Patent Application
20110284083
Not Applicable
Not Applicable
1. Technical Field of the Invention
The present disclosure is related generally to fluid flow control and electro-hydraulic/electro-pneumatic systems, and more particularly, to a valve positioner including a failsafe that maintains the position of the valve to that of a pre-failure state.
2. Description of the Related Art
A control valve regulates a flowing fluid, such as gas, steam, water, or chemical compounds by opening and closing a passageway, through which the fluid flows, with a valve element. The subject flowing fluid is generally referred to as the process. An actuator, in turn, provides the motive force to open and close the valve element. Pneumatic or hydraulic energy is converted by the actuator to rotational or linear motion, depending on the configuration of the valve element.
Typically, pneumatic systems are utilized for valve actuators due to several distinct advantages. For instance, air, rather than fluids such as oil, is exhausted into the atmosphere, and compressed air is better able to absorb excess pressure and pressure spikes. There are other peripheral advantages such as fewer maintenance requirements.
A conventional pneumatic actuator is comprised of a piston sealed within a cylinder, and the piston including a connecting rod that is mechanically coupled to the valve element. Compressed air is forced into and out of the cylinder to move the connecting rod. In a single-acting actuator, the compressed air is taken in and exhausted from one end of the cylinder and is opposed by a range spring, while in a double-acting actuator, air is taken in one end of the cylinder while simultaneously exhausting it out of the opposing end.
Precise and accurate control of the valve actuator, and hence the valve element, can be achieved with a positioner device coupled thereto. Pneumatic valve positioners, which can cooperate with aforementioned pneumatic actuators, are well known in the art. The proportional movement of the actuator is accomplished by the movement of compressed air into and out of the actuator piston, as indicated above. More particularly, valve positioners incorporate a spool (or other devices) that either rotates or slides axially in a housing the port the flow of compressed air to the actuator or to one or more exhaust ports.
In further detail, an electrical control circuit provides a variable current signal to the positioner device that proportionally corresponds to particular states of the actuator and hence a particular position of the control valve. The electrical control circuit and the electrical current signals generated thereby may be part of a broader process managed by a distributed control system (DCS). Generally, the electrical current varies between 4 milliamps (mA) and 20 mA according to industry-wide standards; at 4 mA the valve positioner may fully open the valve element, while at 20 mA the valve positioner may fully close the valve element. The positioner compares the received electrical signal to the current position of the actuator, and if there is a difference, the actuator is moved accordingly until the correct position is reached.
There are a number of operational conditions or exceptions under which it becomes necessary to “freeze” in place the last position of the actuator. These include the complete loss of power to the positioner or other such failure therein, failure in the distributed control system, a wire carrying the actuator signal being cut, and so forth.
Various solutions for such “fail freeze” functions have been developed, though each one is deficient in one or more regards. One involves the use of an external component to monitor the electrical current signal, and driving a solenoid valve upon detection of a failure condition. This tends to be an expensive proposition, however, since a safe external power source is required, along with specialized components that monitors the electrical current such as a current threshold switch and controls the power to the solenoid. Additionally, a further wiring and junction box will be required. Overall, the increased complexity of this solution makes it particularly unsuitable (e.g., too expensive) for hazardous environments. Another solution involves the use of a positioner with normally closed on/off valves. This is also inadequate because the flow capacity of such positioners is typically so low that boosters are necessary to meet the specified stroking time. Furthermore, any leakage from the boosters essentially nullifies the freezing action. Yet another solution involves a pneumatic positioner with a separate fail-freeze electro-pneumatic I/P converter. Again, this solution has proven deficient, as the separate positioner has a slow response time of around six (6) seconds, such that stroking the actuator within the required limits is not possible.
Accordingly, there is a need in the art for an improved valve positioner with a failsafe that maintains the position of the valve to that of a pre-failure state. Moreover, this is a need in the art for a valve positioner that includes a fail-freeze function powered from the electrical current signal loop thereto without an external source. There is also a need for valve positioners with a fail-freeze function that are intrinsically safe.
