Control Valve Actuators

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The control effectiveness of a control loop is unquestionable determined by the weakest element of the chain. This is often the valve actuator. If one presupposes a continuity of the control valve characteristic i.e. an inherent characteristic curve without turning points, the quality of the actuator is, for an attainable control effectiveness, in most cases, more important than the quality of the valve characteristic curve.

The following chapter illustrates the different actuator principles, explains the authoritative parameters for an actuator selection and focuses above all on the calculation of the required actuator thrust and the selection of a suitable actuator type. A purely static or quasi static consideration however is not sufficient to describe the complicated interactions between the control valve and the actuator. Rather dynamic forces must also be considered for critical applications. Unfortunately, there are no obligatory references available in the corresponding literature which will permit a generalization and a simple calculation method with regard to dynamic forces. Therefore the given recommendations are based on the calculation of the SAMSON group experts, their valve sizing software and the authors. The majority of all industrial control valves installed worldwide is even today still driven by pneumatic diaphragm actuators. This actuator type is therefore of course in the center of interest and attention.

9.1.1. Actuator types

A first criterion in the distinguishing of different actuator types is the style and the manner in which the actuator thrust is generated. Here the user can select from the following actuating principles:

Actuator

Pneumatic

with a linear or rotational movement

Diaphragm Piston Rack & Pinion

Scotch Yoke

Scotch Yoke Electric

with a linear or rotational movement

Hydraulic with a linear movement

Electro-mechanical Electro-hydraulic

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9.1.2. Symbols and units for the calculation of Actuators

Symbol Meaning Source Unit

A Required (effective) diaphragm area SVS cm2

c Actuator stem sealing force and other forces SVS

-CS Spring stiffness = A∙(pst100 - pst0) ∙ H/Hnom. SVS

Da Pressure balance cylinder diameter SVS mm

dsd Shaft diameter of rotary valve - mm

f FTC correction factor of fluid force F∆p (open)

f1 Friction coefficient dependent on stem seal SVS N/mm

f2 Coefficient depended on required seat sealing Class SVS N/mm

f3 Friction coefficient for pressure balanced plugs SVS N/mm

Fa Effective actuator force (thrust) SVS N

FB Bellows elastic force SVS N

Ff Ff = pst0 = pre-load spring trust SVS N

F_a/F_o Minimum safety value, Close safety factor SVS

-FS/Fw Open safety factor (SAMSON sizing Ff = FS) SVS

-FM Force due to weight, force due to acceleration) SVS N

Fm Required actuator force (thrust) pressure balanced. SVS N F_max. Actuator trust: Max. allowed trust depends to max. _temperature

F_mreq. Required actuator thrust _F

mrequ. = Fp1+FSF+FR+FRB+FM SVS N

Foreq. Required actuator trust SVS N

Fo Required actuator force (thrust). SVS N

FP Pressure force (Pst ∙A) SVS N

Fp1 Flow (fluid) forces pressure balance f(p_F 1)

p1 = π / 4 ∙ Sd2 SVS N

FR Friction force caused by stuffing box (packing) _F

R = π ∙ f1 ∙Sd SVS N

F_Ra Friction force (negligible in diaphragm actuator) SVS N

F_RB Friction force (pressure balance sealing force) _F

RB = π ∙ f3 ∙ Da SVS N

FS Spring pre-load trust FS = pst0 · A SVS N

F_SE Spring force (elastic)Air to open: F_SE = F_S + C_S ∙ H

Air to close: FSE = FS + CS ∙ (Hnom. – H)

SVS N

FSF Sealing force (seat loading force) when valve closed; _F

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F∆p Flow (fluid) forces SVS N

Fw

Total friction force of valve and actuator system Fw = FR+c

Fw = FR+FRB+c ← in case of pressure balance

SVS N

g Gravity acceleration g = 9.81 m/s2 _m/s2

Hnom. Rated (nominal) travel SVS mm

H Actual travel SVS mm

h Relative travel = H/Hnom. 60534-1

-Relative travel = H/Hnom.· 100 60534-9 %

M Mass of plug and actuator stem system

MA Actuator torque SVS Nm

Mdyn Dynamic torque SVS Nm

ML Bearing friction torque SVS Nm

MSt Shaft sealing torque SVS Nm

∆pA

Specified shut down pressure for actuator sizing: take care to the required seat leakage rate and valve

strength parameter bar

∆p Pressure differential p1 - p2 bar

∆_p0 Pressure differential at 0 % flow or near to min. flow bar ∆p100 Pressure differential at 100 % flow or near to max. flow

p1max

Allowed ∆p depends to the chosen actuator and strength of the stem with pressure balance

(

a SF RB

)

1max. 2 d F F F c p S 10 4 π − − − = ⋅ ⋅ SVS bar ∆pmax

Allowed ∆p depends to the chosen actuator and strength of the stem

∆_pmaxon plug calculation max. force see also ∆_pA ∆pmax > ∆pA > ∆p0

without pressure balance

(

a SF R

)

pmax. 2 b F F F c S 10 4 π − − − ∆ = ⋅ ⋅ SVS bar

∆ps Min. required air supply – pst100 for fail safe ATC SVS bar

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psu Supply pressure SVS barg

p_st0 Lower value: spring range SVS barg

pst100 Upper value: spring range SVS barg

ps0req. Min. required pre-load spring force pst0 for fail safe ATO SVS barg

Sb Seat bore diameter SVS mm

q_vmax. Operation condition m3_/h

qvmin. Operation condition m3/h

Sd Stem diameter SVS mm

S0 Spring trust / torque characteristic _{(assumed linear)} N / Nm SB Actuator trust Fa = pst ∙ A to travel from 0 to 100 % N

Sz Fluid closing force {f (∆p characteristic)} N

SB 1 Required torque M_A = p_st ∙ A (to open rotary valve) N

SB 2 Required torque MA = pst ∙ A (to close rotary valve) N

SF Safety factor SVS

-S stat. Break-off and friction torque (±) f(∆p characteristic) N S dyn. Dynamic torque with closing tendency (-) _f(_{∆p characteristic)}

Valve-Actuator opening and closing time (See Chapter 20)

acc Index for actuator accessories

-pa Atmospheric pressure SVS bar

pNV Supply network pressure SVS barg

t Stroking time of pneumatic actuator SVS s

t_o1, t_o2,

to3 Opening times: for valve opening SVS s

t_o1, t_o2,

to3 Closing times: for valve closing SVS s

T₁ Inlet temperature K

TA Temperature in actuator internal volumes SVS K

VA Actuator internal volume, (diaphragm chamber, filled _{with air)} SVS m3 Actuator internal volume differential quotient SVS m3_d/dt

