Combined flow measuring and valve leakage indicator

Apparatus for detecting fluid leakage past a control valve in a system for controlling and measuring the flow of a fluid in a conduit of the type having first and second control valves, a flow sensing element of the differential pressure type located in the conduit between the control valves and a differential pressure flow rate indicator connected to the flow sensing element through high and low pressure conduits. Fluid leakage past the control valve discharges from the isolated portion of the fluid conduit first through the low pressure conduit and then through a vent conduit causing a pressure drop along the low pressure conduit which causes an indication of flow on the flow rate indicator.

BACKGROUND OF THE INVENTION 
This invention relates to an apparatus for measuring and controlling the 
flow rate of a fluid in a conduit of the type having first and second 
control valves in the conduit with a flow sensing element of the 
differential pressure type located in the conduit between the control 
valves. A flow rate indicating means is connected to the flow sensing 
element through high pressure and low pressure conduits and includes 
apparatus for detecting and indicating fluid leakage past a valve into the 
isolated portion of the fluid conduit between the control valves. 
Fluid flow measuring and controlling apparatus are commonly used in a 
diversity of applications ranging from the control and measurement of the 
flow rate of cryogenic liquids to the control and measurement of the flow 
rate of high temperature gases, and in some cases, supercritical fluids. 
In certain of these applications, it is highly desirable for safety or 
process reasons to be able to insure positive shut-off of the fluid at the 
control valves and to have a positive indication of any leakage past the 
valves. Such conditions occur in applications involving the use of toxic 
or highly reactive fluids, or those where leakage of the fluid past the 
shut-off valve would create hazardous conditions or would damage equipment 
or materials downstream of the flow measuring and controlling equipment. 
Examples of such applications include the supply of oxygen or other gases 
to various processes; the supply of steam to reactions; or the supply of 
various reactants to a reaction vessel. A specific example of the 
application of this invention is to the control and measurement of the 
supply of oxygen gas for enrichment of air used in cupola furnaces. 
In the prior art, fluid leakage past the first control valve when both the 
first and second valve are in shut-off position has been detected by means 
of a vent conduit communicating directly with the fluid conduit at a point 
between the first and second control valves through which the leakage 
fluid discharged. Such vent conduits included valves opened manually or in 
response to a signal from a remote location. In other applications the 
vent conduit included a pressure relief valve which opened to discharge 
the leakage fluid when the pressure in the fluid conduit between the first 
and second control valves exceeded a predetermined pressure, generally 
higher than the normal working pressure of the fluid downstream of the 
first shut-off valve. Detection of the leakage was accomplished by 
inclusion of a flow indicator in the vent conduit on the discharge side of 
the valve. Alternatively, a remote alarm would be actuated by the opening 
of a relief valve. A further method disclosed by the prior art of 
detecting leakage past the valve seats of a shut-off valve is disclosed in 
U.S. Pat. No. 2,430,122, wherein an extraneous fluid is admitted to the 
body of the valve between the upstream and downstream valve seats and any 
leakage past the valve seats is detected by flow of the extraneous fluids 
through the supply conduit. 
In order to provide a positive indication of leakage past the first valve 
in prior art flow measuring and controlling apparatus, connection of a 
vent conduit to the fluid conduit in addition to the flow measuring 
apparatus between the first and second valves is required. A second flow 
indicator, such as a rotameter, was also required on the vent conduit in 
addition to the flow rate indicator connected to the flow sensing element 
in the fluid conduit. Often the flow rate indicator is located remotely 
from the fluid conduit at a control panel. In the prior art, the separate 
vent conduit involved significant additional cost in installing such vent 
conduit to the control panel where the leakage indicator was located, as 
well as the cost attendant with a second flow indicator. 
