BASICS OF HIGH PRESSURE MEASURING AND REGULATING STATION DESIGN Class #1010
E. D. “Rusty” Woomer, Jr., PE El Paso Pipeline Group P.O. Box 2511, Room N731A Houston, TX 77252 Introduction There is more to the design of a measurement facility than the word measurement suggests. Generally, the measurement arena may include any or all of the following: ¾
Metering
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Control
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Pressure regulation Flow control Overpressure protection
Gas Quality
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Primary devices Secondary devices Tertiary devices
Chromatography Spot or composite sampling Analytical instrumentation
Other
Odorization Filtration / Separation Heating
Pneumatic and electronic electronic instrumentation instrumentation is scattered throughout each of the the categories listed listed above. The detailed design of a measurement facility can become quite involved and exceed the space allotted in this paper. However, the fundamentals will be addressed in regard to the considerations for designing natural gas transmission pipeline measurement facilities. facilities. For the purposes of this paper, only metering and regulating (M&R) will be addressed. At a very high level, a good design engineer will focus on three generalities. First, the station must be designed with safety in mind, mind, regarding both personnel and equipment. Secondly, the design design must be cost effective. effective. And thirdly, the station must be operationally functional. Safety: There is a multitude of industry standards and specifications from AGA, ANSI, API, ASME, ASTM, GPA, MSS, NEMA, NEC, NEC, and many others. Those that attract much attention include include but are not limited to: ¾ ¾ ¾ ¾
49 CFR 192 “Transportation of Natural Gas and Other Gas by Pipeline: Minimum Federal Safety Standards” DOT OPS ASME B31.8 “Gas Transmission and Distribution Piping Systems”, B16.5 for flanges, and B16.9 for fittings API 5L “Line Pipe” and API 6D “Pipeline Valves” specifications NFPA 70 “National Electrical Code”
Do not forget the state and local regulations and your company’s safety standards. Cost Effectiveness: When preparing budgets or expenditures, consideration must be given to the cost of the station. Order of magnitude magnitude (± 25%) and preliminary (± 10%) cost estimates help evaluate this matter. matter. Remember, the lowest price is not always the best design when safety and operational functionality are factored.
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Operational Functionality: The station must perform the operations that were intended and as indicated in the scope of work. It must function accurately and efficiently. Since measurement facilities operate unmanned, they must function dependably. The design should adhere to generally accepted industry standards. Also, adherence must be given to your individual company’s standards and specifications. Examples include your O&M Manual, Manual of Engineering Standards, Measurement Manual, Safety and Health Handbook, and FERC Gas Tariff. Keep the design simple and use sound engineering practices. An M&R station design will include engineering disciplines from the civil / structural, mechanical / piping, and electrical / instrumentation arenas. Design Criteria Equipment cannot be selected or sized without knowing the parameters to which the station is to be designed. While some factors can be assumed, the following table lists many items necessary: DESIGN CRITERIA PARAMETER Flow rates (maximum, normal, minimum) Peak hourly flow rates (maximum, normal, minimum) Projected growth Pressure (maximum, normal, minimum) Maximum Allowable Operating Pressure (MAOP) Maximum Operating Pressure (MOP) Overpressure protection Temperature (maximum, normal, minimum) Base pressure (Tariff) Base temperature (Tariff) Atmospheric (barometric) pressure (Tariff) Specific gravity Hydrocarbon dewpoint Water content Carbon dioxide, nitrogen, oxygen Hydrogen sulfide Gas composition and gas quality determination method (e.g., chromatograph, composite sampler, spot sampling) Delivery pressure (downstream of meter) Maximum allowable noise EGM software and hardware specifics per operating company Control method requirements (e.g., flow control with pressure override) Remote monitoring and control requirements Frequency of monitoring required (real time/daily/weekly/monthly) Location (e.g., onshore, offshore, wetland, residential area) Availability of utilities (electricity, telephone) Local building or other permitting requirements Liquid removal, measurement, and re-injection requirements Condensate removal, storage, and handling requirements Critical need for service (i.e., shut-in for inspection allowable or not) Filter separator requirement Heating requirement Odorization requirement
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UNITS MMscfd Mscfh psig psig psig ºF psig ºF psig ºF lbs. / MMscfd mole % grains / 100 scf
psig dBA
When given the peak hourly flow rate and converting it to a daily flow rate, consideration must be given to whether the day is designated a 24, 20, 18, 16, 15, or other hour day. The facility should be designed to accommodate all anticipated flows for the application. Attention should be given to the low flow rates (non-zero minimum flow rates) that may include utility, domestic, or auxiliary fuel uses in an industrial process. Coordination with other departments is paramount regarding corrosion control, communications, legal, regulatory, environmental, and property rights issues. Some of these groups secure the necessary permits for construction. For a practical prospective, receiving input from the field will make life a lot easier on the design engineer as every person and every area has equipment preferences. Do not forget the customer’s requirements.
