Table of Contents
1 What is a gas metering skid?
A gas metering skid is a part of a gas production plant that is pre-assembled on a steel structure, engaged to the measurement of gas products. It can be installed at the onshore or offshore location based on the requirement. Although several parameters are measured, the primary variable to be measured is the flow rate. Depending on the specific technology and user specifications, the measured flow rate can be a mass flow rate or a volumetric flow rate. The metering skid often used for further dealings, such as pressure-reduction by using control valves, filtration, and flow rate control by using different types of flow control valves.
Fig: A typical gas metering skid (Image Source: Exim Engineering).
2 Components of a gas metering station:
A gas metering station may comprise of dispatch station, receiving station, remotely operated sectionalizing valve (SV) stations along with associated instrumentation, filtration skid, ultrasonic metering (USM) skid, pressure letdown skid, protection system, etc. Depending on the volume of gas handled by the gas metering station, the number and size of the components will vary. The function of a dispatch station is to supply gas from the supplier end to the receiver end and vice versa. The dispatch station and receiving stay may or may not locate in the same location. It depends on the system design. For example, one dispatch station can deliver gas to different locations by connecting several flow paths making several dispatch channels. Therefore, the receiving station may locate at various distances from the dispatch station.
Fig: A gas metering skid with different dispatch line (Image Source: S&S Technical Engineering).
Although there are a large number of components in the gas metering skids, the following components are the most common items-
2.1 Flow nozzles
Flow Nozzle is a differential pressure device to measure the flow rate in a closed conduit for heavy applications and a high flow rate. Nozzles are primarily used due to the high velocity of gas and to resist erosion on the gas pipe system. The materials for the flow nozzles can vary depending on the requirement. However, the common materials are stainless steel, carbon steel, brass, fibreglass, spun aluminium, PVC, Hastalloy, Monel, and other materials.
Fig. A typical flow Nozzle (Courtesy ABB).
2.2 Flow conditioners
Due to the flow conditions at the pipe inlet section, the behaviour of flow rate and volume-measuring devices can be affected very strongly. Disturbed velocity profiles caused by pipe configurations such as bends, headers, pressure regulators, and convergent or divergent pipe sections in front of a flowmeter can lead to deviations of the meter reading by up to several percents. So, using a flow conditioner is very helpful in stabilizing the flow. The flow conditioners can be classified into three types –
a) Tube-bundles type flow conditioners which eliminate swirl only
b) Medium category type flow conditioners which remove swirl and non-symmetry, but do not produce pseudo fully developed flow
c) High-performance flow conditioners that remove swirl and non-symmetry and create pseudo fully developed flow.
Flow straightening devices such as honeycombs and vanes inserted upstream of the flow meter can reduce the length of straight pipe required. However, they produce only marginal improvements in measurement accuracy and may still need a significant length of straight pipe, which a cramped installation site may not permit.
Fig. A typical flow conditioner (Source: Wikipedia).
2.3 Gas filters
Gas is used for different purposes, from household to industry. So, the gas is transported from the gas production plant to the user end by millions of miles of pipeline. So, the gas may be contaminated in the pipeline during transportation through long pipelines. As a result, purification of the gas by filtration to remove the foreign particles in the gas is important. This is of utmost necessity for the safety of the expensive process materials or equipment such as gas turbine nozzles, blades, etc. as well as the safety system. Also, the raw gas from the ground requires processing and refining before it is ready for use. The installation of gas filters makes it possible to improve process efficiency and provide optimal process protection. Another purpose of the gas filter is to remove suspended liquids and dirt from pipelines. The filters are usually mounted on a rigid permanent filter holder with a vibration-resistant removable tube retainer. The filter cartridge is self-gasketing, and the filter holder is designed so that a perfect seal is easily made, even when the tube is replaced by an operator unfamiliar with the equipment.
Fig. A typical flow conditioner (Source: Wikipedia).
2.4 Venturi tubes
The Venturi tube is a robust device, designed for the use in low pressure-loss condition. The venturi tube is a differential pressure device which is used to measure flow rate in a closed conduit. The venturi flow meter gives reliable flow readings with the wet natural gas flow. When applied to meter wet gas flows, the venturi meter’s gas flow rate reading is known to have a positive bias, induced by the presence of liquids. The wet gas flow correlation, to correct for this bias or so-called over-reading, plays a critical role in determining the gas flowrate measurement uncertainty.
2.5 Venturi cone meters
The Venturi Cone Meter is a differential pressure device to measure flow rate in a closed conduit with the minimum pipe straight length and with high rangeability. There are various types of venturi cone meters, such as V-cone, VM V-cone, Wafer cone, etc. Depending on the design of the cone, the features of the cones also varies.
