Advantages and Limitations of Valve Types - Valve Type Comparison

Pressure Relief Valves *

The following summary of Pressure Relief Valve (PRV) type advantages and limitations is offered to provide relative information. The summary is not intended to be an absolute list of valve pros and cons.

Otherwise unacceptable valve types might be used if the following circumstances dictate:

specific application
prior experience
available commercial and special valve configurations
various optional accessories for pilot operated valves
rupture disc in series with the PRV
special valve location

Weighted Pallet Type
*Advantages*	*Limitations*
Low Cost	Set Pressure not readily adjustable
Very low set pressures available (down to 0.5 ounce/in²)	Extremely long simmer and poor tightness
Simple	High overpressure necessary for full lift (100% or more in some cases)
	Seat easily frozen closed at cryogenic temperatures

Conventional Metal-Seated
*Advantages*	*Limitations*
Lowest Cost (in smaller sizes and lower pressures)	Seat leakage, often resulting in lost product and unacceptable emissions, causing environmental pollution
Wide chemical compatibility	Simmer and blowdown adjustment is a compromise, which may result in intolerable leakage, product loss and high maintenance costs
High temperature compatibility	Vulnerable to effects of inlet prssure losses
Standardized center to face dimensions (API 526)	Sensitive to effects of back pressure (set pressure and capacity)
Modulating action during small pressure relief excursions may result in reduced product loss	Not normally able to obtain accurate, inplace set pressure verification
General acceptance for most applications

Balanced Bellows, Metal-Seated
*Advantages*	*Limitations*
Protected guiding surfaces and spring	Seat leakage, often resulting in unacceptable emmissions, causing loss of product and environmental pollution
Set pressure stability with superimposed back pressure	Simmer of blowdown may be unacceptable
Capacity reduced only with higher levels back pressure	Bellows life limitations
Good chemical and high temperature capabilities	High maintenance costs
	Vulnerable to effects of inlet pressure losses
	Not normally able to obtain accurate, in-place set pressure verification

Conventional or Balanced Soft-Seated
*Advantages*	*Limitations*
Good seat tightness before relieving	Temperature limited to seat material used
Good reseat tightness after relieving	Chemically limited according to soft goods used
Good cycle life and maintained tightness	Vulnerable to effects of inlet pressure losses
Low maintenance costs	Limited back pressure capability

Soft-Seated, Pilot Operated - Piston Type
*Advantages*	*Limitations*
Smaller, lighter valves at higher pressure and/or with larger orifice sizes	Not recommended for polymerizing type services without pilot purge
Excellent seat tightness before relieving	Vital to match soft goods with process conditions
Excellent reseat tightness after relieving	Limited low pressure setting (about 15 psig)
Ease of setting and adjusting set pressure and blowdown	Not generally used in dirty services without options to eliminate introduction of particles into the pilot
Pop or modulating action available	Code restricted by ASME Section I
In-line maintenance of main valve	More wetted parts exposed to fluids. Exotic materials can result in an expensive valve.
Adaptable for remote pressure sensing
Short blowdown obtainable
Set pressure can be field tested while in service
Remote unloading available
Lift not effected by back pressure (when pilot discharges to atmosphere or is balanced)

Soft-Seated, Pilot Operated-Low Pressure (Diaphragm or Metal Bellows Type)
*Advantages*	*Limitations*
Good operation at very low set pressure (3-inch wc)	Not recommended for polymerizing type services without pilot purge
Excellent seat tightness before relieving	Vital to match soft goods with process conditions
Excellent reseat tightness after relieving	Limited high pressure setting (about 50 psig)
Ease of setting and adjusting set pressure and blowdown	Liquid service limitations
Pop or modulating action available	Not generally used in dirty services without options to eliminate introduction of particles into the pilot
Adaptable for remote pressure sensing	More wetted parts exposed to fluids. Exotic materials can result in an expensive valve.
Short blowdown obtainable
Set pressure can be field tested while in service
Remote unloading available
Lift not effected by back pressure (when pilot discharges to atmosphere or is balanced)
Fully open at set pressure with no overpressure
In-line maintenance of main valve

Rupture Discs
*Advantages*	*Limitations*
Absolute tightness when disc is intact	Relatively wide burst pressure tolerances
Available in exotic materials	Non-reclosing
Minimum space required	Can prematurely burst wtih presence of pressure pulsations

Metal-to-Metal Seated, Pilot Operated - Pressure Relief Valves
*Advantages*	*Limitations*
Excellent seat tightness before relieving	Only pop action available
Excellent seat tightness after reclosing	Pressure limited to 1200 psig
Ease of setting and adjusting set pressure and blowdown	Temperature limited to 1000°F
Adaptable for remote pressure sensing
Short blowdown obtainable
Set pressure can be field-tested while in service
Excellent chemical and temperature compatibility
Dual pilot option allows in-service pilot replacement

Control Valves **

Gate Valves
*Advantages*	*Limitations*
High Capacity	Poor Control
Tight Shutoff	Cavitate at low pressure drops
Low Cost	Cannot be used for throttling
Little resistance to flow
Recommended Uses
Fully open/closed, non-throttling
Infrequent operation
Minimal fluid trapping in line
Applications
Oil
Gas
Air
Slurries
Heavy liquids
Steam
Noncondensing gases
Corrosive liquids
Best Suited For:
Frequent on-off service
Processes where "instantly" large flow is needed (ie. safety systems or cooling water systems)

Globe Valves
*Advantages*	*Limitations*
Efficient throttling	High pressure drop
Accurate flow control valves	More expensive than other valves
Available in multiple ports
Recommended Uses
Throttling service / flow regulation
Frequent operation
Applications
Liquids
Vapors
Gases
Slurries
Corrosive Substances
Best Suited For:
Liquid level or flow loops
Systems where the pressure drop across the valve is expected to remain fairly constant (ie. steady state systems)
Processes where large changes in pressure drop are expected
Processes where a small percentage of the total pressure drop is permitted by the valve
Temperature and pressure control loops

Ball Valves
*Advantages*	*Limitations*
Low cost	Poor throttling characteristics
High capacity	Prone to cavitation
Low leakage and maintenance
Tight sealing with low torque
Recommended Uses
Fully open/closed, limited-throttling
Higher temperature fluids
Applications
Most liquids
High Temperatures
Slurries
Best Suited For:
Frequent on-off service
Processes where "instantly" large flow is needed (ie. safety systems or cooling water systems)
Liquid level or flow loops
Systems where the pressure drop across the valve is expected to remain fairly constant (ie. steady state systems)

Butterfly Valves
*Advantages*	*Limitations*
Low cost and maintenance	High torque required for control
High capacity	Prone to cavitation at lower flows
Good flow control
Low pressure drop
Recommended Uses
Fully open/closed or throttling services
Frequent operation
Minimal fluid trapping in line
Applications
Liquids
Gases
Slurries
Liquids with suspended solids
Best Suited For:
Frequent on-off service
Processes where "instantly" large flow is needed (ie. safety systems or cooling water systems)
Processes where large changes in pressure drop are expected
Processes where a small percentage of the total pressure drop is permitted by the valve
Temperature and pressure control loops

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