By Andrew R. Obertanec, L.J. Star Inc.
In addition to obvious uses and advantages of glass as a working material in process industries, the failure of standard inspection ports and sight glass windows has always been a major problem. To enable visual inspection of fluid flow, chemical reactions, mixing and other operations, process equipment is often fitted with sight glasses.
When observing a process, a common method is to employ a tempered borosilicate (thermally prestressed) glass disc. However, these sight glasses are subject to instant failure throughout the entire cross section of the glass with the possibilities of catastrophic consequences.
For years, attempts have been made to construct safe sight glasses incorporating sealed units, dual windows, laminated glass, wire mesh, etc. These attempts, however, overlook the true cause of the problem; the characteristics of the glass itself.
Contents What is Mechanically Prestressed Glass? Why is Metaglas So Strong? Test Results Materials in Metaglas Construction Metaglas Products Additional Disc Products Special Adaptations for Metaglas Advantages of Metaglas Summary
What is Mechanically Prestressed Glass?
The distinct characteristics of thermally pre-stressed (tempered) glass and Metaglas mechanically pre-stressed (fused glass to metal) can be explained by the created stress conditions particular to the method of manufacturing. In the case of tempered glass, the heated disc surface is chilled by air. When the hot core area cools, it contracts forcing the cooler surface area under compressive stress and the core area under tensile stress.
During the manufacture of the metal fused glass, the steel ring and glass are first heated to the proper fusing temperature at which the glass moves towards the steel ring. Because of differing but precise thermal expansion coefficients of steel and glass during the cooling process after fusion, a compressive stress condition is created in the glass, proportional to the normal unconfined expansion at the fusion temperature. The steel ring is simultaneously placed in a tensile stress condition. In the fused glass disc, a homogeneous compressive stress condition is created through the entire cross section.
Why is Metaglas So Strong?
When a bending moment (Illustration 1) is superimposed on the existing residual stress created in the manufacturing process, the result is curve c. It can be seen that in the case of the thermally pre-stressed (tempered) glass disc, a tensile stress area is formed.
In the case of Metaglas fused glass, the load causes the glass to be placed in the compression stress area. This results in the value of the critical stress Kc (ultimate or fracture stress) being exceeded with differing and important conditions in the two glass discs. Because the tempered disc falls within the tensile area, it fails completely by shattering or bursting causing unsafe or even catastrophic results, almost always without warning.
Having an over-stressed condition in the Metaglas disc, whereby the value Kc is exceeded, the stress falls in the compression area and the stress value becomes smaller than Kc once again and results in stopping the crack. For every continuation of a crack, a renewed application of kinetic energy is created from residual compression (potential energy) thereby preventing a sudden bursting of the glass.
Rigorous tests by major international chemical companies have demonstrated Metaglas's extreme resistance to impact, bending, pressure, over-tightening and rupture. Rupture tests at pressures far exceeding the design pressure have shown that even after formation of a large number of cracks due to extreme pressures, Metaglas safety glass remained leaktight.
Pressure and Temperature Cycling:Gradual overpressurizing of a Metaglas disc rated at 100 PSIG resulted in slight crack formation on the outside of the glass at 1200 PSIG; however, the glass remained leaktight. At 1950 PSIG (20 times the rated pressure) thin layers of glass on the atmosphere side began to separate but still the glass remained leaktight.A Metaglas disc rated at 150 PSIG was pressurized to 5700 PSIG (38 times rated pressure) with the same result; internal cracks but leaktight.Metaglas discs rated 375 PSIG were successfully cycled 2500 times between atmospheric pressure and 375 PSIG for 3 months at temperatures from 68°F to 428°F.A Metaglas disc rated 150 PSIG was successfully cycled from atmospheric pressure to 150 PSIG every 3 seconds during six hours, then to 220 PSIG every 3 seconds for one hour.
Impact Testing:An impact of 36 Nm (maximum capacity of the test apparatus) on a 15 mm thick Metaglas disc using a 30 kg drop hammer resulted in local pitting of the glass which affected neither its functioning nor its tightness. By contrast, an impact of 15 Nm on a conventional glass disc resulted in complete shatter.
Bending:A 20mm thick Metaglas disc was mounted between flanges using 2mm thick IT-300 gaskets plus one short 2mm thick gasket fragment to create unevenness. Tightening the flanges with bolts and nuts with a torque of 130 Nm resulted in cracks in the glass but the glass remained leaktight. The same test repeated with loose glass discs resulted in complete shatter between 40 and 55 Nm.
Erosion:After being aggressively eroded from a thickness of 15mm down to 6mm, a Metaglas disc resisted bursting pressure of 85 bar before the gasket failed.
