Table of Contents
1 Introduction to Gas Metering and Regulator Stations
Measurement and regulation form the groundwork of business, more fundamentally, any exchange. Within the oil and gas industry, the foundation of measurement lies in metering and regulator stations. Without measurement or regulation customers and producers alike would have no basis for conducting business or exchanging goods. As critical mediators from late upstream production to midstream processing and refinement, down to individuals heating their homes and cooking their food, metering and regulating stations must be as accurate and consistent as possible.
If their measurements are inaccurate, tens of millions of USD can be lost per year with as little as 0.1% error. Tens, even hundreds of thousands of dollars would be lost per day on many pipelines.
Since these gateways often handle hundreds of thousands or millions of dollars in hydrocarbon resources per day, their accuracy and reliability are crucial. Expert design and planning is necessary to ensure these elements exchange and regulation within the supply chain remain intact. The impact of failure, even if the errors are marginal, has dramatic consequences on short- and long-term profits. Legal liabilities, loss of customers, and the resulting publicity would be virtual fallout. Lasting damage would be incurred on the reputations of those involved.
Mitigating errors in metering are for these reasons, a priority. As custody-exchange success and failure hinge on these metering stations, strict documentation of and adherence to proper standards is just as essential as proper design and maintenance of said stations.
2 Natural gas supply chain
2.1 Upstream
Following the exploration and production of natural gas is the process of transporting, refining, storing, marketing, metering, and delivery. Gas gathering systems first separate liquid condensate with separators to produce single-phase gas, which is promptly measured and sorted from condensate. Here, condensate takes a separate journey to another processing facility. This process generates intermediate products (condensates among them) and the finished product of transmission quality natural gas.
Gas is metered again upon exiting the processing facility. Compressors are used to move gas, under pressure, through pipelines to LNG facilities or midstream facilities, toward downstream users. Typically, these compressors are powered by a combustion engine or turbine and use natural gas from the lines as a fuel source.
Pipeline infrastructures that frequently span countries or even continents transport the gas further down the supply chain. Intervals of compressors, established roughly 50 miles apart, ensure the continuous transmission of natural gas.
2.2 Midstream
Touching aspects of the upstream and downstream value chain, midstream processes begin to use metering stations for a wide variety of mid and downstream processes, whether supercooled at an LNG facility or used to power industrial processes, power plants or serve other ends. Midstream or transmission pipelines are referred to as mains. These mains transmit natural gas at significantly higher pressures than localized downstream areas.
A useful analogy to midstream value chain operations is that of a national interstate system where high rates of speed are permitted for travel. Once an exit is taken, a significant change speed (pressure) and the operating procedure is required. In the case of natural gas transmission, the segue to surface-roads mired in potential hazards. This is where metering stations display their tremendous value in the natural gas supply chain (or interstate system, if you prefer).
2.3 Downstream
Closest to the end-user, downstream processes (distribution) encompass the end-users of natural gas products. It is often the case that distribution pipelines become smaller and less pressurized as they approach the consumer. Discussed in detail below, a classification of ‘gateway’ or tiered metering and regulator stations await the pressurized gas.
The first of these ‘gateway’ metering stations are often referred to as a ‘city gate’ metering station, or simply a gate metering station. This station meters and reduces the pressure of transmitted gas from midstream-transmission pressure to a more palatable level for high-pressure customer metering stations. Typically, these high-pressure metering stations are used for custody transfer, power plants, industrial processes, and in other larger-scale applications.
The design of distribution pipelines begins to differ more significantly the further downstream we follow the process. Distribution lines may change in dimension, materials, elemental exposure, or other ways. Subsequent ‘gateway’ metering and regulator stations are encountered.
Further pressure reductions are made for mid-pressure applications or continuation further downstream. District regulator stations are the final ‘gateway’ meters that stand between consumers and affordable, effective natural gas. Commercial, industrial, and residential customers are then free to take advantage of the numerous transmission systems designed to reach them.
Great care and planning are given to each aspect of these value chain phases, yet everywhere you may care to look, metering and pressure regulator stations provide exact, highly replicable services.
