UFC 3 -430-11
14 February 2001
5-2.10.1 Gauges. Provide blowout discs for pressure gauges in services with
pressures greater than 1.03 bar (15 psig). Provide a sa fety wall between the dial and
the bourdon for service pressures above 68.9 bar (1000 psig). Provide pigtail siphons
for all gauges in steam service. Pigtail siphons should be installed perpendicular to the
gauge. Provide pulsation dampers and diaphragm seals where required by service
conditions and for all gauges in steam and condensate service. Provide a gauge
isolation valve for each gauge.
Pressure gauges are usually direct connected and field mounted. The size
and range is specified by the user. Locate local gauges so that they are visible from the
operating area and are readable from grade or a platform. Local mounted gauges give
a "backup" reading and also help operators in determining if equipment or pressure
systems are working satisfactori ly. Pressure gauges should conform to ASME B40.100,
Pressure Gauges and Gauge Attachments.
5-2.10.2 Switches. Pressure switches are used for monitoring alarm conditions and
providing safety shutdowns. They are typically mounted directly to the process pipe.
5-2.10.3 Transmitters. Pressure transmitters convert the measured pressure to an
analog or digital signal that is monitored by the control system (e.g. PLC). Provide a
three-valve manifold to accomplish block, drain, and test functions for all pressure
transmitters. Provide a five -valve manifold to accomplish block, equalize, drain, and
test functions for all differential-pressure transmitters. Refer to paragraph 5 -2.11 for
additional transmitter requirements.
5-2.10.4 Draft. Pressure instruments for the measurement of draft in furnaces require
careful attention as to range and sizing. Note that the draft in a balanced draft boiler
furnace is slightly negative, around -2.54 millimeter (-0.1 inch) WC, at the top of the
furnace. Too wide an inst rument range will result in the loss of accuracy. Too narrow a
range will not cover all operating conditions.
Verify that the connections and sensing lines are adequately sized for the low
negative pressures. Use larger sizes than for lines sensing positive and high pressures.
5-2.11
Electronic Transmitters. Electronic transmitters are used to transmit an
electronic signal from a local measuring device to a remotely located device such as a
panel mounted controller. Typical transmitters include flow, level, pressure and flue gas
oxygen. A typical electronic signal is 4 -20 mA DC.
Use stainless steel material for all transmitter components in contact with the
stream. Provide local indicators for all electronic transmitters. Locate the local indicator
at the transmitter for non-controlling loops such as a signal to an indicator or recorder.
Locate the local indicator at the controlling device for controlling loops such as
diaphragm-operated valves. Provide an integrally mounted junction box with a metal
cover and a terminal block for blind electronic transmitters to allow for the connection of
a plug-in ammeter.
5-8
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