Ultrasonic Flow Meters in

Pipeline Leak Detection

Systems-User’s Perspective

by

L.R.Krishnan

L.R. KRISHNAN Engineer Design, Kuwait Oil Company

Working as Engineer Design-Instrumentation with M/s Kuwait Oil Company. Responsible for project engineering including FEED preparation. Executed several projects with Flow Meters especially Ultrasonic Flow Meters. More than 30 years experience in the field of Instrumentation, Control and Engineering with specialization in DCS, ESD, Fire & Gas and SCADA systems.

Why do we need a Pipeline Leak Detection System (PLDS)?

• Monitor leaks in long distance pipelines conveying crude oil, gases, etc.

• Minimize losses, prevent hazards and accidents.

• Comply with the environmental laws & regulations, maintain Company’s reputation.

Accurate PLDS provides early warning of the process leak.

KOC has successfully implemented PLDS for various process applications.

DATA TAKEN ON 29.9.2015 SOURCE: PHMSA

PLDS can be either external or internal based. We will focus on Internal Continuous Online Leak Detection (Computational Pipeline Monitoring (CPM)).

Internal PLDS is based on the common philosophies

• Line Balance Methods such as Modified Volume Balance/Compensated Mass Balance Method

• Real Time Transient Modelling (RTTM)

• Pressure/Flow Change Method

• Acoustic/Negative Pressure Wave

• Statistical Analysis

EFFICIENT PLDS • Less false alarms • Accurate leak location detection • Proper Leak quantity estimation and minimum time for leak

detection • PLDS by Volumetric method- Extensively used by PLDS Vendors. • Technique uses Flow Meters at the start and end of the pipeline and

also in branch over lines for volume measurement. • The efficiency of the PLDS depends upon the accuracy of the

instruments especially the Flow Meters and the signal transmission speed.

• Ultrasonic Flow Meters (UFMs) are widely used for flow measurement and not meters such as Magnetic or Coriolis.

• Advantage of using UFMs-Available for large line sizes such as 80 Inch, no moving parts, can be used for piggable lines, etc.

Flow and Pressure-Inlet Flow and Pressure-Outlet

Efficiency of a Volume based PLDS- Proper Design & Installation of Ultrasonic Flow Meters.

Several issues have been experienced in the design of UFMs for PLDS.

• Size of UFM need not be the same as the size of the pipeline and can be less. Pipeline size reduction for UFM may cause pressure loss, surge, etc.

• This fact to be considered during the FEED stage.

• Accuracy and Repeatability of UFM need not be the same throughout the flow range. Accuracy at lower flow may be less than the specified accuracy.

• Less accurate UFMs-Affect the performance of the PLDS. Better the accuracy of the UFMs more efficient will be the PLDS performance.

• Best possible accuracy available for UFMs is ±0.15%- Used for Custody Transfer applications.

• It may prove difficult to achieve this accuracy either due to

difficult process conditions or due to larger size of the meter.

• Accuracy of ±0.15% may be achieved by reducing the meter size than the size of the line. OR

• Accuracy has to be sacrificed to say ±0.5% if size reduction is not acceptable.

• Repeatability is an important factor.

• Material of construction of UFMs, if not proper, will lead to meter failure and inefficient flow metering and PLDS performance.

Project Design has to consider all the above factors.

Installation of UFMs plays an important role. Minimum of Upstream 10D and Downstream 5D is required.

Picture Source: www.ge-mcs.com

• UFMs may not be available or cannot be properly designed for the process conditions-This affects the PLDS performance.

• Pressure-Temperature compensation is required.

• Meter proving facility shall be provided in the pipelines- Meter size versus mobile prover size should be examined.

• Piggable lines-Consider installing UFMs in a bypass line and not in the piggable portion.

• Alternatively Clamp-on flow meters can be

used-Less accurate such as ±1%- PLDS

cannot be expected to be efficient.

• Reverse Flow case shall be considered if applicable.

• Bi-directional flow, if possible, shall be accounted in PLDS algorithms.

• PLDS on Gravity lines is a challenge- Pressure profile will vary and flow rates will not be consistent-Slack or no flow case.

• Signal transmission for UFMs- Analog, Digital, SCADA-Signal Transmission lag has to be considered.

• Project FEED specifications may not account for pipeline branches or crossover lines- Absence of UFMs result in errors in leak detection. Cover all paths/lines with flow measurement.

• Testing of UFMs especially larger sizes is an issue. Availability of Test Labs with capability for dynamic testing to be ensured.

• Without dynamic testing, it is difficult to establish Accuracy and Repeatability and hence PLDS cannot be validated.

CONCLUSION

Consider all aspects of Ultrasonic Flow Meter in the project design. Better to interact with Manufacturers of UFMs and PLDS during the FEED stage.