Basic Fluid Dynamics - Control Valves
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Setpoint Integrated Solutions is a industry leader in providing control valve solutions across industry segments. Brannon Gant - Regional Sales Manager
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Basic Fluid Dynamics - Control Valves
- 1. WHEN ACCURACY MATTERS IntroductionIntroduction Basic Fluid DynamicsBasic Fluid Dynamics
- 2. WHEN ACCURACY MATTERS Liquid Applications (incompressible media) Considerations: -Specific Gravity -Viscosity -Vapor Pressure -Critical Pressure -Temperature -P1 - Upstream Pressure -P2 - Down Stream Pressure or Delta P (P1 - P2) -Min. Flow Rate / Max Flow Rate -Clean or dirty media -Corrosive potential -Erosion Potential
- 3. WHEN ACCURACY MATTERS Valve Capacity or Rated CvValve Capacity or Rated Cv
- 4. WHEN ACCURACY MATTERS Required Flow Capacity = CRequired Flow Capacity = Cvv One COne Cvv is equal to a flow rate of oneis equal to a flow rate of one gallon per minute of water atgallon per minute of water at 60 degrees F at one pound60 degrees F at one pound per square inch pressure differentialper square inch pressure differential
- 5. WHEN ACCURACY MATTERS Required Flow Capacity = CRequired Flow Capacity = Cvv P Q Cv
- 6. WHEN ACCURACY MATTERS Bernoulli’s LawBernoulli’s Law Bernoulli's principle states that for an Incompressible flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure.
- 7. WHEN ACCURACY MATTERS Conservation of Energy & MassConservation of Energy & Mass Mass 1 2 Energy Bernoulli’s Equation for Incompressible Flow ½ρV2 + P = Constant V P ρ1A1V1 = ρ2A2V2 A V Higher velocity through a smaller area Pressure decreases as velocity increases
- 8. WHEN ACCURACY MATTERS Control Valve Fluid MechanicsControl Valve Fluid Mechanics Letdown Path Control valves throttle by converting static pressure to kinetic energy. Pressure is reduced by creating resistance along the fluid’s flow path. Dynamic Pressure P1 P2 Pvc FL
- 9. WHEN ACCURACY MATTERS Conservation of Mass/EnergyConservation of Mass/Energy Vena Contracta Pressure Pressure P1 P2 A1 A2 Vena Contracta Area V1 V2 Velocity at Vena Contracta Velocity Area
- 10. WHEN ACCURACY MATTERS • Cavitation is defined as the phenomenon of formation of vapor bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapour pressure. • Cavitation is usually divided into two classes of behavior: inertial (or transient) cavitation and Incipient cavitation. Inertial cavitation is the process where a void or bubble in a liquid rapidly collapses, producing a shock wave. • Incipient cavitation is the point at where cavitation begins, but has not reached a destructive state What is Cavitation?
- 11. WHEN ACCURACY MATTERS Cavitation Phenomena P1 V1 V2 P2 Pvapor Vvc CavitationFlashpoint Pvc Pressure Velocity Local Fluid Pressure Drops Below Fluid Vapor Pressure Formation of Vapor Bubbles Downstream Pressure Recovery Above Vapor Pressure Bubbles Collapse
- 12. WHEN ACCURACY MATTERS What is Cavitation?What is Cavitation? 1. Local Pressure Drops Below Fluid Vapor Pressure, Pv 2. Vapor “Bubble” Forms 3. Pressure Recovers Above Pv 4. Collapse of Vapor Back to Liquid 5. Addition of Turbulence & Noise P<Pv P>Pv Flow Vena Contracta
- 13. WHEN ACCURACY MATTERS Why Worry About Cavitation? (Bubble implosion)
- 14. WHEN ACCURACY MATTERS PvPv StaticStatic PressurePressure CavitationCavitation P1P1 P2P2 Cavitation
- 15. WHEN ACCURACY MATTERS Valve Cavitation DamageValve Cavitation Damage • Pitting and erosion of exposed surfaces – Accelerates valve wear – Increases maintenance and process down-time • Larger Scale Cavitation: potential catastrophic failure of valve body & pressure vessel walls
- 16. WHEN ACCURACY MATTERS Localized Erosion Sounds like rocks Will attack any material Worse with high pressure Cavitation
- 17. WHEN ACCURACY MATTERS Why Worry About CavitationWhy Worry About Cavitation High maintenance Poor control System shutdown Lost production Noise Pipe vibration Trim & body wear Downstream pipe erosion
