This article is about Flow elements, it’s types and Minimum Requirements For Design, Materials, Fabrication and Sizing of flow elements in plants, oil and gas projects and refineries.
1. SCOPE
2. REFERENCES
3. DEFINITIONS
4. FLOW ELEMENTS
4.1 General
4.2 Orifice Plate
4.3 Averaging Pitot Tube
4.4 Venturi Tube
4.5 Flow Nozzle
5. INSPECTION AND TESTING
6. REVISION HISTORY
What are Flow Elements?
Flow elements are devices or components used in fluid flow measurement systems to create a pressure differential or change in flow velocity, which can be related to the flowrate of the fluid. These elements are strategically placed within the flow path to obtain accurate measurements and provide valuable data about the fluid’s rate of flow.
Flow elements are essential in various industries, including oil and gas, water treatment, chemical processing, and HVAC systems, to monitor and control fluid flows effectively. Common types of flow elements include orifice plates, venturi tubes, flow nozzles, pitot tubes, and flow meters.
1. Scope
This standard establishes the minimum requirements for the design, materials, fabrication, and sizing of flow elements.
2. Internal Codes used for Flow Elements
Codes References is made in this standard to the following documents. The latest issues, amendments and supplements to these documents shall apply unless otherwise indicated.
- Engineering Standards Articles
- Process Industry Practices (PIP)
- PCEFL001 Flow Measurement Guidelines
- American National Standards Institute (ANSI)
- ANSI 2530/API 14.3/ AGA 3/ GPA 8185 Manual Gas Fluids Measurement Standards Chapter 14 – Natural Gas Fluids Measurement – Concentric Square Edged Orifice Meters
- American Society of Mechanical Engineers (ASME)
- B16.36 Orifice Flanges
- MFC-3M Measurement of Fluid Flow in Pipes using Orifice, Nozzle and Venturi
- International Organization for Standardization (ISO)
- 5024 Petroleum Liquids and Liquefied Petroleum Gases – Measurement – Standard Reference Conditions
- ISO 5167 Measurement of Fluid by means of Pressure Differential Devices
- 5168 Measurement of Fluid Flow – Procedures for the Evaluation of Uncertainties
- Other References
- Miller RW Flow Measurement Engineering Handbook
3. Flow Elements Technical Definitions
Beta Ratio: The ratio of the diameter of the orifice bore to the pipe internal diameter, β = d/D.
Corner Taps: The differential pressure signal location in an orifice flanges defined by the corner formed between the orifice plate and the internal diameter of the flange.
Flange Taps: The differential pressure signal location in an orifice flange at 1 inch upstream and 1 inch downstream from the plate face including the gasket thickness.
Orifice Plate: A disc or plate like member, with a sharp-edged hole in it, used in a pipe to measure flow or to reduce static pressure.
Orifice Run: The differential pressure producing arrangement consisting of selected pipe, orifice flanges and orifice plate.
Primary Flow Element: Any device placed in a flow line to produce a signal for flow rate measurement.
Pulsating Flow: Irregular or repeating variations in fluid flow, often due to pressure variations in reciprocating pumps or compressors in the system.
Reynolds Number: A dimensionless criterion of the nature of flow in pipes. It is proportional to the ratio of dynamic forces to viscous forces: the product of diameter, velocity and density, divided by absolute viscosity.
4. Flow Elements
- General
- Refer to SES-R01-E01 for general design guidelines and specification.
- Orifice plates, pitot tubes, venturi tubes, or flow nozzles shall be used for flow measurement in clean fluids with large pipes depending upon application.
- Restriction orifice shall normally be used to create pressure drop, i.e. minimize the flow, in the system for clean services.
- Nozzle, venturi, and other differential pressure device calculations shall be in accordance with Miller’s Flow Measurement Engineering Handbook.
- Flow element sizing calculation shall use base conditions of 15 deg C and 101.325 kPa. Flow values to be used shall be integers.
- Alternative to the differential producing flow measurement shall be considered for extreme conditions of high viscosity, low static pressure or poor physical condition of the fluid, e.g. fluids containing solids, dirt, heavy slurries and sewage.
- The flow element differential pressure range shall be either 1250 or 2500 mm of WC. A range of 2500 mm of WC is preferred. Other ranges can be considered to meet the application requirements with SABIC approval, e.g. low pressure gas or to meet beta ratio constraints.
- Direction of flow shall be clearly marked on the flow element body.
- If project requires, meter runs and calibrated flow elements should be used, for high accuracy applications.
- Refer to SES-R11-C01 for flow element installation requirements.
