Remote seal installation improvements

A process control system includes a transmitter having transmitter circuitry, a sensor, a remote sensing apparatus, a linkage and a slack take-up device. The sensor senses a process variable of a process fluid. The remote sensing apparatus communicating with the process fluid. The linkage communicates between the transmitter and the process fluid. The slack take-up device selectively adjusts a relative length of the linkage between the transmitter and the remote sensing apparatus.

BACKGROUND OF THE INVENTION

This invention relates generally to process instruments used in industrial process control systems. More particularly, the present invention relates to transmitters having a slack take-up device for remote sensing apparatuses.

In one type of process control system, a pressure transmitter is used to remotely monitor the pressure of a process fluid. The pressure transmitter includes circuitry that conditions an electrical output of a pressure sensor and transmits it to a remote location where it can be monitored as representing the magnitude of the pressure. Remote seals, or remote diaphragm assemblies, are often used to distance the pressure transmitter from hazardous measurement environments, or for linking the pressure transmitter with inconveniently located process fluids. For example, remote seals are often used with corrosive or high temperature process fluids such as in chemical plants or oil refineries. Typically, in those situations, a mechanical remote seal having a diaphragm assembly and a capillary tube is used to relate the pressure transmitter to the process fluid through a hydraulic fill fluid, while the pressure transmitter is located a safe distance away. The flexible diaphragm isolates the process fluid from the fill fluid used in the capillary tube. As the diaphragm flexes, the incompressible fill fluid translates pressure change through the capillary tube to a diaphragm located in the pressure transmitter. Deflection of a pressure transmitter diaphragm is transmitted through another fill fluid to a pressure sensor, which produces a signal relating to the pressure of the process fluid.

Capillary tubes can extend tens of meters in order to couple the pressure transmitter with the process fluid. Because of costs and difficulty associated with customizing the length of the capillary tube, remote seal assemblies are typically made available with stock lengths of capillary tube. Often times, however, the remote seal comes with an excessive length of capillary tube for some applications. Also, for differential pressure measurement in balanced configurations, where two remote seals are used with equal lengths of capillary tubing in order to equalize back pressure, one of the capillary tubes is typically longer than necessary for the application. Due to the sensitive nature of the sensors and remote seals, which are pre-filled with a precise amount of fill fluid at the factory, it is impractical to adjust the length of capillary tubes in the field. Thus, it becomes necessary in field environments to deal with excess lengths of capillary tube in order to ensure their security and pressure transmission performance. However, it is often the case that the capillaries are jumbled up or stashed such that they may easily become crimped, cut or otherwise compromised, which also affects their pressure transmission capabilities. As such, there is a need to eliminate the problems associated with excessive capillary lengths in remote seal assemblies.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed toward a process control system having a slack take-up device for a remote sensing apparatus. The process control system includes a transmitter having transmitter circuitry, a sensor, a remote sensing apparatus, a linkage and a slack take-up device. The sensor senses a process variable of a process fluid. The remote sensing apparatus communicates with the process fluid. The linkage communicates between the transmitter and the process fluid. The slack take-up device selectively adjusts a relative length of the linkage between the transmitter and the remote sensing apparatus.

DETAILED DESCRIPTION

FIG. 1shows process control system10, which includes control room12, control loop13, process transmitter14, flexible linkage15, slack take-up device16, remote sensing apparatus18, vessel20and process fluid21. Vessel20may be any container used to accommodate a process fluid, such as a pipeline or a storage tank. Process fluid21may, therefore, be a petrochemical, an acid or any other fluid. Process transmitter14is used to sense a process variable, such as temperature or pressure, of process fluid21and transmit the magnitude of that variable to control room12over control loop13. Process control loop13can be, for example, a 4-20 mA control loop, a wired digital communication network, a wireless network or any other suitable communication system. Thus, process transmitter14can be situated a distance away from control room12, such as on a factory floor or on a pipeline, or be otherwise remotely situated.

Remote sensing apparatus18extends the reach of transmitter14through flexible linkage15. Remote sensing apparatus may be, for example, an RTD probe for a temperature transmitter or a remote seal assembly for a pressure transmitter, but could also be any device extendable from a process transmitter by way of cable or tubing. Flexible linkage15can be any mechanical or electric communication means, such as wiring, cabling or tubing. In various embodiments, linkage15comprises stainless steel hydraulic tubing or electrical wiring encased in a flexible sheathing. Thus, process transmitter14can be installed at a safe and secure position, while remote sensing apparatus18can extend into more hazardous or inconveniently located positions.