- 18. WHEN ACCURACY MATTERS Cavitation Prediction MethodsCavitation Prediction Methods 1. Method based on the Valve Pressure Recovery 2. Method based on the Valve Cavitation Index ISA-RP75.23-1995: “Considerations for Evaluating Control Valve Cavitation” P1-P2 P1-PVC FL = σσ (Sigma) =(Sigma) = (P1-PV) (P1-P2)
- 19. WHEN ACCURACY MATTERS Cavitation ManagementCavitation Management StrategiesStrategies • Prevention – Control Pressure Recovery by Reducing Velocity • Increase Flow Resistance via Staging • Containment – Control “Bubble” Collapse Location & Size • Removed From Surfaces • In Low-Impact Regions (e.g. Cage Hole) • High FL Not Necessary • Reduce orifice size ( Small hole diameter-Drilled cages)
- 20. WHEN ACCURACY MATTERS Control Valve GeometriesControl Valve Geometries • Where is the Vena Contracta? • How do we use FL to assess the potential for cavitation?
- 21. WHEN ACCURACY MATTERS Technology SelectionTechnology Selection • Single Stage Pressure Letdown • Double Stage Pressure Letdown • Multi-Stage Pressure Letdown
- 22. WHEN ACCURACY MATTERS Energy ManagementEnergy Management • Single Step Letdown DP DP=P1–P2
- 23. WHEN ACCURACY MATTERS Energy ManagementEnergy Management • Single Step Letdown • Multiple Step Letdown DP DP
- 24. WHEN ACCURACY MATTERS Single Stage Cavitation
- 25. WHEN ACCURACY MATTERS Multi Stage No Cavitation
- 26. WHEN ACCURACY MATTERS Hydrodynamic Flow RegimesHydrodynamic Flow Regimes Static Pressure P1 Pressure Letdown Path Subcritical Cavitating Flashing Vapor Pressure
- 27. WHEN ACCURACY MATTERS Single Stage Cavitation Containment
- 28. WHEN ACCURACY MATTERS Turn Recombin e Split Two Stage Cavitation Containment
- 29. WHEN ACCURACY MATTERS Premier High Pressure Liquid Letdown Separates Flow Re-combines Flow Constricts Flow Expands Flow Turns Flow
- 30. WHEN ACCURACY MATTERS Lincoln Log Flow Path
- 31. WHEN ACCURACY MATTERS VARIABLE NUMBER OF STAGES Drilled Hole Stacked Plate Design Pressure Drop Control Technology (Cavitation Prevention) Pressure Drop Always Distributed Across Various Stages Stages Disengage with Increasing Plug Lift High Staging at Low Lift Low Staging at High Lift Wide Cv Ranges are Possible Axial Flow Multi-Stage Variable Resistance Trim (VRT® ) PLUG P1 P2
- 32. WHEN ACCURACY MATTERS Multi Stack Plate
- 33. WHEN ACCURACY MATTERS V-LOG OperationV-LOG Operation • Multiple Turns and Flow Restrictions • Gradual Pressure Reduction • Maintains Low Fluid Velocities 90O TURNS
- 34. WHEN ACCURACY MATTERS 77000 Series Multistage Adiabatic Trim • Product Highlights – Two-Phase Flow – Heavy Guiding – Model 84/85 Piston Actuator • Compound Leverage 4:01 & 6:01 • Spring Return
- 35. WHEN ACCURACY MATTERS Flashing in ControlFlashing in Control ValvesValves
- 36. WHEN ACCURACY MATTERS What is Flashing?What is Flashing? P1 Pv P2 Vapor Bubble Formation Outlet Pressure Inlet Pressure Vena Contracta Flow Pressure
- 37. WHEN ACCURACY MATTERS What is Flashing?What is Flashing? P1 Pv P2 Outlet Pressure Inlet Pressure Vena Contracta Flow Pressure Vapor Pressure High Pressure Recovery = Low FL Low Pressure Recovery = High FL
- 38. WHEN ACCURACY MATTERS • Smooth cuts • Material Loss • Body Damage • Trim Damage • Piping Damage Effects of Flashing
- 39. WHEN ACCURACY MATTERS Treatment of Flashing SurfaceTreatment of Flashing Surface • Flashing cannot be prevented by valve design • Armoring of valve trim and body – Upgrade body to Chrome-Moly or Stainless Steel – Hardening of trim parts or hardfacing • Back pressure devices to increase P2 pressure and postpone flashing • Expanded outlet valve to slow impact of water droplets • Control the valve and line velocities via selection of valve and line size • Multistep valves to dissipate initial energy release within the valve trim
- 41. WHEN ACCURACY MATTERS • Erosion • Seat leakage at high pressure • Cavitation secondary Wire Drawing
- 42. WHEN ACCURACY MATTERS • Chemical induces • All wetted parts are affected • Material Corrosion
