Schneider Electric EN-205 Application Information
Engineering Document
EN-205
Water and Steam System Guidelines
01
INTRODUCTION
All heating and cooling systems are susceptible to
valve and system problems caused by improper
fluid treatment and system storage problems.
These guidelines are provided to help avoid valve
and water system problems from improperly
treated water or storage procedures in cooling, hot
water, and steam systems, and to obtain maximum
life from Schneider Electric valves. While all cooling
and heating systems are susceptible to problems,
closed chilled water systems, including those
containing brine or glycol, are especially prone to
system and valve problems.
The best way to avoid problems is to follow the
advice of professional water treatment and control
specialists.
LEAK PREVENTION
Durability of valve stems and packing is dependent
on maintaining non-damaging fluid conditions. Inadequate treatment or filtration, not in accordance
with the recommendations of a qualified treatment
specialist or the ASHRAE handbook recommendations, can result in corrosion, scaling, or abrasive
particle formation. Scale and corrosion products
can migrate from pipe walls to control valves,
resulting in stem and packing scratches, and can
adversely affect packing life and other parts of the
hydronic system. This condition can be avoided by
the use of proper cleaning, treatment chemicals,
and storage procedures.
SYSTEM COMMISSIONING AND STORAGE
Cleaning
New systems usually contain dirt, solder flux, and
weld and pipe scale. Thorough flushing with a 1%
to 2% solution of trisodium phosphate and thorough rinsing is necessary.
Wet Storage
If the system is stored wet, it should be completely
filled with properly treated water and isolated to
avoid slow leaks, which can contribute to serious
corrosion problems.
Dry Storage
If drained, the system should be air dried, sealed,
and treated with a desiccant to prevent “atmospheric corrosion” of pipes, a major source of “pipe
scale.” Pipe scale is dried rust which will slough off
the pipe walls as abrasive particles and migrate
throughout the system.
Water must be treated and soft. Trace leaks of hard
water result in hard calcium carbonate particles
on the outside of the valve, which after time will
scratch the sealing members creating leak potential.
To maintain non-damaging conditions, the system should be cleaned prior to start-up. Filtration
equipment should be used where needed, and a
regularly scheduled program of water condition
monitoring and/or treatment should be followed.
Control valve operation should be stable and not
hunt at any time. Excessive stroking of the valve
stem due to improper system setup can result in
premature wear.
Engineering Document
EN-205
02
STRAINERS AND FILTERS
Many closed water systems have slow leaks or•
seepage, resulting in water loss without particulate removal. Consequently, particulate solids
often build up in closed systems, resulting in
deposits. In open systems like cooling towers,
particulate solid build up is not as common because continuous “blowdown” is used to remove
solids from the system.
Side stream water filtration is often needed in•
closed systems because there is no regular
blowdown to remove pipe scale, sand, grit, and
other abrasive or sticky particulate matter. Abrasive particles must not be allowed to circulate
through the system.
To determine whether a filtration system is•
required, perform a visual inspection of the
water. Flush a line with turbulence to assure that
a representative water sample is collected and
observe the turbidity. Let the water settle for
5 minutes and inspect for particulate that has
dropped out.
If chip scale and particulate are found in circula-•
tion, install some type of filtration device such
as a “Y” strainer, a cartridge filter, an automatic backwashing side stream sand filter, or a
“chemical pot feeder” packed with cheesecloth
that can replaced periodically. Backwashing
sand filters (sized at 1/2% to 3% of system circulation rate) are often a good choice, because
they are simple, inexpensive, and effective.
Lines carrying water to and from the filtration•
system should be sized for high flow rates to
make sure the particulate matter is carried into
the filtration system.
Filtration is often necessary when chemical•
treatment is started in a system which has not
previously been chemically treated. The treat
ment often dislodges old deposits, which then
migrate to heat exchangers and valves unless
removed by filtration.
Before installing a sophisticated filtration sys-•
tem, make sure strainer baskets are emptied
regularly. Also make sure the baskets have not
been permanently removed — a common practice when they “fill up” quickly and too much
work is required to keep them clean.
Before installing filters or strainers in systems•
containing glycol, consult the glycol vendor for
the proper type.
CHEMICAL WATER TREATMENT
If the make-up water hardness is greater than 50 ppm (3 grains per gallon)•
as calcium carbonate, the water should be softened or a treatment should
be used that contains a polymeric “dispersant” material which forms a soft
sludge not allowing the formation of hard scale or gritty residue.
Make-up water iron should be less than about 1.0 ppm. Manganese should•
be less than 0.1 ppm (0.05 ppm if the system has significant leakage). If
not, an iron/manganese removal system or a new water source should be
used.
Water treatment control addresses four problem areas: corrosion, scale,•
deposition, and bacteria. For control, a nitrite or molybdate based program
is typically used in conjunction with testing and monitoring.
The corrosion control program most commonly used is 600 to 1200 ppm•
sodium nitrite or 100 to 300 ppm molybdate, at a pH of 9.5 to 10.5. Include
a copper corrosion inhibitor such as Tolytriazol (TTA) or Benzotriazole (BZT)
since uncontrolled copper corrosion can lead to corrosion of steel.
The addition of glycol, especially automotive antifreeze, does not assure•
corrosion protection. Specify industrially inhibited ethylene glycol (phosphate based) without silicates to ASTM D1384. Refer to the manufacturer’s
literature for specific requirements, including concentrations and materials
of construction.
Control of bacteria is important because bacteria can break down the ni-•
trites. The level of bacteria should be kept at less than 10,000 CFUs (colony
forming units) per ml of water. Follow your supplier’s instructions for bacterial control.
Operate your chemical treatment program within the guidelines set by your•
water treatment supplier. Monitor results monthly, switching to weekly if
problem resolution is necessary.
Boiler water treatment for steam systems should be continuous. Follow•
industry guidelines such as “Marks Standard Handbook for Mechanical
Engineers.” For oxygen removal, catalyzed sodium sulfate is usually recommended.
Warning: Be ca reful using hydrazi ne that results in ammoni a and must be controlled to
prevent stress co rrosion and embrittl ement leading to fractu re of certai n brass alloys.
CONTROL LOOP OPERATION
Valves should not be oversized. Refer to CA-28, Control Valve Sizing, F-13755,
for information on proper valve sizing and selection. Set the control system
operating parameters so that hunting does not occur, even at light load conditions such as fall, spring, and morning operation. Valves which cycle often
or continuously require a preventative maintenance program to replace worn
parts.
Schneider Electric
F-26080 -5 April 2011 ao
© 2011 Schneider Electric. All rights reserved.
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