Slack take-up device16provides a means for adjusting the relative length of flexible linkage15between transmitter14and apparatus18without interfering with the operation of process control system10. Device16comprises a spool or reel for winding the length of flexible linkage15in excess of what is needed to span the distance between transmitter14and apparatus18. Slack take-up device16is typically manually operated, but, in other embodiments can be automated with, for example, an electric motor. Thus, slack take-up device maintains flexible linkage15in a neat and orderly fashion and prevents damage from being inflicted upon flexible linkage15from improper storage or handling, amongst other advantages that are apparent in the various embodiments of process control system10.

FIG. 2shows one embodiment of process control system10in which process transmitter14comprises a pressure transmitter, flexible linkage15comprises a capillary tube and remote sensing apparatus18comprises a remote seal. Pressure transmitter14is comprised of transmitter circuitry22, sensor24, sensor diaphragm26, passageway28and process diaphragm30. Remote seal18includes capillary15, flange32, remote diaphragm34and fill fluid38. Take-up device16is disposed on capillary15between transmitter14and remote seal18. Process control system10is used to remotely measure pressure P1of process fluid21with transmitter14, such that the information obtained can be monitored at control room12through control loop13. Remote seal18is used to distance pressure transmitter14from hazardous environments or for linking pressure transmitter14with inconveniently located process fluids, such as at vessel20. Slack take-up device16adjusts the relative length of capillary15between transmitter14and remote seal18without interrupting capillary15, thereby allowing continuous operation of pressure transmitter14. Process transmitter14is shown as an absolute pressure gauge having one remote seal18. In other embodiments, process transmitter14is fitted with a second remote seal so that differential pressure can be sensed. In such a configuration, an additional capillary may be used in either a tuned or balanced configuration, which may have an associated slack take-up device.

Sensor24of process transmitter14is mechanically connected with process fluid21through hydraulic fill fluids present in passageway28and capillary15. Remote diaphragm34separates process fluid21from capillary15, process diaphragm30separates passageway28from capillary15, and sensor diaphragm26separates passageway28from sensor24. Capillary15is filled with a first fill fluid, and passageway28is filled with a second fill fluid. Sensor24senses a change in pressure P1of process fluid21through the first and second fill fluids. Pressure P1exerts a force on remote diaphragm34, which is transmitted from remote diaphragm34by the first fill fluid of capillary15to process diaphragm30of passageway28, such that the pressure in capillary15equals pressure P1. The force associated with P1is transmitted from process diaphragm30to pressure diaphragm26by the second fill fluid, such that the pressure in passageway28equals pressure P1and is thus applied to sensor24.

Typically, sensor24is a transducer that produces an electrical signal in response to a change in pressure P1as presented through the fill fluids. Sensor24is in electronic communication with transmitter circuitry22, which processes and transmits the output of sensor24to control room12over control loop13. Alternatively, circuitry22can display the output of sensor24on a local LCD screen contained within transmitter14. In other embodiments, transmitter circuitry22communicates over a wireless network, or is not connected with control room12. In yet another embodiment, the output of circuitry22is readable by a handheld device linked by wires or wirelessly with process transmitter14. Thus, pressure P1is transmitted from vessel20to transmitter14through capillary15, and is then transmitted electronically to control room12through control loop13.

In order to maintain the accuracy of process control system10, the integrity of capillary15must be carefully maintained. Capillary15comprises pliable stainless steel tubing that is enclosed in a flexible steel sheathing. The inner tubing provides a sealed link between remote diaphragm34and process diaphragm30and is easily damaged. The outer steel casing serves to protect the inner tubing, while maintaining a degree of flexibility. The magnitude of the electrical output produced by sensor24is based on the pressure of process fluid21, as presented to sensor24through the first and second fill fluids. The amount of force that is transmitted to sensor24depends on the quality and quantity of the first fill fluid present in capillary15(and the second fill fluid present in passageway28), and its ability to convey pressure between remote diaphragm34and process diaphragm30without obstruction. Pressure transmitter14is calibrated, typically at the factory, having a precise, fixed amount of first fill fluid in capillary15. In the event any fill fluid leaks out of capillary15, the accuracy of pressure transmitter14is reduced, and inaccurate output is produced by sensor24.

The integrity of capillary15can become compromised in the field through accidental damage or, in extreme circumstances through excessive wear and tear. Capillary15also, however, becomes damaged due to mishandling or carelessness in storage and installation of control system10. Capillary15often times is jumbled up or stashed in such a manner as to become crimped, cut or otherwise compromised. In addition to causing leaks, which obviously affect the accuracy of sensor24, crimps or other sharp bends in capillary15impede the ability of the first fill fluid from relaying pressure P1to process diaphragm30.

These problems are compounded by capillary15being made available in standard lengths from the factory, which often results in excess amounts of capillary tube15for some applications. Due to the complexity of the factory sealed capillary assembly and the precision factory calibration of control system10, it is impractical to shorten the length of capillary15after installation so that excess lengths do not need to be stored or stashed and thereby avoiding unnecessary opportunity for damage. Therefore, capillary15is fitted with slack take-up device16. Slack take-up device provides a neat and orderly way for shipping and installing capillary15and storing excess amounts of capillary15without interfering with its operation or calibration.