3 Gas metering and regulator operations
3.1 Custody transfer
Among the most stringent requirements of any metering station are the measurements for custody transfer. Errors made in these measurements can be extreme in cost. Most countries strictly regulate fiscal metering. Several industry associations and standards institutes including the American Gas Institute, American Petroleum Institute, US National Institute for Standards and Technology, and several others are involved in creating and maintaining these requirements.
The use of master meters on site, parallel meters, flow computers, and a host of other sensors and quality measurements are employed to accurately and reliably obtain these metrics. Eliminating or reducing uncertainty is paramount for collecting this data. Flow provers, metering skids, expert planning, and flow meters all play pivotal roles in reducing measurement uncertainty.
3.2 Conditioning
The quality of natural gas can have a significant impact on the operation of regulating and metering stations. The gas should be pipeline quality gas, free of most impurities and condensates. The conditioning of natural gas is the set of techniques used to remove any remaining or subsequently formed liquids, condensates, or solids from gas flows. Using indirect heaters and separators to achieve and maintain necessary gas temperatures prevents operators from incurring damage to metering and regulator stations.
3.3 Metering
Accurate and precise, repeatable measurement is the basis of any metering system used in the oil and gas industry. Measurement systems, regardless of the sensor (meter) type are based on mass measurement. Thus, based on the principle of the conservation of mass.
Flow meters also rely on the law of similarity, that is a dynamic and geometric similarity to minimize the risk of errors between flow meter calibration and field conditions. Inaccurate measures are frequently a result of violating the law of similarity, producing inaccurate measurements and generating loss at facilities.
Flowmeter assemblies are ideally comprised of high performing flow conditioners on upstream pipe sections, flow meters, and subsequent downstream pipe sections. Flowmeter assemblies are precisely calibrated instruments, where the primary device is the flow meter itself.
Secondary devices are those which measure gas composition, pressure, temperature, and other metrics. Auxiliary or tertiary devices are electronics such as flow computers, SCADA systems, or other components which receive process, and store collected data. Complex, proprietary algorithms are programmed into these auxiliary devices for flow calculation within designated parameters. These devices also serve in providing a detailed trail of data which may be audited for analysis.
3.4 Regulating
Achieving and then maintaining the desired system pressure (control) is the primary goal of regulating operations. Each metering station is designed not only to measure but to monitor pressure and flow. Aiding in the pressure regulating efforts at these stations are filters, heaters, reducers, regulators, drains (for condensate collection and disposal), storage tanks, and metering skids.
Filter units are designed to sort and remove entrained liquids and solids that form in the natural gas flow stream. Often, they make use of cyclonic elements to trap particles and liquids along the sides of pressure vessels designed for holding condensates. These sumps may be emptied as required.
Heaters aid in preventing the formation of hydrates and liquids during pressure reduction. Indirect gas-fired water bath heaters are designed to safely and efficiently maintain gas temperatures above the dew point while maintaining desired flow rates.
Each regulator system maintains at a minimum of two trains of pressure reduction. While one operates, the other stands by as a contingency. Two valves, an active and a monitor valve, each equipped with a controller operating the valve at predetermined discharge pressure values.
4 Gas metering and regulator station types
Gate stations, mentioned above, act in a manner somewhat resembling speed limit signs for interstates, surface roads, and school zones. These stations are designed to regulate pressure infirm, categorical steps, from one pressure to another. Each speed limit, step, or pressure level has distinct operating concerns, often reflected in the changes of pipeline size and materials (metal, plastic, etc.) if not location.
4.1 City gate and high-pressure customer metering stations
In most instances, city gate stations are the among first pressure reduction and metering stations encountered by natural gas flowing through pipelines. Transmission pipelines frequently operate between 200 and 1,500 pounds per square inch (PSIG). Distribution pipelines operate anywhere from a quarter pound to 200 PSIG, a transition of pressure made by gate stations (city gate stations).
High-pressure customer metering stations operate above 60 PSIG to required specifications. These high-pressure stations are used for custody transfer metering, gas conditioning and treatment, for industrial applications, power plants, and other specialized applications.
4.2 Town border stations and mid-pressure metering stations
While city gate metering stations distribute large volumes of natural gas to whole areas or for industrial purposes, town border stations distribute gas to areas of a city.