V0 Dead volume f(A) SVS m3

Table 9.1.2.-1: Symbols and units for the calculation of Actuators

AV V

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9.2 Pneumatic actuator with a linear or rotational movement

The majority of final control elements in the classical process industries are still today controlled by pneumatic actuators. Either continuously regulated or activated according to a desired On-Off service, i.e. simply driven into one of the two possible end positions. Pneumatic actuators can be divided into two main categories:

Pneumatic diaphragm actuators

Pneumatic piston actuators

9.2.1. Pneumatic diaphragm actuator

The biggest percentage of all control valves installed worldwide in process industries are activated by pneumatic diaphragm actuators. This actuator type essentially consists of two metal shells (the diaphragm housing), the actuator stem and the diaphragm plate. The actuator thrust is transmitted via the high-strength, fabric-inserted diaphragm and the diaphragm plate onto the actuator stem (Figure 9.2.1.1.-1 and Figure 9.2.1.-2).

Unlike a piston actuator, whose travel depends on the length of the cylinder and the piston-rod, the travel of the pneumatic diaphragm actuator is limited by the shape of the case and the diaphragm. Within a limited travel range, the diaphragm area remains almost constant.

High flexibility of the diaphragm ensures a minimum friction and hysteresis. These qualities predestine the pneumatic diaphragm actuator particularly for control tasks where a high actuator thrust, a good sensitivity and low supply pressures (2.5 to 4.0 bar) are important.

Pneumatic diaphragm actuators are usually designed for maximum

supply pressures of approximately 6.0 bar.

If one were to increase the supply pressure at room temperature, up to the destruction of the actuator, it would work out that, in most cases, the diaphragm would prove to be by no means the weakest part of the actuator. Rather, the actuator housing and the supporting diaphragm plates would normally deform beyond the permissible range before we reached a rupture of the strong diaphragm.

Temperature limits for Diaphragm actuator

Diaphragm area Diaphragm material: _NBR Diaphragm material: _EPDM

80 to 120 cm2 _{- 35** to + 90 °C} _{On request}

240 to 700 cm2 _{- 35** to + 90 °C} _{- 50 to + 120 °C}

1400 to 2800 cm2 _{- 40** to + 90 °C} _{On request}

** In on/off service, the lowest value for the NBR diaphragm increases to -20 °C. Table 9.2.1.-1: Temperature limits for Diaphragm actuator

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There are some reasons to limit the air supply at values < 6 bar to avoid over-stressing of actuator parts and also the valve trim-stem system

_{On-off operations at higher frequencies}

In case of travel stops or other accessories, not designed for 6 bar

Larger actuators with fail safe ATC need to be handled with care regarding too high closing trust: Fa = A ∙ (psu - pst100).

Supply pressure regulators (mass production) or supply service units are responsible to limit the max. actuator trust to avoid mechanical damage. If this relatively inexpensive devices are responsible for the safety function of “high end” special valves (like compressor anti surge valves to protect expensive flow machines) long term reliability must be guaranteed. If at some locations commissioning, proper maintenance and instrumentation air quality cannot be guaranteed such dirt and oil sensitive pressure reduction equipment should be avoided. In case of too high air supply the actuator can be sized with spring bench rates which reduce the closing trust of the ATC application, e.g. spring bench rate 0.8 to 2.4 bar or 1.6 to 3.2 bar.

SAMSON AG developed worldwide the first actuator with parallel spring

design which seems to be the state-of-the-art on long term.

The SAMSON Types 3271 and 3277 Pneumatic Actuators contain a rolling diaphragm and internal springs. A maximum of 30 springs can be installed, partly fitted inside one another.

Special features

Low overall height

Powerful thrust at high response speed

Low friction

Various bench ranges by varying the number of springs or their compression

No special tools required to change the bench range and to reverse the actuator action (also version with handwheel)

Permissible operating

temperatures from –50 to +120 °C

Direct attachment of accessories on additional yoke for

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9.2.1.1 Forces in valve and actuator Fp FRa FSE Fa FR F∆p FSF FM

Figure 9.2.1.1.-1: Valve Type 3241 with pneumatic actuator Type 3271

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

Actuator force: Fa (or actuator torque Ma)

Friction force: FR (stem seal, possibly pressure balance seal)

Inertial force: FM (force due to weight of plug, force due to acceleration)

Flow force: F_∆p (due to the shut-off pressure)

Sealing force: F_SF (seat loading force) when valve is closed)

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

Actuator force: Fa (or actuator torque Ma)

Friction force: FRa (negligible (0) however)

Inertial force: FM (force due to weight, force due to acceleration)

_{Elastic force:} _F_SE_{(due to the springs elasticity)}

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5 1.1 1 6 6.1 3 8 12 12.1 11 10 12.1 13 2 15 16 4 7 a 9 1 Nut 1.1 Nut 2 Actuator stem 3 Vent plug 4 Loading pressure connection 5 Top diaphragm case 6 Springs 7 Diaphragm plate 8 Diaphragm 9 Nuts and bolts 10 Yoke with bottom

diaphragm case 11 Loading pressure connection 12 Stem seal 12.1Dry bearing 13 Wiper 15 Ring nut 16 Stem connector Dimension a 350 cm² = 209 mm 700 cm² = 246 mm Spring pressure Signal pressure pst Travel

Figure 9.2.1.-2: Sectional diagram of Type 3277 with 240, 350, and 700 cm2_{effective diaphragm area} The Type 3277 pneumatic actuators with an effective diaphragm area of 240, 350 or 700 cm2_{are primarily mounted to control valves from the Series 3240, 3250 and 3280.}

The actuator is made up of two diaphragm cases, a rolling diaphragm and springs. The lower diaphragm case is permanently fixed to the yoke which allows the direct attachment of either a pneumatic or electropneumatic positioner or a limit switch.

The signal pressure creates a force at the diaphragm surface which is balanced by the springs (6) arranged in the actuator. The number of springs and their compression determine the bench range (signal pressure range) while taking the rated travel into account which is directly proportional to the signal pressure.

