Pressure Profile and Pressure Diagram

The pressure diagram plots the pressure along the network length from the feed-in point to the critical point and superimposes static pressure, rest pressure, and dynamic pressure loss to verify that the maximum operating pressure (MOP) is not exceeded and that at least 0.4 to 1.0 bar differential pressure is available at every transfer station. It is the central planning tool for ensuring safe pressure conditions in thermal networks, covering both the design case (maximum load) and part-load operation. A correct understanding of the key pressure terms — MOP, MIP, rest pressure, and minimum operating pressure — is essential for its construction.

Pressure Terms Overview

Maximum Operating Pressure (MOP)

At no point in the network may the pressure exceed the maximum operating pressure of the pipeline and installed equipment. The MOP depends on the nominal pressure rating (determined by the chosen pipe system), the operating temperature, and the geodetic conditions.

Typical MOP values:

• Thermal networks with KMR and MMR: PN 16 (MOP ≤ 16 bar)
• Networks with PMR (standard): PN 6 (MOP ≤ 6 bar)

Test Pressure

The test pressure is applied during the pressure test for leak tightness. It typically corresponds to 1.5 times the maximum operating pressure and is maintained for a duration of 24 hours.

Maximum Incidental Pressure (MIP)

The maximum pressure that can occur briefly in a system (Maximum Incidental Pressure), e.g., in the event of network control malfunctions.

Minimum Operating Pressure

At the highest point of the network, the pressure of the district heating water must be at least 0.5 bar above the vapor pressure at the network temperature. An additional safety margin of 0.5 to 1.0 bar is recommended for control tolerances and transient processes.

Structure of the Pressure Diagram

The pressure diagram shows the pressure profile as a function of network length from the feed-in point to the critical point. It is composed of several components:

Static Pressure

The static pressure results from the difference between the highest and lowest point in the network (water column pressure). It is independent of the operating state and is determined by the geodesy.

Rest Pressure (pressurization)

The rest pressure is the pressure in the network with the pump switched off. It is determined by the pressurization system and must ensure the minimum operating pressure at every point in the network.

Network Pressure Loss

The dynamic pressure component arises from friction and local resistance losses during flow through the network (see pressure loss calculation). It is generated by the pump and changes with the volume flow rate:

• Supply pressure profile: Pressure decreases from the feed-in point to the critical point
• Return pressure profile: Pressure increases from the critical point back to the feed-in point
• Transfer station differential pressure: Difference between supply and return pressure at the customer connection

Total Pressure

The maximum network pressure (superposition of static and dynamic components) occurs at maximum load at the design point and at maximum network temperature. This value must not exceed the MOP at any point.

The Critical Point

The critical point (also referred to as the network’s weakest point) is the location in the network with the lowest differential pressure between supply and return. Supply shortages are most likely to occur at this point.

Characteristics:

• The critical point shifts depending on the current heat demand in the network
• It is typically the most remote customer
• The minimum differential pressure at the transfer station must typically be 0.4 to 1.0 bar

The network pump must be sized (see pump sizing) so that the required differential pressure at the critical point is guaranteed even under the most unfavorable operating conditions.

Pressure Boosting and Network Separation

Booster Station

In larger and more extensive networks (PN 25 or higher), it can be economically advantageous to install a booster station at the periphery of the network rather than generating the entire pressure loss centrally through the main pump.

Advantages:

• Reduced operating pressure in the central network
• Lower nominal pressure rating possible
• Smaller central network pumps

Network Separation

Network separation refers to the hydraulic decoupling of two otherwise connected circuits. Possible reasons:

• Protection against excessive pressures (geodetic elevation differences)
• Separation when using different media (e.g., steam-water)
• Different network parameters (pressure, temperature, water quality)
• Secondary networks with their own pressurization

Network separation is achieved via heat exchangers, where the exergy loss should be minimized through the lowest possible temperature approach in the heat exchanger.

Practical Guidelines

The following points should be considered when creating a pressure diagram:

1. Elevation profile of the network: survey and calculate static pressure
2. Pressure losses of individual pipe sections at design load
3. Pressurization selected so that the minimum operating pressure is maintained at the highest point
4. MOP is not exceeded at any point in the network (also check at part load)
5. Differential pressure at the critical point is sufficient for the transfer stations

The pressure diagram should be created for the design case (maximum load) and the part-load case, as the pressure conditions change with the volume flow rate. The results can be validated using thermo-hydraulic simulation.

Conclusion

The pressure diagram is an indispensable planning tool that ensures compliance with all pressure limits and enables the correct sizing of pumps, pressurization systems, and, where applicable, booster stations. It should be updated during every planning phase and forms the basis for safe network operation.

Further reading: Pressure Loss Calculation — the pressure losses in individual pipe sections form the basis of the pressure diagram, Pump Sizing — the pump generates the pressure profile in the network, Pressurization and Expansion — the static pressure superposition from the pressurization system.

References and Standards

• AGFW FW 440 — Hydraulic Calculation of Hot Water District Heating Networks
• DIN 4747-1 — District Heating Systems — Safety Requirements
• Nussbaumer, T.; Thalmann, S.; Zaugg, D.; Cueni, M. (2025): Planning Handbook for Thermal Networks. Version 2.0, EnergieSchweiz / Swiss Federal Office of Energy.