selecting valves for variable hydronic flow

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.


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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


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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.

selecting valves for variable hydronic flow

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