A maximum of 30 springs can be installed, partly fitted inside one another. The stem

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The self contained, spring-opposed pressure governor which was introduced decades ago and initiated the beginning of automation in process industries was the real predecessor of the contemporary pneumatic diaphragm actuator. Modern pneumatic diaphragm actuators developed from them over the course of the years. An essential break through happened with the introduction of high-strength fabrics of polyamide or polyester and oil and wear & tear resistant synthetic rubber. Both materials make a firm bonding via a specific vulcanization process and this guarantees the long service life of the diaphragm. Pneumatic diaphragm actuators compare the input signal, which affects the diaphragm,with the compression force from one or several return springs. Through a linear relationship between actuator travel and trailing force this also results also in a proportionality between actuator input signal and actuator travel. If one presupposes a linear valve characteristic and a constant differential pressure a proportionality also exists between the controller output signal and the process variable, i.e. the flow rate. Because of this feature, the pneumatic diaphragm actuator soon found wide-spread distribution. Another advantage of this simple and robust principle is the possibility of avoiding the use of a valve positioner, if it cannot be justified for economic reasons or in cases of extremely harsh environmental conditions.

Advantages:

Compact, only few parts required, very reliable.

_{High actuator thrusts at low air supply pressures (< 6 bar).} Proportional behavior via one or several return springs.

Simple reversal on site possible (direct or reverse).

Automatic failure position in case of auxiliary power loss.

Low hysteresis and excellent sensitivity.

Very broad application range (-35 °C to 90 °C NBR; -50 °C to +120 °C EPDM), lower temperatures with special diaphragm material on request.

Relatively immune to shock and vibrations.

Mounting in any position possible.

High internal tightness allows “interlocking” for several hours.

Lowest thrust-weight ratio of all actuators.

Best cost/thrust ratio of all actuator types.

Disadvantages:

Limited valve travel (to approx. 1/8 of the diaphragm diameter)

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9.2.2. Lay-out of a suitable valve actuator

The lay-out of a suitable control valve actuator requires that we consider a great number of parameters. The actuator indeed works only in an optimal manner, if both the process and environmental conditions as well as the corresponding characteristics and requirement profiles are exactly known. Based on this information, a calculation of the required actuator thrust (or torque) can be made and a suitable actuator selected. This depends in the first place on the main criteria: required actuator thrust, stroking speed and failure position. In addition to these main parameters, there are a number of further actuating variables, which have additionally to be considered during the calculation and specification of a suitable actuator.

The following parameters and conditions are noted without suggesting that the list is definitive:

_{Type of movement (linear or rotational).} Transient response (proportional or integral).

Action of movement (direct or reverse).

Failure position in case of auxiliary power loss: (Open, Closed, Hold).

Actuator signal (electrical or pneumatic).

Input signal range (e.g. 4 to 20 mA).

Nominal travel or nominal angle of rotation.

Mounting interface between valve and actuator.

Required actuator thrust with closed valve.

Required actuator thrust with open valve.

Required travel rigidity (stiffness) for a stable control service.

Permissible accuracy deviation from the characteristic curve.

Hysteresis, reversal error and sensitivity.

Regulating time for full stroke travel or angle of rotation.

Switching frequency (on time in percent for of electric actuators).

Actuator volume (in case of pneumatic actuator).

Time constant and corner frequency (with positioner).

Resistance against shock and vibration (fatigue strength of actuator).

Permissible ambient temperatures.

Required protection class (against dust, water, moisture, e.g. IP65).

_{Requirements with regard to corrosion resistance.}

Actuator and all accessories like positioners, boosters, solenoid valves and air supply pressure reduction device need proper air quality acc. to ISO 8573-1 Class 4 regarding micro-parts ( 1.0 < d < 5 μm; < 1000 parts/m3_{) and Class 3 (≤ 1 mg/m}3_{) regarding oil} contamination otherwise filter systems are needed to protect each device.

Required auxiliary energy for actuator (e.g. min. and max. pressure).

Typical characteristics (e.g. effective diaphragm area).

Actuator connections (e.g. G⅛ to G1 or ⅛ to 1 NPT).

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◊ Ambient temperature.

◊ Auxiliary energy.

◊ Process conditions.

Other requirements, e.g. manual operation required for emergency situations?

Therefore the user of valve actuators should prepare a “check list”, which contains all the important requirements and limiting conditions before he begins to specify the actuator. By this method it is ensured that all required functions have been considered, and an optimal actuator type for the respective task can be selected.

9.2.3. Required actuator thrust for globe valves

The fluid causes static and dynamic reaction forces which act on the plug and which must be absorbed, to a large extent, by the valve actuator. Static forces occur only with the valve closed i.e. if the flow is zero. In this case the internal pressure of the fluid attempts - in the normal flow direction - to open the valve and push the valve stem outwards. Dynamic forces result from heavy turbulence of the fluid. These forces cause vibrations and oscillations which often lead to early wear and tear in the valve or even failure of specific valve and/or actuator parts.

Static forces of a globe valve

A final control element, as represented schematically in Figure 9.2.3.-1, with a flow direction against the plug from below, tends to open, since the fluid pressure tries to push the plug upwards against the thrust of the actuator. This flow direction has been proven to be most effective and is used, in particular, with spring opposed pneumatic diaphragm actuators which have normally only limited travel rigidity. With a reversed flow, i.e. „flow-to-close“, instability might occur as will be demonstrated later. The total static thrust required in order to close a globe valve against the actual pressure differential, results from the sum of the individual force components which is briefly explained below. The necessary equations for a detailed calculation, the symbols and common units are shown.

Flow force F∆p

Assuming a normal flow direction from below against the plug (Figure 9.2.3.-1), the flow force can be computed if inlet pressure and outlet pressure as well as the characteristic values of the final control element are known. This is firstly the cross-sectional area of the valve seat and stem. For valves with bellows seal its mean diameter should be used. Since the section of the seat ring is usually large compared to the stem cross-section, the flow force increases approx. proportionally with the differential pressure.

Sealing force F_SF(Closing force)

Today one expects from a final control element not only a good rangeability, but often also tight shut-off capabilities are required. The permissible leakage is defined in accordance with IEC-60534-4, where different leakage classes apply (II to VI). A correct lay-out of a suitable actuator requires not only a compensation of the flow force which results from the differential pressure in the closed position, but beyond there this a considerable closing force with which the plug must be pressed into the valve seat required, in order to achieve

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the specified leakage rate or tight shut-off. As experience shows, a leakage-dependent specific seating force is required (per millimeter seat ring circumference) in order to enable satisfactory tightness. This means that the required closing force is approximately proportional to the seat ring diameter.