These metering and regulator stations operate from 15 PSIG to 60 PSIG on average, though some mid-pressure stations operate from 2 PSIG to 15 PSIG. If not added before these stations, odorants are mixed with the natural gas for leak detection purposes.
4.3 District regulating stations and low-pressure metering stations
District regulator stations operate with the least transmission pressures. Typically, these regulator and metering stations function at service levels of pressure or less than 2 PSIG.
Low-pressure metering stations provide natural gas to individual residences or buildings. Natural gas utility companies maintain and operate the supply line up to the residential or business meter. Once metered at the residence, the pressure is frequently reduced to less than one-quarter PSIG, allowing for easy access to customers.
5 Metering and regulator station components
5.1 Condensate tank
Separators (discussed below) help collect and store liquid hydrocarbons and water within the condensate tank. These tanks are equipped with the necessary safety valves to prevent overpressure. High-level alarms are installed to indicate excessive pressure levels. Condensate tanks may be manually emptied as necessary.
5.2 Chart recorder
Perhaps underestimated as a form of record-keeping, manual chart recorders are sometimes employed to measure and record pressures and temperature. Often, they are used in conjunction with digital technologies as an inexpensive and valuable addition to record-keeping practices. They serve as a straightforward and reliable means of recording at-a-glance information.
5.3 Gas heaters and heat exchangers
Gas heating systems or heat exchangers increase the temperature of natural gas indirectly. Usually, the gas first goes through a filter, then is heated, before pressure reduction occurs. These systems are designed to keep natural gas above the condensation or dew point to prevent the formation of hydrates due to the Joule-Thompson effect.
Gas-fired water bath heaters are a type of indirect heater used to prevent hydrate formation as pressure drops. Indirect water bath heaters utilize tubular elements immersed in a heated liquid to maintain gas temperatures above the hydrate point.
Electric immersion heaters may be placed in smaller gas stream flows within a pressure vessel to serve the same purpose. Low input piping may be insulated on the exterior and wrapped with heating tape to prevent condensate formation.
5.4 Inlet and outlet connections
These connections are designed to keep pipeline velocity, and therefore noise, at desirable levels. As each pipeline requires tailor-made solutions, inlet and outlet connections should be designed around calculated pipe velocities to achieve desired noise levels. Noise, regarding metering stations, can severely disrupt accurate measurements from some metering devices.
5.5 Insulating joints
These joints are specially designed to prevent damage to the metering and regulating stations due to eddy currents. Frequently located at the inlets and outlets of stations.
5.6 Inlet emergency shut down (ESD) valve
Emergency shut-down valves (ESD) are designed to isolate the station immediately in case of a high alarm or emergency scenario.
5.7 Liquid separation system
Designed to accommodate the station’s maximum flow rate, liquid separation systems filter liquid condensate from the gas stream. Most often there is a second, identical filter awaiting deployment, should it become necessary. Manually operated valves are generally used to engage these filters.
5.8 Valves and valves actuators
Whether automatic or manually operated, valves serve the vital function of opening, obstructing, or sealing passageways. Ball, butterfly, check, diaphragm, gate, globe, and plug valves are all used in a wide variety of flow resistance environments.
5.8.1 Slam shut valve
These valves are intended to protect the station from high pressure. Unexpected pressure increases can be cut off to prevent downstream damages. Slam shut-valves may be automatically activated when low-pressure or high-pressure conditions are met.
5.8.2 Relief valves
Relief valves are designed to mitigate internal overpressure. Created with a specific pressure value, these valves prevent excess inlet pressure rise. They are pilot operated or directly engaged.
5.8.3 Pressure switching valves
Pneumatic logic systems utilize this type of valve for two or three-way switching. This allows the direction of inlet pressure between outlet ports at preset pressure limits.
5.9 Pressure gauge
Though many advanced pressure measuring instruments are available, the simplest is a pressure gauge. The Bourdon tube pressure gauge remains the most commonly used. It employs a Bourdon tube, a thin, metal tube closed on one end. When pressure is internally applied to the tube, the closed-end moves linearly, allowing for measurement. Some of these pressure gauges may reach accuracies of 0.25%.
