The Challenge – Measuring Level When Process Properties Vary

Tank Gauging Refinery DesaltersComplex level measurement presents several different challenges in many kinds of applications, but one large refinery was able to overcome significant desalter issues using guided wave radar level technology.

 A desalter is designed to remove natural suspended salts and water from crude feed. If not removed, the salts can be extremely corrosive to refinery equipment. Water is mixed with incoming crude to scrub the salts, creating an emulsion; the water and oil separate and the “brine” is drained from the bottom. A high voltage electrostatic grid in the desalter vessel ensures crude dehydration, optimizing coalescence. However, if water reaches the grid, it will cause an overload and shutdown, and water carryover to the distillation tower can cause extreme pressure surges when exposed to high temperatures. Bottom line – desalting is critical to reduce operating and maintenance costs, but it’s fraught with challenges. 

 One of the significant issues in the petroleum industry is that crude oil density varies based upon supplier, making standardization of measurement very complex. The refinery had been using displacer technology in the desalter. Displacer technology is dependent on density and as the density changes, the instrument shifts and requires recalibration, which is maintenance intensive. Interface has to be determined by manual samples that present a health and safety risk. Without a reliable interface measurement, however, operations run a low interface level reducing efficiency and throughput in order to prevent water carryover that can have dire results – a unit trip and possible tower damage.

 The refinery had very low confidence in the level reading presented by the displacer. Troubleshooting doubled the lack of confidence because it showed inconsistent product composition in the vessel, failing torque tubes, and salts solidifying inside the bridle, which could cause the displacer to hang up or bottom out.

 The refinery switched to guided wave radar (GWR) level measurement to address measurement difficulties with changing process properties. Uniquely, GWR measures independent of density by change in process dielectric constant. Its principle of operation is TDR – time domain reflectometry, a top-down, direct level and interface measurement. Microwave pulses travel down along the probe and part of the energy reflects back when reaching media with a different dielectric constant. This was critical to solving the refinery’s problem because measuring the top of the emulsion ensures water does not enter the grid and shut down the desalter.

 GWR technology eliminates having to recalibrate the instrument as crude properties change. Its reliable interface measurement Increases efficiency, maximizes throughput, greatly reduces salt and water carryover, and most importantly, prevents trips of the desalter.

 For the refinery, the choice of the right level technology:

  • Increases operational efficiencies
  • Maximizes refinery throughput
  • Reduces unplanned shutdowns
  • Decreases operating and maintenance costs

Does your application require level measurement of changing process properties? Share how you address it in the comments below.

More information on guided wave radar can be found here.

2 Replies

  • We use Magnetrol GWR to measure interface level in our desalter at the refinery. However, recently we have been having trouble with its measurement freezing at a fixed value with no apparent reason. Probe has been unmounted for inspections but nothing relevant was found. Is there something else we should check?
    Thanks in advance.
  • In reply to Mark Nicolas Yupari Ruiz:

    Hello Mark! I am not so familiar with the Magnetrol guided wave radar, but I know for sure that a Rosemount 5300 guided wave radar is a very valid solution for desalters - and my answer will thus be based on GWR technology as such. First of all; what probe are you using in this application? Note that a coaxial probe may have internal supports to keep the inner rod centered - and salt and sludge may build up around those supports, causing false echoes - thus; ensure that those supports are clean. If you use a still pipe with a single lead probe; ensure that the probe is centered within the still pipe in order to avoid false echoes when the probe is close to/in contact with the pipe wall. If you use a single lead probe; ensure that you have sufficient distance to the grid in order to avoid false echoes from that. If you have managed all that; this can be a matter of threshold settings: If the thresholds are set to "tight", even a small disturbance may cause the radar to lock on the wrong echo. Best regards/Ingemar