Flow, level & control
The merits of pressure transmitters for accurate interface level measurement
M
any processes use water as a means of transporting product from one point
to another. For example, in oil production, water or steam is often used to lift oil out of a well. In chemical production, water is sometimes a by-product or a tool used to clean a vessel. In these situations, the water and hydrocarbons will mix together. At some point, it will be necessary to remove the hydrocarbon from the water. If allowed to settle undisturbed in
a tank, the mixture will separate into its two components, with the heavier, denser material sinking to the bottom and the lighter, less dense material rising to the top. This principle is exactly the same as the way in which oil and vinegar separates in an Italian salad dressing. One example of this in a real
application is a separation tank. A control valve regulates the ingress of a liquid mixture of water and hydrocarbon into the vessel. Eventually, the lighter material in the mixture finds it way up to the separation stack, where a water/hydrocarbon liquid interface forms - effectively a dividing line between the two liquids. The position of this liquid interface is critical - too little or too much either way will end up with either water being drawn out
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with the hydrocarbon, or hydrocarbon remaining in the tank. In either situation, the end result is reduced product quality and process efficiency, adding to the product cost. When the mixture gets to the
critical interface point, a pump will pull out the hydrocarbon from the stack while a continuous amount of new mixture is pumped into the tank. The hydrocarbon is then sent on for processing, free of water. For this process to operate at
optimum efficiency, it is vital that the interface level is measured and controlled properly.
Typical meThods used To measure inTerface level
A range of different technologies exists for interface level measurement applications. Many of these technologies can encounter problems when either the interface level becomes too small or the process involves sticky solids. Substances that can coat or leave residue can also present a problem when using these devices. Here, we will look at the advantages and disadvantages associated with the three main methods most commonly employed for interface level measurement, namely – displacers, capacitance probes and differential pressure transmitters.
October 2021 Instrumentation Monthly
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