Pressurization and Expansion

The pressurization system keeps the operating pressure in thermal networks within safe limits by compensating for the temperature-induced volume change of the water and maintaining a minimum pressure to prevent cavitation and evaporation. Common systems are the closed membrane expansion vessel (static) and the pressure-maintaining pump (dynamic), with the connection point located either upstream (pre-pump), downstream (post-pump), or at a mid-point of the pump. A correct pressure profile in the network depends directly on a functioning pressurization system — it is a safety-critical component of every district heating network.

Tasks of Pressurization

1. Pressure limitation: Keeping the pressure at every point in the system within permissible limits to prevent exceeding the maximum operating pressure (MOP)
2. Volume compensation: Compensating for the temperature-induced volume change of the heat transfer medium
3. Ensuring minimum pressure: Maintaining sufficient minimum pressure to prevent cavitation, evaporation, and gas solubility problems

Pressurization Systems

Open Vessel (historical)

In pressurization with an open elevated vessel, an open tank is installed at an elevated location in the network. Pressurization is achieved through atmospheric pressure and the weight of the water column.

This system is rarely used today, as the permanent presence of oxygen in the system promotes corrosion and a suitable elevated location is difficult to find.

Closed Vessel (static pressurization)

In a closed system, a compressible gas cushion is used as the pressurization medium. When water losses occur, the gas pushes against the water surface in the expansion vessel and feeds water from the vessel back into the network.

An inert gas (typically nitrogen) is used as the pressurization agent to prevent corrosion. A membrane separates the gas and water.

Dynamic Pressurization (pressure-maintaining pump)

The pressure-maintaining pump is installed between the vessel and the network. When water is lacking, it pumps the required volume from the vessel into the network. Backflow is prevented by a check valve. When pressure rises, a relief valve opens so that water can flow back from the network into the vessel.

Connection of Pressurization

The position of the pressurization system within the network has a fundamental influence on the pressure profile. Three variants are distinguished:

Pre-pump Pressurization (suction-side pressurization)

The pressurization system is connected upstream of the pump (suction side).

Advantages:

• Low static pressure level
• Simplest and most commonly used variant
• Operating pressure is always greater than static pressure, no risk of negative pressure

Disadvantage:

• With high pump pressure (see pump sizing), the operating pressure in the network can become high

Post-pump Pressurization

The pressurization system is connected downstream of the pump (pressure side).

Advantage:

• Low static pressure level when the full pump pressure does not need to be applied

Disadvantage:

• The required inlet pressure for the pump must be ensured, otherwise there is a risk of cavitation

Mid-point Pressurization

The measurement point for the static pressure level is “relocated” into the system via an analogy measurement section. Static and operating pressure levels can thus be optimally coordinated.

Advantage:

• Optimal, variable coordination of operating and static pressure

Disadvantage:

• Higher equipment complexity, only justified for complex systems

Expansion Volume

The theoretical expansion volume of a thermal network is the make-up volume during heat removal or the overflow volume during heat supply:

where:

• $V_0$: Total water content of the closed system [m³]
• $\rho_0$: Density at fill temperature [kg/m³]
• $\rho(T_1)$: Density at maximum supply temperature [kg/m³]

To calculate the expansion volume, the total water content of the system must be known — including heat exchangers, thermal energy storage tanks, boilers, and pipelines.

The water reserve compensates for any water losses in the system and should amount to approximately 0.5% of the water content $V_0$.

Make-up Mass Flow Rate

The make-up mass flow rate depends on the supplied or removed heat load, the volume expansion coefficient, and the specific heat capacity of the medium. It is relevant for sizing the relief valve and the pressure-maintaining pump.

Sizing Guidelines

A pressure fluctuation of +/- 0.2 bar is considered adequate network control accuracy. Make-up water supply must be ensured within this bandwidth.

Typical nominal pressure ratings for components:

The choice of nominal pressure rating depends directly on the installed pipe system. Pipelines do not fall under the Pressure Equipment Directive, but components such as valves, safety valves, filters, heat exchangers, and vessels do.

Conclusion

Pressurization is a safety-critical component of every thermal network. Careful calculation of the expansion volume, the correct choice of pressurization system, and its proper connection are fundamental prerequisites for safe and trouble-free operation. Further guidance on implementation can be found in AGFW Guideline FW 442.

Further reading: Pressure Profile in the Network — the pressure distribution in the network depends directly on the connection of the pressurization system, Pump Sizing — the pump and pressurization system must be coordinated, Thermo-Hydraulic Simulation — verification of pressure conditions through coupled simulation.

References and Standards

• AGFW FW 440 — Hydraulic Calculation of Hot Water District Heating Networks
• DIN 4747-1 — District Heating Systems — Safety Requirements
• AGFW FW 442 — Pressurization in Hot Water District Heating Networks