When selecting a pressure gauge
consideration should be given to a number of parameters which have an effect on gauge
accuracy, safety, and utility.
Accuracy required
Engineering Consideration
System Design Requirements Specification -
initial system engineered design requirements will dictate the required pressure gauge
accuracy. Systems should be designed with cost and safety in mind. Gauges
which are more accurate generally cost more to build.
Selection Criteria
McDaniel Controls offers - pressure gauges with
ranges of accuracy: ±.25%, ±.5%, ±1%, ±1.5%, ±2/1/2, ±3/2/3% and choice of scale from inventory for many dual scale ranges, we
offer more ranges and special calibrations than any other manufacturer of gauges.
Pressure Gauge Accuracy for newly designed systems
- should be determined for each application and based on desired application system
operational characteristics. Operational parameters for many process environments
are often fine tuned after the process system goes into production use. Accurate
gauge readings are required to maintain efficient process control.
Pressure Gauge Accuracy for replacement components
- should be at least equal to the gauge being replaced unless otherwise determined by
engineering review and in some cases with the component manufacturer's approval.
Selection of replacement gauges requires careful consideration of all the gauge selection
parameters reviewed in this document and careful consideration and review of ASME
B40.1-1991, Gauges -- Pressure Indicating Dial Type -- Elastic Element.)
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Dial size
Engineering Consideration
Pressure Gauge Accuracy - determines dial size
in two ways. First, a requirement for higher accuracy dictates that a larger dial
size be available for display of small and readable pressure scale increments.
Second, by a requirement that the pressure gauge user/operator be able to read the
scale from a distance that physical circumstances may dictate.
Selection Criteria
Pressure Gauge Range Selection - The pressure
gauge selected should have a dial size which permits the desired readable scale increments
from the operator required viewing distance. McDaniel Controls has a wide range of
dial sizes from 1½", 2", 2½", 4", 4½", and 6" dial sizes
(up to 20 inch dial size available).
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Operating pressure range
Engineering Consideration
Overpressure Failure - is caused by the
application of internal pressure greater then the rated limits of the elastic element
(Bourdon Tube) and can occur when a gauge is subjected to pressures greater than design
limits.
Loss of gauge accuracy - occurs when a gauge has been subjected to
pressure exceeding design operation pressure limits. Loss of gauge accuracy results
from extending the elastic element (Bourdon Tube) past its limit for elasticity.
Selection Criteria
Pressure Gauge Range Selection - 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(12 o'clock position is best). The full
scale pressure of the gauge selected should be approximately two times the intended
operating pressure. This will give the operator a visual indication of normal, ideal
operating conditions. This means a "green light" condition and everything
is operating the way it should be. If however, the "visual" of the pointer
is way below or way above this 12 o'clock position, then we have a situation that needs
attention.
McDaniel Pressure Gauges - are designed and manufactured to minimize
failure and to protect personnel and property.
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Chemical compatibility with gauge construction materials
Engineering Consideration
Corrosion Failure - occurs when the
elastic element (Bourdon tube) has been weakened through attack by the corrosive chemicals
present in either the media inside or the environment outside it. Failure may occur
as a pinhole leakage through the element walls or early fatigue failure due to
stress cracking brought about by chemical deterioration or embrittlement of the material.
Selection Criteria
Chemical Compatibility Table - The Use of
McDaniel Controls chemical compatibility table can assist
in selection of both gauge internals and gauge case construction materials.
McDaniel Provided Diaphragm Seals - can be used to isolate gauge
internals from process fluids. McDaniel provided diaphragm seals are
engineered for reliable use with bourdon tube type pressure gauges where separation of the
medium and the control mechanism is necessary. Seals are offered in a wide range of
materials to withstand corrosive conditions.
Diaphragm Seal Operation - The entire
system above the diaphragm is solidly filled with a suitable liquid. When line
pressure is applied, the diaphragm displacement builds an equal and balanced pressure on
the instrument side of the diaphragm, and the gauge then measures line pressure.
When properly filled, attached and calibrated it allows the attached gauge to function
very close to its normal accuracy with only a slight accuracy loss through use of a seal.
McDaniel Gauge Case Style - a wide range of case
styles available from McDaniel Controls provides opportunity to select a case that is
liquid fillable and constructed with materials resistant to external attack by fumes
and/or spillage of measured process media.
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Operating temperature
range
Engineering Consideration
Loss of Gauge Accuracy - temperature change
effects the stiffness of the elastic element (Bourdon Tube). A change in the bourdon
tube stiffness will effect gauge accuracy.
Explosive Failure - for brass gauges in particular, solder is used
to seal the ends of bourdon tubes. Loss of solder seal can occur if the gauge is
subjected to temperatures near the solder melting point. Loss of solder seal would
allow media to enter the pressure gauge case and possibly lead to explosive failure.
