Class 42 control valves - valve positioners, cavitation and flashing

valve positioners, cavitation and flashing

1. ICE401: PROCESS INSTRUMENTATION 
AND CONTROL
Class 42
Control Valves –Valve Positioners, 
Cavitation& Flashing
Dr. S. Meenatchisundaram
Email: [email protected]
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015

2. Valve Positioners:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015
•The positioner is a high-gain plain proportional controlle r that
measures the valve stem position (to within 0.1 mm), compare s
that measurement to its set point (the controller output sig nal),
and,ifthereisadifference,correctstheerror.
•The open-loop gain of positioners ranges from 10 to 200
(proportionalband of10–0.5%), and theirperiodsofoscill ation range
between 0.3 and 10 sec (frequency response of 3–0.1 Hz). In ot her
words, the positioner is a very sensitively tuned, proporti onal-only
controller.
•Themainpurposeofhavingapositioneristoguaranteethatthevalve
does,infact,movetothepositionwherethecontrollerwantsittobe.

3. Valve Positioners:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015
•The addition of a positioner can correct for many variations ,
including changes in packing friction dueto dirt, corrosio n, orlack
of lubrication; variations in the dynamic forces of the proc ess; sloppy
linkages(deadband);ornonlinearitiesinthevalveactuat or.
•The dead band of a valve/actuator combination can be as much a s
5%;whenapositionerisadded,itcanbereducedtolessthan0.5%.
•It is the job of the positioner to protect the controlled vari able
frombeingupsetbyanyoftheabovevariations.
•Actuatorswithoutspringsalwaysrequirepositioners.
•When a valve is in remote manual (open loop) operation, it wil l
always benefit from the addition of a positioner, because a po sitioner
will reduce the valve’s hysteresis and dead band while incre asing its
response.

4. Valve Positioners:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015
•When the valve is under automatic (closed loop) control, the
positionerwillbehelpfulinmostslowloops,whichcontrolanalytical
properties, temperature, liquid level, blending, slow flow , and large
volumegasflow.
Limitations:
•The positioner’s function as a cascade slave, can causeosci llation
and cycling on fast loops if the controller cannot be sufficie ntly
detuned.
•Similarly, negative force reactions on the plug require an i ncrease in
actuatorstiffnessandnottheadditionofapositioner.
•Actuator stiffness can be improved by increasing the operat ing air
pressureorbyusinghydraulicactuators.

5. Valve Positioners:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015
•Thepositionerineffectisthecascadeslaveoftheloopcontroller.
•Inorderforacascadeslavetobeeffective,itmustbe fasterthan the
speed at which its set point, the masteroutputsignal,canch ange.
•The rules of thumb used in this respect suggest that the time c onstant
of the slave should be ten times shorter (open-loop gain ten t imes
higher) than that of the master and the period of oscillation of the
slave should be three times shorter (frequency response thr ee times
higher)thanthatoftheprimary.
•Thecriteriaforpositionersneednotbe this stringent, but still, it is
recommended not to use positioners if the positioned valve i s
slowerthantheprocessvariableitisassignedtocontrol.

6. Valve Positioners:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015

7. Valve Positioners:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015

8. Plug shapes:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015

9. Valve Components:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015

10. Valve Components:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015

11. Valves:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015

12. Valves:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015

13. Globe Valves:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015

14. Globe Valves – Single Port:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015

15. Globe Valves – Double Port:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015

16. Butterfly Valves:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015

17. Butterfly Valves:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015

18. Valve Types:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015

19. Valve Types:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015

20. Valve Types:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015

21. Valve Sizing:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015
              :
V
v
f
q
C q GPM
P
G
=
∆
•Prior to about 1946 control valves were sized using the flow ar ea of
the valve’s orifice in square inches together with some, usua lly
proprietary equations in order to estimate the amount of liq uid or gas
passingthroughthevalve.
•Thereafter, a new coefficient was introduced called Cv, whic h started
standardizationinthisfield.
•This coefficient denotes the number of (U.S.) gallons of wate r that
a valve would pass when thepressuredropacrossitis1lb/in
2
.

