Leak detection and localisation system for pipeline safety
02 July 2013
The use of pipelines is one of the safest and most economical transportation systems for fluids travelling long distances. However, cost-effectiveness and safety, both in new and existing pipelines, is put in jeopardy if reliable technology is not employed to ensure the rapid detection of any pipeline leaks. Control Engineering Europe reports on a recent development in leak detection.
Pipeline leaks can occur for a variety of reasons ranging from earthquakes, corrosion and material failure through to theft. Leak detection can be performed in various ways, from simple visual controls during inspection through to computer-supported systems that monitor conditions, even for underground and undersea pipelines.
Selecting a suitable leak detection system, however, can be difficult. The system will need to meet the needs of the particular application and will also need to comply with relevant regulations. For example the German ‘Technical Rules for Pipelines’ (TRFL) and API RP 1130.
TRFL was published by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety and was originally intended only for applications within the German federal territory. Today, however, it also provides a model for other national regulations.
TRFL provides for two independent processes for leak detection, which continually monitor steady-state operation (flow and pressure remain unchanged over a long period) and are based on various physical parameters. The rules also require leak detection in other operational conditions (paused flow, start-up/shutdown) and leak localisation.
API RP 1130, published by the American Petroleum Institute, is even more detailed with regard to leak detection systems. Among other items, it includes a collection of general recommendations for operating leak detection systems, such as clear presentation of the results for the operator and for maintenance. It also includes performance criteria for selecting a leak detection system outlined below:
• Sensitivity: the leak detection system should detect even small leaks within a short period.
• Precision: the leak detection system should locate leaks precisely. The leakage rate, the quantity of escaped product (leakage rate multiplied by time) and the product that is escaping should all be indicated.
• Robustness: the leak detection system should continue active monitoring despite unsteady or non-ideal conditions. This includes conditions such as temperature fluctuations, changes in viscosity and sensor failure. It also includes unsteady operating conditions – also known as transient operation – for example due to effects triggered by pumps or valves.
• Reliability: the leak detection system should not generate false alarms, even though it is highly sensitive.
When the operator has clarified relevant regulatory requirements, other characteristics that affect the choice of leak detection system can be considered. These may include technical and environmental parameters such as the length of the pipeline, whether it runs above or below ground, and the volume, type and quantity of different products to be transported. The desired type of monitoring can also be considered – internal (using process measurements), or external (using special measurements).
The next step is to review available systems. Making the assumption that an internal system is specified (as this is the globally preferred solution) the options are reduced to a handful of systems based on various mathematical and physical principles. The RTTM (Real-Time Transient Model) is currently the leading technology. RTTM is a mathematical model that compares measurements taken during the actual operation of a pipeline with those of a ‘virtual pipeline’, or a computer simulation of the pipeline, in real-time.
KROHNE has recently extended its PipePatrol E-RTTM system for continuous internal monitoring of pipelines (E-RTTM stands for Extended RTTM) to include a new feature – leak signature analysis using leak pattern detection
The E-RTTM with leak detection system is able to create a virtual image of a pipeline, based on real measured data. Measurement values from flow, temperature and pressure sensors installed at its inlet and outlet and along the pipeline, in places such as pump and valve stations, are crucial. The flow, pressure, temperature and density at each point along the virtual pipeline are calculated from the measured pressure and temperature values. The model compares the calculated flow values with the actual values from the flow meters. If a flow discrepancy is detected, the leak signature analysis module then determines the cause.
Comparison is critical
Comparison of measurement values with the leak signatures is critical to reliability because it provides protection from false alarms. E-RTTM-based leak detection systems are able to handle changing or transient operating conditions. The system works with dynamic values, which also affects robustness: the system can adapt automatically and quickly to changes in the operating conditions such as sensor failure, communications failure, a valve closing or a product change in the pipeline.
The system is based on three different methods of leak localisation – the gradient intersection method, the wave propagation method and the extended wave propagation method. The most probable leak location is calculated by comparing the results of these methods. The gradient intersection method is based on the pressure profile of a pipeline – the occurrence of a leak changes the pressure gradient along the pipeline in characteristic manner.
Without a leak, the drop in pressure in a liquid pipeline is linear. When there is a leak, the pressure gradient changes and two linear segments appear with different slopes. The leak position can be determined by calculating the intersection point.
The second option for leak localisation is the wave propagation method, which analyses the pressure waves that result from a leak. If a sufficiently large enough leak occurs suddenly, a negative pressure wave spreads at the speed of sound in both directions along the pipeline. The leak position can be calculated by comparing the arrival time of the pressure wave at the pipeline inlet and outlet pressure sensors.
The extended wave propagation method is based on the same physical principle as the wave propagation method. It takes into account additional values from pressure sensors installed in measuring and control stations along the pipeline, for example, and speed of sound data for the current product. This enables more precise localisation of the leak by reducing errors due to delayed sensor reaction or slow signal transfer.
The E-RTTM forms the basis of KROHNE’s PipePatrol leak detection system which is suitable for monitoring liquid and gas pipelines and meets the requirements of TRFL and API RP 1130. PipePatrol is installed on a dedicated server and operates autonomously. The user interface can run on a separate workstation, or can be integrated into an existing control system.
In addition to the visualisation of the pipeline operating conditions, PipePatrol can indicate leak positions on a map. Ethernet and serial interfaces support protocols such as OPC, Modbus TCP/lP, Modbus Serial, HART and PROFIBUS.
PipePatrol is also capable of monitoring pipeline networks. One example application from Germany demonstrates how the system functions in practice.
The Heide refinery in Hemmingstedt processes 4.5 million tonnes of crude oil per year. A 31.5 km multi-product pipeline connects the refinery with the tank terminal. In 2005 the refinery sought a second, independent leak detection system for the pipeline to meet the requirements of TRFL. A total of nine different refined, fluid hydrocarbons with various densities and viscosities are transported, one immediately after the other, so mixing can occur. The pipeline runs underground, with a diameter of 10in/DN 250, and is designed for a maximum flow of 600 m³/hr at 40 bar.
The PipePatrol leak detection system was integrated into the existing process control system (PCS) to provide monitoring at all times. The leak tests, for acceptance by the TÜV Inspection Institute, were performed using a valve in the pipeline to give a leakage rate of around 5 m³/hr. PipePatrol detected all leaks within 30 seconds and triggered alarms within 60 seconds. The leak was located to an accuracy of less than 400m, around 1% of the length of the pipeline. This exceeded TRFL requirements.
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