- 43. WHEN ACCURACY MATTERS Gas and Steam Applications (compressible media) Considerations: -Specific Gravity / Density / Molecular Weight -Compresability -Temperature -P1 - Upstream Pressure -P2 - Down Stream Pressure or Delta P (P1 - P2) -Min. Flow Rate / Max Flow Rate -Clean or dirty media -Corrosive potential -Erosion Potential
- 44. WHEN ACCURACY MATTERS Sizing Equation - Liquid P Q Cv
- 45. WHEN ACCURACY MATTERS Sizing Equation – Gas/VaporSizing Equation – Gas/Vapor
Editor's Notes
- In order to understand why different style control valves or internal trim designs are required under different operating conditions a basic understanding of Fluid Dynamics is required.
- Liquids are not compressible, we see this in hydraulic lifts and tools. Gasses on the other hand are compressible due to the great space between molecules, we see this when buying compressed gasses in tanks. When dealing with liquids as related to Control Valves there are many considerations that effects type of valve selected as well as size, type of internal trim components and materials. The list of considerations is what is typically needed to accurately size and select the best fit most cost effective solution. Many times all of this information isn’t available so assumptions must be made based on best engineering practices.
- In general capacity refers to the amount of fluid a valve can pass through it in a given time fully open. Because there are so many factors involved in determining this amount, Masoneilan developed a coefficient that can express the valves capacity with one figure. The world knows this a Cv. Cv will be discussed later on in detail with both literal and mathematical definitions. The size of the valve does not necessarily determine its capacity. The resistance to flow is a big factor. Note that the globe style valve must be much larger than the butterfly to have the came capacity.
- For those who like a more literal definition. In example a control valve with a CV of 12 will flow 12 gallons of water per minute at 60F with a 1 psi max differential pressure.
- We will be discussing Cv in two ways. One will be called Rated Cv which is the valve capacity that we discussed in the previous slide. The other is the required Cv. Required CV or calculated Cv is based on the input service data for each flowing condition stated as minimum flow and maximum flow. This value is calculated and it is the ratio of the flowing quantity to the square rout of the pressure drop. CV = Flow divided by the sq. rt. of P1 – P2.
- As you will see from the gauges representing velocity and pressure as the diameter of the pipe decreases velocity increases and pressure decreases. This is where an energy conversion takes place accelerating the fluid while dropping its pressure. Down stream recovery occurs when the pipe diameter increases recovering pressure while reducing P2 velocity. P2 pressure (down stream pressure) and flow recovers somewhere below P1 (up stream pressure) and flow essentially causing a pressure drop in the system. Without pressure drop you cannot have flow. Control valves are designed to cause a pressure drop in the system.
- This is a model mathematically depicting the relationship between area and velocity.
- This slide depicts graphically the pressure drop that occur as fluid passes through a control valve (flow restriction). P1 and P2 are variables that are set by the design of the process into which the valve is installed. Pvc, the lowest pressure of the fluid at the valve throat as a result of reducing P1, is known as the vena contracta pressure and is a function of valve geometry. The Vena Contracta is the point in the fluid flow stream where the cross sectional area of the stream is the smallest and the pressure is the lowest while the velocity is the highest. The difference between Pvc and P2 is called the recovery pressure and differs for various styles of valves. This recovery factor is characterized by using the variable FL.
