There is always a degree of uncertainty in selecting instrumentation for difficult or corrosive environments. Material selection for chemical compatibility involves a strained decision between cost and equipment reliability and longevity. The other major uncertainty is whether the instrument is appropriate for the application – will it repeatedly give truly representative readings all it’s operating life? This article presents a list of instrumentation successfully used in a range of difficult and chemically demanding environments.
Keywords: differential pressure transmitter, ultrasonic level detector, radar level detector, ph meter, conductivity meter, mechanical flow meter, mechanical level switch, temperature sensor, thermocouple, rtd resistance thermal device, capacitance probe, magnetic flow meter, orifice flow meter.
Always buy instrumentation for reliability and longevity! Trust me on this! If you have ever had to maintain someone else’s buying decision you will know that they should have brought on quality, simplicity of operation, simplicity of maintenance and reliability. Price, over the life cycle of the equipment, is not important at all! If you make a poor selection the purchase price will be nothing (about 10%) compared to the total lifetime maintenance cost and pain it causes you! If you make a good selection then the price will pay for itself many times over in its (and your) lifetime.
|Differential Pressure TransmitterFull details are available in the ‘Process Control And Insrumentation’ ebook available from www.feedforward.com.au.||Chemical compatibility|
Many process chemicals will destroy the internals of a PT. To prevent contact with aggressive chemicals a diaphragm process seal is used to connect to the process. The PT is mounted on the other side of the seal. Select materials for the diaphragm that give many years of corrosion free service. Protect against pitting and stress corrosion cracking in particular. At times exotic alloys or Teflon coatings are required. Check isocorrosion curves for pitting and stress corrosion cracking before specifying the diaphragm.Temperature compensation
As the outside temperature changes the oil fill expands or contracts. If a differential PT is not temperature compensating the change in volume causes the pressure inside the transmitter to also change. This makes the sensor distort and give a false pressure reading. In such cases protect the PT against temperature fluctuations by shading it from heat and direct sunlight. Or better still … specify a PT with temperature compensation.Diaphragm cleanliness
Sediment or crystal build- up on the sensing diaphragm will stop the diaphragm from flexing freely and will produce false readings. Products that crystallise at cooler temperatures need special attention. Keep the liquid temperature at the diaphragm surface hot enough to prevent crystals forming in the cooler crevice between the diaphragm and transmitter body or insulate the body to reduce the heat loss through the walls.
|Ultrasonic Level DetectorFull details are available in the ‘Process Control And Insrumentation’ ebook available from www.feedforward.com.au.||Dry Bulk Products|
Often used in measuring the level of powdered product in vertical silos the dust created during filling caused interference with the reflections and resulted in false level readings. Dust also coats the surface of the detector causing intermittent spurious readings during normal operation.A further complication is the inversion of the product surface shape from conical pyramid to inverted cone as the product is removed from the bottom of the silo. The reflected signal does really represent the amount of product in the silo.Liquid Levels
When used to measure liquid level major issues are stillness of the liquid surface and vapour given off by the process. Problem readings occur with undulating and turbulent liquid surfaces and where a lot of vapor is generated. If the liquid surface is not still the ULD will get changing readings. Where vapour is present the sound signals will encounter interference. This may be overcome by putting the detector at the top of a dip pipe.Process Chemical Compatibility
Selecting the wrong materials of construction will result in the detector’s housing corroding or dissolving from chemical attack. Check that the long term application is suitable for the vapors and maximum operating temperatures. Beware if it seems the manufacturer’s specifications were apparently satisfied by the average process conditions! What you must satisfy is the local temperature and chemical compatibility conditions. The design conditions of the bulk liquid may be known but the ULD must handle its local temperatures and chemical concentrations, not the bulk temperature and process conditions.Vacuum Service
Since ULD’s require sound to be transmitted and reflected they will only operate in environments that contain sound transmitting atmospheres. As such they will not be reliable in a process conducted under vacuum conditions.Internal Structures
The pulse emitted from a ULD will be reflected back by any object in its path. If there are internal structures within a tank below the ULD they will reflect the pulse and produce a false signal. Relocate the ULD to a place where there is clear space below it.Calibrations
You must check and calibrate the high and low levels and the span when a ULD is in its final position. With tanks filling them with water, physically dip, and measure the levels to compare results. Alternately take the ULD to a ‘dummy’ test set-up and then replace it in its required position.
