saipem

Fano 12 Ottobre 2015 Massimo Benedetto - ESAFA

VALVOLE DI REGOLAZIONE

Titolo presentazione luogo presentazione

società società

Fluidi supercritici

saipem

Il punto critico di una sostanza è identificato da una temperatura e da una pressione critica. A temperatura e pressione inferiori a quelle critiche abbiamo la fase liquida e gassosa ben distinte. Con il riscaldamento del liquido, la sua densità diminuisce, mentre la densità del vapore aumenta. Le densità del liquido e del vapore si avvicinano sempre di più fino a che non si raggiunge la temperatura critica, in corrispondenza della quale i valori della densità della fase liquida e della fase gassosa sono equivalenti. Oltre questi due valori il fluido, appunto detto in condizioni supercritiche, è in una fase gassosa caratterizzata però dalla densità del liquido. In queste condizioni il fluido è estremamente reattivo diffonde nella materia come un gas ma ha le capacità di trasporto di massa di un liquido. Un fluido in condizioni supercritiche presenta delle caratteristiche che possono essere molto diverse da quelle che presenta nello stato liquido o gassoso, questa differenza lo rende interessante nell'ambito di applicazioni chimiche ed industriali.

Titolo presentazione luogo presentazione

società società

Fluidi supercritici

Titolo presentazione luogo presentazione

società società

Fluidi supercritici

Titolo presentazione luogo presentazione

società società

Equazioni per il dimensionamento della valvola di regolazione per fluido supercritico

saipem

• The fluid behaves as a compressible, but near the critical point great deviations from the perfect gas laws prevail.

• It is very important to take this into account

through the use of actual specific weight (mass density) from thermodynamic tables (or the compressibility factor Z), and the actual ratio of specific heats.

• In order to size valves handling supercritical

fluids, use a compressible flow sizing equation with the weight (mass) rate of flow with actual specific weight (Gg), or the volumetric flow with actual compressibility factor (Z). In addition, the actual ratio of specific heats (Fk) should be used.

Titolo presentazione luogo presentazione

società società

Il coefficiente di efflusso relativo Cd

saipem

• The flow capacity of a valve (Cv) is a quantitative rating of its ability to pass a fluid flow for a set of given pressure and fluid density conditions, defined as the flow rate of water in gallons per minute through the valve given a constant pressure drop of 1 PSI.

• Cv may be predicted, or empirically measured.

Titolo presentazione luogo presentazione

società società

Il coefficiente di efflusso relativo Cd

saipem

• Not all control valve types exhibit the same Cv coefficients for the same pipe size.

• A 4” butterfly valve, for example, has a much greater full-open Cv rating than a 4” globe valve, due to the much more direct path it offers to a moving fluid.

• A simple comparison of these two valve types clearly shows why this is true

Titolo presentazione luogo presentazione

società società

Il coefficiente di efflusso relativo Cd

saipem

One way to help quantify the valve ability to throttle fluid flow is to introduce the ratio of flow coefficient (Cv) versus cross-sectional pipe area.

The basic principle is that we should expect the Cv of any particular valve design to be proportional to pipe area (e.g. a ball valve with twice the pipe area should have twice the flow capacity, all other factors being equal), and therefore a ratio of these two quantities should be fairly constant for any valve design.

Titolo presentazione luogo presentazione

società società

Il coefficiente di efflusso relativo Cd

saipem

Thus, we introduce the relative flow capacity Cd, defined as:

To calculate the approximate Cv for any type of valve and valve size:

Titolo presentazione luogo presentazione

società società

Caratteristica intrinseca della valvola di regolazione

saipem

Titolo presentazione luogo presentazione

società società

Caratteristica intrinseca della valvola di regolazione

saipem

Titolo presentazione luogo presentazione

società società

Caratteristica quick opening

saipem

Quick Opening

020406080

100

0 20 40 60 80 100

Input

Out

put

Quick-opening valves are often used in pressure-relief applications, where it is important to rapidly establish flow rate during the initial portions of stem travel.