Station Piping and Valving M&R station piping between the station and tie-in points should be designed to minimize pressure drop and gas velocity, usually less than 50 fps. Piping and fittings located between the meter run and pipeline should be welded and coated in accordance with specifications. Piping should be buried wherever possible. The use of screwed piping should be restricted to diameters less than two inches and not buried.
Pipe used for M&R station piping may be sized with Barlow’s Formula: P = [(2 * S * t) / D] * F * E * T P = design pressure, psig S = specified minimum yield strength (SMYS), psi t = nominal wall thickness, inches D = outside diameter (OD), inches F = design (safety) factor E = longitudinal joint factor T = temperature derating factor When E = 1 for seamless or ERW pipe and T = 1 for 20 - 250º F, then the computation reduces to what is known as the Hoop Stress Equation. All meter tubes should have upstream and downstream isolation valves. The isolation valves should be nonlubricated full opening ball valves because, compared to other valve types, they cause less turbulence, pressure drop, noise and grease accumulation in meters and valves. Insulating flange gasket sets should be installed on both ends of the meter tube or tubes. The insulating gasket should be located outside the meter building. If the insulating gasket is installed inside a building, an arc prevention device must be installed. Where multiple meter tubes are used, tube-switching actuators should be installed on the appropriate meter tube isolation valves. For orifice meters, orifice plates should be sized to ensure that the expected range of flow is covered and proper tube switching is performed. To balance the station load, all multiple meter tube installations should require upstream and downstream headers. Headers can be sized per the following equation and rounded to the nearest nominal pipe size diameter: 2
2
2
0.5
Dh = (2(D1 + D2 +… Dn ))
Dh = diameter of header D1, D2, …Dn = diameter of the individual meter run n = number of meter runs
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Note that this equation sizes the header to twice the sum of the cross-sectional areas of the meter runs. In many cases, a factor of 1.5 is sufficient. Gas velocity in the headers should be less than 30 fps. In addition, the assumption is made that the header outlet nozzles, risers, and meter tubes are a ll of equal size. Header configuration (e.g., T, U, and Z arrangements) is a subject unto its own. Headers should be horizontally oriented and equipped with siphon valve assemblies or drains for liquid removal and blowdown. A meter bypass should be employed on single-run delivery meters and should have a piping system utilizing two valves and blow-down capability between the valves or a double block and bleed valve, all with locking mechanisms. Sufficient support should be installed under piping to prevent excess stress and strain on the piping assembly.
Single and multiple meter runs with bi-directional manifold piping should be designed such that gas flow is always in the same direction through the meters. Check valves should be employed for the prevention of un-metered gas. Some meters (e.g., ultrasonic meters) are inherently reverse flow and may not need the bi-directional manifold.
