Heavy Duty Control Valve

Heavy Duty Control Valves

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By H. L. Miller

22591 Avenida Empresa Rancho Santa Margarita, CA 92688 949.858.1877 w Fax 949.858.1878 w ccivalve.com 229

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07/00 w ©2000 CCI w DRAG is a registered trademark of CCI.

Heavy Duty Control Valves n By Herbert L. Miller, Vice President CCI Abstract

T

he definition of a heavy duty control valve is examined through the undesirable effects of the process medium

on a valve improperly applied to the service. A solution for

Velocity Control The problems mentioned above are best solved by controlling the cause of the damage. Many attempts throughout the industry are made to resolve the problems by addressing the effects of the damage. For example, the use of harder materials when erosion and cavitation damage occur. These solutions only marginally prolong the time at which failure will take place. The cause of the undesirable effects is excessive fluid velocity

these undesirable effects is to control the velocity of the

that occurs as the pressure reduction takes place in the control

fluid as its pressure is reduced across the valve. A means

valve. The velocity must be controlled for all valve settings.

of velocity control is presented and typical applications and

One of the first methods of controlling velocity was to place

examples are discussed.

a cage around the plug. This transferred the higher velocites from the plug/seat interface to multiple orifices in the cage.

Introduction

As higher pressure drop was encountered multiple, concentric cages were added. The principle of operation is still to achieve

The use of the label “heavy duty control valves” results in

pressure reduction in steps by contraction through an orifice

many different visions and definitons for the valves to be

without smooth downstream recovery. A superior method is

discussed. Each of these visions is generally dictated by an

to continously control the velocity by dividing the fluid stream

experience with a troublesome valve application.

into multiple but discrete flow passages and to dissipate the

An understanding of why the valve selected is not performing satisfactorily is not always known and, in many

energy continously via turbulence. A multiple discrete passage method is shown by Figure 1. Each

cases, is accepted as the norm. Although each industry

disk has many parallel flow paths and many disks are stacked

may have a different definition of a heavy and severe duty

to form the valve trim. Each disk’s flow passages are opened as

valve, there are some generaliztions that can be made. These

a plug moves across the opening in the center of the disk stack.

are valves that are characterized by erratic control, noise,

Each path is made tortuous by forcing the fluid to make right-

mechanical vibration, cavitiation, erosion, and short life.

angle turns. The pressure drop thus is achieved by a reduction

When the above conditions exist, the pressure ratio across the valve is usally greater than three, that is, the absolute inlet pressure exeeds three times the downstream pressure. The pressure level is frequently greater than 70 Kg/cm 2 (1,000 psi), although there are many cases in which valve life is shortened due to internal damage at lower pressures. Temperature of the flowing fluid is also an important variable

of a velocity head, ρv2/2g, for each right-angle turn. Test results indicate a higher multiple than one velocity head is achieved for each turn. The fluid velocity through each of the flow paths is controlled by the number of paths selected. Additional velocity control can be achieved by varying the flow area within each flow path. This is illustrated in Figure 2, where the outlet area, A2, is greater than the inlet area, A1, with a continually increasing flow area between inlet and outlet.

in the valve’s duty. If the flowing fluid is liquid, it is important to consider the fluid’s temperature and whether the pressure drop within the valve passes through the vapor pressure of the fluid. When there is sufficient heat in the fluid, a vapor change can take place leading to internal valve damage via cavitation and/or erosion. In additon to the fluid phase change problem, absolute temperature level can have signficant influence on the design. The influence is associated with material strength and expansion characteristics with temperature. Materials and design must be such that expansion does not over-stress components and particular attention must be paid to expansion of parts during thermal transients. As a rule, temperature above 300C (600F) require accomodations in the design to minimize thermal expansion effects.

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Heavy Duty Control Valves | 229

Figure 1—Punched Disk Stack

©2000 CCI. All rights reserved.

Any valve using multiple orifice trim will cause this problem, due to the uncontrolled velocities in the area of each vena contracta. As the fluid moves out of the throat of the valve, pressure recovery begins as kinetic energy is converted back to potential. Full recovery to downstream pressure is indicated at P3 and velocity at V3. When the recovery pressure exceeds the fluid’s vapor pressure, Pv, collapse or implosion of the just-formed bubbles takes place, resulting in cavitation damage. The energy released here causes local surface stresses greater than 700 MPa (100,000 psi) which can consume even stellited valve trim rapidly. Figure 2

VALVE

The damaging effects of velocity are thus controlled in two ways. The first, by dividng the flow into many small streams of low mass flow rate and secondly, by forcing the fluid through a series

Inlet Pressure

Inlet

P1

Outlet

VVC

Pressure

vc = Vena Contracta

of right-angle turns to effect the pressure drop steps. The parallel disks form a cylindrical valve trim, which is placed in a valve body as shown in Figure 3. Control of the flow rate is achieved by the up and down movement of the plug across each

Inlet Velocity

V2 Outlet Velocity

V1 Velocity

of the disk openings. Externally, the valve appearance is essentially

P 2 Outlet Pressure

PV Flashpoint

the same as a conventional globe or angle type control valve.

