Development Of A Pipeline Leak Detector
Authors: Flournoy N.E. , Schroeder W.W. 
Published: March 2010

The Texaco leak detector pig is designed to detect leaks in a pipeline by sensing the acoustic energy generated by escaping fluid. This instrument also locates leaks by the on-board recording of odometer and acoustic data as a pipeline is surveyed ultrasonically. Advantages of the Texaco device over similar leak surveying tools are:the operation is simple and can be done by non-specialist personnel,the recorded data are easily ill1erpreted by operating personnel,a specially developed odometer locates leaks accurately anda dual printing rate minimizes data volume, but provides all needed information on a short primed paper tape.

Successful field tests have been conducted in Texas and in Canada and evaluation is continuing through routine testing.

Introduction

Pipeline pigs equipped with ultrasonic leak detectors have been developed and used by European countries since the late 1960's. The primary purpose of such devices is to prove pipelines to be leak-free without resorting to the time-consuming differential-pressure tests required by the authorities of several countries. Pig instruments save downtime, as they can be run at any time without interrupting pipeline operation.

The uncertainty surrounding recent federal laws and the ever-increasing concern for the environment prompted the development of similar instruments on this continent. Development of the Texaco Leak Detector was completed in 1971 and field tests began in 1972. The following patents have been issued: U.S., 3.774,298 and 3.769,711; Canada. 953-016.

The manner in which leaks produce sound has been studied for many years. A generally accepted theory predicts that vortices will almost always he formed just outside the pipeline when fluid flows through a leak. Oscillatory pressure changes or sound waves are produced by the vortices. Because the flow is subsonic, sound waves propagate back through the leak and establish a sound field inside the pipe. The intensity of the field is usually high enough to allow detection by a piezoelectric transducer.

The detectability of leak sounds is influenced by factors such as fluid pressure, leak size and shape, backfill materials and the type of liquid (crude or product). Leaks generate sounds that fall in a broad frequency spectrum ranging from about 6 kHz (in the audio range) to about 80 kHz (in the ultrasonic range). The optimum frequency for detecting leak sounds is about 35 kHz. At this frequency, the ratio of sounds produced by leaks of various configurations to the noises of fluid and pig motion is highest.

Therefore, 35 kHz is usually chosen as the center frequency of a bandpass circuit for leak detection. The frequency and intensity of leak sounds are also dependent on the amount of gas present in the vicinity of a leak. For high gas-oil ratios, the leak sound received by the hydrophone usually has a lower intensity and a higher frequency than that of liquid only. Thus, leak detector pigs have been designed and constructed for liquid lines using state-of-the-art techniques. However, more development work would be needed for gas pipeline leak detector pigs.