When natural gas is let down through a regulator, it expands and cools. With a large pressure drop, that cooling can lead to hydrate formation if water is present, icing of regulators and pilot lines, and gas temperatures below what the equipment is rated for. A combined heating, regulating and metering skid deals with the problem at its source: it warms the gas before it reaches the regulator, reduces the pressure, and then meters the flow with the temperature effect properly accounted for.
This page focuses on where the heating requirement comes from, which heater options exist and how the three functions are sequenced on one skid. Regulator and protective-device selection is covered on the regulating skid page, and the overall layout of a city gate on its own page.
Why does gas cool when pressure drops?
When a real gas expands through a restriction without exchanging heat with its surroundings, its temperature changes; for natural gas under normal operating conditions, it falls. This is the Joule-Thomson effect. How much the gas cools depends on the pressure drop, the inlet temperature and the gas composition, and ISO 20765-5 provides a method for calculating the Joule-Thomson coefficient for metering use. In practical terms, a station stepping down from transmission to distribution pressure will deliver gas well below its inlet temperature unless it is heated. The heat duty needs to be calculated for each project from the inlet conditions and the target outlet temperature.
What problems does cooling cause?
· Hydrate formation: When water is present, low temperature combined with high pressure favors solid, ice-like hydrates. Hydrates can restrict flow, plug equipment, damage valves and interrupt supply.
· Regulator and pilot icing: Small-bore pilot circuits and sensing lines are the points most prone to freezing, and icing upsets stable regulator control.
· Material and equipment limits: Regulators and slam-shut devices are specified for a defined temperature range; the relevant European standards, for instance, cover them from −20 °C to +60 °C. Unheated gas falling outside that range can invalidate the equipment selection.
· External condensation and frost: Moisture condenses and freezes on cold pipe surfaces, leading to corrosion, trouble at instrument connections and harder maintenance.
Heater options
The heater type is chosen according to the heat duty, the availability of fuel gas or electrical power on site, hazardous area conditions and the maintenance approach.
Heater type | How it works | Points to evaluate |
Indirect water bath heater | A burner fires into a firetube inside the shell, heating a bath that is commonly a water and glycol mixture. Gas passes through a high-pressure coil submerged in the bath | No direct flame contact with the process gas; a share of the station's own gas can serve as fuel. Bath fluid, burner maintenance and emissions need attention |
Electric heater | Heating elements warm the gas directly or through an intermediate fluid | Requires adequate electrical supply and equipment suitable for the hazardous area; considered for small to medium duties or where fired equipment is unwanted |
Boiler and heat exchanger | Water heated in a separate boiler is circulated to an exchanger on the gas line | The boiler can sit outside the hazardous area; boiler redundancy and circulation pumps are added to the system |
Sequencing the functions on one skid
A typical flow path runs from inlet isolation through filtration, heating, slam-shut and regulation, metering and outlet isolation. The heater must sit upstream of the regulator, because the aim is to add enough heat before the cooling occurs to reach the target temperature at the regulator outlet. Placing the filter ahead of the heater protects the coil and the regulator from particles and liquids.
Where to meter depends on project conditions. Metering downstream of regulation gives a controlled, stable temperature at lower pressure, but flow disturbance from the regulator and, for ultrasonic meters in particular, regulator noise must be taken into account. Metering on the high-pressure side may allow a smaller meter, although temperature variability then depends on whether the meter sits before or after the heater. In either case, redundancy for the heater and regulating streams, and how the heater is taken out of service for maintenance, should be defined from the outset.
Temperature control
Heater control usually targets the gas temperature at the regulator outlet, set to a value that removes hydrate and icing risk while staying within equipment limits. On water bath heaters this is done by adjusting bath temperature and burner firing. Overheating wastes fuel and places unnecessary thermal load on downstream metering and equipment. Since inlet pressure, flow and gas temperature vary through the year, the control strategy has to handle changing conditions, not just the worst case.