F

_a

F

_R

F

_SF

F

_∆p

S

_b

S

_d

Figure 9.2.3.-1: Static forces acting on a globe valve (schematic)

Friction force FR

Most control valves still use a “stuffing box” which is required for the sealing of the valve stem (shaft), PTFE and graphite, which are often combined with special filling materials in order to improve hysteresis and sensitivity, are the most common package materials used today. Friction in the stuffing box can be roughly computed if the stem diameter d, the maximum pressure p and the package type are known. Generally, the frictional force increases proportionally with the stem diameter and the working pressure. A special packing coefficient is associated with the packing material.

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Friction force FRB in pressure balanced valves

The principle of pressure balance is often applied on large valve sizes and high differential pressures. As a result, the actuator thrusts remains small in comparison with a non-pressure balanced control valve. The causes of friction are dynamic forces, which result mainly from the gap flow in the guide bushing and which press the plug firmly against the wall of the guide bushing. It is therefore erroneous to assume that pressure balanced valves require only very small actuator thrusts. These frictional forces in fact increase proportionally with the diameter of the guide bushing and with the differential pressure. Additional piston rings, which are often inserted in the plug to keep the leakage low, make things worse. Ignoring this frictional force is sometimes the reason why the final control element is unable to close, since the differential pressure normally increases with a decreasing flow, so that the friction force becomes highest near the closed position. The jamming effect is especially troublesome strongly with fluids which have no lubrication features e.g. like dry overheated steam or other dry gases.

The pressure balance contoured plug seat configuration of SAMSON AG has not been changed to the non pressure balanced standard trim. The unique balanced design -see Chapter 6 Fig. 6.2.4.-3 – is sealed in the valve bonnet and not in the seat area,

Therefore the risk of jamming is minimized as well as the high dirt sensitiveness known from other cage retained pressure balanced systems.

Weight force F_M

In a case of a very heavy opturator (plug) with a considerable mass, emphasis must naturally give also to the weight of the plug (depending on the mounting position of the control valve), if the actuator has to be able to open or close the final control element against gravity forces.

9.2.4. Dynamic forces need to be considered in case of flow to close tendencies

When considering static forces, the main factors apply to the valve in the closed position, i.e. the actuator must be able to close the valve against the highest differential pressure, On the other hand, an accurate and stable control service should be achieved for pressure balanced and/or double seated valves, in spite of low actuator forces and a relatively small travel stiffness when dynamic forces exist. Provided a comprehensive actuator calculation for a single seated control valve has been carried out, and that it considers all essential factors, the dynamic forces can usually be ignored because static forces normally predominate to a great extent.

A different situation exists, however, in the case of a double seated or pressure balanced control valve. Here the dynamic forces can considerably exceed the static forces. This means that unstable regulating service and/or inadequate control characteristics might occur. The regulation service becomes particularly critical if with “direct action”, the installed characteristic of the pneumatic diaphragm actuator flattens so far that different travel positions can be assigned to the same actuator input signal (Figure 9.2.4.-1). This effect can be observed with control valves with a flow direction „to close“, i.e. the flow enters the valve from entry point on top of the plug. This misapplication often results in

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an uncertain correlation between the actuator input signal and the corresponding valve travel and should be avoided under all circumstances. While the signal range of the actuator becomes greater with the normal flow direction „to open“ (approx. 160 % in Figure 9.2.4.-1), the signal range decreases in the case of re-verse flow direction (approx. 65 %) and this might cause instability.

Considerable instability is often found near the closed position but rarely also instability can be found at opening position if here the total signal pressure characteristic SB turns negative (see Figure 9.2.4.-2 and Figure 9.2.4.-3). The plug usually “flutters” in this range and not only prevents a stable regulation, but also tends to self-destruction through “hammer effects”. The dynamic forces acting on the internals of the control valve can be greatly suppressed, if an actuator with sufficient rigidity (travel stiffness) is employed, Unlike self-locking electro actuators or very “inflexible” electro hydraulics actuators, pneumatic diaphragm actuators have only a limited stiffness, which is determined by the spring rate of the return spring(s). Empirically based findings prove that a satisfactory regulation can be expected even in the case of limited travel stiffness.

Naturally, the requirements increase with an increasing differential pressure and with a larger diameter of plug. A re-calculation of the required minimum travel stiffness is recommended, if (1) high differential pressures occur, (2) the valve is pressure balanced, (3) the flow direction is reverse (flow-to-open). Since the user very seldom knows the precise physical dimensions in the case of an installed valve, the actual flow coefficient (Cv100 value) can also be used in a rough calculation. In addition, a valve specific stability factor may be utilized in order to observe the dynamic behavior of the valve depending on the trim. Cage and contoured plug valves, for instance, behave more favorably in this respect than do valves with parabolic plugs.

% 0 10 20 0.2 30 40 0.4 50 60 0.6 70 80 0.8 90 100 1.0 1.2 1.4 1.6 1.8

Signal range (bar)

Valve open Valve closed Flow _directi on FT_O Bench ran_ge Flow directi_{on FTC} instable

Resulting force of fluid and spring for ATC

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w

FTC + ATO

Y x

p

₁

p

₂ SB SB Sz Sz

Safe control range pst Travel 0.45 p100 f p0 _∆   _∆    p100 f p0 ∆   _∆    pst0 pst100 Dynamic Joukowsky peak „Water hammer“ Spring force Fluid force S0

Signal pressure - Travel diagram with non-stable signal pressure characteristic SB if the slope (gain) turns negative (-) to the positive (+) spring slope.

Risk of hunting (hammering) at small opening < 20 % travel and also possible oscillation (hunting) at > 35 % travel if ∆p_max<< ∆p₀, respectively at 45 % travel if ∆pmax = ∆p0 = constant.

Sz = fluid force characteristic: f(∆p100/∆p0)

In case of high liquid velocities in long larger pipes at valve upstream there could be at least always a dynamic risk of water hammer (Joukowsky peak).

Figure 9.2.4.-2: Signal pressure - Travel diagram with non-stable signal pressure characteristic

SB SB S 0 Sz pst Travel Avoid to control at small

opening (10 to 20 %)

ATO

Sz

Signal pressure - Travel diagram with stable signal pressure characteristic SB characteristic from a static point of view. Higher actuator stiffness keeps the signal pressure characteristic slope positive (+) like the slope of the spring characteristic. See equation (9-10) to optimize actuator stiffness.

