General Requirements for Flow Instruments in Process Industry

This article is about General Requirements for Flow Instruments in Process Industry. Flow Instruments. Differential Pressure Transmitters. Orifice Plates. Turbine meters, Positive Displacement Meters, venturi tubes, flow switches, pitot tube, vortex flow meter, flow gauges, flow indicators, ultra sonic flow meter, corolios flow meter, magnetic flow meter and rota meter variable area meter.

General Requirements for Flow Instruments in Process Industry

Flow Instruments

See PIP PCCFL001 for flow measurement criteria.
Flowmeters selection shall be based on the type of fluid being measured, and accuracy, rangeability and turndown ratio requirements.
Flowmeters shall be smart, 2-wire type and loop powered with 4-20 ma output where possible.
Flowmeters material of construction shall be compatible with the fluid being measured and applicable piping specification requirements.
All in-line flowmeters shall be flanged, unless otherwise specified.
Rotameters shall be used for local flow indication in clean fluids and pipe sizes 2 inches and smaller.
Vortex meters shall be used for flow measurement in clean fluids for up to 8 inches of meter size.
Orifice plates, Annubars, Flow Nozzles, or Venturi tubes shall be used for flow measurement in clean fluids with large pipes depending upon application.
Ultrasonic flowmeters shall be used for flow measurement where significant quantities of suspended solids or entrained gas is present.
Magnetic flowmeters shall be used for flow measurement in highly corrosive fluids with adequate conductivity.
Coriolis mass flowmeters shall be used for flow measurement in custody transfer Applications.
The overall accuracy for flowmeters in accounting and material balance applications shall not exceed ±1 percent of rate.
Upstream and downstream meter run lengths shall be in accordance with manufacturer’s recommendation except where specifically noted otherwise per referenced industry
standards.
Calculation Methods
a. Calculations for sizing concentric, square-edge orifice meters shall conform to ANSI2530/API-14.3/AGA-3/GPA-8185, unless otherwise specified by Owner.b. Nozzle, venturi, and other differential pressure device calculations shall confirm to Flow Measurement Engineering Handbook.
c. For any other proprietary meter, the manufacturer’s sizing and calculation methods shall be used.
d. Sizing calculations shall use base condition of 15ºC and 1.013 bara.

Differential Pressure Transmitters

Differential pressure transmitters shall have a minimum static pressure rating of 140 barg. The minimum static pressure rating shall not be less than 150 percent of operating pressure.
Differential pressure transmitters shall be designed for differential pressure overrange, in either direction, equal to the body pressure rating without affecting the calibration of the instrument. No instrument damage shall result in full-line pressure if connected to the low pressure side of the transmitter.
Pressure tap-sensing lines for differential pressure transmitters on liquid, steam, and heat transfer media shall be installed on the horizontal side or 45 degrees down from horizontal with impulse lines sloped down to the transmitter.
Pressure tap sensing lines for differential pressure transmitters on Gas and Vapor service shall be installed on the vertical from the tap, horizontal or 45 degrees from vertical with impulse lines sloped up to the transmitter.
Differential pressure transmitters shall be furnished with 3-valve manifolds.
In case of liquid flow measurement the piping configuration shall ensure that meter run is always full of liquid under all working conditions.

Orifice Plates

Concentric paddle type orifice plates shall be used. The following minimum information shall be stamped on the upstream side of the handle:
a. Orifice plate tag number
b. Material type
c. Vent or drain hole size
d. Inlet flow side
e. Bore
f. Flange size in inches and ASME pressure rating
g. Inside diameter of pipe
When an orifice meter is the selected flow element and the orifice plate sizing dictates a meter run size less than a nominal 2 inch, an integral orifice shall be used.
Orifice plate material shall be 316 stainless steel minimum unless otherwise required by process service. Where the nature of the fluid is such that it requires another material, it shall be suitable for the service.
Orifice plates shall be fabricated to meet ANSI 2530/API 14.3 Part 2, except plate thickness shall be as follows:
Line Size Thickness
1 inch to 2 inch 2 mm
2-1/2 inch to 6 inch 3 mm
8 inch to 12 inch 5 mm
14 inch to 24 inch 6 mm
Drain holes shall be provided in steam and gas flow service when condensation possibilities exists. Vent holes shall be provided for liquid service where a potential for gas entrapment exists.
Orifice diameter ratio (β) shall be between 0.2 and 0.6 for liquids and between 0.2 and 0.7 for gases and steam. Beta ratio of 0.75 shall be acceptable for orifice plates in 24 inches and larger pipelines.
Orifice plates for 2-way flow shall not be beveled, but shall be counter bored.
Eccentric or segmental orifice plates shall be used to measure the flow of liquids, which contain solids.
Quadrant edged plates or segmental wedges shall be used where viscosity conditions dictate. The Reynolds Number shall be greater than 500 at minimum flow and less than 100,000 at maximum design flow. Quadrant edged plates shall be designed in accordance with DIN 1952.
Acceptable orifice taps shall be as follows:
Nominal Pipe Size Types of Taps
1½ in and smaller Corner taps
2 in and larger Flange taps
Flange type and material shall conform to piping specifications. Orifice flanges shall be rated at a minimum of ASME Class 300 and conform to ASME B16.36 and ANSI 2530/API 14.3/AGA-3/GPA-8185.
Orifice flange tap connections shall be ½ inch when used in ASME Class 600 service and below unless otherwise required by piping specifications.
Orifice flange tap connections shall be ¾ inch when used in ASME Class 900 service unless otherwise required by piping specifications.
Connection type (threaded or socket weld) shall be as per piping specifications.
Other taps, when used, shall be per piping specifications.
Piping design for orifice runs shall confirm to the following:
a. Orifice meters shall be installed in horizontal runs to minimize measurement inaccuracies. Vertical orifice runs shall be used only with Owner’s approval (upward flow for liquids and downward flow for gases).
b. 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.

