Flat glass gages provide direct observation of liquid level in a process vessel. The process liquid level and liquid characteristics can be observed through the glass as it rises and falls in the gage chamber.
The six basic components of flat glass gages are as follows:
Chamber: Pressure retaining element which provides rigidity to the gage assembly and a means to connect the gage to isolation valves or other vessel connections.
Penberthy’s standard chambers have a flat gasket seat to allow for easy removal of gasket residue and fragments. Because the chamber does not have a recessed face, the gasket may be removed quickly with a flat blade.
Penberthy has an optional chamber with a recessed gasket seating surface to meet customer requirements.
Slots are machined into Penberthy’s transparent gage chambers rather than continuous vision slots. These cross ties between vision slots increase the mechanical integrity along the length of the chamber.
The cross tie provides a higher strength chamber due to the reduction of unsupported beam length. Chambers are available with PFA or ECTFE (Halar®) lining.
Gasket: Seals the gap and prevents leaking between the glass and the chamber.
Glass: Contains the process liquid and provides the viewing window to the process liquid level.
Cushion: Protects the glass from mechanical stress points by acting as a buffer between the glass and cover.
Cover: Protects the glass from mechanical impact along the side of the chamber and provides a compression surface for the bolts/studs/nuts to hold the gasket and glass tightly against the chamber.
Bolts/Studs/Nuts: Properly torqued gage bolting applies a uniform compressive load to the gage assembly for pressure retaining purposes.
Multiple Section Gages
To meet visible glass lengths greater than Size 9 glass (12.625" [320 mm]), Penberthy stacks covers along the length of the single chamber. The maximum operating pressure and temperature ordinarily determine the largest glass size that can be used.
Vision slots are cut according to the size of the glass used and the number of sections necessary to meet the desired vision length or minimum center to center (vessel connections). Support brackets should be attached to larger gages to support the weight of the gage assembly.
The maximum number of sections (covers) that can be used on a single gage chamber is ten. If the required liquid level vision length extends beyond 142.750" [3623 mm], multiple section gages may be bridled to a standpipe or alternately top-bottom side connected.
End, side and back (typically reflex only) connections can be machined into the gage chamber to provide end users with desired center to center distance and/or the optimum vision position for liquid level viewing. Connections include NPT, BSP, ANSI flanged, DIN flanged and socketweld.
Pictured is a typical transparent glass boiler feed water level gage as often found on a McDonnell & Miller water controller. This level gage configuration features union vessel and union gage connection allowing maximum flexibility for installation and maintenance.
Reflex and Transparent Gages
Reflex gages have a single vision slot in which light can enter the gage chamber to determine liquid level. Above the liquid level, glass prisms reflect the surrounding light. Below the liquid level the liquid fills the prisms causing the glass to become relatively transparent.
Light striking the area of glass covered by transparent liquid is allowed to pass through to the interior of the gage along the entire length of the covered prism. The light is reflected from the back of the chamber to the observer. Typically the chamber appears dark. An opaque liquid such as milk would directly reflect the light at the surface of the prisms, where it appears as a solid column of white.
Light striking the area of the glass above the liquid level experiences internal reflection. Internal reflection will occur when light attempts to move from a medium having a given index of refraction to one having a lower index of refraction.
When the light attempts to move from a higher index of refraction (glass - 1.47 index of refraction) to a lower index of refraction (air - 1.00 index of refraction through the prism angle), it reflects rather than passes through to the back of the chamber. Except for a focused line of light along the apex of the prism, the surrounding light reflects back to the observer appearing silvery.
The interface between the liquid and gas occurs where the silvery and dark/opaque area intersect.
Transparent gages have a vision slot on both sides of the chamber. Light enters the gage from the side opposite the observer so that both the level of a liquid and its characteristics can be seen. For easier liquid observation in dark environments, illuminators are available for use with transparent gages.
Transparent gages may be used for interface applications.
Armored flat glass gages give users the ability to visually inspect liquid characteristics (color, particulate, striations, turbulence) and monitor relative fill or drain rates in a variety of applications where vibration, temperature and/or pressure extremes rule out other level technologies.
End users interested in establishing a reliable, easy-to-understand level reference often use flat glass gages to verify electronic level device output at plant start-up or routine maintenance (especially pressure vessels).
Other distinct advantages:
- They require no electrical power. Level can be monitored in remote locations where no power is available. In event of a power failure, the flat glass gage is not effected.
- They are not dependent on most liquid properties. Multiple liquids can be processed through the same vessel without concerns for density, surface turbulence, di-electric, conductivity, etc.
- They’re suitable for vacuum application.
- They provide a near-unlimited length of measure.
- They’re non-instrusive.