In accordance with one embodiment of the present disclosure, a valve positioner system is contemplated. The system may have a transducer with a first type output port connectible to a valve actuator, as well as a second type input port receptive to a valve position signal. The valve position signal may be proportional to an output of the first type output port. Additionally, the system may include a monitoring circuit. A pilot activation signal may being generated thereby while predefined conditions are met. There may also be a primary piloted valve in communication with the monitoring circuit. The primary piloted valve may have a first position in absence of the pilot activation signal, and a second position during receipt of the pilot activation signal. The valve positioner system may include a first valve coupled to the primary piloted valve. The first valve may have a first position corresponding to the first position of the primary piloted valve, and a second position corresponding to the second position of the primary piloted valve. The first type output port may be disconnected from the valve actuator while the first valve is in the first position, while the first type output port may be in fluid communication with the valve actuator while the first valve is in the second position. In accordance with another embodiment of the present disclosure, a valve positioner failsafe device is contemplated. The device may include an electro-pneumatic transducer with transducer output ports and an electrical input port receptive to a valve position signal. A pressure value of the transducer output may be proportional to an electrical current level value of the valve position signal. The device may also include an electrical current level monitoring circuit receptive to the valve position signal. A pilot activation signal may generated while the current value of the valve position signal remains greater than a predetermined failure value. There may also be a primary piloted valve including a primary piloted valve output port and a pressure line intake port. The primary piloted valve may be in communication with the current level monitoring circuit. Furthermore, there may be a first valve including a first valve pilot input port connected to the primary piloted valve output port. A first valve input port may be coupled to a first one of the transducer output ports, and a first valve output port may be coupled to a first one of actuator input ports of a valve actuator. The first valve may selectively fluidly couple the transducer to the actuator.
According to yet another embodiment of the present disclosure, a method for fail-safe regulation of a process with a valve positioner including an actuator is contemplated. The method may begin with receiving a valve position signal. Thereafter, the method may include deactivating a pilot signal to a pneumatic piloted valve. This may be in response to the valve position signal having a current value less than a predetermined failure level. The method may also include switching closed the pneumatic piloted valve in response to deactivating the pilot signal. Additionally, the method may include switching closed a first valve selectively coupling a first output of the valve positioner to a first input of the actuator. This may be in response to the switched closed pneumatic piloted valve. Pneumatic pressure to the first input of the actuator existing prior to the deactivation of the pilot signal may be maintained upon the closing of the first valve.
The present invention will be best understood by reference to the following detailed description when read in conjunction with the accompanying drawings.
These and other features and advantages of the various embodiments disclosed herein will be better understood with respect to the following description and drawings, in which:
Common reference numerals are used throughout the drawings and the detailed description to indicate the same elements.
The detailed description set forth below in connection with the appended drawings is intended as a description of certain embodiments of the present disclosure, and is not intended to represent the only forms that may be developed or utilized. The description sets forth the various functions in connection with the illustrated embodiments, but it is to be understood, however, that the same or equivalent functions may be accomplished by different embodiments that are also intended to be encompassed within the scope of the present disclosure. It is further understood that the use of relational terms such as top and bottom, first and second, and the like are used solely to distinguish one entity from another without necessarily requiring or implying any actual such relationship or order between such entities.
The block diagram of
The components of the valve positioner failsafe system 10 are variously described herein as being driven by compressed air, though it will be appreciated that any other inert gasses may be utilized. Along these lines, other fluid power systems such as hydraulics may be substituted without departing from the scope of the present disclosure. As indicated above, however, compressed air offers several advantages with respect to response times and safety in potentially hazardous industrial environments.
