Note: Descriptions are shown in the official language in which they were submitted.
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TESTING OF FLOW METERS
Introduction
This invention relates to testing of flow meters
that measure the air velocity particularly in industrial
stack flue gas flow streams. More particularly, this
invention relates to an integrated test module for
periodic and automatic validation of the accuracy of such
meters.
Background of the Invention
One of the most common forms of flow meter for
use in determining the air flow velocity in industrial
stack exhaust systems in an S-type pitot tube probe set,
consisting of two pitot tubes, facing opposing directions
and positioned in the flue gas stream so that the air
velocity effect causes a differential between the measured
air pressure in each tube. The pressure in each tube is
fed to a differential pressure sensor that provides an
analogue output signal proportional to the pressure
variation which can be equated to the airflow.
National environmental regulators typically
require that all industrial flue gas analyser
instrumentation undergoes performance validation testing
on a regular basis to ensure accurate and reliable
emission reporting. The performance validation tests
usually require a zero test to ensure that the instrument
is not affected by low level drift, as a result of
component failures, ambient conditions, electronic drift,
contamination etc; and a span test. The span, or upscale,
test takes place after low level drift has been
compensated or detected, and is conducted determining the
system accuracy against a known test standard. The span,
or upscale value is typically around 60% of the scale of
the system's operating range. An analyser instrument
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measuring flue gas concentrations would be validated using
a certified test gas or, in the case of an optical dust
monitor, a neutral density test filter.
The test procedures require that all components
of the instrument that are fundamental to the measurement
operation are subjected to the approved test standard. It
is not acceptable to "simulate" a test result that has not
been generated by the normal detection system of the
analyser. Thus it is not acceptable to simply generate a
specific output signal pulse from the instrument that is
not the result of the measuring sensor response to a
certified test gas.
Testing procedures may be manually or
automatically initiated on a regular cycle depending on
the capability of the instrument to automate an acceptable
test method. Manual audit testing on a weekly, monthly,
or quarterly basis may be reduced to an annual test
procedure if the instrument design is capable of automated
self testing on a daily or weekly basis. This
determination will be made by the national regulators.
A differential pressure type flow meter can be
tested in-situ by simply controlling the air pressure
effect to the differential pressure sensor. The zero test
can be carried out when both positive and negative ports
of the differential pressure sensor are vented to an equal
pressure e.g. normal atmosphere, thus providing a zero
response.
The span, or upscale test can be carried out if
an accurate and stable pressure can be provided to the
sensor positive port, referenced against the negative port
while it is vented to atmosphere. Because the typical
operating range of industrial differential pressure flow
meters varies from pressures between 0 and 100mm of water
column, the challenge is to produce a test pressure within
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this normal operating range from a typical plant air
supply that is usually somewhere between 60 to 120psi with
load fluctuations potentially causing pressure changes as
high as 20psi. (It should be noted that a pressure of
100mm of water column equates to less than 0.2psi).
Therefore the span / upscale test pressures required are
significantly lower than the pressure available from the
air plant supply, especially considering that the pressure
fluctuations from an air supply would render the testing
regime totally ineffective.
It is these issues that have brought about the
present invention.
Summary of the Invention
According to a first aspect of the present
invention there is provided a test module for a twin pitot
tube airflow meter including a differential pressure
sensor, the module including a controller coupled to a
series of solenoid valves to control the differential
pressure signal from the pitot tubes to the pressure
sensor and from an air supply source to the pressure
sensor, and an adjustable regulator to control and
regulate the pressure of air from the supply source
whereby in a span test cycle the solenoid valves isolate
the pressure signal from the pitot tubes but open
controlled and regulated pressure to the differential
pressure sensor to test the accuracy of the differential
pressure sensor.
According to a further aspect of the present
invention there is provided a method of testing a twin
pitot tube flow meter coupled to a differential pressure
sensor, the method comprising using solenoid valves to
control flow from the pitot tubes to the pressure sensor,
passing air from an air supply source through a pressure
regulator to reduce the pressure, micro adjusting the
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pressure to a pre-selected reading on the pressure sensor
and operating the solenoid valves to provide a zero flow
test and an upscale or span test where air at a known
pressure is supplied across the differential pressure
sensor.
Description of the Drawings
An embodiment of the present invention will now
be described by way of example only with reference to the
accompanying drawings in which:
Figure 1 is an illustration of a differential
pressure flow meter utilizing a pitot tube set;
Figure 2 is a side elevation view of a pressure
regulator coupled to a span test module;
Figure 3 is a pneumatic circuit diagram showing a
differential pressure sensor coupled to a pitot tube set
and a testing module; the circuit switched to a measure
cycle;
Figure 4 is the same circuit switched to a purge
cycle;
Figure 5 is the same circuit switched to a zero
test cycle; and
Figure 6 is the same circuit switched to a span
test cycle.
Description of the Preferred Embodiment
As shown in figure 1 a standard S-type pitot tube
set 10 is positioned through the wall of the stack S of an
industrial exhaust stream. The pitot tube set 10
comprises a high pressure tube 11 that faces downwardly
into the flow and a low pressure tube 12 that faces
upwardly away from the flow. As shown in Figures 3 to 6
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the pitot tube set 10 is coupled to an pneumatic circuit
that includes a differential pressure sensor 20. The high
pressure pitot tube 11 is connected to the high pressure
side of the sensor 20 through two solenoid valves 21 and
22 and the low pressure pitot tube 12 is connected to the
low pressure side of the sensor 20 via solenoid valves 23
and 24.
