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Control Valve Actuators Failure Modes

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Control valves can be built from various combinations of valve actuator and valve body. The combination of actuator and valve body is usually chosen to provide a particular failure mode should the instrument air supply fail for any reason.
The most common control valve actuator used in the industry is the diaphragm actuator. Diaphragm actuators, as in the case of valve bodies, can be classified as either direct or reverse acting. 


Any failure mode can be obtained with a combination of direct or reverse acting
actuator and direct or reverse acting valve body. The two most common failure mode of control valves are :

1. Fail Open
2. Fail Close

These two failure modes can be achieved by an Air to Close Valve (ATC) and an Air to Open valve (ATO).

Air to Close Control Valve (ATC)
An air to close (ATC) valve and therefore fail open valve, can be obtained with the combination of a reverse acting actuator and a reverse acting valve body or a direct acting actuator and a direct acting valve body.

Air to Open Control Valve (ATO)
An air to open (ATO) valve and therefore fail close valve, can be obtained with a combination of direct actuator and reverse body or reverse actuator and direct Body.

Valve Body and Actuator Combination and Their Failure Modes
The action of an actuator can easily be determined (usually by whether the air is supplied to the upper or lower half of the housing). Direct or reverse acting valve bodies are not always readily identifiable. Most often, reference to the nameplate or flow sheet is usually necessary to correctly identify the action of a valve body – reverse acting or direct acting.

Listed in the table below are all possible combinations of valve body and actuator and their failure modes:

Valve Actuator 
Valve Body
Valve Action
Failure Mode
Direct 
Direct
Air to Close
Fail Open
Reverse
Reverse
Air to Close
Fail Open
Direct
Reverse
Air to Open
Fail Closed
Reverse
Direct
Air to Open
Fail Closed




Basics of Split-Range Control in Control Valve Applications

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In many process control applications in industry, it is sometimes desirable to have multiple control valves respond to the output of a single common controller. Control valves configured in this way to follow the command of a single controller are said to be split-ranged, or sequenced.

Split-ranged control valves may assume different forms of sequencing. Common modes of control valve sequencing seen in the process industry are:  complementary, exclusive, and progressive.

Complementary Split-Range Control
With this form of split-ranging, there is never a condition in the controller’s output range where both valves are fully open or fully shut. Rather, each valve complements the other’s position.  A typical example of complementary split-range control is a situation where two valves serve to proportion a mixture of two fluid streams, such as where base and pigment liquids are mixed together to form colored paint as shown below:
Example of complementary split-range control.


Both base and pigment valves operate from the same controller output signal. While the pigment valve is Air-To-Open, the base valve is Air-To-Close. The following table shows the relationship between valve opening for each control valve and the controller’s output:
Controller Output (%)
I/P Output (PSI)
Pigment Valve (Stem position)
Base Valve (Stem position)
0
3
Fully Closed
Fully Open
25
6
25% Open
75% Open
50
9
Half-Open
Half - Open
75
12
75% Open
25% Open
100
15
Fully Open
Fully Closed

Exclusive Split-Range Control
The nature of valve sequencing in this type of split-range control is to have an “EITHER OR” throttled path for process fluid. That is, either process fluid flows through one valve or through the other, but never through both at the same time.

This type of split-ranged control valves call for a form of valve sequencing where both valves are fully closed at a 50% controller output signal, with one valve opening fully as the controller output drives toward 100% and the other valve opening fully as the controller output goes to 0%.

A practical example of this form of split-ranging is in reagent feed to a pH neutralization process, where the pH value of process liquid is brought closer to neutral by the addition of either acid or caustic:
Exclusive Split-Range Control

The basic operating principle of the above process is:
  1. A pH analyzer monitors the pH value of the mixture and a single pH controller commands two           reagent valves to open when needed.

  2. If the process pH begins to increase, the controller output signal increases as well (direct action)         to open up the acid valve.

  3. The addition of acid to the mixture will have the effect of lowering the mixture’s pH value.

  4. Conversely, if the process pH begins to decrease, the controller output signal will decrease as      well, closing the acid valve and opening the caustic valve.

  5. The addition of caustic to the mixture will have the effect of raising the mixture’s pH value.
The Air-To-Open acid valve has an operating range of 9 to 15 PSI, while the Air-To-Close caustic valve has an operating of 9 to 3 PSI. The table below shows the relationship between valve opening for each control valve and the controller’s output:

Controller Output (%)
I/P Output (PSI)
Acid Valve (Stem position)
Caustic Valve (Stem position)
0
3
Fully Closed
Fully Open
25
6
Fully Closed
Half - Open
50
9
Fully Closed
Fully Closed
75
12
Half - Open
Fully Closed
100
15
Fully Open
Fully Closed

Progressive Split-Range Control
This form of split-range control for control valves is used to expand the operating range of flow control for some fluid beyond that which a single control valve could deliver. In this type of control, one of the valve usually a small valve opens gradually and becomes fully open at 50% of controller output while the large valve will remain shut at until the controller output goes beyond 50% when it starts opening. Both valves become fully open when controller output is 100%.

An example of progressive split-range control is a pH control process where the incoming liquid always has a high pH value, and must be neutralized with acid as shown below:
An example of progressive split-range control

The PH of the incoming water to be treated is measured by the analyzer, AT. As the output of controller AIC increases, the small acid valve starts to open and becomes fully open at 50% of controller output. Meanwhile the large acid valve will remain shut until controller output goes beyond 50%. At 100%, both small and large acid valves are fully open to ensure that the PH of the incoming water is neutralized.

Controller output and valve status for proper sequencing of the small and large acid control valves is shown below:

Controller Output (%)
I/P Output (PSI)
Small Acid Valve (Stem position)
Large Acid Valve (Stem position)
0
3
Fully Closed
Fully Closed
25
6
Half Open
Fully Closed
50
9
Fully Open
Fully Closed
75
12
Fully Open
Half Open
100
15
Fully Open
Fully Open




How a Self Operated Pressure Reducing Regulator Works

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A self operated pressure reducing regulator is a mechanical device that is used to control and reduce pressure especially in natural gas plants. A pressure regulator is essentially a force balanced device that adjusts to changes in the system it is controlling. There are two types of pressure reducing regulators used in natural gas systems:
1. Self operated regulators
2. Pilot operated regulators
Both types of regulators are very common in the gas industry the self-operated regulators are general used in lower flow and lower pressure system, and are less expensive regulators. While the pilot operated regulators are generally use in higher flow situation, like city gates, large customers, industrial accounts etc and where you have higher pressure to control.

Basic Parts of a Self Operated Pressure Reducing Regulators
Self Operated regulators consist of three basic components:
1. A loading element. 
2. A measuring element and 
3. A restrictive element as shown below

Self Operated Pressure Reducing Regulator

As seen above, the loading element is typically a spring but it can also be a weight or pressure from some external source. When the spring is compressed, it exerts a loading force. The measuring element or diaphragm is connected to the process fluid (gas) that is being controlled and creates a force opposing the loading force. The restricting element or valve is connected to the spring and diaphragm assembly and regulates the flow through the regulator.

Operating Principle of Self Operated Pressure Reducing Regulators
In a self operated pressure regulator, as downstream system pressure decreases the spring force overcomes the force of the gas acting on the effective area of the diaphragm and the valve opens increasing flow into the system. When system pressure increases, the measuring force (the force of the system gas acting on the effective area of the diaphragm) overcomes the loading force (spring force) and closes the valve reducing flow into the system.