Module 2. Classification and selection of instruments
Lesson 5
Instrument Selection Criteria and Installation Procedures
5.1 Introduction
The availability of several types of sophisticated instruments and automatic process control techniques has greatly helped in improving not only the product quality in the food processing industries but also in achieving the reduction in cost of processing. Automation has become the essential component of technological and industrial development. A large number of specialized instruments have been developed to measure the variety of process variables. They generally give a range of measurement accuracy and sensitivity and vary vastly in cost factor. Choice between the various types of instruments is available and several alternatives are there with the entrepreneur.
5.2 Instrument Selection Criteria
The selection of an instrument for a specific application is an iterative process, carried out as a joint effort of a process technologist and an instrument engineer. Following are the points that should be considered while choosing the instrument for particular application
1. Identify all operating cases, such as normal operation at minimum, normal and maximum flow, alternative operating modes, start-up, commissioning and emergency operation.
2. Collect all relevant process data for each operating case. Data pertaining to processing fluids, such as, fluid name and phase, physical properties, corrosiveness and toxicity , presence of solids or contaminants, special risks such as fouling, plugging and deposition need to be delineated. Quantify the process operating data, such as, flow rate, pressure, temperature, density and viscosity etc. Specify the application aspects, such as, continuous/batch operation, pulsating flow, unidirectional or bi-directional flow, backflow risk, vibration and hydraulic noise.
3. Specify the environmental conditions that the instrument will be subjected to. As some conditions will immediately either eliminate the possibility of using certain types of instrument or else will create a requirement for expensive protection of instrument. One point to be remembered is that the protection of instruments reduces the performance of some instruments, especially in terms of their dynamic characteristics. For example sheaths protecting thermocouples and resistance thermometers reduce their speed of response. Instrument should be chosen that are as insensitive as possible to operating environment.
4. The extent to which the measuring system will be disturbed during the measuring process is another important factor in instrument choice. For example significant pressure loss can be caused to measured system in some techniques of flow measurement.
5. Consideration of durability, maintainability and consistency of performance are also very important during selecting the appropriate instrument.
6. Published literature is of considerable help in the choice of suitable instrument for a particular measurement situation. It is important to keep abreast of latest developments through appropriate technical publications.
7. Select suitable instrument makes and types for each measurement option from the ‘List of Selected Instrument Vendors’ as prepared for each project. The types of instruments already installed at the Principal's site should be taken into consideration, for the sake of variety control.
Instrument choice is a compromise between it’s performance characteristics, ruggedness, durability, maintenance requirements and purchase cost. To carry out such an evaluation properly, the instrument engineer must have a wide knowledge of range of instruments available for measuring particular physical quantities, and he/she must also have deep understanding of how instrument characteristics are affected by particular measurement situations and operating conditions.
Cost is very strongly correlated with the performance characteristics of an instrument. 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 finding an instrument whose characteristics match those required. To select an instrument with characteristics superior to those required would only mean paying more than necessary for a level of performance greater than that needed.
5.3 Choice Between Different Types of Instruments
5.3.1 Pressure sensors
Choice between the various types of instruments available for measuring mid-range pressure (1.013-7000bar) is usually strongly influenced by the intended application. Manometers are commonly used when just a visual indication of low pressure level is required. For medium pressure or vacuum sensing a Bourdon tube type pressure gauge is used. Bellows-types instruments are also sometimes used for this purpose, but much less frequently. When an electrical form of output is required, the choice is usually any one of the diaphragm type sensor with strain gauge, capacitive or fiber optics are used. If very high measurement accuracy is required, the resonant-wire devices are popular choice. Deadweight gauges, because of their superior accuracy, are used in calibration procedures of other pressure measuring devices.
In the case of pressure measurement in vacuum ranges, that is, below 1.0132bar, adaptation of most of the types of pressure transducer can be used. Special forms of Bourdon tubes measure pressures down to 0.1mbar, and diaphragms can be designed to measure the pressure down to 0.001mbar. However a number of specialized instruments have also been developed to measure the vacuum pressures. For differential pressure measurement, diaphragm-type sensors are the preferred option, with double-bellows sensors being used occasionally. Manometers are also sometimes used to give the visual indication of differential pressure values especially in the liquid flow rate indicators.
5.3.2 Temperature sensors
Choice between the various types of temperature measuring instruments for a given situation depends mainly on the type of medium to be measured. A good contact is essential between the medium and the sensor transducer. If the medium is solid this choice is restricted to thermocouples, thermopiles, resistance thermometers, thermisters, semiconductor devices and color indicators. For the fluid temperatures can be measured by any of the instruments with the exception of radiation thermometers.