5.10 Temperature gauge
Regardless of the scale used, a significant number of common temperature gauges are available for use. Among the most frequently used are:
- Bimetallic devices
- Filled thermal system
- liquid-in-glass thermometer
- Radiation pyrometers
- Resistance temperature detectors (RTDs)
- Smart temperature transmitters
- Thermistors
- Thermocouples
Changes in electrical resistance, volume or pressure of the gas, radiation emissions, and the contraction or expansion of liquids (or metals) are most often used to gauge temperature. Sensor selection will always depend on the unique advantages conferred and constraints levied on operators in a given circumstance. Some stations require the greatest levels of sensitivity and can afford the necessary cost to acquire them. Maintenance and power requirements may be weighty choices for another station.
6 Primary, secondary, and auxiliary devices
6.1 Primary devices
Two standard classifications for flow meters are designated as (energy) additive or extractive. Additive meters are a class of flow meters which supply energy into the gas flow for flow rate measurements. Extractive meters function with energy supplied from the flow stream to determine flow rate measurements. Subsequent flow meter delineations are either inferential or discrete. The former infers flow rate measurements by dynamic properties within the gas flow stream while the latter separates the flow stream, counting individual portions.
Instruments used for measuring the volumetric flow of gas or liquid moving through a pipeline or station are referred to as flow meters. These sensors measure linear and nonlinear mass using different means. Some are ideally suited to high-pressure metering stations while others function best in low-pressure circumstances. The merit of one meter over another largely depends on the needs of a particular station, though the performance of a particular flowmeter should be among the primary considerations when choosing which meters to install.
6.1.1 Coriolis flowmeter
Specializing in direct mass flow measurement, Coriolis flow meters are more than half a century old within the oil and gas industry. Despite their accuracy, early Coriolis meters suffered from vibrations caused by early design flaws. The continual refinement of this meter type—a typical sight within the heavily tech-invested oil and gas industry—eliminated these issues. Capable of measuring liquids and gases with high degrees of accuracy, they require less maintenance but command higher initial expense.
The Coriolis principle is utilized for the measurement of natural gas by observing the twist created from flow moving through two oscillating tubes. The twist generated varies proportionally to the mass flow rate, which is measured and analyzed to determine a linear flow signal.
6.1.2 Orifice flow meter
Widely regarded as the oldest pressure regulator and metering device—invented by the Romans—the orifice flowmeter is a primary device, further classified as an energy extractive, inferential meter. Appropriate filters, separators, and the like should be installed to mitigate condensate or hydrate interference with flow measurement. Particles, if not properly accounted for, may wear the plate seal rings and edges.
Until relatively recent times, orifice flowmeters designed with concentric, flanged, square-edged features were the most widely used primary device upstream of gas processing plants. This is due to its inexpensive nature (both CAPEX and OPEX) and its lack of sensitivity to flow composition.
Frequently accompanying secondary devices include:
- Differential pressure transmitter
- Static pressure transmitter
- Fluid temperature transmitter
- Operator chosen method for determining flow density
- Operator chosen method for determining the base density
- Additional devices for reinforcing quantity and quality of pipeline transmission
- Include but not limited to double block and bleed valves (DB&B), online gas chromatograph (GC), sampling point, etc.
6.1.3 Thermal flow meter
Commonly used in gas transmission lines, thermal meters are designed to sense the rate of heat dissipation within a flowing stream. Once transferred into the stream, the speed which heat dissipates indicates the composition and temperature of the gas, providing the meter with mass flow data.
Thermal mass flow meters are simple, inexpensive, and low maintenance. They have no moving parts. However, these meters suffer accuracy when the natural gas composition is unknown or varied from what is expected. With proper adjustments, the mass flow may be determined accurately once more.
6.1.4 Turbine flow meter
Comprised of bearings, electronic rotor speed sensor, flowmeter body, rotor, and stator, turbine flow meters are primary devices, further defined as an energy extractive, inferential meter when lacking a high-performance flow conditioner. When paired with an adequate flow conditioner, this type of flowmeter is used downstream of gas processing plants (condensate risk). The latter configuration is widely adopted as a primary device.