Temperature effects on metering
Heating changes the gas volume at the metering point. Because the meter measures at operating conditions, the temperature used for conversion to base conditions must represent the actual gas temperature at the meter. The temperature measurement is therefore placed close to the meter where the flow is well mixed; installations very close to the heater outlet should be checked for temperature differences across the pipe section. Any error in temperature measurement carries straight through into the corrected volume.
Design inputs and their effects
Parameter | Why it matters in design | Effect on selection |
Inlet and outlet pressure range | The pressure drop sets the amount of cooling | Heat duty, heater capacity and number of regulation stages |
Inlet gas temperature (seasonal) | The lowest inlet temperature drives the highest heat demand | Heater size and control range |
Flow range | Heat duty rises in proportion to flow | Number of heaters and regulators, need for parallel streams |
Gas composition and water content | Affect the degree of cooling and the tendency to form hydrates | Target outlet temperature, gas quality measurement if needed |
Target outlet temperature | Must remove hydrate and icing risk without exceeding equipment limits | Control set point, material selection |
Energy source | Access to fuel gas, electricity or external heat differs by site | Heater type and layout |
Hazardous area classification | Burners, electric heaters and panels need different protection methods | Equipment layout and separation distances |
Tolerance to interruption | Supply may need to continue if a heater fails | Heater redundancy and bypass strategy |
How TLY Enerji contributes
On skids combining heating, regulation and metering, TLY Enerji can support the evaluation of process data, the selection and supply of temperature, pressure and flow instrumentation, system integration, site installation, testing and commissioning. Bringing heater control and metering data into a PLC or SCADA environment can also be part of the scope. The supply boundary for the heater itself is clarified separately on each project.
Information needed for a heat duty review
· Lowest and highest inlet pressure and temperature expected over the year
· Required outlet pressure and minimum target outlet temperature
· Minimum, normal and maximum flow and its seasonal variation
· Gas composition, water content or water dew point
· Energy sources available on site: gas, electricity, waste heat
· Purpose of metering and preferred meter position relative to the regulator
· Expected redundancy for heater and regulating streams
· Hazardous area classification and layout constraints
Related solutions
· Gas pressure regulating skids: Selecting and arranging regulators, monitors and slam-shut valves.
· Pressure reduction and metering stations (RMS/PRMS): Combined consumer stations where heating is unnecessary or limited.
· City gate stations: Where heating fits in the overall architecture of a transmission-to-distribution station.
· Fuel gas conditioning and metering skids: Heating to meet the fuel specification of a gas turbine or engine.
Questions about heated skids
Does every pressure reduction station need a heater?
No. If the pressure drop is small, the inlet temperature is high enough and the gas is dry, the regulator outlet temperature may stay within acceptable limits. The need for heating is established by calculating the outlet temperature from inlet pressure and temperature, outlet pressure, flow and gas composition, and comparing it with hydrate, icing and equipment limits. The calculation should include the worst seasonal case.
Why is the heater placed upstream of the regulator?
Cooling happens during the pressure drop across the regulator. Heating the gas afterward would not prevent hydrates or icing in the regulator itself or its pilot circuits. The gas is therefore heated beforehand, enough to reach the target temperature at the regulator outlet. With multi-stage pressure reduction, the need for additional heating between stages is assessed separately.
How does a water bath heater compare with an electric heater?
A water bath heater usually burns a share of the station's own gas and keeps the flame away from the process gas; it is common for larger heat duties. An electric heater needs no burner or flue but requires sufficient electrical power and a design suited to the hazardous area. The choice weighs heat duty, energy access, emissions targets and maintenance approach together.
Does heating affect metering accuracy?
Heating itself is not an error source; what matters is that the temperature used in volume conversion represents the actual gas temperature at the meter. The temperature point should be close to the meter where flow is well mixed. A measurement point too close to the heater outlet may give an unrepresentative value because of temperature differences across the pipe.