- Company approval shall be required for any other proprietary meter not specified in this document. The manufacturer’s sizing and calculation methods shall be used for proprietary meter.
4.2 Orifice Plate
Orifice plate is famous flow elements. Standard orifice meters using sharp-edged, concentric paddle type orifice plates shall be used for all process flow measurements unless process conditions require otherwise. If unrecoverable pressure loss or other constraints preclude their use, another measuring device shall be selected. The recommended minimum Reynolds number for a given application may range from 10,000 to 25,000.
4.2.2 Calculation for sizing of concentric, square edged orifice plate shall be in accordance with
the latest ISO, i.e. ISO 5167, ISO 5168, and ISO 5024 or ANSI, i.e. ANSI 2530/API 4.3/AGA 3 / GPA 8185, standards.
4.2.3 The following minimum information shall be stamped on the upstream side of the handle of
all orifice plates:
a. Orifice plate tag number
b. Inlet flow side (“INLET” marked on upstream side of the paddle)
c. Orifice bore
d. Material of construction
e. Flange size in inches and ASME pressure rating
f. Inside diameter of pipe
g. Vent or drain hole size
4.2.4 Orifice plate shall be fabricated to follow ANSI 2530/ API 14.3/ AGA 3/ GPA 8185 Part 2 requirements.
4.2.5 The orifice plate beta ratio shall be between 0.20 and 0.60 for liquids and between 0.20 and
0.70 for gases and steam. Beta ratio of 0.75 is acceptable for orifice plates in 24 inches and
larger pipelines. Refer to PIP PCEFL001 for recommended beta ratio ranges for
measurement performance.
4.2.6 Orifices for liquids are usually sized for full scale reading at a differential pressure of 2500
mm of WC. For gas or steam flow, a general rule is that the meter range, in inches of water,
should not exceed the flowing pressure, expressed in pounds per square inch absolute.
4.2.7 The differential pressure range for measurement of low pressure gas flow and compressive
fluids shall be based on the process pressure balance. For such application, the range shall
not exceed 5 percent of the normal static absolute pressure.
4.2.8 Orifice plate material shall be 316 stainless steel, as a minimum, unless process conditions
require different material.
4.2.9 Orifice flanges type and material shall conform to piping specifications. Orifice flanges shall
be rated at a minimum of ASME Class 300 rating and conform to ASME B16.36 and ANSI
2530/ API 14.3/ AGA 3/ GPA 8185.
4.2.10 Drain or vent holes in orifice plate shall not be provided unless specified otherwise.
4.2.11 Orifice flange tap connections shall be 1/2 inch when used in ASME class 600 service or
below, unless otherwise required by piping specifications.
4.2.12 Orifice flange tap connections shall be 3/4 inch when used in ASME class 900 service,
unless otherwise required by piping specifications.
4.2.13 Orifice tap connection type, i.e. threaded, threaded and seal welded or socket weld, shall be as per piping specifications.
4.2.14 Other taps, when used, shall be per piping specifications.
4.2.15 Acceptable orifice taps shall be as follows:
4.2.16 Piping design for orifice meter runs shall conform to the following:
a. For “flange tap” designated orifice meters, the taps shall be located at 1 inch upstream
and 1 inch downstream from the orifice plate face including gasket thickness.
b. For “pipe tap” designated orifice meters, the taps shall be located 2.5 times internal pipe
diameters upstream and 8 times internal pipe diameters downstream of the orifice plate.
4.2.17 Orifice plate for 2-way flow shall not be beveled, but shall be counter bored.
4.2.18 Eccentric or segmental orifice plates should be used to measure the flow of fluids, which
contain suspended solids.
4.2.19 Quadrant edge or conic type orifice plates shall be considered if the maximum measured
flow pipe Reynolds number is below 10,000 or if the anticipated process viscosity changes
would cause significant errors with standard square edge orifices.
4.2.20 For pipe size less than 2 inch, an integral orifice assembly shall be used.
4.2.21 Integral orifices assembly shall be installed in accordance with manufacturer’s
recommendations. Manufacturer’s designed and fabricated integral orifice assembly should
be considered in these cases.
4.2.22 For custody transfer applications, a prefabricated orifice meter tube, upstream and
downstream runs, shall be provided in accordance with ANSI 2530/ API 14.3/ AGA 3/ GPA
8185. For material balance applications this requirement is also recommended.
4.2.23 Prefabricated orifice meter tubes shall be provided with a weld neck flange on the
downstream end to facilitate inspection of the meter tube internal surface. The exception to
this rule is where a flange is already conveniently located for this purpose. Orifice meter
tubes for all other applications shall be fabricated from standard mill run pipe.