The illustrative example shows a first fluid flow passageway 24 and a second fluid flow passageway 26 defined by the cylinder body 16, which is characteristic of a double-acting actuator in which compressed air is supplied to one side of the chamber 18 while the other side is exhausted. It is expressly contemplated, however, that a single-acting actuator with spring return may be used instead, along with attendant modifications to the configuration of the positioner device 12.
The supplying and exhausting of the compressed air to the valve actuator 14 is governed by the positioner device 12, an exemplary variation of which is illustrated in
With reference again to the block diagram of
Per common industry standards, the valve position signal 42 is an analog current ranging between 4 mA and 20 mA. Although the basic operation of the valve positioner failsafe system 10 does not require it, the valve position signal 42 can carry a digital signal utilized by positioner device 12 for additional functionality such as diagnostics, configuration, and so forth, and is accordingly HART compliant (Highway Addressable Remote Transducer). As will be described in further detail below, the valve position signal 42 also provides electrical power to the positioner device 12 and other associated components.
The valve position signal 42 can be quantified as a percentage of the fully open or fully closed position of the control valve, and more specifically, as the pressure of the compressed air that is ported from the pressure line intake port 30 to the first and second output ports 30, 32 for achieving that position. For example, upon proper calibration, a 0% (4 mA) input signal may be defined as the fully closed position, while a 100% signal (20 mA) input signal may be defined as the fully open position. A 12 mA signal may thus represent a 50% position.
An electro-pneumatic transducer 46, and specifically a microprocessor 48 therein, receives the valve position signal 42. In order to ensure correct positioning of the valve actuator 14, a feedback sensor reads the actual position of the valve actuator and transmits a signal representative thereof to the microprocessor 48. The valve position signal 42 includes a set point or reference value, to which the value of the actual position signal is compared. The transducer 46 is then adjusted to supply more or less compressed air to the valve actuator 14 to position the same to the designated set point. A variety of different algorithms may be used to effect a change in the flow rate of compressed air to the valve actuator 14.
The positioner device 12 is understood to be suitable for hazardous environments where flammable gasses in the environment have the potential to ignite from sparks typical in regular circuits and constituent components thereof. In this regard, the positioner device 12 is understood to be intrinsically safe, in that, among other things, the electrical components and any others devices utilized therein operate on low voltages.
In accordance with one embodiment of the present disclosure, valve positioner failsafe system 10 is contemplated to include a “fail-freeze” function. As described above, “fail-freeze” refers to a function where the position of the actuator device 14 is held to that most recent prior to failure. These failures include loss of power due to the two-wire connection 44 being disconnected from the signal source, a loss of pressure in the pressure line 40, loss of the actuator position feedback signal, and so forth. The present disclosure includes a description of one embodiment where the loss of electrical power triggers the fail-freeze function, and is presented by way of example only and not of limitation. Other failure conditions such as those enumerated above may also trigger the fail-freeze function, and it is understood that other embodiments of the valve positioner failsafe system 10 may be adapted thereto.
Referring to
The monitoring circuit 62 in accordance with one embodiment of the present disclosure continuously evaluates the electrical current level of the valve position signal 42. So long as the electrical current level remains above a predefined failure level, a pilot activation signal 64 is generated on a monitor output line 66. By way of example only and not of limitation, this predefined failure level may be 3.7 mA in where a proper signal has a range between 4 mA and 20 mA. As noted above, other failure conditions besides a loss of the valve position signal 42 can be monitored. In this regard, the pilot activation signal 64 can also remain on while such other failure conditions are not detected. Therefore, appropriate threshold values of monitored conditions such as system-wide compressed air pressure, position feedback error rate, and so forth, can be preset.
