How to Justify

Special
Control Valves
...................................................
By R. E. Self
From Control Engineering, August 1973

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 Noise and vibration sources
How to Justify
Vapor and flashing liquid service—The only significant vapor
Special Control Valves piping system noise is aerodynamic. This noise, whether in
control valves, block valves, relief valves, or piping sections, is
" By R. E. Self, CCI (Control Components Inc.)
Reprinted from Control Engineering, August 1973 attributable solely to shearing action between a high speed fluid
©1973 Dun-Donnelly Publishing Corporation. All rights reserved. jet or flow core and the slower or stagnated fluid surrounding it.
In some references, the influence of velocity on noise runs as high

T he toughest system requirements on noise, vibration,

erratic control, cavitation, and long life, including OSHA,
can be satisfied by using standard control valves with various
as the eighth order of velocity.

Liquid service—Except for very rare cases (very high velocity

muffling, silencing, and vibration and cavitation control piping) the objectionable noise generated in a liquid control
techniques. Or they can be satisfied by using special control valve or piping system is attributable only to cavitation. The
valves that seek to eliminate the sources of these problems, but phenomenon of cavitation is quite simple, but unfortunately
are more expensive than standard valves. The engineer’s job is generally unappreciated. The author feels that the noise in liquid
to know when the special valves are less costly than standard valves is secondary to the real problem, that of cavitation damage.
valves with special treatment. This article tells you. Vibration sources, all services—Vibrations are excited and
Pressures and throughput levels in typical process systems have maintained by a system of forces seeking some balance, but for
grown to where traditional methods of noise reduction are no a variety of reasons never quite attaining synchronization for a
longer economically effective or acceptable. steady state. In the case of control valves, the principal forces
seeking balance are almost always hydrodynamic forces acting on
The mass flow to be controlled by a single valve in many process
the plug. These forces can act either perpendicularly or axially on
systems today almost boggles the mind—flows of over 500,000
the plug, and are the result of excessive fluid velocity impinging
lbs. per hr. are now common. High mass flows such as these, in
on and thus deflecting the plug. This deflection alters the existing
concert with even the slightest excitation, can induce or excite
area-flow relationship which in turn upsets the force balance,
vibrations that can lead to rapid and cataclysmic failure of the
causing the plug to move seeking a new balance. The vibration
valve, the piping system and the related instrumentation.
results from the oscillating plug mass, or from the rapid rise
The problems of cavitation and trim erosion in process control and fall in pressures associated with the rapidly fluctuating flow
valves are not new by any historic review. What is new, however, developed by the plug motion.
is the process industry’s reliance on the single large component
Erratic valve control, all services—Erratic valve control,
and its reliability. This dependence follows from the complex
assuming compatible controllers and correct sizing and system
control problems associated with the use of parallel or redundant
gain, is caused by hydrodynamic forces similar to those which
systems and components.
excite vibrations. The typical high fluid velocities in standard

Left—velocity profile in a typical

process piping system. Right—
logarithmic summation of noise
sources in typical system. No
reduction attempted, traditional
design method.

2 How to Justify Special Control Valves | 226 ©2000 CCI. All rights reserved.
control valves cause plug instability or result in excessive or
unrepeatable flow gain (flow vs. stroke), or even cause a fluid
phase change, which can drastically and instantaneously alter the
flow gain.

For undefined reasons cavitation has not been thoroughly

One approach to noise
understood by all instrument or process control engineers. As reduction in special
a result, many standard control valves are misapplied simply control valves is to turn
because total valve operating conditions and characteristics are fluid jets so they impinge
on one another.
not considered during equipment selection. Cavitation still occurs
due to high velocities.
The phenomenon is quite simple. In any conventional control
valve, the fluid is accelerated to a high velocity at the valve throat
due to the differential pressure across the valve. This increase in
velocity must result in a loss of fluid static pressure because of the
conservation of energy. If this loss in static pressure is greater than
the difference between the upstream pressure and the fluid vapor
pressure, then the fluid flashes into vapor. As the high velocity
and lower pressure vapor moves through the valve and mixes
with the surrounding liquid, the fluid slows down and again
recovers static pressure. When the static pressure recovers to
Another approach
or above the vapor pressure, the vapor condenses violently, or subdivides flow area
implodes, with enormous destructive capability. to reduce apparent
noise levels.
Noise and cavitation prediction