|Radar Level DetectorFull details are available in the ‘Process Control And Insrumentation’ ebook available from www.feedforward.com.au.||Location|
The radar must be positioned to prevent unwanted reflections off tank internal structural beams, wall welds, rivets, etc. Unwanted reflections can be electronically separated but now you have programming complications better avoided if possible. Keep the receiver below the bottom of the nozzle so that it is in clear space. Pulses can bounce off the nozzle walls to the receiver and produce false signals.Foam on the liquid surface deadens the return pulse. If located inside vigorously agitated tanks there is a risk of blocking the opening of the antenna cone from splashed product or condensing vapours. This is a problem with products that sublime (evaporate then solidify on surfaces) like sulphur or that crystallise.Check material compatibles and process condition resistance for many years of operating service.Bulk products
They can be used on bulk products with better success than ultrasonic level detectors since their signals are much stronger and intense, and not so easily distorted and deflected. Dust coating the antenna can become a problem, especially if humidity is present. They are not successful in mixing tanks where dry product is fed into a stirred liquid. Powder from the product eventually coats the antenna and stops it receiving a signal.
|pH MeterFull details are available in the ‘Process Control And Insrumentation’ ebook available from www.feedforward.com.au.||Process variables|
For pH meters to be accurate they require retention, or hold-up time, for chemical interactions within the process to come to completion. Lag occurs because of the transfer time needed for the dosing system to add the dosed agent. Measurement lag is present between the process chemical’s change in pH and the probe’s detection of the change. The mixing rate and agitation available to properly mix the chemical being measured so a pH measurement reflects the bulk chemical pH adds further complication. The process properties such as corrosion, erosion, sediment, caking, etc on the probe can affect the measurement quality.The pH probe requires time for pH to stabilise without rapid changes, stable temperature without extremes, clean electrode surfaces, full probe contact with the liquid being measured, sufficient conductivity through the process chemical, no poisoning or drying of the reference electrode.Operating Issues
Chemical compatibility is critical and the appropriate plastics, elastomers and metals need to be selected for the service. Acids and alkaline vapours will, unless it is suitably protected, work their way into the probe’s internals.The liquid junction of the reference electrode must always be wet. If the liquid junction dries out the pH probe is destroyed. In such situations use probes that have a wetting spray to keep it moist. It is also possible to put the probe in a ‘drop leg’ bend where the pipe work is intentionally run down and then up again to create a catchment of liquid to keep the probe wet. Install a drain valve in the drop leg to empty the pipeline.
|Conductivity MeterFull details are available in the ‘Process Control And Insrumentation’ ebook available from www.feedforward.com.au.||Conductivity meters measure dissolved ions in a liquid. The sensing surface on normal probes scale-up over time and the probe must be removed for cleaning. A direct-contact probe must be chemically cleaned in weak hydrochloric acid and not by scraping or buffing the surface. Scratching the sensor increases the surface area in contact with the solution and alters the cell constant. A self-cleaning probe overcomes much of this problem.The meter must be properly earthed as required by the manufacturer. Since the probe emits and senses electric currents it will display readings from any electric current that presents to it. Badly connected conductivity meter set-ups will give false results even though the meter reads true at the time of calibration. If this happens then check the electrical continuity of the sensing system and meter equipment.|
|Mechanical Flow MeterFull details are available in the ‘Process Control And Insrumentation’ ebook available from www.feedforward.com.au.||For a flow meter to be accurate numerous factors must be right.|
PLC scan rate must be quicker than the meter pulses else pulses can be missed.
Sediment must not be allowed to wear away the meter’s internal clearances.
Solid items get jammed in the meter or protrude into the flow path.
The meter specification are based on water flow in laboratory conditions and a product with different physical properties or temperature is measured.
Software adjustment factors added in to make the meter produce the required results are lost or changed.
Temperature changes in the meter circuit electronics caused by the process fluid or local environment conditions change the behaviour of the meter.
Broken turbine wheel, rotor blades or paddles produce the wrong pulses.
Leaks into the meter destroy internal circuitry.
The meter accuracy varies as the flow rate varies.