Mechanically pre-stressed glass as stated earlier is a glass disc fused in a metal ring. Glass used in this process is either borosilicate or soda lime.
The advantages of borosilicate over soda lime glass are increased strength, better pressure and temperature shock, endurance, higher allowable temperature gradient, better resistance in steam and condensate environments and it is readily accepted by regulatory bodies.
The metal ring encasing the glass disc can be of various materials. Duplex stainless steel is the preferred choice. The duplex stainless steels can be thought of as chromium-molybdenum ferritic stainless steels to which sufficient austenite stabilizers have been added to produce steels in which a balance of ferrite and austenite is present at room temperature. Such grades can have the high chromium and molybdenum content of ferritic stainless steels. In fact, the duplex grades with about equal amounts of ferrite and austenite have excellent toughness and their strength exceeds either phase that are present singly.
The addition of nitrogen to the newer duplex grades restores the phase balance more rapidly and minimizes chromium and molybdenum segregation without annealing. The new duplex grades combine high strength, good toughness, high corrosion resistance, good resistance to chloride SCC, and good production economy in the heavier product forms.
Compared with type 316, the annealed duplex alloys provide improved resistance to chloride stress-corrosion cracking. Another useful characteristic of the duplex grades is that they typically have yield strengths more than twice the conventional austenitic steels. In thicker sections, the duplex alloys are more impact resistance than ferritic alloys. Duplex stainless steels are alloyed with .15 to .20% Nitrogen. This minimizes alloy element segregation between the ferrite and austenite, thereby improving the as welded corrosion resistance compared with the type 329 alloy. The nitrogen addition also increases the precipitation of austenite during casting and welding and prevents high-ferrite content in rapidly cooled welds.
In addition to duplex stainless steel, the metal rings can be manufactured from carbon steel, Hastelloy C and Monel with temperature ranges varying from -76°F to 572°F, depending on the alloy.
Metaglas products include discs, flanges, threaded, sanitary clamp and aseptic sight windows to existing standards or to customer specifications. Metaglas fused glass discs are used by clamping the disc between flanges, e.g. as a replacement for conventional glass discs in sight windows and visual flow indicators.
Sizes (outside diameter and thickness) and tolerances are available to fit: Raised Faced ANSI B16.5 flanges (bolt on) sizes 1/2 to 12 in., DIN 28120 Sight Port (weld type), Visual Flow Indicators, and Sanitary Clamp discs with special configurations to meet Sanitary 3-A specifications sizes from 1in. to 12 in.
Additional Disc ProductsDiscs sized to fit between tongue and groove and RTJ flangesDiscs having dimensions other than standard sizes. Outside diameters range from 20-365 mm (3/4 in. to 14 in.). Pressure ranges can be to several thousand PSI depending on dimensions (outside diameter, view diameter, and thickness).Flange-Type for flat face (with holes) directly bolted to an ANSI flat face flange without a retainer. These discs, however, require a parallel flat face mating flange and a full face gasket. Sizes range from 1-1/2 in. to 8 in. Threaded Metaglas consists of glass inside a smooth-bore threaded bushing, flush on the bottom for threading into a female outlet or cover plate. Configurations are round headed bushings with metric or BSP thread and hexhead Bushings with NPT-thread, with a size range of 3/8 in. to 2 1/2 in. NPT. Back to the Top Special Adaptations for Metaglas Metaglas sight windows may be equipped with accessories such as sight lights, window wipers, etc. Drilling into the metal ring is possible but must be done in a non-critical area as to not interfere with the stress field. A circumferential area can be designated or calculated. Welding can be done only with extreme care to avoid thermal warping and the destruction of the compressive stress field. Welding is not encouraged although it has been successfully accomplished. Back to the Top Advantages of Metaglas Besides its suitability to new installations, Metaglas safety glass is intended for conversion or replacement of conventional glass discs in existing apparatus, containers, tanks, vessels and in visual flow indicators. Like conventional sight glass installations, Metaglas safety glass discs can be clamped between a weld pad or connection flange and a retaining flange. However, unlike conventional sight glasses, the use of a gasket between the disc and cover flange is optional and a torque wrench is not required because the design virtually eliminates the risk of over-torquing. To protect against chemical attack (sodium hydroxide, hot concentrated alkaline solutions, phosphoric acid, fluorine), Metaglas windows can be supplied with a mica or Teflon film shield, double window, etc. Contrary to conventional glass discs, which must be discarded, Metaglas windows which have suffered chemical or mechanical damage (opacity, erosion, scratches) can often be repolished. To date, Metaglas has been approved on vessels to 960 PSIG in accordance with TÜV in the following countries: Austria, Belgium, France, Germany, Netherlands, Switzerland. In