2 Module 4Final control element : I/P converter – pneumatic and electric actuators – valve positioner – control values – characteristics of control valves – inherent and installed characteristics – valve body – commercial valve bodies – control valve sizing – cavitation and flashing – selection criteria.
4 ACTUATORSIf a valve is used to control a flow, some mechanism must physically open or close the valve.For a temperature control some mechanism must turn the heater ON or OFF.these are examples of ACTUATORS.Between Signal conversion unit and Control Valve.
18 ACTUATORS - COMPARISON Actuator typesAdvantagesDisadvantagesElectrical(servomotor or stepping motor)Direct interface with computer system. Simple design.Low thrust.Slow speed.No mechanical fail safe hazardous.Electromechanical(Motors combined with gear boxes)High thrustHigh stiffness coefficientFlexible adaptationComplex designNo mechanical fail safeLarge, heavy structureHazardousHydraulic and Electro hydraulicFast speedSelf lubricationFluid Viscosity SensitivePneumatic and Electro-pneumaticLow costMechanical Fail safeSimple designSmall packageSuitable for highly hazardous areas alsoGood control with control deviceSlow speedLack of stiffnessInstabilityModerate trustQuality air requirement
19 POSITIONERSIt guarantees that the valve does move to the position where the controller wants to be.Can correct many variations, including changes in packing friction due to dirt, corrosion, or lack of lubrication; variations in the dynamic forces of the process; sloppy linkages; or non-linearities in the valve actuator.
20 ELECTRONIC VALVE POSITIONERS (For Electric Actuators) High Gain Proportional controller.It is considered as a slave controller of a cascade loop in which the master controller is the primary controller itself.The primary controller’s output signal is the set point for the slave controller (positioner)
27 CONTROL VALVE CHARACTERISTICS The relationship between control valve capacity and valve stem travel is known as the Flow Characteristic of the Control Valve.Trim design of the valve affects how the control valve capacity changes as the valve moves through its complete travel.Because of the variation in trim design, many valves are not linear in nature. Valve trims are instead designed, or characterized, in order to meet the large variety of control application needs.Many control loops have inherent non linearity's, which may be possible to compensate selecting the control valve trim.
28 CONTROL VALVE CHARACTERISTICS Inherent CharacteristicsInstalled/ Effective Characteristics
29 CONTROL VALVE CHARACTERISTICS Inherent Characteristics (Ideal) These curves are based on constant pressure drop across the valve (pressure difference is determined by the valve alone).based on the design conditions, parameters and experimental results.Different from actual because of the variations in parameters and limitations in manufacturing process.
30 CONTROL VALVE CHARACTERISTICS Inherent Characteristics (Ideal) ASSUMPTIONS:The actuator is linear.The pressure difference across the valve is constant.The process fluid is not flashing, cavitating or approaching sonic velocity (Choked flow).
32 1. Quick Opening CV Used for Full On/ Full OFF applications. Relatively small motion of valve stem results in maximum possible flow rate through the valve.Eg: 30% travel of stem may results in 90% of full flowAlso called ‘DECREASING SENSITIVITY TYPE VALVE’The valve sensitivity (∆Q/∆S) at any flow decreases with increasing flow.
33 2. Linear Valve Flow rate varies linearly with stem position. Q/Qmax = S/ SmaxSensitivity Constant.Eg: if valve open 25% the flow through the valve is 25% of full flow.Process tank level control, De-aerated level control etc.
34 3. Equal Percentage Valve A given percentage change in stem position produces an equivalent change in flow.These valves does not shut off flow completely.RANGEABILITY, R = Qmax /Qmin‘INCREASING SENSITIVITY TYPE’The valve sensitivity at any given flow rate is a constant percentage of the given flow rate, thus equal percentage.
35 3. Equal Percentage Valve… Eg: consider a valve having a maximum flow rate of 60 tonnes/hr.Case 1when valve delivering 10 tonnes/hr, when the valve is permitted to open 10% more, then the incremental flow is 1 tons/hr.Case 2when valve delivering 40 tonnes/hr, when the valve is permitted to open 10% more, then the incremental flow rate is 4 tonnes/hr.
37 CONTROL VALVE CHARACTERISTICS Installed Characteristics (Real) When valve is installed as part of a process plant, its flow characteristics are no longer independent of the rest of the system.Fluid flow through the valve is subject to frictional resistances in series with that of the valve.Distortion Coefficient,
38 CONTROL VALVE CHARACTERISTICS Installed Characteristics (Real) Deviation in inherent valve characteristics.Actuators without positioner will introduce nonlinearities.Pump curves will also introduce nonlinearities.
39 Valve Body & Commercial Valve Bodies Refer Krishnaswamy
40 CONTROL VALVE SIZING Proper sizing of control valve is important. If control valve is oversize,The valve must operate at low lift and the minimum controllable flow is too large.Lower part of the flow-lift characteristic is most likely to be non uniform in shape.If control valve is undersize,the maximum flow desired for operation of a process may not be provided.
