Energy efficiency in gas pressure reduction and metering stations: the future of gas preheating with gas heat pumps

Gas Pressure Reduction and Metering Stations (PRMS) are a fundamental infrastructure within natural gas distribution system. 

Function of a gas pressure reduction and metering station

The main function and purpose of a gas pressure reduction and metering station (PRMS) is to act as an interconnection and control point between the main natural gas transmission pipelines (operating at high pressure) and local distribution networks (operating at lower pressure).

The main functions of these stations are:

• Pressure regulation: gas travels through the national transmission pipelines at very high pressure; at the entrance to urban areas, gas pressure reduction and metering stations reduce this pressure to manageable and safe levels for city distribution networks.
• Volume metering: meters are installed within the stations to record the exact quantity of gas flowing from the transmission network to the distribution network; this measurement is crucial for billing and flow management.
• Filtration: they often include filtration systems to remove any impurities from the gas before it enters local networks.

What happens to the gas when it reaches a PRMS?

When natural gas enters a gas pressure reduction and metering station, it undergoes a sequential and highly engineered process designed to make it suitable for local distribution.

Here are the steps in detail:

1. Arrival at high pressure: the gas arrives from the national transmission pipeline (the “primary network”) at very high pressure, which may vary depending on the point in the network.
2. Filtration (initial cleaning): the first operational phase is almost always filtration. Although natural gas is already refined, traces of dust, solid debris, or liquids (such as condensate or compressor lubricating oils) may still be present and must be removed to protect downstream sensitive equipment and ensure gas quality.
3. Heating (pre-regulation): this is a crucial step. The next phase is pressure reduction, a physical process (known as Joule–Thomson expansion) that causes the gas to cool drastically. If the gas were not heated beforehand, this intense cooling could lead to the formation of ice or solid hydrates inside valves and piping, blocking the system. Condensation could also form on the piping, accelerating corrosion processes.
   Heat exchangers (usually fuelled by natural gas itself or by electricity) are therefore used to heat the gas to an optimal temperature before pressure reduction.
4. Pressure regulation (the heart of the station): the gas passes through a system of sophisticated valves and pressure regulators. These devices reduce the pressure from transmission levels (high or very high pressure) to the level required by the local distribution network (medium or low pressure). This reduction often occurs in several successive stages within the same station.
5. Metering (accounting): after regulation, the gas is sent to high-precision metering systems. Volumetric or ultrasonic meters are used to record exactly how much gas volume has passed through. This data is essential for commercial and fiscal purposes.
6. Odorisation (safety): natural gas is odourless by nature. Before being introduced into urban networks, an odorant (usually a mercaptan, with the characteristic sulphur or “gas” smell) is injected in very small quantities, but sufficient to allow people to immediately detect any leaks by smell.
7. Injection into the distribution network: at this point, the gas is clean, at the correct pressure, metered and odorised, and can be injected into the secondary network pipelines supplying end users.

In summary, the gas entering a gas pressure reduction and metering station is conditioned, secured and fully traceable before being distributed.

Why is it important to heat the gas in a PRMS?

Heating the gas within a gas pressure reduction and metering station is essential for physical and engineering reasons, specifically to prevent operational issues caused by the Joule–Thomson effect.

The Joule–Thomson effect and cooling

Natural gas travels through pipelines at high pressure. When pressure is suddenly reduced, the gas undergoes rapid cooling, a phenomenon known as the Joule–Thomson effect.

The consequences of excessive cooling

Without preheating, sudden cooling would cause severe operational problems:

• Formation of ice and solids (hydrates): very low temperatures can cause the solidification of residual moisture or other components in the gas. Ice crystals or solid “hydrates” would form, which would obstruct valves, filters and piping, blocking the gas flow.
• Damage to equipment: the metal components of the station (valves, seals, measuring instruments) are designed to operate within a specific temperature range. Excessively low temperatures can make metals brittle and cause breakages or malfunctions. There would also be a risk of condensation forming on the station equipment, generating undesirable corrosion phenomena.
• Metering inaccuracies: gas density and volume vary significantly with temperature. For accurate metering (crucial for billing purposes), the gas must be under stable and controlled conditions.

The solution: controlled preheating

To avoid these problems, the gas is passed through heat exchangers before pressure reduction takes place. This preheating “balances” the cooling effect that will occur subsequently, ensuring that the entire process takes place at safe and stable operating temperatures.

Operational advantages and sustainability with Robur gas absorption heat pumps

The natural gas preheating process within gas pressure reduction and metering stations represents a crucial focal point for the operational efficiency of the entire distribution network. As previously analysed, this operation is physically indispensable: it serves to counteract the Joule–Thomson effect, namely the drastic cooling of the gas that occurs during pressure reduction. Without an adequate upstream thermal input, the formation of ice, solid hydrates and the potential embrittlement of metal components would compromise safety and continuity of service.

