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