Excess pressure and absolute pressure
In steam boiler technology, it is routine for all pressures to be stated as excess pressure relative to an atmospheric pressure of 1 bar. The unit [bar] or [barg] is used at these points.
The excess pressure is therefore converted to absolute pressure as follows:
Conversion from excess pressure to absolute pressure
pabsolute = p + 1.01325 bar
Normal temperature and pressure and standard temperature and pressure
Normal temperature and pressure (according to DIN 1343):
pn = 101,325 Pa = 1.01325 bar = 1 atm
Tn = 273.15K = 0°C
Standard temperature and pressure (STP, IUPAC):
p° = 100,000 Pa = 1.0 bar
T° = 273.15K = 0°C
Standard ambient temperature and pressure (SATP, IUPAC):
p° = 100,000 Pa = 1.0 bar
T° = 298.15K = 25°C
The operating pressure of a boiler system is not a constant value, but instead fluctuates around the average operating pressure pavg. The reason for this is that the operating pressure in the steam boiler is used as the input variable for the output regulation of the steam boiler system and therefore fluctuates in a range of roughly ±10% of the average operating pressure used as the set value.
Illustration of pressure curve over time (Boiler Control BCO)
To determine the necessary average operating pressure pavg the following aspects must be taken into account:
To define the average operating pressure pavg, the required pressure or temperature level of the steam consumers must be determined. The maximum pressure level pmax,C of the consumers must be used to design the average operating pressure of the steam boiler system.
The required pressure level of the consumers at a known heating temperature can be determined from the steam table by linking the boiling pressure to the temperature.
Info on Pressure and Temperature
Maximum pressure and temperature required at the consumer
pmax,C = ps (Tmax,C)
Maximum pressure required at the consumer
Boiling pressure at the maximum temperature required
Maximum temperature required at the consumer
In addition, the pressure losses in the steam pipe between the steam boiler and consumer, the pressure losses of valves in the steam pipe and the pressure losses of any existing pressure reducing stations must be taken into consideration.
Info on Pressure loss
If the pressure levels of the consumers are far apart (> 3 bar) or if a constant steam pressure is required with smaller fluctuation range than can be performed by the output control of the steam boiler, the steam pressure at the consumers should be set via pressure reducing stations between the steam boilers and consumers. The control circuits for the load control at the steam boiler and pressure regulation at the consumer are separated by installing pressure reducing stations.
The average operating pressure pavg of the steam boiler is determined taking the maximum control deviation into account as follows:
Formula for determining the average operating pressure
Example calculation for determining the average operating pressure
Average operating pressure [bar]
Maximum pressure required at the consumer [bar]
Pressure losses in the pipework [bar]
Pressure losses of valves and reducing stations [bar]
Lower control level for the output regulation based on the maximum permissible pressure of the boiler
Average control level for the output regulation based on the maximum permissible pressure of the boiler
Setting and modifying the average operating pressure
The average operating pressure pavg of the steam boiler system can be set at the control unit of the boiler control cabinet during commissioning and operation of the system or specified by the automation system.
Safety margins during design
At this point in the design process, it is advised to work without safety margins in order to reflect the actual operating conditions.
If the average operating pressures specified are too high, this may lead, among other things, to the steam valves and steam pipes being sized too small due to the higher density of the saturated steam.
If safety margins are to be taken into account for pressure in the design, this can be done when defining the maximum permissible operating pressure.
Reduced operating pressure
For energy efficiency reasons, the operating pressure should not be set any higher than what is necessary. A typical application for reduced operating pressure can be carried out during the weekend, this enables reduction in radiation losses of the boiler.
However, the operating pressure must not be reduced. If the operating pressure is reduced too much, it can have the following negative effects:
High steam velocities arising from reduced density can also lead to negative effects:
The minimum permissible operating pressure is therefore limited to half of the configured operating pressure as the default setting.
The minimum average operating pressure of a high-pressure steam boiler system should not fall below 5 bar as the steam feed valves and steam pipes must be sized very large due to the larger specific volume of the steam at a lower pressure.
The maximum permissible operating pressure of a boiler is the excess pressure arising as a result of the construction and strength of the materials used.
This information is provided on the boiler manufacturer’s data plate, in the documentation provided and in the acceptance documents. The excess pressure must be limited by a safety valve on the steam boiler. The safety valve opens when the maximum permissible pressure is reached to prevent the pressure in the boiler shell from rising any further. Once the safety valve has responded, it closes again when the pressure in the boiler has reduced.
For the safety valve to function reliably, the valve does not respond or start leaking during operation, the pressures in the steam boiler are graded as follows.
Grading of operating pressures (as a percentage of maximum permissible operating pressure)
Type of pressure
Maximum permissible operating pressure
(safety pressure/safety valve triggering pressure)
pmax,perm = pSV ≙ 100 %
pPL ≙ pmax,perm ∙ 95 %
Combustion system switch-off point
pbu,off ≙ pmax,perm ∙ 91 %
Average operating pressure
pavg ≙ pmax,perm ∙ 83 %
Combustion system switch-on point
pbu,on ≙ pmax,perm ∙ 75 %
Standard value for heat maintenance via heating coil
= average operating pressure / 2
pHMS ≙ pavg ∙ ~ 50 % ≅ 42 %
This results in:
Formula for determining the smallest maximum permissible operating pressure required
Example calculation for determining the smallest maximum permissible operating pressure required
Formula for determining the highest average operating pressure
pavg,max ≤ pmax,perm ∙ 83% = pSV ∙ 83%
Example calculation for determining the highest average operating pressure
Diagram of pressure curve with switching points when using steam boilers with an average operating pressure of 13.3 bar
Other gradings for the average operating pressure can also be selected in the boiler control, subject to certain limits.
Other system components
In addition to the boiler shell itself, the maximum permissible operating pressure and maximum permissible temperature of all components and assemblies directly connected to the boiler (e.g. valves, sensors, pipework, gaskets, apparatuses) must be taken into account. Please also note the Pressure/temperature curve for flange according to EN 1092-1.
If consumers with a lower permissible operating pressure than the steam boiler are connected in the steam system, they must be equipped with their own safety valves or pressure reducing stations with safety valves connected in-between them.
In reference, to the difference between the excess pressure determined pmax,perm and the maximum permissible excess pressure of the next higher boiler pressure stage, a design safety margin may effectively already exist. If an additional design safety margin is required for the pressure range, an appropriate higher boiler pressure stage can be selected.