The most commonly used devices in the industry for the temperature measurement is the base-metal thermocouple. Typical inaccuracy is ±0.5% of the full scale over the temperature range -250°C to +1200°C. Noble metal thermocouples are much more expensive, but are chemically inert and can measure temperature up to 2300°C with an accuracy of ±0.2% of full scale.
Resistance thermometers are also in common use within the temperature range -270°C to +650°C, with a measurement inaccuracy of ±0.5%. They have a smaller temperature range than thermocouples. Thermisters are other commonly used sensors. They are small and cheap. They give a fast outputs response to temperature changes with good measurement sensitivity, but their measurement range is quite limited. Dual diverse devices are new development that includes a thermocouple and a resistance thermometer inside the same sheath.
Semiconductor devices have better linearity than thermocouples and resistance thermometers and similar level of accuracy. Thus they are viable alternative to these in many applications. For non contact, non invasive mode of measurement radiation thermometers or optical pyrometers could be used. They are used to monitor temperature above 600°C in industrial furnaces etc. The instruments working on the thermal expansion principle, such as bimetallic thermometers, are used as temperature indicating devices as well as components within automatic control system.
5.3.3 Flow sensors
The number of relevant factors to be considered when specifying a flow meter for a particular application is very large. These include the temperature and pressure of the fluid, its density, viscosity, chemical properties and abrasiveness, whether it contains particle, whether it is liquid or gas, etc. This narrows the field to a subset of instruments that are physically capable of making the measurement. Next, the required performance factors of accuracy, range ability, acceptable pressure drop, output signal characteristics, reliability and service life must be considered. Accuracy requirements vary widely across different applications and range from ±0.5% to ±5%. Finally, the economic viability must be assessed and this must take account not only of purchase cost, but also of reliability, installation difficulties, maintenance requirements and service life.
Where only a visual indication of flow rate is needed, variable-area meter is popular. Where a flow measurement in the form of an electrical signal is required, the choice of available instrument is very large. It ranges from an orifice plate, various forms of differential pressures meter and electromagnetic flow meters. The currently trend in flow measurement is limiting the use of rotating devices such as turbine meters and positive displacement meters and at the same time increased use of ultrasonic and vortex meters.
5.4 Installation Requirements of Instruments
Every instruments manufacture provides a set of instrument manufacturer’s instructions for installation of that equipment. It is mandatory to comply with such instructions. It is to be ensured that the instruments that are flow direction sensitive e.g. Control Valves, Regulators, Vortex meters, Magnetic Flow meters, Orifice Plates, Corrosion Coupons, Pitot Tubes, Venturi Tubes etc. are checked with marking on instruments before installation. Ensure that dummy holes on instruments supplied with temporary protection for transportation etc (e.g. extra taps on venture tube supplied with plastic plugs etc.) are blocked with properly rated valves, plugs or blinds
5.4.1 Pressure instruments
Pressure gauges should have a high quality block valve, followed by a bleed valve and then followed by the gauge. If the instrument is located at a significant distance from the process piping location, the pressure take-offs installations should have a double block and bleed arrangement between the process piping and the instrument. The bleed valve shall be so located that it may act as the vent valve if the instrument impulse line must be flushed or filled with a sealing fluid like glycol or a purging fluid. The block valve immediately upstream of the instrument should be part of a manifold assembly to facilitate mounting the instrument. When a sealing fluid is required, a drain valve downstream of the instrument is needed to be installed to allow flushing the tubing or filling the tubing with the isolating fluid.
5.4.2 Temperature instruments
The sensor should be located at a place where it will have limited exposure to direct or reflected sunlight and open night sky. It should be placed at least 2 metres from man-made sources of heat because the heat from this surface may affect air temperature readings in the vicinity.
Keep the sensor away from AC power lines. Sensor and most of the cable should be at least 3m from 220 V A.C. 50 Hz power supply. Do not run the sensor cable parallel to house wiring. Mount the sensor at least 10 m from high-voltage power lines and transformers.
5.4.3 Flow instruments
The flow meter must be installed according to the manufactures written instructions. The meter must be installed in the correct direction to flow. It must be ensured that there are no air pockets in the section of the pipe and the pipe runs full of water or fluid. 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. If it is likely that air will become entrapped near the meter, an air valve must be installed upstream of the meter.
Appropriate liquid filtering device must be installed on the intake side of the meter. Suitable backflow preventer must be installed on the discharge side of the meter. If the meter is of electro-magnetic type, it is essential that the meter is earthed correctly as per the manufacture’s specifications. Where the meter is to be fitted above ground to plastic, PVC or polythene pipelines, it must be suitably supported to ensure its stability. The meter must be mounted in such a way that it allows for both easy access and reading of display unit.