41 Control valve sizing FACTORS AFFECTING SIZING Pressure Drop across the control valveFlow Rate through the control valveSpecific GravityOther factors such as type of fluid, gas or liquid, critical flow conditions for gases and vapours and viscosity of liquids influence valve size
42 FLOW COEFFICIENT, CVThe flow coefficient indicates the amount of flow the control valve can handle under a given pressure drop across the control valve.‘The number of US Gallons of cold water at 60◦ F flowing through a fully open control valve for 1 minute at a pressure difference of 1 psi.’US Gallon = ltrImperial Gallon = 4.53 ltr
43 FLOW COEFFICIENT, KV CV = 1.17 KV KV = 0.86 CV Whenever the flow coefficient is mentioned in metric units.The flow rate of water in m3/hr at about 300C flowing through the fully opened control valve at a pressure drop of 1 kg/cm2 across the valve.CV = 1.17 KVKV = 0.86 CV
44 FLOW COEFFICIENT, KVQ = flow rateSG = Specific Gravity
45 Guidelines for Sizing of Control Valves The valve shall be sized for the actual flow condition and not for the ultimate design capacity of the system. Normal maximum flow rate is normally about 70% of the ultimate design capacity.Most of the pressure drop of the system should be across the control valve. (about 70%)When the pipe line is dimensioned with normal allowable velocities the control valve will be a few sizes smaller than the pipe line. Only in extreme cases where very high velocities have been used in the pipe line, the size of the control valve will be the same as that of the pipe line.
46 Guidelines for Sizing of Control Valves… The selection must be done such that the calculated CV is attained at about 75 to 80% of the full wave travel.Regardless of the application such as flow control or pressure control the valve sizing is done on the basis of flow coefficient CV.
47 CAVITATIONRelated to Bernoulli’s theorem which describes the pressure profile as the fluid passes through a narrower passage, restriction, orifice or control valve.As the fluid accelerates pressure head is converted into velocity head.Fluid accelerates to maximum velocity, minimum pressure known as Vena Contracta.Fluid then slows down as it again expands to full pipe area.Static pressure recovers but a part is lost due to friction.
48 CAVITATION…If the pressure head drops below the liquid vapour pressure at that temperature, then vapour bubbles will form downstream of the restriction.As the static pressure recovers to a point greater than vapour pressure, the vapour bubbles collapse back into their liquid phase.This collapse of bubbles produces high-energy implosions which is called ‘CAVITATION’.These implosions generate noise, fluid shock cells, and gets that impinge upon the trim metal parts.It generates a tremendous and concentrated impact force that destroys the metal as it fractures out tiny metal particles.
51 CAVITATION…Cavitation is usually coupled with vibration and a sound like rock fragments or gravel flowing through the valve.
52 Elimination of CAVITATION No known material can withstand continuous cavitation without damage and eventual failure.The length of time it will take is a function of the fluid, metal type and severity of the cavitation.The greatest damage is caused by a dense pure liquid with high surface tension.Density governs the mass of the micro jet stream, and surface tension governs the jet velocity.
53 Cavitation Elimination Methods Revised Process ConditionsRevised ValveGas InjectionRevised Installation
56 FLASHINGCavitation occurs when P2˃PV , while flashing takes place when P2˂PV.Liquid flashes into vapour and stays in the vapour phase.The specific volume increases as liquid changes to vapour which in turn causes an increase in the fluid velocity.If enough vapour is formed, the resulting high velocities can erode metals.So the piping downstream of a valve needs to be much larger than the inlet-piping in order to keep the velocity keep the velocity of the stream low enough to prevent erosion.
57 SELECTION OF CONTROL VALVES Need for a Control ValveCollection of Process DataAssigning Valve Pressure DropControl Valve PerformanceValve characteristicsGain of control loop components like process, sensor, controller and valve gain.Process NonlinearityValve RangeabilityControl Valve SequencingSplit-ranging or floating
58 SELECTION OF CONTROL VALVES… Control Valve SizingValve Actuator SelectionWhether electrical, pneumatic or hydraulicActuator Speed of ResponseActuator power or torqueValve failure position (Fail safe operation)Valve PositionerWhen not to use positionerTo eliminate dead bandSplit range operation
59 SELECTION OF CONTROL VALVES… Process Application ConsiderationsHigh Pressure serviceHigh differential pressure usageVacuum serviceHigh temperature serviceLow temperature serviceCavitation and erosionFlashing and erosionViscous and slurry serviceLeakageSmall flow valuesControl valve noisePiping and installation considerationsClimate and atmospheric corrosion
60 SELECTION OF CONTROL VALVES… 9. Control Valve Specification form10. Test report and test specification