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Jul-2019

Comparison of interface level between differential pressure and multiphase level measurement

This paper will explain the difference in technology in measuring interface level between Differential Pressure (DP) level and Multiphase Level Measurement (MPLM).

Sabrina Nees and David Williams

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Article Summary
DP is an economical way of measuring levels in tanks and vessels It is the second most common use of pressure measurement behind flow measurements.

DP levels use Pascal law to determine level by showing the relationship between level, pressure and density of the fluid, simply stated, P = ρ * h, where P = pressure (pascals, Pa), ρ = density of fluid (kg/m3) and h = height of the column of fluid (meters, m). You can also solve h by rearranging Pascal law, h = P/ρ. For example, a tank with 2 m of water will exert a pressure of 2000 mmH2O or 0.196 bar. If the fluid was kerosene with a density of 0.82 spg, the pressure exerted would be 1640 mmH2O or 0.161 bar. As you can see the density of the fluid has a great influence on the pressure. If this was a level application calibrated on water to measure the 2 m span, the error of the level output at 50% level would be 18%, or the output would read 50% but the actual level would only be 32% if the density of fluid changed from 1.0 spg to 0.82 spg.

The errors are more complicated for DP levels when trying to measure interface levels in vessels, especially if emulsions are present. The difference pressure is the difference in pressure exerted by the vessel full of the higher density fluid and the vessel full of the lower density fluid. Since two fluids are being measured, the error can be compounded if both fluid densities change. Another point that can cause an issue is the height of the emulsion (see Figure 1). Emulsion is actually not the true term to use in the case of the desalter since emulsion implies a fluid of constant density, but this term describes the transition zone from clear oil to clear brine. Actually in this range there is a gradient change in density from the lighter density fluid to the heavier density fluid. The DP level will only measure the average density across the full span of the measurement and cannot tell the operator whether the level output is the bottom, the middle or top of this emulsion.

The MPLM system consists of radiometric density devices at different elevations through the measurement span (see Figure 2, left). This arrangement allows to accurately measure the density at each of these positions and provide a density profile (see Figure 2, right) of the fluids as they separate inside the vessel. This allows the operator to see where the top of the emulsion is located as well as the bottom of the emulsion. This helps not only to control the water outlet valve but also ensures for example, that the water level does not become too high inside the vessel where it can cause the electrostatic grids of a desalter or coalescer from shorting out. This information is also useful for chemical treatment companies since it allows them to monitor the effectiveness of chemical treatment.

The graphic of Figure 2 shows where the break points are between clear oil, emulsions and water. This allows for better control of the water outlet valve to ensure that the water level neither becomes too low nor too high inside the vessel.