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:
Normal temperature and pressure and standard temperature and pressure
Normal temperature and pressure (according to DIN 1343):
Standard temperature and pressure (STP, IUPAC):
Standard ambient temperature and pressure (SATP, IUPAC):
Average operating pressure
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:
- Required pressure/temperature level at the steam consumers
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.
- Pressure losses
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.
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.
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:
- Cavitation can occur in the pumps if throttling is insufficient.
- The pumps can run in an unstable range of the characteristic map.
- The pumps can run with a lower pump efficiency.
High steam velocities arising from reduced density can also lead to negative effects:
- Erosion at valves and elbows
- Loud flow noises
- Increased rate of droplet entrainment from the steam boiler
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.
Maximum permissible operating 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.
This results in:
- The lowest maximum permissible operating pressure pmax,perm of the boiler required based on the average operating pressure pavg which has been derived from the operational requirements
- The highest average operating pressure pavg,max of the boiler based on the maximum permissible operating pressure pmax,perm of the boiler:
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.
Safety margins during design
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.