Liquid Static Measurement and Tank Gauging

Liquid Static Measurement and Tank Gauging

Every time the product changes ownership measurement of the product and custody of the product are transferred to another party. Quantities typically measured for custody transfer and monitoring/controlling facilities: · Volume flow rate accumulated at transfer points · Pressure at custody transfer and pressure control points (pump stations/peak points) · Temperature at custody transfer and temperature control points ( injection/delivery) · Density at the custody transfer and batch lifting/delivery points · Quality at the custody transfer and batch lifting/delivery points Measurement Systems are installed for: · Custody transfer · Inventory control · Batch size determination · Leak detection and line balancing purposes Measurement is done statically or dynamically. Static: involves measuring tank volume and its changes over time Dynamic: involves measuring hydrocarbons under flowing conditions by a meter Process Control and Internal Product Transfer Measurement: can use vortex shedding and orifice meters. Static Measurement: requires exacting attention to the detail and precision in the calculation process and the underlying supporting processes. Supporting processes include: · Tank calibration · Calibration of temperature and pressure transmitters · Densitometer calibration · Tank gauging equipment verification or calibration Tank Calibration: Manual Tank Strapping Method (MTSM): · Tanks must be calibrated after they have been filled at least one with a liquid of density equal to or greater than that of the liquid it will hold in the future. · Hydrostatic test is applied to all new tanks · In order to determine tank capacities dimensional measurements and determination of weights are necessary using the following methods o Circumferential measurement: uses steel tape aka tank strapping o Height o Deadwood o Floating roof – weight of the roof material o Tank bottom survey · Wall thickness of the circumferential plates and of the paint used on the tank walls is necessary to be able to achieve the correct internal volume Optical Reference Line Method (ORLM) · Uses an optical device but requires manual strapping of a reference circumferential course. · The strapping of the first ring is set 20% below the top horizontal weld seam and acts as a datum Optical Triangulation Method (OTM) · A laser is used to externally calibrate vertical cylindrical tanks by measuring angles. · No access to the tank roofs is necessary · A measured reference is necessary by strapping at a location near the bottom ring · Two stations are present per ring and is established at 20% of the distance from the upper and lower horizontal weld seams for each tank ring. Electro-Optical Distance Ranging Method (EODRM) · Uses internal measurement procedures outline in API MPMS chapter 2 · An electro-optical distance-ranging instrument capable of precision angular measurement and a laser beam emitter are involved. · Distances and slope angles are measured at a predetermined number of target points around the internal circumference of the tank · Placed on the tank floor and stabilized to prevent movement. It should be located at or near the center of the tank. All target points are sighted along the horizontal plane at each course location. Tank Capacity Table: They’re determined from the dimensional measurements obtained through the calibration procedures. Recalibration of tank required establishing revised tank capacity tables Liquid Calibration of Tank: If the tank shapes are irregular or unknown or inaccessible liquid calibration may be required this takes place using a positive displacement meter. Tank Gauging: Manual Tank Gauging: manual gauge tape must be calibrated and certified to a national standards. Some operations don't allow open access to tank contents because of hazardous vapors or for emission control purposes. In these cases it is desirable to implement manual closed tank gauging systems. Servo Tank Gauge: Automatic tank gauge (ATG) involves a physical connection of the servo instrumentation to a level sensor or float or a physical connection to a floating roof. Servo tank gauge can provide a product level indication as well as product density, can be used to determine Net Standard Volume of the tank contents (NSV) When using Servo tank gauge on a floating roof tank, a gauging platform is required in order to mount the gage over the product. (Tank roof for IFR or actual platform) Stilling Well: provides stability and calm to the product surface allowing for more accurate measurements. Accurate temperature information is accomplished by the installation of multiple resistance temperature detector RTD elements each of a different length in a prove that traverses the entire height of the tank. Microprocessors determine the RTDs and average their temperature. Radar Tank Gauge: Very reliable and suitable for custody transfer purposes in tanks with open visibility to the liquid. Non contact device Needs to be installed with instruments of similar accuracy in the measurement of product temperature and density. Stilling well is recommended for radar level measurements. Hybrid Tank Measurement System (HTMS): Combines accurate level gauge instrumentation with temperature sensors and pressure transmitters. Uses level-based and mass-based systems it can obtain tank level, temperature, mass and density. Components of Hybrid Tank Measurement System: · An automatic tank gauge · Tank thermometer system · Tank bottom pressure sensor and transmitter · A processor that provides an accurate conversion of pressure to density These four components calculate NSV. Calculation of Tankage Volumes: NSV is the primary unit of measurement for custody transfer and product inventory control. It is the equivalent volume of a liquid at its base temperature and pressure conditions. Doesn't include non-merchantable items To obtain NSV you must: Obtain TOV; total observed Volume, determined by measuring the liquid height of the tank. You reference the liquid height in the tank to the calculated volume from the tank capacity table. GSV: Gross standard volume, the total volume of petroleum liquids and S&W TCV: Total Calcualted volume TOV: total observed volume