Cavitation P VC

Figure 4—Conventional Control Element

An approach widely used in the industry concerns itself only with the flow rate. Simply stated, the principle is that if the valve flow rate is not significantly reduced by the bubble volume entrained in the fluid, the cavitation potential that exists is not damaging. A more thorough approach is to control the velocity through the pressure reduction. Using the velocity control technology, the pressure is reduced in a large number of steps. The steps are so numerous that the energy of velocity is dissipated through turbulence at the same rate the pressure energy is being converted into velocity. Therefore, the velocity can be maintained virtually constant at each step if Figure 3

Velocity Control Examples

desired. The local pressure can also be maintained above the vapor pressure of the fluid so that bubbles are not formed during the pressure reduction across the valve. This is shown by Figure 5.

A number of examples in which velocity control is essential are

VALVE

presented. The first example involves the cavitating fluid situation. Cavitation is the process of formation and subsequent collapse of microscopic vapor bubbles in a flowing fluid. In order for cavitation to occur, the fluid must pass through a low-pressure region, then

Inlet Inlet Pressure

P1

Inlet Velocity

V1

Outlet

Pressure

into a region of higher pressure. In control valves, the low-pressure region is caused by local high velocities, usually in the vena contracta immediately downstream of the seating area or port area. This process, for a conventional valve design, is shown in Figure 4. Fluid enters at pressure P1 and velocity V1. As the fluid moves through the reduced area of the valve trim, it accelerates to velocity V2 and its static pressure drops suddenly to P2, a level at or below the fluid’s vapor pressure Pv. At this point, the fluid boils. ©2000 CCI. All rights reserved.

Velocity

Pv

V2

Outlet Velocity

P2

Outlet Pressure

Flashpoint

Figure 5—Velocity Control Element

229 | Heavy Duty Control Valves

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Because of the ability to completely control the velocity, it is

of excessive fluid velocities acting either perpendicularly or axially on

practical to let the velocity increase by using a constant area

the plug. By controlling these velocities to acceptable levels during the

passage when the pressure is considerably above the vapor

pressure reduction and throttling function, the vibrations and erratic

pressure. Then, as the pressure reduces, convert to an expanding

control that may result are eliminated.

passage so that the velocity will drop and the local pressure be maintained comfortably above the vapor formation pressure. Another example of the need to control velocity is for cases in which erosion damage occurs as a result of liquid droplets

From the above discussion, it is seen that, if needed, almost any velocity can be achieved with the multi-path turns concept. Variables that can be varied are: 1.

Number of passages per disk.

be avoided by maintaining the expansion of the fluid above the

2.

Number of disks.

saturation curve as shown in Figure 6. The low pressure and high

3.

Flow area of each passage.

trim, allows the fluid to expand polytropically from P1 to P3. At P3,

4.

Amount of expansion of the flow passage.

with velocity at its peak, droplets are formed which will rapidly

5.

Number of turns per flow passage.

entrained in the fluid vapor. The formation of the liquid can

velocity inherent in flow through a conventional or multi-orificed

erode valve trim and cause impingement damage to the valve body. Pressure recovery is completed in the valve outlet at P2 with the fluid in a superheated phase. In contrast, velocity control elements operating at a low velocity allow the fluid to expand

6.

The resistance, flow impedance, across each disk may change with stroke so as to characterize the flow versus stroke performance of the valve.

in an isenthalpic manner. Thus the moisture droplets are not

With such a side range of variability, designs could be made that

formed during the throttling across the velocity controlling trim.

would achieve velocities that exactly follow the specific volume expansion of a fluid as the pressure decreased. Control Sensitivity Most of the above discussion has dealt with the problems that result in physical damage to the valve hardware. There are cases in which the control valve performs without noise and vibration, but still has a significant impact on the processs. This would occur where the process requires a very fine control of the flow rate, or pressure, and small deviations are magnified. One solution is to solve the problem with multiple valves in which one valve, the smallest, acts as a trimming control in parallel with larger valves which carry the major load changes. The principle of operation in this case is that a relatively large change in the position of the small valve plug results in only a small change in the combined affect of all the parallel valves. As control systems improve in their ability to provide an accurate and discrete signal, the possibility exists that the multi-valve approach may not be required. Much depends upon the ability of the valve actuation and position system. Even though these systems have improved over the years, special care must be exercised to achieve a 1% resolution of the valve stem position. In most cases a 2% resolution of position results, which is many times larger than the accuracy of the control signal.