- Again, this slide graphically depicts the relationship between Pressure, Velocity and area. The Vena Contracta is the point at which the cross sectional area of the flow stream is the smallest velocity is the highest and pressure is the lowest. For liquids: Mass Conservation: A1 x V1 = A2 x V2 …Area decreases/Velocity increases Energy: ……Velocity increases/ Pressure decreases Limits at the vena contracta are Vapor pressure for liquids & Sonic velocity for gas/vapors. Usually A1 = A2 on liquid application… V1=V2
- Read the slide
- As area decreases through the orifice of the control valve velocity increases converting static pressure into Kinetic energy. As velocity increases as pressure decreases below the liquid media vapor pressure forming bubbles in the fluid stream. Down stream of the control valve orifice pressure increases or recovers above liquid vapor pressure forcing the collapse of the bubble. In the event pressure does not recover above liquid vapor pressure a condition known as flashing occurs which is equally damaging to the control valve trim and parts. Flashing will be discussed later.
- We follow the Fluid pressure as the liquid passes through an orifice. This picture very simplistically models a valve orifice, a single seat valve Seat Ring orifice for example. As previously shown the fluid velocity increases dramatically as the flow converges to pass through the orifice & its static pressure decrease similarly. Somewhere AHEAD of the vena contracta the liquid pressure reaches its VAPOR PRESSURE & vapor bubbles start to form…not all at the same time. Beyond the theoretical vena contracta (the fluid never reaches that Min. Area when cavitating) the pressure increases (recovers) & the vapor bubbles implode in various locations downstream of the orifice, including against metallic surfaces.
- Why worry about CAVITATION? If Vapor Collapses Near a Solid Surface, It Generates Extremely High Surface Pressures. This is like Numerous, Repeated Small and Sharp Blows on Surface. Damages to Valve Trim, Body, Piping is likely to result in time Potential Complete Part Failure may also occur! Increased Maintenance Costs -- $$$! Cavitation noise, if loud & continuous, ought to be a warning of failures to come i.e The noise is not the problem, cavitation is. Let’s show a typical failed piece.
- This slide graphically depicts cavitation where control valve trim, body and piping begin to see damage How does it sound? Turn you speakers on in animation mode.
- Typical “Lava” like look of cavitation damaged metal surface. The picture: A seat ring that has suffered extensive cavitation damage (note the pitted look of the surface). Clearly, this has severely limited throttling control, & completely destroyed shut-off capability. Further damage would lead to complete destruction of the seat ring and potential damage to downstream components. The cavitation process seems to be self feeding i.e. once damage is initiated the material removal seem to accelerate.
- Camflex body with cavitation down stream of trim. Is this application the flow orientation was flow to open or fluid flowing into the face of the plug.
- Two prediction method are being used by the industry. The oldest is FL based, the one we are using & recommending is Sigma based. The valve pressure recovery coefficient equation definition has been discussed previously. Sigma definition/equation to be discussed later. On this slide note the similarity between Fl & Sigma i.e.. Sigma = 1 / Fl2 when Pv = Pvc ISA had to assure differentiation between the two terms i.e.. the inverted ratio Let’s review the FL method & its limitations
- Masoneilan uses two general Cavitation Management Strategies Prevention where no cavitation is allowed utilizing multi-hole trim and perhaps multistage trim to reduce velocity and eliminate cavitation. Containment where some cavitation is allowed but throttling limitations are applied to assure no damage occurs. Use of multi-hole trim that will temper the violence of cavitation by containing where the cavitation occurs, that is, within the flow stream and away from valve components.
- Additional emphasis on Valve & Valve Trim Geometry. Where is the vena contracta where”ALL” vaporization takes place? With both the Camflex & the 10,000 series (& others) there is at least two well defined flow paths. One of those will be in full cavitation before the other BUT the valve WILL NOT be fully choked, & FL would not warn of potential cavitation damages. That is one of the major draw back of the FL method. Furthermore, it can be shown that two valve geometry with similar FL as cavitation index, for example a 10,000 series and the drilled cage 41000 series would show dramatically different damage level on the same application i.e. the 10,000 trim could be destroyed by cavitation while the Cavitation Containment, discussed later, would “Live forever”.
