6. Static actuator sizing

6. Static actuator sizing

6.1. Forces in the control valve

The following forces act upon the throttle trim including the plug stem:

Actuator force (or actuator torque )

Friction forces (stem seal, possibly pressure relief seal)

Inertial forces (force due to weight, force due to acceleration)

Flow forces

Sealing force (seat loading force) when valve is closed

Residual forces are present on the actuator stem in the pneumatic actuator:

Actuator force (or actuator torque )

Friction forces (actuator stem seal), negligible (0) however

Inertial forces (force due to weight, force due to acceleration) (0)

Elastic force

Pressure force (signal pressure , diaphragm area )

6.2. Linear control valve

Valve

Friction force

: Friction coefficient [N/mm] dependent on stem seal

: Friction coefficient [N/mm] for pressure balanced plugs

: Stem diameter

: Seat hole diameter

: Travel velocity

sign: sign

Inertial force

M: Mass of plug as well as plug and actuator stem

g=9.81 : Gravity acceleration

Acceleration negligible as a rule compared with g

Flow force
(FTO without pressure relief)

(FTC with pressure relief)

(with pressure relief)

: Inlet pressure

: Outlet pressure

: Seat hole diameter

: Stem diameter

FTO: (Flow To Open)

FTC: (Flow To Close)

Sealing force when H=0: , otherwise = 0

: Coefficient [N/mm] dependent on required sealing class

: Seat hole diameter

Bellows elastic force

Actuation force

Parameters

Stem seal Friction coefficient [N/mm]

PTFE 1.6

Graphite 10

Bellows 1.6

Insulating section 1.6

Plug sealing edge (leakage class) Closing force coefficient [N/mm]

Metallic 2

Ground-in 20

Soft sealing 0.3

Pressure relief Friction coefficient [N/mm]

Without -

With PTFE seal 1.6

With graphite seal 10

Pneumatic actuator

Spring stiffness

: Diaphagm area

Spring range (in bar overpress.)

: Nominal travel of control valve

Spring bias force

: Diaphagm area

Lower spring range value (in bar overpress.)

Spring force

(valve spring closing)

(valve spring opening)

: Spring bias force

H: Valve position

: Nominal travel of control valve

: Spring stiffness

Pressure force

: Diaphagm area

: Signal pressure

Actuator force

(valve spring closing)

(valve spring opening)

Actuator + control valve combination

(valve closing spring)

(valve opening spring)

The required actuator force from the maximum actuator force (valve side) for the chosen configuration (leakage class, stem seal, valve seat hole diameter, stem diameter, pressure relief with/without, flow direction) is obtained from the operating data or the pressure data when the valve is closed.

The actuator is minimized

in its diaphragm area for a valve closing actuator spring including
the respective maximum possible spring range

in its diaphragm area for a valve opening actuator spring including
the respective maximum possible spring range .

The results of static actuator sizing beside the diaphragm surface Aw and spring range are:

the required actuator force (without pressure relief) and (with pressure relief)

permitted actuator force (stiffness, max. actuator force)

permitted pressure difference at valve plug (stiffness, max. actuator force)

required signal pressure lower range value

required pressure difference between signal pressure upper range value and inlet air pressure

Minimum thrust of actuator

Minimum safety value

percentage figure of hysteresis (friction) hyst.

Safety value for

6.3. Butterfly valve and plug valve

This option is currently not yet implemented, sizing is, however, similar.