The valve positioner failsafe system 10 also includes a primary piloted valve 68 that is in communication with the monitoring circuit 62. With further reference to
With the power supplied to the microprocessor 48, which is also in series with the two wire connection 44 (parallel with the piezoelectric pilot element 70), a low or an open value is output to the digital output line (OP1) on the digital output terminal group 58 as the pilot activation signal 64. By outputting a low value, electrical current flows through the piezoelectric pilot element 70, thereby activating the primary piloted valve 68. Thus, during normal operation, the pilot activation signal 64 and hence the primary piloted valve 68 remains on. However, by outputting an open value, to the extent there is any electrical power remaining on the positive line 72 after a failure is detected, the piezoelectric pilot element 70 is powered off and the primary piloted valve 68 is deactivated.
The primary piloted valve 68 is understood to be a conventional normally closed three/two way valve with spring return. Power consumption is understood to be approximately 6 millwatts (mW), and while having a very low fluid flow rate (CV), further work may be performed with its output. Such low power devices are also known to be intrinsically safe and suitable for use in hazardous environments.
As best illustrated in
The primary output port 78 is in fluid communication with a first pneumatic pilot 82 of a first valve 86, as well as a second pneumatic pilot 84 of a second valve 88. The first and second valves are understood to be normally closed two-position valves with spring return that are interposed between the positioner device 12 and the valve actuator 14. More particularly, the first valve 84 has a first input port 90 in direct fluid communication with the first output port 32 of the positioner device 12 over the first pneumatic connecting line 36, and a first output port 92 in direct fluid communication with the first fluid flow passageway 24 of the valve actuator 14. Along these lines, the second valve 88 has a second input port 94 in direct fluid communication with the second output port 34 of the positioner device 12 over the second pneumatic connecting line 38, and a second output port 96 in direct fluid communication with the second fluid flow passageway 26 of the valve actuator 14.
Without the compressed air flowing from the primary output port 78 of the primary piloted valve 68, the first valve 84 and the second valve 88 remain in a first closed position in which the first input port 90 and the second input port 94 are obstructed from the first output port 92 and the second output port 96, respectively. Once the first pneumatic pilot 82 is activated by a flow of compressed air from the primary output port 78 of the primary piloted valve 68, the first valve 84 and the second valve 88 are turned on, thereby connecting the first input port 90 and the second input port 94 to the first output port 92 and the second output port 96, respectively. When the first valve 84 and the second valve 88 are deactivated, the pressure at the first fluid flow passageway 24 and the second fluid flow passageway 26, respectively, are maintained at levels immediately prior to such first and second valves 84, 88 being triggered off.
With reference to the flowchart of
Once the pilot activation signal 64 is turned off, the method continues with a step 204 of switching closed the primary piloted valve 68. Turning off the flow of compressed air through the primary piloted valve 68 also deactivates the first pneumatic pilot 82 and the second pneumatic pilot 86. Thereafter, according to step 206, the first valve 84 and the second valve 88 are switched closed. This, in turn, has the effect of cutting off the flow of compressed air from the positioner device 12 to the valve actuator 14, and holding the pressure to the valve actuator 14 from just before the deactivation of the pilot activation signal 64.
As long as the valve position signal 42 has an electrical current value less than the predetermined failure level or threshold, the state of the valve positioner failsafe system 10 as of step 206 is maintained, that is, the valve actuator is kept in a “fail freeze” position. After evaluation step 207 is found true, in which the electrical current value is greater than or equal to the predetermined failure level or threshold, the method continues with a step 208 of generating a delay. This delay is understood to correspond to the delay in restarting the positioner device 12. Then, according to step 210, the primary piloted valve 68 is reactivated. This, in turn, activates the first pneumatic pilot 82 and the second pneumatic pilot 86, switching the first valve 84 and the second valve 88, respectively, to the opened second position. The flow of compressed air from the positioner device 12 to the valve actuator 14 therefore resumes.
The particulars shown herein are by way of example only for purposes of illustrative discussion, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the various embodiments set forth in the present disclosure. In this regard, no attempt is made to show any more detail than is necessary for a fundamental understanding of the different features of the various embodiments, the description taken with the drawings making apparent to those skilled in the art how these may be implemented in practice.
