Mica Shields are used with process media that will corrode glass. The most common is steam above 2069 KPaG (300 PSIG) Working Steam Pressure (WSP). Transparent glass must be used to provide a flat surface to back the shield. Mica is a laminate type, naturally occurring material resistant to alkaline and can be used in high temperatures. As a layer of mica degrades in service, a new layer is exposed.
PCTFE (Formerly Kel-F) Shields
PCTFE shields are a common alternative to mica shields and extremely resistant to chemical corrosion but ineffective in steam service. These shields are most commonly used in hydrofluoric acid service. It should be noted that PCTFE is available in varying degrees of optical quality. While a PCTFE shield is not completely transparent, Jerguson shields are extremely clear and contain a minimal milky-white appearance. This milky-white appearance is prevalent with third-party product and can make it extremely difficult to read the level in the gage.
Jerguson Belleville Spring Washers are recommended for process temperatures over 500°F (260°C) or 600# steam. Belleville Spring Washers help maintain constant bolt load during thermal expansion and contraction, thereby maintaining both gasket load and an effective seal.
Jerguson Support Brackets are typically recommended for gages in excess of four or five sections in length. This length recommendation can vary based on gage type (reflex or transparent) and the amount of piping between the glass level gage and the vessel.
Jerguson Expansion Loops relieve piping stresses which can be transmitted to the gage thru thermal expansion and contraction of the vessel. Expansion loops are typically recommended for steam applications above 750 WSP.