Common Valve problems.pdf

(2436 KB) Pobierz
http://boilersinfo.com/
Source: Valve Handbook
9
Common Valve
Problems
9.1
High Pressure Drops
9.1.1 Introduction to High Pressure
Drops
Flow moves through a valve due to a difference between the upstream
and downstream pressures, which is called the
pressure drop
(
P)
or the
pressure differential.
If the piping size is identical both upstream and
downstream from the valve and the velocity is consistent, the valve
must reduce the fluid pressure to create flow by way of frictional loss-
es. A portion of the valve’s frictional losses can be attributed to friction
between the fluid and the valve wall. However, this friction is minimal
and is not sufficient to create enough pressure drop for an adequate
flow. A more effective way to create a significant frictional loss in the
valve is through a restriction within the body. Because many valves are
designed to allow a portion of the valve to be more narrow than the
piping, they can easily provide this restriction in the fluid stream.
Because of the laws of conservation, as the fluid approaches the valve,
its velocity increases in order for the full flow to pass through the
valve, inversely producing a corresponding decrease in pressure (Fig.
9.1). The inverse relationship between pressure and velocity is shown
by Bernoulli’s equation, which is
V
12
2g
C
P
1
V
VC2
2g
C
P
VC
339
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.
http://boilersinfo.com/
Common Valve Problems
340
Flow
P
1
P
2
Chapter Nine
P
1
P
2
High recovery
P
2
Low recovery
Vena contracta
P
1
Pressure
P
2
P
VC
Velocity
V
1
Distance downstream
Figure 9.1
Location of vena contracta from
point of orifice restriction and pressure and
velocity curves. (Courtesy
of Fisher Controls
International, Inc.)
V
2
where
V
1
g
C
V
VC
P
VC
P
1
density units
upstream velocity
gravitational units conversion
velocity at vena contracta
pressure at vena contracta
upstream pressure
The highest velocity and lowest pressure occur immediately down-
stream from the narrowest constriction, which is called the
vena con-
tracta.
Figure 9.2 shows that the vena contracta does not occur at the
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.
http://boilersinfo.com/
Common Valve Problems
Common Valve Problems
341
Figure 9.2
Relationship between orifice restriction and turbulence generation.
(Courtesy
of Fisher Controls International, Inc.)
restriction itself but rather downstream some distance from the restric-
tion. This distance may vary according to the pressures involved. At
the vena contracta the flow velocity is at a maximum speed, while the
flow area of the fluid stream is at its minimum value.
Following the vena contracta, the fluid slows and pressure builds
once again, although not to the original upstream pressure. This differ-
ence between the upstream and downstream pressures is caused by
frictional losses as the fluid passes through the valve, and is called the
permanent pressure drop.
The difference in pressure from the pressure at
the vena contracta and the downstream pressure is called the
pressure
recovery.
A simplified profile of the permanent pressure drop and pres-
sure recovery is shown in Fig. 9.3.
The flow rate for a valve can be increased by decreasing the down-
stream pressure. However, in liquid applications the flow can be limit-
ed by the pressure drop falling below the vapor pressure of the fluid,
which will create imploding bubbles or pockets of gas (called cavita-
tion or flashing, respectively).
Choked flow
occurs when the liquid flow
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.
http://boilersinfo.com/
Common Valve Problems
342
Chapter Nine
Figure 9.3
Flow curve showing pressure recov-
ery and permanent pressure drop. (Courtesy
of
Fisher Controls International, Inc.)
is saturated by the fluid itself mixed with the gas bubbles or pockets
and can no longer be increased by lowering the downstream pressure.
In other words, the formation of gas in a liquid crowds the vena con-
tracta, which limits the amount of flow that can pass through the
valve. With gases, as the velocity approaches sonic speeds, choked
flow also occurs and the valve will not be able to increase flow despite
a reduction in downstream pressure.
9.1.2 Effects of High Pressure Drops
As discussed in Sec. 9.1.1, the flow function of the valve is dependent
on the existence of a pressure drop, which allows flow movement from
the upstream vessel to the downstream vessel or to atmosphere.
Because a pressure drop generated by the valve absorbs energy
through frictional losses, the ideal pressure drop allows the full flow to
pass through the body without excessive velocity, absorbing less ener-
gy. However, some process systems, by virtue of their system require-
ments, may need to take a larger pressure drop through the valve.
A high pressure drop through a valve creates a number of problems,
such as cavitation, flashing, choked flow, high noise levels, and vibra-
tion. Such problems present a number of immediate consequences:
erosion or cavitation damage to the body and trim, malfunction or
poor performance of the valve itself, wandering calibration of attached
instrumentation, piping fatigue, or hearing damage to nearby workers.
In these instances, valves in high-pressure-drop applications require
expensive trims, more frequent maintenance, large spare-part invento-
ries, and piping supports. Such measures drive up maintenance and
engineering costs.
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.
http://boilersinfo.com/
Common Valve Problems
Common Valve Problems
343
Although users typically concentrate on the immediate conse-
quences of high pressure drops, the greatest threat that a high pressure
drop presents is lost efficiency to the process system. Because high
pressure drops absorb a great deal of energy, that energy is lost from
the system. In most process systems, energy is added to the system
through heat generated by a boiler or through pressure created by a
pump. Both methods generate energy in the system, and as more energy
is absorbed by the system—including that energy lost by valves with
high pressure drops—larger boilers or pumps must be used.
Consequently, if the system is designed with few valves with high
pressure drops, the system is more efficient and smaller boilers or
pumps can be used.
9.2
Cavitation
9.2.1 Introduction to Cavitation
Cavitation is a phenomenon that occurs only in liquid services. It was
first discovered as a problem in the early 1900s, when naval engineers
noticed that high-speed boat propellers generated vapor bubbles.
These bubbles seemed to lessen the speed of the ship, as well as cause
physical deterioration to the propeller.
Whenever the atmospheric pressure is equal to the vapor pressure of
a liquid, vapor bubbles are created. This is evident when a liquid is
heated, and the vapor pressure rises to where it equals the pressure of
the atmosphere. At this point, bubbling occurs. This same phenome-
non can also occur by decreasing the atmospheric pressure to equal the
vapor pressure of the liquid. In liquid process applications, when the
fluid accelerates to pass through the narrow restriction at the vena
contracta, the pressure may drop below the vapor pressure of the
fluid. This causes vapor bubbles to form. As the flow continues past
the vena contracta, the velocity decreases as the flow area expands and
pressure builds again. The resulting pressure recovery increases the
pressure of the fluid above the vapor pressure. This phenomenon is
described in Fig. 9.4.
As a vapor bubble is formed in the vena contracta, it travels down-
stream until the pressure recovery causes the bubble to implode. This
two-step process—the formation of the bubble in the vena contracta
and its subsequent implosion downstream—is called
cavitation.
In simple
terms, cavitation is a phase that is characterized by a liquid–vapor–
liquid process, all contained within a small area of the valve and with-
in microseconds. Minor cavitation damage may be considered normal
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika:

Zgłoś jeśli naruszono regulamin