Measuring product in oilfield storage tanks can pose significant risks to workers if proper precautions are not taken. This video discusses the risks and ways they can be reduced to protect workers. Italian oil major Eni said on 18 April it had decided to temporarily shut down a treatment plant serving its biggest domestic oil field in southern Italy to meet a local order.
Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. SPE disclaims any and all liability for your use of such content. The near solid or highly viscous residuals at the bottom of an oil storage tank, generally composed of a large amount of paraffins, silt, heavy ends, etc.
When product vapor pressure is greater than 0.5 psia (more in some states) but less than 11.1 psia, the U.S. Environmental Protection Agency permits the use of a floating-roof as the primary means of vapor control from the storage tank. Floating-roof tanks are not intended for all products. In general, they are not suitable for applications in which the products have not been stabilized (vapors removed). The goal with all floating-roof tanks is to provide safe, efficient storage of volatile products with minimum vapor loss to the environment. Design requirements for external floating roofs are provided in Appendix C of the API Standard 650.
Specially designed pressure/vacuum vent valves should be provided to protect the tank against overpressure or vacuum conditions. Safety should be a primary concern when selecting a storage tank vent system for a specific application. In production operations, this normally means that a safe way of handling vapors that evolve from the liquid must be designed into the system, and air must be excluded from entering the tank and mixing with hydrocarbon in the vapor space. Fixed-roof tanks should be configured to operate with a suitable gas blanketing system that maintains the tank at positive pressures under all operating conditions. Tank vent piping should include flame arrestors such as that shown in Figure 1, which protect the tank against ignition of the vent gases owing to lightning strike or a discharge of static electricity at the vent location.
Fixed roof tanks are common in production facilities to store hydrocarbons with vapor pressures close to atmospheric pressure. In this use, they should be equipped with pressure-vacuum valves and purged with natural gas to eliminate air intake into the vapor space. In crude oil terminals and pumping stations, internal floating roofs may be added to the fixed-roof tank to reduce product vapor losses if the crude oil has been stabilized to vapor pressures less than 11 psia. Examples of fixed-roof tanks are shown in Figure 1. The most common fixed-roof design contains a shallow cone roof utilizing a single center column plus internal (or external) framing to support the roof plates.
Production, refining, and distribution of petroleum products require many different types and sizes of storage tanks. Small bolted or welded tanks might be ideal for production fields while larger, welded storage tanks are used in distribution terminals and refineries throughout the world. Product operating conditions, storage capacities, and specific design issues can affect the tank selection process. Storage tanks come in all sizes and shapes. Special applications might require tanks to be rectangular, in the form of horizontal cylinders, or even spherical in shape.
Liquid loss from a storage tank is generally caused by localized material failure in the form of localized corrosion. Tank bottom leaks can be a result of improper foundation design or operating a tank outside the recommended design pressure or temperature boundaries. Product liquid leakage remains a significant environmental concern. Any tank used to contain a hydrocarbon product can be prone to develop leaks sometime during the service life. Tank design options that reduce the risk of a leak can be considered, or in the event of a leak, any product that escapes is contained and detected in a realistic time frame.
Fixed-roof tanks should have a quick opening gauge hatch in the roof, which allows the operator access to the tank to "gauge" the tank, determine if water is present, measure the height of the oil/water interface, and take samples of the crude oil. Standards for manual gauging of petroleum and petroleum products are given in the API Manual of Petroleum Measurement Standards, Chap. When a volatile product is stored in a freely ventilated fixed-roof tank, the concentration of volatile vapors in the vapor space can vary depending on the tank operating conditions. During holding periods, when no liquid is added or removed from the tank, the vapor space comes to equilibrium conditions based on product temperature and vapor pressure. Emissions during holding are generated by the vapor space breathing process.