The following article was printed in the September 1997 issue of Chemical engineering
Magazing
CE
Publication :
Date : September, 1997
Copyright : Copyright 1997 The McGraw-Hill Companies, Inc.
Volume : 104
Issue : 9
Page : 130
Section : ENGINEERING PRACTICE
A NEW LOOK AT SIGHT GLASS GAGES
The ubiquitous glass port is actually a finely engineered product. Don't
neglect its care and feeding
Scott Markman Schott Corp.
For the chemical process industries (CPI) and related applications, the sight glass is a
favored attachment for providing visual observation and control of processes involving
liquids or slurries. No other method of gauging fluid levels, properties, reactions or flows
matches sight-glass gages (aka sight gages) for accuracy. With glass, ``What you see is
what you get.''
Engineers generally agree that gages based on a sight glass remove certain risks inherent
with magnetic or mechanical gages where views are always indirect. Those rely on
magnetic links and separate mechanical devices.
The pricing of glass is generally 25-33% less than that of magnetic gages. The advantage
of no moving parts leads to low-cost maintenance. Glass gages are not dependent upon
outside power sources, and continue to operate reliably if there is an outage.
Glass is not practical for every gauging application. For example, magnetic gages are
preferred when operating conditions involve steam above 1,200 psig working steam
pressure, where conditions dictate a closed system, or where highly aggressive or lethal
substances, such as certain metallic hydroxides or hydrofluoric acid, are being handled. It
is impractical to install sight gages in locations that are not visually accessible.
The shape of the gage
The most common configurations for gage glasses are long-form and round (or discs).
Long-form gage glasses are available in transparent or reflex versions. I will discuss all
three.
We call the garden-variety round gage glass option the welding-pad gage. Its prime use is
to permit visual inspection into process vessels (Figure 1). These gages are welded
directly to the tank or vessel shell and become an integral part of it. Some types of
welding-pad gages can be isolated from the contents of the process vessel by valves and
pipes.
Long-form, also called transparent and reflex (prismatic) gages, usually are isolated from
vessels by pipes and valves (Figure 2). Users base specifications for long-form gages on
their particular application.
Long-form transparent glass gages. These gages are constructed to allow the liquid to be
seen through a glass ``sandwich'' (Figure 3). Consider their use when:
-- The liquid being viewed is opaque
-- There is a need to see through the liquid to check for an interface or the presence of
suspended particles or color
-- The substance being observed is boiling, viscous, volatile or will attack glass
-- Observing process media whose refractive index is similar to glass or for processes
where surging occurs. In this case specify large-chamber gages
Where aggressive substances will attack the glass, mount flat shields of mica or Daikin1
brand polychlorotrifluoroethylene (PCTFE) on the process side of the glass. Mica resists
the corrosive effects of hot alkaline or acidic solutions and protects the glass from
condensate-caused leaching.
PCTFE resists attack by many acids and other corrosives. This extends the life of the
glass. Mica or polymer sheeting are recommended to protect the exterior glass surface
when gages are outdoors or in locations where spraying or wetting can cause thermal
shock.
Long-form reflex-glass gages. These gages have been designed with multiple 90-deg
angle prisms molded into the glass on the process side, which permit viewing from
various angles (Figure 4). Reflex-glass is specified when clear or semi-clear liquids are
involved or when glass attack is minimal. Due to total refraction and absorption, the
liquid appears black in contrast to the reflective mirror-like surface above the level of the
liquid.
The application of reflex-glass gages is limited to specialized situations even though there
is a simplicity advantage with only one sealing gasket. They must be used singly, not in
``sandwich'' form. We do not recommend them for viewing liquid-liquid interfaces or in
areas where ambient lighting is low. Shields cannot be used to protect reflex sight glasses
from corrosive substances because of the prism design.
Chemical properties
Resistance to high pressure, sharp temperature changes and chemical attack are important
requirements for most gages. Given the extreme conditions under which gage and safety
sight glasses find application, Maxos2 is one tempered borosilicate gage glass that meets
these criteria. It exhibits excellent resistance to attack by water, neutral, acidic and saline
solutions as well as chlorine, bromine, iodine and organic substances. In combination
with mica, Maxos provides even higher total resistance against extreme pressure,
chemical attack, temperature and thermal changes.
