Valve Selection & Sizing


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Brief description about sizing consideration of valves in process industries & selection as per control strategy.

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Valve Selection & Sizing

  1. 1. Valve Sizing & Selection Ranjeet Kumar M.Tech – Chemical
  2. 2. Steps in Sizing <ul><li>Science with many rules of thumb </li></ul><ul><li>Define the System. </li></ul><ul><li>Maximum Pressure Drop for the Valve. </li></ul><ul><li>Calculate the valve Characteristics (C v ). </li></ul><ul><li>Preliminary valve selection. </li></ul><ul><li>Check the C v and stroke percentage at minimum flow. </li></ul><ul><li>Check the gain across applicable flow rates. </li></ul>
  3. 3. Define the System – Key Variables <ul><li>Total pressure drop, </li></ul><ul><li>Design flow, </li></ul><ul><li>Operating flow, </li></ul><ul><li>Minimum flow, </li></ul><ul><li>Pipe diameter, </li></ul><ul><li>Specific gravity </li></ul>
  4. 4. Maximum Allowable Pressure Drop <ul><li>The usual rule of thumb is that a valve should be designed to use 10-15% of the total pressure drop or 10 psi, whichever is greater . </li></ul><ul><li>Investigate the Pump & its maximum available head. </li></ul><ul><li>MAPD = NPHSA – NPSHR. </li></ul><ul><li>Trade off- </li></ul><ul><ul><li>larger pressure drops increase the pumping cost (operating) & </li></ul></ul><ul><ul><li>smaller pressure drops increase the valve cost because a larger valve is required (capital cost). </li></ul></ul>
  5. 5. Valve Characteristics – C v <ul><li>Note – Check thumb rules before referring valve chart or characteristic curve. </li></ul>
  6. 6. Preliminary Valve Selection <ul><li>Thumb Rules – </li></ul><ul><ul><li>Never use a valve that is less than half the pipe size. </li></ul></ul><ul><ul><li>Avoid using the lower 10% and upper 20% of the valve stroke. The valve is much easier to control in the 10-80% stroke range. </li></ul></ul><ul><li>Select the type of valve & use its corresponding valve chart. </li></ul><ul><li>Valve chart is supplied by manufacturer. </li></ul><ul><li>Check C v for minimum flow with selected size of valve & check for minimum flow should not fall below 10% of valve stroke. </li></ul>
  7. 7. Valve Chart / Characteristics Curve Table 1 – Chart for Equal Percentage Globe Valve
  8. 8. Gain across applicable flow rates <ul><li>Gain #1 = 85/38 = 2.2 Gain #2 = 40/12 = 3.3 </li></ul><ul><li>Acceptance criteria :– </li></ul><ul><ul><li>Gain should never be less than 0.5. </li></ul></ul><ul><ul><li>Gain#2 – Gain#1 < 50% of (max of Gain#1 or Gain#2) </li></ul></ul><ul><ul><ul><li>Here 0.5 (3.3) = 1.65 and 3.3 - 2.2 = 1.10.  Since 1.10 is less than 1.65 so it can be a choice </li></ul></ul></ul>85 – 73 = 12 150-110 = 40 85 150 73 110 73 – 35 = 38 110 – 25 = 85 35 25 Change in stroke (%) Change in Flow(GPM) Stroke (%) Flow (GPM)
  9. 9. Choke Flow F L <ul><li>At max restriction – flow rate is max & pressure is minimum. </li></ul><ul><li>Vapor bubbles flashes if liquid pressure falls below Vapor Pressure of liquid. </li></ul><ul><li>Bubbles has (a) no effect on flow, (b) increases pressure drop across valve, (c) cavitations </li></ul><ul><ul><ul><ul><ul><li>Decreased efficiency </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Noise </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Vibration </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Material loss – sand blasted surface </li></ul></ul></ul></ul></ul><ul><li>F L checking is recommended when difference in Max & Min flow exceed 90% of Max flow. </li></ul>
  10. 10. Basic Valve Types – mechanical characteristics <ul><li>Ball valves; </li></ul><ul><li>Diaphragm valves; </li></ul><ul><li>Gate valves; </li></ul><ul><li>Globe valves; </li></ul><ul><li>Butterfly valves; </li></ul><ul><li>Plug valves; </li></ul><ul><li>Check valve; </li></ul><ul><li>Safety/relief valve </li></ul>
