©Copyright 2003 Dwyer Instruments, Inc. Printed in U.S.A. 4/03 FR# 17-440819-00 Rev. 1
DWYER INSTRUMENTS, INC.
Phone: 219/879-8000
www.dwyer-inst.com
P.O. Box 373 • Michigan City, IN 46361-0373, U.S.A.
Fax: 219/872-9057
e-mail: [email protected]
The following material is excerpted from a standard
titled
Gauges-Pressure Indicating Dial Type-
Elastic Element
(ANSI/ASME B40.1-1985) as pub-
lished 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 infor-
mation to guide users, suppliers, and manufacturers
toward minimizing the hazards that could result from
misuse or misapplication of pressure gauges with elas-
tic elements. The user should become familiar with all
sections of this Standard, as all aspects of safety can-
not be covered in this Section. Consult the manufac-
turer or supplier for advice whenever there is uncertain-
ty about the safe application of a pressure gauge.
4.2 General Discussion
4.2.1
Adequate safety results from intelligent plan-
ning and careful selection and installation of gauges
into a pressure system. The user should inform the
supplier of all conditions pertinent 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 m use of
pressure gauges has been excellent. Injury to person-
nel and damage 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 subjected to high internal stresses, and
applications exist where the possibility of catastrophic
failure is present. Pressure regulators, chemical
(diaphragm) seals, pulsation dampers or snubbers,
syphons, and other similar items, are available for the
use in these potentially hazardous systems. The hazard
potential increases at higher operating pressure.
4.2.4
The following systems are considered poten-
tially hazardous 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)
nonsteady pressure systems
(h)
systems where high overpressure could be acci-
dentally 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 (liq-
uids 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 supplier or manufacturer with infor-
mation relative to the application 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 pressure gauge. Violent effects are also
produced when failure occurs due to:
(a)
hydrogen enbrittlement
(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 expect-
ed to produce violent effects.
4.2.7 Modes of Elastic Element Failure.
There
are four basic modes of elastic element failure, as fol-
lows.
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 pro-
viding pressure relief openings in the gauge case.
However, in high pressure elastic elements where the
yield strength approaches the ultimate strength of the
element material, fatigue failure may resemble explo-
sive 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.
Overpressure fail-
ure 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 of system. The effects of overpres-
sure 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 secondary 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 pro-
tection 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 the attack by corrosive chemicals present in
either the media inside or the environment outside it.
Failure may occur as pinhole leakage through the ele-
ments walls or early fatigue failure due to stress crack-
ing brought about by chemical deterioration or embrit-
tlement 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 adiabat-
ic compression of oxygen occurs in the presence of
hydrocarbons. It is generally accepted that there is no
known means of predicting the magnitude or effects of
this type of failure. For this mode of failure, a solid wall
or partition between the elastic element and the win-
dow will not necessarily prevent parts being projected
forward.
4.2.8 Pressure Connection.
See recommenda-
tions in para. 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 pressure occurs in the middle half (25 to
75%) of the scale. The full scale pressure of the gauge
selected should be approximately two times the intend-
ed operating pressure.
Should it be necessary for the operating pressure to
exceed 75% of full scale, contact the supplier for rec-
ommendations.
This does not apply to test, retarded, or sup-
pressed 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 per-
centage 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 ±50% of the applied
pressure. In addition, the scale of a gauge is often laid
out with takeup, which can result in further inaccura-
cies 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 atmospheric pressure. Depending on the
accuracy and the span of the gauge and the possibili-
ty 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 per-
forming 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 condi-
tions and must, therefore, be carefully selected for
compatibility with the pressure medium being mea-
sured. 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, sil-
ver brazing, and welding. Joints can be affected by
temperature, stress, and corrosive media. Where appli-
cation questions arise, these factors should be consid-
ered and discussed by the user and manufacturer.
4.3.5
Some special applications require that the pres-
sure element assembly have a high degree of leakage
integrity. Special arrangement should be made
between manufacturer and used 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 para. 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 gener-
al practice to use glycerine or silicone filling liquids.
However, these fluids may not be suitable for all appli-
cations. They should be avoided where strong oxidiz-
ing agents including, but not limited to, oxygen, chlo-
rine, 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 application of gauges having liquid filled cases and
solicit the advice of the gauge supplier prior to installa-
tion.
Consideration should also be given to the instanta-
neous hydraulic effect that may be created by one of
the modes of failure outlined in para. 4.2.7. The
hydraulic effect due to pressure element failure could
cause the window to be projected 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 immediate effect of failure, but will help con-
trol flow of escaping fluid following 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. The gauge may bear the inscrip-
tion ACETYLENE on the dial.
4.3.8.3 Ammonia Gauge.
A gauge designed to
indicate ammonia pressure and to withstand the corro-
sive effects of ammonia. The gauge may bear the
inscription AMMONIA on the dial. It may also include
the equivalent saturation temperature scale markings
on the dial.
4.3.8.4 Chemical Gauge.
A gauge designed to
indicate the pressure of corrosive or high viscosity flu-
ids, or both. The primary material(s) in contact with the
pressure medium may be identified on the dial. It may
be equipped with a chemical (diaphragm) seal, pulsa-
tion damper, or pressure relief device, or a combina-
tion. These devices help to minimize potential damage
to personnel and property in the event of gauge failure.
They may, however, also reduce accuracy of sensitivity,
or both.
4.3.8.5 Oxygen Gauge.
A gauge designed to
indicate oxygen 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 para. 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 conse-
quences of incompatibility can range from contamina-
tion to explosive failure. For example, moving an oil ser-
vice gauge to oxygen service can result in explosive
failure.
4.4.2 Partial Fatigue.
The first installation may
involve pressure pulsation that has expended most of
the gauge life, resulting in early fatigue in the second
installation.
4.4.3 Corrosion.
Corrosion of the pressure ele-
ment assembly 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 the Standard relative to
application of gauges should be followed in the same
manner as when a new gauge is selected.
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