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Inside
Insights
Is Bigger Better for
Control Valves?
Right-sizing combines art,
science, experience
Steve Tredinnick, PE, Infrastructure Project Engineer/Manager, Affiliated Engineers Inc.
Editor’s Note: : “Inside Insights” is a column designed to address ongoing issues
of interest to building owners, managers
and operating engineers who use district
energy services.
I
, for one, will never complain about a
TV that is too large, a stomach that is
too flat, a bank account that is too
great, a car that is too fast, or having too
much vacation time! Yeah, baby! I am not
all that different from any other American
– we want it all, and we want it big! But
having the biggest and ‘baddest’ is not
always the best alternative. Just like having
too much of a good thing is not always
good, having something too big may not
be appropriate in all situations.
In the fourth quarter 2006 edition of
this column, I addressed the size of decouplers in primary/secondary chilled-water
pumping systems and how some installation/design deficiencies benefit from ‘supersizing.’ Can this philosophy be applied to
other devices in a hydronic system such as
control valves? Is bigger better all the time?
To explore the answers to these questions, let’s start by defining the main parameters required to correctly determine the
size of a hydronic control valve:
● Flow rate (i.e., load)
● Pressure drop (coil, branch piping,
valve authority component)
● Pipe size
Rangeability (minimum to maximum
flow or turn-down)
● Valve-sizing coefficient (C )
v
● Shut-off pressure
● Actuator selection
● Valve body-type configuration
(two-way or three-way)
● Valve type (ball, globe, butterfly, etc.)
● Control type – two position or throttling
● Flow characteristics (linear, equal
percentage, etc.)
There are other valve selection parameters such as allowable noise/cavitation,
materials of construction, temperature and
pressure ratings, connection types, allowable leakage, normally open/closed, etc.,
but these do not really affect the performance of the valve. Correct flow and pressure-drop calculations are the two key
selection parameters for pressure-dependent valves.
●
Evaluating Flow and
Pressure Drop
The flow rate through the valve and
coil are usually sized for the peak-load
condition on the hottest or coldest day –
an event that occurs less than 1 percent of
the total heating/cooling hours in a year
(based on ASHRAE climatic data) – so a little
oversizing occurs here, since one does not
want to undersize the coil. However, grossly
overestimating the loads will oversize the
control valve.
Reprinted from First Quarter 2007 District Energy magazine with permission of IDEA.
The pressure drop is comprised of the
friction losses of the components within the
piping circuit, including the coil, valving,
pipe and fittings. Without getting in too
much detail, this typically amounts to 30
percent to 50 percent of the total system
pressure drop at design conditions. To a
certain extent, the temperature control
system may correct for errors in estimated
pressure losses by intelligently modulating
the control valve; however, if the pressure
drops are grossly overestimated, the result
will be an oversized valve.
If a valve is oversized, a small increment of valve movement varies the flow
greatly; hence, the valve is always modulating or hunting trying to satisfy a setpoint that is constantly being over- and
undershot. It’s just like trying to drink from
a fire hose: It is just too much flow to handle, and it’s very hard to control properly.
Uncontrolled and unstable
flow will often lead to
occupant discomfort and
system inefficiency...
The constant modulation will lead to system control instability. Uncontrolled and
unstable flow will often lead to occupant
discomfort and system inefficiency (low
Delta T). In addition, at low loads, the
valve barely lifts off its seat. That creates a
high-velocity fluid stream that produces
noise from the turbulence and wears a
thin channel or groove into the valve seat,
reducing its life. The groove becomes a
‘leak’ in the system even when the valve is
closed, which results in a wire-drawn valve.
Such traditional methods of selecting
a valve use the valve-sizing coefficient (Cv).
A Cv is a unit-less parameter that is calculated from the flow and differential pressure
duty of the valve. (See equation below.) Cv
is used in manufacturers’ valve charts to
more accurately select a valve.
For water at 60 degrees F,
Cv = GPM x √1/∆P
Not only does control valve oversizing
affect system performance, but actuator
sizing does as well. The actuator should be
sized to close against system pressure greater
First Quarter 2007
53
than the distribution system pump head. If
the actuator cannot close the valve against
the system pressure, then the valve will be
lifted of its seat and create a ‘leak’ in the
system. System leaks resulting from oversized valves and undersized actuators are
probably the largest contributor to low Delta
T. Further, excessive modulation can lead to
premature actuator failure and valve-seal
failure, creating maintenance issues.
A Look at Other Valve Types
The above discussion pertains specifically to pressure-dependent valves – probably the most prevalent valve type used. The
selection process may sound complicated,
but it is a mixture of art, science and experience. Other valve types may be easier to
select and specify. Pressure-independent
control valves (PICVs) are being used increasingly in the HVAC industry.
PICVs are selected using a much simpler
method based on the maximum flow rate
expected in the coil and not on differential
pressure or valve-sizing coefficient. A PICV
does not allow a change in flow rate when
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District Energy
the pressure differential across the valve
changes. Although not a panacea for all
applications, the PICV’s use has improved
the Delta T performance of many water
systems and does not require any balancing.
Traditionally, throttling valves are one
size smaller than the pipe size serving the
branch piping, and a line-sized valve may be
oversized depending on its duty. However,
just because a valve is line-sized does not
mean that it is oversized. Line-sized valves
are acceptable in some applications, such
as two-position valves (e.g., actuated isolation valves) where lower system pressure
drop is required. Line-sized valves are usually
not acceptable in throttling applications.
Right-Sizing Is Best
Since it regulates the system’s flow and
provides comfort to building occupants, the
control valve is probably the single-most
important element in a hydronic system. A
valve must be ‘right-sized’ for its specific
application: A valve that is too small will not
have sufficient capacity, and a valve that is
too large will cause instability issues and add
cost. A successful control-valve selection
procedure that follows acceptable engineering practice and manufacturer’s recommendations will help ensure that occupant
comfort and efficient operation are achieved.
Right-sizing wins every time!
Steve Tredinnick, PE, , is
a project engineer/manager
for Affiliated Engineers in
Madison, Wis., with more
than 20 years’ experience
related to building HVAC
systems. The past 10 years
of his work have been
focused on district energy systems. Tredinnick
is a graduate of Pennsylvania State University
with a degree in architectural engineering. He
is a member of IDEA and ASHRAE and is currently chair of ASHRAE TC 6.2 District Energy.
Tredinnick may be reached at stredinnick@
aeieng.com.
Column and previous columns available at
www.districtenergy.org/de_magazine.htm
Reprinted from First Quarter 2007 District Energy magazine with permission of IDEA.