Titolo presentazione luogo presentazione

società società

La caratteristica lineare

saipem

Linear

020406080

100

0 20 40 60 80 100Input

Out

put

Titolo presentazione luogo presentazione

società società

Caratteristica equipercentuale

saipem

Titolo presentazione luogo presentazione

società società

Caratteristica equipercentuale

saipem

Equal Percentage 50:1

020406080

100

0 50 100

Input

Out

put

Titolo presentazione luogo presentazione

società società

Caratteristica equipercentuale

saipem

Equal Percentage 30:1

020406080

100

0 50 100

Input

Out

put

Titolo presentazione luogo presentazione

società società

Caratteristica equipercentuale

saipem

Flow Characterization

020

4060

80100

0 20 40 60 80 100Input

Out

put

Equal Percentage 30:1

Equal Percentage 50:1

Titolo presentazione luogo presentazione

società società

Relazione tra forma dell’otturatore e caratteristica della valvola

saipem

L’otturatore equipercentuale, nella prima parte della corsa, è più restrittivo della sezione di passaggio del fluido rispetto all’otturatore lineare.

L’otturatore lineare, nella prima parte della corsa, è più “aggressivo” rispetto all’equipercentuale.

Titolo presentazione luogo presentazione

società società

Relazione tra forma della gabbia e caratteristica della valvola

saipem

Titolo presentazione luogo presentazione

società società

Relazione tra forma della sfera e caratteristica della valvola

saipem

Titolo presentazione luogo presentazione

società società

Importanza del corretto dimensionamento della valvola

While control valve selection is an "art," control valve sizing is closer to a "science".

Valve sizing procedures are based on accepted mathematical equations that are used to model flow through ideal restrictions such as orifice plates and flow nozzles. While control valves do not always resemble ideal restrictions, the mathematical models generally give useful results if the process inputs data are accurate. However, if the service conditions and fluid properties that are used as inputs to the sizing process are not accurate, this may led to the selection of a control valve that is either undersized or oversized for the application.

saipem

Titolo presentazione luogo presentazione

società società

Problemi di sottodimensionamento

Limited Flow Capacity is the primary concern of control valves that are too small. Limited capacity may have economic impact, such as the

inability to meet production quotas. Limited capacity may result in process failure because of the

inability to supply needed fluids in sufficient quantity. Limited capacity can also result in safety hazards; for

example, an undersized control valve that is used in a relief application may allow upstream pressure to reach unsafe levels.

saipem

Titolo presentazione luogo presentazione

società società

Problemi di sovradimensionamento

saipem

Excessive Seat Wear is a common result of oversizing control valves.

A valve with excess capacity may spend most of its life throttling near the seat. Sustained throttling with the plug near the seat causes high velocity flow that impinges on and around the seating surfaces. Rapid wear and premature valve failure can result.

Titolo presentazione luogo presentazione

società società

Problemi di sovradimensionamento

saipem

Safety is also a key issue; for example, if an oversized valve feeds a relief system, the relief system may have insufficient capacity to control the excess input to the relief system.

Titolo presentazione luogo presentazione

società società

Problemi di sovradimensionamento

saipem

Stable Control is another problem that is associated with oversized valves.

Process gain is typically quite high when the valve closure member operates near the seat.

The high gain can cause large changes in the process variable, which results in instability.

In addition, any friction or deadband in the valve has a pronounced effect on performance at extremely low valve lifts.

Titolo presentazione luogo presentazione

società società

Trim ridotto

saipem

A practical solution to the problem of valve over-sizing is to initially order the control valve with a body size suitable for the largest flow capacity it will need in the future, but equipped internally with reduced port (or restricted-capacity) trim. This means trim having smaller holes (“ports”) through which the fluid must flow. Such “reduced” trim is under-sized for the valve body, making the control valve’s Cv rating significantly less than it would be with normal-sized trim installed. The benefit of installing reduced-port trim in a control valve is that the flow capacity of the valve may be upgraded simply by removing the reduced trim components and replacing them with full-port (full-sized) trim. Upgrading a control valve’s trim to full-port size is significantly less expensive than replacing the entire control valve with a larger one.

