Transportation of fluids in pipelines is increasing all over the world, and with good reason: pipelines are among the safest and most economical transportation systems over long routes.
To ensure this cost-effectiveness and safety, both new, and especially existing pipelines must reflect the standard of the most current technology. Leaks pose a potential safety risk. Leaks occur for a wide variety of reasons, from earthquakes, corrosion and material failure to drilling by product thieves. Special leak detection systems are often used to limit these risks. In general, leak detection in pipelines refers to the recognition and quick localisation of product leaks. Reasons to employ leak detection include the following:
• To minimise the effects of accidents.
• To minimise downtime.
• To minimise product loss.
• Regulatory compliance.
Leak detection in pipelines can be performed in various ways, from simple visual controls during inspections to computer-supported systems that monitor conditions, even for underground and undersea pipelines.
Selection criteria
Selecting a suitable leak detection system is not an easy task for pipeline operators. The system must meet the needs of the particular application and comply with relevant regulations. In this article, regulatory requirements will be discussed first. The most important existing regulations are the German ‘Technical Rules for Pipelines’ (TRFL) and API RP 1130.
The TRFL are published by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety. They were originally intended only for applications within the German federal territory, but now they provide a model for other national regulations. The 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 is published by the American Petroleum Institute, which is the largest trade association in the oil, gas and petrochemical industries in the USA with influence that reaches far beyond America. API RP (recommended practice) 1130 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: these criteria are very detailed and explain how leak detection systems work. The criteria are outlined below, and it is easy to see that they are linked and interdependent:
• 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 may also be considered: internal (using process measurements), or external (using special measurements).
System reviews
The next step is to review systems available on the market. Assuming an internal system has been specified, the preferred type all over the world, the options are reduced to a handful of systems based on various mathematical and physical principles. The RTTM (real-time transient model) is the leading technology at the moment. 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 expanded its product range to include E-RTTM (extended RTTM), which also features leak signature analysis using leak pattern detection.
Extended RTTM
An E-RTTM leak detection system creates a virtual image of a pipeline based on real measured data. Measurement values from flow, temperature and pressure sensors installed at the inlet and outlet of the pipeline 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 flowmeters. If the model detects a flow discrepancy, the leak signature analysis module then determines whether it was caused by an instrument error, a gradual leak or a sudden leak.
The performance criteria from API RP 1130 provide a useful guide to the detailed functions of the E-RTTM. It provides a high degree of sensitivity and quick leak detection with real-time comparison of existing measuring results against leak signatures, which are stored in a database. Comparison of measurement values with the leak signatures is also critical to reliability because it provides a high degree of protection from false alarms. E-RTTM-based leak detection systems are able to handle changing or transient operating conditions that are not recognised by less sophisticated internal leak detection systems. An E-RTTM-based leak detection system works with dynamic values, which also affects robustness: the system can adapt automatically and very quickly to changes in the operating conditions such as sensor failure, communications failure, a valve closing or a product change in the pipeline.
The precision of the E-RTTM is based on three different methods of leak localisation: the gradient intersection method, the wave propagation method and the extended wave propagation method. The leak detection system calculates the most probable leak location(s) 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 a 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.
Wave propagation method
The second option for leak localisation is the wave propagation method, which analyses the pressure waves that result from a leak. If a large enough leak occurs suddenly, for example if the pipeline is damaged by an excavator, 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 introduced here is the basis of the PipePatrol leak detection system by Krohne. The system is suitable for monitoring liquid and gas pipelines (including liquefied gas and supercritical products) and meets all the requirements of TRFL and API RP 1130.
PipePatrol is easy to use as it is installed on a dedicated server and operates completely autonomously. The user interface can run on a separate workstation, or be integrated into an existing control system. The user interface features intuitive operation: only the information that the current user needs for his scope of work is displayed.
Sample application
In addition to standard applications (liquid, gas, liquefied gas and supercritcial products), PipePatrol is also capable of monitoring pipeline networks. Based on Krohne’s many years of experience in the area of pipeline monitoring, the system offers outstanding performance and adapts quickly to changing operating conditions. An example application in Germany demonstrates how quickly and precisely leak detection functions in practice.
The Heide refinery in Hemmingstedt processes 4,5 million tonnes of crude oil per year into finished products and feedstocks for the chemical industry, among others. The plant produces gasoline, diesel and heating oil, as well as heavy fuel oil for ships and jet fuel.
A 31,5 km multi-product pipeline connects the refinery with the tank terminal in Brunsbüttel. The refinery sought an independent leak detection system for the pipeline to meet the requirements of TRFL. It transports middle distillates bidirectionally: a total of 9 different refined, fluid hydrocarbons. The products have various densities and viscosities and are transported one immediately after the other, so mixing can occur. The pipeline runs underground, with a diameter of 10”/DN 250, and is designed for a maximum flow of 600 m3/h at 40 bar. The leak detection system was to be integrated into the existing process control system (PCS 7) and provide reliable monitoring at all times.
Following thorough consultation, the Heide refinery opted for the PipePatrol leak detection system. Krohne commissioned, configured and tuned the system on site. PipePatrol used the measurement values provided by the process control system and was integrated into the pipeline monitoring system at the customer’s request. The leak tests performed for acceptance by the TÜV inspection institute were performed using a valve in the pipeline to give a leakage rate of around 5 m3/h. PipePatrol detected all leaks within 30 s and triggered alarms within 60 s. The leak was located to an accuracy of less than 400 m, around 1% of the length of the pipeline. This exceeds the TRFL requirements.
Installation of the PipePatrol leak detection system has guaranteed safe monitoring of this multi-product pipeline.
Conclusion
Modern leak detection systems are based on various mathematical and physical models. The pipeline properties as well as regulatory requirements must be taken into account when selecting a system. The most advanced technology currently available is the E-RTTM model. E-RTTM-based leak detection systems guarantee reliable leak monitoring for various types and lengths of pipelines, even under transient operating conditions. Krohne supplies the PipePatrol E-RTTM-based leak detection system either installed on separate hardware or for integration into an existing control system and measurement installation.
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