INTRODUCTION

The third essential function of a control loop is the correction of the measurement by a component called the Corrective Component (CC). The best-known type, in continuous control, is the Automatic Valve or Control Valve (CV), the main types are:

• PCV for pressure control
• TCV for temperature control
• LCV for level control
• FCV for flow control

DEFINITION

As the terminal element of the control loop, the Automatic Valve is a device that:

Transforms
The variation of the Signal emitted by the controller following a deviation between measurement and setpoint.
Into
a proportional movement of the stem of a valve, in other words, a variation in the passage section offered to the control fluid.

In summary, an Automatic Valve is a VARIABLE ORIFICE and therefore its first characteristic is a FLOW characteristic.

SPECIFIC CRITERIA

A valve is identified by its type, diameter, and specific criteria established at the end of the control, such as: the "CV", and the "Sensitivity".

"CV" OR VALVE COEFFICIENT

Established by MASONELLAN, in 1944, this coefficient defines:

• The flow rate, expressed in US gallons per minute
• Of water
• At a temperature of 60 °F
• Passing through the valve at MAXI opening
• Under a ∆P (upstream flange - downstream flange) of 1 psi.
  • Its expression is \( CV = Q\sqrt {d\over\Delta P }\)  , \(Q = K \sqrt {\Delta P\over d}\)
    According to the definition:
    \(\sqrt {d\over\Delta P } = 1\)  hence 

     

    CV = Q MAXI

    The American notation, the standard basis for calculating the sizing of a valve, has been widely developed, as well as metric corollaries, and in particular, For Liquids: \(CV = 1,16 Qm^3 /h \sqrt {d\over √ ∆P _{bar}} \)

    « SENSITIVITY » or « LAW »

    This is the ratio between the flow rate through the valve and the stroke of the valve:

    Law = \(\Delta Q\over\Delta L\)

    In fact, the LAW is a characteristic more specific to the valve than to the valve.

    It should be considered as that of the valve, provided that the relationship between the variation in valve lift / variation in signal (∆L/∆S) is linear.

    TECHNOLOGIES

    

    The automatic valve is composed, for the most part, of two main parts, one motor, called a servomotor or actuator (pneumatic, hydraulic or electric), the other activated, called a body assembly, and comprising:

    • A cap and gland
    • The body itself
    • The seat (s) - valve (s) assembly

    Accessories, such as:

    • Greaser
    • « Safety lock »
    • Manual control
    • Position indicator
    • Position transmitter
    • Positioner
    • etc.

    

  • It should be noted that the automatic valve of the so-called "conventional" type is made of:

  • • A pneumatic diaphragm servomotor with a return spring
    • A straight body, with a "sliding" stem

    THE SERVOMOTOR (ACTUATORS)

    This is the control organ of the valve seat/flap system and is composed of:

    • A 2-part cap, separated by a membrane (diaphragm)
    • A spring whose tension can be adjusted
    • A membrane support plate connected to the stem
    • An arch serving as a link between the servomotor and the body
    • A nut connecting to the polished stem of the valve
    • A vernier

    There are two types:

    1. Direct action servomotor: the air pressure acts on the membrane and makes the stem go down.
    2. Indirect action servomotor: the air pressure makes the stem rise upwards.

    Operating pressure: the servomotors operate between 3 and 15 PSI or between 6 and 30 PSI. At 3 PSI, the valve starts its stroke, at 15 PSI, the valve is at 100% of its stroke. The same applies for 6-30 PSI.

    A manual control can be mounted on the arch, allowing the valve to be operated in case of a failure in the automatic control.

    Position indicator: these valves can be equipped with an electrical or pneumatic signaling device that will indicate the valve's position in the control room: open or closed.

    Flow variation (at constant P), in % CV / Valve stroke variation, in % lift
    Some valves are equipped with a lubrication port communicating with the guide ring
    of the flap...

    THE BODY

    This is the driven part of the whole valve, ensuring the variation of the fluid flow through the various flap and seat models.

    Flap and seat sealing: the sealing of double-seat valves is not perfect; the two flaps being difficult to adjust, the precision should be of the order of 1/100 mm, there will always be a slight offset. Since these valves are regulating devices, therefore used, the flaps will wear out due to whistling, and this wear will not be distributed in the same way.

    Valve body sealing: the valve's sealing and the flap stem is achieved using the cap which houses the packings and the gland system.

    The flaps can be reversed to change the action of the valve.

    Some valves are equipped with a grease fitting for lubrication between the body and the actuator.

    

    TYPICAL SCHEMAS

    Level Control

    

    Pressure Control

    

    Flow Control