Specifications

The following material is excerpted from a standard titled
Gauges-Pressure Indicating Dial Type-Elastic Element
(AN-SI/ASME B40.1-1985) as published by The American
Society of Mechanical Engineers, 345 East 47th St., New
York, NY 10017. This information is furnished to assist the
user of Dwyer Spirahelic
®
gages in properly evaluating their
suitability for the intended application and conditions.
4 SAFETY
4.1 Scope
This Section of the Standard presents certain information to
guide users, suppliers, and manufacturers toward minimizing
the hazards that could result from misuse or misapplication of
pressure gauges with elastic elements. The user should
become familiar with all sections of this Standard, as all
aspects of safety cannot be covered in this Section. Consult
the manufacturer or supplier for advice whenever there is
uncertainty about the safe application of a pressure gauge.
4.2 General Discussion
4.2.1 Adequate safety results from intelligent planning and
careful selection and installation of gauges into a pressure sys-
tem. The user should inform the supplier of all conditions per-
tinent to the application and environment so that the supplier
can recommend the most suitable gauge for the application.
4.2.2 The history of safety with respect to the use of pres-
sure gauges has been excellent. Injury to personnel and dam-
age to property have been minimal. In most instances, the
cause of failure has been misuse or misapplication.
4.2.3 The pressure sensing element in most gauges is sub-
jected to high internal stresses, and applications exist where
the possibility of catastrophic failure is present. Pressure reg-
ulators, chemical (diaphragm) seals, pulsation dampers or
snubbers, syphons, and other similar items, are available for
use in these potentially hazardous systems. The hazard
potential increases at higher operating pressure.
4.2.4 The following systems are considered potentially haz-
ardous and must be carefully evaluated:
(a) compressed gas systems
(b) oxygen systems
(c) systems containing hydrogen or free hydrogen atoms
(d) corrosive fluid systems (gas and liquid)
(e) pressure systems containing any explosive or flammable
mixture or medium
(f) steam systems
(g) non-steady pressure systems
(h) systems where high overpressure could be accidentally
applied
(i) systems wherein interchangeability of gauges could result
in hazardous internal contamination or where lower pressure
gauges could be installed in higher pressure systems
(j) systems containing radioactive or toxic fluids (liquids or
gases)
(k) systems installed in a hazardous environment
4.2.5 When gauges are to be used in contact with media
having known or uncertain corrosive effects or known to be
radioactive, random or unique destructive phenomena can
occur. In such cases the user should always furnish the sup-
plier or manufacturer with information relative to the applica-
tion and solicit his advice prior to installation of the gauge.
4.2.6 Fire and explosions within a pressure system can
cause pressure element failure with very violent effects, even
to the point of completely disintegrating or melting the pres-
sure gauge. Violent effects are also produced when failure
occurs due to:
(a) hydrogen embrittlement
(b) contamination of a compressed gas
(c) formation of acetylides
(d) weakening of soft solder joints by steam or other heat
sources
(e) weakening of soft soldered or silver brazed joints caused
by heat sources such as fires
(f) corrosion
(g) fatigue
(h) mechanical shock
(i) excessive vibration
Failure in a compressed gas system can be expected to
produce violent effects.
4.2.7 Modes of Elastic Element Failure. There are four basic
modes of elastic element failure, as follows.
4.2.7.1 Fatigue Failure. Fatigue failure caused by pressure
induced stress generally occurs from the inside to the outside
along a highly stressed edge radius, appearing as a small
crack that propagates along the edge radius. Such failures
are usually more critical with compressed gas media than with
liquid media.
Fatigue cracks usually release the media fluid slowly so case
pressure buildup can be averted by providing pressure relief
openings in the gauge case. However, in high pressure elas-
tic elements where the yield strength approaches the ultimate
strength of the element material, fatigue failure may resemble
explosive failure.
A restrictor placed in the gauge pressure inlet will reduce
pressure surges and restrict fluid flow into the partially open
Bourdon tube.