SUMMARY OF THE INVENTION 
We have discovered that in an apparatus for measuring and controlling the 
flow rate of a fluid utilizing a differential pressure type of flow 
measuring system between two control valves in a fluid conduit, a positive 
indication of leakage of small amounts of fluid past the first control 
valve into the isolated portion of the fluid conduit when both control 
valves are in shut-off position can be realized by connecting a vent 
conduit in communication with the low pressure conduit of the flow 
measuring system at a point between the flow rate indicator and the flow 
sensing element in the fluid conduit. The point of connection of the vent 
conduit to the low pressure conduit is preferably in close proximity to 
the flow rate indicator. The vent conduit includes a valve closed to block 
the vent conduit during the time when the first and second control valves 
are open allowing normal fluid flow through the fluid conduit. Upon 
closing of the control valves, the vent valve is opened to allow discharge 
of any leakage fluid from the fluid conduit through the low pressure 
conduit and the vent conduit. Fluid leaking past the first control valve 
creates an increase in pressure in the portion of the fluid conduit 
isolated by the control valves. With no flow through the high pressure 
conduit, this pressure is transmitted undiminished to the high pressure 
connection of the flow rate indicator. Discharge of the leakage fluid from 
the fluid conduit through the low pressure conduit and through the vent 
conduit creates a pressure drop between the point of connection of the low 
pressure conduit to the flow sensing element and the junction with the 
vent conduit. This reduced pressure is communicated to the flow rate 
indicator through the low pressure conduit between the points of 
connection of the low pressure conduit to the vent conduit and the flow 
rate indicator, thus causing a positive indication of flow on the flow 
rate indicator. The leakage fluid is discharged from the vent conduit at a 
safe location. 
Operation of the vent valve may be manual or may be accomplished 
automatically in response to closing of the first control valve. 
The present invention offers a significant improvement over the prior art 
in that venting of fluid in the conduit between the first and second 
control valves, and indication of leakage past the first valve is 
accomplished with the flow rate indicator installed in the system to 
measure the normal flow in the conduit. Where the flow rate indicator and 
means for actuating the control valves are located remotely from the fluid 
conduit, the present invention eliminates the need for a vent conduit 
extending from the fluid conduit to the control panel and the installation 
in the control panel of an additional flow indicator in the vent conduit. 
A further advantage of the present invention over the prior art is the 
utilization of a single flow indicator for both measuring fluid flow in 
the conduit during a normal operation and indicating the presence of 
leakage past the first control valve during the period the first and 
second control valves are in shut-off position. This advantage is most 
appreciated when operation of the fluid supply system is generally 
unattended and a recording type fluid flow rate indicator is installed. In 
such applications, the recording flow rate indicator will provide a 
graphic recording of any fluid leakage during unattended operation. To 
provide such capability, the prior art sould require the installation of 
an additional expensive flow rate recorder. 
Therefore, it is an object of this invention to provide a combined flow 
measuring and valve leakage indicator wherein a flow rate indicator of the 
differential pressure type is utilized to measure fluid flow in a conduit 
and to detect leakage past a control valve upstream of a flow sensing 
element when such control valve is in a closed position. 
A further object of this invention is to provide a means for automatically 
venting the isolated portion of the fluid conduit between the first and 
second control valves and to automatically indicate leakage of small 
volumes of fluid past the first control valve. 
A still further object of the invention is to provide means for indicating 
positive shut-off of the fluid in the fluid conduit at the first control 
valve.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates diagrammatically a preferred embodiment of the present 
invention. A fluid conduit 1 includes a first control valve 2, a second 
control valve 3, and a flow sensing element 4 of the differential pressure 
type shown as an orifice element located in fluid conduit 1 between 
control valves 2 and 3. A flow rate indicator 12 of the recording 
differential pressure type, such as the Model 202A Flow Recorder 
manufactured by ITT Barton, communicates with fluid conduit 1 through high 
pressure conduit 7 and low pressure conduit 8, which are connected to 
flange taps 5 and 6, respectively, on either side of flow sensing element 
4. Pressure conduits 7 and 8 are generally constructed of a tubing 
material compatible with the fluid and having a nominal diameter of from 
1/8to 1 inch. A preferred material for use in oxygen service is copper 
tubing of 1/4 inch nominal diameter. Connected to and communicating with 
the low pressure conduit 8 at a point between flange tap 6 of flow sensing 
element 4 and the flow indicating recorder 12, is a vent conduit 9 
terminating at a vent valve 10. Vent conduit 11 extends from vent valve 10 
to a remote location where any leakage fluid can be safely discharged. 
Advantageously, vent conduits 9 and 11 are constructed of materials 
similar to low pressure conduit 8. 
In the preferred embodiment, valve 10 is a solenoid valve of the normally 
open type manufactured by Asco Valves and described in ASCO Bulletin 8263. 
In operation, the solenoid coil of this valve is electrically energized by 
the control panel power supply 17, thus closing vent valve 10 during flow 
of fluid through conduit 1 and open control valves 2 and 3. Interruption 
of the panel power supply 17 deenergizes instrument air supply 15 causing 
valve controllers 13 and 14 to close control valves 2 and 3, thus shutting 
off the flow of fluid in conduit 1. Simultaneously, vent valve 10 opens 
allowing any fluid pressure in the isolated portion of fluid conduit 1 
between control valves 2 and 3 to discharge through low pressure conduit 8 
and vent conduits 9 and 11. While in the preferred embodiment, vent valve 
10 is electrically energized through the panel power supply, there exist 
alternative arrangements for actuation of vent valve 10. These include 
actuation by means of the instrument air supply 15 by suitable connection 
of vent valve 10 to instrument air supply manifold 16. 