The testing module comprises an air supply 30
usually coming from a compressor that flows through a
solenoid valve 31 via a cross over 39 to a pressure
regulator 32 through a micro pressure control unit (MPC)
40 to a test solenoid valve 35. The test solenoid valve
35 has one outlet point 36 coupled to the high pressure
sensor solenoid valve 32 and the other 37 to vent. The
solenoid valves 21, 22, 23 and 24 and 35 are known as 3/2
solenoid valves meaning three ports and two positions.
The solenoid valve 31 is a two port two position valve
(2/2). The cross over 39 allows air to flow to the high
and low pressure solenoid valves 21 and 23 and also to the
pressure regulator 32, when the AIR solenoid 31 is
energised.
The air supply 30, pressure regulator 32 and MPC
unit 40 are shown in greater detail in Figure 2. The air
supply 30 produces air at typical pressures between 40 and
100 psi. The air supply is fed into an off-the-shelf
pressure regulator 32 that is adjustable to provide an
output pressure of between 3 and 4 psi. The pressure
regulator 32 is connected to the MPC unit 40 that
comprises a housing 41 with a through duct 42 to outlet 43
with a needle adjust valve 44 having a tapered valve
member 45 that projects into an orifice port 51. The
needle valve 44 is adjustable through rotation to vary the
flow through the duct 42 by diverting a portion of the
flow through the orifice port 51 to bleed to exhaust 46.
The diversion of flow is possible because of the back
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pressure buildup created by a plug 50 with a fixed small
orifice 52 located along the through duct 42. A pressure
gauge 48 is mounted across the entry of the duct 42 to
measure the input pressure from the regulator 32.
The MPC unit 40 can be adjusted to ensure that
the output air pressure is from 5 to 300 mm of water
column. The needle valve 44 can be adjusted to provide
the desired output test pressure to the differential
pressure sensor 20 and a manometer connected temporarily
in parallel with the sensor 20. The needle valve 44 is
then locked into position so that repeatable testing can
be achieved. A control panel (not shown) is coupled to the
solenoid valves that are operated in accordance with the
tests described hereunder.
In the normal measure cycle shown in Figure 3
the pressure from the high pressure pitot tube 11 is fed
through the first high pressure solenoid 21 to the high
pressure sensor solenoid 22 and to the high pressure side
of the differential pressure sensor 20. Similarly the
pressure from the low pressure pitot tube 12 is fed
through the low pressure solenoid 23 to the low pressure
sensor solenoid 24 to the low pressure side of the
pressure sensor 20 so that the pressure sensor 20 can
measure the pressure difference. In this cycle, the
testing module is disconnected from the circuit and not in
operation.
In a purge cycle shown in Figure 4, flow between
the high pressure solenoid valves 21 and 22 and low
pressure solenoid valves 22 and 24 is disconnected. The
air supply is thus fed through the high and low pressure
solenoid valves 21 and 23 through the pitot tubes 11 and
12 and back into the stack S, thus purging the tubes 11
and 12. During this cycle, both sides of the differential
sensor 20 are vented to atmosphere via the sensor solenoid
valves 22 and 24.
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The zero test cycle is shown in Figure 5 where
the only open solenoid valves are the two sensor solenoid
valves 22 and 24. The energised solenoid valve 22 vents
the DP sensor high port to atmosphere via the de-energised
test solenoid 35, and the energised solenoid 24 vents the
DP sensor low side direct to atmosphere. This condition
creates equal pressures to both ports of the DP sensor
thus providing a zero DP sensor response. There is no
pressure effect from the isolated stack 5 or from the air
supply 30.
Figure 6 shows the span test cycle, in this cycle
the pressure effect from the stack 5 is disconnected when
solenoids 22 and 24 are energised. When air solenoid 31 is
energised the air supply 30 is fed through the regulator
32 and the MPC unit 40 and through the energised test
solenoid valve 35 to the energised sensor solenoid valve
22 to the high pressure side of the sensor 20. The low
pressure side of the sensor 20 is vented to atmosphere via
the energised low pressure solenoid valve 24. In this way
air at a controlled test pressure from the MPC unit 40,
referenced to atmosphere, is passed to the differential
pressure sensor 20 and the pressure reading by the sensor
can be compared with the known input pressure from the MPC
unit 40 to determine the accuracy of the sensor 20. The
inputted test pressure can be varied across the whole span
or scale of the differential sensor 20 but it is usual to
carry out this test at about 60% of the DP sensor full
scale value.
The four operations of the flow meter can be
controlled through and can be carried at whatever interval
is considered appropriate. The testing does not affect
the operation of the stack and is simply an automated
means of
(a) ensuring ongoing measurement;
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(b) providing a purge facility;
(c) a periodic zero test cycle; and
(d) a periodic span test cycle.
It is understood that the electronics of the assembly will
include appropriate warning lights or alarms should the
accuracy of the differential pressure sensor 20 fall
outside pre-set limits for both the zero test cycle and
the span test cycle.
In the claims which follow and in the preceding
description of the invention, except where the context
requires otherwise due to express language or necessary
implication, the word "comprise" or variations such as
"comprises" or "comprising" is used in an inclusive sense,
i.e. to specify the presence of the stated features but
not to preclude the presence or addition of further
features in various embodiments of the invention.