Titolo presentazione luogo presentazione

società società

Trim ridotto per valvole steam-guided

saipem

Titolo presentazione luogo presentazione

società società

Trim ridotto per valvole cage-guided

saipem

Titolo presentazione luogo presentazione

società società

Limiti inferiore e superiore del grado di apertura della valvola

saipem

While it is sometimes tempting to select and size control valves for the maximum flow condition only, it is equally important to calculate Cv requirements at normal and minimum flow conditions.

Sizing for maximum flow ensures adequate capacity.

Sizing for normal flow conditions allows ensure that the valve will normally throttle in a range of travel (or percentage of maximum valve Cv) that provides good control and sufficient reserve capacity.

Sizing for minimum flow conditions allows ensure that the valve is capable of providing stable control at the low-flow condition.

Most valves are designed to provide good control down to about 10 % of rated travel.

Throttling below 10% travel can cause system instability because of the high valve gain at low lifts.

Throttling below 10% travel can cause high velocity flow that results in accelerated seat wear.

Titolo presentazione luogo presentazione

società società

Limite inferiore del grado di apertura della valvola

One should remain aware that the most common control valve problem is the oversized valve, and one should strive to use actual service conditions when sizing control valves.

Most valve manufacturers use a rule of thumb that establishes acceptable percentages of travel for the minimum, normal, and maximum flow conditions. The flow versus travel recommendations that are shown in the following table are common.

Section 5.2 of SAES-J-700 contains guidelines for the percentage of valve travel that produces the normal and maximum flow rates. The recommended percentages vary with the inherent valve characteristics as shown in the following table.

saipem

Titolo presentazione luogo presentazione

società società

Rangeability della valvola

saipem

Titolo presentazione luogo presentazione

società società

Condizione di stabilità della variabile di processo

saipem

Titolo presentazione luogo presentazione

società società

Condizione di stabilità dell’anello di regolazione

saipem

Titolo presentazione luogo presentazione

società società

La caratteristica installata della valvola di regolazione

saipem

Most real-life valve installations do not provide the control valve with a constant pressure drop. Due to frictional pressure losses in piping and changes in supply/demand pressures that vary with flow rate, a typical control valve “sees” substantial changes in differential pressure as its controlled flow rate changes. Generally speaking, the pressure drop available to the control valve decreases as flow rate increases.

Titolo presentazione luogo presentazione

società società

La caratteristica installata della valvola di regolazione

saipem

The result of this pressure drop versus flow relationship is that the actual flow rate of the same valve installed in a real process will not linearly track valve stem position. Instead, it will “droop” as the valve is further opened. This “drooping” graph is called the valve’s installed characteristic, in contrast to the inherent characteristic exhibited in the laboratory with constant pressure drop.

Titolo presentazione luogo presentazione

società società

La caratteristica installata della valvola di regolazione

saipem

Titolo presentazione luogo presentazione

società società

Comprendere il concetto di caratteristica installata

saipem

Linear

020406080

100

0 20 40 60 80 100Input

Out

put

Titolo presentazione luogo presentazione

società società

Comprendere il concetto di caratteristica installata

saipem

This tells us our control valve will respond linearly when operating with a constant pressure drop).

Titolo presentazione luogo presentazione

società società

Comprendere il concetto di caratteristica installata

saipem

Suppose the valve is not closely coupled to the dam, but rather receives water through a narrow (restrictive) pipe.

In this installation, the narrow pipe generates a flow-dependent pressure drop due to friction between the turbulent water and the pipe walls, leaving less and less upstream pressure at the valve as flow increases. The control valve still drains to atmosphere, so its downstream pressure is still a constant 0 psig, but now its upstream pressure decreases with increasing flow.

How will this affect the valve’s performance?

Titolo presentazione luogo presentazione

società società

Comprendere il concetto di caratteristica installata

saipem

A doubling of stem position (from 25% to 50%, or from 50% to 100%) no longer result in a doubling of flow rate.

Titolo presentazione luogo presentazione

società società

Comprendere il concetto di caratteristica installata

saipem

We see a definite nonlinearity in the control valve’s behavior!

