Crude oil

From Active Pharmaceutical Ingredients (APIs) to Vapor Phase Corrosion Inhibitors (VCIs), any one of a thousand additives and agents can be injected into the process stream of a given industry to alter or impart new product properties or enhance processing dynamics. Injection systems and chemical skid systems offer a wide range of dosing control options. Challenge A chemical injection system typically consists of one or more chemical supply tanks or drums, a metering tank, a vessel with a mixer (if required), a variable pump, and process controls. Chemical tanks and chemical skid packages require level monitoring to ensure that the tanks do not overfill or run out of feed chemicals.

Not all water is removed from crude oil during the first stage of gravity separation. Separated crude may contain up to 15% water which exists in an emulsified form that is difficult for a separator to remove. The oil and water emulsion must be broken down so that the water can be removed before the crude is shipped. Deemulsification processes are accomplished using chemical agents and heat. Challenge Level control is found on two-phase and three-phase water knock out drums, heater treaters and chemelectric dehydrators. Interface measurement is critical in dehydration as it keeps the water-emulsified oil from flowing over the separator weir.

Salt in the crude stream presents serious corrosion and scaling problems and must be removed. Salt is dissolved within the remnant brine of the crude oil. Desalting removes both salt and the residual free water. Though the refinery is the most economical place for desalting, pipeline requirements often necessitate field desalting. Challenge Level instrumentation is integral to single and two-stage desalting systems, multiple orifice plate mixers, and the settler tank of a chemical desalter. Interface level control keeps free water from hitting the desalter electrodes and prevents expensive damage. The Interface level should be kept constant otherwise electrical field changes will disturb electrical coalescence.

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. Challenge 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.

By removing dissolved gases and hydrogen sulfide, crude stabilization and sweetening processes diminish safety and corrosion problems. Gases are removed by a stabilizer. Sweetening employs stabilization or vaporization processes along with a gas or steam-based stripping agent. Challenge Removing dissolved gases by stabilization requires level control in the reboiler unit. Sweetening by stage vaporization and trayed stabilization require level control in a series of staged separators. Sweetening by reboiled trayed stabilization requires additional level control in a reboiler.

A cement slurry prepared on location (or trucked in) cements the well casing. Drilling fluids lubricate the drill bit, remove cuttings, prevent open wellbore collapse and maintain hydrostatic equilibrium for blowout protection. Water- and oil-based “muds” are used. Dispersants, flocculants, surfactants and rust inhibitors may be added to the drilling fluid. Challenge Drilling fluids are typically stored in a series of partitioned, rectangular steel tanks. The tank fluid volume should be continuously monitored since level variations may indicate a pending blowout. Tank conditions include agitated media, suspended solids and media that will coat floats, displacers and probes.

Hydrocarbon gases are often flared in a high-temperature oxidation process which burns combustible components of waste. Environmental laws and restrictions mandate the precise monitoring of flared gases on oil and gas platforms. A flow meter is required to monitor the waste gases. Challenge Consideration must be given to abruptness of flow change, low pressures, and a wide range of velocities. Thermal dispersion flowmeters are ideal instruments for flare flow measurement due to low flow sensitivity and high turndown.

Crude oil, natural gas liquids (NGLs) and water are stored in oil and gas fields. Unlike midstream tank farms at terminals and refineries, field storage consists of smaller vessels associated with oil, gas and water processing. Diesel generator fuel, potable water, and fire water are also stored offshore. NGLs are stored at atmospheric pressure in double-walled tanks. Challenge Tank level monitoring can be provided with overflow control and alarm systems or shutdown pumps when level falls below the specified low level. Interface controls will sense the beginning of an oil/water interface during tank dewatering and control the water draw-off. API 2350 New recommended practices regarding tank overfill protection for above-ground storage tanks that receive Class I (flammable) liquids outline that careful selection and application of level controls can effectively protect against tank overfills.

A variety of chemicals are typically stored in the field or processing facility to expedite processing time by preconditioning an inlet fluid. These fluids may first enter into a hold tank to allow upstream solids and liquids time to separate prior to production, which allows the facility tobetter handle upset conditions without stopping production. Stored additive chemicals include dispersants, flocculants, surfactants, glycols, diluents and rust inhibitors. Challenges Fluids are typically stored in a series of outdoor steel tanks. The tank fluid volume should be continuously monitored since level variations may lead to upsets. Tanks contain agitated media with suspended solids that can coat floats, displacers and probes.

Nitrogen – the most widely used commercial gas – is the ideal tank blanketing gas when injected into the vapor space of a storage tank. It prevents ignition of flammable liquids, inhibits vapor loss, and protects chemicals, pharmaceuticals and foods from oxygen and moisture degradation. Nitrogen is also used as a purging agent and in cryogenic applications. Challenge Mass flow measurement will monitor the nitrogen blanketing gas. A mass flow meter can track usage as a cost control measure and determine the when, where and by whom of gas usage. Flow monitoring of feed lines can prevent unsafe conditions that may arise when gas supply is insufficient.

If allowed to escape into the atmosphere, hydrocarbon vapors diminish income through loss of hydrocarbon volume and create fire hazards and pollution problems. A Vapor Recovery Unit (VRU) collects vapors from storage and loading facilities, reliquefies the vapors and returns the liquid hydrocarbons back to storage. Methods to recover vapors include absorption, condensation, adsorption and simple cooling. Challenge A VRU is a simple, economical process unit that provides EPA compliance and improves operating economies by capturing up to 95% of fugitive emissions. Critical to the VRU is the flash drum where vapors are reliquefied. Liquid level control of the flash drum is essential.

The first step in processing the well stream is to separate the crude oil, natural gas and water phases into separate streams. Oil and gas fields utilize two-phase gas/oil and gas/condensate separators as well as three-phase gas/oil/water separators. Units are classified according to horizontal or vertical configuration, operating pressure, turbulent or laminar flow, and test or production separation. Challenge Interface level measurement will actuate a valve to adjust vessel level. An emulsion layer along the oil/water interface can contaminate the oil with water or the water with oil. Foaming along the gas/liquid interface, if entrained, can cause liquid carryover or gas blow-by.