Process Control & Instrumentation Book

The instrumentation technician's reference manual for process control.

Electric motor, variable speed drive, pressure transmitter, ph meter, proximity sensor, manometer, mac valve, pneumatic actuator, differential pressure transmitter, capacitance probe and much more.

Contents

Table of Contents

• Ultrasonic level detectors - Consider these experiences.

  • About the ultrasonic level detectors section: These instruments use high-frequency sound bounced off a surface to determine distance. Level detectors in tanks and silos are often ultrasonic level detectors. Long-lived, valuable service depends on the configuration of the tank roof and internals, the material of construction compatibility with process chemicals, position on the roof, stillness, and surface orientation measured.

• How control valves operate.

  • About the how control valves work section: Control valves regulate the flow of a liquid or gas by opening or closing internal passages. They form part of a control loop used to control a process. The control valve responds to instructions from the controller and adjusts the internal openings accordingly.

• Isolation valves for different chemical services.

  • Regarding the isolation valves for different services, the valves isolate and regulate liquid and gas flow. The valve used for isolation duties must provide a secure, tight shut-off. The nature of the chemical process, the quality of sealing, and the impact of outside environmental occurrences are considerations.

• Radar level detectors.

  • Radar-level detectors are becoming a popular technique for tank-level detection of liquids. The radar sends a sub-millionth of a second pulse from an emitter, and a receiver detects the return signal. The reflection’s time-lapse is measured and electronically converted to a distance from the liquid surface. Installation peculiarities must be understood and appreciated to provide a reliable operating setup.

• Differential pressure transmitters.

  • About the differential pressure transmitters section. These are instruments to detect pressure. The sensing element is connected to the process by pipework and flexes proportionately to the pressure. The resulting distortion produces an electric signal amplified and converted to a value on a read-out.

• Magnetic flow meters.

  • About the magnetic flow meters section. Mag-flow meters measure the flow velocity of conductive liquids. When powered, a magnetic flow meter creates a magnetic field in an electromagnet on either side of the bore. A voltage develops across the liquid as the conducting liquid flows through the magnetic field. Insulated electrodes are flush mounted on the wall of the meter to detect the voltage. The magnitude of the induced voltage is proportional to the liquid velocity, the meter's length, and the magnetic field's strength.

• Control systems can trick operators.

  • How can control systems trick operators? Operators are required to run their plants from control panels. They view the control panel dials, lights, and readouts and interpret the plant and equipment operating conditions from them. Every indicator on the panel must be a true reflection of the equipment’s state out in the plant.

• Pneumatic actuator experiences.

  • Experiences with pneumatic actuators section. Pneumatic (air-operated) actuators are preferred to open and close quarter-turn valves automatically. They consist of a ram moved by air pressure within a cylinder. Long-lived actuator operation depends on the air quality, the compatibility of construction materials, the design's simplicity, and the installation's quality.

• Capacitance level probes.

  • Capacitance level probes section: These instruments consist of a long rod or cable that protrudes into a vessel and its contents. The instrument sets up an electric field between the probe and the tank wall using the tank's contents as a dielectric (a nonconductor that allows an electric field to exist within itself). The electrical properties vary between the portion of the probe under and above the liquid. The position of the change is detected and represents the level of the tank contents.

• Temperature sensing elements.

  • Temperature sensing elements section: The temperature of a process is an important measure to know as it indicates whether or not the process is in control. When high temperatures exist and accurate measurements are needed, thermocouples and Resistance Thermal Devices (RTD) are two commonly used industrial temperature sensing methods.

• Plastic air pipelines for instruments and control valves.

  • Plastic pipelines for instruments and control valve section: Some instruments and many actuated valves require an air supply. Control panels are purposely pressurized with air to stop the ingress of dust or hazardous vapors. Losing the supply of air to these plant items causes the process to fail or the level of risk to rise dangerously. Buying and using high-quality compressed air fittings and piping is sensible to protect against lost production time and unwanted dangers.

• Water hammer and its control.

  • About the water hammer and how it's controlled. When a control or manual valve is shut fast in a full pipeline of moving liquid, the liquid suddenly stops. If the pipe suddenly starts banging and thrashing about, you can be sure a water hammer was created. A water hammer can cause great noise and damage. The pressure surge can deform pipes and valves and start leaks.

