Reprinted From July 2008 Page 1 P.O. Box 1116 Twinsburg, OH 44087 330-405-3040 www.ljstar.com Page 2 best practices By Andrew Obertanec Are You Gambling with Your Sightglass? Selection Strategies for When Failure Is Not an Option ightglasses are sometimes called “the weakest link” in a S processing system because of the fragility of the glass. Actually, that’s not necessarily true. A sight- glass need not be a weak link in a processing system because glass, when uniquely designed, is not necessarily fragile. General Properties of Glass In fact, strength is actually a key characteristic of glass. In its purest form, glass is one of the strongest composites available in the world — even stronger than steel. The tensile stress of virgin (or untouched) glass is one million PSI, Glass Types as compared to steel’s 60,000 PSI. Common types of industrial glass because soda lime glass is some- In practice, untouched virgin include borosilicate, soda lime, times used — often unknowingly — glass is not a practical option since quartz, and sapphire. Quartz and sap- we will compare borosilicate glass it is sensitive to even tiny imperfec- phire are actually crystals and exhibit to soda lime glass in some detail. tions. Just the touch of your fingers great strength. But sapphire is not would reduce its tensile strength used in processing and quartz only from one million PSI to 1,000 PSI. Glass Selection where especially high temperature is The type of glass best suited to In addition, glass almost always needed. any industrial sightglass application fails under a tensile stress rather Borosilicate glass is the type best should be based on the conditions of than compression. The compressive suited for use in most chemical and service — in particular, working strength is nearly 200 times its ten- pharmaceutical applications, and temperature, chemical exposure, sile strength. therefore this article devotes primary and operating pressure. But even the 1,000 PSI figure attention to this type of glass. Also, does not tell the whole story. Glass is not ductile like steel, so it cannot stretch under tension to absorb the stress. Therefore, imperfections in the glass will create areas of stress concentrations, hastening failure. Unfortunately, when glass fails under pressure, failure is often sud- den and catastrophic. Research has shown that glass can fracture at five miles per second. That’s why safety is a critical factor in sight- glass selection. So, the goal in selecting glass elements for sightglasses is to take advantage of the inherent qualities of glass for a particular application. P.O. Box 1116 Twinsburg, OH 44087 330-405-3040 www.ljstar.com Page 3 USP Glass Standards for Pharmaceutical Use The USP (United States Pharmacopoeia) provides standards that govern the classi- fication of glass into types suitable for specific pharmaceutical uses. USP Type I borosilicate glass is the least reactive glass available and is required for many applications. It can be used for all applications and is most commonly used to package pharmaceuti- cal materials. All lab glass apparatus is generally Type I borosilicate glass. To meet this standard, a sightglass must pass test procedures defined by USP29- 661. Many sightglasses on the market fail to meet this important standard, and some are not even Type II or Type III; rather they are non-parenteral and should not be used in applications where chemi- cal durability and heat shock are factors. USP Type II de-alkalized soda-lime Standard DIN 7080 rates silica with boric oxide. It is, then, com- glass has higher levels of sodium borosilicate glass to 280 C (536 F) prised of silica, boric oxide, and alu- hydroxide and calcium oxide. It is less and Standard DIN 8902 rates soda mina. The commonly accepted resistant to leaching than Type I but more lime sightglass to 150 C (302 F). borosilicate glass standard is an aver- resistant than Type III. It can be used for Very high-temperature equipment, age of 12 percent boron. In addition to products that remain below pH7 for their such as ovens and furnaces, may sightglass windows, borosilicate glass shelf lives. require windows made of quartz. is used in the manufacturer of cook- USP Type III soda-lime glass is The following chart compares the eries, laboratory equipment, and glass acceptable in packaging some dry pow- maximum operating temperatures pipe. ders which are subsequently dissolved to of glass commonly used in sight make solutions or buffers. It is also suit- windows. Chemical Resistance able for packaging liquid formulations In the past, all sightglasses For chemical, biotech, and pharma- that prove to be insensitive to alkali. Type were made of soda lime glass. That ceutical applications, one must con- III glass should not be used for products all changed when Corning sider the effect of chemical exposure that are to be autoclaved, but can be (www.corning.com) developed