Fluid handling – Line condition change responsive valves – Pilot or servo controlled
Reexamination Certificate
2000-04-05
2001-01-30
Hepperle, Stephen M. (Department: 3753)
Fluid handling
Line condition change responsive valves
Pilot or servo controlled
C137S492500
Reexamination Certificate
active
06178997
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to fluid pressure regulators and more particularly, to an improved fluid pressure regulator having intelligent electronics and software to enhance performance.
BACKGROUND OF THE INVENTION
In general, the four basic elements of a process control loop include a process variable to be controlled, a process sensor or measure of the process variable's condition, a controller, and a control element. The sensor provides an indication of the process variable's condition to the controller, which also contains an indication of the desired process variable condition, or the “set point.” The controller compares the process variable's condition to the set point and calculates a corrective signal, which it sends to the control element to exert an influence on the process to bring it to the set point condition. The control element is the last part of the loop, and the most common type of final control element is a valve, though it may also comprise a variable speed drive or a pump, for example.
A pressure regulator is a simple, self-contained control system that combines the process sensor, the controller and the valve into a single unit. Pressure regulators are widely used for pressure control in fluid distribution applications and the process industries, for example, to maintain a desired, reduced outlet pressure while providing the required fluid flow to satisfy a variable downstream demand. Pressure regulators fall generally into two main categories: direct-operated regulators and pilot-operated regulators.
A typical prior art direct-operated regulator
11
is illustrated in FIG.
1
. Typical applications for direct-operated regulators include industrial, commercial, and gas service; instrument air or gas supply; fuel gas to burners; water pressure control; steam service; and tank blanketing. The direct-operated regulator
11
includes a regulator body
12
which has an inlet
13
and an outlet
14
. A fluid flow passage area
15
having a restriction area
16
connects the inlet
13
and outlet
14
. The restriction area
16
has a throttling element
17
, such as a plug, membrane, vane, sleeve or similar restricting device which, when moved, limits the flow of the fluid (gas or liquid). An actuator including a sensing element having two sides responds to variations in the fluid pressure being controlled. Examples of sensing elements include membranes, diaphragms or pistons. The embodiment illustrated in
FIG. 1
uses a diaphragm
18
for the sensing element. Control pressure is applied to the first side, or control side
19
of the sensing element via a control line or a passage
20
internal to the regulator body
12
. If a control line is used for this purpose, it may be integral to the regulator body
12
or located in the adjacent piping. The second side, or reference side
21
of the sensing element is typically referenced to atmosphere. An additional force such as a spring
22
may be applied to the actuator, which biases the throttling element into a predetermined position representing a set point.
The direct-operated regulator
11
illustrated in
FIG. 1
is considered a “pressure reducing” regulator because the sensing element (diaphragm
18
) is connected by an internal passage
20
to pressure downstream of the regulator (on the fluid outlet-side)
14
. An increase in downstream pressure is applied to control side
19
through the internal passage
20
, applying pressure to the diaphragm
18
, and forcing it up against the force of the spring
22
. This, in turn, moves the throttling element up into the flow restriction area
16
, reducing the fluid pressure to the regulator outlet
20
.
Pressure reducing regulators regulate flow by sensing the pressure downstream of the regulator. A typical application of a pressure reducing regulators is on steam boilers, where pressure reducing regulators provide the initial pressure regulation. If the diaphragm
18
were connected to upstream pressure and the throttling element
17
were moved to the other side of the restrictor
16
, the direct-operated regulator
11
would be considered a “back pressure” regulator. Back pressure regulators are applied, for example, in association with compressors to ensure that a vacuum condition does not reach the compressor.
A pilot-operated regulator is similar in construction to a direct-operated regulator. A typical prior art pressure reducing pilot operated regulator
23
is illustrated schematically in
FIG. 2A
, and a prior art back pressure pilot operated regulator is illustrated in FIG.
2
B. The pilot operated regulator includes all the structural elements of the direct operated regulator with the addition of the pilot
24
(also called a relay, amplifier, or multiplier). The pilot is an auxiliary device which amplifies the loading pressure on the regulator actuator to regulate pressure. The pilot is similar in construction to a self operated regulator, having essentially the same elements as the self operated regulator.
In the pilot operated regulator
23
illustrated in FIG.
2
A and
FIG. 2B
, inlet pressure is supplied via a pressure tap
27
in the piping upstream of the regulator
23
. In the back pressure pilot operated regulator
23
in
FIG. 2B
, the pressure tap
27
further may include a restriction
26
therein. Inlet pressure to the pilot may also be supplied through an integral pressure tap to the regulator body. Outlet pressure is fed back through piping
20
connected downstream of the regulator
23
. The downstream pressure is connected to the pilot
24
and the main regulator
10
. The pilot
24
amplifies the pressure differential across the main regulator diaphragm
18
in order to control either the upstream (back pressure) or downstream (pressure reducing) fluid pressure.
Pressure regulators have many advantages over other control devices. Regulators are relatively inexpensive. They generally do not require an external power source to perform the pressure control function; rather, regulators use the pressure from the process being controlled for power. Further, the process sensor, controller and control valve are combined into a relatively small, self-contained package. Other advantages include good frequency response, good rangeability, small size, and there is generally little or no stem leakage.
There are also disadvantages associated with known regulators. Significant problems associated with existing pressure regulators include “droop” and “build-up,” also referred to as offset or proportional band. Droop is defined as the decrease in controlled pressure in a pressure reducing regulator and build-up is defined as an increase in controlled pressure for a back pressure regulator that occur when moving from a low load to full load flow condition. They are normally expressed as a percent. Droop and build-up are especially prevalent with direct-operated regulators, but it also exists to a lesser degree with known pilot-operated regulators.
Regulators are often required to go to a no flow condition which is referred to as “lock-up” or “reseat.” In a pressure reducing regulator such as the self operated regulator
11
in
FIG. 1
or the pilot operated regulator
23
in
FIG. 2A
, down stream pressure may reach a point where it is desirable for the regulator
11
to completely stop fluid flow. At this down stream pressure, the control pressure fed back to the diaphragm
18
moves the throttling element
17
completely into the flow restriction area
16
, thereby blocking flow. This condition is known as “lock-up.” In a back pressure regulator such as the pilot operated regulator
23
shown in
FIG. 2B
, pressure up stream of the regulator may drop to a level where the regulator is required to shut off flow. In this case, the up stream control pressure falls to a level where the load spring and/or the pilot pressure cause the throttling element
17
to move to a position completely blocking fluid flow. Internal parts problems, contamination or binding in the movement of the internal parts can all contribute to a l
Adams Paul R.
Gabel Karl J.
Roper Daniel G.
Fisher Controls International Inc.
Hepperle Stephen M.
Howrey Simon Arnold & White , LLP
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