Process Piping Connection:

a. Primary orifice taps shall have a separate isolation (root) valve in addition to any manifolds.
b. Piping from taps to and through first isolation (root valve) shall confirm to applicable piping specifications.
c. Unused orifice taps shall be plugged per piping specifications.

For accounting and material balance applications, straight run piping lengths before and after an orifice plate flow element (meter tube) shall be provided in accordance with ISO 5167. For all other applications, straight run piping lengths shall be in accordance with ANSI 2530/API-14.3/AGA-3/GPA-8185 unless additional lengths are specifically required for a particular process application.
For accounting and material balance applications, a pre-fabricated meter tube (upstream and downstream runs) shall be provided by an Owner approved fabricator in accordance with ANSI 2530/API 14.3/AGA-3/GPA-8185. Pre-fabricated 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. Meter tubes for all other applications shall be fabricated from standard mill run pipe.
The overall accuracy for orifice meters in accounting and material balance applications shall not exceed ±1 percent of rate. The overall accuracy for all other meters shall not exceed ±5 percent of rate.
Differential pressure ranges shall be selected in accordance with the following:
a. Orifices for liquids are usually sized for full-scale reading at a differential pressure of 2500 mm WC. This allows an increase or decrease in the meter range for different flow rates without changing the plate. 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.
b. For differential pressure instruments with a continuously adjustable range, the range shall be selected from the table shown below; however, 250 mbar shall be considered. c. When a fixed range differential pressure type instrument is selected, the range shall be similar to the values listed above, and shall also conform to the manufacturer’s standard.
d. The 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.

Rotameters / Variable Area Meters (VA)

Rotameters may be used in the following services:
a. Chemicals with small volumetric flows
b. Chlorinators
c. Instrument purging
d. Small rates to analyzers
e. Small rates on air streams
Sizing shall be done in accordance with Vendor’s literature; however, proper compensations shall be applied to correct for liquids other than water or gases other than air. Normal flow
should fall in the 50 to 60 percent ranges.
In general, rotameters should be used for line sizes smaller than 2 inches in cases of low flow rates where integral orifices are not practical and the process is clean. Rotameters typically offer assurance in the range of 2 percent, have a flow range of 5:1 and are not suitable for pressures over 400 bar.
Specifications for process rotameters shall be as follows:
a. When the fluid is expected to freeze or congeal at ambient temperature, steam tracers or steam jackets shall be provided around the periphery of the rotameter and, when necessary, steam jackets shall be fitted around the meter body based on the properties and conditions of the fluids to be measured.

b. When a metal tube type rotameter is utilized, the material for the float and tapered tube shall be stainless steel 304, unless the service requires a different material.
c. Rotameters, which measure pulsating fluids, shall be equipped with pulsation dampeners.
d. Variable area meters shall be installed in a manner, which permits easy meter or float removal for cleaning or repair purposes. Type and rating of end connections shall be in accordance with the piping specifications.

Variable area meters (rotameters) shall be armored for all hazardous services and shall have end connections of the type and rating in accordance with the piping material specifications. Glass tube meters shall be used only in air, inert gas and water services at temperatures of 60 deg C (140ºF) or less and pressures 6.5 barg (100 psig) or less. The following uses are typical:
a. Two inches and larger rotameters may be used for conditions where there is an advantage over using an orifice meter or other type of flowmeter.
b. For process flows, scales shall be graduated in percent units. Meter factor for maximum flow at 100 percent shall be engraved on scale. c. Scales for purge meters shall read directly in Standard Cubic Meter per Second (Std m³/s) of air for gas service or kilogram per hour (kg/h) of water for liquid service.
d. For purge meter applications, a check valve shall be installed on the outlet of the meter to prevent back flow. Needle valve shall be installed on the inlet side for manual flow control.
e. The meter shall be selected so that the normal flow falls at 50 to 60 percent of the maximum range.