The instrument air supply 15 is shown in the preferred embodiment of FIG. 1 
to be energized by the panel power supply. Advantageously, a supply of 
instrument air may be admitted to the instrument air supply manifold 16 
through a solenoid valve (not shown) of the normally closed type. In such 
case, interruption of the panel power supply, for example, through a 
switch (not shown) for shutting off fluid supply will close the instrument 
air supply solenoid valve thus causing valve controllers 13 and 14 to 
close control valves 2 and 3. 
Flow sensing element 4 is shown in FIG. 1 of the preferred embodiment as an 
orifice plate with upstream and downstream flange pressure taps. However, 
other flow sensing elements of the differential pressure type such as 
venturi or pitot elements can be used. For example, a pitot type flow 
sensor is manufactured by the Ellison Instrument Division of Dieterich 
Standard Corp. of Boulder, Colo., and sold under the Annubar trademark. A 
necessary requirement for the use of such flow sensing elements is that 
there exist direct high and low pressure connections between the flow 
sensing element and the flow indicator. 
Flow rate indicator 12 communicates with the high and low pressure taps of 
the flow sensing element through conduits 7 and 8 respectively. Flow rate 
indicator 12 may be a gauge type instrument or, advantageously, a 
recording type instrument. The use of a recording type flow rate indicator 
provides a graphic record of the fluid flow rate in conduit 1 during 
normal flow and permanent graphic indication of any leakage past the first 
of control valves 2 and 3 during the time such control valves are in the 
closed position, and vent valve 10 is open. 
In the preferred embodiment, fluid conduit 1 generally ranges in diameter 
from 1/2 inch nominal diameter upwards. Pressure conduits 7 and 8 and vent 
conduits 9 and 11 are generally constructed of tubing or pipe with a 
nominal diameter between 1/8 and 1 inch, with a common range being 1/8 
through 1/2 inch nominal diameter. Utilizing common construction 
materials, the cross-sectional flow area of low pressure conduit 8 ranges 
generally from 10% to less than 0.2% of the cross-sectional flow area of 
fluid conduit 1. 
While the location of fluid conduit 1 containing control valves 2 and 3 and 
flow sensing element 4 is generally determined with regard to the location 
of the fluid supply system (flow direction indicated by arrow in FIG. 1) 
upstream of control valve 2 and the use point downstream of control valve 
3, the flow indicator 12, panel power supply 17, instrument air supply 15, 
valve controllers 13 and 14 and vent valve 10 are advantageously located 
within a single control panel located at the point of maximum convenience 
and safety for the operator. The control panel may be located within a few 
feet of flow sensing element 4 or may be located remotely by a distance of 
100 feet or more. Thus, pressure conduits 7 and 8 will range generally in 
length from about 2 feet to 200 feet. Vent conduit 9 is connected to the 
low pressure conduit 8 advantageously at a point near flow indicator 12 
within the control panel assembly. Vent valve 10, similarly, is preferably 
located within the control panel assembly so as to provide for 
fabrication, operation, and maintenance efficiencies. Vent conduit 11 
connects vent valve 10 with a discharge region into which any fluid 
leaking past control valve 2 is safely discharged. In the case of 
atmospheric gases such as oxygen or nitrogen, such discharge can be to the 
atmosphere at a location where the gases will be safely dispersed. Where 
the fluid in conduit 1 is toxic or normally a liquid, vent conduit 11 
should discharge to adequate facilities for safely handling the maximum 
expected volume of discharging fluid. 