Figure 6

In some valve applications, it is necessary to maintain very low

With the velocity control valve concept it is still possible to be able to do the job with one valve for many applications. Thus the

velocities. Such an example would be a pump test loop application,

process control system would not need the extra control logic nor

where it is very important that the control valve not impose

extra valves to utilize a multi-parallel valve system. The approach

vibration and control instability prints upon the pump characteristics

is achieved with a highly characterized trim whereby, at certain

being measured. Vibrations/transitions are caused and maintained by

positions of the valve stroke, a 1% change in stroke would result

unbalanced forces acting on the valve plug. These forces are the result

in less than a 1% change in the flow rate.

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Heavy Duty Control Valves | 229

©2000 CCI. All rights reserved.

To illustrate the above principle of characterization, consider a valve in which control of the process pressure upstream of the valve is required. Downstream of the valve the pressure is essentially constant because a large receiver, tank or atmosphere, is used for discharge. Then any change in valve position will change the upstream pressure and flow rate throughout the valve. Such an application is quite common, especially in process start-up systems. The magnitude of the pressure swings can be estimated by using small perturbaton theory on the equation for the valve CV. For the incompressible flow situation this results in:

1 Figure 7

or

Since the downstream pressure is constant, the above values of dP 2

represent the instantaneous swing in process pressure for the two valves. If the valve resolution is 2%,the above values would double. The potential for oscillation of system pressure then exists as the control system attempts to correct the pressure causing in the linear

The above equation says that for a small change in the CV of the valve, a corresponding change in flow and pressure drop will

valve trim, a swing of + 12 Kg/cm2 (+ 173 psi). This value is

follow. For the case in which the flow is constant, the above

significantly reduced by characterizing the valve trim.

equation reduces to:

The characterization shown by Figure 7 has three unique flow resistant regions. In each of these regions, the disk used to form 3

the trim would be different. Primarily in the number of rightangle turns and flow passages per disk. Via characterization, it

A constant flow would result for an upstream pump running at

is possible to significantly amplify the resolution accuracy of the

constant speed or a flow rate delay results as the mechancial

actuator and positioner controls.

pumping equipment tries to respond.

Even if a trim is not characterized, the approach of many flow

The percentage change in CV is related to the valve stroke from the manufacturer’s valve curve as shown on Figure 7. From Figure 7, for a 1% change in stroke with a linear trim, a change of 2.6 occurs in CV. For the characterized trim, an 0.8 change in CV results for the range of CV from 40 to 65. For the valves operating at a CV fo 60, a system pressure of 175 kg/cm (2,500 psi), and 2

a downstream tank pressure of 35 kg/cm2 (500 psi) the impact from equation 3 of a 1% change in stroke is:

can be made, but deviations from linear exist. These deviations are due to flow distribution patterns in the valve which are not ideal, as well as manufacturing compromises and economic tradeoffs. In most cases, these deviations do not have a noticable impact on the process control, however, when they are significant, changes in the control system is not always a fix and valve replacement is required.

FIGURE 7 CURVE Characterized

Linear

% Stroke at CV = 60

45

22

δ CV/CV

0.8/60

2.6/60

%dP

2.67

8.67

dP, Kg/cm2

3.7

12.2

dP, psi

53

173

©2000 CCI. All rights reserved.

disks results in a straight flow versus stroke curve. In conventional valves, whether there is a ported cage or not, a linear valve design

Aerodynamic Noise The most signficant noise in control valves is aerodynamic. If a valve is noisy, it generally identifies major problems that will result in shortened life and high maintenance costs. In addition to the valve damage that results, consideration must be given to plant operating personnel, who work in the noisy field environment. In many countries, the noise design constraint is imposed by law.