To avoid any risk of unpredictable “hammering” near the seating area avoid any control function below 20 % or alternatively use a pressure balance valve or an electrical actuator.

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Typical trim configurations with flow to close tendencies:

Without going to details some graphics of the total force characteristic, which may turn from flow to open (stable) to flow to close (non stable tendencies) are sometimes causing trouble in liquid applications, if actuator sizing with “know how” not taken account stability in all travel positions.

Angle type valves:

(FTC in case of safety reasons; abrasive fluids; severe service) Perforated plugs need for liquids FTC for effective sound reduction

Double seated valves:

If flow goes from outside (above the plugs) to inside

Three way valves:

For dividing service with compact plug design or mixing service with two plugs above the seats.

This trim selection must be avoided in case of after sale trim replacements. SAMSON AG avoid FTC tendencies with a compact trim for mixed service and a splitted two plug trim for dividing service.

Rotary control valves:

For butterfly – and most of other rotary control valves the actuator sizing should take care of all static- and dynamic forces for all travel positions and need a special look to the actuator stiffness, if the dynamic force, which general has a FTC tendency is not negligible.

Only a larger stiffness - which is (pst100 - pst0) ∙ A - can minimize the break off torque and carefully looking to travel start and travel end and the necessarily spring force or air supply can avoid bad surprises.

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SB 1 S 0 ≈ linear SB 2 (-) S d yn. ± S stat. 10° 20° 30° 40° 50° 60° 70° 80° 90° Opening angle Break off

Fail safe: ATO (closed by spring force) Min. air supply p_su

p_st open ཱ ི ི p_st close Instable opening for control > 70° p_su ≥ 2∙p_st0 pst (p_su - p_st100_{)·MA ≥ S dyn} p_st0· M_A p_su ∙ M_A ≥ S stat. M_ATorque

Signal pressure - opening diagram ATO (Closed with spring force) S0 Spring characteristic

(±) ML= friction (break off) torque (against direction of travel) (±) MSt = Shaft sealing torque (against direction of travel) (-) Mdyn = dynamic torque f(size, ∆p) (closing direction)

SB1, SB2 Signal pressure characteristic to open and to close the rotary valve. Take care of the necessarily min. air supply and min. pre-load of the spring To check: ② pst0 ∙ MA > ML +MSt

③ (psu – pst100) ∙ MA > Mdyn. + MSt

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6≈ linear (-) S dy n. s6_V WDW 10° 20° 30° 40° 50° 60° 70° 80° 90° Opening angle Break off torque

Fail safe: ATC (open by spring force)

p_st open p_st100ཱ ིp_st0 p_su - p_st100 p_su = 2∙p_st100 S_VWFORVH Instable opening for control > 70° SB₂ SB₁

Min. spring pre-load torque

Min. air supply p_su pst

M_ATorque

཰

Signal pressure - opening diagram ATC (Closed with air pressure).

The dynamic torque (-) Mdyn. in some applications like “flow machines anti-surge” control with high performance low noise butterfly valves can get much higher values than shown here in case of high ∆p at max. load.

This closing torque need to be balanced safety with a necessarily spring pre-load torque.

See sizing example

To check: ① (p_su – p_st100) ∙ M_A > M_L + M_St

② pst00 ∙ MA > ML + MSt

③ pst0 ∙ MA > Mdyn. + MSt

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9.2.5. Calculation and selection of an actuator

For the selection of a suitable pneumatic diaphragm actuator, it is vital to work through the previously mentioned “check list”. The following questions, which have a decisive influence on the actuator selection procedure, are to be answered without fail in this respect:

Failure position: valve either closed or open?

Nominal valve travel: actuator travel must be ≥ valve travel.

_{Prescribed flow direction: flow to open or flow to close?} Important physical dimensions: seat and stem diameter?

Packing type: PTFE or graphite?

Flow coefficient (Cv100 value)?

Required actuator thrust in closed valve position?

Required minimum full stroke regulating time for nominal valve travel?

Is a positioner required? If not: required signal range?

Does the interface match the actuator and the control valve?

Minimum air supply pressure of the plant?

Supply air quality can have a major impact to safety functions. Filter systems are recommended for each devise to keep air quality acc. to ISO 8573-1 in locations with lower standards.

Minimum and maximum ambient temperatures?

Required shock and/or vibration resistance?

Special requirements with respect to corrosion resistance?

Other demands of the customer?

When all these items have been considered, a corresponding calculation and selection of a suitable valve actuator can be made.

9.2.5.1 General notes for the calculation and selection of actuators

A sufficient additional safety factor should always be planned, since an exact calculation of the required actuator thrust is quite complex and there could be a difference in the force balance from final inspection to after commissioning under process conditions as well as after long term operations.

For DIN/DVGW valves and other quick closing and safety control valves (TÜV certificated) the actuator safety factor must be about 2.

SAMSON Valve Sizing (SVS and SVSS) creates a general warning, if the safety factor become < 1.1. This min. safety factor expects ideal environmental condition like an in-house installation- and equates to the competitive conduct.

Environmental conditions at delivery and plant location are often unknown as well how long valves are stored before commissioning. Valves may be delivered in hazardous areas and stored less protected. Then sealing material will be attacked from rain, sand and petrochemical dust and sometimes it can take month before the plant is ready for start up. Any packing-stem system will suffering more under static not pressurized conditions.

(30)

The valve system friction may increase also from lack in commissioning (pipe installation without force compensation due to temperature elongation). Long term age-hardening of packing and sealing material occurs in case of no proper frequently maintenance.

It is recommended to use a higher than 1.1 safety factor in case of those unknown (not ideal) random conditions.

Therefore the required actuator force (trust): Fo = 1.25 ∙ Fa

_{Large effective areas reduce the negative influence of friction forces on control valve}

performance by minimizing hysteresis and dead band.

At request of the highest control controllability for expensive products like insulin also a low price actuator of smallest size may close against a moderate low shut down pressure. The valve performance and controllability interacting to the production quality and quantity, which significantly can improved with a larger actuator size selected in the first priority for lowest hysteresis and not for the shut down pressure.

Diaphragm actuators have a considerably smaller hysteresis than piston actuators (up to 40 % hysteresis due to piston seals), which the positioned is likely to be unable to compensate on long term.

