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Valve Selection & Sizing

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Information about Valve Selection & Sizing
Education

Published on January 13, 2009

Author: ran_nwd

Source: slideshare.net

Description

Brief description about sizing consideration of valves in process industries & selection as per control strategy.
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Valve Sizing & Selection Ranjeet Kumar M.Tech – Chemical

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.

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.

Define the System – Key Variables Total pressure drop, Design flow, Operating flow, Minimum flow, Pipe diameter, Specific gravity

Total pressure drop,

Design flow,

Operating flow,

Minimum flow,

Pipe diameter,

Specific gravity

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).

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).

Valve Characteristics – C v Note – Check thumb rules before referring valve chart or characteristic curve.

Note – Check thumb rules before referring valve chart or characteristic curve.

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.

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.

Valve Chart / Characteristics Curve Table 1 – Chart for Equal Percentage Globe Valve

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)

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

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.

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.

Basic Valve Types – mechanical characteristics Ball valves; Diaphragm valves; Gate valves; Globe valves; Butterfly valves; Plug valves; Check valve; Safety/relief valve

Ball valves;

Diaphragm valves;

Gate valves;

Globe valves;

Butterfly valves;

Plug valves;

Check valve;

Safety/relief valve

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.

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.

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

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

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

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

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

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

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

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

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