALLED?

•       When a coriolis flow meter is installed, typically dual bent-tube designs are used to ensure the highest possible performance, causing pressure to drop and the risk of vaporization resulting multiphase fluid, which makes it difficult to measure and can introduce significant measurement uncertainty.
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•            Coriolis meters, while immune to changes in flow profile, are only available in diameters up-to 10” sizes.
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•             While a coriolis meter is a good choice for smaller line sizes and highly accurate in mass measurement several flow meters would be required to maximize throughput.

WHAT IF AN ULTRASONIC FLOW METER IS INSTALLED?

•       Ultrasonic flow meters on other hand, have a straight flow path and do not obstruct or impede the gas or liquid flow through a pipeline.
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•             The result is essentially a no pressure drop and reduced risk of vaporization, results in good accuracy of measurement.
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•       They are also available in larger diameter sizes than coriolis meters, making them a more cost effective alternative for LNG applications, where pressure drop and maximizing throughput are prime considerations.

CONCLUSION

Ultrasonic flow meters offer the right technology and value proposition for custody-transfer applications. The high accuracy and low maintenance profile for capital investment in the oil & gas industry, will propel the ultrasonic flow meter market to double digit growth rates.

Expected Benefits

- Safe installation at hazardous remote sites. 

- Auto fleet identification at unmanned fuel stations.  

- Auto compensation to base temperature using a customized equation. 

- Automatic e-mailing of compiled daily dispense report to office via GSM/GPRS. 

- Web-based HMI that displays dispensing and tank status at remote site and enables upload and   set-up of  daily report.

Process Description

When a truck refuels at an unmanned station, the driver presses the start button and the message "Enter ID number" is displayed on the local HMI. When the ID entered at the local HMI is authenticated by the controller, the LNG dispensing sequence starts. The controller starts a pump to circulate liquefied gas and cool down the flowmeter. LNG is then dispensed to the truck. Then, the controller compensates the flow volume using a customized equation for custody transfer based on the measured temperature and pressure. On completion of each dispense, flow data is saved in the controller as a file. At the end of the day a report is created by compiling all the day' s dispenses on the controller and is automatically e-mailed to the office for billing purposes.

Application Solutions

Yokogawa provides an innovative automatic custody transfer system for unmanned LNG fuel stations that uses the STARDOM FCN controller and a cryogenic version vortex flowmeter.

Safe installation
LNG that vaporizes and mixes with air in the proper proportions can ignite. To prevent accidents at remote gas stations, the controller is stored in a cabinet with an intrinsically safe barrier.

Fleet identification
An ID file storing fleet ID information has been loaded onto the controller and can be modified online by accessing the controller web page. When the truck driver presses the start button, "Enter ID number" is displayed on the local HMI. When the fleet ID number is entered into the local HMI, the ID number is sent to the controller and compared with the ID information in the file. When the ID is authenticated, the controller initiates the LNG dispensing sequence. However, if the ID is not authenticated, an error message is displayed on the local HMI.

Liquefied gas dispensing
1. After the ID is authenticated, the controller starts a pump to circulate liquefied gas through the pipe until the temperature falls low enough for single-phase liquid flow. If this cannot be achieved within a given time frame, the controller stops the pump, displays an "error" on the local HMI, and returns to idle.

2. When the temperature is low enough, the controller opens the valve and slowly ramps up the pump speed to avoid tripping due to sudden increases in back pressure. During the dispensing, the pressure is monitored and, if it goes high (due to back pressure problems or the tank being full), filling stops. If the truck has more than one tank to fill, the local HMI prompts the driver to connect to the next tank and continue filling. Once completed, the local HMI displays the total amount dispensed, and a report is created in the controller.

Flow volume measurement
The Yokogawa DY cryogenic version vortex flowmeter handles process temperatures as low as -200 deg C with high accuracy (liquid: ±0.75%).

Flow correction
Flow (measured by the Yokogawa cryogenic version vortex flowmeter) as well as temperature and pressure are transmitted to the controller. The flow is compensated to the base temperature using a customized equation for custody transfer.

Logging file
On completion of each dispense, the corrected flow, temperature, and pressure are saved as an Excel compatible CSV file in the controller's compact flash memory. This log file can be uploaded on demand to a PC using the FTP function and can also be viewed in trend format by accessing the controller web page.

Report file
At the end of the day, a report file is created by compiling all the day's dispense files. As a default, two daily reports are provided. These are (1) Daily Dispense Report: summary of the day's dispenses, and (2) Daily Status Report: tank level and pump usage. These reports are in Excel compatible CSV format and are completely user definable. The created daily reports are e-mailed to an office for billing purposes via GSM/GPRS or some other Internet connection. Also, these reports can be uploaded on demand to a PC using the FTP function and can be opened in Microsoft Excel.

Web-based HMI
The following pages can be accessed at the controller URL: Tuning Parameter Setting: Without the modification of applications, parameters can be set on this parameter web page. These include timer set points, pressure set points, alarm set points, and PID tuning parameters. Also, data logging and report creation configuration can be set.
Dispense Trend: The dispensed quantity and historical trend can be monitored in real time.
Site Status: Gas station status information such as storage and dispensing quantity and status can be monitored.

Alarm notification via e-mail
If an alarm occurs, the controller immediately sends to a specified person an alarm e-mail that contains a log of all process parameters at the time of the alarm.