Metering The types of meters (primary measurement devices) that are generally accepted in the industry for custody transfer are: ¾ ¾ ¾
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Orifice Turbine Positive Displacement Rotary Diaphragm Ultrasonic Coriolis
Orifice, turbine, ultrasonic, and coriolis meters are inferential meters. While orifice meters are non-linear, the others are linear meters. The type of meter selected depends upon the application, range of pressures, and range of flow rates. As an example, for an electrical power generating plant, a small turbine meter may be used for the utility load and ultrasonic meters for the combustion turbine generator load. For maximum rangeability, meters should be sized as follows: Q(min.) @ P(max.) Q(max.) @ P(min.) As appreciated by Operations, this procedure will allow for “elbow room” on both ends of the flow rate range. To measure all flow rates within the given range, multiple runs must have overlap between the maximum flow of a run and the minimum flow of the next run. Orifice Meters: Orifice meters should conform to the requirements of AGA Report No.3 “Orifice Metering of Natural Gas and Other Related Hydrocarbon Fluids” (API MPMS 14.3). Generally, orifice meters are used in moderate to high pressure and flow applications. Orifice meters have a rangeability of 3:1 on a plate basis and 30:1 on a tube basis. Uncertainty is ± 0.5% when fabricated and operated within AGA-3 standards. The orifice meter should be sized to operate on a range of beta ratios of 0.2 ≤ β ≤ 0.6, even though the physical tube may be designed for a 0.75 β. The orifice meter should be sized to operate on a range of differential pressures of 10% - 90% of the calibrated span of the secondary instrument. With EGM, differential pressure ranges may go as high as 300 In.WC or more. Consideration must be given to the maximum allowable orifice plate deflection.
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The design engineer must decide if the range of flows can be measured without frequent orifice plate changes. Perhaps multiple meter runs will accomplish the task. Square root error (SRE) must be addressed in regard to gauge line layout. The sharing of custody meter taps or gauge lines should not be permitted. If the orifice fitting is rotated 90 degrees, the tap holes must be on the top center and the opening for the orifice plate holder on the side. A ball valve should be installed on the bottom upstream differential pressure tap for a drain. If the orifice fitting is oriented upright, a ball valve should be installed on the drain tap. A blowdown valve should be installed on the meter tube. All meter tubes should be installed and supported in the horizontal position. Instrument stands, meter tube leveling supports, tie down lugs, or other items should not be welded to the meter tube. Adjustable leveling supports may be required. For inspection, the use of tees with end closures or blind flanges with davits is helpful. A spacer plate should be installed to facilitate tube breakdown. Turbine and Positive Displacement Meters: Turbine meters should conform to the requirements of AGA Report No.7 “Measurement of Gas by Turbine Meters.” Turbine and positive displacement meters should be sized per the manufacturers’ tables. To allow for operating room at both ends of the flow rate range, perhaps the sizing should be limited to 10% - 90% of the published maximum flow rate. Generally, turbine meters are used in moderate to high pressure and high flow applications. Rangeability varies and increases with increasing operating pressure. Accuracy will range from ± 0.25% to ± 1%. Most diaphragm meters are used in low pressure and low flow applications. Rangeability is almost infinite and accuracy near 100% when the meter is in proof. Some diaphragm meters must be de-rated at elevated pressures to prevent over speed. Rotary meters are used in low to high pressure and moderate flow applications. Rangeability varies and increases with increasing operating pressure. Accuracy is ± 1%. A full port critical flow proving tap should be required for turbine and positive displacement meter installations smaller than 4”. The proving tap should be located downstream of the meter and upstream of control valves and flow restrictor devices. Piping pressure losses between the meter and the proving tap should be minimized. Transfer provers are another option. Static pressure should be obtained from a pressure tap on the meter. A flow limiting or restricting device (e.g., orifice or sonic nozzle) should be installed downstream of the turbine or positive displacement meter to prevent over ranging. Automatic oilers are good for meters that have lubrication ports and allow for the installation of such appurtenances. A strainer should be installed upstream of any turbine, rotary, or diaphragm meter to protect the meter’s internals from pipeline debris. When using rotary meters, consideration should be given to the impact of meter failure which blocks gas flow. Some rotary meters have built-in bypasses that open upon meter lock-up. Ultrasonic Meters: Ultrasonic meters should conform to the requirements of AGA Report No.9 “Measurement of Gas by Multipath Ultrasonic Meters.” AGA Report No.9 is vague compared to AGA-3 and the design of an ultrasonic meter tube is generally at the engineer’s discretion. However, a new revision is forthcoming. Generally, ultrasonic meters are used in moderate to high pressure and flow applications with a rangeability of 7:1 and a flow calibrated accuracy of ± 0.25%. Only multi-path ultrasonic meters should be used for custody transfer measurement.
