5/13/15 Pressure Gauge Selection
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Engineering Reference Gauge Selection
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
Dial size
Operating pressure range
Chemical compatibility with gauge construction materials
Operating temperature range
Humidity
Vibration, pulsation, and shock
Pressure fluid composition
Method of mounting
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
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5/13/15 Pressure Gauge Selection
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
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5/13/15 Pressure Gauge Selection
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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5/13/15 Pressure Gauge Selection
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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.
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5/13/15 Pressure Gauge Selection
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
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5/13/15 Pressure Gauge Selection
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,
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5/13/15 Pressure Gauge Selection
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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McDaniel Controls, Inc., P.O. Box 187 Luling, LA 70070
Street address: 14148 Highway 90 West, Boutte, LA 70039 U.S.A.
T el: 985-758-2782 New Orleans: 504-467-1333 Fax: 985-758-1688
E-mail: Click HERE to send mail Web address: http://www.mcdanielcontrols.com
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