Diaphragm Control Valve

Control Valve – A valve that uses a signal, electronic or pneumatic, to adjust its opening. Failures, FO, FC - Fail Open or Closed ATO - Air To Open ATC - Air To Close Two Basic Designs: Sliding Stem, globe gate, needle etc. Rotating Stem, Ball, Plug, Butterfly, Disk, etc

Valve Trim Flow Characteristics, Throttle Plug and V-Port are older designs. Equal Percentage is the most common. Used to compensate for the flow losses in a pipeline. Rangebility (Paul Wing “Control-Valve Characteristics” from Process / Industrial Instruments and Controls Handbook, 1957 ) The term rangeability has been used for many years to describe the flow range of a control valve. It is more commonly used in a loose way to indicate a particular equal-percentage characteristic Such as 50 to 1. Rangeability of a control valve is often defined as the ratio of maximum to minimum controllable flow. The definition is unusual in that neither maximum nor minimum controllable flow can be accurately determined. A control valve with a soft seat, installed on an open- and-shut application, could be said to have a controllable flow range from zero to maximum and, therefore, according to the definition, infinite rangeability, This obviously is not the intent, but without proper qualification all control valves would have a rangeability limited only by the leakage flow in the closed position. A better definition of the rangeability of a control valve might be the flow range through which a particular inherent characteristic is maintained within prescribed limits. The factors forming this possible definition now can be described briefly. Positioner (Bert J. Peterson “Process Control Valves” from Process / Industrial Instruments and Controls Handbook, 1999) One aspect of flow characteristic is its rangeability, which is the ratio of maximum and minimum controllable flow rates. Exceptionally wide rangeability might be required for certain applications to handle wide load swings or a combination of start-up, normal, and maximum working conditions. Rotary valves, especially partial ball valves, normally have greater rangeability than sliding-stem varieties.

Valve Sizing Equations – Very Vendor Dependant

Research Control Valve Sizing Equations

FISHER SIZING EQUATIONS Cv is the liquid sizing coefficient. Fisher uses Cg for gas, steam or vapor sizing. Also Define C1 = Cg / Cv Liquid Sizing Equation:

PG

QCv ∆=

Q in GPM; G is Specific Gravity; ∆P is the pressure difference in psid.

radian

ghrlb PP

CSINCPdQ

∆

=

1111/

64.5906.1

d1 = Density in lb/ft^3; P1 inlet Pressure psia

For a reduced size valve installed in a larger line size, the total Cv is reduced by:

d Valve diameter D Pipe diameter Reference: Ward & Sheldon; Fisher Controls ISA FCE 1973 Older equations will produce the same answer as the newer ones, easier to understand.

22

22

___*_

_fittingCvvalveCvfittingCvvalveCv

TotalCv+

=

−= 2

2

1*50.0Dd

Kinlet

−= 2

2

1*0.1Dd

Koutlet

outletinletTotal KKK +=

TotalKd

fittingCv2*9.29

_ =

Masoneilan “Camflex” Rotary Design

USES Good for Slurry Service Can Use for Steam Valves

Low Cost in Alloy Construction Use Flangeless if Possible. Watch for Pipe Clearance if piping at a Tee or with a reduced size or lined pipe!

Masoneilan Sizing Equations

From: L. Driskill, “Control Valve Sizing with ISA Formulas”; Instrumentation Technology; July, 1974 Cavitation and Flashing problems When a Cv test is made, great care is exercised to be sure that the liquid remains a liquid from one pres- sure tap to the other. If any vaporization occurs in the valve, the flow rate is less than would otherwise exist; Standard S39.2 provides a means of predicting the flow under such circumstances. Before proceeding, an explanation of the vaporization phenomenon is in order. Assuming a fixed up- stream pressure and temperature, the flow rate through a valve will increase as the square root of the differential pressure-until a point is reached where the pressure at the valve orifice is so low that the liquid begins to vaporize. Tiny cavities of vapor form at this point and then promptly collapse as the stream expands to fill the larger area beyond, where the static pressure begins to rise again as the fluid slows down. The formation and collapse of these bubbles is known as cavitation. The collapsing of very small bubbles at the inception of cavitations causes a hissing sound. As the downstream pressure is further decreased, the bubbles grow larger and noise increases to the point where it sounds like gravel passing through the line. When this happens mechanical damage and vibration are almost sure to occur. If the downstream pressure is reduced to where it is less than the vapor pressure of the entering liquid, the bubbles persist in the stream, and both vapor and liquid will exist at the valve out- let. This condition is known as flashing. Although some materials last longer than others, there is no known engineering material that can withstand the ravages of cavitations for a reasonable period of time. The destructive power of this process is quite dramatic. Cavitation, like the boiling of a liquid, is an example of the phenomenon of nucleation where every nucleus has a critical size above which the new phase grows spontaneously. Below this critical size, an inordinate amount of energy must be supplied to over- come the large force of surface

