DESIGN CONSIDERATIONS

Environmental and Classified Area Requirements.

• Pneumatic field-mounted instruments shall generally be provided with NEMA Type 3 "weatherproof" enclosures.
• Electrical and electronic field-mounted instruments shall be, as a minimum, provided with NEMA Type 4 "watertight" enclosures for envirormental protection. For non-intrinsically safe systems installed in hazardous area, enclosures shall also meet the requirements imposed by the electrical area classification.

FLOW INSTRUMENTS

Selection.

• Magnetic Flowmeters may be employed where it is desirable to measure the flowrate of conducting fluids or slurries over ranges of 20:1 with negligible permanent pressure losses.
• Turbine flowmeters may be  employed were accuracy requirements cannot be  met by ny other method, and where sufficiently high permanent pressure loss is available.
• Accuracies with turbine meters are generally ± 0.25% of the rate of flow.

Thin Plate Orifices.

• A differential range of 0-2500 mm H2O shall be specified. If Beta ratio limits of 0.25 to 0.75 can not be met with this range, 0-500 mm H2O to 0-5000 mm H2O ranges will be specified.

Venturi Tubes And Flow Tubes.

• Materials of construction shall be chosen in accordance with material specifications for the lines in which the tubes will be installed.
• Where possible, a differential range of 0-250 mm H2O shall be specified, however, ranges of 0-50 mm H2O to 0-500 mm H2O may be specified where necessary.

D/P Flow Transmitters.

• Body pressure rating shall be 105 bar (1500 psig) minimum and measuring capsules shall be able to withstand pressure up to the body rating in either direction without damaging the instrument or shifting the zero more than 0.5K of the span.
• Process connections shall be ½ " National Pipe Thread (NPT) on standard 2 1/8 " centres.
• Accuracy of measurement shall be ± 0,20 % of differential span, or better.
• For application in measuring very low flow rates normally handled in pipe sizes smaller than l inch, consideration may be given to the specification of D/P flow transmitters with integral orifices.

Magnetic Flowmeter Systems.

• These systems shall consist of an in-line mounting primary device with an integral mounted signal converter. Where high fluid temperatures prohibit integral mounting, separately mounted converters shall be used.
• System accuracy shall be ±1.0% of flow rate or better.
• The primary device generally shall have a stainless steel body with an insulating liner chosen to meet the service conditions.
• End connections shall be determined by piping specifications.
• The primary device shall be mountable in any orientation to meet piping requirements.
• It shall be piped in such a manner that is always full of liquid.
• In sizes up to 24 inches it shall not require additional support.
• Primaries shall be able to withstand accidental submergence in up to 10 m of, water for two hours without adverse effects.
• Straight runs for pipe before and after the mag flowmeter are not necessary.
• Damages to meter pipe and flange liner shall be avoided installing pipe spool pieces on each end of the  magnetic flow meter.
• Proper electrical grounding and jumper arrangement shall be provided.

Turbine Flowmeter Systems.

• These systems shall consist of an in-line mounting primary device with an integral mounted signal converter. Where high fluid temperatures prohibit integral mounting, separately mounted converters shall be used.
• System accuracy shall be ± 0.5% of flow rate for normal applications; however, special circumstances may require accuracies of up to ± 0.1% of flow rate.
• Primary device connections shall be determined by applicable piping specification.
• Materials of construction shall be stainless steel where applicable throughout the primary device; bearing type and material shall be chosen with due regard to the lubricating qualities of the fluid being metered.
• Primary devices shall be specified for horizontal mounting wherever possible.

Rotameters (Except Purgemeters).

• Accuracy of measurement shall be within ± 2% of full scale.
• Armoured metal tube meters shall be used on hydro-carbon (liquid or gas} service, steam or other hazardous applications.
• "0"-ring construction shall be considered acceptable unless pressure, temperature or flowing medium considerations require packing.
• Floats shall be designed for maximum immunity to viscosity variation, maximum self-cleaning effect and dimensional stability.
• Float stops shall be provided on the inlet and outlet of the tube.

LIQUID LEVEL INSTRUMENTS

Displacement Type Level Instruments.

• External cage units shall be furnished with 1 ½ " minimum flanged connections with flange rating and specification matching that of the mating vessel nozzles.
• Side connected instruments shall have a ¾ " NPT plugged vent and a ¾ " NPT valved drain connection, as minimum size.
• Radiation fins shall be provided for temperature service over 200ºC, or according to manufacturer’s recommendations.
• A plain extension shall be provided for temperatures below -20ºC.

