Optical method for flow patterns discrimination, slug and pig detection in horizontal gas liquid pipe

• Flow patterns of gas–liquid flow can be distinguished by infrared ray method.
• Slug frequency and velocity can be measured by infrared ray method.
• The pig moving in pipe can be detected by laser method.
• The infrared ray and laser methods do not disturb flow in pipe.

An optical method including infrared ray and laser was developed to discriminate flow pattern, and detect liquid slug and pig in horizontal gas–liquid pipe. Based on the principle that infrared ray attenuates differently during penetrating gas and liquid, the infrared ray method was developed to discriminate flow pattern and detect liquid slug. In experiment, infrared ray was emitted on one side of the transparent pipe, and detected on the other side. Simultaneously, a signal of output voltage that is proportional to the intensity of infrared ray detected was generated and recorded. A series of experiments in horizontal air/water loop were carried out to generate bubble, stratified smooth, wavy and intermittent flow, and the output voltages under the four flow patterns were analyzed. The flow patterns can be discriminated by characteristics of output voltage. Meanwhile, the velocity and frequency of liquid slug were measured by this method, and the results were consistent with that calculated by formulas. However, infrared ray is easily affected by interface between gas and liquid, a laser method was explored to detect pig. The laser method is similar to infrared ray, a laser beam was emitted and detected and then a signal of output voltage was recorded. The results from pigging experiments show that the laser method could correctly detect the passing of a pig. The combined use of infrared ray and laser method could rightly detect pig and pigging slug during pigging operation.