tension in the tiny bubble. When the static pressure of a liquid is suddenly reduced to a value less than the vapor pressure of the liquid, bubbles begin to grow at nucleation sites in the superheated liquid. If the pressure is reduced sufficiently, the bubbles grow to exceed the critical size, and once across this energy barrier their rate of growth is greatly accelerated. As the stream passes beyond the vena contracts, the pressure rises, and may again become greater than the liquid vapor pressure. With this increase in pressure, the process is reversed and the bubbles become smaller. Once a bubble is smaller than the critical size, it collapses spontaneously, giving up a relatively, large amount of energy. The effect is greatly intensified by the fact that the bubble does not collapse symmetrically, but tends to become a flattened sphere, which in many cases forms a torus that collapses with a central jet. When this jet impinges upon a solid surface, it acts like a shaped charge, concentrating the release of energy on an exceedingly small area. This is believed to be the primary damaging mechanism in cavitating flows. When a liquid flashes, the bubbles do not collapse; however, the two-phase stream occupies a much greater volume and consequently travels at a much greater velocity than the liquid which entered the valve. This high velocity stream causes a scrubbing action which can erode certain valve materials and downstream piping. Cavitation and flashing affect the flow through a valve as shown in the figure below. Again, with fixed up-stream conditions and increasing pressure drop, cavitation begins at some location of high local velocity, then extends about the orifice, choking off more and more of the passage until the condition known as choked flow is attained. Increasing the differential pressure will not increase the flow because all parts of the valve orifice are cavitating, and still lower down- stream pressures will only serve to increase the vaporization and volume of the bubbles.

Special problems involving gas in the case of liquids, cavitation, flashing, laminar and transitional flow are special conditions which occur frequently. Compressible fluids also encounter special conditions of flow, but the occurrence is much less common, and the new standards make no provision for them. Low Reynolds number in gas flow is one situation that has already been discussed. Let us now consider several others. When a gas passes through a restriction such as a valve orifice, the flow is adiabatic (there is no los's or gain of heat). The overall process is also essentially isenthalpic, that is, the unit energy content of the outlet fluid is the same as that of the inlet. However, it should be noted that the flow process from the valve inlet to the vena contracts is not isenthalpic but approaches the isentropic state. The effect which this entire throttling process has upon the temperature and state of the fluid depends upon the thermo- dynamic properties of the particular fluid.

Auto-refrigeration and condensation If the fluid is a dry gas, the outlet temperature may be considerably lower than the inlet temperature, so much so that the low temperature rating of the pipe may he exceeded. In addition, a still lower temperature may exist at the vena contracts because the flow to this point is isentropic. Parts of the valve trim may thus be exposed to extremely low temperatures. Figure 5 shows the conditions which existed in an actual application where carbon monoxide gas was let down from 1,000 psig to atmospheric pressure. The problem was solved by heating the gas at the inlet when the temperature approached the limiting condition. With some fluids, expansion through a valve may create the possibility of condensation in the valve body. Figure 6 shows a situation involving ethylene which not only produces very low temperatures, but creates the possibility of condensation around the vena contracts. High pressure letdown of ethylene frequently requires multistage pressure reduction with interstage reheating. Steam-wet, saturated or superheated-may con- dense as it passes through the valve, forming water droplets which travel at velocities exceeding 1,600 ft/s. The valve material must resist the scrubbing action of these high velocity droplets. Figure 7, a Mollier diagram, shows a probable path which steam takes as it travels through a single-stage valve. Some of these special conditions which occur inside a valve body can be mitigated by the use of valves which have a high permanent pressure loss (high FL factor), or multistage or labyrinth type valves. But the conditions downstream of the valve are not affected by the valve style, so that problems in the downstream piping resulting from low temperatures or change of state cannot be improved by valve selection.

