April 2008

Level: A visual concept

By Don Gillum

Level applications have changed somewhat in recent years because   of the dynamic economic constraints and stipulations brought about by material cost, international market strategies, and product changes. Increased costs of raw materials have led to requirements of more power and precise level measurement.

You can measure level by direct and inferred methods. Direct methods employ physical principles, such as fluid motion, floats and buoyancy, and optical, thermal, and electrical properties. Inferential methods involve the use of hydrostatic head, weight quantities, radioactive properties, density, and sonic detectors. 

Level is one of the few variables you can detect by sight observation, where you would not want/need constant monitoring.

Dipsticks, lead lines

The earliest instruments used for level measurement were perhaps sticks or poles with calibrated scales to test the depths of streams. Although the dipstick and lead-line methods of level measurement are unrivaled in terms of accuracy, reliability, and dependability, there are drawbacks. First, these techniques require you to perform an action, which means an operator must interrupt other duties. Also, there cannot be a continuous representation of the process measurement. Another limitation is the difficulty of successfully and conveniently measuring level values in pressurized vessels. Such disadvantages limit the effectiveness of these types of visual level measurement.

Application of a tape and bob (weight) assembly for level measurement is simple and straightforward. The weight lowers through the fluid in a vessel until it reaches the bottom and then reels back up on a spool. Operators would note the point at which the liquid in the vessel marked the tape and then determine the level. An obvious problem you might encounter is the difficulty of knowing when the weight hits the tank bottom, especially if the fluid is highly viscous or the depth is great enough for the liquid to create a buoyant force that offsets the force of gravity on the weight. Inaccuracies caused by thermal expansion of the tape are probably negligible, offset by the thermal expansion of the liquid you are measuring.

Sight glasses

A sight glass, also called a gage glass, is an important instrument for visually determining level quantities. To understand the simple principle of operation, consider a U-tube manometer. With equal pressure on both legs of the manometer, the levels in the two legs will have the same amount of vertical displacement. One of the legs is represented by the process, and the other is a transparent tube on the outside of the process vessel that is available for visual inspection. As the process level fluctuates, the level in the transparent tube changes accordingly and is a true representation of the process level.

We can classify gage glasses in terms of low- and high-pressure processes. A low-pressure gage consists of a clear, round tube fitted between two special types of valves. 

A shield constructed from metal or plastic rods usually protects the tubular sight glass. Be careful when working around this device, to avoid mechanical shock and breaking the tubes, as the shield provides only minimum protection. The tube is normally limited to about 70 inches in length and a pressure of a few hundred pounds per square inch. As the length decreases, the pressure rating can increase. You can provide extended ranges by mounting several shorter tubes in series end to end.

To avoid imposing piping strains on the gage chamber, especially for the low-pressure non-armored type, the mounting should be such that the gage does not support the piping. Also, differential rates of thermal expansion between the valve and gage can cause severe mechanical stress, especially for extremely high or low process temperatures. Install an expansion loop between the gage and the vessel, or use a long run of piping. Install support brackets for long non-armored gages greater than 4 ft in length or weighing more than 100 lb.

Provide shutoff valves to isolate the gage from the vessel, and provide drain valves to depressurize the gage for maintenance. When placing the gage in service, open the valves slowly, especially in hot fluid applications, to avoid damage caused by thermal shock. Additional loads and stress on the gage reduces its ability to withstand resistance and thermal shock. While the gage is in operation, the isolation valve should be fully open; partially restricted valves can prevent the ball checks from seating, which could result in injury or loss of product should the glass break.

A recent innovation in sight-glass technology involves the use of fiber-optic cables to transport light generated by a bicolor boiler-water-level gage system. The sensing portion of the fiber-optic system is a gage that operates on the principle of light refraction. Light is refracted by different amounts when it travels through different mediums. When light from a high-intensity lamp is strategically positioned to pass through steam and water, the amount of light refraction will determine the path direction through red- and green-colored filters. Light traveling through steam shows red, and when traveling through the water shows green. A fiber-optic cable transmits the light to a fiber-optic readout for remote induction. Fiber-optic cables can see use for transmission lengths of 3 to 500 ft, depending on the configuration.

SOURCE: Industrial Pressure, Level and Density Measurement, 2nd Edition, by Don Gillum. Coming soon to the ISA bookstore at www.isa.org/pressureleveldensity.