Instrument Selection Criteria, Installation Procedures & Cost

Instrument Selection Criteria, Installation Procedures & Cost Presented to: Sir Naveed Ramzan Presented by: 07-chem-44 Chemical Engineering Department, U.E.T Lahore

Introduction • Today the techniques of measurement are of immense importance in the process industry. • The progress of chemical reaction is indicated by measurement of temperatures and pressures at various points and such measurements allow the operator to take the correct decisions. Chemical Engineering Department, U.E.T Lahore

Selection of Instruments The selection of an instrument for a specific application is an iterative process. Following points should be considered while their selection: • Identify all operating cases • Collecting all relevant process data • Environmental conditions • The extent to which the measured system will be disturbed • Durability • Maintainability • Consistency of performance Chemical Engineering Department, U.E.T Lahore

Selection of Instruments • Selection of pressure sensors • Selection of temperature sensors • Selection of flow sensors • Selection of level sensors Chemical Engineering Department, U.E.T Lahore

Selection of Pressure Sensors • Pressure-sensing devices are chosen on the basis of following requirements: • Pressure range • Accuracy • Temperature operating range • Line-of-sight reading or electrical signal • Response time • In some applications there are other special requirements. Parameters, such as hystersis and stability, should be obtained from the manufacturers’ specifications. Chemical Engineering Department, U.E.T Lahore

Selection of Pressure Sensors • Manometers are used when just a visual indication of pressure level is required. • Dead weight gauges, because of their superior accuracy, are used in calibration of other pressure measuring devices. • When an electrical form of output is required, the choice is electric pressure transducer. • Resonant wires are used for higher accuracy. Chemical Engineering Department, U.E.T Lahore

Selection of Temperature Sensors • In process control a wide selection of temperature sensors are available. However, the required range, linearity, and accuracy can limit the selection. • In the final selection of a sensor, other factors may have to be taken into consideration, such as calibration, vibration sensitivity, response time, maintenance requirements, and cost. • The choice of sensor devices in instrumentation should not be degraded from a cost standpoint. Process control is only as good as the monitoring elements. Chemical Engineering Department, U.E.T Lahore

Selection of Temperature Sensors • The most commonly used device for temperature measurement is thermocouple. • RTD are also used frequently but they are very sensitive and gives fast response. • Pyrometers are non contact and used for high temperature applications. • Quartz thermometers provides very high resolution but they are very expensive. • Fiber optic devices are used in very inaccessible locations and they are very accurate. Chemical Engineering Department, U.E.T Lahore

Selection of Flow Sensors • The selection of a flow meter for a specific application to a large extent will depend on the required accuracy and the presence of particulates, although the required accuracy is sometimes down graded because of cost. • One of the most accurate meters is the magnetic flow meter. The meter is good for low flow rates, with high vis-cosities and has low energy loss, but is expensive and requires a conductive fluid. • The Venturi tube has the highest accuracy and least energy loss followed by the flow nozzle and the orifice plate. • Gas flow can be best measured with an anemometer. Chemical Engineering Department, U.E.T Lahore

Selection of Level Sensors • A number of factors affect the choice of sensor for level measurement, such as pressure on the liquid, liquid temperature, turbulence, volatility, corrosiveness, accuracy needed, single-point or continuous measurement, direct or indirect, particulates in a liquid, free flowing solids, etc. • Floats are often used to sense fluid levels because they are unaffected by particulates, can be used for slurries. • Bubbler devices require certain precautions when being used. To ensure a continuous air or gas supply, the gas used must not react with the liquid. • Radiation devices are used for point measurement of hazardous materials. Chemical Engineering Department, U.E.T Lahore

Installation requirements This section defines the detailed installation requirements for specific types of instruments such as: • Pressure Instruments • Flow Instruments • Temperature Instruments • Level Instruments Chemical Engineering Department, U.E.T Lahore

Installation requirements All instruments shall be installed so that they are • Accessible • Capable of removal • Replacement and repair • In-line instruments shall be mounted and secured in place under the mechanical subcontract. • Remainder of the installation shall be completed in accordance with the installation detail(s) specified for each instrument in the instrument index provided by suppliers. Chemical Engineering Department, U.E.T Lahore

