PRESSURE INSTRUMENTATION
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The document discusses various types of pressure measurement instruments and concepts. It describes pressure gauges, transmitters, and transducers, explaining their measuring principles, components, installation considerations, and common terms. Diagrams illustrate typical configurations and components of differential pressure transmitters and loops.
PRESSURE INSTRUMENTATION
- 1. 1# Pressure Measurement • Pressure is the force exerted per unit area • Pressure is the action of one force against another force. Pressure is force applied to, or distributed over, a surface. The pressure P of a force F distributed over an area A is defined as P = F/A TOTAL VACUUM - 0 PSIA PRESSURE ABSOLUTE GAUGE COMPOUND BAROMETRIC RANGE ATMOSPHERIC PRESSURE NOM. 14.7 PSIA
- 2. 2# Pressure Measurement Pressure Units psi 100 bar 6.895 mbar 6895 mm of Hg 5171 mm of WC 70358 in of WC 2770 Kg/cm2 7.032 Pascal 689476 kPa 689.5 atm 6.805
- 3. 3# Pressure Measurement Terms • Absolute Pressure Measured above total vacuum or zero absolute. Zero absolute represents total lack of pressure. • Atmospheric Pressure The pressure exerted by the earth’s atmosphere. Atmospheric pressure at sea level is 14.696 psia. The value of atmospheric pressure decreases with increasing altitude. • Barometric Pressure Same as atmospheric pressure. • Gauge Pressure The pressure above atmospheric pressure. Represents positive difference between measured pressure and existing atmospheric pressure. Can be converted to absolute by adding actual atmospheric pressure value. • Differential Pressure The difference in magnitude between some pressure value and some reference pressure. In a sense, absolute pressure could be considered as a differential pressure with total vacuum or zero absolute as the reference. Likewise, gauge pressure (defined above) could be considered as Differential Pressure with atmospheric pressure as the reference.
- 4. 4# Pressure Gauge PRESSURE GAUGES: • A Pressure Gauge is used for measuring the pressure of a gas or liquid. • A Vacuum Gauge is used to measure the pressure in a vacuum. • A Compound Gauge is used for measuring both Vacuum and Pressure. • Pressure Gauges are used for Indication only.
- 6. 6# Measuring Principle Bourdon tube measuring element is made of a thin-walled C-shape tube or spirally wound helical or coiled tube. When pressure is applied to the measuring system through the pressure port (socket), the pressure causes the Bourdon tube to straighten itself, thus causing the tip to move. The motion of the tip is transmitted via the link to the movement which converts the linear motion of the bourdon tube to a rotational motion that in turn causes the pointer to indicate the measured pressure. Coiled Bourdon Helical Bourdon Pressure Gauge “C” Type Bourdon
- 7. 7# When selecting a Pressure Gauge, care should be given to a number of parameters which have an effect on the gauge’s accuracy, safety, and cost. • Accuracy required • Dial size • Operating pressure range • Chemical compatibility with gauge construction materials • Operating temperature range • Vibration, pulsation, and shock • Pressure fluid composition • Mounting requirement Pressure Gauge Selection Guideline
- 8. 8# Pressure Gauge Installation Gas Service Liquid Service • Top connection preferred for gas installations & side connection preferred for liquid installations. • The pressure gauge can be connected to the pipe by individual block and bleed valves or a two way manifold.
- 10. 10# Pressure Transmitter • A Pressure Transmitter is used where indication and/or record of pressure is required at a location not adjacent to the primary element. • A Pressure Transmitter is used for both indication and control of a process. • A Pressure Transmitter is used where overall high performance is mandatory. • Both Electronic and Pneumatic Transmitters are used. • These can be either Gauge, Absolute or Differential Pressure Transmitters.
- 11. 11# Transmitter Measuring Principle • The diagram shows an electronic differential pressure sensor. This particular type utilizes a two-wire capacitance technique. • Another common measuring technique is a strain gauge. • Process pressure is transmitted through isolating diaphragms and silicone oil fill fluid to a sensing diaphragm. • The sensing diaphragm is a stretched spring element that deflects in response to the differential pressure across it. • The displacement of the sensing diaphragm is proportional to the differential pressure. • The position of the sensing diaphragm is detected by capacitor plates on both sides of the sensing diaphragm. • The differential capacitance between the sensing diaphragm and the capacitor plates is converted electronically to a 4–20 mA or 1-5 VDC signal. • For a gauge pressure transmitter, the low pressure side is referenced to atmospheric pressure.
- 12. 12# Pressure Transmitter • Typical Outputs • 4 to 20 milliamp (mA). analog signal • Smart HART digital signal (superimposed on analog signal) • Fieldbus digital signal • 3 to15 psi pneumatic signal
- 13. 13# When selecting a pressure transmitter care should be given to a number of parameters which have an effect on transmitter accuracy, safety, and utility. • Accuracy required • Power supply • Operating pressure range • Operating temperature range • Body Material • Pressure fluid composition and Internal parts • Mounting requirement • Process connection size Pressure Transmitter Selection Guideline
- 14. 14# Pressure Transmitter installation • Mounting above tap is typical for gas service and mounting below tap typical for liquid and steam services. • Direct mount is possible for low temperature services.