Glossary Bernoulli coefficient-If the area of a stream is changed, as by a reducer, there is a corresponding change in the static pressure, or "head." This pressure change is measured in units of "velocity head." The dimensionless number used for this purpose is the Bernoulli coefficient Ka . Chokedflow-This condition exists when, with a constant upstream pressure, the flow through a valve cannot be further increased by lowering the downstream pressure. Coefficient of discharge-This ratio of actual flow to theoretical flow includes the effects of jet contraction and turbulence. Compressibility factor-This factor compensates for deviation from the laws of perfect gases. If the gas laws are used to compute the specific weight of a gas, the computed value must be adjusted by the compressibility factor Z to obtain the true specific weight. Critical flow-This somewhat ambiguous term signifies a point where the characteristics of flow suffer a finite change. In the case of a liquid, critical flow could mean the point where the flow regime changes from larninar to transitional. It is more often used to mean choked flow. In the case of a gas, critical flow may mean the point where the velocity at the vena contracts attains the velocity of sound, or it may mean the point where the flow is fully choked. Laminar flow-Also known as viscous or streamline flow, this non-turbulent flow regime in which the stream filaments glide along the pipe axiallv with essentially no transverse mixing. This condition occurs at low Reynolds numbers, is usually associated with vi scous liquids, and rarely occurs with gas flows in valves. Flow rate varies linearly with AP. Reynolds number-This dimensionless criterion of the nature of flow in pipes is the ratio of dynamic forces to viscous forces: The product of diameter, velocity and density, divided by absolute viscosity.

Specific heat-The ratio of the thermal capacity of a sub- stance to that of water is called "specific heat." The specific beat at constant pressure of a gas is designated Cp, and the specific heat at constant volume of a gas is designated Cv the ratio (Cp/Cv, = k) is called the ratio of specific heats. Streamline flow-See " laminar flow." Transitional flow-This flow regime lies between turbulent flow and laminar flow. Turbulent flow-This flow regime, characterized by random motion of the fluid particles in the transverse direction as well as motion in the axial direction, occurs at high Reynolds numbers and is the type of flow most common in industrial fluid systems. Flow varies as the square root of AP. Velocity of approach-This factor F is determined by the ratio m of the valve orifice area to the inlet pipe area. Velocity head-This is the pressure, measured in height of fluid column, needed to create a fluid velocity. Numerically, velocity head is the square of the velocity divided by twice the acceleration of gravity (U2/2g). Vena contracta-The place along the axis of flow, just be- yond the orifice, where the jet stream contracts to its minimum cross-sectional area. Viscous flow-See " laminar flow." Valve Positioner (Bert J. Peterson “Process Control Valves” from Process / Industrial Instruments and Controls Handbook ) Positioners are instruments that help improve control by accurately positioning a control valve actuator in response to a control signal. Positioners receive an input signal either pneumatically or electronically and provide output power, generally pneumatically, to an actuator to assure valve positioning. A feedback linkage between valve stem and positioner is established so that the stem position can be noted by the instrument and compared with the position dictated by the controller signal (Fig. 40). Use of positioners is generally desirable to linearize the control valve plug position with a control signal. Positioners will often improve the performance of control valve systems. There are situations, however, where process dynamics eliminate the use of positioners. On very fast loops it has been found that the use of positioners will degrade performance because the response of the positioner might not be able to keep up with the system in which it is installed. Positioners operate with a pneumatic input and output signal or with an electronic input signal and pneumatic output. Some of the electronic versions accept an analog input signal, and others accept a digital input signal.

Electropneumatic positioners combine the function of a current-to-pressure transducer with those of a positioner. It receives an electronic input signal and ensures valve position by adjusting output pressure. (Fisher Controls International, Inc.)

Volume Booster: Used to increase the stroking speed. These are pneumatic device that has a separate air supply to deliver higher volume of air to move the actuator faster. A Positioner is actually a feedback controller in itself. Frequently fast flow loops can interact with the dynamics of the positioner. Do not confuse a positioner with a volume booster! A Positioner will control the valve position while a volume booster will just provide more air supply to the actuator.