The working principle of the venturi cone meter is- the reduction in the cross-sectional area available to the flow will increase the flow velocity and a corresponding decrease in pressure. By measuring the upstream pressure, temperature, static pressure difference between the upstream and the minimum cross-sectional areas, the flow rate can be determined as long as the fluid properties are known. The flow rate determination is done by applying the laws of conservation of mass and energy.
2.6 Pressure regulators
The function of the gas pressure regulator is to control the pressure of the gas in the pipeline. Very often, the reducers or expanders are used as gas pressure regulators. A wide range of materials is used to handle different types of gas and operating environments. Common regulator component materials include brass, plastic, and aluminium. Various grades of stainless steel (such as 303, 304, and 316) are also used very often. Springs used inside the regulator are typically made of carbon steel or stainless steel.
Brass is suited to most common applications and is usually economical. Aluminium is often specified when weight is a consideration. Stainless Steel is often chosen for use with corrosive fluids, use in corrosive environments, when cleanliness of the gas is a consideration or when the operating temperatures will be high.
2.7 Multiport averaging pitot tube
The multiport averaging pitot tube is a general-purpose and affordable device. It is a differential pressure device suitable to measure the flow rate in a closed conduit for general applications. It is used in big pipes with the low flow velocity.
The working principle of a multiport averaging pitot tube can be explained as – it is a head-type device that calculates the flow rate from the measured differential pressure. This modernized pitot tube produces a differential pressure signal proportional to the square of the flow rate per Bernoulli’s equation. The signal has two components, the upstream impact pressure (high pressure) and the downstream static pressure (low pressure).
2.8 Wedge flowmeters
Wedge flow metes are used for slurries and highly viscous media. The application of wedge meters is in the mining and basic materials industry, petrochemical industry, oil and gas refineries, etc. The wedge flow meters are robust in design with low maintenance costs. This type of meters has a low unrecovered pressure loss unique feature. Another convenient feature is the metering can be bi-directional. The constructional design of the wedge flow meter enables bi-directional measurement, which differentiates it from other flow meters. The functional principle of this measuring instrument is based upon the Bernoulli principle (continuity and energy balance equations). Via an engineered wedge, a differential pressure is generated, which can be equated with the mass or volume flow.
2.9 Orifice plate assembly
Orifice plate assembly is an integral part of a flow-control valve-control circuit. It provides the differential pressure to power the flow control pilot. The opening and closing of the pilot cause the flow control valve to throttle accordingly. Total head loss across the valve is reduced by locating sensing ports close to the orifice plate, to sense downstream pressure before it recovers. The orifice plate’s internal diameter is calculated and machined according to valve size and required flow limitation.
There are several types of orifice plates, such as square edge (standard) bore, quadrant edge bore, eccentric bore, segmental bore, RTJ type orifice plates, paddle-type orifice plates, etc. The materials of the orifice plates are 316 Stainless Steel, 304 Stainless Steel, 310 Stainless Steel, Hastelloy® C276, Hastelloy® B3, Duplex Stainless Steel, Super Duplex Stainless Steel, Monel® 400, Carbon Steel, Titanium, Incoloy® 800, Incoloy® 825, Inconel® 600 and Inconel® 625.
2.10 Temperature sensor
A temperature sensor is one of the vital components of the gas skid because each gas has a different condensation temperature. If there is any liquid in the gas pipeline that will damage the control valve, make corrosion in the nozzles, piping system, and related instrumentation. So, temperature sensors are employed to monitor the temperature of the gas line. The location of temperature sensors is critical for better accuracy. For gas, American Gas Association (AGA) recommends the temperature sensors should be installed between two and five diameters downstream of the flowmeter in a uni-directional system and three diameters from the meter in a bidirectional installation.
2.11 Pressure transducer
An accurate pressure determination is required since the measured gas is always metered at a pressure higher than atmospheric pressure (one standard atmosphere) and is in a compressed form. A compressibility factor must be established and applied to the metered volume. A pressure transducer is essential due to the safety system of the gas metering skids.
2.12 Air eliminators
Air or vapour eliminators should be installed preceding the meter since large compressed volumes will tend to expand through the meter, causing over-speed and severe damage. An air eliminator is usually an enlarged section of pipe or vessel that is vented and provided with a level control device. The air eliminators serve to protect the meter against large air bubbles. The materials for air eliminators are cast iron, iron, malleable iron, steel, etc.
Fig: Typical unions.