Positive Displacement Type and Turbine Type Flowmeters

Positive displacement type meters should be used when (1 percent – 5 percent) accuracy and moderate rangeability (5:1) is required. The positive displacement meter is suitable for high viscosity fluids and is frequently used for custody transfer applications. The meter should not be used for dirty, non-lubricating or abrasive liquids.
Turbine Meter – Turbine meters should be used for high accuracy (±0.25 percent) and wide rangeability (10:1) of clean fluids. The turbine meter is not suitable for high viscosity liquids.
Specifications for positive displacement and turbine flowmeters shall be as follows:
a. Suitable gas eliminators shall be installed where vapor is expected to affect the meter accuracy, high accuracy is required or vapor can mechanically damage the instrument, such as for custody transfer and to meet customs regulations, or where their installation is required by local laws and regulations. Strainer shall be provided with differential pressure indicator.
b. Temperature compensation should be considered when necessary.
c. To ensure accurate measurement by the turbine meter, the minimum straight run shall be fifteen times diameter upstream and five times diameter downstream of the meter. Straightening vanes should be generally provided to stabilize inlet flow.
d. Mating flanges shall be of the same internal bore as the turbine meter. Gaskets shall not obstruct free flow of fluid in line.
e. Pipe supports shall be provided for easy removal of meter. It shall be possible to isolate meter section if necessary. Proper drains/vents shall be provided.

Turbine meters

Turbine meters shall be installed so they are always in single phase flow, liquid full in liquid applications, and liquid free in gas applications.
a. The installation setup shall avoid creating a pressure drop condition for the fluid to flash in meter. Flashing can damage the meter and severely impact measurement.
b. Turbine meters shall be marked to indicate proper flow direction.
c. In-line type turbine meters 1.0 inches and above shall be flanged.
d. A strainer shall be installed upstream of turbine meter system. Dual strainers shall be provided if the process flow cannot be interrupted for maintenance. Strainers shall be installed far enough upstream to prevent the distortion of flow profile at the turbine meter.
e. Liquid turbine meters shall be installed in accordance with API MPMS 5.3.
f. In-line gas turbine meters shall have rotor module removable from top of meter through a special flange or through bore of meter for meters larger than 3 inches.

Positive Displacement Meters

a. Inlet and/or outlet ports shall be plainly marked to indicate proper flow direction.
b. Liquid positive displacement meters shall be installed in accordance with requirements of API MPMS 5.2, for high accuracy applications. As a minimum manufacturer’s recommended installation shall be followed.
c. The meter shall be installed so that the fluid passing through the meter is in a single phase and the meter is liquid full.
d. A strainer shall be installed upstream of the meter.
e. Meters in line sizes 1-1/2 inches and above shall be flanged body style.

Venturi Tubes

Venturi tube shall be classical venturi type.
Only industrial quality and design low-loss venturi-type flow elements shall be considered.
The size of pressure taps shall be between 5/32 inch and 25/64 inch, inclusive, but never greater than 10 percent of pipe diameter or 13 percent of the throat diameter, respectively.
Sizing shall be done in accordance with Seller’s literature.
The body configuration shall be so constructed to minimize pressure drop through the meter.
Body materials shall conform to the project piping specification as specified on datasheets.
Venturi tube shall be manufactured and used in accordance with ISO 5167.
The Beta Ratio (β) of venturi tube shall be 0.30 ≤β≤0.75 and Reynolds number shall be ≥ 200,000.
Direction of flow shall be clearly marked on the venturi tube.
Piezometer ring tapings shall not be used for steam on vapor services. Provision shall be made for venting entrapped gases on draining condensated liquids.

Flow Switches

Flow switches shall be the flapper types with a microswitch, the magnet moving type with a reed relay, or any other similarly suitable type. Flow switches shall not be used without prior approval
of OWNER.

Pitot Tubes

Pitot tubes shall only be used for the following services:
Clean services in large line sizes, refractory lined piped or ducts.
Low pressure or low pressure drops available.
Restriction Orifices Restriction orifices shall normally be used for the following:
Clean services and minimum flow. To create pressure drop in the system.

Vortex Flow Meter

Flow direction shall be permanently marked on the meter body.
Vortex flow meter internal inside diameter shall be matched as closely as possible to the adjoining process pipe inside diameter. Proper gasket sizing shall be followed to avoid protrusion into line.
Manufacturers upstream downstream requirements and installation shall be followed.

Flow Gauges and Flow Indicators

Local flow indicators shall be pneumatic differential pressure transmitters with local receiver instrument mounted where it is visible. When an instrument air supply is not readily available in the area, bellows type differential pressure gauges with a fixed range may be used.
Flow gauges with glass windows shall be used in only utility and non-critical services, such as fluid purge or lube oil system.

Ultrasonic Flow Meter

Ultrasonic flow meters shall be of Doppler type with clamp on design.
Transducer location shall be carefully selected to avoid turbulence in the pipe.
Manufacturer installation and upstream downstream requirements shall be followed.

Coriolis Mass Flow Meter

Meter shall be sized so the maximum full scale flow rate is in the upper one-third of the meter range.
Manufacturer installation and upstream downstream requirements shall be followed.

Magnetic Flow Meters

Magnetic flow meters shall be installed so that the meter is always liquid full.
Transmitter shall be integrally mounted.
Flow meter liner and electrode material shall be compatible with process fluid.
Manufacturer installation and upstream downstream requirements shall be followed.