Gas Metering Skid for the Application of Gas Supply System

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 and used to measure gas products. Based on the requirement, it can be installed at the onshore or offshore location. Although several parameters are measured, the flow rate is the primary variable to be measured. Depending on the specific technology and user specifications, the measured flow rate can be a mass or volumetric flow rate. The metering skid is often used for further dealings, such as pressure reduction using control valves, filtration, and flow rate control using different valves.

2 Components of a Gas Metering Station

A gas metering station may comprise a dispatch station, receiving station, remotely operated sectionalizing valve (SV) stations, 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 be located 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 and making several dispatch channels. Therefore, the receiving station may be located at various distances from the dispatch station.

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

A Flow Nozzle is a differential pressure device used to measure the flow rate in a closed conduit for heavy applications and high flow rates. 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 others.

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 in the meter reading by up to several percent. 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 the 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.

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. Installing 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.

2.4 Venturi Tubes

The Venturi tube is a robust device designed for use in low-pressure-loss conditions. The venturi tube is a differential pressure device that is used to measure the 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 flow rate measurement uncertainty.

2.5 Venturi Cone Meters

The Venturi Cone Meter is a differential pressure device to measure the flow rate in a closed conduit with the minimum pipe straight length and high rangeability. There are various types of venturi cone meters, such as V-cone, VM V-cone, Wafer cone, etc. Depending on the cone’s design, the cone’s features also vary.

The working principle of the Venturi cone meter is that reducing the cross-sectional area available to the flow will increase the flow velocity and correspondingly decrease the pressure. As long as the fluid properties are known, the flow rate can be determined by measuring the upstream pressure, temperature, and static pressure difference between the upstream and the minimum cross-sectional areas. The flow rate determination is done by applying mass and energy conservation laws.

2.6 Pressure Regulators

The function of the gas pressure regulator is to control the gas pressure in the pipeline. Reducers or expanders are often used as gas pressure regulators. Many materials are 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 often used. Springs inside the regulator are typically 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, in corrosive environments, when the cleanliness of the gas is a consideration or when the operating temperatures are 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 for measuring the flow rate in a closed conduit for general applications. It is used in big pipes with low flow velocity.

The working principle of a multiport averaging pitot tube can be explained as follows: 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 Flow Meters

Wedge flow meters 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 and have low maintenance costs. This type of meter has a unique feature for low, unrecovered pressure loss. 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 on the Bernoulli principle (continuity and energy balance equations). A differential pressure is generated via an engineered wedge, 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, it will damage the control valve and cause 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, the American Gas Association (AGA) recommends the temperature sensors 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 before the meter since large compressed volumes 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.

2.13  Unions

Unions are the joints of gas-metering skid piping systems. They connect two pipes. Orifice flange unions are also the most cost-effective for orifice measurement and have a simple operating procedure. The outlet materials for gas skid unions are cast iron, iron, lead-free brass, malleable iron, steel, etc.

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; in others, any meter type would perform satisfactorily.

All meter installations must meet specific fundamental requirements such as adequate protective devices such as strainers, relief valves, 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, when selecting a meter and its auxiliary equipment, the following criteria should be considered:

• Type of gas and its 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. Different gas metering systems are employed depending on the system requirement and handling of the gas property. 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.

The Orifice Meter System is designed to allow inspection or replacement of the orifice plate without interrupting the flow. By using the orifice meter system, all replacement parts can be changed without removing the fitting from the line. The slip-lock design prevents the cover plate from blowing 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 gas generation plants, gas-fired power generation plants (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 maintain the safety of valuable machinery and equipment by filtering the gas and controlling the flow pressure, temperature, and velocity. In other words, the revenue of the gas production plant is calculated 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 various reports and standards that various organizations publish. 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 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 with 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 and 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, 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.

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