Mounted within the pipeline, turbine flowmeters combine magnetic sensors and with rotor movement to accurately determine volume within gas lines. Magnets located on the blades provide external sensors with accurate data in steady flow conditions. These flow meters are capable of providing mass flow data if the gas composition is known.
Middling in terms of expense, their mechanical parts require replacement or repair once worn. Turbine flow meters can be adapted for a wide variety of circumstances, particularly when operating conditions are well accounted for.
Frequently accompanying secondary devices include:
- Static pressure transmitter
- Fluid temperature transmitter
- Operator chosen method for determining flow density
- Operator chosen method for determining the base density
- Additional devices for reinforcing quantity and quality of pipeline transmission
- Include but not limited to double block and bleed valves (DB&B), online gas chromatograph (GC), sampling point, etc.
6.1.5 Ultrasonic flow meter
This type of primary device is comprised of a flowmeter body, electronics, and transducers. Transit time technology is employed to determine mass volume with ultrasonic flowmeters. Transit times are affected by the turbulent structures within a flowing stream, the flow profile, as well as mean velocity and the speed of sound along the chordal path. Chordal velocity measurements may prove unreliable depending on the path length, transmitted acoustic pulse form, electronic timing and gating performance, the configuration and radial position of the acoustic path, and the calculations involved in reducing measurements to the mean chordal velocity.
Transducers can be mounted externally or internally along transmission lines. Depending on how the transducers are mounted, this primary device may be further classified by each instalment. When internally mounted, ultrasonic flowmeters are considered as either intruding or non-intruding if the transducer intrudes into the gas flow. Acoustic paths can be arranged in a reflective, non-reflective, or hybrid geometry and vary by commercial designs. When used in combination with a high performing flow conditioner, this primary device is considered an energy extractive, inferential flow meter. It is widely employed inflow stream measurement downstream of natural gas processing facilities.
Rotary displacement flow meters are frequently accompanied by these secondary devices:
- Static pressure transmitter
- Fluid temperature transmitter
- Operator chosen method for determining flow density
- Operator chosen method for determining the base density
- Additional devices for reinforcing quantity and quality of pipeline transmission
- Include but not limited to double block and bleed valves (DB&B), online gas chromatograph (GC), sampling point, etc.
Due to their use of ultrasonic pulses, outside noise can significantly disrupt accurate metering. Buildup within the pipeline can interfere with accuracy as well, and they are more costly than some other meters. Notwithstanding, ultrasonic meters are evaluated as durable and low maintenance.
6.1.6 Rotary displacement flow meter
The method of measurement employed by this primary device requires a particulate filter to maintain flowmeter accuracy. Oil deposits may develop an unwanted, accumulated presence given enough time, exaggerating measured flow quantities. Exceptionally precise machining of a rotary displacement flow meter is required for optimal performance. Underreporting of flow quantities can occur due to internal friction and slippage.
Proper installation design of rotary displacement flow meters requires considering potential stresses incurred on the primary device cause by connecting piping. Abnormal stresses on the housing of this primary device may develop if installers fail to account for this. Rotary displacement flowmeters are further classified as energy extractive and discrete meters.
Comprised of two-lobe impellers rotating in opposite directions, these elements maintain a fixed relationship to one another within the cylindrical housing of the meter. The impellers are held fast by precise timing gears, while the housing maintains flat end plates on both the inlet and outlet. Natural gas flows turn the impellers. As a result, a measurement chamber is created between the cylinder, headplate, and impeller. This primary device is best used in conjunction with electronic impellor sensors to totalize displaced volume most accurately. Combined with a particular filter, this primary device is routinely employed in metering stations found downstream of gas processing facilities.
Rotary displacement flow meters are frequently accompanied by these secondary devices:
- Static pressure transmitter
- Fluid temperature transmitter
- Operator chosen method for determining flow density
- Operator chosen method for determining the base density
- Additional devices for reinforcing quantity and quality of pipeline transmission
- Include but not limited to double block and bleed valves (DB&B), online gas chromatograph (GC), sampling point, etc.
6.2 Secondary devices
A host of secondary devices are available for utilization in metering and regulator stations. The principle function of secondary devices is to provide tertiary devices with inputs. Generally, they analyze pressure (differential and static) temperature, gas composition, and other metrics.
