In general, block valves that are used to isolate equipment or process units shall be of gate, ball or plug type. Hand control valves shall be of globe type. Gate valve should not be used in throttling service due to high erosion rate of gate & seat Butterfly valves should be used for cooling water service generally, 6” & larger Use of globe valves in sizes 3” and larger should be avoided due to high cost
If valve is used for particular service and it is not shown in piping clas s, then it is treated as piping speciality item.
Comparison of Basic Type of valves Valve Type
Characteristics
Use
Limitations
Gate
Gate like disc actuated by stem, screw & hand wheel move at right angle to flow
On-off service
Not good for throttling service. Throttling can cause erosion.
Comparison of Basic Type of valves
Comparison of Basic Type of valves Valve Type
Globe A) Standard
B) Y Pattern
Characteristics
Use
Disc attached to stem Preferred for seats on circular throttling service. opening. Fluid changes direction in passing through valve.
Y-type valve produces lower pressure drop & turbulence than std. globe & preferred for high P/T service
Limitations
Not recommended for on-off service. Cost & throttling efficiency above 6” become unfavorable. Not recommended where solids or corrosives are present Same as above
Comparison of Basic Type of valves
Comparison of Basic Type of valves
Comparison of Basic Type of valves Valve Type
Characteristics
Use
Limitations
Butterfly Valve
Circular disc is pivoted at centre & opened by ¼ turn of external handle.
Where minimum pressure drop & face to face dimensions is required. May be used for throttling
Positive shut-off is not always possible except when disc or seals are coated with elastomers which may limit max allowable temperature
Comparison of Basic Type of valves
Comparison of Basic Type of valves Valve Type
Angle (90)
Characteristics
Similar to globe valve except inlet & out make 90 angle
Use
Same as globe. Used for high pressure drop service.
Limitations
Bends in piping system are subject to strains that should be placed on valves.
Comparison of Basic Type of valves
Comparison of Basic Type of valves Valve Type
Characteristics
Plug Valve
Tampered plug with hole same shape as interior of valve body.
Use
Limitations
On-off service. More May be difficult to positive shut-off than operate. gate valve. Also used for throttling but characteristics not as satisfactory as globe for such service.
Comparison of Basic Type of valves
Comparison of Basic Type of valves Valve Type
Characteristics
Use
Ball Valve
Related to plug valve except movable element is ball. ¼ turn from full open to full closed
Similar to plug valve. Low pressure drop across valve. Full port valve reduces holdup of solids.
Limitations
Limited temp range because of elastomer seats. Gives tighter shut-off than plug
Comparison of Basic Type of valves
Comparison of Basic Type of valves Valve Type
Characteristics
Check valve
Use
Limitations
General: prevent back flow in lines
a) Swing check
Flow keeps swing gate open while reverse flow close it.
Best for liquids & less affected by solids in fluid.
Not suitable for pulsating flow. Some style operate only in horizontal position.
b) Dual flapper check
Dual flapper
Lower pressure drop than swing check
Not suitable for pulsating flow.
a) Piston check
Flow pattern as globe valve
Good for vapor, steam, water. Suitable for pulsating flow
Design for horizontal service. Not recommended over 6” line size & service where solids
Comparison of Basic Type of valves Valve Type
d) Ball check
Characteristics
Use
Stops flow reversal more rapidly than others. Good for viscous fluids. Vertical & horizontal installation is possible
Limitations
Not common over size 6” not suitable for pulsating flow can act as flow limiting device.
Comparison of Basic Type of valves
Comparison of Basic Type of valves Valve Type
Automatic Control
Characteristics
Similar in principle to globe valve but precision built for accurate automatic control. Air pressure actuates the diaphragm causing stem to move, opening or closing valve orifice. Double port valve gives better control range & require only small force to move stem.
Use
Limitations
First cost is high. Double port valves are very good for tight shut-off. Control valves which normally throttle are subject to wear and are normally not preferred for emergency shut-off purposes. Control valves often have bypass or hand wheel. Control valves require tight shutoff (TSO) or bubble tight shut-off (BTSO) must be indicated in P&ID.
Comparison of Basic Type of valves Valve Type
Characteristics
Self-regulating automatic control
Sensing element & control valve in single unit
Use
When local control is satisfactory as fixed pressure reduction stations
Limitations
Require local readjustment for any required changes in downstream flow or pressure conditions
Comparison of Basic Type of valves Valve Type
Characteristics
Packless Diaphragm seal the Diaphragm valve bonnet, seating member may be separate disc diaphragm or solid diaphragm
Use
For corrosive, volatile , toxic fluids service where leakage cannot be tolerated
Limitations
Diaphragm may be rubber like or plastic material which cannot withstand temp above 40 °C Cannot considered as ‘fire safe’ due to use of elastomers
Comparison of Basic Type of valves Valve Type
Relief valve
Characteristics
Valve opens automatically when force on seat exceeds that of spring. Return to closed position when pressure drops below set pressure
Use
Limitations
For protecting Require periodic equipment from inspection to insure overpressure operability
Valve Arrangements Around Pumps •
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Gate valves on suction & discharge for isolation Check valve to avoid back flow Check valve location: upstream of discharge block valve & close to pump nozzle
Exchanger Valving •
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Upper detail is cheaper, however it requires complete shutdown of cooling system Lower detail is preferred by most of clients as it allows service users without complete shutdown of drainage of cooling system
Valve Around Flow meter •
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Globe valve in bypass to maintain proper throttling & regulation Downstream valve shall be either globe or needle valve if needle valve is integral with rotameter then downstream valve should be gate valve.