During the past several years, many control valve manufacturers

have aggressively pursued and developed quantified predictive 4. Prediction of noise for relief or vent valves:
techniques which have proved quite accurate for system design SPL = SPL10ft + ΔSPL valve size
and analysis, problem identification, and valve design. More + ΔSPL sg + ΔSPL distance
important, these techniques assure that a system and its (See Burgess-Manning Silencing Handbook, 1968 edition)
components can be designed before construction to conform to 5. Prediction of noise for standard liquid valve:
specifications. Typical techniques are: SPL =10 log (C v Cf )
+ 8 log (P2crit – P2)
1. Prediction of noise for standard valves:
+20 log (P2 – P v)+ 33
SPLdBa = SPL ΔP + SPLcq
(See Masoneilan Bulletin 340E)
+ ΔSPL ΔP/P1 + ΔSPLk
(See Fisher Tech Manual 24) 6. Noise reverberation or reflection:
SPL = PWL – 10 log (α) + 16.6
2. Prediction of noise for standard valves:
(See ISA Paper 3-4-210: “Control Valve Noise”)
SPLdBa = 10 log (Xη 10” CvC4P1P2 ) – TL + Sg
(See Masoneilan 340E) 7. Cavitation prediction:
ΔPcritical = Cf2 (P1 – P v)
3. Prediction of noise for closed piping (Assumes design
No cavitation at ΔPactual < ΔPcritical
and use of DRAG trim valves compatible with piping
system noise level): 8. Adding levels together:
SPL dBa = 40 log W – 88 (See Burgess-Manning Silencing Handbook)
- 20 log (R/100)
Qualified prediction techniques
- 70 log d – 10 log (R/3)
Two of the problems discussed, vibration and erratic control,
- 1,880 d2 (p/W) 0.68 – TL
unfortunately remain beyond any quantifying analysis
(See CCI Bulletin 120)
techniques so far developed. But there are some starting points
for qualifying these problems in the background or experience of
most control engineers.
©2000 CCI. All rights reserved. 226 | How to Justify Special Control Valves 3
 In the case of vibration, one can look at just
the energy levels involved in the pumped or
compressed horsepower behind the fluid stream
and then consider the potential-to-kinetic-to-
potential energy conversion that occurs in the
standard control valve. In very many of today’s
process loops, control valves may well have
over 1,000 horsepower in kinetic energy available
to excite vibrations! It is therefore wise to
evaluate all experience with various valve types,
configurations, and power levels against the new
application. When scaled up, almost any old
or existing problem application is certain to be
intolerable or even disastrous.

In the case of erratic control, if the industry

experience with standard valves suggests some
Multistage valves are in effect several valves in series on one stem;
pressure drop is low in each stage. Left and center—vapor service maximum turndown ratio, it would be foolish to
designs. Right—liquid service valve for high-pressure recovery. expect that a larger ratio could be met by a single
standard control valve in a higher flow situation.
Experience with plug oscillation or jumping near
the seat strongly suggests more difficult problems
when similar standard control valves must be
scaled up in a new application.

Noise and vibration reduction

The sources of noise in process plants are

multitudinous. For the sake of this discussion,
only those attributable to the piping system
are covered. The concept of reverberation,
logarithmic addition, and shifting dominance
should be evident, indicating that all noise
contributors to the area of interest must be
considered to achieve noise control.
Addition of a few turns to the multistage valve has the effect of
adding another stage. Left—high-recovery design. Right—very low Typical noise sources are listed in the illustration
recovery. on page 2.

