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Pipeline Technology Journal 2-2015

Latest developments and news from the pipeline industry

RESEARCH / DEVELOPMENT / TECHNOLOGY “IN PRINCIPLE, PIPEPATROL CAN BE INTEGRATED INTO ANY NEW OR EXISTING INFRASTRUCTURE; THIS APPLIES TO BOTH THE CONTROL SYSTEM AND THE MEASURING TECHNOLOGY USED.” >DanielVogt,KROHNE An E-RTTM leak detection system creates a virtual image of a pipe- line 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 meas- ured pressure and temperature values. The model compares the cal- culated flow values with the actual values from the flow meters. If the model detects a flow discrepancy, the leak signature analysis mod- ule then determines whether it was caused by an instrument error, a gradual leak or a sudden leak. The increased capacities of modern computers allow leak signature analysis to apply powerful statistical hypothesis testing, outperform- ing the detection times of A. Wald’s old fashioned SPRT algorithm from 1942, while still providing the same confidence into the results. Based on modern statistical tests, the signature analysis decides, whether the pipeline is affected by a leak or not. Signature analysis is critical to reliability because it provides a high degree of protection from false alarms. 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 ex- isting measuring results against leak signatures, which are stored in a database. Comparison of measurement values with the leak signa- tures 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 recognized 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 automatical- ly 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 prop- agation 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 intersec- tion method is based on the pressure profile of a pipeline: the occur- rence of a leak changes the pressure gradient along the pipeline in characteristic manner (see Figure 2: Leak localisation by the gradient intersection method). Without a leak, the drop in pressure in a liquid pipeline is linear (blue line). When there is a leak, the pressure gradi- ent changes and two linear segments appear with different slopes (orange). The leak position can be determined by calculating the in- tersection point. The second option for leak localisation is the wave propagation method, which analyzes the pressure waves that result from a leak. If a sufficiently 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 (see Figure 3: Leak localisation by the wave propagation method). The extended wave propagation method is based on the same phys- ical 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 (see Figure 4: Leak localisation by the extended wave propagation method). Figure 2: Leak localisation by the gradient intersection method 26 PIPELINE TECHNOLOGY JOURNAL

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