Co-axial control valve

Details Co-axial control valve Co-axial control valve

2001-05-24

2004-06-01

Michalsky, Gerald A. (Department: 3753)

Fluid handling

Larner-johnson type valves; i.e., telescoping internal valve...

C137S220000

Reexamination Certificate

active

06742539

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to control valves and, more particularly, to a new and highly advantageous precision control valve for providing precise, selective control of fluid flow through the valve, while additionally providing greatly improved response, significantly increased fluid flow rates, and dramatically reduced overall size and weight.
Instrumented control valves, also called final control elements, regulate the flow or pressure of a fluid which affects many controlled process. Control valves are usually operated by remote signals from independent devices using control mechanisms, and may be powered electrically, pneumatically, electro-hydraulically, etc. Control valves are extensively used in many industries, including chemical, petrochemical, oil, water, and gas. Many companies in all industries are having major problems in the areas of energy consumption and wasted product, due to the complexities of their process control systems.
Industry research has revealed that of all the components and instrumentation used in process control systems, it is the control valve that has resulted in a major portion of energy consumed. Moreover, the slow or erratic response of conventional control valves are directly responsible for most of the wasted product. This is reflected in the responses from systems engineers that most plants cannot process a consistent quality product, without them having to constantly adjust the system. The unfortunate result is that after spending a great deal of time setting up a controlled process, the systems engineer out of frustration, may adjust the valve to a compromised setting or operate it manually, resulting in millions of dollars of low quality or wasted product.
The mechanics of control valve design have not changed much over the years. New electronic devices controlled by computers have generally been unable to improve response times of control valves to stabilize controlled systems. Although valves with rapid response times have been used in the aerospace industry, i.e., to shuttle fluids between the wings, they are incapable of regulating, and are used only for on/off or as relief valves. Often the electronic accessories have only made the process system control worse because of their high speed feedback. Such rapid feedback is normally desirable. Constraints in existing control valves cause them to be much too slow to react to the rapid process signal. Moreover, in this situation, the control valve continues to receive additional signals before it can respond to the initial signal, causing the control valve to become erratic and unstable.
A conventional control valve, is typically a globe valve due to its ability to more precisely control flow, uses a totally external driving means, also referred to as control components, to actuate the valve member that is exposed to and effects control of the fluid in the pipeline. This external driving means generally consists of a cumbersome, elongated structure usually much larger than the valve itself, containing a linkage to the control member that is in communication with the fluid in the pipeline, also referred to as the working fluid. This linkage generally includes a shaft and a spring for urging the valve member toward a valve opening position. A large diaphragm, generally located at the top of the elongated structure and supplied by an external pneumatic source, urges the linkage toward a closed position. The design of this external driving means has remained basically unchanged for many years. A conventional control valve generally works as follows: after receiving a signal indicating that the control member needs to actuate to a different position, the pneumatic source adjusts the pressure in a chamber above the diaphragm. The speed of pneumatic line flow to effect this change is limited to the speed of sound. Additionally, the force generated by the diaphragm to effect this change in position of the valve member must first overcome the resisting forces acting on the valve member by the fluid in the pipeline, the inertia in the linkage itself (including, but not limited to the shaft and friction associated with O-rings and packings installed to prevent valve leakage; the term “striction” has been coined and used by those skilled in the art to refer to any type of mechanical valve restriction), and the spring force (if the valve member is to be moved toward a valve closing position).
Moreover, after sufficient pressure is built up in the chamber above the diaphragm to effect valve member movement, unless the target position is to completely open or to completely close the valve, the linkage inevitably drives the valve member past the target position, commonly referred to as overshoot or gain. The valve member must then be repositioned in the opposite direction, with the overshoot in this direction commonly referred to as droop. This combination of gain and droop is referred to as dead band, which may cause hysteresis. Hysteresis is the tendency of the valve to give a different output for a given input, depending on whether the input resulted from an increase or decrease from the previous value. Hysteresis is distinguished from dead band in that some reversal of output may be expected for any small reversal of input. To then correct gain or droop, the pressure above the diaphragm must again be adjusted, again with the same pneumatic line flow limitations, and again the inertia of the linkage must be overcome. Valve hysteresis must also be taken into consideration. Additionally, a change in pressure may affect the working fluid flow rate, especially if the fluid is a gas, due to compressibility, which is the fractional change in volume of a fluid per unit pressure change. To make matters worse, if the driving means is not mounted in a substantially vertical position, which generally requires that the control valve be installed in a horizontal position, unbalanced forces are introduced into the system due to side loads caused by gravity. These unbalanced forces produce additional friction forces, which further worsen an already excessive response time. Worse yet, even properly installed control valves may experience other adverse effects due to circumstances requiring component re-routing, e.g., clearance problems with an existing plumbing system. This may be due to the installation of geared linkages necessary to achieve the desired vertical position, which produces its own backlash.
A goal of system engineers is to design a control system so that when it is disturbed, the controlled process variable will come under control again as quickly as possible. The time required for a control system to regain control after it has been upset and temporarily goes off control is commonly referred to as recovery time. The time delay between two related events is commonly referred to as dead time. An example of dead time as it relates to the control valve is the delay encountered between the time the signal is sent to effect valve member movement, and the time actual valve member movement is effected. Generally, the recovery time of a control system will increase in direct proportion to the dead time. That is to say, if the dead time is doubled, the control system will take twice as long to stabilize, or regain control. Dead time is the antithesis of effective process control. A 7-8% dead time has been associated with a conventional control valve. A recently conducted study of an industrial control process using such a valve revealed that costs associated with waste could approach US$750,000 annually from just a 2-inch valve above. Moreover, based on flow area (&pgr;r
2
), waste from an 8-inch valve could approach 16 times that of a 2-inch valve, or US$12,000,000 annually.
In addition to process control problems, both the valves, including the valve housing and control components, as well as downstream components are subjected to fluid flow pulsation surges. These pulsation surges, which invariably occur during any control process, create fluid stresses that act on the valves and

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