Design and Implementation of HC900 Control System - Allowable Function Blocks for Process and Safety Functions
Revision 1.9 HC900 Process & Safety Controller Safety Manual 29
01/14
AGA8 Gross
The Gross method is used to approximate natural gas
by treating it as a mixture of three components,
equivalent hydrocarbon component, Nitrogen and
Carbon Dioxide. It is typically used for dry, sweet (no
H2S) natural gas. There are two methods used.
Gross Method 1 calculates the super-compressibility
and gas density from knowledge of the relative
density, heating value and carbon dioxide, hydrogen
and carbon monoxide components. Gross Method 2
calculates the super-compressibility and gas density
from knowledge of the relative density, Nitrogen,
carbon dioxide, hydrogen and carbon monoxide
components. The Gross Method only works over a
limited range of conditions but requires less
instrumentation to implement.
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AGA3 Orifice
Meter Calculations for Orifice Metering
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AGA7 Turbine
Meter
Calculations for gas measurement by Turbine Meters
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AGA9 Ultrasonic
Meter
Calculations for gas flow measurements from multi-
path Ultrasonic Meters.
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Alarm/Monitor
Blocks
High Monitor
Monitors two analog input values (X and Y) and turns
ON a digital output if X exceeds Y. A hysteresis
adjustment is provided to prevent output cycling. If X
> Y, then OUT = ON. If X < or = (Y – Hysteresis),
then OUT = OFF. If (Y – Hysteresis) < X < Y, then
OUT = Previous State.
X
X
Low Monitor
Monitors two analog input values (X and Y), and turns
ON a digital output if X is less than Y. A hysteresis
adjustment is provided to prevent output cycling. If X
< Y, then: OUT = ON. If X > or = (Y + Hysteresis),
then: OUT = OFF. If (Y + Hysteresis) > X > Y, then:
OUT = Previous State.
X
X
System Monitor
(FSYS)
The Fast Logic Status Block (FSYS) is a function
block and is part of the Fast Scan Alarm/Monitor
Blocks category. It provides read access to controller
status values including those related to the Fast Scan
execution cycle. The output may be connected to
function block inputs. The outputs may also be
connected to signal tags for operator interface
monitoring.
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