Several spring assemblies with smaller springs distributed over the circumference offer a greater accuracy and mechanical stability than fewer large springs arranged in the center. In addition, the variability of the spring range and the stiffness of the actuator are increased

The actuator characteristic should, ideally, be linear and the hysteresis small. Linearity errors stem mostly from a changing diaphragm area which might possibly result in an inadequate actuator thrust in the closed position.

The actuator selection should also consider durability and maintenance.

An application in the following climatic conditions should be guaranteed:

◊ Moderate

◊ Cold

◊ Dry warm

◊ Humid and warm

Constructional arrangement of the valve interface and general firmness of the actuator should meet all requirements considering at the same time ambient influences. Demands regarding earthquake safety and/or radiation resistance in nuclear power plants are examples of this point.

The selection of a suitable actuator should always consider possible future enhancements. Subsequent mounting of a hand wheel or positioner might be taken as examples. Mounting possibilities of further accessories should also be considered.

Fatigue strength and impact resistance ought to be considered e.g. as they occur on ships.

Corrosion-resistant paint coatings should be applied, in order to protect the actuator and guarantee the specified service life.

All aspects of accident safety must be considered, in order to protect users against injuries.

(31)

An adequate quality assurance system (ISO 9001) and attention paid to valid standards and regulations is necessary, to guarantee correct „state of the art“ products.

9.2.6. Fail-safe action

When the signal pressure fails, the fail-safe action of the actuator depends on whether the springs are installed in the top or bottom diaphragm chamber.

Actuator stem extends (ATO)

When the signal pressure is reduced or the air supply fails, the springs move the actuator stem downwards and close the valve.

The valve opens when the signal pressure is increased enough to overcome the force exerted by the springs.

Actuator stem retracts (ATC)

When the signal pressure is reduced or the air supply fails, the springs move the actuator stem upwards and open the valve.

The valve closes when the signal pressure is increased enough to overcome the force exerted by the springs.

ATC

pst

ATO

pst = Signalpressure Black arrow = Spring force Blue arrow = Air pressure

pst FTC + ATC pst FTC + ATO pst FTO + ATC pst FTO + ATO Figure 9.2.6.-1: Fail-safe action

(32)

9.2.6.1 Major application cases

One must distinguish between the following application cases before beginning the relevant calculation procedure:

(a) Valve without pressure balance,

FTO flow direction to open, ATO (air failure→spring CLOSED).

_{(b) Valve without pressure balance,}

FTO flow direction to open, ATC (air failure→spring OPEN).

_{(c) Valve with pressure balance,}

FTO flow direction to open, ATO (air failure→spring CLOSED).

_{(d) Valve with pressure balance,}

FTO flow direction to open, ATC (air failure →spring OPEN).

_{(e) Valve without pressure balance,}

FTC flow direction to close, ATO (air failure→spring CLOSED.

_{(f) Valve with pressure balance,}

FTC flow direction to close, ATO (air failure→spring CLOSED.

_{(g) Valve with - or without pressure balance,}

FTC flow direction to close, ATC (air failure→spring OPEN.

The application mentioned under (g), i.e. valve with - or without pressure balance, flow direction to close and valve on air failure OPEN, must be avoided, because full stability can hardly be expected under these conditions (see Figure 9.2.4.-1).

The required effective area of the actuator diaphragm and therefore the required actuator size is derived from the previously calculated actuator thrust. It is always recommended to add an additional safety factor of approx. 1.25 in order to ensure that the actuator is able to close the final control element, under all conditions, against the max. upstream pressure p1 or against the specified max. differential pressure ∆pA. The following equations consider the relationships between actuator and valve in the different application examples.

9.2.6.2 Flow force

Depending to the customer specification ∆p is declared as ∆pA; ∆p0 or p1 max. For actuator sizing this value has the highest cost influence and must be clarified before starting the offer.

Without pressure balance trim: FTO flow to open

Flow force F∆p = trust is opening (+) and need Fa trust to close (-)

(

2

) (

2

)

b d 2 F _p S p S p 4 10

π

  = ⋅ ∆ + ⋅ ⋅ ∆   _⋅ (9-1)

(33)

If in the majority of trim variations the seat diameter Sb is essential larger than the stem – or bellows seal diameter Sd the influence of Sd is negligible.

(

2

)

b

F

_p

S

p

4 10

π

=

⋅ ∆ ⋅

∆

_⋅

(9-1.2.1) FTC flow to close

F_∆p trust is closing (-) and need Fa trust to open (+)

(

2 2

)

(

2

)

b d d 2

F

_p

S

p S

p

4 10

π





=

−

⋅ ∆ −

⋅

∆





_⋅

(9-1.2.2)

The term for the closing force F∆p = (Sb2 – Sd2) ∙ ∆p ∙ π/(4 ∙ 10) need to have a force correction for the open position with factor f:

f = 0 if Δp100/Δp0 tends to zero; f = 0.4 if Δp100/Δp0 tends to 1

f = 0.4 based on measurements with parabolic plugs (mentioned in VDI/VDE 3844)

Remark: F_∆p at open position = F_∆p∙ f

With Pressure balance trim:

FTO flow to open: (Standard flow direction)

F∆p and Fp1 trust result to open (+) and need Fa trust to close (-)

In case of Da > Sb

(

2 2

)

(

2

)

a b d 1 F _p D S p S p 4 10

π

  = − ⋅ ∆ + ⋅ ⋅ ∆   _⋅ (9-1.3.1)

In case of D_a = S_b (Standard for SAMSON pressure balance)

2 p1 d 1

F

S

p

4 10

π

=

⋅ ⋅

⋅

(9-1.3.2)

FTC flow to close (on request for an angle type valve)

F_∆p and F_p1 trust result to close (-) and need F_a trust to open (+)

In case of D_a > S_b

(

2 2

)

(

2

)

a b d 1

F

_p

D

S

p S

p

4 10

π





=

−

⋅ ∆ −

⋅

∆





_⋅

(9-1.4.1)

In case of Da = Sb (Standard for SAMSON pressure balance)

2 p1 d 1

F

S

p

4 10

π

=

⋅ ⋅

⋅

(9-1.4.2) b d

S

>>

S

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9.2.6.3 Sealing, friction and other forces Sealing forceFSFif valve closed H = 0

(9-2)