4.2.7.2. Overpressure Failure. Over pressure failure is
caused by the application of internal pressure greater than the
rated limits of the elastic element and can occur when a low
pressure gauge is installed in a high pressure port or system.
The effects of overpressure failure, usually more critical in
compressed gas systems than in liquid filled systems, are
unpredictable and may cause parts to be propelled in any
direction. Cases with pressure relief openings will not always
retain expelled parts.
Placing a restrictor in the pressure gauge inlet will not reduce
the immediate effect of failure, but will help control flow of
escaping fluid following rupture and reduce potential of sec-
ondary effects.
It is generally accepted that solid front cases with pressure
relief back will reduce the possibility of parts being projected
forward in the event of failure.
The window alone will not provide adequate protection
against internal case pressure buildup, and can be the most
hazardous component.
4.2.7.3 Corrosion Failure. Corrosion failure occurs when
the elastic element has been weakened through attack by cor-
rosive chemicals present in either the media inside or the envi-
ronment outside it. Failure may occur as pinhole leakage
through the elements walls or early fatigue failure due to stress
cracking brought about by chemical deterioration or embrittle-
ment of the material.
A chemical (diaphragm) seal should be considered for use
with pressure media that may have a corrosive effect on the
elastic element.
4.2.7.4 Explosive Failure. Explosive failure is caused by the
release of explosive energy generated by a chemical reaction
such as can result with adiabatic compression of oxygen
occurs in the presence of hydrocarbons. It is generally
accepted that there is no known means of predicting the mag-
nitude or effects of this type of failure. For this mode of failure,
a solid wall or partition between the elastic element and the
window will not necessarily prevent parts being projected for-
ward.
4.2.8 Pressure Connection. See recommendations in
paragraph 3.3.4.
4.3 Safety Recommendations.
4.3.1 Operating Pressure. The pressure gauge selected
should have a full scale pressure such that the operating pres-
sure occurs in the middle half (25 to 75%) of the scale. The
full scale pressure of the gauge selected should be approxi-
mately two times the intended operating pressure.
Should it be necessary for the operating pressure to exceed
75% of full scale, contact the supplier for recommendations.
This does not apply to test, retarded, or suppressed scale
gauges.
4.3.2 Use of Gauges Near Zero Pressure. The use of
gauges near zero pressure is not recommended because the
accuracy, tolerance may be a large percentage of the applied
pressure. If, for example, a 0/100 psi Grade B gauge is used
to measure 6 psi , the accuracy of measurement will be ±3 psi,
or ±50% of the applied pressure. In addition, the scale of a
gauge is often laid out with takeup, which can result in further
inaccuracies when measuring pressures that are a small per-
centage of the gauge span.
For the same reasons, gauges should not be used for the
purpose of indicating that the pressure in a tank, autoclave, or
other similar unit has been completely exhausted to atmos-
pheric pressure. Depending on the accuracy and the span of
the gauge and the possibility that takeup is incorporated at the
beginning of the scale, hazardous pressure may remain in the
tank even though the gauge is indicating zero pressure. A
venting device must be used to completely reduce the pres-
sure before unlocking covers, removing fittings, or performing
other similar activities.
4.3.3 Compatibility With the Pressure Medium. The
elastic element is generally a thin walled member, which of
necessity operates under high stress conditions and must,
therefore, be carefully selected for compatibility with the pres-
sure medium being measured. None of the common element
materials is impervious to every type of chemical attack. The
potential for corrosive attack is established by many factors,
including the concentration, temperature, and contamination
of the medium. The user should inform the gauge supplier of
the installation conditions so that the appropriate element
materials can be selected.
4.3.4 In addition to the factors discussed above, the capability
of a pressure element is influenced by the design, materials, and
fabrication of the joints between its parts.
Common methods of joining are soft soldering, silver brazing,
and welding. Joints can be affected by temperature, stress,
and corrosive media. Where application questions arise, these
factors should be considered and discussed by the user and
manufacturer.
4.3.5 Some special applications require that the pressure ele-
ment assembly have a high degree of leakage integrity. Special
arrangement should be made between manufacturer and user
to assure that the allowable leakage rate is not exceeded.