• Proximity switches.

  • Proximity switches. A proximity switch is the common name for a range of detectors that generate an electrical signal when a physical mass cuts through their sensor field. Upon detection of an object, the electrical signal is sent to a controlling device. It is used to monitor and control the operation of the equipment.

• Using pH meters.

  • Using pH meters section: A pH meter determines if a liquid is acidic or alkaline. It is essential to know if a chemical is corrosive to human tissue and engineering materials so that the company can provide appropriate protection. The pH measures the amount of free H+ ions in a solution. The pH measure ranges from 0 at the acid end to 14 at the base end. Each division is ten times stronger than the previous one (a logarithmic scale).

• Using Liquid Manometers for Measuring Pressure.

  • Using manometers for measuring pressure section: A U-tube manometer is the simplest of the pressure measurement devices. Its name comes from the U-shape formed when the two ends of a flexible tube full of liquid are elevated so that the liquid will not come out of the ends. A U-tube manometer is a ‘liquid’ balance. Using liquids of different densities can measure a range of pressures simply by measuring the height difference of the liquid columns.

• Electrical Motor Current Protection Saves Your Plant. (<< View full section)

• Control Loops – masters of automatic control.

  • Control loops – masters of automatic control section: A control loop senses, monitors, and adjusts a process to maintain it within preset requirements. The loop consists of instruments, computers, and computer-controlled equipment that interact to make continuous fine adjustments.

• Flow Meters - Don't ever believe what they tell you.

  • Flow meters – don’t believe what they tell you. A flow meter is an instrument that measures the amount of fluid (liquid or gas) that passes through it. The meter in the pipe is the detecting device. It increases the count when a specific quantity of fluid passes by. There are several types of commonly used flow meters. They include magnetic flow meters, vortex flow meters, mass flow meters, turbine flow meters, rotor blade meters, paddle flow meters, and ultrasonic flow meters. Usually, the quoted accuracy on the flow meter specification sheet is for water at 20°C. However, installing the meter in a process application will likely meet the quoted accuracy with site-specific calibration.

• Electric motor variable speed drives operation.

  • Electric motor variable speed drives operation. Alternating current electric motor speeds can be changed by using a frequency converter. This device changes the frequency of the alternating current to the motor. This then changes the rotating speed of the motor’s internal magnetic field. The speed of the rotor responds accordingly, allowing for variable speed control of connected items. When these devices are used, several issues need to be addressed to ensure successful selection and long-term operation.

• Orifice plate flow meter measurement.

  • The orifice plate meter flow measurement section: An orifice meter is a circular piece of metal plate placed between flanges in a pipe. It is a square-edged round hole machined 0.5 to 0.8 pipe diameter. Pressure tapping points are placed on either side of the plate at specified distances. The orifice causes a flow restriction and produces a pressure drop from one side of the hole to the other. The pressure difference is proportional to the fluid flow through the hole. The flow is calculated using accurate mathematical formulas from the pressure difference and flow areas.

• Effect of process changes on electric motors.

  • Effect of process changes on electric motors. This section explains the impact of changing process loads on AC induction electric motor performance. The motor’s behavior responds to the load imposed on it by the equipment connected to it. The equipment’s behavior responds to the duty and service it has to perform. Where the operator adjusts the equipment duty and service or fluctuates as the process changes, the motor will react to those changes. When the changes are so excessive that they are beyond the motor’s ability to handle them, the motor stops, and the equipment or process stops with it.

• Conductivity meter operation & use.

  • Conductivity meter operation and use. Water-based solutions' electrical conductivity (and its opposite, resistivity) indicates their electrical current-carrying ability. High conductivity occurs when many charged atoms and ions are in the water. That high conductivity typically means the presence of dissolved metals, salts, and acidic or alkali chemicals. Conductivity probes measure the total level of charged particles present. This article explains how conductivity probes work and their application in boiler water treatment and management.

• Mechanical level sensor types and use.

  • Mechanical level sensor types and use. In some process situations mechanical level sensors are the best choice for the environment or are stipulated by law (as in boilers). To operate these devices require the physical contact of the process condition being measured. They provide solid indisputable proof that a process condition has been achieved. Provided the mechanical mechanism is connected and working properly these devices provide the most believable proof that a level in a vessel, tank or silo is actually what the instrument is saying it is.

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