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 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 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 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 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 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. 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; (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.