Glass Lined Equipment Repair and Maintenance

• Corrosion resistance. Glass has the widest range of corrosion resistance of any material of construction. Most processes are developed in the laboratory in glassware. If the laboratory equipment shows no sign of corrosion or etching, it is likely that glass-lined equipment will be resistant. Glass resists corrosion in most acid solutions, except for hydrofluoric acid. It resists corrosion in alkali solutions at moderate temperatures.
• Cleanliness. Glass does not contaminate products, and protects their color and purity.
• Smoothness. Glass linings minimize friction, reducing demands on agitation equipment. Glass also resists viscous or sticky product buildup.
• Superior vacuum service. Although glass linings are comparable in pressure and temperature ratings to other types of premium lined equipment, the tight bond between glass and steel imparts superior vacuum performance.

The design and fabrication of glass-lined equipment is substantially different from corrosion resistant alloys. Designs and fabrication practices cannot be transferred from alloy to glass. Some of the differences are as follows:

Nozzles

• Alloy nozzles are welded to the top head and may have reinforcing pads. Glass-lined steel nozzles are first swaged to create a protrusion from the vessel head to which nozzles are welded with split lap ring backup flanges. Larger diameter glass-lined steel nozzles such as manways and dome covers use a J-clamp or J-bolt closure design instead of conventional bolting. The J-bolt closure design is used to minimize the bending moment created in the larger diameter flanges, versus a conventional ANSI or DIN bolted flange. Nozzle reinforcing pads are not permitted because they create glassing difficulties. 
• Threaded couplings can be welded to alloy vessels for pressure gauges, inert gas addition, and other such uses. They are not permitted with glass-lined vessels because they cannot be glassed. All nozzles must be flanged. 
• Alloy vessels may have any size nozzle. Glass-lined steel nozzle size is based on the steel thickness and the distance between the nozzles. The fabricator can advise the minimum allowable nozzle size, but 1-½ in. diameter is the minimum in certain applications.
• Nozzle lengths are limited in glass-lined steel, to minimize the amount of thermal movement or distortion that can occur, due to the required high temperature firings.
• Nozzle loadings are limited in glass-lined steel to avoid damaging the lining. As a rule of thumb, maximum nozzle load is typically set at 100 pounds per inch of nozzle diameter (for example, 400 pounds for a 4-inch nozzle). Most glass-lined equipment manufacturers have published tables of allowable nozzle forces and moments. Expansion joints are useful in decoupling piping loads from vessel nozzles. However, expansion joints create a weak link (or an additional failure mode) in the piping system and should be scrutinized very carefully from a process hazards analysis standpoint. In higher pressure systems, expansion joints can also create unacceptably high thrust loads if not properly constrained. Generally speaking, for systems that operate above ambient temperature, a piping designer should evaluate the piping layout and ensure that nozzle allowable loads are not exceeded.

Internals

• Alloy vessels are normally supplied with four baffles bolted or welded to the vessel wall. Glass-lined steel vessels are normally supplied with one or two baffles, depending on vessel size. Each baffle must be installed through a nozzle, reducing the number of nozzles available for other purposes. There is a relatively new design with built-in baffles.
• Internal coils can be supplied in alloy vessels, but not in glass-lined steel.

Jackets

• Alloy vessels have jackets with built-in baffles to introduce turbulence. Glass-lined steel vessels use jacket agitation nozzles located in the bottom head and shell of the jacket. This creates turbulence and high flow rate in the jacket to counteract the lower thermal conductivity of the glass.
• Glass-lined steel vessels are equipped with a diaphragm in the bottom head of the jacket to compensate for expansion and contraction of the jacket, which limits thermal stress to the lining.

Pressure/Temperature Rating

• The upper temperature limit of standard glass-lined steel is 500º F (260º C); specially formulated glass for high-temperature applications can withstand 650º F (345º C). The lower temperature limit is -94º F (-70º C). Any temperature below the minimum requires the base metal to be changed to stainless steel.
• The maximum pressure rating for glass-lined steel is determined by the vessel size. Normal pressure rating is 100 to 150 psi, but higher pressure ratings are available on special order.
• The glassed shell of a standard glass-lined steel vessel already contains a built-in corrosion allowance. The shell is relatively thick (above design allowable thickness) to maintain dimensional stability during multiple firings). The steel jacket is usually fabricated with a 1/16 -in. corrosion allowance.

Gaskets

• PTFE envelope gaskets are the most commonly used and recommended gaskets for glass-lined steel. It is important to note that a wide range of designs and quality of envelope gaskets are available and not all are equal. Therefore, the user should consider issues like maximum operating temperature, presence of permeants, consequences of failure, and expected service life when selecting a gasket manufacturer and envelope gasket design. Additionally, for smaller nozzles (12-in. diameter and lower), monolithic expanded PTFE gaskets have been used successfully. The advantage of a monolithic gasket is largely related to greater permeation resistance and better conformability, along with elimination of some failure modes that envelope gaskets experience. See Appendix C, Bolting and Gasketing, for guidance on flange makeup and gasket selection.

Externals

• Gear boxes with motors are flange-mounted on alloy vessels. In glass-lined steel vessels they must be mounted on a series of bosses located on the top head or main cover. Likewise, mechanical seals for glass-lined steel agitators are more specialized and in many cases more expensive and larger in diameter than their metallic mixer counterparts.

NONMEMBER PRICE: $205