4.3.6 Cases
4.3.6.1 Cases, Solid Front. It is generally accepted that a
solid front case per paragraph 3.3.1 will reduce the possibility of
parts being projected forward in the event of elastic element
assembly failure. An exception is explosive failure of the elastic
element assembly.
4.3.6.2 Cases, Liquid Filled. It has been general practice
to use glycerine or silicone filling liquids. However, these fluids
may not be suitable for all applications. They should be avoid-
ed where strong oxidizing agents including, but not limited to,
oxygen, chlorine, nitric acid, and hydrogen peroxide are
involved. In the presence of oxidizing agents, potential hazard
can result from chemical reaction, ignition, or explosion.
Completely fluorinated or chlorinated fluids, or both, may be
more suitable for such applications.
The user shall furnish detailed information relative to the appli-
cation of gauges having liquid filled cases and solicit the advice
of the gauge supplier prior to installation.
Consideration should also be given to the instantaneous
hydraulic effect that may be created by one of the modes of fail-
ure outlined in paragraph 4.2.7. The hydraulic effect due to
pressure element failure could cause the window to be project-
ed forward even when a case having a solid front is employed.
4.3.7 Restrictor. Placing a restrictor between the pressure
connection and the elastic element will not reduce the immedi-
ate effect of failure, but will help control flow of escaping fluid fol-
lowing rupture and reduce the potential of secondary effects.
4.3.8 Specific Service Conditions
4.3.8.1 Specific applications for pressure gauges exist where
hazards are known. In many instances, requirements for
design, construction, and use of gauges for these applications
are specified by state or federal agencies or Underwriters
Laboratories, Inc. Some of these specific service gauges are
listed below. The list is not intended to include all types, and the
user should always advise the supplier of all application details.
4.3.8.2 Acetylene Gauges. A gauge designed to indicate
acetylene pressure. It shall be constructed using materials that
are compatible with commercially available acetylene.
4.3.8.3 Ammonia Gauge. A gauge designed to indicate
ammonia pressure and to withstand the corrosive effects of
ammonia. The gauge may bear the inscription AMMONIA on
the dial. It may also include the equivalent saturation tempera-
ture scale markings on the dial.
4.3.8.4 Chemical Gauge. A gauge designed to indicate the
pressure of corrosive or high viscosity fluids, or both. The pri-
mary material(s) in contact with the pressure medium may be
identified on the dial. It may be equipped with a chemical
(diaphragm) seal, pulsation damper, or pressure relief device, or
a combination. These devices help to minimize potential dam-
age to personnel and property in the event of gauge failure.
They may, however, also reduce accuracy or sensitivity, or both.
4.3.8.5 Oxygen Gauge. A gauge designed to indicate oxy-
gen pressure. Cleanliness shall comply with Level IV (see
Section 5). The dial shall be clearly marked with a universal
symbol and/or USE NO OIL in red color (see paragraph
6.1.2.1).
4.4 Reuse of Pressure Gauges
It is not recommended that pressure gauges be moved from
one application to another. Should it be necessary, however,
the following must be considered.
4.4.1 Chemical Compatibility. The consequences of incom-
patibility can range from contamination to explosive failure. For
example, moving an oil service gauge to oxygen service can
result in explosive failure.
4.4.2 Partial Fatigue. The first installation may involve pres-
sure pulsation that has expended most of the gauge life, result-
ing in early fatigue in the second installation.
4.4.3 Corrosion. Corrosion of the pressure element assem-
bly in the first installation may be sufficient to cause early failure
in the second installation.
4.4.4 Other Considerations. When reusing a gauge, all
guidelines covered in this Standard relative to application of
gauges should be followed in the same manner as when a new
gauge is selected.
DWYER INSTRUMENTS, INC.
Phone: [phone] www.dwyer-inst.com
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©Copyright 2003 Dwyer Instruments, Inc.
Printed in U.S.A. 9/03
FR# 17-443027-00 Rev. 2
E-105 9/11/03 8:13 AM Page 4