Resistive Sensing Elements
a. Resistance Temperature Detector (RTD) The RTD incorporates pure metals or certain alloys that increase in resistance as temperature increases and, conversely, decrease in resistance as temperature decreases. RTD act somewhat like an electrical transducer, converting changes in temperature to voltage signals by the measurement of resistance. |
b. Bellows Resistance Transducer
Bellows Resistance Transducer is resistance type transducers which has moving contacts (slide wire variable resistors). The pressure change causes a resistance change due to the distortion of the wire. The value of the pressure can be found by measuring the change in resistance of the wire grid. The following equation shows the resistance relationship. |
Capacitive Sensing Elements
A common method of level measurement is to use a capacitance bridge. A typical arrangement is shown in Fig. in which the sensor consists of two concentric metal cylinders. In the case of a circular tank, the wall of the tank can be employed as the outer cylinder of the sensor. The capacitance of the sensor is: |
Thermo-electric Sensing Elements
Thermocouple Thermocouples are the most popular temperature sensors. They are inexpensive, interchangeable, have standard connectors and can measure a wide range of temperatures. Their main limitation is accuracy as the system errors of less than 1°C can be difficult to achieve. Following figure represents internal construction of thermocouple and its circuitry. A thermocouple is constructed of two dissimilar metal wires joined at one end. |
Differential Pressure Sensing Elements
Differential Pressure Cell (DPC) The differential pressure cell (DPC) measures the difference between two or more pressures introduced as inputs to its sensing unit. The sensing unit consists of a diaphragm and a pressure cavity to create a variable capacitor which detects strain due to applied pressure. The following figure presents a schematic of the sensing unit of the DPC. The cell contains two compartments separated by a diaphragm. |
Valve Selection & Characterization:
1. Equal Percentage: equal increments of valve travel produce an equal percentage in flow change 2. Linear: valve travel is directly proportional to the valve stoke 3. Quick opening: large increase in flow with a small change in valve stroke Field of application for the above mentioned valves (Selection for Application): 1. Equal Percentage (most commonly used for valve control) - a. Used in processes where large changes in pressure drop are expected b. Used in processes where a small percentage of the total pressure drop is permitted by the valve c. Used in temperature and pressure control loops |
A variable frequency drive generally consists of following:
a. AC motor b. Main controller assembly c. Operator interface a. AC motor: It is usually a 3 – phase induction motor driven by an alternating current. The magnetic field on the rotor is either generated by current delivered through slip rings or by a permanent magnet. b. The main controller assembly: It is a solid state power electronic conversion device & consists of three main parts - i. Bridge rectifier ii. DC link iii. Inverter c. Operator interface : It often includes an alphanumeric display and/or indication lights and meters to provide information about the operation of the drive. A serial communications port is also often available to allow the VFD to be configured, adjusted, monitored and controlled using a computer |
Relief Valves
A relief valve (RV) is a type of valve used to control or limit the pressure in a system or vessel which can build up by a process upset, instrument or equipment failure, or fire. The pressure is relieved by allowing the pressurised fluid to flow from an auxiliary passage out of the system. Working: The safety valve will start to open; reach full lift and close over a range of pressures. There are three pressures that are important when specifying a relief valve. Set pressure – This is the pressure at which the safety valve will begin to open. Relieving pressure – This is the pressure at which the safety valve is fully open and working at full capacity. Re-seat pressure – The pressure at which the safety valve closes after a relieving event. Types of Relief Valves
Pressure relief valve (PRV) or pressure safety valve (PSV): The difference between PRV & PSV is that PSVs have a manual lever to activate the valve in case of emergency, while most PRVs are spring operated. At lower pressures some use a diaphragm in place of a spring. The oldest PRV designs use a weight to seal the valve. Relief valve (RV): A valve used on a liquid service, which opens proportionally as the increasing pressure overcomes the spring pressure. Safety valve (SV): Used in vapor/gas service. Most SVs are full lift or snap acting, in that they pop completely open. Safety relief valve (SRV): A relief valve that can be used for gas or liquid service. However, the set pressure will usually only be accurate for one type of fluid at a time. |
Safety Integrity Level (SIL) is defined as a relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction.
In simple terms, SIL is a measurement of performance required for a Safety Instrumented Function (SIF). In the European Functional Safety standards based on the IEC 61508 standard four SILs are defined; with SIL 4 being the most dependable and SIL 1 being the least. A SIL is determined based on a number of quantitative factors in combination with qualitative factors such as development process and safety life cycle management. |
Layers of protection analysis (LOPA) is a semi-quantitative methodology that can be used to identify safeguards that meet the independent protection layer (IPL) criteria established by CCPS1 in 1993.
LOPA is not just another hazard assessment or risk assessment tool. It is an engineering tool used to ensure that process risk is successfully mitigated to an acceptable level. It can be used at any point in the lifecycle of a project or process, but it is most cost effective when implemented during front-end loading when process flow diagrams are complete and the P&IDs are under development. Application LOPA is typically applied after a qualitative hazards analysis has been completed, LOPA Process 1) Record all reference documentation, including hazards analysis documentation, pressure relief valve design and inspection reports, protection layer design documents, etc. 2) Document the process deviation and hazard scenario under consideration by the team. 3) Identify all of the initiating causes for the process deviation and determine the frequency of each initiating cause. 4) Determine the consequence of the hazard scenario. This evaluation should include an examination of safety, environmental, and economic losses. 5) List the IPLs that can completely mitigate all listed initiating causes. The IPLs must meet the independence, specificity, dependability, and auditability requirements. 6) Provide specific implementable recommendations. The recommendations from the LOPA team must be considered options for implementation. |
Prevention System includes the following:
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Mitigation System includes the following:
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