D/P Level Transmitters.

• Differential pressure level transmitters shall be of the force balance, or capacitance diaphragm or strain gage type, equipped with suppression or elevation capabilities adjustable up to the maximum possible span calibration.
• Minimum accuracy requirements are ± 0,20% of measuring span.

Level Gauge Glasses.

• Gauge glasses shall be of the armored type having single piece back construction and shall be flat tempered borosilicate glass, reflex or transparent type as required by application.
• Gauge cocks, when provided, shall be quick closing, lever operated, off-set angle type, with either vacuum tailpiece or ball checks furnished according to the service conditions.
• A drain valve shall be provided as part of the installation detail.
• Heated level gauges shall be used with highly viscous liquids which tend to block level measurement at ambient temperatures.

Magnetic Level Gauges.

• Magnetic level gauges may be employed where glass, gaskets or threaded connections cannot be used because of corrosive, toxic, hazardous or other difficult services or for interfaces between fluids of similar colors.

Level Switches.

• Float and displacer type level switches shall preferentially be of the side mounted, external chamber type. If operating conditions warrant it, level switches may be of the internal float or displacer type.
• Switch differential (difference between actuation and reactuation values) shall be selected as follows:
• For alarm purposes only, the differential shall be as low as possible and fixed.
• For on-off control, shutdown or other purposes, differential shall be selected according to process requirements and, in general, adjustable.
• Electrical switches shall preferentially be of the snap-action type.Electrical contacts shall be SPDT or DPDT with a minimum rating of l A at 24 VCC.
• Pneumatic switches shall be:snap-áction type with a minimum air vent capacity of 1,6 Nm3/H at 1,4 bar (1 scfm at 20 psig).

PRESSURE INSTRUMENTS

Pressure Transmitters.

• Pressure transmitters shall have accuracies of ± 0,20% of the span of the instrument, minimum.
• Ranges shall be adjustable, and shall be specified so that the normal operating pressure is within 50% to 85% of the calibrated range.
• Overrange protection to 1.3 times the maximum range of the instrument shall be provided, as a minimum.
• Transmitters in vacuum service shall be capable of withstanding full vacuum regardless of range, without damage or calibration shift
• All elements shall be heat treated and stress relieved for  minimum hysteresis.
• Pressure transmitter process connections shall generally have a ½ " NPT thread.

Pressure Switches.

• Pressure switches shall generally be of the bourdon, diaphragm or piston types, according with operating and surge pressures requirements.
• Where long expected service life and/or fast cycling are required, piston type pressure switches shall be considered.
• Generally, pressure ranges shall be selected such that set pressures are within the middle 30% of the adjustable range.
• Proof pressure must exceed the maximum anticipated pressure by, at least, 30%.
• Pressure connections shall preferably be ½" NPT for process services and ¼" NPT for instrument air signals.
• Switch differential (difference between actuation and reactuation values) shall be selected as follows:
• For alarm purposes only, the differential shall be as low as possible and fixed.
• For on-off control, shutdown or other purposes, differential shall be selected according with process requirements and, in general, adjustable.
• Electrical switches shall preferentially be of the snap-action type.
• Electrical contacts shall be SPDT or DPDT with a minimum rating of 1A at 24 VCC.
• Pneumatic switches shall be snap-action type with a minimum at air ver capacity of 1.6 Nm3/h at 1,4 bar (1 scfm at 20 psig).

Pressure gauges.

• Ranges of gauges shall be selected from manufacturer’s standards such that the normal operating pressure indication is approximately at mid-scale.
• Accuracy of local pressure gauges shall be ±1% of full scale.
• Pressure gauges shall be capable of withstanding intermittent overpressures of 1.3 times the maximum scale reading without damage or calibration shift.
• Gauge connections shall be ½" NPT, with wrench flats or square shoulder.
• Where range requirements cannot be met by bourdon tubes or where the nature of the fluid is such as to present a corrosion hazard, other types of elements and other materials of constructions shall be considered.
• Sockets and tips shall be of the same material as the measuring element and shall be welded to the element using the same filler material (or silver-soldered on the case of brass/bronze elements).
• Dial face shall be white with black figures and graduations.
• Gauge pointer shall be balanced and with micrometer adjustment.

