Fluid handling – Removable valve head and seat unit – Retained by bonnet or closure
Reexamination Certificate
2002-02-14
2004-12-07
Hepperle, Stephen M. (Department: 3753)
Fluid handling
Removable valve head and seat unit
Retained by bonnet or closure
C137S501000
Reexamination Certificate
active
06827100
ABSTRACT:
FIELD OF THE INVENTION
The invention is an automatic pressure control valve which can be used for pressure control of a liquid or gas media flow. It can be used to control the gauge pressure, or the differential pressure between two points in a system.
It can control the pressure in for example; compressed air, water or steam lines, oil and fuel supplies and air handling systems. It can also be used to control the liquid level in tanks.
An important application is together with control valves, for automatic flow rate control. The automatic pressure control valve is piped in series with the control valve and arranged so it maintains a constant differential pressure across the control valve. The two valves works together as a pressure independent control valve.
Description of Prior Art
Automatic pressure control valves are used to control liquid or gas media flow so the pressure is essentially constant. A common type of automatic pressure control valve has the controlled media acting against one side of a diaphragm, and the opposite side is connected to the atmosphere. The differential pressure over the surface over the diaphragm produces a force which is opposed by a spring. Typical examples are shown in U.S. Pat. Nos. 4,044,792 and 5,009,245.
Pressure variations causes unbalance between the two forces and produces a net force that moves the diaphragm. The diaphragm operates a valve mechanism, which increases or decreases the pressure of the media until there is balance between the diaphragm and the spring. Thus, the spring tension determines the set-point of the automatic pressure control valve. In the following text the automatic pressure control valve is referred to as APCV.
The diaphragm is connected to the valve mechanism via a stem that passes through a packing or similar. Its friction together with the friction of the valve mechanism must be overcome by the net force from the diaphragm and spring.
In order to get a smooth control, with only a small hysteresis, the diaphragm and spring need to be relatively large so already a very small pressure variation produces a net force strong enough to overcome the friction.
This is not the only reason for using a large diaphragm. The control surface of the valve mechanism has an area against which the media pressure acts and produces a force. This force typically counteracts the spring force. Therefore, variations in the media pressure will change the set-point. This is especially true if the control mechanism must have a large flow capacity.
When a large flow capacity is needed, the control surface must be large, and the force counteracting the spring is quite large. Therefore the diaphragm and spring need to be large so the influence of the media pressure on the set-point will not be too large.
Instead of increasing the area of the control surface its movement can be increased. This will also increase the flow capacity. The drawback is that the movement changes the spring tension, which also changes the set-point. The change depends upon how much the control surface needs to open, which is a function based upon both the flow and the pressure.
The change in set-point is reduced by using a long spring, so the movement is small compared to the length of the spring. However, this increases the size and cost of the APCV.
The above described APCV balances the controlled pressure against the atmosphere and a spring. Many other types of valves exists. For example, instead of connecting to the atmosphere, both sides of the diaphragm are connected to the media, but to different points of the system. A built in spring acts against the diaphragm and the valve mechanism regulates the media flow so a controlled differential pressure is maintained between the two points. This is an automatic differential pressure control valve. In the following text referred to as ADPCV.
From the above it is understood that in order to achieve a good accuracy APCVs ADPCV's need large diaphragms and springs. This of course means that also the housing surrounding the diaphragm need to be quite large and costly.
There are some valve mechanisms that have not the above described problems. However, many of these valves (sleeve type, for example) tend to leak, so the very small flows can not be controlled.
It is also possible to use pilot valves to operate the diaphragm to improve the accuracy. However, this an added complication which increases the cost.
The above is a brief summary of the some of the problem associated with APCVs.
Automatic control valves in HVAC and industrial process applications are fitted with actuators that operates the control valves in response to signals from controllers, so the correct flow is provided. The problem is that the flow not only depends upon how much the valves are open, but also upon the differential pressure across the valve.
The differential pressure depends upon the operating conditions of the whole piping system.
A sudden pressure variation in the piping system changes the flow through a control valve and the control is upset. It takes some time before the control system signals the actuator to change the opening of the valve so the correct flow is obtained and stable control is restored.
Control valves are made with a certain flow characteristics, which defines how the flow changes as the valve opens.
The flow characteristics is designed with a curvature that compensates for the non-linear characteristics of the control object (often heat transfer devices). The objective is that the total characteristics is linear, from the signal to the actuator to the output of the control object. This is very beneficial for stable control.
The flow characteristics of a valve is laboratory tested at a constant differential pressure.
Pressure variations due to load changes distorts the flow characteristics of the control valves, which is detrimental for stable control.
It is very difficult to correctly size control valves. The flow coefficient needs to be calculated. It is calculated by multiplying the flow rate (GPM) by the square root of the specific gravity of the liquid and then divide by the square root of the differential pressure at the maximum load conditions. Unfortunately, it is very difficult to obtain a correct information about the differential pressure that reflects the actual conditions. One of the reasons is that the “as built conditions” deviate form the specification.
Without correct information, the control valves will not be sized correctly. Undersized control valves can not supply the needed flow and must be replaced. To avoid this the tendency is to install oversized control valves. However, it is very detrimental for stable control, especially at low loads.
The problem can be solved by combining the control valve with an ADPCV, and arrange it so it maintains a constant differential pressure across the control valve.
With a constant differential pressure across the control valve a well defined flow rate is provided for each degree of opening of the control valve. The flow rate is independent of pressure variations in the piping system before and after the valve combination. Therefore, the combination of an ADPCV and a control valve is referred to as a PRESSURE INDEPENDENT CONTROL VALVE (in the following text called PICV).
Because of the constant differential pressure the control valve will always operate with a perfect valve authority and therefore the flow characteristics will not be distorted by pressure variations in the piping system.
The PICV can be applied in different ways.
It can be used as an automatic flow rate controller, with a manually adjusted set-point, and can have a handle and a graduated indicator disk to adjust the flow rate. Applications are where a constant, or manually adjusted flow rate is needed. It can also be used as a high limit in applications with a variable flow
The PICV can be operated by an actuator, which responds to signals from a controller.
The maximum flow through the PICV can be set by limiting the maximum opening of the control valve. This can be done by limiting the stro
Belimo Holding AG
Carvis Thaddius J.
Hepperle Stephen M.
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