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Installation,
Operation & Maintenance |
Pressure Gauge Operating Principle
Most standard dial type pressure gauges use a bourdon tube-sensing
element generally made of a copper alloy (brass) or stainless steel for
measuring pressures 15 PSI and above. Bourdon tube gauges are widely
used in all branches of industry to measure pressure and vacuum. The
construction is simple yet rugged and operation does not require any
additional power source. The C-shaped or spirally wound bourdon tube
flexes when pressure is applied producing a rotational movement, which
in turn causes the pointer to indicate the measured pressure. These
gauges are generally suitable for all clean and non-clogging liquids and
gaseous media. Low pressure gauges typically use an extremely sensitive
and highly accurate capsule design for measuring gaseous media from as
low as 15 INWC to 240 INWC (10 PSI). Digital gauges use an electronic
pressure sensor to measure the pressure and then transmit it to a
digital display readout.Pressure Gauge Installation, Operation & Maintenance
Selecting a Pressure Gauge
Pressure Range
Installation
Operation & Maintenance of Pressure Gauge
Recalibration / Assembly & Disassembly Procedures
Selecting a Pressure Gauge
I t is important to consider the various information outlined in this manual to
ensure appropriate pressure gauge selection and usage. Improper application can
lead to instrument failure and possible personal injury and/or property damage.
The following information in this manual is meant to be a guide in the selection
process. Please feel free to consult Ningfeng Instruments for further assistance
and clarification.
Types of Pressure Gauges
Ningfeng offer a full range of pressure gauges suitable for virtually all types
of applications. Some specific types of pressure gauges that will be covered in
this manual are general-purpose Bourdon tube, capsule-type, duplex, and
differential pressure gauges.
General-Purpose Bourdon Tube Gauge
T his common type of gauge utilizes a C-Bourdon tube, which usually has an arc
of 250 degrees. The process pressure is connected to the fixed socket end of the
tube, while the tip end is sealed. Because of the difference between inside and
outside radii, the Bourdon tube tends to straighten when pressure is applied.
The non-linear motion at the tip of the Bourdon tube is converted to linear
rotational pointer response through the use of gears and pinion movement. This
type of pressure gauge is used in medium to high-pressure measurements with
accuracies of 0.1 to 3% of full span. The pressure range capabilities starts
from 15 psi (100 kPa) to 20,000 psi (137.8 Mpa).
Capsule-Type Gauge
Capsule-type are formed from welding two pre-formed plates together. Input
process pressures cause the capsule to expand or contract which is then
translated into a linear rotational pointer response through the use of gears
and pinions. Bellows gauges are typically used in low to intermediate and vacuum
pressure ranges with 0.1 to 1% of full span accuracy. Ningfeng Instruments
capsule gauges have pressure ranges from 0-15" H2O (0-3.7 kPa) to 0-10 psi
(0-68.9 kPa).
Duplex Gauge
A Duplex gauge indicates two related pressures in a process. There are two
Bourdon tubes inside the gauge casing which results in two separate readings.
The principal of the operation is as described in the General-Purpose Bourdon
Tube Gauge section for Ningfeng' Duplex Gauges.
Differential Gauges
The purpose of a differential gauge is similar to a duplex gauge, but this gauge
will indicate the difference between two related pressures. Ningfeng Instruments
provide three different types of differential gauges; parallel Bourdon tube,
magnet and piston, and opposing Bourdon tube designs.
Parallel Bourdon Tube Differential Gauge
The placement of the Bourdon tubes is similar to that of the duplex gauges. The
gears and pinions are calibrated to measure the difference between the flex of
the two Bourdon tubes and this is translated to a single pointer. This type of
differential gauge is used when the indicating scale is 2/3 of the process
static pressure due to the limitations of the Bourdon tubes.
Piston Differential Gauge
The sensing element is a spring-biased diaphragm, which moves linearly in
proportion to the difference between two basic pressures. A magnet on the HI
pressure side of the piston assembly moves with the diaphragm and rotates a
follower magnet located adjacent to the pressure cavity. The gauge pointer is
located at the end of the rotary magnet shaft and rotates with the magnet to
provide gauge readings proportional to the differential pressures. This type of
instrument can indicate small values of differential pressure even when used in
high-pressure processes.
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Pressure Range
Select a pressure gauge with a full-scale pressure range of approximately twice
the normal operating pressure. For Bourdon tube gauges, the maximum operating
pressure should not exceed 75% of the full-scale range. The exception to this
rule is the opposing Bourdon tube differential gauge. Failure to select a gauge
within these criteria may result in premature fatigue and failure of the Bourdon
tube.