- When approaching a high pressure liquid letdown application the order of progression is from single stage to double drilled hole cage, axial flow up to 9 stages and finally multi-stack plate design for extreme high pressure drop conditions up to 36 stages.
- In a simple waterfall, what do you find at the bottom? A hole, a puddle, or a bunch of salmon trying to go up over it. But typically, you find a washed out hole because water dropping a long distance creates a tremendous amount of energy, and that energy causes severe erosion.
- The simple solution is multistep drops. Bring the water down in small steps preventing the washout at the bottom. Is this simple example we are slowing the velocity by imposing multiple drops along the flow path.
- This would be an example of the pressure drop in a ball valve or butterfly valve. There isn’t a tortuous path to reduce velocity in these types of valves so they are susceptible to cavitation.
- In contrast a multi-stage drop would prevent cavitation.
- Most substances have well known physical properties which determine the temperatures at which they will be solid, liquid or gas. Water is the best known fluid and its boiling and freezing points are well known under atmospheric pressure conditions. This assumes that you are raising or lowering the temperature of water while the pressure remains relatively constant. Now for a moment suppose you are lowering the pressure of a liquid while keeping the temperature relatively constant. There is a pressure at a given temperature for most liquids below which the liquid will change (flash) to gas. This point is called the vapor pressure. This slide depicts differing pressure drops in a single stage valve relative to the vapor pressure of the liquid flowing through it. For the yellow curve, both P1 and P2 are above the vapor pressure and the Pvc (the lowest portion of the curve) is also above Pvc. This valve will have no problems in this service as the fluid undergoes no state change. For the red curve, P2 and Pvc are both below the vapor pressure line, so the liquid flashes to vapor and remains vapor. The valve type and materials must be selected to accommodate this flashing which can be erosive for certain types of fluid. For the green curve, P2 is above the vapor pressure but Pvc is below it. In this case, the fluid flashes to vapor as the valve throttles to Pvc, then &quot;recovers&quot; back to P2 (liquid again). This change of state is very violent and can cause severe damage to any metal in its path. This condition is called cavitation and must be controlled or eliminated.
- This type of trim (single stage anti-cavitation) would eliminate or contain cavitation by breaking the flow into multiple discrete streams, contracting the fluid stream as it passes through the small holes then expanding the fluid in the center of the fluid stream then colliding the stream against diametrically opposed streams imploding the bubbles in the center of the flow path away from metallic surfaces. Drilled hole technology would be used in clean fluids. This technology is considered radial flow.
- • 2-stage Anti-cavitation trim options in the 41005 Series are also based on Drilled Hole Technology. This trim design is generally applied in the Flow-to-Close (FTC) direction in cavitating liquid applications. As general rules-of-thumb for applying this trim option, the Single Stage configuration can be used in applications with an operating delta P up to 1,000 PSI or less. 2-stage trim should be used for applications with a operating delta P up to 2,000 PSI. The Multi-Stage design consists of a drilled hole plug and cage, both of which control the throttling characteristics through the valve. These are theoretical maximum operating pressure drop limits. Actual limits will be dependent upon the specific valve trim, trim velocities, Sigma index, and hydrodynamic noise or sound power levels. These trims are designed to contain cavitation by directing and collapsing the high-pressure bubbles away from the valve trim and valve body surfaces. * Specific cavitation containment performance can be predicted by using the Masoneilan Sizing & Selection Program.
- The Lincoln Log is Masoneilan premier high pressure liquid let down offering. This design goes beyond 2-stage drilled hole technology utilizing up to 9-stages of letdown in clean or dirty media. Each stage is capable of handling up to 800 psi drop in continuous duty up to 1000 psi drop in intermittent duty.
- This trim technology is considered axial flow trim utilizing a 3-dimensional multi-step flow path as the fluid passes along the length of the plug in concert with the cage increasing resistance and friction. Lincoln Log trim utilizes 5 methods of reducing velocity preventing pressure lose eliminating cavitation. Fluid stream splitting, fluid stream contracting, fluid stream expansion, turning the fluid stream and merging the fluid stream. All stages throttle in unison sharing the pressure drop where the last stage has the lowest pressure drop.