Titolo presentazione luogo presentazione

società società

Comprendere il concetto di caratteristica installata

saipem

This is how the valve responds when installed in a non-ideal process, compared to the straight-line response it exhibits under ideal conditions of constant pressure. This what we mean by “installed” characteristic versus “ideal” or “inherent” characteristic.

Questo fenomeno si definisce drooping

Titolo presentazione luogo presentazione

società società

Confronto tra i guadagni della caratteristica lineare ed equipercentuale

saipem

• The control system will tend to over-react at low flow rates and under-react at high flow rates, simply because the control valve fails to exert the same degree of control over process flow at different flow rates.

• Oscillations may occur at low flow rates.

• Excessive deviations from set-point at high flow rates as a result of this “distorted” valve behavior.

Titolo presentazione luogo presentazione

società società

Cause di distorsione della caratteristica installata

Pressure losses due to fluid friction as it travels down pipe Pump characteristic curve Frictional losses in other system components such as filters

and heat exchangers.

Whatever the cause, any piping system that fails to provide constant pressure across a control valve will “distort” the valve’s inherent characteristic in the same “drooping” manner, and this must be compensated in some way if we desire linear response from the valve.

saipem

Titolo presentazione luogo presentazione

società società

Regole empiriche per la selezione della curva caratteristica

saipem

Titolo presentazione luogo presentazione

società società

Regole empiriche per la selezione della curva caratteristica

saipem

Titolo presentazione luogo presentazione

società società

Le funzioni dell’attuatore.

saipem

All actuators have several distinct purposes. They must: 1. Move the valve closure member to the desired position. Not only must the actuator provide enough torque or thrust to move the closure member under the most severe conditions, it must also be fitted with the appropriate controls to direct it. 2. Hold the valve closure member in the desired position. Particularly in throttling applications where fluids may create a dynamic torque, actuators should have adequate spring or fluid power or mechanical stiffness to overcome this phenomenon. 3. Seat the valve closure member with sufficient torque to provide the desired shutoff specification. 4. Provide a failure mode in the event of system failure. This may be fully opened, closed, or as-is depending upon the application. 5. Provide the required rotational travel (90°, 180°, etc.). 6. Provide the required operating speed. All actuators may be regulated in cycle speed depending on the control circuit elements used.

Titolo presentazione luogo presentazione

società società

Criteri di selezione dell’attuatore

saipem

1. Compatibility (Power Source) 2. Temperature range 3. Hazardous Area 4. Performance characteristics .

Titolo presentazione luogo presentazione

società società

Compatibilità

saipem

First and foremost in the selection of an actuator type (pneumatic or electric) is to determine the most effective power source for the actuator. Points to consider are: • Power source availability • Torque at the valve stem • Failure mode • Speed of operation • Frequency of operation • Plant environment • Size of valve • System component costs • System maintenance

Titolo presentazione luogo presentazione

società società

Campo di temperatura

The standard temperature range of a pneumatic actuator is from -20 to 80 °C but may be extended to -40 to 121°C with optional seals, bearings and grease. If control accessories are used (limit switches, solenoid valves etc.)

they may not have the same temperature rating as the actuator and this should be considered in all applications. In low temperature applications the quality of the supply air in

relation to dew point should be considered. Condensate may freeze and block air supply lines making the actuator inoperable. Electric actuators are available in a temperature range of -40 to

65°C. Electric actuator for external service must be hermetically sealed

and internally heated to prevent condensation. saipem

Titolo presentazione luogo presentazione

società società

Aree pericolose

It is sometimes difficult to justify the use of electric actuators in a hazardous environment, but if compressed air is not available or if a pneumatic actuator will not provide the operating characteristics required, then an electric actuator with a properly classified enclosure may be used.

saipem

Titolo presentazione luogo presentazione

società società

Caratteristiche di funzionamento

Duty cycle. Pneumatic actuators have a 100 percent duty cycle. In fact, the harder they work, the better they work.

Electric actuators are most commonly available with 25 percent duty cycle motors: this means that to prevent overheating in high cycle applications the motor must rest frequently.

