A very simple means by which liquid level is measured in a vessel or a tank is by the gauge glass method (Figure 19).
In a gauge glass, a transparent tube or a glass is attached to the bottom and top section of the tank or vessel that is monitored. The height of the liquid in the glass is equal to the height of water in the tank or vessel. Gauge glasses made from tubular glass or plastic are used for service up to 450 psig and 400°F.
In critical service and to measure the level of a vessel at higher temperatures and pressures, a different type of gauge glass is used. The type of gauge glass utilized in this instance has a body made of metal with a heavy glass or quartz section for visual observation of the liquid level. The glass section is usually flat to provide strength and safety. Figure 20 illustrates a typical transparent gauge glass.
Another type of gauge glass is the reflex gauge glass (Figure 21). In this type, one side of the glass section is prism-shaped. The glass is formed such that one side has 90-degree angles which run lengthwise. Light rays strike the outer surface of the glass at a 90-degree angle. The light rays travel through the glass striking the inner side of the glass at a 45-degree angle. The presence or absence of liquid in the chamber determines if the light rays are refracted into the chamber or reflected back to the outer surface of the glass. When the liquid is at an intermediate level in the gauge glass, the light rays encounter an air-glass interface in one portion of the chamber and a water-glass interface in the other portion of the chamber. Where an air-glass interface exists, the light rays are reflected back to the outer surface of the glass since the critical angle for light to pass from air to glass is 42 degrees. This causes the gauge glass to appear silvery-white. In the portion of the chamber with the water-glass interface, the light is refracted into the chamber by the prisms. Reflection of the light back to the outer surface of the gauge glass does not occur because the critical angle for light to pass from glass to water is 62-degrees. This results in the glass appearing black, since it is possible to see through the water to the walls of the chamber which are painted black.
12.1 Installation / Maintenance Of Gauge Glass
Do not proceed with any maintenance unless the gauge has been positively isolated, relieved of all pressure or vacuum and has reached ambient temperature. Gauge should be flushed out to remove any hazardous liquids. Strictly follow the Oxy safety guidelines and work permit system.
Improper installation or maintenance of gauge glass and connections can cause immediate or delayed breakage resulting in bodily injury and/or property damage. DO NOT use glass if it contains any scratches, chips, or any other visible signs of damage. DO NOT reuse any tubular glass packing or gaskets. DO NOT subject gauge glass to bending or tensional stresses and ensure that the mating flanges are in correct alignment. Never attempt to clean or repair a gauge glass under pressure or when in operation.
While replacing a gauge glass ensure that the vendor has supplied correct replacement glass and gaskets. Examine gauge glass and packing carefully for damage before installation. Install protective guards where necessary to help prevent injury in case of glass breakage.
To prepare for installation of new glass, remove old gaskets and clean the gasket seating surfaces. This should be done using a soft metal scraper (preferably brass) to remove all burrs, rust and remnants of the previous gasket.
Exercise extreme care to avoid gouging or scarring gasket-seating surfaces. Failure to prepare the gasket surfaces will result in leaks and/or glass breakage. Check flatness of each glass-seating surface of the sight window. If the glass seating surface is warped or bent and is not flat the entire sight window must be replaced. If tolerances are met, proceed with re-assembly of the sight window.
Before installation, inspect the replacement glass for imperfections. During inspection, and during any subsequent handling of the glass, keep the glass from contacting other surfaces. Bumping or sliding of glass against other surfaces can result in glass breaking, scratching or chipping.
Once a sight window has been disassembled, all glass must be disposed off because of wear; and all gaskets and cushions must be disposed off since they are permanently deformed by compression during service.
Carefully assemble the entire assembly as described in the manufacturer instructions. Apply oil or anti-seize to threads of studs. Tighten all nuts by fingers equally and then tighten by a torque wrench. Refer to manufacturer instructions for the correct torque setting. Using a torque wrench, tighten the bolts to the proper value in a sequence as shown in the vendor instructions. Tighten bolts in increments of 3 ft/lbs. or 50% of torque value, whichever is smaller.
Never attempt to tighten the bolts or carryout any maintenance when the gauge glass is in operation and is under pressure. Tightening under pressure could result in severe personal injury and property damage.
Gauge glass should be brought into service slowly to avoid excessive shock or stress on the glass. Rapid pressurization of a sight window can cause glass breakage/ fragmentation and fluid leakage. Failure to take proper precautions could result in severe personal or property damage.
Detection of any damage, problem areas or surface wear is sufficient evidence to take the sight window out of service. DO NOT proceed with operations until the glass has been replaced with a glass replacement kit supplied by the gauge glass supplier.
DO NOT under any circumstances reuse glass or gasket items previously in service. They can cause leaks or high stress points resulting in glass breakage and severe personal and property damage. Glass that is broken is dangerous and should be disposed of in a safe manner.
SHIELDS showing any sign of clouding, wear, or deterioration is an indication that the sight window glass has been or could soon become exposed to the contained fluid. Immediately take the sight window out of service and replace the shield, glass, and gasket supplied by the gauge glass vendor
GASKET LEAKS must be repaired immediately. Use gaskets supplied by the gauge glass manufacturer. Never use old or shop-cut gaskets. Only qualified and trained personnel on gauge glass repairs and servicing should undertake servicing or maintenance of a sight or gauge glass.
Failure to properly service or repair a gauge glass could result in serious personal injury and property damage. Read all warnings and manufacturers instructions before performing installation or maintenance. Proper PPE, such as safety glasses and gloves should be worn at all times when working with or examining gauge glass and connections.
Always work within the Occidental HES guidelines and the field work permit system. If in doubt stop work and contact the departmental supervisor or the HES department for guidance.