- As pressure drops increase, containment trim no longer provides an adequate solution, so the valve designer must provide unique ways to add additional stages to prevent cavitation, while still maintaining enough valve capacity (Cv) to meet required flow conditions. The trim pictured, called VRT, which stands for variable resistance trim, was engineered in the late 1970&apos;s. It features plates arranged in a brazed stack, with a small number of holes drilled in the bottom plate nearest the seat ring and increasing number of holes on subsequent plates. For many applications, the greatest pressure drops occur at low flow rates with decreasing pressure drops as the flow increases. This design forces the fluid through the greatest number of stages at low lifts corresponding to low flow rates, while it reduces the number of stages used as its travel increases, thus increasing capacity. Because the drilled holes were small, this design could not be used on a dirty or particulate entrained fluid.
- This trim design utilizes from 6 to 36 stages. To attain the same capacity as a two stage drilled hole cage design, a V-Log stack requires more than twice the travel and often a larger body gallery area diameter to accommodate 16 or more channel turns in the stack. For these reasons, V-Log trim cannot usually be packaged into more standard products like the 21000 or 41000 series bodies.
- V-Log stacks can be designed to flow axially in a single plane or multiplanar. The slide above shows an 8 turn multiplanar flow design. The number of turns can be as few as one or nominally, as many as 50. Each flow path through a V-Log stack consists of a series of fluid turns, each associated with a contracting flow area to cause a drop in pressure, followed by an expanding area to limit the velocity and allow for fluid expansion. V-Log stacks should be at the top of the application hierarchy. Brazed stacks are more expensive to manufacture than drilled hole cages, while Drilled hole cages also have about twice the capacity for a given stroke length than a brazed stack.
- This type of trim enables the valve to maintain a constant fluid velocity throughout the complete throttling process, avoiding high velocities at low turndowns which are associated with radial flow designs, therefore the possibility of erosion from wet steam, cavitating, flashing or a combination of all is eliminated completely. The flow area of the valve trim is gradually increased toward the downstream section giving optimum noise abatement and velocity control capabilities in high pressure steam letdown service. The valve plug and stem assembly is guided from the top and bottom to insure the integrity of the valve trim to high turbulence and vibration which are associated with high fluid pressure drop applications. This insures that valve plug and stem assembly maintains alignment which enables good throttling control and seat/plug alignment when valve is required to shutoff.
- Flashing is a simpler concept than Cavitation & generally it is well understood by Customers in the Instruments field. The quantity of liquid being flashed is key. From it derives the mixed phase velocity & consequent valve outlet size & the selection of the exposed material of construction. Flashing CANNOT be eliminated.
- What is Flashing? Using a familiar graphic: As the liquid is being throttled (& accelerated) through the valve orifice its static pressure drops & eventually reaches the Fluid vapor pressure Pv. Vaporization STARTS then & continues as the pressure drops towards the outlet pressure P2. The combined (mixed) fluid phases velocity is mainly that of the vapor which has by far the largest volume & entrains the liquid phase to a possibly dangerously erosive/ corrosive speed level. Let’s see if pressure recovery is critical here...
- Pressure recovery impact on flashing In flashing application pressure recovery has little impact on the resulting volume of vapor as demonstrated graphically. The geometry of the valve however has a great deal to do with it being damaged or not as flashing occurs. Why worry about Flashing?
- Read Slide Flashing must occur if P2&lt;Pv, no matter what the valve geometry is. Increasing P2 artificially through back pressure devices is ineffective as its effectiveness will vary with flow rate & more importantly may cause Cavitation. Expanding valve outlet to slow impact of water droplets can be effective in reducing velocity if other solutions are not available. Multi step must for the same reason as cavitation be carefully selected & sized. Velocity is key for the selection of a solution with each specific flashing application.
- Move to harder materials or boronizing
- High pressure liquid seat leakage is the cause. Cavitation is secondary. Change to class V
- May look like cavitation sometimes but affects all wetted parts. Cavitation is a localized damage. Change to non corrosive material
- Gases or compressible behave differently so there are different considerations for control valve selection.