Tempering is required to create a particularly high resistance to sudden temperature
changes. It is also required to absorb pressures resulting from mounting the gage and
from the vessel pressure itself. For borosilicate sight glasses, the quality of tempering and
the overall tempering process is extremely important. For safety reasons, the stress to the
glass caused by internal forces, thermal stress, mounting conditions, and vessel pressures
must be absorbed by the surface compressive strength of the glass. Control of flatness,
surface quality, and the lack of chips, cracks, bubbles and striae are crucial in meeting
these criteria.
The useful ranges for tempered borosilicate glass specially produced for sight glass
applications are pressures to 6,000 psi (41,400 kPa) and temperatures ranging from -300
degrees F (-184 degrees C) to +608 degrees F (320 degrees C). See Table 1 for
application conditions and Table 2 for specifications for corrosives. For higher
temperatures, aluminosilicate glass protected with mica sheeting is normally specified.
Even over long periods of time at temperatures of approximately 212 degrees F (100
degrees C) borosilicate glass exceeds the chemical resistance of most metals and other
materials. Exposure to water and acids results in only small amounts of mostly
monovalent ions leaching from the glass. In this process, a very thin, highly resistant
layer forms on the outer surface of the glass. This inhibits additional attack.
For steam service, apply mica shielding to the gage glass. We recommend mica shielding
for gages used in high-pressure, high-temperature process applications. But in such
instances, consult the gage manufacturer first.
Certain minimum amounts of surface compressive strength3 must be built into the glass
and should be guaranteed by the manufacturer. Maxos, for example, guarantees at least
90 to 140 N/mm2or 13,000 to 20,000 psi. The compressive strength and a means to check
it are specified in various standards [1-10].
Installation
Properly tempered borosilicate gage glass has high resistance to chemical attack and to
thermal shock. Additional stress loads placed on it during installation can adversely
impact thermal shock resistance. But like any glass, it is much stronger in compression
than it is in tension. Therefore, users should be careful to avoid any bending of the glass
during installation and to avoid setting up any local stresses during operation.
Normal expansion and contraction of pressure and process vessels from heating and
cooling can place stress on sight gage glass. Generally, thermal expansion problems do
not occur when sight glass gages are attached to, and supported by, vessels and associated
piping. They expand and contract in concert with the vessel under normal operating
conditions.
Differential thermal expansion can occur if the gage glass is isolated -- such as behind
closed valves -- allowing gage glass to cool, resulting in a reduction in its length. This
can impose stress on the valves, piping and glass itself. The longer the gage, the worse
the stress on the glass.
To avoid this, add a flexible member such as an expansion loop, to the assembly.
Bracketing is recommended if gages are more than 50 in. (19.7 cm) in length or weigh
over 100 lb (45 kg), especially if they are subjected to vibration. Accommodate
expansion and contraction when brackets are used to support gages.
High-temperature applications call for high-temperature materials, provisions for thermal
expansion, and insulation material to reduce temperature variation. As noted earlier,
mica-protected aluminosilicate glass should be used in applications above 608 degrees F.
Gages used in high-pressure or high-temperature applications should be positioned so
that viewing can be through mirrors, fiber optic links or monitors which would provide
an extra margin of safety in case of failure.
Operations and maintenance
In operation, avoid rapid opening of connecting valves, which could result in breakage
and injury. If process streams are hot, warm the gages slowly by carefully cracking open
the shutoff valve. Gage manufacturers recommend the use of valves with safety shut-off
mechanisms when permitted by prevailing codes and standards.
Stresses can be placed on gage glasses from sources other than improper installation and
maintenance. For example, gages exposed to corrosive atmospheres (such as found in
marine environments) may experience corrosion build-up on metal parts. They in turn
expand, thereby distorting gaskets and placing pressure points on the glass surface
(Figure 5).
Don't just install it and forget it. Ongoing maintenance is also important. Gage glass
requires a maintenance schedule depending on service conditions. Maintenance includes
retorquing of the bolting, changing glass and gasketing as well as checking flatness
specifications. In dirty services, periodic internal cleaning is required. We recommend an
occasional blow-down to remove sediment. Remember to isolate the gages from the
process system by closing upper and lower valves, then drain the gage to relieve pressure
before performing any maintenance operations. A troubleshooting checklist, as in Table
3, is useful.
Do regular visual inspection for signs of clouding or scratching. In new processes, inspect
glass daily until the need for replacement becomes apparent. This will help establish a
routine inspection cycle.
When inspecting, use a very bright concentrated light held at a 45-deg angle to the glass
surface and look carefully for crescent-shaped marks or at any scratch that glistens or
catches a fingernail. These wear indicators signal a need for replacement.
