Effective Interface Control in Multi-Phase Separators

Various measurement technologies have been used over the years to measure interface in a separation vessel. Interfaces vary from clean and well-defined, to a small rag layer or a large emulsion band, right through to multiple interfaces of oil, emulsion, water, solids and associated build-up. Different technologies have shown varying degrees of success, but rarely cope with all of the conditions that can occur. Doug Anderson of VEGA Controls Ltd asks how can this measurement be made more reliable?

‘Traditional’ technologies used for interface measurement have shown degrees of unreliability, especially with heavy emulsions and/ or varying quality of crude inputs.

There are three main reasons:

They need consistent ‘measurement properties’ of the liquid phases they are measuring in order to function, e.g. electrical property or density/specific gravity. If it is not constant or stable, it introduces another (unwanted) ‘unknown’ into the measurement ‘equation’.
Measurement devices can also be affected by build-up or contamination from the process over a period of time.
As well the requirement for stability, most technologies for interface monitoring also require a minimum ‘measurement property’ differential between the upper and lower fluids to detect a difference (interface), otherwise they start to ‘guess’ or fail completely. The problem with emulsions is that they are not defined, they are diffuse in their nature and literally a ‘grey area’ to many interface detection technologies.

Often the end user may be unaware of the characteristics of the process/ interface they are trying to measure, especially if there is no possibility for the use of externally mounted sight glasses on the vessel to try and ‘see inside’ the process. It therefore makes it doubly important to have a measurement technology that is proven and reliable in managing the throughput of any separation process.

Why consider an MDA type profiler system? The benefits to the end user are as follows:

Density profile across the elevation span monitors changes in the separation process on a real-time basis, enabling automatic control and optimal throughput.
Each of the density gauges independently transmit their signal to the control room.
No external algorithms on a separate computer are required. Easy for fault finding.
Safe withdrawal of sources using extremely simple and robust retractable mechanisms. Relatively small vessel process connection.
Reliable, even with process build up inside, through higher energy output and longer measurement paths – better performance and reduces outages for cleaning or recalibration.
System can also support extra independent units for total level and point level trips (SIL)
Externally mounted SMART detectors are reliable by design, withstand higher process temperatures for low operating costs.
Fast calibration of sensors and system. Monitoring each fixed point permits trending optimising inputs and throughput of the process.
If it should happen, a single density detector outage does not compromise the system. If hardware exchange is needed, it is faster due to external mounting and back up of calibration.
When no clear interface exists, the system shows valuable data to regain process control in the quickest time using optimal inputs.