| Flow Rate
Changes With Pipeline Pressure |
|
Both
Flowmeters and
Flow Control Orifices alter flow
rate as pipeline pressure varies. Depending on the pipe size and
configuration, pipeline pressure changes occur as additional welding
machines are placed in use. Just how much does the flow change?
We conducted the following Laboratory tests to demonstrate these flow
changes. |
|
TEST PROCEDURE:
A pipeline flowmeter was connected to a
test pipeline that had its inlet pressure controlled by an inert gas regulator
connected to an Argon/CO2 shielding gas
cylinder. The cylinder regulator allowed pipeline pressure to be varied from 20 to 75
psi. To assure pressure reading accuracy a calibrated 2 inch diameter gauge
was installed in the regulator outlet gauge port.
The flowmeter selected
for the test was a commonly used Victor* FM372 (photo right.) As noted in the Victor
literature, all their flowmeters are calibrated at 25 psi (accept one for CO2
which is calibrated at 80 psi to avoid dry ice particles in the needle
valve.) They note that the pressure is used to proved what we refer to
as "Automatic Flow Compensation"
that requires a pressure of 25 psi or greater. Therefore
the flow reading scale will only be accurate at a pipeline pressure of
25 psi. Increased pressure will produce a higher flow than what is
read on the flow tube. These Laboratory tests demonstrate just how much more
flow variation occurs with pipeline pressure changes with a fixed flow control knob setting. |
|
MEASURING ACTUAL FLOW:
 To
measure the actual flow we used a portable flowmeter, our part number
WAT- PFM. This uses the same flow
measuring principle defined by
Bernoulli in the 1700's. It is accurate and very repeatable. It was used in a way
recommended in our Lean Welding
Manufacturing Self Study Training Programs which is to measure flow with
the wire feeder gas hose fitting placed in the gauge (photo left.)
Therefore there is no possibility of leaks between the Victor FM372 and the
portable flowmeter. Instructions shipped with the WAT-PFM define how this
measurement approach in combination
with measuring at the MIG torch nozzle quantifies leaks in wire feeder plumbing, torch to feeder connections,
backflow through wire outlet guide etc and what differences are excessive.
As we do with each
batch of flowmeters, we checked the one used for these tests with calibrated
regulator/flowmeters placed on the cylinder. The WAT-PFM was accurate and correlated
with the regulator/flowmeters within our measurement precision from 20 to
40 CFH.
The following are the
results of the tests: |
| Pipeline
Pressure |
Reading on FM372 Flowmeter Flow Tube |
Published Correction Factor |
Flow Using
Correction Factor |
Actual Flow
Measured on Portable Flowmeter |
| 25 psi |
20 CFH |
1.00 |
20 CFH |
20 CFH |
| 30 psi |
23 CFH |
1.05 |
24 CFH |
23 CFH |
| 35 psi |
24 CFH |
1.12 |
27 CFH |
28 CFH |
| 40 psi |
26 CFH |
1.18 |
31 CFH |
32 CFH |
| 45 psi |
27 CFH |
1.23 |
33 CFH |
34 CFH |
| 50 psi |
31 CFH |
1.28 |
40 CFH |
40 CFH |
|
|
INTERPRETING TEST RESULTS:
Looking at the first
row of data. The pipeline pressure was set at 25 psi which is the
calibration pressure for the FM372 flowmeter. The 2nd column shows the
reading on the FM372 flowmeter which was 20 CFH since the flow knob was
adjusted to that flow setting. The next column is the published factor used to
correct for pressure differences between the actual pipeline pressure and
the calibration pressure. Since 25 psi is the calibration pressure
the factor is one. Therefore applying the factor shows 20 CFH in the
third column.
The last column is the flow measured on the WAT-PFM which, as expected, is also
20 CFH. This also validates the calibration of the
FM372 and the WAT-PFM flowmeter are the same. |
|
Examining the third row
of data; the pipeline pressure was raised to 35 psi with NO CHANGE IN
THE INITIAL
FLOW KNOB SETTING. The FM372 flow now read 24 CFH. The published correction
factor for the increase in gas density is 1.12. Applying this
correction factor shows an actual flow for an observed 24 CFH reading to be 28 CFH.
Therefore although the gauge on the Victor FM372 was reading 24 CFH the
actual flow was theoretically 28 CFH. The extra flow is because the
density difference in the gas from the 25 psi calibration pressure and the
35 psi in the pipeline. The actual
flow as measured on the WAT-PFM was 27 CFH which is within our
measurement precision of the theoretical 28 CFH. |
|
Looking at the last
row of data. Pipeline pressure was raised to 50 psi and the FM372
flowmeter now read 31 CFH. The correction factor for 50 psi is 1.28
yielding a theoretical flow of 40 CFH (31 X 1.28) for a 31 CFH
FM372 flowmeter reading. The calculated flow is the same as the actual flow
measured on the WAT-PFM that was also 40 CFH. That is 29%
more actual flow then measured on the FM372 flowmeter [(40-31)/31=29%. ]
Therefore if this flowmeter (or any calibrated at 25 psi, as are many) is
operated at a pipeline pressure of 50 psi and gas flow set at a reading of 31 CFH
using the flowmeter scale, the actual flow would be 40 CFH! That is
29% more flow
than read on the pipeline flowmeter scale. Not only is this wasting
gas but it could exceed
the
allowable range defined in a Welding Procedure Specification (WPS.)
|
|
TEST OF FLOW CONTROL ORIFICE:
Similar tests were
conducted using variations in pipeline pressures with our Standard Flow Control
Orifice, WAT-35AO. We
compared the theoretical flow rate based on orifice size and pressure with
that read on the WAT-PFM. Here are the results: |
| Pipeline
Pressure |
Theoretical Flow
Based on Orifice Size and Pipeline Pressure |
Actual Flow
Measured on Portable Flowmeter WAT-PFM |
| 25 psi |
25 CFH |
25 CFH |
| 30 psi |
30 CFH |
30 CFH |
| 35 psi |
33 CFH |
33 CFH |
| 40 psi |
35CFH |
36
CFH |
| 45 psi |
40 CFH |
41 CFH |
| 50
psi |
43 CFH |
44 CFH |
|
|
As seen from the data, the flow measured on the WAT-PFM through a WAT-35AO
Flow Control Orifice is very predictable and within the measuring precision of the theoretical flow based
on orifice size, pressure and flow calculations. |
|
BOTTOM LINE:
 If
employing pipeline supplied shielding gas and using an Orifice to Control
 Flow,
a Portable Flowmeter is the way to verify the actual flow. The flow
through the Orifice changed about 8 CFH with a 10 psi pipeline pressure
change. This is as predicted by
"critical flow" flow equations.
When using a Flowmeter to control
shielding gas flow from a pipeline, when pressure
changed 10 psi the actual flow changed a similar 8 CFH.
Therefore pipeline pressure changes produced essentially the same flow variations in
both Flowmeters and with Orifices used to set flow.
Note:
when using Regulator/Flowmeters on cylinder shielding gas supply, these flow
reading variations DO NOT exist as the regulator maintains the proper
calibration pressure of 25, 50 or 80 psi. |
|
Want a detailed report
on this test data?
 It
includes a table with correction factors to calculate actual from
measured flow when pipeline pressure varies over a range from 20 to 70 psi.
CLICK HERE
or the ICON to Download a PDF Copy: |
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| *Victor is a registered trademark of
Thermaydyne Corporation |
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