π

= ⋅ ⋅

SF 2 b

F

f S

f2 = for required leakage class N/mm

Metallic 2 Class IV

Lapped in 20 Class V

Soft sealing 0.3 Class VI

Packing/Sealing friction FR

without pressure balance

R 1 d

F

= ±

π

⋅ ⋅

f S

(9-3)

Stem seal → Friction coefficient f1 N/mm

Series 3240 3250 DN up to 150 DN > 150 Packing number 1 2 2 PTFE- packing 1.6 3.2 3.2 Graphite-packing 5 10 10 Bellows 1.6 1.6 Insulated section 1.6 1.6 Univerdit Alchem 3.2 3.2

Figure 9.2.6.3.-1: Stem seal → Friction coefficient f1

Friction in pressure balanced valves

RB 3 a

F

= ⋅ ⋅

π

f D

(9-4)

Stem seal → Friction coefficient f3 N/mm

With PTFE seal 3.0

With Graphite seal 10

Weight force of plug and actuator stem system

M

F = ± ⋅M g (9-5)

Remark: - M normal installation; + M reverse hanging installation M = Mass of plug and actuator stem system

g = Gravity acceleration g = 9.81 m/s2

Of interest only at larger sizes: DN 200 approximately 200 N

DN 300 approximately 450 N

Weight forces (FM) for small sizes are negligible.

In this way, the following basic equations will apply for the calculation of the required actuator thrust considering the valve type, the flow direction and the respective failure position of the final control element:

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9.2.6.4 Actuator force Fa in all cases of (a) to (f) Chapter 9.2.6.1

Actuator force Fa

Case (a) and case (b):

No pressure balance, flow to open,

failure position: ATC - OPEN or ATO - CLOSED,

( )

a p SF R

F

=

F

_∆

+

F

+

F

+

FM

(9-6)

From the variables of equation (9-1) equation (9-2) equation (9-3) we get:

(

2

) (

2

)

a b d 2 2 B 1 d

F

S

p

S

p

f S

M g

4 10

π

_π





=

_

⋅ ∆ +

⋅

_

⋅

+ ⋅ ⋅

+ ⋅

⋅

(9-6.1)

Case (c) and case (d):

With pressure balance, flow to open, failure position OPEN or CLOSED

(9-7) a p1 SF R RB

F

=

F

+

F

+

F

+

F

+

(FM)

(9-7.1)

(

2

)

a d 1 2 B 1 d 3 a

F

S

p

(f S

f S

f D ) M g

4 10

π

_π

=

⋅

+ ⋅

⋅

+ ⋅

⋅

Case (e):

No pressure balance, flow to close, failure position CLOSED

( )

∆

=

+

a p R M

F

(9-8)

(

2 2

)

(

2

)

a b d d 2 1 d

F

S

p S

p

f S

M g

4 10

π

_π





=

_

−

⋅ ∆ −

⋅

_

⋅

+ ⋅ ⋅

+ ⋅

⋅

(9-8.1)

If Sb >> Sd equation (9-8.1) could be simplified:

(

2 2

)

a b d

F

S

p

M g

4 10

π

=

−

⋅ ∆ ⋅

+ ⋅

⋅

(9-8.2)

Fa trust here is needed to open the valve from 0 to 100 % travel.

Important to check the actuator stiffness for control stability with equation (9-10) and check the closing position if the closing pressure differential is too small.

st0 SF R 2 b 1 d

(36)

Case (f):

With pressure balance, flow to close, failure position CLOSED

( )

a p1 R RB

F

=

F

+

F

+

F

+

FM

_(9-9) in case of S_b = D_a

(

)

2 a d 1 1 d 3 a

F

S

p

f S

f D

M g

4 10

π

_π

=

⋅ ⋅

+ ⋅

⋅

+ ⋅

⋅

(9-9.1)

Fa trust here is needed to open the valve from 0 to 100% travel.

Important to check the actuator stiffness for control stability with equation (9-10) and check the closing position if the closing pressure differential is too small.

(

)

st0 SF RB 2 b 1 d 3 a

10 p

⋅

⋅ >

A F

+

F

= ⋅ ⋅

π

f S

+ ⋅

π

f S

⋅

+ ⋅

f D

With a reverse flow direction (fluid tends to close the valve), the plug is pressed into the seat by the differential pressure. This application should be limited to special cases only, in order to avoid instability with fine control requirements. A few exceptions are applications where it is either favorable

To exploit the reaction force of the differential pressure for secure closing of the valve and/or to achieve tight shut-off, or

To limit wear and tear inside the valve body caused - in the case of angle type valves - by erosion or/and abrasion.

One main criterion for achieving adequate actuator stability is the difference of force gradients, when a reverse flow direction is chosen. Generally there exists in every travel position of the actuator, a balance between the actuator thrust which is determined by the effective diaphragm area multiplied by the actuator input signal and the trailing force of the return spring(s).

Under dynamic conditions high fluid forces can occur, which act on the plug and considerably disturb this balance. From Figure 9.2.4.-1 it becomes clear, that the effective signal range in the case of “direct action” and a flow direction „to close“ is considerably decreased which leads, unavoidably, to instability, since the force gradient resulting from fluid forces is higher that of the return spring(s). This makes, in such a case, a satisfactory regulation impossible. For this reason, the combination („direct action“ and „flow to close“ service) should be excluded under all circumstances.

However, the flow direction „to close“ is even with “reverse action” (valve closed in case of air failure) not without problems. Near the closed position the reaction forces of the control valve normally increase greatly and may be superposed by a so called “Joukowsky peak” at larger sizes, long upstream pipe and high pipe velocity, so that the force balance is suddenly disturbed considerably. This imbalance can be slightly improved by the application of a positioner. The response time is, however, not fast enough by far, so that a continuous “hammering” of the plug might occur near the closed position.

(37)

This must be in all circumstances avoided for wear and tear reasons. Essential for stable actuator operation is a sufficiently high spring rate whose force gradient must be higher at any travel position than the force gradient resulting from dynamic valve reaction forces. Furthermore, it should be noted that the actuator force direction of flow „to close“ applications is reverse, i.e. the actuator force hat to “pull” the plug out of the seat. One should further differentiate between “control service” and “ON-OFF service. In the latter case, a continuous “hammering effect” cannot occur, since the critical range is usually passed through very quickly. Important with reverse flow conditions is an examination of the required actuator rigidity or travel stiffness, in order to assure a stable control service. This stiffness depends on the spring rate and the diaphragm area of the pneumatic actuator: A ∙ (pst100 - pst0).