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Ultrasonic meters should be sized based on velocity, where 10 fps ≤ velocity ≤ 70 fps and: Q = A * V * Pm * Tm * F pv
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Q = flow rate A = cross sectional area of pipe ID V = velocity Pm = pressure multiplier Tm = temperature multiplier Fpv = supercompressibility factor Caution should be taken regarding the resonant frequencies of thermowells at high gas velocities. Ultrasonic meters should be dry calibrated by the manufacturer. At an approved third-party test facility, they should be flow calibrated, including a zero flow verification test. Flow calibration should be performed at the expected operating pressures and temperatures. The flow points tested should be 2.5, 5, 10, 25, 50, 70, and 100 % of range. Each flow point should be repeated a minimum of six times. While not specifically indicated in AGA-9, the following are good ultrasonic meter tube specifications. The ultrasonic meter should be installed with a flow conditioner. The flow conditioner should be installed with 10 ND of straight pipe upstream, including the flow conditioner. Downstream of the flow conditioner should be 10 ND before the meter. Downstream of the meter should be 5 ND to the first thermowell. The average internal roughness of the pipe should be less than a 300 μinch finish. Pipe out of roundness should not exceed 0.5% in the piping sections immediately upstream and downstream of the meter. The entire tube should be honed after all welding. Insulating gasket sets should be installed on both ends of the meter tube and the meter tube should be electrically grounded. Changes in internal diameters and protrusions should be avoided at the ultrasonic meter inlet because they create local disturbances to the velocity profile. The ultrasonic meter bore, flanges, and adjacent upstream tube should all have the same inside diameter to within 0.5% of each other and be carefully aligned to minimize flow disturbances, especially at the upstream flange. For alignment, the flange pairs on the upstream and downstream side of the meter should be female-faced companion flanged or dowel pinned. No part of the upstream gasket or flange face edge should protrude into the pipe internal diameter. Coriolis Meters: Coriolis meters should conform to the requirements of AGA Report No.11 “Measurement of Gas by Coriolis Meters.” The use of and applications for this meter are still evolving. Control Generally, regulators are direct spring-operated or pilot-operated, while control valves require external controllers. Regulators perform pressure regulation, while control valves perform either pressure regulation or flow control. Control valves are for throttling or positional applications, not open-close. The type of regulator or control valve selected depends upon the application, range of pressures, and range of flow rates. Linear motion control valves are globe body and gate valves. Rotary motion control valves are ball, plug, and butterfly or disc valves. Regulators are usually of the diaphragm or boot style. Control modes are pressure, backpressure, differential (pneumatic), and flow (electronic) with various combinations of override or under ride. Generally, the attributes of diaphragm or boot style regulators are moderate pressure drop, moderate noise, and moderate capacity. The attributes of ball control valves are low pressure drop, high noise, and high capacity. The attributes of globe body control valves are high pressure drop, moderate to high noise, and moderate capacity.