If the process side surface appears cloudy or roughened and will not respond to cleaning
procedures, it is evidence of chemical attack and cause for replacement. Gasket or
connection leaks should be repaired immediately.
When cleaning a sight glass do not use harsh abrasives that can scratch the surface.
Cleaning should be done without removing the glass. If the process side of the glass is
inaccessible, cleaning can be accomplished by recirculating the cleaning material in the
vessel and gage. We give this a label; flow-cleaning.
Safety precautions
-- Personnel removing or installing sight gages must wear safety glasses. Contained fluids
may unexpectedly exit the vessel connections due to apparatus or material failure
-- At replacement time, old sight glasses, shields and gasketing should never be reused,
and should be disposed of in a proper manner
-- Clean metallic surfaces that mate with gasketing with a scraper made of soft-metal
such as brass, to remove all traces of old gasketing. Surfaces must be flat to within 0.002
in. (0.051 mm) and have a surface finish of approximately 450 to 500 A.A.R.H.
(arithmetic average roughness height)
-- If metal surfaces that mate with new sight-glass gaskets are corroded, eroded, pitted or
deeply scratched, consult the manufacturer before remachining as this may void
performance warranties of the gage
-- When handling new sight-gage glass prior to installation, use extreme care to avoid
scratching or chipping, as these can result in failure points. Do not lay new gage glass on
work surfaces because detrimental foreign material may adhere to its surface. Do not
place the glass in service if scratches or chips are observed
-- Follow the manufacturer's instructions on the proper torquing sequence and bolt torque
values during the installation process to avoid abnormal stress to the glass
Other considerations
Sight-gage glass, while crucial, is but one part of an assembly where specifiers have a
virtually infinite choice in materials, components and sizes. Fluids or slurries being
handled, corrosive conditions (both internal and external), temperatures, atmospheres
(inside the plant and outdoors) and other criteria will govern not only the type of sight
gage, but alloys and protective coatings used for the gage as well as the type and
construction of supporting valves and piping. Insulators, non-frosting extensions and
illuminators are available to accommodate special conditions. Manufacturers'
recommendations should be sought in written specifications when sight glass gages are
required for a new application.
1. Formely Kel-F, a trademark of 3M Co.
2. Maxos is a registered trademark of Auer-Sog, a Schott Group Co., and is distributed in
the U.S. by Schott Corp.
3. Surface compressive strength is introduced into the glass by means of a thermal
process, but calculated without the inherent resistance of the glass itself.
Edited by Peter M. Silverberg
Acknowledgments The author thanks the following for assistance in the preparation of
this article: Jerguson Gage & Valve Co., Div. of Clark-Reliance Corp., Strongsville,
Ohio, and Penberthy, Inc., Prophetstown, Ill.
[Table]
TABLE 1.
UPPER LIMITS OF SUGGESTED APPLICATION CONDITIONS
Maximum
Maximum
Application Conditions
Permissible Pressure
Permissible
Temperature
bars
psia
C
F
Saturated steam or hot water
in direct contact with reflex
35
507
243
470
or transparent sight glass
Saturated steam or hot water
in direct contact with
103
1,490
320
608
transparent glass protected
with mica
Transparent sight glasses in con-
tact with media with no techni- 345
100
cally significant glass attack
High-pressure transparent sight
glasses in special armatures
414
100
5,000
38
6,000
38
DIN 7080 governs dimensional tolerances for disc sight glasses as well
as provides the formula to calculate glass thickness. The working
pressure values given for DIN-glasses guarantee 5-fold safety. That is,
they are subjected to a test pressure that is least 5 times higher than
the working pressure.
[Table]
TABLE 2.
TESTS FOR CORROSION RESISTANCE
Chemical Characteristic
Resistance
Test according to [11-17]
52322 or
Hydrolytic
Resistance
Acid
Resistance
Alkali
DIN 12111 or
DIN 12116
DIN
ISO 719 Class 1 Class 1
ISO 695
Class 2
DIN 28817 or
ISO 720 Class 1
Maximum Abrasion (mm)
according to DIN and ISO
than
0.1 or 0.031
0.7
greater
75, less
than
175
[Table]
TABLE 3.
