VALVES AND ACTUATORS
39
Overheated Water Systems
For this application the valves can be two- or three-way type.
The valve pressure drop must be high, in order to have a good control flow characteristic
and a properly working system.
The principles and rules for correct sizing are the same as “WATER SYSTEMS”.
Steam Systems
For low pressure steam systems (up to 2 bar), the pressure drop through the valve must
be from 60 to 80 % of the pressure available upstream the valve.
Steam pressure upstream the valve
Valve pressure drop
0.5 bar (50 kPa)
40 kPa
1.0 bar (100 kPa)
70 kPa
For high pressure steam systems (above 2 bar) the pressure drop through the valve must
be from 30 to 40% of the pressure available upstream the valve.
Steam pressure upstream the valve
Valve pressure drop
200 kPa
80 kPa
600 kPa
200 kPa
1,000 kPa
300 kPa
For on/off valves there are no particular rules to follow: pressure drop may be 10 to 20%
of inlet pressure, but the valve is normally pipe sized.
Note:Do not size valves for high pressure steam with pressure drop higher than 50%
of absolute pressure upstream: beyond this percentage thermodynamic problems
could affect valve efficiency and life.
Diathermic Oil Systems
The most commonly used valve type is three-way with linear characteristics, in order to
ensure a constant flow to the boiler by constant speed.
Two-way valves can be used for several low-power users and wherever a balanced-plug
valve is mounted between supply and return boiler.
The pressure drop of three-way valves must be at least equal to or higher than the one
of the heat exchanger.
For a single user control, the valve must have a pressure drop from 30 to 50% of the
system pressure drop.
For two-way valves, see also the “WATER SYSTEMS” section.
