Upon arrival at the refinery terminal, crude oil is pumped into above-ground storage tanks with capacities of thousands to millions of gallons. Raw crude is stored in floating- or fixed-roof tanks field-built to API standards. Tank level measurement by noncontact radar has gained share over mechanical float type and servo gauges due to its accuracy, low maintenance, no moving parts and fast set-up.
Tank level is maintained by valve actuation. By triggering an emergency cutoff, level controls prevent overflows and shut down pumps when level falls below low level. Safety-certified controls may be necessary due to crude’s low flash point.
All unrefined crude oil stored in tanks has a percentage of water entrained within it, and while stored in tanks, separation naturally occurs with water collecting at the bottom of the tank beneath the oil. The two fluids are very distinct except for a “black water” or “rag” interface layer which is an emulsion of mixed oil and water. To dewater the tank, water is drawn off of the bottom of the tank and is then sent off to water treatment.
Level controls designed for interface detection will sense the beginning of the oil/water interface during dewatering procedures and provide feedback to a control system which will terminate water draw-off when appropriate.
Inorganic chlorides, suspended solids, and trace metals found in untreated crude must be removed by chemical or electrostatic desalting. This reduces the risk of acid corrosion, plugging, fouling and catalyst poisoning in downstream units. Measurement of the oil/water interface in the desalter is crucial in separating the cleansed crude from contaminants.
Coating and build-up on probes may create interface measurement errors. Instruments susceptible to electrostatic grid interference may require special filters. Interface-dedicated level transmitters fitted with quick-disconnect probes provide optimum performance while reducing cleaning and maintenance time.
Located in the preheat train of the distillation column, a preflash drum system separates the vapors generated by preheating before entering the heater or main column. This prevents higher heater firing or pressure drops and reduces vapor loading of the column to avoid flooding.
Preflash drums create moderate foam that can affect measurement accuracy of liquid levels and decrease distillate production in the atmospheric column. Too low of a pre- flash drum level will cause pump cavitation of the flashed crude. Too high of a level will cause liquid carryover to the distillation column.
Following desalination, crude oil enters the distillation column where fractional distillation separates hydrocarbons into separate streams, cuts or fractions. For optimum operation of the distillation column, level controls must contend with occurrences of foaming, bubbling and moderate-to-high temperatures.
Sight glasses and displacer systems mounted in external chambers have traditionally provided distillation level measurement. Today, radar retrofitted in these existing chambers is gaining popularity due to radar’s less demanding maintenance schedule and ease of retrofit. High product temperatures necessitate temperature-tolerant level sensors.
A heat exchanger removes vapor from the upper parts of the fractionator, cools it to a liquid, and pumps it into an accumulator (reflux drum). Reflux pumps then draw liquid from the bottom of the accumulator and pump part of it back (reflux) where it is reintroduced at a lower point in the column. This refluxing process improves separation in the column by assuring sufficient downward liquid flow meeting the rising vapor.
Accurate and reliable level monitoring and control is necessary for the reflux accumulator to serve as a distribution point for reflux and distillate, and prevent excessive reflux from returning back to the tower.
A heat exchanger positioned near the bottom of the distillation column re-heats and vaporizes liquid and reintroduces the vapor several trays higher. This improves separation by introducing more heat into the column. For effective functioning of the reboiling process, level monitoring of the reboiler is required.
In some steam reboilers, the level must be controlled so that only a percentage of tubes are covered. This allows a control scheme to regulate the heat transfer in the reboiler by controlling the percentage of the reboiler tubes covered by liquid. This is a critical control loop as heat transfer into the liquid is a strong function of the percentage of tubes covered.
The heavy fraction remaining following the distillation of crudes is called petroleum resids. A variety of solvent-extraction processes yield deasphalted oil (DAO) from these resids. These oils serve as downstream feedstocks for catalytic crackers and hydrocrackers. Depending upon the system configuration, level monitoring of the separator, pre-flash, stripper and hot oil phases may include surge and flash drums, separators and strippers.
Level control is critical because interface level control of the separator feeds the flash drum, whose level feeds the stripper, etc. Application extremes include high temperatures, high pressures, and the presence of steam.