Adequate control stability can be expected if the required maximum actuator thrust is in a certain relationship to the signal range of the actuator. The required signal range which, in turn, determines the travel stiffness can be roughly checked by means of the following equations. The disadvantageous case that the valve authority ∆p₁₀₀/∆p₀ tends to zero, (means ∆pqmax << ∆p0) need the highest actuator stiffness and from safety point of view expressed with a factor 2 in equation 9-10.

For a continuous stable control service the following rule applies for FTC: Without pressure balance:1

(

st100 st0

)

p A p⋅ −p ⋅10 2 F> ⋅ _∆ (9-10)

(

2 2

)

b d 0

F

_p

S

p

4 10

π

=

−

⋅ ∆

⋅

∆

_⋅

(9-10.1)

for cases Sd << Sb and ∆p0 / ∆p100 < 0.2

(

)

2

st100 st0 b 0

A p

⋅

−

p

>

0.016 S

⋅

⋅ ∆

p

(9-10.2)

With pressure balance:

(

st100 st0

)

p1 A p⋅ −p ⋅10 2 F> ⋅ (9-10.3) 2 p1 d 1

F

S p

4 10

π

=

⋅ ⋅

⋅

(9-10.4)

In case of Da = Sb (Standard for SAMSON pressure balance.

The factor 2 in equation (9-10) tends to factor 1 if the valve authority ∆p100/∆p0 also tends from 0 to 1.

∆p0 roughly can be substitute with the max. ∆p at qmin. but not with the specified ∆pA for actuator closing, which could be much higher than ∆p0. Control actions are not recommended below 20 % travel.

(38)

For ON-OFF service the following rule applies for FTC

(

_st100 _st0

)

_p

A p⋅ −p ⋅10 1.33 F> ⋅ _∆ (9-11)

Based on long term experience with perforated plugs (FTC) a proven rule-of-thumb for the maximum permissible differential pressure ∆p0 has been derived from equation 9-10 and 9-11 for applications with a flow direction to close.

∆p0 roughly can be substitute with the max. ∆p at qmin. but not with the specified ∆pA for actuator closing, which could be much higher than ∆p₀.

For control service:

(

)

(

)

0 st100 st0 ₂ ₂ b d 100 p 0.5 A p p 4 S S π ∆ = ⋅ ⋅ − ⋅ ⋅ − ⋅ (9-12)

The factor 0.5 tends to factor 1 if the valve authority Δp100/Δp0 also tends from 0 to 1.

For ON-OFF service:

(

)

(

)

0 st100 st0 ₂ ₂ b d 100 p 0.75 A p p 4 S S

π

∆ = ⋅ ⋅ − ⋅ ⋅ − ⋅ (9-13)

9.2.6.5 Safety factors to actuator sizing

Important requirements and safety factors to actuator sizing published in SAMSON VALVE SIZING SVS and SVSS.

Figure 9.2.6.5.-1: SAMSON Actuator sizing

Fa : delivered actuator trust kN

Foreq. : Required actuator trust kN

Fmreq. : Required actuator trust (Pressure balanced) kN

F_max. : Actuator trust: Max. allowed trust depends to max. temperature kN

∆pmax. : Allowed ∆p depends to the chosen actuator and strength of the stem bar

ps0req. : Min. required pre-load spring force pst0 for fail safe ATO bar

∆pS : min. required air supply – pst100 for fail safe ATC bar

To check in case of pressure balance trim:

(39)

Handling safety factors SF:

Safety factor SF delivered actuator trust / required actuator trust.

The factor SF is the result of selecting the actuator type (e.g. actuator data sheet information from Chapter 9.2.22 to Chapter 9.2.22.2 with higher trust than calculated. The responsible engineer should have information about environment, installation, local maintenance and warrantee conditions as well as special customer requirements. See also Chapter 9.2.5.1. If this random conditions are critical it is recommended to size SF higher than 1.1 (SVSS software warning value).

If 1.1 < SF < 1.25 select an actuator type with SF equal or near to 1.25. (Is the SVSS software warning 2.0 than select near to 2.0)

Fa/Fo or Fa/Fm without or with pressure balance warning in SAMSON Valve Sizing

Specialist SVS / SVSS if SF < 1.1 → recommended in hazardous areas = 1.25

Open safety factor: only for fail safe ATC

F_f /F_w warning in SAMSON Valve Sizing Specialist SVS / SVSS if SF < 2.0

Ff = pst0 = pre-load spring trust, Fw = system friction

9.2.6.6 Calculation examples (pressure balance):

Fp1 forces can support to open (FTO) or to close (FTC) the valve. In case of pressure balance this forces are small in comparison to the total system friction forces

w R RB

F

=

F F

+

c

Flow to open FTO

Fail safe ATO

Closing safety factor:

(

)

a st0 2 m d 1 2 b 1 d 3 a F A p 10 F _S _p _{f S} _{f S} _{f D} 4 10

π

_π

⋅ ⋅ = ⋅ ⋅ + ⋅ ⋅ + ⋅ ⋅ + ⋅ ⋅

Opening safety factor:

(

)

(

su st100

)

a m 1 d 3 a

A p

p

10 F

F

π

f S

f D

⋅

−

⋅

=

⋅

+ ⋅

Warning in SAMSON Valve Sizing Specialist SVS / SVSS if SF < 1.1 recommended in hazardous areas = 1.25

Control Valve Actuators

Table of contents

9. Control valve actuators

Table of contents

Table of contents

Table of contents

Table of contents

Table of contents

Table of contents

Table of contents

Table of contents

9. Control valve actuators

(

)

(

)

Pneumatic diaphragm actuators are usually designed for maximum

supply pressures of approximately 6.0 bar.

SAMSON AG developed worldwide the first actuator with parallel spring

design which seems to be the state-of-the-art on long term.

F

F

F

F

S

S

FTC + ATO

p

p

(

) (

)

π

(

)

F

p

S

p

4 10

π

=

⋅ ∆ ⋅

∆

⋅

(

)

(

)

F

p

S

S

p S

p

4 10

π





=

−

⋅ ∆ −

⋅

⋅

∆





⋅

(

)

(

)

π

F

S

p

4 10

π

=

⋅ ⋅

_p

_⋅

_p

_⋅

_p

_⋅