Temperatures
The normal ambient temperature ranges are -40C to 70C (-40F to 158F) for dry
gauges and -25C to 65C (-13F to 149F) for glycerin filled gauges. The process
temperature for dry gauges can be up to 400C (752F). In situations where the
process temperature is extreme, utilizing a syphon or remote mounting the
pressure gauge with a capillary and diaphragm seal are reasonable
alternatives.In general, a gauge is unduly hot if it cannot be grasped by hand
without discomfort. It should be noted that gauges used on water might burst if
exposed to frost. Please consult Ningfeng for your specific requirements.
Materials
W inters provide a full range of casing and wetted parts to accommodate all
types of applications. The most common wetted parts used are brass and 316
stainless steel. Brass wetted parts are suitable for use on the following types
of media; air; oil, water and other non-corrosive fluids. For corrosive fluids,
stainless steel should be specified. For special applications, other materials
such as Monel can be used as well. Diaphragm seals may also be considered.
Please consult Ningfeng Instruments for assistance.
Vibrations
Vibrations and extreme ambient temperatures can affect the dial reading of the
pressure gauges. These areas should be avoided as much as possible. Vibration
effects can be minimized by the use of a dampening liquid such as glycerin or
silicone. If vibration is extreme, then a flexible tube connected to an
appropriate diaphragm seal should be considered.
Pulsations
If pressures are expected to pulsate violently, oscillate with high frequency or
occur with sudden shock, a snubber should be considered. Ningfeng snubbers
incorporate sintered porous 316 stainless steel snubbing element with a large
surface area to ensure long term effectiveness on most pressure media. Snubbers
are available in the three viscosity classifications; heavy oil, water and air.
Brass, stainless steel or Monel housing can be specified depending upon the
media used.
Isolating Valves
It is recommended that isolating valves be fitted with the gauge. This enables
the gauge to be removed at any time for checking, recalibration or replacing
without interruption to the process. The valves should be opened or closed
slowly to avoid sudden changes to pressure being applied to the gauge. It is
recommended for systems that have an abrupt pressure surge at start-up to close
the valves during initial start-up.
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Installation
Location
V ibrations and extreme ambient temperatures can affect the dial reading of the
pressure gauge. These areas should be avoided as much as possible. Vibration
effects can be minimized by the use of a dampening liquid such as glycerin or
silicone. If vibration is extreme, then a diaphragm seal connection should be
considered.
Mounting
A suitable thread sealant is required for N.P.T. threads such as pipe
dope or Teflon tape. Never use any part of the pressure gauge other than
the wrench flats that is on the gauge socket. Always tighten with an open
end or adjustable wrench on the wrench flats. Failure to do so will severely
damage the pressure gauge. Please note the following special requirements
for the piston differential gauge.
Piston Differential Gauge
Check instrument & identify HI and LO markings.
HI identifies the high-pressure port; LO the low pressure port. If the
instrument is installed backwards, it will neither operate nor be damaged.
It is recommended that the instrument be located above the pressure source
to allow for drainage of the unit.
Please note that since there is a magnetic movement, the piston differential
gauge should never be mounted in direct
contact with a steel surface. Failure to do so will cause a calibration
shift. Mount the gauge so that the gauge body is at least 1" (25.4 mm)
away from metal surface with non-magnetic spacers or an aluminum-mounting
bracket.
Venting Procedures
Due to pressure ¡°build up¡±, some gauges (usually lower pressure ranges such as
vacuum, up to 100 psi) may reflect a reading that is slightly ¡°off zero¡±. To
properly ¡°vent¡± the pressure gauge, please cut off tip of fill plug after you
have installed the instrument.
Operation & Maintenance of Pressure Gauge
Storage
A ll pressure gauges should be placed in dry storage under ambient room
temperatures. It is recommended to store the pressure gauges in their original
packing boxes.
Inspection Frequency
W inters pressure gauges are rugged instruments featuring simplicity of design
to provide dependable and efficient service. The frequency of inspection is
dependent on the severity of service and how critical the accuracy of the
indicating pressure is. The inspection frequency can range from monthly to
annually basis.
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Recalibration / Assembly & Disassembly Procedures
Positive Pressure Gauge
Inspect the pressure gauge for "zero reading" when there is no applied pressure.
In many cases, the gauge pointer can simply be adjusted and then the gauge will
work within specifications. However, if the gauge requires calibration, then
follow the outlines procedures below.