FIG. 3shows a perspective view of the front of one embodiment of slack take-up device16of the present invention. Device16comprises first plate40and second plate42, between which capillary15is wound around spool44(shown in phantom). First plate40includes first guide46and second guide48, carry handle50, pedestal52and locking pin53. Second plate42includes spool44and crank handle54. Carry handle50, which provides a ready way of transporting or mounting device16, and pedestal52, which provides a simple means for storing device16, are optional features that are included for ease of use and convenience. Other features, such as mounting bores, hooks or brackets, can be included on first plate40or second plate42to facilitate use, handling and storage of device16. For example, pedestal52or first plate40may include mounting bores so that device16may be mounted to a wall or mounting post. First plate40and second plate42can be comprised of any material having suitable properties for the particular environment in which it will operate. Such properties include resiliency to harsh conditions, structural stability, heat resistance, or oil resistance. Such materials include aluminum, plastic or wood.

Second plate42is rotatably secured to first plate40with, for example, threaded fastener56, and can be manually rotated with handle54. Capillary15uninterruptedly passes through spool44, and is comprised of stainless steel sheathing57and flexible steel tubing58. First end15A of capillary15passes through guide46and second end15B passes through guide48. Guide46and guide48are positioned on flanges extending out from opposite ends of first plate40. Guide46and guide48are comprised of, for example, a pair of threaded fasteners60that span the distance between first plate40and second plate42and prevent the circumferential windings of capillary15from expanding beyond the perimeter of device16.

As spool44is rotated (clockwise for the embodiment and orientation of device16inFIG. 3) by handle54and second plate42, capillary15is wound around spool44from both first end15A and second end15B simultaneously. The width of spool44can be slightly larger than the width of capillary15to allow for a single, concentric winding of capillary15. In other embodiments, the width of spool44is wider to allow parallel windings of capillary15in order to, for example, accept longer lengths of capillary15with the same diameter of plates40and42. Steel sheathing57protects steel tubing58as they flex and are drawn past guides46and48. Guides46and48prevent capillary15from unwinding and maintain capillary15wound in a bi-directional configuration. Guides46and48keep first end15A and second end15B extending outwardly from opposite ends of spool44such that capillary15can be easily extended and contracted between transmitter14and remote seal18. The shape of spool44permits capillary15to be wound and extended in an orderly fashion without interrupting the operation of control system10. Locking pin53prevents movement of second plate42with respect to first plate40such that capillary15cannot be unwound, such with handle54, pulling on either end15A or15B, or some other force. Locking pin53is inserted through a hole in second plate42and into and through a hole in first plate40to prevent unwinding of spool44. In one embodiment, first plate40includes a plurality of holes such that second plate42can be locked in a plurality of positions. Locking pin53can be any suitable fastening means such as a cotter pin, threaded fastener, clevis pin or a spring biased detent.

FIG. 4shows a front view of slack take-up device16taken through spool44of second plate42. Spool44, guide46and guide48are shown in section, handle54is shown in phantom. The shape of spool44permits capillary15to wind-up without slipping when handle54is cranked (counterclockwise for the embodiment and orientation of device16inFIG. 4). Spool44also allows controlled unwinding of capillary15when first end15A and second end15B are pulled outward away from spool44through guide46and guide48. To aid in winding and unwinding of capillary15, threaded fasteners60of guides46and48include rotatable bushings62that facilitate movement of and prevent damage to capillary15.

Spool44comprises a “yin-yang” type shape that allows capillary15to be placed through spool44. Spool44is comprised of two opposing teardrop shaped projections that form a central channel. Capillary15can be laid through channel64without interrupting, impeding or otherwise altering the shape or flow of capillary15. First end15A and second end15B of capillary15are then wound around spool44in the same direction such that one end each can be extended through guide46and guide48. The yin-yang shape facilitates the bi-directional unwinding of capillary15. The yin-yang shape grabs capillary15to prevent slipping of capillary15around spool44during winding up of capillary15, but also prevents sharp bending or capillary15in order to prevent kinks. Additionally, this type of configuration allows device16to be installed or removed from system10without uninstalling system10or any of its components by simply un-securing fastener56. Since channel64runs generally through the center of spool44, fastener56is offset from the center so that it is positioned within one of the tear drop shaped projections. In other embodiments, channel64need not run through the center of spool44, and fastener56can extend through the center of spool44. Channel64can be comprised of any shape in which capillary15can lay uninterruptedly and which facilitates gripping of capillary15during winding. Thus, spool44can be comprised of any suitable shape in different embodiments of the present invention.