2.13 Unions
Unions are the joints of the gas-metering skid piping systems. They are used to connects two pipes. Orifice flange unions are also the most cost-effective for orifice measurement with a simple operating procedure. The outlet materials for gas skid unions are cast iron, iron, lead-free brass, malleable iron, steel, etc.
Fig: Typical unions.
3 Gas meter selection criteria:
The selection of a specific gas meter type depends on its application. In some applications, a particular type is preferred, while in others, any meter type would perform quite satisfactorily.
All meter installations must meet specific fundamental requirements such as adequate protective devices such as strainers, relief valves, and air or vapour eliminators, dependable pressure and flow controls, and accuracy proving facilities. Accurate instrumentation for measuring the physical properties of the flowing fluid, such as temperature, density, etc. is also required. Also, physical conditions during normal metering operations must be identical. Therefore, selecting a meter and its auxiliary equipment, the following criteria should be considered:
- Type of gas and it’s characteristics.
- Amount of foreign matter in fluid(gas) streams.
- Flow range and whether the flow is intermittent or continuous.
- Temperature range and accuracy of temperature compensation.
- Maximum operating pressure & maximum permissible differential pressure.
- Type of volume registration device.
- The required accuracy of the meter.
- Type and method of proving required.
- Applicability of auxiliary meter registration equipment.
- Maintenance requirements.
- Installation space available.
4 Types of gas metering systems:
There are several types of gas metering systems. Depending on the system requirement, handling gas property, different gas metering systems are employed. Different types of gas metering systems are-
- Turbine meter system
- Positive-displacement meter system
- Ultrasonic meter systems,
- Coriolis meter systems
- Orifices plate meter systems
- Venturi tube meter system
- Pitot tube metering system etc.
Orifice Meter System is designed to allow inspection or replacement of the orifice plate without interrupting the flow. All replacement parts can be changed without removing the fitting from the line by using the orifice meter system. The slip-lock design prevents the cover plate to blow-off in the event the upper chamber has not been depressurized.
Direct mass measurement sets Coriolis flow meters apart from other technologies. Mass measurement is not sensitive to changes in pressure, temperature, viscosity, and density. With the ability to measure liquids, slurries, and gases, Coriolis flowmeters are universal meters.
The Bernoulli Equation is used for the differential pressure flow meter system. In a differential pressure drop device, the flow is calculated by measuring the pressure drop over an obstruction inserted in the flow.
Volumetric meter systems excel at measuring clean, steady, medium-to-high speed flows of low-viscosity fluids. Volumetric metering systems are widely used for liquid and gas flow measurement but cannot be used to measure steam flow. Turbine meters are one of the main types of Volumetric meters used for custody transfer of natural gas & liquid.
5 Application of gas metering skid:
The purpose of the gas metering skid is to control the gas flow rate, distribute the gas flow, and measure the amount of gas flow in the systems. So, the gas metering skids are used in the gas generation plant, gas-fired power generation plant (open cycle or combined cycle), gas distribution stations, etc. Gas metering skids can be found in the following locations-
- Onshore/offshore gas custody
- Transfer metering
- Pipeline bidirectional measurements
- Gas terminals and mixing stations
- Gas power plants
- Pipeline junctions
- Compressor stations
- Chemical companies
- City gate facilities etc.
The gas metering skids help to maintain the safety of the valuable machinery and equipment by filtering the gas, controlling the flow pressure, temperature, and velocity. In other words, the revenue of the gas production plant is calculated by using the gas metering skids.
6 Gas metering standards:
For metering installation and measurement of natural gas quantities, one must continually track the progress of a broad array of reports and standards that are published by various organizations. E.g., the American Gas Association (AGA), American Petroleum Institute (API), and Gas Processors Association (GPA) or other local authorities. American Gas Association (AGA) reports detailing the specification, installation, and operation of meters that are approved for custody transfer in the American gas industry. AGA publishes reports and other organizations such as the API and the GPA publish standards and have adopted or endorsed AGA reports; this could cause some confusion.
Measurement personnel in natural gas pipeline and distribution utility companies must keep up-to-date of several AGA reports and at least one chapter in the API Manual of Petroleum Measurement Standards (MPMS), Chapter 21.1. These standards cover audit trail requirements for a gas measurement system as well as some orifice measurement calculations. In the U.S., end-user natural gas operating companies typically require manufacturers of gas measurement devices (e.g., flow computers) to pass testing by a third-party laboratory such as Colorado Experiment Engineering Station Inc. (CEESI) or Southwest Research Institute® (SwRI®).
In other regions of the world, countries use the International Organization for Standardization (ISO) publications such as ISO 5167. Some countries maintain their own, unique measurement standards. For instance, China uses a modified version of AGA3 for orifice metering applications.
