Globe Valve?
Or
Ball Valve?
What the market Experts says ….
Globe valve VS Ball valve
Globe valve
Equal percentage flow
control
This applies from the start of the opening
of the valve
Suitable for proportional control of hot
water application & in other applications
where wide load variations can occur
Ball valve
Not fully equal
percentage
Though it has equal percentage flow
characteristic, for the first few degree
of the stem movement from closed
position, there is no equal
percentage flow control
Approximately 10% - 12 % of the
stem rotation (as show in the
figure)
Inherent ball valve characteristic
Suitable for fully open and fully closed
application
Reference: September 1998 HPAC Heating/ Piping/ Airconditioning
Globe valve VS Ball valve
Globe valve
Pipe reducer not necessary
Globe valve does not result in decrease in
effective Cv
Therefore it is a better choice for
modulating control that require small Cv
capacities (around 1)
Ball valve
Require pipe reducer
Due to High inherent capacity
Pipe reducers must be considered when
sizing the valve
Pipe reducers result in decrease in
effective valve capacity due to added
pressure loss created by the pipe reducers
With pipe reducers, they do not normally
allow stable regulation of thermal capacity
Ball valve with very small reduced ports
are more suitable for on/ off control
Reference: September 1998 HPAC Heating/ Piping/ Airconditioning
Modulating Valve Authority
Valve Port
Coil
1. Large
Valve Cv
Flow is
Limited by
Coil Cv
2. Small
Valve Cv
Flow is
Limited by
Valve Cv
3. Equal
Valve Cv
Flow is
Controlled by
Valve Cv
Globe valve VS Ball valve
Globe valve
Leak proof
Lower maintenance & downtime
Globe valve has a bonnet which
provides leak proof closure for the body
Screw-in bonnet: Offers a durable,
pressure high seal
Union bonnet: Used for application with
frequent inspection/ cleaning
Bolted bonnet: Used for higher pressure
rating
Globe valve
Ball valve
May have leakages
High maintenance & downtime/
short lifetimes
Seal degradation due to wear and
tear
Repeated opening and closely of ball valve
wears the inner surfaces of the valve
(typically teflon)
Consequence: Leakage of valve
Lifetime will shorten further under non-ideal
situation, for example when handling “dirty”
steam or water.
Repeated opening and closely of ball valve
wears the inner surfaces of the valve
(typically teflon)
Ball
valve
Reference: September 1998 HPAC Heating/ Piping/ Airconditioning/ http://pennwell.printthis.clickability.com
http://www.answers.com/topic/globe-valve
Globe valve VS Ball valve
Globe valve
Performance not affected by
idling time
Lower maintenance & downtime
Globe valve has a bonnet which provides
leakproof closure for the body
Ball valve
Performance impaired by idling
time
Repeated opening and closing of ball
valve wears the inner surfaces of the
valve (typically teflon)
When ball valves are not reposition for
extended period of time, the breakaway
torque increases
Due to seat material creeping
Eg. Valve of normal operating torque of 25in-lb might
require a torque of 37in.-lb and 49in.lb respectively
after idling for 1 week or 1 month
Consequence: May require valve to be manually
actuated – building startup/ seasonal changeover
Globe valve
Reference: September 1998 HPAC Heating/ Piping/ Airconditioning
Globe valve VS Ball valve
Globe valve
No noise
Ball valve
Noise
Ball valves creates noise because the
fluid flow cannot be quantified
Depends on the pressures and flow
conditions of the medium and the size and
port configuration of the ball valve
Less hysteresis
Actuator coupled directly to the valve
stem
Thus, inherent mechanical tolerances
does not exist
Reference: September 1998 HPAC Heating/ Piping/ Airconditioning
Greater hysteresis
Due to inherent mechanical tolerances
(clearances) between the stem extension
and the ball valve stem and between the
ball valve stem and ball
Depends on the pressures and flow
conditions of the medium and the size and
port configuration of the ball valve
What is the take away?
Recent introduction of motorized ball valves provides
a significant cost effectiveness alternative to globe
valves
However, it has inherent disadvantages and when
cost is not a significant factor
Globe valve should be still considered as a more
reliable valve in precision HVAC control
The difference between ball valve and
globe valve is RELIABILITY!
CONTROL VALVE
CLOSE-OFF RATING
The close-off rating of a valve refers to the maximum pressure drop that
a valve can withstand without leakage while in full closed position.
The close-off rating is a function of actuator power to hold the valve
closed against pressure drop.
Since most HVAC systems utilize valves as the final control elements for
regulating flow of a cooling or heating fluid, the close-off rating of valves can
affect energy usage and precise temperature control.
If control valves are unable to close-off against high differential water
pressure, especially under lowload conditions, over cooling and energy
wastage can result.
Cv vs. Close-off Rating
HEAD
PRESSURE
LARGE
FORCE
LOW
CLOSE-OFF
v
C
gh
i
Force = Pressure * Area
H
HEAD
PRESSURE
LOW
FORCE
Lo
v
C
w
High Cv = Low Close-Off
HIGH
CLOSE-OFF
DIFFERENTIAL PRESSURE CONTROLLER
IMPORTANCE OF CONTROL
VALVE CLOSE-OFF
AT THE CLOSED POSITION, ALL THE
PRESSURE DROP ACROSS THE AHU (X)
APPEARS ACROSS THE VALVE. ONLY A
VALVE WHICH HAS GOOD CLOSE-OFF
RATING
CAN
PERFORM
PRECISION
TEMPERATURE CONTROL UNDER LOW LOAD
OR LOW FLOW CONDITIONS.
P-X
CHILLED
WATER
PUMPS
CHILLED WATER
BYPASS VALVE
CHILLER
CHILLER
SUPPLY
HEADER
CHILLER
RETURN
HEADER
Proportional Control
Equal percentage valve characteristic gives
stable, linear temperature control in mild weather