Pressure Calculation in the Network

Overview

VICUS Districts computes network pressures consistently as gauge pressure — as pressure above the ambient one, the way a manometer reads it and the way a technical connection specification states it. The computed node pressures are therefore directly comparable with the nominal pressure rating of the pipes, with the MOP and with the requirements of a network operator; no conversion is needed.

Absolute pressure matters in exactly one place: proving that the fluid does not evaporate. The saturation pressure of water is an absolute pressure, so the ambient pressure has to be accounted for there. Nowhere else does it appear in the calculation model.

This page describes which pressure is computed and which parameters determine it. How the results are displayed and checked is covered by Results of the Steady-State Calculation and Network Report. The engineering background of the limits is covered by Meeting Operating Pressure Limits in Heat Networks.

The pressure level: setpoint of the pressure maintenance

The pressure level of the whole network is set by a single value: the setpoint of pressure maintenance, applied at a freely selectable reference node. Every other node pressure follows from it through the pressure losses along the way and through its geodetic height.

Field	Meaning
Setpoint of pressure maintenance	Gauge pressure in [bar] that the pressure maintenance holds at the reference node
Position of pressure maintenance	Energy plant the value is applied at; At node inlet / At node outlet selects the connection side

Both settings appear in the project wizard, in the dialog of the steady-state calculation and in the simulation settings — with the same meaning and the same field name everywhere.

In engineering terms this value is the resting pressure: the level that establishes itself when circulation stops. With the pump running, the pressure losses are superimposed on it, but the level itself stays tied to the reference node. How a pressure maintenance system is sized, and why the resting pressure cannot be chosen arbitrarily low, is covered by Pressurization and Expansion.

The geodetic component

Differences in elevation shift the pressure at every node by the geodetic component:

$$\Delta p_{geo} = \rho \, g \, \Delta z$$

For water this amounts to roughly 1 bar per 10 m of elevation difference, that is about 0.1 bar per metre. What counts is the elevation difference relative to the reference node of the pressure maintenance — not relative to the lowest point of the network and not relative to sea level.

The component does not enter the solution of the hydraulic system of equations. Mass fluxes, pressure losses and the operating point of the pumps are established without it; it is added to the pressure quantities of the result display afterwards. The option Consider geometric height switches this on and off in the path profile and in the steady-state analysis — a pure display option, both variants are stored in the results.

For the closed loop this is correct: supply and return run over the same elevations, the static column cancels around the circuit and does not load the pump. A large elevation difference therefore does not change the pump sizing — but it does change the pressure level and thus compliance with the operating pressures. Where the node elevations come from is covered by Elevation Query.

The limits that are checked

What is checked is the operating pressure, that is the gauge pressure including the geodetic component — the pressure that actually stands in the pipes.

Limit	Criterion
Highest operating pressure	≤ lowest nominal pressure rating PN of the pipes on the checked path
Lowest operating pressure	≥ min. operating pressure from vapour pressure, safety margin and ambient pressure

The upper limit compares gauge against gauge: PN is a gauge pressure rating and the computed node pressures are gauge pressures. The comparison is therefore exact — neither on the safe nor on the unsafe side. If the path runs over pipes of different ratings, the lowest one governs — a profile of mixed products is only as strong as its weakest one. A pipe with no nominal pressure assigned in the database gets no upper limit; a check is then missing rather than failing.

The lower limit prevents the fluid from evaporating at the hottest point of the path:

$$p_{min} = p_{sat}(T_{max}) + p_{margin} - p_{amb}$$

where:

• $p_{min}$: checked minimum operating pressure as gauge pressure [bar]
• $p_{sat}(T_{max})$: saturation pressure at the hottest fluid temperature of the path, an absolute pressure [bar]
• $p_{margin}$: Minimum pressure margin [bar]
• $p_{amb}$: ambient pressure at the project altitude [bar]

The saturation pressure is determined over the whole temperature range according to the industrial formulation IAPWS-IF97, so supply temperatures above 100 °C are evaluated correctly. The hottest fluid temperature is not an input parameter — it comes from the computed operating point. If that operating point provides no temperatures for the path, the lower limit is omitted.

The ambient pressure — the only place with absolute pressure

Because the saturation pressure is an absolute pressure and the checked profile a gauge pressure, one of the two has to be converted. VICUS Districts subtracts the ambient pressure from the limit rather than adding it to every point of the profile — the result is the same, but all displayed pressures stay gauge pressures.

The ambient pressure is not an input field. It is computed from the altitude of the project origin above sea level according to the standard atmosphere, that is from the z coordinate that the Elevation Query sets to the absolute terrain elevation:

Project altitude	Ambient pressure
0 m (sea level)	1.013 bar
500 m	0.955 bar
1000 m	0.899 bar

It is shown read-only under menu Network > General settings … next to the safety margin; in the network report the note below the limit table states it together with the project altitude it was derived from. A project that was never georeferenced sits at 0 m — which is also where the standard atmosphere applies, so the two cases coincide harmlessly. The difference is not negligible: at 1000 m roughly 0.11 bar are missing compared with sea level, a fifth of the usual cavitation margin.

The parameters at a glance

Quantity	Where to enter	Default
Setpoint of pressure maintenance	project wizard, steady-state calculation dialog, simulation settings	2 bar; 3 bar with the Conventional network preset
Position of pressure maintenance	the same dialogs	first energy plant, node inlet
Minimum pressure margin	menu Network > General settings …	1.5 bar (range 0 to 100 bar)
Ambient pressure	not enterable, shown read-only there	from the project altitude
Nominal pressure (PN)	pipe database	16 bar; no upper limit without an assignment
Consider geometric height	path profile and steady-state analysis	enabled; pure display option
Hottest fluid temperature	not a parameter — from the operating point	–

The margin of 1.5 bar is composed of 0.5 bar protection against evaporation and cavitation — on which there is broad professional consensus — plus roughly 1.0 bar for the control tolerance of the pressure maintenance and for transients; the second part is a planning recommendation and not a standard. It applies to the whole network.

Scope: steady state and dynamic

The two limits are not checked everywhere alike:

	Steady-state calculation	Dynamic simulation
Upper limit (nominal pressure rating)	checked, with a written verdict	checked, as the result quantity Pressure exceedance
Lower limit (evaporation)	checked	not checked

The reason is a substantive one: the lower limit depends on the ambient pressure and on the safety margin, and both are properties of the project rather than of the solver model. The evaporation check is therefore performed exclusively at the steady-state operating points — in the status line of the path profile and in the network report. The dynamic simulation says nothing about low pressure or evaporation.

Related topics

• Network Model and Terminology
• Fundamentals of the Steady-State Calculation
• Results of the Steady-State Calculation
• Network Report
• Elevation Query
• Meeting Operating Pressure Limits in Heat Networks