a over time. Glass can suffer chemical used in dry heat sterilization. process to produce borosilicate degradation from caustic or acidic USP Type NP soda-lime glass is a glass, offered under the brand substances and suffer pitting from general-purpose glass and is used for name Pyrex, and today borosilicate mild abrasion. This may weaken nonparenteral applications (noninjectable is a popular choice for sightglass glass, dim its transparency, and possi- drugs) where chemical durability and construction. The key characteris- bly create crevices where bacteria can heat shock are not factors. As containers, tics of borosilicate glass include hide or leach into the process media. it is frequently used for capsules, tablets, strength, high temperature capabil- Borosilicate glass is significantly more and topical products. ities, corrosion resistance, high resistant to chemicals than soda lime thermal shock resistance and, of glass, as shown in the following chart. Toughened (Tempered) Glass course, the main reason for its use “In investigating for chemically In addition to chemical and thermal – good transparency. induced disintegration of soda lime stresses, sightglasses are also subjected Soda lime glass owes its popu- and borosilicate glass, it was found to stress from pressure and bending larity to its status as a low cost that borosilicate was much more moments in many applications. To help solution. Soda lime glass is used in resistant. This occurred not only at withstand this, the glass can be tough- the manufacture of bottles, light slightly acidic conditions (pH 6), but ened. How it is toughened may affect its bulbs, and windows. It is comprised also when the pH was raised to a suitability for a particular application. of silica, soda, lime, magnesia, and value of 10 (alkaline). As shown on Flat glass is produced utilizing the alumina. the chart, the degradation of soda float glass process, where molten glass is Borosilicate glass is similar, but lime glass is 10 times greater than floated on a bed of molten tin. Then, to it is made by replacing some of the that of Borosilicate glass.” — Translated from VGB KRAFTTWORK- STECHNIK, Dr. A. Peters, Feb. 1979. P.O. Box 1116 Twinsburg, OH 44087 330-405-3040 www.ljstar.com Page 4 anneal it, the hot glass passes Fused-Glass Sightglass the glass, but the glass remains elastic through an “annealing lehr,” where Tempering is not the only way to and able to tolerate bending as long as it is slowly cooled to prevent toughen glass. Glass can also be the compressive force applied by the buildup of internal stresses. strengthened by placing it in mechani- steel ring exceeds the tensile stress on Annealing is commonly used in the cal compression. The most commonly the glass. production of ordinary glass. used mechanically prestressed sight- Taken to the extreme, bending force Further, strength improvement can glass consists of a stainless steel ring can theoretically exceed the compressive be accomplished by toughening or encircling a glass disc. force. But even then, the fused-glass win- tempering the glass in one of two During manufacturing, the glass dow will not shatter. Rather, it will failsafe ways. is melted within the metal ring as the because it retains elasticity. When the Chemical Strengthening: A ring expands. Then, temperature is surface stretches beyond its elastic capa- chemical solution may be applied raised to the point where the glass bility, small cracks usually develop near to the glass in order to increase the and the metal ring fuse together. the surface. The cracks can interfere with mechanical resistance of the glass. When the unit cools, the glass hard- visibility, but they do not compromise Chemically treated glass is com- ens before the metal ring can shrink window integrity. The subsurface glass monly used in applications where back to its original size. This places (product side) is still under compressive thin, strong glass (stronger than the metal ring in tension and the glass stress and can withstand the pressures. window glass) is required. in uniform radial compression. The same mechanism relieves the stress Heat Treating: Annealed glass This compressive force is so great of over-pressurization. In addition, the may be subjected to additional heat that if the metal ring is cut the com- compressive force on the glass can also treating to further increase its pressive force will be released, and resist the effects of thermally induced strength. This process makes heat- the ring will shear from the glass. bending, also known as thermal shock. tempered glass three to five times Some mistakenly believe that this This sometimes happens with a hot ves- as strong as annealed glass. indicates the glass was not fused to sel and a cold-water wash down. Thermally tempered glass is often the ring. Actually, it proves only that used in conventional sightglasses.