Hydrostatic tank gauging

A hydrostatic tank gauge applied to a pressurized, spherical tank. (Courtesy of The Foxboro Co.)... 

A second is hydrostatic tank gauging, where level is inferred from pressure. Again, density, particularly if two phases exist (aqueous and foam), may render this approach questionable. 

Hydrostatic tank gauging. Hydrostatic tank gauging (HTG) is one of the oldest techniques to measure the tank contents. In the process industry, level measurement using differential pressure transmitters is very common. Normally this method uses analog pressure transmitters, with a 1 percent accuracy. Inventory measurement requires a much better accuraty therefore, analog transmitters are not suitable for this purpose. 

FIG. T-12 Hydrostatic tank gauging system. (Source Enraf.)... 

Hybrid inventory measurement system. The hybrid inventory measurement system (HIMS) combines the most modern level gauging techniques with hydrostatic tank gauging (Figs. T-13 and T-14). It utilizes an advanced radar or servo level gauge for accurate level measurement, with a smart pressure transmitter (PI) and a temperature measurement instrument. On nonatmospheric tanks a second transmitter for the vapor pressure compensation is required. 

Hydrostatic tank gauging. Recent developments of smart transmitters opened a new era for HTG. The development of smart pressure transmitters with microcomputers... 

A mass-based tank-gauging system quantify assessment based on hydrostatic pressure of the liquid column measurement. 

In many cases it is inconvenient to use a dipstick, due to the position or location of the tank, and there are a variety of direct and remote contents gauges available, including gauge glasses, float and weight, float and swing arm, float and indicator, hydrostatic, electrical capacitance, etc. 

Liquid level measuring devices are classified into two groups (a) direct method, and (b) inferred method. An example of the direct method is the dipstick in your car which measures the height of the oil in the oil pan. An example of the inferred method is a pressure gauge at the bottom of a tank which measures the hydrostatic head pressure from the height of the liquid. 

Figure 7-11 shows the tank after the complete roof structure had been assembled and welded (dosed as it was prepared for a hydrostatic test. In this test the tank was filled with water and then pressurized to give the equivalent of a 30-lb internal pressure at all points within the tank. Strain gauges were attached at many points so that the complicated stress pattern could be studied in some detail for assurance that the tank was safely within design limits. 

Level gauging instruments measure the liquid level in the tank. Pressure transmitters measure the hydrostatic pressure of the liquid column. Both level and pressure are primary functions for the calculation of volume and mass, respectively. Hybrid sj stems, such as HIMS, use both inputs in one system. Conversions from volume to mass or vice versa are made using density and temperature as secondary inputs. The density input may be obtained from an outside source, such as a laboratory, or may be measured in the tank by using pressvue transmitters or servo density. The temperature input is obtained from a temp ature-measuring system in the tank. 

---