Meter position or orientation allows bubble inclusion in the liquid (two-phase flow).
|Mechanical Level SwitchFull details are available in the ‘Process Control And Insrumentation’ ebook available from www.feedforward.com.au.||Mechanical level switches are in direct contact with the process chemicals. This can corrode components, product can jam moving parts, materials used in construction and methods of construction can fail due to pressure surges (floats can leak at joints and fill up with process fluid) and moving parts gradually wear. Internal electrical components can fail from dust, moisture, chemical attack and temperature effects.A raising/lowering cable sensor or a cable capacitance sensor is the most effective level detection device for bulk product silos. When specifying the cable lengths keep them shorter than the distance to the bottom discharge valve, so the cable is not caught if the valve is shut.With rotating paddles and vibrating forks install larger surface areas that contact a greater amount of product to produce the needed amount of drag.Vibrating fork level sensors can get material trapped, caked, glued or gummed into place between the forks and stop them from operating properly. Often it is best to put these vibrating sensors in at a steep angle to allow the vibrations to self-clean product of the instrument.|
|Temperature SensorFull details are available in the ‘Process Control And Insrumentation’ ebook available from www.feedforward.com.au.||Thermocouple|
Two wire thermocouples quickly detect a difference in temperature between the ends measuring end and meter end. It is critical to have both wires in firmly in contact with what is being measured. This is best done by tack welding the wires to the surface being measured.Resistance Thermal Device
In a Resistance Thermal Device (RTD) the sensor is a fine wire wound around an insulated core and the lot is encapsulated in epoxy resin or glass and protected by an outside sheath of stainless steel. Encapsulating the wire significantly increases the response time (lag time) and makes RTD’s less favourable for control purposes where process temperature changes quickly. RTD’s can be damaged by being subjected to excessive temperatures, by being chemically attacked by the process chemicals and from physical damage.An RTD is designed to cover a span of temperature and emits a signal in proportion to the temperature. This signal is then converted into an electrical signal. It is critical that the signal converter has the same temperature span range as the RTD to insure a true reading.
|Capacitance ProbeFull details are available in the ‘Process Control And Insrumentation’ ebook available from www.feedforward.com.au.||The accuracy of capacitance probes depends on the electrical properties of the liquid or solid being measured. To insure an electric field develops between the probes the medium in which the probe sits must not conduct electricity and short circuit the probe.These instruments are not affected by changes in the content’s pressure, temperature or specific gravity (density). They can detect the presence of separate layers of different liquids within the same tank.False level readings can arise if the dielectric properties change along the length of the probe and care is needed when calibrating the device in tanks containing layered liquids. Large changes in the vapour content within the space above the contents can lead to false level readings. When used to measure the level of solids there must be sufficient moisture content within the solid. In corrosive liquids insure the probes are protected from attack by plastic sheaths.|
|Magnetic Flow MeterFull details are available in the ‘Process Control And Insrumentation’ ebook available from www.feedforward.com.au.||For a magflow meter to work correctly and with confidence it must always be full of only liquid. It cannot recognise the presence of bubbles and will give false flow rates if they are present. Modify the pipe work to produce full-bore flow in the meter.If the electrodes transmitting the small voltages created in the liquid become coated with process contaminants false readings occur. Long signal transmission cable runs or outside electrical interference affecting cables can produce false readings.Calibration accuracy tests are usually done with water and when the meter is used on non-water fluids wide errors are possible.Correct earthing is critical. The meter will read voltage from any source. To insure only voltage across the liquid is measured keep the meter body in electrical contact with the liquid. In plastic pipes this is done with graphite gaskets between the body and flanges or by an earthing ring of chemically compatible metal connected to the meter body. In metal pipes clamping to a metal tag on the meter body and on the pipe flanges makes the electrical contact.|
|Orifice Flow MeterFull details are available in the ‘Process Control And Insrumentation’ ebook available from www.feedforward.com.au.||The accuracy of an orifice plate flow meter depends on the square edged remaining round and sharp. Wear or damage will produce errors in the pressure reading. It is also necessary to insure that no partial blockages occur upstream of the orifice that produce changes to the flow profile and pressure gradient. Blockages can occur in the sensing lines from the orifice to the transmitter or pressure gauge. The position at which the meters are located should be where the flow pattern in the pipe is straight and no turbulence exists.|