Remove the ring and lens
Slowly pressurize the gauge to its full scale and slowly release the pressure
back down to zero
Check the gauge at a minimum of four equal pressure intervals against a
Certified Test Gauge
Adjust the movement accordingly by removing the pointer with a pointer
extractor. Be careful not to bend the pointer shaft
Removing the dial plate by loosening the holding screws
When adjustments are complete, replace the dial plate and pointer
Re-check the gauge for accuracy
Check lens assembly gasket for any deformation, tears or cracks. If there are
any visual defects, the gasket will need to be replaced
Re-assemble the lens assembly
Capsule-Type Pressure Gauge
The same inspection procedure applies as per the positive pressure gauge section
above. The micrometer adjustable pointer is accessible by removing the white
plastic knob on the lens and adjusting the screw. This is done without having to
remove the lens. If calibration is required, please follow the outlined
procedure below.
Remove lens by prying open the lens window with the notches provided on the
sides of the case
Slowly pressurize the gauge to its full scale and slowly release the pressure
back down to zero
Check the gauge at a minimum of four equal pressure intervals against a
Certified Test Gauge
Adjust the movement accordingly by removing the pointer with a pointer
extractor. Be careful not to bend the pointer shaft.
Remove the dial plate by loosening the holding screws
When adjustments are complete, replace the dial plate and pointer
Re-check the gauge for accuracy
Replace the lens window
Piston Pressure Gauge
Other than replacing broken lens, there is only one area where the gauge may
need attention. Erratic pointer action may indicate that cleaning is required.
Calibration is not required. However, if the range spring is damaged or a new
dial is required, the gauge should be returned to the factory for parts and
recalibration.
For cleaning:
Remove the low pressure end cap with 1" (25.4 mm) spanner wrench
Remove the range spring and the spacers at the bottom of the spring pocket. BE
CAREFUL NOT TO LOSE STACKING SPACERS, IF ANY
Remove the four ¼-20 socket head screws and separate the body parts
Remove the diaphragm assembly
To replace the diaphragm, remove the screw at the top of the diaphragm disc to
separate the assembly. DO NOT REMOVE THE SCREW IN THE MAGNET ASSEMBLY
Clean parts in a solvent solution after removing the "O"-ring seals since
solvents will attack the seal material
Place diaphragm disc on magnet shaft with "O"-ring groove facing away from
magnet. Place new diaphragm over shaft with the open end of the convolution
facing the magnet. Place the second disc over diaphragm so that centre "O"-ring
groove faces diaphragm "O"-ring
Replace the spring cup, and the #8-32 assembly screw. Tighten to finish assembly
Place the diaphragm assembly back into the diaphragm pocket with the magnet end
of the assembly at the high pressure side
Reassemble both body parts with the four ¼-20 socket head screw
Insert the spacers and the range spring into the spring pocket of the end cap.
While holding the instrument with the low pressure port down, insert the end cap
back into the low pressure side of the body
Tighten the LO end cap and the instrument is now ready for service
No recalibration is required.
For replacing the lens:
To replace a broken lens, check to see if the lens is held on by a bezel or
snap-ring. To remove a bezel, which is a pressed on cover, either twist off by
hand or pry off with a screwdriver. To remove a snap-ring, pry out the ring with
a small screwdriver. Remove all glass chips, insert new lens and re-insert the
bezel or snap. With snap-rings, locate the ring joint at the bottom of the
gauge.
For replacing the pointer:
Remove bezel or snap-ring as previously described and clean out glass chips.
Remove old pointer with pointer extractor or two small screwdrivers opposite
each other under pointer hub. Pry off evenly being careful not to bend the
pointer shaft. Install new pointer on zero. NOTE: Gauges with a zero peg must
have the pointer set at a reference pressure (preferable mid-scale) to offset
the preload against the zero pegs. Re-install the lens, as described under lens
replacement.
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Gauge Mounting & Connection
Lower mount (LM), center back mount (CBM), and/or lower back mount (LBM)
are available for most gauge types. Most of our standard gauges have NPT,
BSP or BSPT threaded connections. Many gauges can be panel mounted using
either a U-clamp bracket or a front flange option. Pressure gauges
should be mounted in the upright position if possible.
L:Lower connection
B:Center back connection
N:Lower back connection
R:Lower connection with back flange
V:Center back connection with front flange
W:Lower back connection with front flange
U: Center back connection with U-clamp
F: Left connection
G: Right connection
H: Higher connection
Thread is used mainly like:
2: 1/2"NPT; 2B:1/2"BSP; 2T:1/2:BSPT connection thread
4: 1/4"NPT; 4B:1/4"BSP; 4T:1/4"BSPT connection thread
8: 1/8"NPT; 8B:1/8"BSP; 8T:1/8"BSPT connection thread
3: 3/8"NPT; 3B:3/8"BSP; 3T:3/8"BSPT connection thread |
Copyright © 2003-2005,
Cixi Ningfeng Pressure Gauge Factory,
TBP
All rights reserved.
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