TROUBLESHOOTING
Problem
Glass quickly becomes cloudy
or etched when placed in service
shields
Glass continually breaks
despite attention to
maintenance
Slow visual response to known
changes in process fluid levels
Problem
Glass quickly becomes cloudy
shields that will not be
or etched when placed in service
Cause
Fluids being handled are not
compatible with the glass or
Thermal or hydraulic shock, high
mechanical loads, exceeding
design specs or a combination
of these
Clogged valves or piping to
or from the gage
Solution
Replace glass, use suitable
affected by the process fluid
Glass continually breaks
cause
despite attention to
maintenance
Slow visual response to known
changes in process fluid levels
piping
Check entire system to determine
of excessive stress, and contact
gage manufacturer
Shut down the process, remove
and clean or replace valves or
[Photograph]
Photograph: FIGURE 1. A welding-pad gage allows observation into a process vessel
[Photograph]
Photograph: FIGURE 2. Long-form gages are mounted where observation is convenient
[Illustration]
Illustration: FIGURE 3. Transparent gages are used when the liquid being viewed is
opaque or there is a need to see through the liquid to check for interface, the presence of
suspended particles or color. This cross-section of a long-form sight-glass gage uses
transparent glass in a ``sandwich'' configuration
[Illustration]
Illustration: FIGURE 4. Reflex-glass gages have prisms that permit viewing from various
angles and are used when clear or semi-clear liquids are involved, or when glass attack is
minimal
STEPHEN WALSH
[Photograph]
Photograph: FIGURE 5. A build-up of corrosion products can lead to stresses that can
cause failure
[References]
1. Deutsches Institut fur Normung, Pressure Resistant Circular Toughened Borosilicate
Glass Panes, DIN 7080, Berlin, February 1996.
2. Deutsches Institut fur Normung, Pressure Resistant Oblong Toughened Borosilicate
Glass Panes for Sight Glasses, DIN 7081, Berlin, February 1996.
3. British Standards Institution, Specification for Observation and Gauge Glasses for
Pressure Vessels, BS 3463, London, 1975.
4. ON Osterreichisches Normungsinstitut, Pressure Resistant Circular Sight Glasses,
ONorm M 7353, Vienna, December 1, 1979.
5. ON Osterreichisches Normungsinstitut, Pressure Resistant Oblong Sight Glasses,
ONorm M 7354, Vienna, October 1, 1977.
6. Japanese Industrial Standards Committee, Gauge Glasses for Boilers, JIS B 8211,
Tokyo, 1975 -- reaffirmed 1978.
7. U. S. Military Specification - Gauges, Boiler-Water, Direct Reading, MIL G 16356 D,
Washington D.C., Dec. 18, 1987.
8. ESSO/Exxon, Corp. Standard, Level Instruments, RP 15-5-1, Section 8.1, Houston,
April 1995.
9. American Soc. of Mechanical Engineers, Boiler and Pressure Vessel Code, ASME B
& PV, Section I, New York, 1996.
10. American Petroleum Institute, Process Measurement Instrumentation, API RP 551,
Washington, D.C., May 1993.
11. Deutsches Institut fur Normung, DIN 12111, Obsoleted - replaced by ISO 719
12. International Organization for Standardization, Glass, Hydrolytic resistance of glass
grains at 98C; Method of test and classification, ISO 719, Geneva, Switzerland, Dec.
1989.
13. Deutsches Institut fur Normung, DIN 28817, Obsoleted - replaced by ISO 720
14. International Organization for Standardization, Glass; Hydrolytic resistance of glass
grains at 121C; Method of test and classification, ISO 720, Geneva, Switzerland, Oct.
1985.
15. Deutsches Institut fur Normung, Testing of Glass; Determination of the Acid
Resistance (Gravimetric Method) and Classification of Glass into Acid classes, DIN
12116, Belin, May 1976.
16. Deutsches Institute fur Normung, DIN 52322, Obsoleted - replaced by ISO 695.
17. International Organization for Standardization, Glass; Resistance to attack by a
boiling aqueous solution of mixed alkali, Method of test and classification, ISO 695,
Geneva, Switzerland, May 1991.
[Biography]
Scott E. Markman is product manager with the Technical Glass Div. of Schott Corp. (3
Odell Plaza, Yonkers, NY 10701; Phone: 914-378-3812, Fax: 914-968-4422). He is
responsible for these product lines: Maxos safety sight-gage glasses, dichroic coated
parabolic reflectors for architectural lighting, stage, film and studio fresnel lenses, traffic
and railroad signal reflectors and medical and dental lighting. Mr. Markman holds an
M.B.A. from George Washington University and a B.A. from the State University of
New York at Stony Brook. Before joining Schott, he worked in marketing at Grumman
Corp.