However, this technical necessity clashes with a significant economic and environmental challenge. Traditionally, gas heating in PRMS is achieved by using simple direct-fired gas boilers, which burn part of the transported natural gas or consume electrical energy drawn from the grid. Although functional, this approach presents notable inefficiencies: conventional boilers have modest efficiencies (often well below 90–95%) and entail significant operating costs, which directly impact the balance sheets of distribution companies. In a regulatory context that increasingly pushes towards energy efficiency and decarbonisation, maintaining obsolete heating systems is no longer sustainable nor economically advantageous.

It is in this scenario that the technological superiority of Robur gas absorption heat pumps emerges. These solutions represent a step change in preheating efficiency. Unlike traditional boilers, which generate heat by burning fuel, absorption heat pumps use natural gas as an energy source to transfer heat from a low-temperature source (such as ambient air or groundwater) to the gas that needs to be heated. They exploit a virtuous thermodynamic cycle (based on the water/ammonia mixture) that makes it possible to achieve a very high nominal efficiency (GUE), equal to 172% with respect to the calorific value of the gas used.

The installation of Robur gas heat pumps within the PRMS system translates into tangible benefits for network operators:

1. Higher energy efficiency: significantly less primary energy is consumed to obtain the same desired thermal effect, optimising the use of natural gas.
2. Reduced operating costs: the reduction in gas consumption for auxiliary station services drastically lowers the plant’s energy bill.
3. Environmental sustainability: lower fuel consumption results in reduced CO₂ and NOx emissions, aligning station management with the ESG (Environmental, Social, and Governance) objectives of utilities.

Integrating Robur technology means transforming a fixed and inefficient operating cost into a strategic investment that ensures rapid economic payback and state-of-the-art management of PRMS infrastructure.

Practical demonstration: 46% operating cost savings with gas heat pumps

The table above shows the concrete results of a real case study, comparing the operational efficiency of a traditional heating system based exclusively on existing boilers with a hybrid solution that integrates a gas absorption heat pump (GAHP A 35 kW). The data analysis, based on 196 days of measurement, unequivocally highlights the economic benefits resulting from the adoption of GAHP technology, confirming the superiority of the gas utilisation efficiency (GUE) of absorption heat pumps compared to conventional boilers.

The most significant figure is the drastic reduction in self-consumption of gas in standard cubic metres (Scm), which decreases from 16,668 Scm to 8,306 Scm. This 50% reduction in consumption is the core of the savings, as the required thermal energy is now supplied with much higher efficiency, exploiting ambient heat rather than exclusively burning gas. Although the integration of the GAHP introduces a new electrical consumption (2,800 kWh, for a total cost of EUR 846), the savings on total gas costs are clearly predominant. The total annual cost is in fact reduced from EUR 16,668 to EUR 9,152.

The final result is an overall saving of 46% on total operating costs of the system. These real-world data clearly demonstrate how investing in high-efficiency gas heat pumps, such as Robur solutions, is not only an environmentally responsible choice, but a strategic operational and economic decision for gas distributors, capable of generating a significant and immediate ROI.

Typical PRMS installation

The adoption of Robur GAHP A heat pumps (single units or cascaded) makes it possible to achieve significant energy efficiency improvements in PRMS, ensuring more sustainable and higher-performing thermal system management.

The technology has already been successfully implemented in more than 140 PRMS in Italy operated by leading gas distribution companies, delivering outstanding results in terms of energy savings, emission reductions and operational reliability. Today, Robur’s solution for PRMS positions itself as a benchmark standard for the future energy efficiency upgrading of gas distribution infrastructures in Italy.

Observed benefits

• Water–ammonia absorption cycle that eliminates the compressor, ensuring negligible electrical consumption, below 1 kW, and avoiding modifications to the electrical system present in the PRMS.
• Natural refrigerant (NH₃) with zero GWP, exempt from restrictions due to F-Gas regulations.
• Flow temperatures up to 65 °C with notable performance even at very low ambient temperatures. Particularly short defrost cycles that do not interrupt unit operation. Performance perfectly compatible with any sizing of the heat exchangers installed in the PRMS.
• Easily installable outside the PRMS, in non-classified areas and without the need for flue systems, offering maximum ease of integration.
• A solution suitable both for equipping new PRMS and for upgrading existing ones, guaranteeing in both cases significant energy savings.
• Ensures operation always prioritised on the base load of the preheating service, making it possible to achieve very significant energy savings even with a heat pump thermal capacity in the order of 30–50% compared to that of the boiler already installed in the PRMS.