Negative pressure wave description
The system is based on the energy conservation law. The onset of a leak creates a flow of liquid or gas into to the environment, which in its turn releases the pressure inside the pipeline, generating a negative pressure wave (NPW). The NPW propagates downstream and upstream at the speed of sound.
The first task in pipeline leak detection is to detect this negative pressure wave by high performance pressure transmitters, placed along the pipeline at LDS measurement stations. The station detection algorithm is a result of PAS own continuous improvement since ‘97, addressing multitude of applications on natural gas, gasoline and diesel fuel, multi-product, LPG, ethylene, crude oil, methanol, water and condensate pipelines. The algorithm is running right at the local measurement station, i.e. right by the source of raw data. It analyses the pressure signal over different time spans, processing the pattern recognition of negative and positive pressure waves:
- Pressure signal is acquired using very fast scan rates, up to 1 ms.
- Data are time stamped using precise GPS time synchronization.
- Pressure signal is archived and processed.
- During the process the self-learning, adoptive thresholds are calculated and the background noise is filtered out from the signal.
- The algorithm then performs the identification of pressure abnormalities - the search of leak-like patterns.
- The NPW direction is determined, comparing the signal from two pressure transmitters.
As soon as a leak-like pattern is verified to be an NPW, the measurement station issues a time stamped event, which will be further analyzed at the central station. This small packet of data (100 Bytes per 20-120 second) is sent to the central station. The communication data packet contains only NPW processing results (not raw data) and some station diagnostic, security and other information, especially when the LDS measurement station is used as a SCADA RTU.