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; the limits it has to observe while doing so are derived in Meeting Operating Pressure Limits in Heat Networks. 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

Pre-charge Pressure and Geodetic Elevation

The pre-charge pressure of a pressurization system is not a free parameter. It follows from the static height of the system and is then checked against the minimum operating pressure at the highest point of the network. The classical design chain starts with the rest pressure at the connection point:

where:

• $p_0$: rest pressure as gauge pressure at the connection point of the pressurization system [bar]
• $H_\text{st}$: static height between the connection point and the highest point of the system [m]

The denominator of 10 m per bar is the usual approximation for the geodetic component: 10 m of elevation difference correspond to roughly 1 bar, 1 m to roughly 0.1 bar. Setting the fill pressure adds a further 0.3 bar:

In total, at least 0.6 bar of gauge pressure against the atmosphere is then present at every point of the network. This reserve keeps air from being drawn into the system at vents, glands, and seals, and has to be maintained independently of temperature.

Cross-check Against the Minimum Operating Pressure

Excluding air is only one of the two conditions. At the highest point of the network, the absolute pressure must in addition stay above the saturation pressure at the hottest point of the supply line:

where:

• $p_\text{min,abs}$: required lowest absolute pressure in the network [bar]
• $p_\text{sat}(T_\text{max})$: saturation pressure at the highest supply temperature occurring [bar]
• $0.5$ bar: margin against evaporation and cavitation — broad professional consensus
• $1.0$ bar: margin for the control tolerance of the pressurization system and for transients — a planning recommendation, not a standard

Together this gives a safety margin of 1.5 bar above the saturation pressure. Which of the two conditions is the stricter one is decided by the supply temperature: at low network temperatures the air exclusion criterion with its 0.6 bar of gauge pressure governs, at high ones the evaporation limit. The derivation of both margins and the saturation pressure table are given in Meeting Operating Pressure Limits in Heat Networks; the pre-charge pressure has to be chosen so that both conditions are met at the same time.

Connection at the High Point or at the Low Point

The pressurization system dictates the pressure level at its connection point; every other network point sits above or below it by its own elevation difference. Where the system is connected therefore decides which point is set directly and which one can only be calculated afterwards.

If the plant sits at the geodetic low point — the normal case, since it is usually housed in the energy center — then $H_\text{st}$ is the full elevation difference up to the high point. The rest pressure level at the connection point already contains the entire static column before the network pump adds its head on top. Vessel, make-up unit, and valves of the pressurization station accordingly work at a high pressure level and need a matching nominal pressure rating.

If the plant sits close to the high point, $H_\text{st}$ becomes small and the rest pressure level at the connection point low. The minimum pressure at the critical high point is then set directly instead of resulting from a calculation, and the pressurization station itself gets by with a lower pressure rating. The price is a location out in the network that needs access, a power supply, and frost protection.

In both cases the low point carries the same static column, and in both cases the nominal pressure rating there has to be checked against the sum of rest pressure, geodetic component, and pump head. How reliable the underlying elevation values are is covered in Terrain Elevation and Topography in Network Planning.

NPSH of the Network and Make-up Pumps

The lower limit applies not only to high points in the network but also to the suction side of every pump. What governs there is the NPSH condition: the NPSH value available from the system must exceed the value required by the pump by at least 0.5 m of water column. This holds for the network pump just as much as for the make-up or pressure-maintaining pump of the pressurization system. If this reserve is undercut, the pump cavitates even when the pressure is sufficient everywhere else in the network. With post-pump pressurization this is the critical point, because the pressurization system only sets the level downstream of the pump and the inlet pressure therefore has to be verified separately.

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.

Before vessel and pre-charge pressure are sized, the static height $H_\text{st}$ has to be settled. It feeds through the rest pressure straight into the pre-charge pressure, the vessel size, and the nominal pressure rating of the pressurization station, and cannot be corrected afterwards without rebuilding. What governs is the elevation of the pipe axis and of the highest components of the system, not the terrain surface — data sources and achievable accuracy are covered in Terrain Elevation and Topography in Network Planning.

Two quantities need to be kept apart for the nominal pressure rating: the system limit, that is the highest rating the pipe system itself permits, and the usual network design, the rating networks of that type are designed for in practice. For the pressurization system it is the usual network design that is the reference; the system limits of all pipe families are listed in the comparison of pipe systems:

The usual network design regularly stays below the system limit, because the pipe is not the weakest component: valves, fittings, heat exchangers, and transfer stations carry their own, often lower pressure ratings. What governs the upper limit is always the weakest component on the path considered, see Meeting Operating Pressure Limits in Heat Networks. Pipelines do not fall under the Pressure Equipment Directive, but components such as valves, safety valves, filters, heat exchangers, and vessels do.

Getting pressurization right

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: Meeting Operating Pressure Limits in Heat Networks — the pressure limits against which pre-charge pressure and rest pressure level have to be checked, Terrain Elevation and Topography in Network Planning — the data basis of the static height and of the geodetic component, 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