22. Flashing and Erosion:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015
•When vaporization occurs, the proportionality between flo w rate and
thesquarerootofpressuredropceases.
•The flow rate reaches a maximum (chocked) value, which is con stant
andthoughthedownstreampressureisfurtherreduced.Thecurewill
be

23. Flashing and Erosion:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015
•This condition is referred as “chocked flow’. If large amoun t of
dissolvedgasescomeoutofsolutionsasthepressuredropintheflow,
thesereleasedgaseswillalsorestricttheflow.
•Vaporization can cause cavitation or flashing. Cavitation occurs when
static pressure anywhere in the valve drops to or below the va pour
pressureoftheprocessliquid.
•Vaporization begins around microscopic gaseous nuclei. The
cavitation process includes the vapour cavity formation and
sudden condensation (collapse) of the vapour bubble drivenby
pressurechanges.
•The basic process of cavitation is related to the conservati on of
energy and Bernoulli’s theorem, which describes the pressu re profile
ofaliquidflowingthrougharestrictionororifice.

24. Flashing and Erosion:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015

25. Flashing and Erosion:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015
•Cavitationoccurswhenp
2>p
v,whileflashingtakesplacewhenp
2<
p
v.
•Whenaliquidflashesintovapour,thereisalargeincreaseinvolume.
•In this circumstance, the piping downstream of a valve needs to be
much larger than the inlet piping in order to keep the velocit y of the
two-phasestreamlowenoughtopreventerosion.
•The ideal valve to use for such applications is an angle valve with an
oversizedoutletconnection.

26. Flashing and Erosion:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015
•In order to accelerate the fluid through the restriction, som e of
thepressureheadisconvertedintovelocityhead.
•This transfer of static energy is needed to maintain the same mass
flow through the reduced passage. The fluid accelerates to its
maximum velocity, which corresponds to the point of minimum
pressure(venacontracta).
•The fluid velocity gradually slows down as it expands back to t he
full pipe area. The static pressure also recovers somewhat, but part of
itislostduetoturbulenceandfriction.
•If the static pressure at any point drops below the liquid vap our
pressure (Pv) corresponding to the process temperature, th en vapour
bubbleswillform.

27. Flashing and Erosion:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015
•If enough energy is imparted to the growing vapour bubble to
overcome surface tension effects, the bubble will reach a cr itical
diameterandexpandrapidly.
•As the static pressure recovers to a point greater than the va pour
pressure, the vapour will condense, causing the bubbles to
collapseviolentlybackintotheirliquidphase.
•The growth and collapse of the bubbles produce high-energy
shockwavesinthefluid.Thecollapsestageoftheprocess(thebubble
implosion)produces the more severe shock waves.
•Shock waves and liquid microjets radiate for short distance s from
implodingcavitiesanderodenearbysurfaces.

28. Flashing and Erosion:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015
•Cavitation can cause erosion, noise, and vibration in piping
systems and thereforemust be avoided.
•Extensive cavitation also causes choked flow conditions in t he
valve.
PredictingandMitigatingCavitation:
•Sizing a valve in liquid service for choked flow allows one to
determineitsmaximumflow capacity, but this is of limited va lue,
because most liquid-service valves should not be operated u nder
chokedconditions.
•Special trim designs with multiple stages or multiple flow pa ths are
typically used to prevent severe cavitation and are better a ble to
operateatornearchokedconditionswithoutdamage.

29. Flashing and Erosion:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015
PressureRecoveryFactor(F
L):
•F
Lis the pressure recovery factor, which indicates the size of the
pressurerecoveryrelativetothevalvepressuredrop(Figu re).

30. Flashing and Erosion:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015
PressureRecoveryFactor(F
L):
•F
Lis the pressure recovery factor, which indicates the size of the
pressure recovery relative to the valve pressure drop (Figu re). F
Lis
definedas

31. Flashing and Erosion:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015

32. Flashing and Erosion:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015
•Cavitation can cause erosion, noise, and vibration in piping
systems and thereforemust be avoided.
•Extensive cavitation also causes choked flow conditions in t he
valve.
PredictingandMitigatingCavitation:
•Sizing a valve in liquid service for choked flow allows one to
determineitsmaximumflow capacity, but this is of limited va lue,
because most liquid-service valves should not be operated u nder
chokedconditions.
•Special trim designs with multiple stages or multiple flow pa ths like
those in Figure are typically used to prevent severe cavitat ion and are
betterabletooperateatornearchokedconditionswithoutdamage.

33. References:
• Instrument Engineers' Handbook: Volume Two: Process Control
And Optimization (Fourth Edition) - Bela G. Liptak, CRC Press,
2005.
• Process Control – Principles and Applications, Surekha Bhanot,
Oxford Higher Education Press, 2012.
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug 
–
Nov 2015