With optional motors and/or capacitors an electric actuator may be upgraded to 100 percent duty cycle.

Stalling. Pneumatic actuators may be stalled indefinitely without overheating.

Electric actuators should not be stalled. Stalling an electric actuator draws excessive current, which generates heat in the motor and can cause damage. Torque switches or heat and current sensors are often installed in electric actuators to protect the device.

saipem

Titolo presentazione luogo presentazione

società società

Caratteristiche di funzionamento

Speed control. The ability to control the speed of a pneumatic actuator is an important advantage of the design. The simplest way to control the speed is to fit the actuator with a variable orifice (needle valve) at the exhaust port of the air pilot.

Since electric actuators are geared motors it is impossible to make them cycle faster unless a gearing change is made. For slower operation a pulsing circuit may be added as an option.

Modulating control. In modulating service an electric actuator interfaces well with existing electronic control systems and eliminates the need for electro-pneumatic controls. A pneumatic or electro-pneumatic positioner is used with pneumatic actuators to provide a means of controlling the valve position.

Torque-to-weight ratio. Electric actuators have a high torque-to-weight ratio above 450 Nm. Pneumatic actuators have an excellent torque-to-weight ratio below 450 Nm.

saipem

Titolo presentazione luogo presentazione

società società

Dettaglio costruttivo attuatore elettrico

Titolo presentazione luogo presentazione

società società

Attuatori – dimensionamento e selezione

saipem

Valve actuators provide force to move control valve trim. For precise positioning of a control valve, there must be a calibrated relationship between applied force and valve position. Most pneumatic actuators exploit Hooke’s Law to translate applied air pressure to valve stem position:

F = Force applied to spring k = Constant of elasticity, or “spring constant” in newtons per meter (metric) x = Displacement of spring in meters

Titolo presentazione luogo presentazione

società società

Attuatori – dimensionamento e selezione

saipem

WARNING The effective area (A) of the diaphragm upon act the pressure is variable along the stroke

Titolo presentazione luogo presentazione

società società

Attuatori – dimensionamento e selezione

saipem

• The pressure acting to the left of the horizontal tangent point can only be transmitted to the diaphragm plate through tension in the diaphragm material.

• At the horizontal tangent point, this tensile force is horizontal, as shown in figure, and therefore can contribute nothing to the up and down motion or the thrust of the actuator.

• The area outside of the point of horizontal tangency is not effective area. Thus, when we use the formula (F = PA) for the force developed by a pressure acting on an area, we must use the effective area in the calculation.

• This effective area is calculated using the diameter between the points of horizontal tangency on the convolution.

• The position of the point of horizontal tangency is variable with the stroke.

Titolo presentazione luogo presentazione

società società

Attuatori – dimensionamento e selezione

saipem

When a control valve is assembled from an actuator and a valve body, the two mechanisms must be coupled together in such a way that the valve moves between its fully closed and fully open positions with an expected range of air pressures. A common standard for pneumatic control valve actuators is 3 to 15 PSI (0.2 – 1 bar).

Multi-spring arrangement allows for 4 standard spring ranges. 3 - 15 PSI 6 - 30 PSI (0.4 – 2.0 bar) 11 - 23 PSI (0.7 – 1.6 bar) 21 - 45 PSI (1.4 – 3.1 bar)

Titolo presentazione luogo presentazione

società società

Taratura dell’attuatore

saipem

There are really only two mechanical adjustments that need to be made when coupling a pneumatic diaphragm actuator to a sliding-stem valve: the stem connector and the spring adjuster.

Steam connector

Spring adjuster

Titolo presentazione luogo presentazione

società società

Taratura dell’attuatore

saipem

If the stem connector is set with the actuator and valve stems spaced too far apart (i.e. the total stem length is too long), the actuator diaphragm will bind travel at the upper end and the valve plug will bind travel at the lower end. The result is a valve that cannot ever fully open.