Installation requirements Pressure Instruments: • Distance between sensor and source should be kept to a minimum. • Sensors should be connected via valves for ease of replacement. • To eliminate errors due to trapped gas in sensing liquid pressures, the sensor should be located below the source. Chemical Engineering Department, U.E.T Lahore

Installation requirements Pressure Instruments (Contd.) : • When measuring pressures in corrosive fluids and gases, an inert medium is necessary between the sensor and the source or the sensor must be corrosion resistant. • Resistance and capacitance can be added to electronic circuits to reduce pressure fluctuations and unstable readings. Chemical Engineering Department, U.E.T Lahore

Installation requirements Flow Instruments : • The meter must be installed according to the manufactures instructions . • The meter must not be installed in a section of pipe where there may be air pockets or the pipe does not run full of water. If it is likely that air will become entrapped near the meter, an air valve must be installed upstream of the meter. • A meter approved for operation in full flowing pipes shall be installed so that it is completely filled with fluid under all conditions during operation. Chemical Engineering Department, U.E.T Lahore

Installation requirements Flow Instruments (Contd.): • Flow nozzles may require a vertical installation if gases or particulates are present. To allow gases to pass through the nozzle, it should be facing upwards and for particulates, downwards. • Any backflow preventer must be installed on the discharge side of the meter and >5 pipe diameters downstream of the flow meter. • The meter must be installed in the correct direction to flow. • The meter must be mounted in such a way that it allows for both easy access and reading of display unit. Chemical Engineering Department, U.E.T Lahore

Installation requirements Temperature Instruments : • Care must be taken in locating the sensing portion of the temperature sensor, it should be fully encompassed by the medium whose temperature is being measured, and not be in contact with the walls of the container. • The sensor should also be placed downstream from the fluids being mixed, to ensure that the temperature has stabilized, but as close as possible to the point of mixing, to give as fast as possible temperature measurement for good control. • Place the sensor in a location where it will not be in direct sunlight. Chemical Engineering Department, U.E.T Lahore

Installation requirements Temperature Instruments (Contd.): • Place the sensor in a location at least 10 feet away from lights or lamps. • Place the sensor at least 5 feet from man-made sources of heat. • Keep the sensor away from AC power lines. • Limit the exposure of the sensor to the open night sky. • If you are unsure about a location's exposure to the night sky, check for dew at that location on a light dewy morning. If the area is dry, the location should work well. Chemical Engineering Department, U.E.T Lahore

Installation requirements Level Instruments: • Displacer Level Transmitters/Switches and Level Gauges, which must be removed/repaired without shutting down the process are capable of being isolated from the process. • All instruments should be installed so that they are accessible. • In-line instruments shall be mounted and secured under the mechanical subcontract. Chemical Engineering Department, U.E.T Lahore

Cost • Cost is very strongly correlated with the performance of an instrument, as measured by its static characteristics. Increasing the accuracy or resolution of an instrument, for example, can only be done at the penalty of increasing its manufacturing cost. • Instruments choice therefore proceeds by specifying the minimum characteristics required by a measurement situation and then searching manufactures’ catalogue to find an instrument whose characteristics match those required Chemical Engineering Department, U.E.T Lahore

References • Control Engineering (Editorial), Pg. 119-128, 1998. • Figiola R.S. and Beasley D.E. “Theory and Design of Mechanical Measurement”, John Wiley, 1995. • Instrument Society of America, “Flow meters- a comprehensive survey and guide to selection”, Pittsburgh, 1988. • King N.W., “Multi phase floe measurement at NEL, Measurement and Control, Pg. 237-239, 1988. • Medlock R.S., “Measurement and Control”, Pg. 293-298, 1985. • http://www.hbrc.govt.nz/LinkClick.aspx?fileticket=QQkdmyb9AcY%3D&tabid=1038, retrieved on 20.10.2010 Chemical Engineering Department, U.E.T Lahore

Instrument Selection Criteria, Installation Procedures & Cost