Measurement tank gauges

A high integrity overfiii prevention system shouid, as a minimum, provide a ievei of SiL 1 as defined in BS EN 61511 -1. To reduce risk as iow as reasonabiy practicabie the overfiii prevention system shouid preferabiy be automatic and shouid be physicaiiy and eiectricaiiy separate from the tank gauging system. Automatic overfiii prevenfion may inciude, but not be restricted to, measures such as automatic shutdown of the suppiy iine or automatic diversion of the flow to another tank. 

Dutyhoiders wiii need to prepare a robust demonstration that aiternative measures are capabie of achieving an equivaient ALARP oufcome fo an overfiii prevention system that is automatic and physicaiiy and eiectricaiiy separate from fhe tank gauging system. 

Normally, tank levels were controlled from a control room using an automatic tank gauging (ATG) systan. A level gauge measured the liquid level. The operator in the control room used the ATG systan to monitor levels, temperatures and tank valve positions, and to initiate the remote operation of valves. 

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

Custody transfer. Many installations used their tank-gauging system for the measurements of product transfers between ship and shore and/or pipeline transmission systems. A tank-gauging system is a very cost-effective and accurate solution compared to flow metering systems, especially when high flow rates are present and large quantities are transferred. When flow measuring systems are used, however, the tank-gauging system offers a perfect verification tool. 

For oil movement and operations, either mass or volume measurement techniques can be used. Volume can be derived from level only mass can be measured directly by means of pressure transmitters. Additional information can be obtained by measuring vapor temperature and pressure. Density measurement can also be added, with accuracies from 0.5 percent up to 0.1 percent. Whichever technique is selected, it should be compatible with the operations of all parties using the data from the tank-gauging system. 

Tank gauging has a long history. Since each user and every application has its own specific requirements, several measurement techniques and solutions to gauge tank contents are currently available. 

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. 

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. 

The uncertainties of quantity assessment of a tank-gauging system depend on the measuring uncertainties of the installed instruments, tank capacity table (TCT) and installation. 

Hazards of fire and explosions. The majority of tank-gauging instruments are installed on tanks containing flammable products. The instruments on such tanks or in the surroimding hazardous area must be explosion proof. Circuits entering the tank atmosphere, like temperature-measuring systems, should be intrinsically safe. In the past, each coxmtry had its own safety standards, but an international harmonization of standards has become a reality. 

Radar gauges. Radar gauges play an important role in tank gauging (Figs. T-23 and T-24). Their nonintrusive solid-state natiure makes them very attractive. The accuracy of the newest generation radar gauge meets all requirements for custody transfer and legal inventory measurements. 

These are needed in order to transmit and receive the typical tank-gauging measuring data. Standard protocols as Extended MODBUS, Standard MODBUS, and others are also available for smooth communication between tank inventory systems and third-party control systems. Modem DCS or other systems have sufficient power to handle inventory calculations, but often lack the dedicated programming required for a capable inventory management. 

Unexpected product movements can then be signaled to the operator by an alarm. Statistical analysis of static data from the tank-gauging system and dynamic data from flow meters could also be used to improve the accimacy of the tank capacity table. Cross-correlation of gauges versus fiow meters could further reduce measurement uncertainties. With high-accuracy tank-gauging instruments combined with powerful computing platforms, automatic reconcihation becomes realistic. 

In summary, a wide range of different tank-gauging instnunents is available. The employed techniques are more complementary than competitive as each measuring principle has its own advantages. See Table T-3. Modem servo and radar gauges have improved considerably. They hardly need any maintenance and can provide trouble-free operation if applied correctly. The possibility of mixed installations with servo, radar, HTG, and HIMS provides optimal flexibility and utilizes the capability of each gauging technique. 

Fig. 10. Pressure gauge, caHbrated in terms of Hquid height, for measurement of level in open tanks.

The gauge has proved equally accurate for constant pressure and blowdown systems, and can also be adapted to vented systems. A typical curve for normalized krypton concn vs the amt of proplnt remaining in the tank is shown in Fig 1. Also shown is the analytical relationship between tracer concn and proplnt remaining in the tank. Statistical error analyses showed the typical average gauging error thruout the entire range of proplnt expulsion to be less than . 3% with a one sigma deviation of less than . 4%. This illustrates the consistency and reproducibility of this measurement technique... 

Bourdon gauges are used on gas cylinders and are also considered a type of aneroid gauge. These devices have a coiled tube (shown in Figure 3.5) and are used to measure the pressure difference between the pressure exerted by the gas in a cylinder and the atmospheric pressure. The coiled tube is mechanically coupled to a pointer (shown in red). As a gas at a pressure above atmospheric pressure enters the coiled tube, it causes it to slightly uncoil, kind of like those New Year s Eve paper noisemakers. This causes the pointer to move over a numerical scale, thereby indicating the gauge pressure in the tank. 

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. 

A very simple means by which liquid level is measured in a vessel is by the gauge glass method (Figure 1). In the gauge glass method, a transparent tube is attached to the bottom and top (top connection not needed in a tank open to atmosphere) of the tank that is monitored. The height of the liquid in the tube will be equal to the height of water in the tank. 

On hard-to-handle services, such as the fluidized-bed level measurement in combustion processes, there is little choice but to use radiation gauges. On slurry and sludge services, d/p units with extended diaphragms eliminate the dead-ended cavity and bring the sensing diaphragm flush with the inner surface of the tank. Other level transmitters that can be considered for hard-to-handle services include the capacitance/RF, laser, radar, sonic, and TDR types. 

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