Chromatography sensors allow for detailed analysis of gas quality. Calorific value measurement, process control, quality control, and several other important functions are greatly aided by the use of sophisticated gas chromatographs.
Modern designs are compact and modular, allowing relatively simple installation at sample points.
6.3 Auxiliary devices
Auxiliary devices are the electronic devices programmed to properly calculate flow within narrowly defined parameters, as they receive information from primary (flow meters) and secondary devices. Flow computers, SCADA systems, and other devices which gather store, and process data are classified as auxiliary devices for metering and regulator stations.
Algorithmic calculations performed by these tertiary components are frequently proprietary by nature, though generally known within the industry. Output signals originating from primary and secondary devices are fed into the algorithms. Auxiliary devices receive these signals as input and address it according to their programming.
Employed with the diaphragm, rotary, and turbine meters, an electronic volume corrector (EVC) is an auxiliary device designed to record signal, pressure, and temperature data from the gas flow. Including an EVC increases the accuracy and precision of data gathering to an extent beyond what flow meters alone could achieve.
Similar to, yet more sensitive than an EVC, flow computers are capable of advanced programming and calculation. They possess a wider variety of alarm settings, are reprogrammable, and can control odorant injection systems. This power and versatility make them desirable for use with differential pressure meters (introduced below), as they provide highly necessary tracking of multiple flow variables. Though they are undoubtedly more powerful than EVCs, they are infrequently used in hazardous environments.
Acting as a remote control, data gathering, monitoring, and processing system, Supervisory Control and Data Acquisition Systems (SCADA) provide station operators with several advantages. Responding to unexpected events, overseeing operations regardless of location, maximizing resources, monitoring data, and report generating are some of the many improvements SCADA systems offer.
7 Odorants
Along the oil and natural gas supply chain, an odorant is added to natural gas, typically as it nears its destination. Natural gas is predominantly odourless until an odorant is incorporated. This practice arose from the incidents where gas leaks went undetected, resulting in fires or explosions.
THT (tetrahydrothiophene), mercaptan, or non-sulfur odorants are added for their distinctive smell, which resembles that of certain cooked vegetables (cabbage, leeks, onions).
7.1 Overpressure protection
At a minimum, all pipelines which could exceed their determined maximum allowed operating pressure (MAOP) must have reliving or limiting devices which meet international standards. These regulating devices must meet the load, pressure, and other service condition requirements. Overpressure protection devices must be designed to prevent accidental over-pressuring and accurately limit during both normal and no-flow conditions.
8 Future developments
Metering stations have steadily incorporated new technologies and manufacturing processes throughout the decades. Automated systems, electronic sensors, and data collection were just the beginning. Modular systems, skids, and turnkey projects have become a greater part of developing metering and regulator solutions within the oil and gas value chain.
These systems are continually improving, adopting smarter, more interconnected data analysis—capable of being viewed in real-time and analyzed in depth. The growing interest in Biogenic gas and the ability to effectively meter quality biogas (sans CO2) into distribution networks will promote interest in metering stations that perform this function. Specialized, niche modular metering stations are garnering increased levels of attention as the ability to provide tailor-made solutions is improving within the industry.
9 Conclusion
Far from simple gateways, metering and regulator stations are at the very core of the oil and natural gas sectors. Metering and regulator stations not only symbolize voluntary exchange and reliable service, but they also embody the very principles. Reiterating a salient point, these stations are the hinges of the entire oil and gas value chain. Their ability to precisely and accurately make measurements affects everyone from stakeholders to end users, should they fail even marginally in their efforts. Many companies are keenly aware of their central role, consequently, they invest in expert design, installation, and operation.
Natural gas metering and regulator stations are rapidly becoming tailor-made, modular solutions, highly adapted to the specific needs of distributors and customers throughout the value chain of oil and natural gas. Without these efficient, meticulously designed stations the oil and natural gas distribution system could not keep up with demand.
Metering and regulator stations are the essential link in the oil and gas supply chain. As such, they hold a place of prominence that is only growing as the demand for inexpensive, reliable, and life-improving natural gas grows.
10 Further reading
Modularization for Hydrocarbon Pipelines in Africa
Where Natural Gas Is Found and How It is Obtained