Titolo presentazione luogo presentazione

società società

Taratura dell’attuatore

saipem

If the stem connector is set with the actuator and valve stems too closely coupled (i.e. the total stem length is too short), the actuator diaphragm will bind travel at the lower end and the valve plug will bind travel at the upper end. The result is a valve that cannot ever fully close.

Titolo presentazione luogo presentazione

società società

Risposta dell’attuatore pneumatico al segnale del controllore

saipem

A limitation inherent to pneumatic valve actuators is the amount of air flow required to or from the actuator to cause rapid valve motion. This is an especially acute problem in all-pneumatic control systems, where the distance separating a control valve from the controller may be substantial.

Titolo presentazione luogo presentazione

società società

Risposta dell’attuatore pneumatico al segnale del controllore

saipem

If the pneumatic valve actuator is driven by an I/P transducer instead of directly by a pneumatic controller, the problem is lessened by the ability to locate the I/P close to the actuator, thus greatly minimizing tube friction and thus minimizing the “time constant” (τ ) of the control valve’s response.

Titolo presentazione luogo presentazione

società società

Risposta dell’attuatore pneumatico al segnale del controllore

saipem

If the pneumatic actuator is particularly large in volume, an I/P transducer may experience trouble supplying the necessary air flow rate to rapidly actuate the control valve. The problem of time delay is reduced, but not eliminated, by the close-coupled location of the I/P transducer to the actuator. One way to improve valve response in either type of system (full-pneumatic or I/P-driven) is to use a device known as a volume booster to source and vent compressed air for the valve actuator.

Titolo presentazione luogo presentazione

società società

Il booster pneumatico

saipem

A “volume booster” is a pneumatic device designed to reproduce a pneumatic pressure signal (1:1 ratio), but with far greater output flow capacity. A 3 to 15 PSI pneumatic pressure signal applied to the input of a volume booster will result in an identical output signal (3 to 15 PSI), but with greatly enhanced flow capacity.

Titolo presentazione luogo presentazione

società società

Dettaglio costruttivo del booster pneumatico

saipem

• Bypass Valve • Signal Pressure • Input Diaphragm Chamber • Upper Diaphragm • Diaphragm Plate • Exhaust Seat • Seal Plate Orifice • Plug • Inlet Port • Lower Diaph. Assembly • Plug Return Spring • Plug Retaining End-Cap

Supply Cylinder

Pilot

Exhaust

Titolo presentazione luogo presentazione

società società

Come funziona il booster

saipem

• Without pilot pressure, Actuator flows to Exhaust through the diaphragm seal plate orifice.

• Pilot signal pressure actuates the upper diaphragm which closes Actuator to Exhaust when Pilot pressure and Actuator pressure are in equilibrium.

• When Pilot pressure is greater than Actuator pressure, diaphragm assembly pushes downwards and opens plug.

• Allowing pressure Air Supply to Actuator flow.

• Pilot signal pressure when decreased at Actuator pressure, closes plug (balanced) then opens exhaust plug.

• Needle valve adjusts sensitivity level Air

Supply Actuator

Pilot

Exhaust

Titolo presentazione luogo presentazione

società società

Normative UNI-EN per gli attuatori

saipem

UNI EN 15714-1 - Industrial Valves – Actuators - Part 1: Terminology and definitions. UNI EN 15714-2 - Industrial Valves – Actuators - Part 2:

Electric actuators for industrial valves - Basic requirements UNI EN 15714-3 – Industrial Valves – Actuators - Part 3:

Pneumatic part-turn actuators for industrial valves - Basic requirements UNI EN 15714-4 - Industrial Valves – Actuators - Part 4:

Hydraulic part-turn actuators for industrial valves - Basic requirements.

Le norme prevedono i requisiti di base per gli attuatori per valvole industriali utilizzati come on-off e per regolazione. Esse includono linee guida per la classificazione, la progettazione, la protezione dalla corrosione e metodi per la valutazione della conformità.

Titolo presentazione luogo presentazione

società società

Termini e definizioni per attuatori PN ed EH

saipem

Titolo presentazione luogo presentazione

società società

Termini e definizioni per attuatori EL

saipem

Titolo presentazione luogo presentazione

società società saipem