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8/2/2019 Selecting Valves for Variable Hydronic Flow
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12 A S H R A E J o u r n a l a s h r a e . o r g S ep tem b er 2005
By Gil Avery, P.E., Fellow/Life Member ASHRAE
About the Authori very, . ., is irector of systems researc wit
The Kele Companies in Memphis, Tenn.
Valves, such as common globe, butterfl y, and ball valves,
that performed satisfactorily for constant fl ow or variable
fl ow primary-secondary systems generally are not satisfactory for
primary-only variable fl ow systems because of the great variations
in differential pressures across modulating control valves in direct
return hydronic systems.
The valve requirements for primary-
only systems are more severe than those
or primary-secon ary systems ecause
the pump head on primary-only systems
o ten is 20% to 00% greater t an t e
pump head of the secondary pump of
a primary-secondary system. For ex-
amp e, inFigure t e coi va ves must
be able to operate properly under all
con itions an wit a pump ea 0 t
(120 kPa) greater than the head required
for a secondary pump on a primary-sec-
on ary system.
his article reviews the limitations of
commonly used control valves for these
systems an speci es t e requirements
necessary for valves to meet the high
i erentia pressures preva ent in pri-
mary-only systems.
wo-way modulating valves have
our operationa ratings t at s ou e
considered when selecting the valve for
t ese systems.
Close-off: determined by the
power of the actuator and the valve
construction.
Static: etermine y t e strengt o
the valve body and generally avail-
able with ANSI 150 or 300 ratings. Rangeability: t east 200 to . (T e
ratio of the maximum flow with the
va ve wi e open to t e minimum
controllable flow.)
Dynamic: the maximum differential
the valves wetted parts will with-
stand. This rating has not been de-
ne y our in ustry. Manu acturers
sometimes refer to it as the modulat-
ing rating. Dynamic rating is more
descriptive and will be used in this
article. A valve with a close-off rating
o 00 psi ( 90 Pa) an a ynamic
(modulating) rating of 25 psid (172
Pa) s ou not e use on systems
where the valve differential pressure
is ever greater than 25 psid (172 kPa)
even t oug it wi c ose o against
100 psid (690 kPa). Differential pres-
sures greater t an 25 psi ( 72 Pa)
2005, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Reprinted by permission from
ASHRAE Journal, (Vol. 47, No. 9, September 2005). This article may not be copied nor distributed in either paper or digital form without
ASHRAEs permission.
8/2/2019 Selecting Valves for Variable Hydronic Flow
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8/2/2019 Selecting Valves for Variable Hydronic Flow
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14 A S H R A E J o u r n a l a s h r a e . o r g S ep tem b er 2005
start up, va ves are avai a e wit various evices, inc u ing
flow limiting regulators, pressure independent actuators, and
automatic and manual balancing valves. Each of these devices
a s pump ea to t e system an increases t e operating an
maintenance costs for the life of the system.
Over raw is rare y a pro em on y ronic systems. T emaximum flow on start up, when the valves on Coils A through
F inFigure are fully open, is about 690 gpm (44 L/s); or an
over ow o a out 90 gpm (5.7 L s). ny increase in c i e
water flow will just decrease the cool-down time.
W en over raw to a coi oes occur, t e supp y air tempera-
ture will drop and the temperature sensor will start to close
the valve to minimize the overdraw. Most systems that do not
operate continuous y are starte in t e morning w en t e coo -
ing load is less than design. The internal load (lights, people,
equipment, etc.) an transmission oa are a e ow esign an
overdraw is improbable. On some systems, no outside air load
exists until the building is occupied.
Valves once considered industrial and deemed too costly for
HVAC systems are now available for about the same price per
Cv as common g o e va ves. T e va ve c art inTa e 2 ists t e
features and relative prices for six types. The rotary-segmented
p ug va ve an ou e-seate g o e va ve ave c aracteristics
that negate the need for any flow control accessory. The plug
valve also is priced lower than globe or ball valves furnished
wit ow contro evices.
Industrial engineers generally select control valves with the
proper c aracteristics to matc va ve per ormance wit sys-
tem operation. Valves with flow accessories are not normally
considered. The HVAC industry should follow this approach
Pressure Rise Ratio*
2 5 0 20
inimum Contro a e F ow mValve
Rangea i ity
10 14.1 22.4 31.6 44.7 10
5 7. .2 5.8 22. 202.5 3.5 5. 7.9 .2 0
1 1.4 2.2 3.2 4.5 100
0.5 0.7 . .58 2.2 200
0.3 0. 7 0.75 .05 . 9 300
* The ratio of the maximum to minimum pressure drop across the control valvewhen handling required flow under varying dynamic conditions.
Table 1: Minimum controllable flow100 gpm coil.
Figure 1 (left): Chilled water variable primary distribution system for a multistory building. Figure 2 (right): Chilled water variable primarydistribution system for a typical small coil zone.
since many of the valve standards and tests have come from
t e in ustria mar et.