Leakage Past Gauge Case Seal - can
result from exposing the gauge case to elevated temperatures which may bake the seal and
cause the seal to loss its elasticity.
Selection Criteria
McDaniel provided guidelines - for high temperature gauge use
can be referred to when high temperatures or extreme environments are expected.
Mounting Options - are important when
consideration is given to use of a siphon or pigtail which can trap condensed steam or
other vapor to isolate the gauge from the heat source. Use of a siphon, or pigtail,
makes a liquid seal in the line to the gauge and prevents the high temperature from
reaching the gauge internals.
McDaniel Provided Diaphragm Seals - are another
option to stem flow of measured media to the bourdon tube measuring element.
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Humidity
Engineering Consideration
Loss of Gauge Accuracy - condensation inside
pressure gauge lens can make the gauge hard or impossible to read.
Selection Criteria
McDaniel Designed Liquid-Fillable gauges - All
McDaniel Gauges are designed to avoid formation of condensation inside the gauge lens.
In environments where humidity and moisture are still a problem, McDaniel
Controls liquid-fillable gauges are an excellent solution. Glycerin or other choice
of fillings prevent moisture from entering the case body and condensing on the inside of
the case lens. An added benefit of a liquid-filled pressure gauge is extended gauge
life due to reduction of wear to the movable gauge internal parts.
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Vibration,
pulsation, and shock
Engineering Consideration
Vibration Failure - the most common mode
of vibration failure is that where the movement parts wear because of high cyclic loading
caused by vibration, resulting in gradual loss of accuracy, and, ultimately failure of the
pointer to indicate and pressure change.
Vibration-Induced Fatigue Failure - vibration may in some instances,
result in high loading of various parts of the pressure element assembly. This
loading could cause cracks in the element itself, or in joints. Case pressure
buildup may be slow, but it is possible that a large hole may suddenly develop, with a
high rate of case pressure rise, which could result in a failure similar to an explosive
failure.
Selection Criteria
McDaniel Designed Liquid-Fillable gauges - are
used for applications involving vibration or pulsation. Glycerin is an
excellent damper of bourdon tube movement. Emersion of the entire assembly in a
protective liquid envelope provides lubrication and shuts out corrosive or dirty
atmospheres -- thereby minimizing wear of moving parts and prolonging gauge life
significantly. McDaniel Controls gauges can be filled with Glycerin, Silicone,
depending on application needs. See choice
of fillings.
McDaniel provided pulsation snubbers - are used to minimize effects from
pulsation and surges. The snubber helps keep accurate data flowing by dampening
pulsation and surges. Gauge life is also extended by reducing gauge wear.
Mounting Options - may become an important consideration for elimination
of mechanical vibration between mechanical equipment and the pressure gauge. In some
cases it may be appropriate to connect the pressure gauge to the mechanical device using
flexible hose and tubing so that equipment mechanical vibration is dampened by structural
components. McDaniel Controls offers a wide range of dial sizes, mounting options,
and case styles to fit customer gauge mounting needs.
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Pressure fluid
composition
Engineering Consideration
Pressure Gauge Accuracy - can be degraded
when the measured media contains high solids or is such that the measured media changes
between states of gas, liquid, or solid, due to varying process temperature and
media chemical compositions. Measured media solids can restrict flow of the media to
the elastic measuring element (Bourdon tube) or completely block flow. Consequences
of such loss of accuracy may be a pressure gauge which stops responding to pressure
changes or is slow or sporadic in response to pressure changes in the measured media.
Selection Criteria
McDaniel provided diaphragm seals - are
engineered to prevent mixing of the measured media with liquid used to transmit diaphragm
seal pressure to the Bourdon tube. Use of diaphragm seals are recommended where
there is a possibility of problems with process media solids or process media change of
state.
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Method of mounting
Engineering Consideration
Vibration Control - can be achieved by
remote mounting of gauges monitoring equipment with high levels of mechanical vibration.
Available Structure and Design - available
building structure and system design presents a need for variable mounting requirements
and case styles.
Selection Criteria
McDaniel designed pressure gauges - offers a
wide range of NPT sizes and case mounting styles as follows:
NPT - 1/8",
1/4", 1/2" NPT
Other connection
sizes available
1/4"
MPF, 1/4" MPM,
1/4" HPF,
7/16
S.A.E., 9/16" MPM, 9/16"
HPM
Mounting Styles
- Standard Bottom Connection
- Center Back or Lower Back Connections
- Panel Mount (Center and Lower Back Connections)
- U-Clamp Center Back
- Front Flange (Center or Lower Back Connection)
- Rear Flange (Bottom or Lower Back Connection)
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