CEMP-E
TI 811-12
18 August 1998
(10) Gas utility flow meters are diaphragm or bellows type (gas positive displacement meters)
for flows up to 2,500 standard cubic feet per hour (SCFH) and axial-flow turbine type for flows above
2,500 SCFH. These meters, which are designed specifically for natural gas supply metering, have
electrical impulse dry contact outputs for input to UMCS.
(11) Flow switches are operated by input flow to open or close contacts at a selected flow
setting. Flow switches must be adjustable over the operating flow range.
f.
Level instruments.
(1) For vented tanks with accessible bottom taps, a pressure transducer connected to a bottom
tap will be used for level measurement. The pressure measurement is converted to a level measurement
by the UMCS based on the density of the liquid in the tank. In certain cases, where temperature is
expected to vary widely and the density of the liquid varies significantly with temperature, a temperature
measurement is required for compensation of the engineering units conversion. If the tank is pressurized
and both bottom and top taps are accessible, level will be measured using a differential pressure
transducer and engineering units conversion based on density.
(2) For sumps or tanks without accessible taps, capacitive liquid level sensors will be used.
For measurement of non-conductive liquids or where sloshing of the liquid is expected, the liquid level
sensor will be installed in a perforated steel stilling well.
(3) Bubbler type liquid level sensors will be used for level measurement of fuel oil or extremely
caustic or corrosive liquids. Compatibility of the wetted tubing with the liquid will be assured by the
designer.
(4) Liquid level switches are combinations of displacer floats suspended from a stainless steel
cable attached to the switch housing. Changes in liquid level near the elevation of the displacers results
in varying downward force on the stainless steel cable, which actuates the switch mechanism. Liquid
level switches will be used where the UMCS is required to actuate specific alarms or controls at defined
liquid levels, but continuous monitoring of liquid level is not required.
(5) Float switches will be utilized for sewage lift station pits or similar applications with
corrosive liquids and floating solids. Float switches are mercury-free tilt switches rigidly mounted in
bouyant polypropylene (or other corrosion-resistant material) floats. The floats are secured at the
elevation where switch actuation is desired, and the tilt switches actuate when the liquid level tilts the
float.
g. Electrical power instruments.
(1) Electrical energy consumption measurements require the use of voltage and current
transformers whose proportional outputs are connected to a dedicated watt-hour meter or transducer, or
to a field equipment panel where the watt-hour consumption calculations are performed. Where
dedicated watt-hour meters are used, a dry contact pulse output is required from the meter for input to the
field equipment panel. Where watt-hour transducers are used, an analog output is required for input to the
field equipment panel.
(2) Electrical peak demand is calculated from the output of potential (voltage) and current
transformers used for the electrical energy measurements or by the use of dedicated electrical peak
demand transducers with an analog output to a field equipment panel.
(3) Voltage and current measurements for ranges which do not match field equipment panel
input requirements will require the application of voltage and current transformers.
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