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JUL-AUG 2018

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INTECH JULY/AUGUST 2018 35 SPECIAL SECTION on a measuring wire from a drum is accurately positioned and balanced in the liquid medium using a servomotor. These devices can meet the inventory and custody accuracy requirements. Radar tank gauging Radar tank gauging (RTG) is the most common solution for tank gauging. It used to be the Saudi Aramco standard tank gauging solution for in ventory tanks. This technology is microwave- based, which measures the distance from the top connection to the liquid surface. The two available techniques are frequency modulated continuous wave (FMCW ) and time-of-flight time domain reflectometry (TDR). These de - vices can meet the inventory and custody accu- racy requirements. Hydrostatic tank gauges A hydrostatic tank gauge (HTG) sys- tem has up to three pressure transmit- ters and one tem- perature transmit- ter. Two pressure transmitters are in- stalled close to the bottom of the tank and are used to cal- culate the density. A third transmitter measures the vapor pressure at the top of the tank to increase the accuracy. Project initial scope and challenges One recent tank gauging project at Saudi Aramco Company was scoped to install radar tank gauging with multiple points of tempera- ture measurement on 42 tanks for crude and refined products. Three types of tanks are avail- able: external float, internal float, and fixed-roof tanks. The main challenges addressed during the design stage were: l Each RTG required excavation for running new and long cables for the signals and for the exter- nal power supply, in the large tank farm area. l A nozzle size of 6 to 8 inches was required for every tank. l A nozzle was required for the multiple spot temperature transmitter. l Tanks with internal roofs had to be empty, cleaned, and ventilated to install the new stilling pipes. This meant at least three-to-five years to execute the project. l New cabinets, hardware, and software instal- lations in the process interface buildings were required, which could mean expansion of ex- isting buildings or construction of new ones. l RTG required factory acceptance testing (FAT), which adds to the project schedule and cost. l The project execution would take years, in- cluding its high cost. GWR technology for inventory tank gauging Principle of operation Guided wave radar (GWR) or time domain re- flectometry is a two-wire microwave level in- stru ment that transmits a radar signal down a metal lic wave guide (single-rod/cable, twin rod, cable, or coaxial rod). The instrument principle of operation is time of flight. V = C / √K Where V = pulse velocity in service C = pulse velocity in air = 300,000 km/sec K = dielectric constant The signal transient time to and from the liquid surface will determine the level measurement. GWR advantages GWR technology has many advantages, which make it an attractive option for inventory tank gauging: l The GWR level instrument is a two-wire trans- mitter and does not require an external power supply. l GWR is a very economical choice in terms of capital and operating expenditures. l GWR can be installed while the atmospheric storage tank is in service. l GWR is "plug and play" and a maintenance- friendly technology. l The GWR cable version can be extended up to 75 meters high. l GWR is insensitive to dielectric changes and works for fluid with a low dielectric constant. l GWR is available with different process con- nection types and sizes. l A GWR rod/cable can be cut to fit the length (or height) required. l A GWR instrument can provide both total hy drocarbon level and water interface level. P3 P2 P1 RTD Figure 2. Float and tape gauge Figure 3. Servo gauge Figure 4. RTG Figure 5. HTG Vapor phase Liquid phase Cone roof (CR) No confined vapor phase Liquid phase Potential vapors Liquid phase External floating roof (EFR) Internal floating roof (IFR) Figure 6. Three storage tank types

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