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Appurtenances include pilots, valve actuators, positioners, PID controllers, filter dryers, and catalytic heaters. Valve actuators may be spring and diaphragm or piston type. The use of low bleed or no bleed appurtenances is always recommended. Sufficient piping is required between the meter run and regulators or control valves to minimize the acoustic error effects on metering equipment. Regulators and control valves should be sized to handle the normal expected range of flows and normal expected range of pressures, at 10% - 90% of valve capacity. If this condition cannot be met, additional parallel control valves can be added using split range control. This arrangement can also improve low flow performance. When designing pressure regulation, each cut should not exceed 250 psig. The gas temperature will cool approximately 7º F per 100 psig drop in pressure (or 1º F per 15 psig drop in pressure). If necessary, adequate heating facilities should be installed to prevent regulator freezing. Depending upon customer requirements, use of fail-open or fail-closed regulators and control valves should be determined on a case-by-case basis. If noise levels exceed local, state, or federal requirements, adequate measures must be taken to attenuate such noise to the acceptable level. Multiple pressure cuts, heavy wall pipe, pipe insulation, diffusers, quiet trim valves, buildings with insulation, or buried control valves may address this matter. Gas velocities in regulator or control valve run piping should be limited to a maximum of 100 fps. Upstream and downstream piping and valves should be of the same diameter or larger than the control valves or regulators. Isolation full-opening ball valves should be installed upstream and downstream of all regulators and control valves. To reduce turbulence and for sufficient pressure recovery, at least 5 ND of straight pipe upstream and 7 ND downstream should be installed on each run. A bleed valve should be installed between the control valve and any isolation valve, upstream or downstream. A bleed valve or instrument tap should be installed between multi-cut regulators. Blow-offs should be installed within any section, which may be isolated. A bypass should be installed around all regulators and control valves unless two or more are installed in parallel. Locking devices should be provided on all bypass valves. Taps should be provided on the bypass for pressure sensing and instrument supply/power gas. Each pressure control device should have independent sensing lines. Pressure and flow control systems should have redundant filter dryers for the instrument supply/power gas. If necessary, catalytic heaters may be used on the instrument gas. Overpressure Protection Continuity in ratings and MAOP in material is required. Any piping design causing discontinuity of MAOP will require overpressure protection. Overpressure protection design may accommodate for one failure of the pressure regulator or control valve. Overpressure protection devices must be independent of primary control devices.
Relief valves, fail-closed spring-return actuated valves, and monitor regulators are some of the more common devices recommended for overpressure protection. The type of overpressure protection device selected depends upon the application. Relief valves are direct spring-operated or pilot and piston operated. Monitor regulators have the same attributes and appurtenances as regulators and control valves.
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Monitor sensing lines and supply/power gas must be independent and dedicated to the overpressure protection purpose. Monitor regulators should be fail-closed. Relief valves should be sized for a setpoint capacity that is greater than the capacity of the pressure regulator when the regulator is wide open and with a P1 of MAOP. Relief valves are usually recommended for use on diaphragm meters. Other Considerations There are a multitude of other considerations when designing a high pressure M&R station. Some of the more notable items include site location, buildings and structures, secondary measurement devices (transmitters), and tertiary measurement devices (GFCs). Also, other considerations include pulsation dampening, dehydration, and in-ground versus skid designs. The electrical design should address matters such as NEC hazardous area location classifications, conduit and wire and cable, grounding and ground grid, isolation, signal or data interfacing, and adherence to API MPMS 21.1 “Flow Measurement Using Electronic Metering Systems, Electronic Gas Measurement” for EGM. The gas quality design should include equipment and devices such as spot sampling, composite or continuous sampling, chromatographs, gas analyzers (e.g., H2O, H2S, S, CO2, O2, etc.), and other analytical instrumentation. All of these considerations are subjects unto themselves and are beyond the scope of this paper. Conclusion The more time spent gathering data in the conceptive, investigative, and preliminary stages, the better the design and the better will be all other aspects of the M&R station. Motto: “Measurement Services (Operations) sets the standards by which Project Management (Engineering) designs and constructs measurement facilities.”
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References Meter Station Design Specifications and Measurement Practices, Measurement Facility Design Specifications, El Paso Corp., 2000 thru 2006 “Basics of High Pressure Measuring and Regulating Station Design,” S. Craig Blake, TXU Pipeline Services, th Proceedings of the 75 International School of Hydrocarbon Measurement, 2000 “Basics of High Pressure Measuring and Regulating Station Design,” Maureen E. Kolkmeier, Texas Utilities Corp., th Proceedings of the 75 International School of Hydrocarbon Measurement, 1998 “Design and Installation of a Complete Measurement and Control Facility,” T. G. Quine, Northstar Industries, th Proceedings of the 35 American School of Gas Measurement Technology, 2000 Natural Gas Measurement & Control, A Guide for Operators & Engineers, Lohit Datta-Barua Ph.D., McGraw-Hill, 1991
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