By Lauren DeLorenzo, Journalist, Pump Industry magazine
Demand for natural gas projects is set to increase substantially over the coming years – but that can’t happen without compressor stations driving these projects forward. No matter the size or location of the project, compressor stations are essential for the movement and optimal efficiency of natural gas pipelines.
With a variety of functions and features, compressor stations are the pulse of natural gas pipelines, making sure gas travels from the wellhead to your BBQ without interruption. Australia’s natural gas network is currently made up of over 47,000km of transmission pipelines.
When travelling over long distances, natural gas loses pressure and movement slows. This is especially common when gas is travelling to an end user living in a rural or mountainous area, and changes in terrain or elevation can lead to drops in pressure along the pipeline.
The main function of a compressor station is to propel the medium forwards by boosting pressure and keeping natural gas moving through the network. Compressor stations are usually centrifugal compressors powered by turbines, electric motors or reciprocating compressors.
Compressor stations are often powered by the gas that they are compressing, making them extremely efficient. They may also be electronically powered for environmental or security reasons.
These stations are positioned at intervals along the pipeline, and are typically needed every 60-100km. Some modern compressors stations are operated remotely, with only a small crew or maintenance team on staff.
It’s clear that compressor stations are an integral part of maintaining efficient operations in natural gas pipelines. Here, we break down the wide range of components and functions that they can entail, why they are so essential and how to optimise compressor station efficiency in the future.
What are the functions of compressor stations?
Compressor stations perform several important functions beyond boosting gas pressure. As natural gas travels through the pipeline, it can pick up contamination, such as water molecules or small pollutants like clay or soil. Compressor stations use technology to filter through impurities and ensure that clean, uncontaminated gas is moving through the pipelines and back to the end user.
Natural gas enters a compressor station through station yard piping, and then is passed through scrubbers to strip hydrocarbons out of the gas stream, removing particles of water and filtering through the gas. Compressor stations also use strainers or filter separators to remove liquids that may condense out of the gas as it moves through the pipeline.
By-products, such as natural gas liquids, can be collected in tanks and trucked off-site to be sold. Some of these liquids can then be used as a blend in motor gasoline or other chemical mixes. Once the gas has been cleaned, it is directed to individual compression units, where computers regulate the flow and number of units that are needed to pressurise the gas.
Some compressor stations include odourisation equipment, which infuses the odourless natural gas with mercaptan, a substance that gives gas its pungent “rotten egg” smell. Although unpleasant to be around, this odour is an extremely important safety measure.
The smell can quickly alert end users to gas leaks, without needing to wait for someone to routinely check equipment.
Types of compressor units
Most compressor units operate independently alongside each other. However, when the incoming natural gas needs a very high boost in pressure, several compressor units can be used in series to achieve the high pressure in a number of stages.
Typical operating pipeline pressure can range from 200 to 1,500psi, depending on the elevation, terrain and diameter of the pipeline. This means that the compressor unit may compress gas at different levels.
Compressor units typically operate in one of three ways. The first is where the turbine turns a centrifugal compressor, which is powered by natural gas from the pipeline itself. The second is where a high voltage electric motor makes a turbine turn a centrifugal compressor.
Centrifugal compressors increase gas pressure by adding velocity to the gas as it moves through an impeller, forcing the flow to spin faster. As the gas leaves the impeller, it travels at an accelerated rate.
Kinetic energy increases the pressure of the gas by then slowing the flow through a diffuser. Centrifugal compressors have better isothermal efficiency than reciprocating compressors, but are not as flexible.
The last type of compressor unit is a reciprocating compressor, which is also fueled by natural gas from the pipeline and uses large piston engines to turn reciprocating pistons located on the side of the unit. The movement of the piston changes the pressure, and the
reciprocating pistons then compress the gas.
These units can accommodate all volume capacities and pressures, making them particularly valuable in natural gas projects, where gas pressures can fluctuate. However, because these reciprocating compressors have a larger number of moving parts, they require increased lubrication of cylinders and pistons.
Compressor valves, which are responsible for controlling the flow of gas through the machine, can easily get damaged or worn, and therefore these compressor units can require high levels of maintenance.