Diaphragm Seals.

• Diaphragm seals shall be of the welded diaphragm type close coupled to the instrument, or connected to it with capillary tubing.
• In general, bottom housings shall have ¼ " NPT flushing connection.
• For viscous fluid services clean-out type diaphragm seals with removable bottom housing shall be considered.
• The entire system above the diaphragm and including the element, shall be evacuated and entirely filled with an inert liquid, suitable for operation at the process temperature and an ambient temperature in accordance with project specifications.
• Generally, diaphragm seals shall not be specified for use with instruments having a range less than 0-2 bar.

Pulsation Dampeners.

• Pulsation dampeners shall have ½" NPT female connections and be of the externally adjustable type.

Siphons.

• Siphons shall have ½ " NPT connections, pig tail type, and schedule 80 as minimum, in accordance with piping spec.

Receiver Gauges.

• Receiver gauges shall be equipped with elements providing full scale deflection for pressure inputs of 0.2-1.0 bar.
• Receiver gauges shall have a white dial with black figures, and an accuracy of ±0.5% of full scale.
• Locally mounted receiver gauges shall generally be 90 mm (3 ½") diameter.
• Panel mounted receiver gauges dimensions shall be selected to match other panel instruments.
• Process connection shall be ¼" NPT.

Draft Gauges.

• Direct connect gauges for indication of low pressure or vacuum in the range of 0-5 mm WC to 0-150 mm WC shall be vertical scale gauges utilizing a slack diaphragm (rubberized nylon or equivalent) as the sensing element.
• Draft gauges shall be designed for flush panel mounting on a vertical panel face or for "freize" faces at up to 15 degrees from vertical.
• Where draft gauges are for use in the field, a suitable housing shall be provided.
• Draft gauges shall be required to have an accuracy of ± 2% of scale or better and shall be capable of withstanding pressures or vacuums equivalent to 100% overrange.
• Process connection shall be ¼" NPT.

TEMPERATURE INSTRUMENTS

Selection.

• Local temperature indication shall be obtained by means of  bimetal actuated thermometers.

Thermocouples.

• Thermocouple assemblies shall be completely factory built.
• Field fabricated thermocouples are not acceptable.
• Thermocouples terminal blocks shall be enclosed in a waterproof head having a female threaded gasketed cover with captive chain.
• Thermocouple heads shall have ¾ " NPT conduit connections and ½" NPT thermocouple ends.
• A drain wire grounding screw shall be provided inside the  head assembly.
• In general, thermocouples shall be of the conventional type, using solid alloy wire and porcelain insulators.
• Mineral insulated thermocouples may be used where environmental or economical considerations recommend to do so.
• Mineral insulated TC's shall be of the hard packed magnesium  oxide insulation type, with ¼" OD sheath.

Temperature Converters and Transmitters.

• Electronic TC/I or R/I converters and transmitters and electropneumatic TC/P or R/P converters shall be of the solid-state electronically balanced type.
• Output shall be linear and directly proportional to the measured temperature.
• Input-output isolation shall be provided.
• Each converter or transmitter shall be designed to give the fail--safe direction signal (up-scale burn-out) in the event of an open-circuit in the measuring element.
• Minimum accuracy requirements are ± 0.25% of span for electronic TC/I or R/I converters and transmitters, and ± 1% span for electropneumatic TC/P or R/P converters.

Filled System Instruments.

• Filled system elements shall be liquid-filled (SAMA Class IA) or gas-filled (Class III) as required by temperature limit and span considerations.
• Thermal bulbs shall be AISI 316 SS with bendable extensions, AISI 316 SS capillary with stainless steel armour.
• Filled-system instruments shall not be used if process contamination due to system failure is not acceptable.
• Filled-system instruments shall have an accuracy of ± 1% of span, minimum.

Bimetallic Thermometers.

• Local temperature indicators shall be heavy duty bimetallic actuated thermometers with minimum 114 mm (4 ½ ") diameter dials.
• Stem  lengths shall be chosen so that they may be fitted into thermowells matching project specifications.
• Ranges shall be selected from manufacturer's standards such that normal operating temperature indication is approximately at midscale.
• Accuracy of measurement shall be ± 1% of dial range.
• Standard overrange protection shall be specified.

Temperature Switches.

