Measuring and testing – With fluid pressure – Dimension – shape – or size
Reexamination Certificate
2001-11-13
2003-07-29
Kwok, Helen (Department: 2856)
Measuring and testing
With fluid pressure
Dimension, shape, or size
C138S040000, C219S121830
Reexamination Certificate
active
06598462
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and apparatus for the measurement and calibration of fluid flow through work pieces having one or more apertures and, in particular, to a method and apparatus for testing such work pieces and verifying that the apertures have been adequately formed.
2. Related Background Art
It is well known to form apertures, bores, etc. in work pieces, such as gas turbine blades and vane cooling holes, fuel nozzles, combustion chamber cooling holes, and the like. A variety of different processes are used to form such apertures, including casting, mechanical machining, for example drilling, electrical machining, for example electrical discharge machining (EDM), electrochemical machining such as capillary drilling, or combination of such processes.
It is usually desirable and, in some applications, essential to verify that such aperture or bores have been correctly formed so that they provide the desired amount of fluid flow under specified conditions, thereby ensuring correct operation of the work piece in question and minimizing the probability of failure of a component during use.
The present invention is based on a modification of a known type of airflow test system for the above purpose and in order to fully understand the invention, the known system will now be described with reference to
FIGS. 1 and 2
which show an embodiment of the present invention including parts of the known system. The known system is based on the use of critical flow nozzles
100
,
102
. The system comprises an air inlet
104
through which pressurized air, typically at 7 Bar absolute (for example from a factory compressor or the like) passes to an air/water filter
106
via a ball valve
108
. The air then passes to an accumulator
110
, which may be a carbon steel receiver typically of 127 liter capacity, and from there to a fine particle/oil filter
112
. Thus, pressurized clean, dry air passes to the critical flow nozzles
100
,
102
via a pilot operated pressure regulator
114
. The pilot operated pressure regulator
114
can be adjusted to control the pressure P
1
A at the inlet to the critical flow nozzles
100
,
102
, and thus the mass flow rate through the nozzles
100
,
102
to a test station
116
. Airflow to the test station
116
is selectively controlled via respective ball valves
118
,
120
, and a pressure relief valve
122
is provided at the inlet to the test station
116
, to protect the work station pressure transducers from damage due to accidental over pressurization.
FIG. 2
is a schematic view of the system of
FIG. 1
, with the air/water filter accumulator and fine particle/oil filter omitted. Further, for clarity, only one of the critical flow nozzles
100
is shown. Thus, in use, pressurized clean, dry air flows through the critical flow nozzles
100
,
102
via a pressure regulator
114
. The absolute air pressure P
1
A and the temperature T
1
at the inlet of the critical flow nozzles
100
,
102
are measured and the mass flow of air from the outlet of each critical flow nozzle
100
can be calculated using the equation:
WFN
=
(
K
·
P1A
)
T1
where:
WFN=flow nozzle mass flow;
P
1
A=absolute air pressure;
T
1
=absolute temperature; and
K=flow nozzle calibration constant (usually provided for the nozzle by its manufacturer).
The total mass flow through the critical flow nozzles
100
,
102
is the sum of the mass flows calculated for each of the critical flow nozzles through which air is flowing, i.e. the nozzles whose respective valves
118
,
120
are open.
This known total mass airflow then passes to the work test station
116
, which typically includes a flow straightener
124
. The work test station
116
is designed to support, seal and clamp the work piece
126
to be tested so that all of the air from the critical flow nozzles
100
,
102
passes through it, but it will be apparent to persons skilled in the art that such supporting, sealing and clamping arrangements (not shown) will be different for each type of test piece, as each type has its own specific requirements.
The gauge air pressure P
3
G and the absolute temperature T
2
are measured at the inlet to the test piece
126
, as is the absolute (barometric) air pressure PA of the air as it exits the test piece
126
. It will be appreciated that the absolute pressure PA of the air as it exits the test piece
126
will be atmospheric pressure if the system vents to atmosphere.
The pressure ratio PR can be calculated using the following equation:
PR
=
PA
+
P3G
PA
and various test piece characteristics can be determined. For example, the effective area of the test piece can be calculated using the following equation:
AEFF
=
WTP
2.
D2
.
(
P3G
)
where:
AEFF=effective area of test piece;
D2=test piece inlet density; and
WTP=test piece mass flow=A.CD {square root over (2.D2.G.P3G)}
where:
A=total discharge area of test piece;
CD=discharge coefficient for test piece; and
G=gravitational constant.
This assumes that the cross-sectional area of the flow straightener is sufficiently large compared to the test piece cross-sectional area that the total absolute pressure at P
3
G tapping can be assumed to be equal to the static absolute pressure (P
3
G+PA), i.e. the flow velocity at the tapping is very low. If this is not the case the equation needs to be corrected for the dynamic pressure (kinetic head).
Thus, with kinetic head correction, this becomes:
AEFF
=
WTP
2.
D2
.
(
PD
)
where:
PD=test piece total differential pressure drop, i.e. including dynamic pressure.
The flow parameter of the test piece can be calculated using the following equation:
FP
=
WTP
T2
P1
=
AEFF
2.
(
PR
-
1
)
PA
.
R
where:
R=gas constant
FP=flow parameter of test piece; and
P
1
=test piece absolute inlet pressure.
There are, in fact, a wide range of test piece characteristics which can be measured, and those chosen to be measured and/or calculated within any particular system are dependent upon user requirements.
A typical test specification requires the fluid pressure at the inlet to the test piece
126
to be adjusted to a particular pressure ratio (or equivalent parameter), and then the desired characteristics of the test piece to be determined, for example, the effective area, discharge coefficient, flow parameters, etc.
In conventional systems, the desired pressure ratio is obtained by manual or automatic adjustment of the fluid flow rate through the test piece. It will be appreciated that in a typical test, where the test piece
126
is vented to atmosphere, the inlet pressure required to give the desired pressure ratio depends on the atmospheric (barometric pressure) and therefore with time. Further, the altitude at which the test is conducted can be very significant. In any event, it is relatively difficult to achieve a stable exact setting, and a setting tolerance is therefore allowed. Even then, manual setting is quite skilled and time consuming. In addition, as the flow characteristics of a typical test piece are quite often very sensitive to pressure ratio (due, for example, to the complexity and variations in size of their internal passages) the error due to incorrect setting can be very significant, for example, +1-0.5% compared to an overall error budget of 1%.
We have now devised an arrangement which overcomes the problems outlined above.
SUMMARY OF THE INVENTION
Thus, in accordance with a first aspect of the present invention, there is provided fluid flow measurement apparatus for verifying one or more apertures in an object, such as a work piece, the apparatus comprising a source of pressurized fluid and adjustment means for adjusting the fluid flow from the source, means for measuring said fluid flow, means for mounting or otherwise arranging a test piece in the fluid flow from the source such that fluid flows through the at
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Kwok Helen
Leamount Limited
Politzer Jay L
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