Valve Sizing Considerations. Steven Hocurscak Metso Automation

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1 Valve Sizing Considerations Steven Hocurscak Metso Automation

2 Oversizing Your Valves Issue with oversized valve A valve that is oversized can cause premature failure of valve trim or body - Oversized valves have low opening angles, and lower opening angles increase the velocity of the liquid - Properly sizing control valves can dramatically improve performance and life E, loss of material EROSION, E 2 FLOW VELOCITY, v

3 Guideline for Proper Valve Sizing Solution to oversizing your valves Maintaining larger opening angles (above 20%) is ideal for flow conditions that are seen often - If the condition is a startup case that will only be seen a few times a year, it s okay to size the valve in this application. - On a normal flow condition ideally we want the valve to be open around 50% 3

4 Guideline for Proper Valve Sizing Death by oversizing Seat is eroded away Segment shows signs of erosion 4

5 Guideline for Proper Valve Sizing Death by oversizing Hole in body Seat is eroded away Erosion pattern in body 5

6 Cavitation Definition of Cavitaion Each liquid has a different pressure and temperature at which it boils (or vaporizes). Water for example at atmospheric pressure will boil at 100 C (212 F). Dropping the pressure inside of a valve for example can effect the temperature at which a liquid will boil. 1 atm Pressure.006 atm Ice (solid) Water (Liquid) Triple Point Water Vapor (gas) 6.01 C Temperature 100 C

7 Cavitation P 1 P VC P 2 A valve is represented as a simple orifice plate to represent a restriction in the flow path Pressure Velocity Vapor Pressure Because of the law of conservation of energy, when the velocity of the liquid increases through the valve, the pressure must decrease 7

8 Cavitation P 1 P VC P 2 Pressure P 1 P 2 Velocity Vapor Pressure Cavitation occurs when the pressure inside the valve drops below the vapor pressure of the liquid, allowing the liquid to turn into a gas. 8

9 Pressure Shock Generation Mechanisms 9

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11 Cavitation The damage caused by cavitation is very rough and pitted and can occur quickly. 11

12 Pressure Recovery Factor F L The Pressure Recovery Factor of the valve indicates the size of the pressure recovery relative to P VC and P 2. P 1 P VC P 2 A valve is represented as a simple orifice plate to represent a restriction in the flow path P 1 P 1 P 2 F L = P 1 -P 2 P 1 -P VC P 2 Pressure Recovery P 2 P VC P VC 12

13 Pressure Recovery Factor F L A high recovery valve (similar to the red line) has a greater recovery from P VC to P 2. This can mean the valve is more likely to experience problems with cavitation. The higher the pressure recovery factor (F L ) the lower the recovery of the valve and the better the valve is able to avoid cavitation and noise issues. P 1 Higher F L (Low Recovery) Lower F L (High Recovery) P 2 Pressure Recovery P 2 P VC P VC 13

14 Pressure Recovery Factor F L P 1 = 15 P 1 -P 2 P 2 = 10 Pressure Recovery F L = P 1 -P 2 P 1 -P VC P 2 -P VC P VC = 5 = = 5 =.5 =

15 Pressure Recovery Factor F L F L = P 1 -P 2 P 1 -P VC P 1 -P 2 P 1 = 15 P 2 -P VC P 2 = 10 P VC = 8 P VC = 5 = = 5 =.714 =

16 Valve Geometry Effects F L Turbulence and friction cause permanent pressure loss VELOCITY CHANGE DIRECTION CHANGE P 1 P 2 P 1 P 2 P V Vapor Pressure P V Vapor Pressure

17 Valve Geometry Effects F L MOSTLY VELOCITY CHANGE DIRECTION & VELOCITY CHANGE P 1 P 1 P2 P2 P V Vapor Pressure P V Vapor Pressure

18 Pressure Recovery Factor F L 1 Recovery Factor, FL H.P. Butterfly Globe Ecc. Plug Ball Percent Open 18

19 Sizing w/ Orifice Plates p2 P1 Pi P2 Vapor pressure Lowest system pressure w/o plate Lowest system pressure with plate 19

20 Design Features Variable Resistor in Valve 20

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22 Design Features Variable Resistor vs. Fixed Resistor A typical defined process tells us the largest dp a valve will experience will typically be on the minimum flow A fixed resistor must be sized for the maximum flow, so it s most effective on lower dp s If a liquid application is going to cavitate often times they do on a minimum / startup flow Pressure P O P Pump Curve P m System Curve System Pressure Drop 22 Q min Q max Flowrate

23 Flashing Issue with oversized valve Caused when the downstream pressure is below the vapor pressure of the liquid. Similar to cavitation, vapor bubbles form, but unlike cavitation the vapor pressure never recovers high enough for the vapor bubbles to implode Vapor bubble remain mixed in the media and travel downstream 23

24 Flashing (Liquid) P1 Pvc P2 Pressure P1 Velocity Vapor pressure P2 24

25 Flashing Damage Typically, damage potential of flashing is smaller than in cavitation. Damages are erosion type wear, smooth grooves and cavities. 25

26 Flashing Solutions It s important to understand flashing is caused by the process, not the valve. Therefore there isn t anything you can do to stop the valve from flashing, it has to be a process change. To prevent flashing damage to the body we turn the valve to the FTC or non-preferred direction. When flashing occurs it will hit the seat of the valve, instead of the body. The seat is made of a harder material, and easier to replace, thus less damaging. 26

27 Flashing - conclusions Rotary valves FTC direction Hardened trim, SST body Discharging into vessel if possible Wide valve openings Limit flow velocities Enlarger downstream pipe No Q-trim in clear flashing service (P2 << Pv) FTC direction 27 Valve Discharging Directly into Vessel