Pressure Reducing Stations; Mixing Station; Key Points; Heat Transfer Substations - Honeywell AUTOMATIC CONTROL Engineering Manual

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
PRESSURE
MAXIMUM
SAFETY
LIMIT
SUPPLY
FLOW
RETURN
FLOW
MINIMUM
SAFETY
LIMIT
HEAT SOURCE
Fig. 127. Network Pressure Profile.
A booster pump station overcomes the pressure drop in areas
with considerable differences in altitude (to overcome 165 ft
difference in altitude a pressure of 71.5 psi is required). They are
often applied where going under or over obstacles is necessary.
Equip the pipeline with an emergency shut down system (ESD)
in areas with considerable differences in altitude to protect the
system from high pressures in case of power failure.

PRESSURE REDUCING STATIONS

A pressure reducing station is the counterpart to the booster
pump station. A pressure reducing station is used in lines
located in mountainous areas to protect the pipeline from over
pressure and to keep the pressure in the return line lower than
the supply line. For this application pressure reducing valves
are control valves.

MIXING STATION

A mixing station (Fig. 128) is used in hot water networks. It is
a variable speed (mixing) pump which mixes cooled return flow
directly into the supply flow to reduce the supply flow line
temperature to the required level. These facilities are used to
provide different maximum temperatures in the network pipeline.
SUPPLY FLOW (HIGH TEMP)
RETURN FLOW
Fig. 128. Principle Of A Mixing Station.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
MINIMUM
PERMISSIBLE
DIFFERENTIAL
PRESSURE
DISTANCE
CONSUMER M11443
SUPPLY FLOW (LOW TEMP)
MIXING PUMP
M11444

KEY POINTS

Key points are locations where pipelines branch off (Fig. 129).
They consist of valves for supply and return flow which can
separate the branch line from the main line. If a branch line is
shut down, the pressure ratio in the entire network is affected.
To prevent pressure spikes in the main system, the shutdown
must be performed slowly and carefully. Often two different
sized surge tanks in parallel are used to damp the pressure peak
during shutdown. Usually key point temperatures and pressures
are monitored.
SUPPLY FLOW
MAIN LINES
RETURN FLOW
Fig. 129. Typical Key Point.

HEAT TRANSFER SUBSTATIONS

In general heat transfer substations link district heating
networks with the consumer. The consumer side can be either
another network or the end user.
Heat transfer can be either direct and indirect. Direct transfer
uses mixing valves, jet pumps or two way valves to supply the
heating medium directly to the consumer. Indirect substations
use heat exchangers and physically decoupled or independent
heating circuits.

Direct Heat Transfer Substations

The main parts of a direct substation are:
– Primary side.
– Heat flow regulation unit.
– Circulating pumps.
– Secondary side.
Direct heat transfer substations:
– Transfer the required heat from the supply (primary) side
to the consumer (secondary) side.
– Meter heat.
– Provide safety functions to protect consumer and
equipment against overheating, frost, and harmful agents
in hot tap water.
– Provide optimization functions to reduce energy
consumption to the lowest possible level.
386
BRANCH
SURGE
TANKS
M11445