229 | Heavy Duty Control Valves

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The aerodynamic noise generated in a gas or steam valve is the

The most logical and economical way of dealing with control valve

noise associated with moving of turbulent compressible fluid and

noise is to treat it at the source. This is accomplished through

interaction of the moving compressible fluids with solid boundaries.

two mechanisms: frequency shift and velocity control. Small passage design greatly increases the frequency of the noise over that found in

At subsonic flow speeds, noise is created by one or more of

conventional or large passage designs. It has been found through testing

three basic types of aerodynamic sources: monopole, dipole and quadrupole. The aerodynamic monopole is like a pulsating sphere where the acoustic pressure waves are always in phase. Typical monopole sources are pulse jets, sirens, or propellers at zero pitch. The dimensional relation of radiated sound power and the parameters that product it are:

that the greatest portion of the noise energy generated in a disk stack is well above the frequency range sensitive to human hearing. This high-frequency noise is easily attenuated by the pipe wall and does not contribute to the “A” weighted total sound level. Velocity is another major contributing factor to noise generation in a valve as is clearly seen in the sound power and velocity relationships disccussed earlier. For aerodynamic dipole and quadrupole, which are the major noise

4 Note from the above relationship that the radiated sound power varies as the fourth power of velocity.

generating mechanisms in control valves, the sound power increases as the sixth to eight power of velocity. Therefore, in dealing with control valve noise the most effective way is velocity control. The velocity control trims can be custom designed to meet the noise requirement

The aerodynamic dipole is two monpoles pulsing 180 degrees

at virtually any pressure ratio and flow capacity. And, since the fluid

out of phase with each other. This type of radiation occurs when

velocity is maintained well below sonic at all points in the trim, there is

a fluid interacts with a solid body to produce unsteady forces.

no need for any downstream diffusers, insulations, or silencers.

Typical of dipole sources are compressors, fluid flowing past grids TYPICAL APPLICATIONS

or rods, flow through valves, and flow in pipes or ducts. The dimensional dependence on radiated sound power is:

As discussed at the beginning of the paper, a definition of heavy 5

Note from the above relationship that the radiated sound power

duty control valves is best recognized from the problems encountered. Parameters to be considered for predicting that a special valve is needed were absolute pressure level, pressure ratio, noise requirements, temperature level and control. These parameters are present in one form

varies as the sixth power of velocity.

or another in the many applications that are listed in Table 1. New

The aerodynamic quadrupole is two dipoles pulsing in opposing

applications arise as designers increase pressures and temperatures in

pairs. This type of radiation is typical of high speed jets mixing with still media in the absence of obstacles. The dimensional dependence on radiated sound power is:

their processes to increase efficiency or to commercialize new processes. In the latter case there will be much activity in the future in coal gasification/liquification and heavy crude refining processes, as we learn to expand our known energy resources.

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NOMENCLATURE

Note from the above relationship that the radiated sound power

ρ Density of Fluid

varies as the eighth power of velocity.

V Velocity of Fluid

At sonic speed flow, or choked flow, the noise generation is based

g Gravitational Contact

on two noise producing mechanisms. The first mechanism is the turbulent mixing process downstream from the device. The mechanism is associated with quadrupole radiation. The second

A Flow Area P Pressure

mechanism results from the interaction between the turbulence

W Flow

and the complex flow field that forms downstream from the

Wam

Radiated Sound Power, Monopole

Wad

Radiated Sound Power, Dipole

less than three across the conventional valves, both mechanisms

Waq

Radiated Sound Power Quadrupole

must be considered. With pressure ratios greater than three, the

Cv Valve Sizing Coefficient

device. This mechanism is called “shock noise” and is not associated with the three basic source types. For pressure ratios

shock noise mechanism predominates and the turbulent mixing mechanism may be neglected. In valves with velocity control trim, sonic conditions and their related noise mechanisms are avoided.

dPPressure drop δ Change C Speed of Sound in Fluid

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Heavy Duty Control Valves | 229

©2000 CCI. All rights reserved.

TABLE 1

Velocity Control Applications Typical Power Plant Applications • Feedwater Regulation - Full Range • Pump Recirculation - Minimum Flow Systems • Reheat, Attemperator and Desuperheater Spray • High Pressure/Temperature Bypass • Steam Vents • Start-Up System Steam Valves • Auxiliary Steam Control --------

Process Applications • Compressor Anti-Surge (Kick Back) • Process Letdown • Process Bypass • Process Vents • Level Control • Flare Header • Steam Valves • Water Valves --------

Oil and Gas Production/Separation • Gas and Surge Drum Vent to Flare • Gas and Reinjection Compressor Anti-Surge • Flow and Pressure Control-Separators to Compressors • Separator Level Control • Turbo Expander Bypass • H.P. Scrubber - Letdown and Vent • Injection Pump and Mol Pump - Discharge and Recirculation • Well Head Choke

©2000 CCI. All rights reserved.

229 | Heavy Duty Control Valves

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