• Field mounted temperature switches shall be of the bimetal or filled system types.
• Switch differential (difference between actuation and reactuation values) shall be selected as follows:
• For alarm purposes only, the differential shall be as low as possible and fixed.
• For on-off control, shutdown or other purposes, differential shall be selected according to process requirements and, in general, adjustable.
• Electrical switches shall preferentially be of the snap-action type.
• Electrical contacts shall be SPDT or DPDT with a minimum rating of a 1A at 24 VCC.
• Pneumatic switches shall be snap-action type with a minimum air vent capacity of 1,6 Nm3/h at 1,4 bar (1 scfm at 20 psig).

CONTROL VALVES

Selection.

• Angle type control valves shall only be used for applications where no guarantee can be obtained for globe type, e.g. slurries, critical flow with cavitation, etc.
• Eccentric rotating plug valve shall be considered as alternates to globe valves for those specific services listed on each project.
• Butterfly valves shall be considered for large sizes with low pressure drop and low line pressures. Butterfly valve bodies shall be of the heavy duty wafer type, with outside bearings and packing glands.
• Saunders patent type valves may be used where fluid characteristics (viscosity, suspended solids, corrosiveness) make impractical the application of other valve types.
• Other special types of valves (ball, gate, etc) may be used on on-off control applications where so required by the piping specifications.

Valve Trim.

• Control valve seat leakage specification and classes shall be according ANSI B 16, 104-1976.

Valve Actuator.

• Generally, control valve actuator shall be of the spring and diaphragm, pneumatically actuated type.
• Piston type actuators, with or without helper springs and/or failsafe capacity tanks will only be considered if the force requirements fall beyond the normal range of diaphragm operators.
• Where piston type actuators must be furnished the vendor shall supply the accessories necessary to achieve fail safe action on air failure.
• Piston type actuators supply pressure shall generally be equal to main instruments header supply pressure.
• Servo-operated electro-hydraulic actuators shall be considered for services involving extremely high pressure or pressure drops with very fast response requirements.

Valve Electro-Pneumatic (I/P) Converters.

• Electro-pneumatic shall be supplied separately.
• Converters for separate mounting shall be complete with 2" pipe mounting bracket.
• Input signal shall be 4-20 mA DC.
• Output signal shall be 3-15 psig.
• Air supply and output gauges shall be supplied.
• Only one I/P converter shall be used when multiple valves are operated from a single control signal.

Pilot Solenoid Valves.

• Generally, pilot solenoid valves shall be furnished completely mounted and piped to control valves.
• Body material for solenoid valves used in instrument air service shall generally be forged brass.
• Solenoid valves body connections shall normally be ¼" NPT or 3/8" NPT, depending on application.
• Solenoid valves internal parts shall be manufacturers' standard stainless steel with resilient seat materials.
• Solenoid valves coil insulation class shall be selected according with the maximum ambient temperature and solenoid temperature rise conditions.
• Solenoid valves shall be energized during normal operation.

Valve Limit Switches.

• When required, limit switches shall be furnished completely mounted on control valves.
• In general, limit switches shall be provided to actuate at the fully open or closed position of control valves.

Valve Air Locks.

• Air lock-up devices shall be used when a control valve is required to hold its last position in the event of instruments air failure.
• Air locks shall be provided mounted and piped with control valves.

Valve Air-Sets.

• Air-sets shall be furnished as part of the control valves supply.

Valve Piping and Connections.

• Valve mounted accessories shall be completely mounted and piped by vendor.
• Pneumatic tubing shall generally be PVC coated tubing ¼" OD or 3/8 " OD.
• Pneumatic connections shall be ¼" NPT female or 5/8" NPT female to suit.
• Electrical conduit connections shall generally be ¾" NPT female.
• Limit switches and/or solenoid valves, shall be wired to a junction box, completely mounted and furnished by control valve vendor.

Valve Noise.

• Sound Pressure Level shall be expressed in db (referred to 0.0002 u bar), on the "A" weighted frequency scale, according to ANSI S1.4 1961.
• SPL shall be 90 dbA maximum for an 8 hour per day exposure period.
• These requirements do not apply to non-controlling valves, e.g. valves used for safety/relief services.
• Control valves noise performance data shall be provided and guaranteed by manufacturer with the quotations.
• For noise levels above 90 dbA, control valves manufacturers shall indicate the means to reduce the SPL.

Valve Nameplate.