Noise treatment techniques vary with noise power

level:

Type I (low power, 85-90 dBa)

" Change standard valve trim or style—gain a

possible 5 dBa

" Reduce pressure drop across valve, piping

vent or restriction —gain a reduction of
approximately 6 log (P1/P2) dB, where P1 = old,
Labyrinth disk valves are P2 = new
multistage (many turns within each
disk) and modulable (disks of varying type are paralleled as plug
" Reduce mass flow through valve or vent
opens). Internal velocities can be lower than in pipe. Left—liquid
service. Right—vapor service, high pressure to atmosphere vents. — gain reduction of approximately 10 log
(W1/W2), where W1 = old, W2, new

4 How to Justify Special Control Valves | 226 ©2000 CCI. All rights reserved.
 " Insert a silencer downstream
of valve—gain over 15 dB
reduction

Type III (high power, >100dBa)

" Use silencer or mute

downstream of control valve –
gain 20dB or more if certain
criteria can be maintained

" Use special purpose low noise

control valves and increase pipe
size as required—gain up to 50
dBa reduction.

Vibration levels certain to be

destructive are those that are
associated with very high noise
levels (over 120 dBa, 10 ft. from
the device, with the exception of
relief or vent exhausts). In these
cases, even where the noise may be
tolerated, the only certain solution
is to use a special purpose control
valve.

Cavitation reduction
techniques
Total cost (hardware, installation, and operation) of standard control valves and
labyrinth disk-type valves for increasing mass flow rates, in various types of " Utilize special valve trim in
applications. All figures relative to cost of standard valves for application. A=cost, standard valves—gain
SelfDrag valve system; B=cost, standard valve system; B1 =standard control
valve; B2 =maintenance; B3 =muffler, sound treatment; B4 =structure, erection costs; confi nement of cavitation
B5 =power savings; B6 =piping costs; C=parallel standard valve system; C1 =parallel damage to the trim
standard control valves; Mpph=millions of pounds per hour; kpph=thousands of
pounds per hour; Mscfh=millions of standard cubic feet per hour. " Operate the valve in an on-off
mode with line restrictors to
" Increase the distance between the source and the observer—
divide pressure drop—reduce or eliminate cavitation
gain a reduction between 10 and 20 log (X, /X 2), where X1 =
old, and X 2 = new " Utilize special control valves—eliminate cavitation

" Increase the wall thickness of the valve, pipe or vessels—gain " Change process or valve drops and temperature—eliminate
up to 6 dB for each doubling of wall thickness. cavitation.

Type II (medium power, 90-100 dBa) Erratic control problems can only be qualified by experience.
Techniques for reducing erratic control are:
" Lag the valve, piping or vessel with acoustical material—gain
over 10 dB " Use equal percentage trim

" Change standard trim to special trim—gain up to 15 dB " Use cage-guided or semibalance type trim to minimize
reduction hydrodynamic forces

" Insert a special line restriction downstream of valve to " Use parallel control valves, split-ranged to increase turndown
prevent lower-frequency propagation down the piping—gain
" Use single special purpose semibalanced “long-stroke” cage-
up to 15 dB, generally effective only for fi xed flow
type-trim.
©2000 CCI. All rights reserved. 226 | How to Justify Special Control Valves 5
Identifying the real problem Labyrinth disk valves