addition, has also been approved by Factory Mutual in the USA. Back to the Top Summary Metaglas safety glass is FM approved and suitable for use in many applications using circular flange assemblies for welding into vessels or tanks, for weld neck or nozzle flanges, for sight flow indicators, and as cover flanges for flat-face base flanges. For ANSI flange connections, DIN and British Standard installations, Metaglas safety glass can easily be retrofitted. Sanitary clamp designs are 3-A approved for use in sterile processing applications. Due to its superior safety ratings, Metaglas is already in use in over 500 international companies in the chemical, pharmaceutical, biochemical, petrochemical, paper and food industries. Universities, research facilities and NASA also specify Metaglas to enhance safety factors. Other users include engineering and construction firms as well as manufacturers of equipment for underwater research, sterilization, centrifuges, separators, crystallizers, extraction apparatus, turbidity meters, autoclaves, filters, mixers, pressure vessels, housings for measuring scales, condensate traps, pumps, compressors and electrical switch housings. Aside from the obvious safety responsibilities, any plant which uses sight windows can eliminate potential leaks, spills, contamination, torque failure and glass replacement down time by fitting equipment with Metaglas mechanically prestressed safety glass sight windows. In addition, Metaglas addresses environmental concerns by minimizing the potential for hazardous spills, pollution and contamination. For more information: Andrew R. Obertanec, VP of Operations, L.J. Star Inc., P.O. Box 1116, Twinsburg, OH 44087. Tel: 330-405-3040. Fax: 330-405-3070. Back to the Top
Threaded Metaglas consists of glass inside a smooth-bore threaded bushing, flush on the bottom for threading into a female outlet or cover plate. Configurations are round headed bushings with metric or BSP thread and hexhead Bushings with NPT-thread, with a size range of 3/8 in. to 2 1/2 in. NPT.
Metaglas sight windows may be equipped with accessories such as sight lights, window wipers, etc. Drilling into the metal ring is possible but must be done in a non-critical area as to not interfere with the stress field. A circumferential area can be designated or calculated.
Welding can be done only with extreme care to avoid thermal warping and the destruction of the compressive stress field. Welding is not encouraged although it has been successfully accomplished.
Besides its suitability to new installations, Metaglas safety glass is intended for conversion or replacement of conventional glass discs in existing apparatus, containers, tanks, vessels and in visual flow indicators. Like conventional sight glass installations, Metaglas safety glass discs can be clamped between a weld pad or connection flange and a retaining flange. However, unlike conventional sight glasses, the use of a gasket between the disc and cover flange is optional and a torque wrench is not required because the design virtually eliminates the risk of over-torquing.
To protect against chemical attack (sodium hydroxide, hot concentrated alkaline solutions, phosphoric acid, fluorine), Metaglas windows can be supplied with a mica or Teflon film shield, double window, etc. Contrary to conventional glass discs, which must be discarded, Metaglas windows which have suffered chemical or mechanical damage (opacity, erosion, scratches) can often be repolished. To date, Metaglas has been approved on vessels to 960 PSIG in accordance with TÜV in the following countries: Austria, Belgium, France, Germany, Netherlands, Switzerland. In addition, has also been approved by Factory Mutual in the USA.
Metaglas safety glass is FM approved and suitable for use in many applications using circular flange assemblies for welding into vessels or tanks, for weld neck or nozzle flanges, for sight flow indicators, and as cover flanges for flat-face base flanges. For ANSI flange connections, DIN and British Standard installations, Metaglas safety glass can easily be retrofitted. Sanitary clamp designs are 3-A approved for use in sterile processing applications.
Due to its superior safety ratings, Metaglas is already in use in over 500 international companies in the chemical, pharmaceutical, biochemical, petrochemical, paper and food industries. Universities, research facilities and NASA also specify Metaglas to enhance safety factors. Other users include engineering and construction firms as well as manufacturers of equipment for underwater research, sterilization, centrifuges, separators, crystallizers, extraction apparatus, turbidity meters, autoclaves, filters, mixers, pressure vessels, housings for measuring scales, condensate traps, pumps, compressors and electrical switch housings.
Aside from the obvious safety responsibilities, any plant which uses sight windows can eliminate potential leaks, spills, contamination, torque failure and glass replacement down time by fitting equipment with Metaglas mechanically prestressed safety glass sight windows. In addition, Metaglas addresses environmental concerns by minimizing the potential for hazardous spills, pollution and contamination.
For more information: Andrew R. Obertanec, VP of Operations, L.J. Star Inc., P.O. Box 1116, Twinsburg, OH 44087. Tel: 330-405-3040. Fax: 330-405-3070.
Get the latest articles from Pharmaceutical Online delivered to your inbox.