Operation of the apparatus is as follows: 
During normal fluid flow through conduit 1, control valves 2 and 3 are 
maintained in open position by valve controllers 13 and 14. Control of 
valves 2 and 3 may be in response to pressure or flow rate in conduit 1 or 
other control parameter, and can be accomplished by any number of means 
well known in the art. In the preferred embodiment as shown in FIG. 1, 
fluid flow through conduit 1 is shut-off by interrupting the panel power 
supply 17 by means of a switch. Interruption of the panel power supply, in 
turn, interrupts the instrument air supply to valve controllers 13 and 14 
through manifold 16. Upon loss of instrument air, valve controllers 13 and 
14 close control valves 2 and 3. In one preferred embodiment of the 
apparatus, valve controller 14 operates in a manner to delay final closing 
of valve 3 for a period of time after the closing of control valve 2. This 
delay allows fluid in the portion of conduit 1 between control valves 2 
and 3 to initially discharge through the downstream portion of conduit 1, 
reducing the time required to equalize pressure in the isolated portion of 
conduit 1 with the ambient pressure existing at the discharge end of vent 
conduit 11. At the same time that power to the instrument air supply is 
interrupted, power to the solenoid of vent valve 10 is interrupted, 
causing vent valve 10 to open. During a brief period of time, residual 
fluid contained in the isolated portion of fluid conduit 1 between control 
valves 2 and 3 will discharge through the low pressure conduit 8, thence 
through vent conduit 9, vent valve 10, and finally vent 11 to atmosphere. 
If no leakage past control valve 2 exists, pressure in the isolated 
portion of the fluid conduit 1 will equalize with the pressure at the 
discharge end of vent conduit 11 and no flow will take place through low 
pressure conduit 8. In this instance, the pressure communicated to the 
flow indicator by high pressure conduit 7 will be equal to that existing 
at the point of connection of low pressure conduit 8 and vent conduit 9 
and communicated to the low pressure side of the flow indicator by low 
pressure conduit 8, thus causing the flow indicator to register an absence 
of flow. If, during the period control valves 2 and 3 are in closed 
position, fluid leaks past control valve 2, such leaking fluid will cause 
an increase in pressure in the isolated portion of fluid conduit 1 causing 
a flow of the leakage fluid through low pressure conduit 8, vent conduit 
9, vent valve 10, and finally vent conduit 11 to discharge. The pressure 
of the fluid in the isolated portion of the fluid conduit 1 will be 
communicated to the high pressure side of flow indicator 12 by high 
pressure conduit 7. Since no flow will exist through high pressure conduit 
7, this pressure will be transmitted undiminished to flow indicator 12. 
Flow of the leakage fluid through low pressure conduit 8 will cause a 
pressure drop between the point of connection of low pressure conduit 8 to 
the fluid conduit at 6 and the point of connection of vent conduit 9 to 
the low pressure conduit 8, resulting in an intermediate pressure. A 
further pressure drop will occur through vent conduits 9 and 11 with the 
pressure at the discharge of vent conduit 11 being equal to that existing 
in the discharge region. Since no flow will exist in the portion of low 
pressure conduit 8 between the point of connection with vent conduit 9 and 
the flow rate indicator 12, the intermediate pressure existing at the 
connection of low pressure conduit 8 and vent conduit 9 will be 
transmitted undiminished to the low pressure side of flow rate indicator 
12. The difference between the pressure existing in the isolated portion 
of fluid conduit 1 and the intermediate pressure transmitted to the low 
pressure side of the flow rate indicator 12 will cause a positive reading 
on the flow rate indicator, indicating the presence of leakage past contol 
valve 2. 
In practice, it will be noted that sensitivity of the flow indicator to the 
flow of leakage fluid through low pressure conduit 8 increases with 
increasing length of low pressure conduit 8 between the point of 
connection with fluid conduit 1 and vent conduit 9, and with decreasing 
diameter of low pressure conduit 8. If it is desirable, the length of low 
pressure conduit 8 between the fluid conduit 1 and the connection with 
vent conduit 9 and the flow diameter of low pressure conduit 8 may be 
selected utilizing customary fluid flow design principles such that at a 
predetermined leakage flow rate, the pressure drop along low pressure 
conduit 8 from the point of connection with fluid conduit 1 to the point 
of connection with vent conduit 9 corresponds to the full scale pressure 
differential of the flow rate indicator 12. Such commonly used ranges are 
10, 20 and 40 inches of water. An alternative method of controlling the 
pressure drop along low pressure conduit 8 consists of installing a fldow 
resistance device in low pressure conduit 8 between flange taps 6 and the 
junction with vent conduit 9. Such flow resistance devices include 
instrument gauge snubbers, orifices, or valves. 
While the system has been described as detecting leakage past the first 
control valve, it is to be understood that the apparatus will operate to 
give a positive indication of leakage past either the upstream or 
downstream control valves into the isolated portion of the fluid conduit. 
Such leakage past the downstream valve may occur when the system pressure 
downstream of the flow measuring apparatus is maintained at a pressure 
higher than that of the region to which vent conduit 11 discharges.