  11. 11. Basic Valve Types – control / openness <ul><li>Equal Percentage :  equal increments of valve travel produce an equal percentage in flow change. </li></ul><ul><ul><li>Large changes in pressure drop are expected </li></ul></ul><ul><ul><li>Minimum pressure drop due to valve </li></ul></ul><ul><ul><li>In temperature & pressure control loop </li></ul></ul><ul><li>Linear :  valve travel is directly proportional to the valve stoke </li></ul><ul><ul><li>In liquid level or flow loops </li></ul></ul><ul><ul><li>Pressure drop across valve is almost constant. </li></ul></ul><ul><li>Quick opening :  large increase in flow with a small change in valve stroke </li></ul><ul><ul><li>For frequent on-off services </li></ul></ul><ul><ul><li>Instantly large flow is needed. </li></ul></ul>
  12. 12. Gate Valve <ul><li>Best Suited Control:  Quick Opening </li></ul><ul><li>Recommended Uses: </li></ul><ul><ul><li>Fully open/closed, non-throttling </li></ul></ul><ul><ul><li>Infrequent operation </li></ul></ul><ul><ul><li>Minimal fluid trapping in line </li></ul></ul><ul><li>Advantages:    </li></ul><ul><ul><li>High capacity </li></ul></ul><ul><ul><li>Tight shut off, Low cost, Little resistance to flow   </li></ul></ul><ul><ul><li>                      </li></ul></ul><ul><li>Disadvantages: </li></ul><ul><ul><li>Poor control </li></ul></ul><ul><ul><li>Cavitate at low pressure drops </li></ul></ul><ul><ul><li>Cannot be used for throttling </li></ul></ul><ul><li>Applications:   Oil, Gas, Air, Slurries, Heavy liquids, Steam, Non-condensing gases, and Corrosive liquids </li></ul>
  13. 13. Globe valve <ul><li>Best Suited Control:   Linear and Equal percentage </li></ul><ul><li>Recommended use- </li></ul><ul><ul><li>Throtteling services/flow regulation </li></ul></ul><ul><ul><li>Frequent operation </li></ul></ul><ul><li>Advantages: </li></ul><ul><ul><li>Efficient throttling </li></ul></ul><ul><ul><li>Accurate flow control valves </li></ul></ul><ul><ul><li>Available in multiple ports   </li></ul></ul><ul><ul><li>                       </li></ul></ul><ul><li>Disadvantages: </li></ul><ul><ul><li>High pressure drop </li></ul></ul><ul><ul><li>More expensive than other   </li></ul></ul><ul><li>Applications :   Liquids, vapors, gases, corrosive substances, slurries </li></ul>
  14. 14. Ball valve <ul><li>Best suited control – Quick opening linear . </li></ul><ul><li>Recommended uses – </li></ul><ul><ul><li>Fully open/closed limited throttling </li></ul></ul><ul><ul><li>Higher temperature fluids </li></ul></ul><ul><li>Advantages – </li></ul><ul><ul><li>Low cost </li></ul></ul><ul><ul><li>High capacity </li></ul></ul><ul><ul><li>Low leakage & maintenance </li></ul></ul><ul><ul><li>Tight sealing with low torque </li></ul></ul><ul><li>Disadvantages – </li></ul><ul><ul><li>Poor throttling characteristics </li></ul></ul><ul><ul><li>Prone to cavitation </li></ul></ul><ul><li>Applications – Most Liquids, high temperatures, slurries </li></ul>
  15. 15. Butterfly valve <ul><li>Best Suited Control:   Linear, Equal percentage </li></ul><ul><li>Recommended Uses : </li></ul><ul><ul><li>Fully open/closed or throttling services </li></ul></ul><ul><ul><li>Frequent operation </li></ul></ul><ul><ul><li>Minimal fluid trapping in line </li></ul></ul><ul><li>Advantages : </li></ul><ul><ul><li>Low cost and maint. </li></ul></ul><ul><ul><li>High capacity </li></ul></ul><ul><ul><li>Good flow control </li></ul></ul><ul><ul><li>Low pressure drop </li></ul></ul><ul><li>Disadvantages – </li></ul><ul><ul><li>High torque required to control </li></ul></ul><ul><ul><li>Prone to cavitation at lower flows </li></ul></ul><ul><li>Applications :  Liquids, gases, slurries, liquids with suspended solids </li></ul>