• Control valves nameplate shall be stainless steel and show the following information as a minimum:
• Manufacturer's name or trademark.
• Valve model and serial numbers.
• Maximum valve body pressure rating.
• Valve body material and nominal pipe size.
• Trim material, size and characteristic.
• Stem travel.
• Valve action, signal range and bench settings.

PRESSURE REGULATING VALVES

Selection.

• If required by process conditions, fanged diaphragm control valves with yoke mounted pressure pilots shall be used for reducing or back-pressure control applications.
• Pressure pilots shall normally be of the non-indicating type, with control action to suit the application.
• Pressure pilots measuring elements shall be AISI 316 stainless steel minimum.
• Pressure pilots air supply and output gages shall be provided.
• On applications where steam or electrical tracing is required for the pressure pilot, it shall be mounted separately from the control valve.
• Self-contained pressure regulators with threaded connections may be used, if required by process conditions.

SAFETY RELIEF VALVES, RUPTURE DISCS AND VENTING DEVICES

Selection.

• Safety relief valves, except for thermal relief of liquids, shall generally be of the flanged, full nozzle, adjustable blowdown type.
• Direct spring-loaded safety relief valves shall generally be used.
• Self-acting, fail safe pilot operated safety relief valves shall be considered for large sizes and operation pressures close to set pressures.
• Conventional non-balanced type safety relief valves shall be provided:
• When the discharge is vented directly to the atmosphere.
• When the back-pressure  (gauge) is variable and does not exceed 10K  of the set-pressure (gauge).
• When the back-pressure (gauge) is constant and the valve is set accordingly.
• Balanced bellows type safety relief valves shall be provided:
• When variable back-pressure (gauge) exceeds 10K of the set-pressure (gauge).
• When guiding area and upper valve parts must be sealed off against corrosive or viscous fluids.
• When highly toxic or expensive fluids are handled.
• Balanced bellows with auxiliary balancing piston type safety relief valves shall be employed when the following conditions simultaneously exist:
• Back pressure, either constant or variable, exists.
• Excessive pressure may build in the bonnet as a result of pressure build-up in the bonnet vent piping.
• Resultant buid-up of pressure in the bonnet would cause a dangerous condition.

Valve Body and Bonnet.

• Flange rating and facing shall be as determined by the piping specifications.
• Safety relief valves used for thermal expansion of liquids shall generally be ¾" NPT male x 1" NPT female, minimum.
• Conventional type safety relief valves shall have closed bonnets with screwed caps.
• Balanced bellows safety relief valves shall have vented bonnets with screwed caps.
• Safety valves for steam service shall be of the yoke (open) type with screw stted caps.

Valve Accessories.

• Plain lifting levers shall only be provided for steam and air services.
• Packed lifting levers shall be used when a lifting mechanism is required and protection against leakage is necessary.

RECEIVER INSTRUMENTS

Feedback Controllers.

• Miniature type controllers shall permit the independent remotion of the automatic or manual section while the other remains in operation or equivalent back up facilities.
• Field mounted, large case type controllers shall be of the conventional type.
• Automatic-manual air transfer switches shall be integrally mounted, preferably internally.
• Field mounted, pneumatic type controllers shall be provided with air supply and signal output gauges.
• Feedback Control Modes

Feedback control modes generally used for typical process applications shall be according the table below:

NOTES: 1. Use fast integral (typically 0.01-5 min/rep.)
2. Use integral when resulting proportional offset is unacceptable or undesired.
5. Use normal (slow) integral (typically 0.1-50 min/rep.)
4. Extra-wide proportional bands (up to 1000 %) may be required.
5. Use derivative if process variable measurement is inherently noise-free.

• All controllers shall be provided with suitable means to easily reverse the control action from direct to reverse and vice-versa.

ALARM ANNUNCIATOR SYSTEMS

• The alarm annunciator system shall include black-lighted windows (color-coded if required), logic units, flasher unit, horn or buzzer, acknowledgement, test and reset buttons, as required.
• Alarm annunciator system shall be of the solid state design.
• Alarm annunciator sequences shall be defined in accordance with ISA-S18.1 standard.
• Generally operating sequence ISA F2 A-14 shall be used.
• Alarm annunciators shall be provide with 20% spare points.
• Trouble contacts actuating alarms shall normally be closed, energized, and shall open on the abnormal condition or on power failure.
• A separate pair of wires shall be provided from each trouble contact to the respective alarm annunciator point.