Once it has been pressurised, the natural gas is directed back into the pipeline to be delivered to end users.
Components of compressor stations
Natural gas compressor systems contain several safety systems, monitoring implementations, and backup measures to ensure safety and reliability. Heat is generated when gas is compressed, with every 100psi increase in pressure creating a seven to eight degree increase in natural gas temperature.
If the gas is sent back into the pipeline at a high temperature, it could potentially damage the infrastructure of the pipeline and associated equipment. Compressor stations need to have safety measures in place to cool the gas to a manageable temperature before the gas is sent out into the pipeline again.
Most compressor stations include an aerial cooler system to dissipate heat of the gas. The heat generated by the compressor itself is dissipated through a sealed coolant system. All of these processes tend to generate a lot of noise, and so most compressor stations are also fitted with muffler systems, especially when located near residential areas.
Muffler systems aim to suppress sound levels to around 55 decibels while operating at full capacity. Because of this, compressor
stations are often housed in a building (typically including one compressor per building) to mitigate the loud noise with insulated walls.
Steps to deaden the sound of operating compressor stations can include insulation of turbines, shielded exhaust systems, advanced fan technology, strong weather stripping, air inlet and air discharge mufflers.
Compressor stations also help manage the supply and demand of natural gas through pipelines with their storage functions. Sometimes, supply of natural gas through the pipeline can exceed end user demand, and the excess gas can be stored in compressor stations.
On the other hand, natural gas stores in compressor stations can fill gaps in supply when demand increases. Because compressor stations play such a critical role in the supply of natural gas, they are usually fitted with backup generators to keep gas flowing.
Emergency shutdown systems are usually featured to allow gas to be rerouted if necessary. Compressor stations can also include infrastructure to allow PIGs to be launched and received at the station to inspect and clean the pipeline.
PIGs can inspect and clean the pipeline without stopping the flow of natural gas, and are therefore extremely useful in compression stations. PIGs can be inserted into a funnel-shaped Y section in the pipeline – the PIG launcher – which is then closed, and the pressure of the gas in the pipeline is used to push it along the pipe until it reaches the receiving trap.
Metering systems are standard for compressor stations, allowing for monitoring of gas storage levels and the amount of gas going in and out of the compressor station. Monitoring is vital to determine the amount of compression required at each station, which will change depending on the pressure of the gas as it arrives.
Monitoring systems also help determine whether compressor stations need to burn off excess gas to dispel vapours recovered during the removal of impurities from the gas. It’s important that these monitoring systems are accurate to ensure that stations are complying with emissions regulations.
This is especially critical for compressor stations that are powered by combustion engines, which emit exhaust gases into the atmosphere and can be a potential source of methane emissions. Because many compressor stations in Australia are located in remote areas, it is critical that these systems are designed to be operated remotely.
This includes having real-time monitoring of process controls for compressor units, especially when multiple compressors are operating at the same time.
Safety and regulations
Compressor stations include emergency shutdown systems which are connected to control systems that can detect risks or abnormalities, such as leaks or unanticipated pressure drops. These systems will stop the compressor units and isolate the compressor station piping in case of an emergency.
Compressor stations are regulated in Australia by AS 2885: Pipelines – Gas and liquid petroleum series and must meet environmental standards in their area of operation.
Optimising compressor stations
In Australia, it is typically more costeffective to install a compressor station compared to installing a looping or additional parallel pipeline. There are a number of ways to optimise efficiency of compressor stations.
Placing compressor stations close together can reduce the fuel cost, but will increase maintenance requirements. Planners should also consider whether the station will deal with a relatively consistent gas supply, or whether the supply is expected to fluctuate.
A station that runs under a wide fluctuation of supply will often operate in part load, meaning a higher fuel consumption. Spreading the load across all units evenly, and running the least amount of units possible, can make a station more fuel and cost-efficient.
This can be done by running gas turbines at the same load setting or by running compressors at the same distance from their surge lines. To take optimisation of a compressor station even further, modelling the entire pipeline network can allow for further efficiency.