4.2.24 Where maximum accuracy is important, it is recommended that the maximum to minimum
flow ratio should not exceed 3:1.
4.2.25 For installations requiring frequent orifice plate inspection or replacement, e.g. custody
metering, dual chamber orifice fitting shall be used to allow plate removal without
depressurizing the process.
4.2.26 In case minimum straight run pipe length requirements are not in accordance with SESR11-C01, conditioning orifice plate or equivalent technology can be considered with Company approval.
4.3 Averaging Pitot Tube
- Averaging pitot tubes which is one of flow elements, have better velocity profile and are preferred over the conventional Pitot tube due to higher accuracy. Other types of pitot tubes shall require Company approval.
- Averaging Pitot tube is a low cost alternative to the orifice plate where low head loss is required and lower accuracy is acceptable.
- Applications of averaging Pitot tube is restricted somewhat by fluid characteristics and operating conditions. However it is of value under certain conditions where other measurement devices are impractical or inoperable due to low-operating pressure, low-fluid velocity, excessive pipe size, and limited room.
- Averaging Pitot tube can be used for high velocity applications without developing excessive permanent pressure loss.
- Averaging Pitot tube may vibrate at high fluid velocities, resulting in elastic fatigue. Maximum expected fluid velocity should not exceed manufacturer’s design limit.
- Averaging Pitot tubes shall be provided with a wake frequency calculation as standard.
- Averaging Pitot tubes are subjected to plugging and their application should be limited to clean gas and liquids.
- Retractable type averaging pitot tubes shall have suitably designed restraint system to prevent the blowout of the tube from the live process line. The isolation valve shall have proper inside diameter to allow passage of the element. The manufacturer’s recommendations shall be followed and shall meet applicable piping specifications.
- Element type, design, size, manufacturing tolerances, size of impact port, fluid characteristics, and capabilities of secondary instruments are some of the selection influencing factors which should be considered when fluid velocities are low.
- Installations shall be leak tested, particularly on gas applications where differential pressures are low.
- If the element is provided with a flow rate calibration curve, the element shall be positioned according to the manufacturer’s specifications. Calibration curves may be based upon positioning the element either at the center of the pipe or at the theoretical average velocity.
- Averaging Pitot tube elements shall be permanently marked to indicate proper installation position and flow direction.
- Averaging Pitot tube material shall be 316 stainless steel, as a minimum, unless process conditions require different material
- If failed element could cause damage to the downstream rotating equipment, consideration shall be given to the mechanical design and end supports or alternative type of flow element.
4.4 Venturi Tube
- Venture tube is also type of flow elements. Venturi tube shall be used in special flow metering applications where high cost is justified by the low permanent pressure loss, higher accuracy and ability to measure fluids containing suspended solids.
- Venturi tube may be used in low viscosity, non-abrasive fluids at high flow rates where only a small pressure drop or permanent head loss is allowed.
- Venturi tube design shall be classical, industrial grade and low pressure loss type.
- Classical venturi tubes shall be fabricated according to ASME MFC-3M specifications.
- Venturi meter material shall be compatible with the process conditions.
- Flow calculation for the venturi shall be supplied by the manufacturer.
- Beta ratio of the venturi tube shall be between 0.3 and 0.75.
4.5 Flow Nozzle
- Flow nozzle is another types of flow elements. Flow nozzles can be used in low viscosity, non abrasive fluids at high flow rates in which.
- a. Lower head loss than orifice plate is desired.
- b. A contoured element is needed for long service life where a sharp edge would wear due to erosive fluids
- The design commonly used is the long-radius ASME flow nozzle.
- Flow nozzles shall be fabricated of 316 stainless steel, as a minimum, unless process conditions require different material. Fluid contact surface finishes in the order of 6-10 microns result in more predictable nozzle coefficients and high repeatability of measurement. The outlet or the discharge side is normally beveled and is one of more critical point of manufacture.
- In high pressure applications above 60 kg/cm2, flow nozzle should be welded directly into the pipe.
- Tap connections shall be installed at 1 ID upstream and 0.5 ID downstream in accordance with ASME-MFC-3M.
- Flow nozzles should be used for measurement of boiler feed water, superheated steam flow and other high velocity fluid flows where erosion is foreseen. Since the exact contour is not particularly critical, the flow nozzles are expected to retain precise calibration for a long time under adverse conditions.
5. Inspection and Testing
Vendor shall submit following tests and reports of flow elements for purchaser’s review, approval and records.
a. Sizing calculation
b. Wake frequency calculation for Pitot tubes
c. Dimensional drawings
d. Hydrostatic test report
e. Material certificate