(128d) Droplet Detecting Probe at High Pressures Verified with Stereoscopic Technique

Increasing capacity in a process plant or a gas/oil processing facility may result in higher concentrations of droplets in a gas stream and subsequently these droplets should be quantified in terms of size and mass flow rates. For upstream equipment such as pumps, compressors having large droplets in the gas stream may result in poorer performance and eventually breakdown of the equipment. Having a technique that can identify droplet size and mass rates is thus invaluable. This will facilitate the proper method of action for removal of droplets suspended in the gas stream. In most cases this will result in installation of demisting equipment such as inlet vanes, mesh pads or cyclones. The different devices are constructed to operate in a specific droplet size range and at a certain rate of liquid. Knowing the exact droplet size distribution and mass rates will allow personnel to choose the proper demisting device.

The technique consists of a detecting probe composed of a small impact surface and a wave-guide, which carries the particle impact signal to a piezoelectric transducer where the impact is converted to an electrical signal. The signal is then amplified and sent to data acquisition software on a computer. The method is based on monitoring of minute shock waves produced by the impact of liquid particles on the probe tip. A transducer converts these waves, which are proportional to the kinetic energy of the impacting particles, into electrical signals. The instrument is capable of continuous monitoring particle size. This makes it suitable for on-line monitoring of a particle laden gas. It measures the number of particles per second, the mass flow rate, and generates the particle size distribution in the gas. The instrument has been verified at atmospheric conditions with air and waterdroplets. In order to verify the probes performance at higher pressures a stereoscopic Lagrangian particle tracking (SLPT) technique has been utilized. Investigations also include tests on hydrocarbon liquid drops in gas. Hydro carbon droplet identification at higher pressures at industrial scale is a challenge and attempts are scarce in literature.

The SLPT method implements two high speed cameras to record a succession of events from two different viewpoints. The events are projected on two different planes corresponding to each camera and the image recognition procedure is able to record the shape of a droplet in the stereoscopic images and each 2D trajectory. From the information of both trajectories, a 3D trajectory is computed. This reconstructed trajectory can supply information such as collision impact parameter, incident angles, velocities, and accelerations at the desired instant of the event. The technique is able to measure quantities that can not be controlled or measured by other techniques such as droplet mean radius distribution, impact velocity, and angle. In addition, the information stored in a two dimensional image as a projection of the three dimensional event is usually easy to obtain. Having two simultaneous images from different projections allows a reconstruction of the three dimensional event. The SLPT method is thus a good method for measuring size and speed of a droplet hitting the impact surface of a probe. Similar techniques have been applied in the literature to three dimensional flow measurements, known as stereoscopic particle image velocimetry.

The droplet probe collisions were done in a small high pressure cell. The images were taken by a set of cameras with optical access to the event through one of the high pressure cell windows. Through the opposite window the back light technique required a powerful light source. The high pressure cell was designed for the particular droplet probe impact event. In this specific droplet probe experiment the main chamber consisted of a stainless steel structure with access holes to introduce external pipes, several internals and probe. Fused silica windows were used to gain optical access to the droplet probe collision event.

These experiments lay the foundation for further experiments in droplet separation units operating with a natural gas system at elevated pressures. The purpose is to calibrate a tool for obtaining droplet size distribution in a scrubber at several measuring locations and characterise the different internals, such as inlet vane, mesh pad and cyclone.

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