Modulating valves for variable flow systems should:
Be selected to operate and close tight at a valve differential
pressure as great as the pump cutoff head or 1.5 times the
pump design head.
Have a o y rating to wit stan t e static system ea
plus the pump cutoff head plus the fill pressure. InFigure
t e o y pressure may e as ig as 500 t (2 psi
[1500 kPa]).
Have a rangeability of at least 200 to 1 when tested per
NSI IS -75. .0 -2002 stan ar .
Meet ANSI 150 or 300 standard body ratings that conform
to t e SME B .3 - 99 stan ar .
Meet ANSI Class VI leakage rating.
Have actuators with enough power to close the valve tight
8/2/2019 Selecting Valves for Variable Hydronic Flow
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S ep tem b er 2005 A S H R A E J o u r n a l 15
a ve ype ipe ize, yp. ax D yp. ax emp.Cv Typical Rangeability
(Construction) Connection (psid) (F)
Rotary 1 in. NPT 24 200 400 $$ 300:1
Segmented Plug 1.5 in. NPT 55 200 400 $$
Valve (Stainless 2 in. NPT 77 200 400 $$
Trim, Graphite 3 in. Flanged 207 200 400 $$
Seat, Teflon 4 in. Flanged 350 200 400 $$Gaskets) 6 in. Flanged 507 200 400 $$
Standard Globe 1 in. NPT 12 100 340 $$ Approx. 20:1 to 50:1
Valve (Stainless 1.5 in. NPT 30 100 340 $$
Trim, Metal Seat, 2 in. NPT 47 100 340 $$
Teflon Gaskets) 2.5 in. NPT 63 60 340 $$
3 in. Flanged 100 45 340 $$
4 in. Flanged 160 25 340 $$
6 in. Flanged 400 12 340 $$
HVAC Ball Valves 1 in. NPT 19 50 212 $ ?
(Chrome Ball) 1.25 in. NPT 25 50 212 $
1.5 in. NPT 37 50 212 $
2 in. NPT 46 50 212 $
Pressure Independent 1 in. NPT N/A 50 212 $$$ ?Ball Valves 1.25 in. NPT N/A 50 212 $$$
(Chrome Ball) 1.5 in. NPT N/A 50 212 $$$
2 in. NPT N/A 50 212 $$$
Butterfly Valve 2 in. Flanged 44 75 275 $ Approx. 10:1 (Not Recommended(Stainless Disc, 2.5 in. Flanged 75 75 275 $ For Modulating Service at High DP)
EPDM Seat) 3 in. Flanged 116 75 275 $
4 in. Flanged 230 75 275 $
6 in. Flanged 605 75 275 $
Double Seated 2.5 in. Flanged 75 150 300 $$$$ Approx. 50:1 to 200:1
Globe Valve 3 in. Flanged 116 150 300 $$$$
(Stainless Trim, 4 in. Flanged 178 150 300 $$$$
Metal Seat, 5 in. Flanged 318 150 300 $$$$
Teflon Gaskets) 6 in. Flanged 390 150 300 $$$$Table 2: Rangeability and $/Cv comparisons. (More dollar signs equals more expensive.)
and positively position the valve plug at the highest dif-
ferential pressure.
Include a pressure recovery chamber to minimize noise.
Valves With Flow Coefficients (Cvs) Less Than 2
Small valves with a flow coefficient (Cv) of less than 2 are
enerally used on fan-coil units and small zone heating coils.
Most are two-position, two-way valves controlled by room
temperature sensors. They are used because:
Mo u ating va ves wit ow coe cients ess t an 2 mayhave ports that are so small they are susceptible to getting
clogged by debris in the hydronic system.
On recirculating fan coil units with no outdoor air connec-
tion, the space humidity will be less when a two-position,
rat er t an a mo u ating, va ve is use on a coo ing coi .
The modulating valve does less dehumidification in the
partia y open position.
Actuators on two-position valves are generally smaller
and fit better in the small end compartment of fan coil
enc osures.
The units are often located near or in occupied areas where
noise is a consi eration. It may e necessary to connect t ese
units to a separate piping circuit, as shown inFigure 2 to
reduce the noise and differential pressure across the valves.
T e DP sensor contro s t e mo u ating two-way supp y
valve V-1. On direct return systems the supply and return
mains s ou e i era y size to minimize t e i erence
in pressure across the branches.
The branch differential pressures are less. Therefore, the
port sizes are arger.
The comfort achieved by on-off room temperature sensors
wi e as goo or etter t an rom t e temperature sensorsfound in most residences where the sensor controls a two-
position heating valve and direct expansion cooling unit.
Summary
Now cost effective valves are available for variable flow
systems t at ont pena ize t e owner wit ig er operating
and maintenance costs for the life of their system. All control
valves must be tested and rated for the severe service that ex-
ists in t ese systems. Speci ying ig per ormance va ves wit
high rangeabilities will reduce the initial system cost by using
a ower ea pump an e iminating a t e ranc a ancing
and/or flow control devices.