Hi Flow
Sampler
Operation
Instruction 0055-9017
9
2 Operation
2.1 Operating Precautions
2.1.1 Limits of Use
The Hi Flow Sampler was developed for use on natural gas streams with high methane
content. After introduction and industry acceptance, the instrument has been applied to
additional areas in the natural gas industry, including upstream locations. These
locations potentially contain less methane as a percentage of the overall mixture. As
methane becomes less of the overall mixed gas stream, a response correction factor is
recommended.
The response correction factor modifies Equation 1 as follows, and must be determined
and applied by the user (to the measurements reported by the instrument):
Leak = (Flow x (Gas
sample
– Gas
background
) x 10
–2
) x k
Eq. 2
where:
Leak = rate of gas leakage from source (cfm)
Flow = sample flow rate (cfm)
Gas
sample
= concentration of gas from leak source (%)
Gas
background
= background gas concentration (%)
k = response correction factor (user determined)
An alternative approach for use in mixed gas streams is to calibrate the instrument on
the actual gas, in which case the response correction factor (k) is not required.
The Hi Flow instrument includes a temperature sensor and compensates for changes in
density associated with temperature in the reported measurement. The Hi Flow does
not however measure absolute pressure and as a result does not compensate for the
effects of altitude.
2.1.2 Calibration
The instrument is calibrated to pure methane at the factory. Calibrate the instrument
about every 30 days to assure its accuracy. More frequent calibrations may be required
depending on how often the instrument is used and the amount of gas that was
sampled. Create a maintenance log to keep track of when an instrument was calibrated,
and then use this log to help develop a calibration schedule. In the case of mixed gas
streams, it is recommended to calibrate prior to use each day.
2.1.3 Sensor Flooding and Poisoning
The Hi flow sensors operate in catalytic mode, from 0 up to 5% (by volume) methane
concentration, and in thermal conductivity mode from 5% up to 100% methane by
volume. The rapid introduction of high methane concentrations may inhibit the sensors
operation in catalytic mode by starving the catalytic reaction of oxygen. Flooding the
sensor in this way can result in erroneously low readings and prevent transition to
thermal conductivity mode. In cases where very large leaks are suspected (approaching
the limits of the instrument’s measurement range, 8SCFM) the Hi Flow should be turned
on sampling air adjacent to the leak and slowly moved toward the largest source of the
leak. Please refer to section 2.3 for specific details on the attachments provided for leak
capture.
