Honeywell AUTOMATIC CONTROL, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
377
HIGH TEMPERATURE WATER SAFETY
A well designed HTW system with proper installation of
piping to prevent undue stress rarely fails.
HTW drops in temperature very rapidly with a minor, low
mass, flow leak into space. In low or medium temperature water
the escaping fluid remains at nearly the same temperature. HTW
flashes to steam and the turbulent mixing of liquid and vapor
with the room air rapidly drops the fluid temperature below
212F and within a short distance to 125 to 140F.
Minor leakage at valves is usually imperceptible except for
deposits of scale at the point of leakage. Valves should be
inspected periodically and scale removed from valve stems to
prevent damage to packing and to allow free valve movement.
HTW CONTROL SELECTION
The features of good temperature control for low and medium
temperature water heating systems also apply to high
temperature water heating. Special considerations are needed
for controls to withstand the temperatures and pressures
encountered in a HTW system. The large temperature difference
between HTW and the heated fluid (air, water, or steam) means
that most of the control selection precautions are to provide
fail safe operation. Careful consideration must be given to what
happens to the secondary fluid of a converter and its controls
when flow is reduced or stopped, or equipment is shutdown.
Secondary fluid can overheat immediately after or during
prolonged shutdown. Controls near a HTW supplied coil can
be damaged by high temperature if airflow stops.
Fig. 117 Typical Nitrogen Pressurized High Temperature Water System.
Controls can be pneumatic, electric, or digital. The low mass
of electronic temperature sensors (thermocouple or resistance
bulb) provides faster response than fluid-filled or bimetal
elements. Pneumatic actuators generally have the faster
response, higher temperature ratings, and greater reliability
required for HTW applications.
Standard commercial quality controls with integral (automatic
reset) and derivative (rate) action are satisfactory for most
applications when used with industrial quality valves. An
industrial controller or a digital system may be required where
local recording of a control value is specified. The control
system selected should:
1. Function from zero to full load.
2. Close the HTW valve if there is failure in the control
system, loss of power or air, or shutdown of the system
being controlled.
HTW VALVE SELECTION
Valves for HTW must be selected to ensure suitability for
high temperature and pressure conditions. A control valve must
have wide rangeability (50 to 1 is desirable) and the ability to
position accurately and hold that position. Rangeability is the
ratio of maximum flow for a valve to the minimum controllable
flow. These requirements dictate use of industrial quality valves.
For additional information on rangeability see the Valve
Selection and Sizing section.
BOILER
EXPANSION
TANK
PRESSURE CONTROL
MANUAL AIR VENTS
AT HIGH POINTS OF
SUPPLY AND RETURN
SYSTEM
CIRCULATING
PUMP
CHECK
VALVE
MAKE-UP
FEED PUMP
PIPING FOLLOWS
CONTOURS OF
LAND.
ALL DISTRIBUTION
PIPING IS WELDED.
INSTANTANEOUS
CONVERTER
STORAGE
CONVERTER
STEAM GENERATOR
REVERSE
CONDENSATE
RETURN
DOMESTIC HOT WATER
FOR BUILDING
LOAD
UNIT
HEATERS
OTHER HEATING COILS
EXPANSION
LOOPS
NITROGEN BOTTLES
STEAM FOR PROCESS
OR AIR CONDITIONING
C2581