Until recently all that could be done to reduce the problems A quick review of the previously described special control
of noise, vibration, cavitation, and erratic control in valves was valve approaches makes it plain that three things must be
to use mufflers, silencers, lagging, special trim, friction tubes, accomplished by a successful control valve. First, the velocity
orifices, series valves, and so on. All of these “solutions” treat reduction technique must be effective enough to reduce the
only the symptoms, not the cause. velocity within the valve to levels comparable with those in the
piping system as required by the system. Second, the technique
The real problem, in every case, is the high fluid velocity
must be effective (modulable) over the full range of control,
developed inside the component! Basically, these velocities
from shut-off to full flow. And third, the technique must lend
follow one of two expressions: V = K vΔP for liquids, or V =
itself to convenience of use and small size of package. The last
f(P1/P2)fkC for vapors where ΔP or P1 – P2 is the component
characteristic will determine the price, especially in larger sizes.
differential pressure, generally a system requirement that cannot
be designed out. The Self DRAG valve (bottom illustrations, page 4), meets all
of these requirements. It is a truly multistage valve, since each
Several approaches offered by control valve manufacturers appear
disk can incorporate almost any number of turns to reduce
to recognize that velocity is the real problem, but then attempt
the internal velocity to even less than pipe velocity. It is truly
only to contain its effects or to shift the problem into a
moduable, since the center cylindrical plug uncovers any number
more tolerable area. This is true, for example, in designs which
of disks as the valve is opened, and disk types can be varied
divide the flow into many parallel paths and then direct these
throughout the range to produce any desired pressure drop-
against one another (top figure, page 3), the theory being that
velocity characteristic.
impingement of fluid against fluid is less destructive than fluid
against moving parts. Cavitation is still present due to the basic In vapor service, considerably more turns (stages) are required
high velocity, but damage can be confined to the valve plug and in each disk, because very low stage pressure ratios must be
cage. maintained to limit velocities to low mach numbers. Also the
change in specific volume must be accommodated after each
Another approach is to configure the zones of high velocity
stage in vapor service in order to keep the velocity relatively
to obtain a shift in the noise characteristic—for example, less
constant.
apparent noise is generated by many small areas compared to
a single equal area (lower figure, page 3). In this case, test Price vs. cost of special valves
data indicates that noise is reduced even though the nozzle is It is seen that there are many alternative solutions to the
supersonic. problem of providing quiet, long-life control valves. The
Multistage valves offered by control valve makers are indeed engineer’s problem, however, is to choose the one that meets the
attempts to reduce velocity, by packaging several valves in series specifications and operates as required at the lowest possible cost.
on a single stem with a common control and positioner. (Top Cost is cost and price is price and seldom are the two equal. In
illustrations, page 4). Unfortunately, this multi-valve solution the case of special control valves, the price difference between
entails much expense and difficult control problems. In special and standard valves is not the cost difference, but rather
vapor applications, the very low vapor drops which result in only one contributor to the total cost. The illustrations on this
high velocities limit multi-stage valves to low pressure ratio page show how all of the various contributors affect total cost
applications. In liquid service applications, high recovery trim (installation and operation) as mass flow increases, for a number
causes higher fluid velocity which almost negates the effects of of different valve applications.
multi-stages.
For a successful choice between standard and special valves:
Other multistage liquid service valve designs go a step farther and quantify those problems that can be; consult experience in
add twists and turns to the fluid path (center illustrations, page situations that cannot be quantified; and consider all costs,
4). The shortcoming of these designs is that the turn stages do hardware, installation, and operation.
not modulate and are effective only near wide-open conditions.
With the typical process control valve running at 60 percent of
its design flow, or less than 50 percent of supplied capacity, the
turn stages are less than 25 percent effective, equivalent to just
one more stage.

6 How to Justify Special Control Valves | 226 ©2000 CCI. All rights reserved.
Bibliography

1. “Why Velocity Control?” R.E. Self, Bulletin 100, Control

Components, Inc.

2. Control Components, Inc., Bulletin 120, R. Zarate.

3. “Control Valve Noise Prediction and Its Relation to Fluid

Velocity,” D.A. Gettman, Control Components, Inc.

4. “Smoothness Effects Noise Generation in Valve Manifolds—

Fact or Folklore?” J.G. Seebold, ISA Final Control Elements,
Volume 2, 1973.

5. Masoneilon Bulletin 340E.

6. Fisher Technical Manual 24.

7. Burgess-Manning-Silencing Handbook, 1968 Edition.

8. “Noise and Vibration Control,” L.L.Beranek, McGraw-Hill

Book Co., 1971.

9. “How to Cope with Control Valve Noise,” J.B. Arant,

DuPont Co., Instrumentation Technology, March 1973.

10. “Special Control Valves Reduce Noise and Vibration,” J.B.

Arant, DuPont Co., Chemcial Engineering, March 6, 1973.

11. “Applications and Economics for Control Valve Noise

Pollution,” J.B. Arant, DuPont Co., ISA Paper 72-724, 1972.

12. “Noise and Noise Control for Process Plants,” Bolt, Beranek
& Newman, September 1970.

13. “Considerations in the Specification of Pipeline Noise,” J.H.

Cooper and L.L. Aldridge, Leslie Co., ISA Symposium on
Flow, Paper No. 3-4 210.

14. “Energy Transmission in Piping Systems and Its Relation

to Noise Control,” R.J. Sawley and P.R. White (B, B & N),
ASME Paper No. 70-WA/Pet-3.

©2000 CCI. All rights reserved. 226 | How to Justify Special Control Valves 7