- 15. PRESSURE TRANSDUCERS HOW IT WORKS Pressure sensors either convert the pressure into mechanical movement or into an electrical output. Complete gauges not only sense the pressure but indicate them on a dial or scale. 1) Mechanical movement is produced with the following elements 1) Bourdon Tube 2) Spring and Piston 3) Bellows and capsules 4) Diaphragm
- 16. PRINCIPLE,CONSTRUCTION & WORKING 1.1) BOURDON TUBE • The Bourdon tube is a hollow tube with an elliptical cross section. When a pressure difference exists between the inside and outside, the tube tends to straighten out and the end moves. The movement is usually coupled to a needle on a dial to make a complete gauge. It can also be connected to a secondary device such as an air nozzle to control air pressure or to a suitable transducer to convert it into an electric signal. This type can be used for measuring pressure difference.
- 17. PRINCIPLE,CONSTRUCTION & WORKING 1.2 PISTON TYPE The pressure acts directly on the piston and compresses the spring. The position of the piston is directly related to the pressure. A window in the outer case allows the pressure to be indicated. This type is usually used in hydraulics where the ability to withstand shock, vibration and sudden pressure changes is needed (shock proof gauge). The piston movement may be connected to a secondary device to convert movement into an electrical signal. 1.3 CAPSULES AND BELLOWS A bellows is made of several capsules. These are hollow flattened structures made from thin metal plate. When pressurized the bellows expand and produce mechanical movement. If the bellows is encapsulated inside an outer container, then the movement is proportional to the difference between the pressure on the inside and outside. Bellows and single capsules are used in many instruments. They are very useful for measuring small pressures.
- 18. 1.4 DIAPHRAGMS These are similar in principle to the capsule but the diaphragm is usually very thin and perhaps made of rubber. The diaphragm expands when very small pressures are applied. The movement is transmitted to a pointer on a dial through a fine mechanical linkage.
- 19. ELECTRICAL PRESSURE TRANSDUCERS There are various ways of converting the mechanical movement of the preceding types into an electric signal. The following are types that directly produce an electric signal. 1) Strain Gauge types 2) Piezo electric types. 3) Other electric effects Strain Gauge- The principles of electric strain gauges are covered later. Strain gauges are small elements that are fixed to a surface that is strained. The change in length of the element produces changes in the electrical resistance. This is processed and converted into a voltage. A typical pressure transducer would contain a metal diaphragm which bends under pressure.
- 20. Typical DP Transmitter Construction & Installation A DP transmitter is used to measure the gas pressure (in gauge scale) inside a vessel. In this case, the low-pressure side of the transmitter is vented to atmosphere and the high-pressure side is connected to the vessel through an isolating valve. The isolating valve facilitates the removal of the transmitter. The output of the DP transmitter is proportional to the gauge pressure of the gas, i.e., 4 mA when pressure is 20 kPa and 20 mA when pressure is 30 kPa.
- 21. Differential Pressure Transmitters • Most pressure transmitters are built around the pressure capsule concept. They are usually capable of measuring differential pressure (that is, the difference between a high pressure input and a low pressure input) and therefore, are usually called DP transmitters or DP cells. • A typical DP transmitter. A differential pressure capsule is mounted inside a housing. One end of a force bar is connected to the capsule assembly so that the motion of the capsule can be transmitted to outside the housing. A sealing mechanism is used where the force bar penetrates the housing and also acts as the pivot point for the force bar. Provision is made in the housing for high- pressure fluid to be applied on one side of the capsule and low-pressure fluid on the other. Any difference in pressure will cause the capsule to deflect and create motion in the force bar. The top end of the force bar is then connected to a position detector, which via an electronic system will produce a 4 - 20 ma signal that is proportional to the force bar movement.
- 22. TYPES OF DRAWINGS IN INSTRUMENTATION • Plot Plan It is often helpful to look at the plot plan to get an overview of how a plant is physically organized. By examining the plot plan, it is possible to get an idea of where a piece of equipment is located in the plant as well as understanding the physical layout of the plant and the distances between pieces of equipment can often provide insight into the expected transport delay associated with material or product flow between pieces of equipment.
- 23. Process flow diagram Drawing that shows the general process flow between major pieces of equipment of a plant and the expected operating conditions at the target production rate. Since the purpose of the process flow diagram is to document the basic process design and assumptions, such as the operating pressure and temperature of a reactor at normal production rates, it does not include many details concerning piping and field instrumentation. In some cases, however, the process engineer may include in the PFD an overview of key measurements and control loops that are needed to achieve and maintain the design operating conditions. The process engineer then documents the design in a process flow diagram (PFD). The process flow diagram typically identifies the major pieces of equipment, the flow paths through the process, and the design operating conditions—that is, the flow rates, pressures, and temperatures at normal operating conditions and the target production rate.
- 25. Piping and Instrumentation Diagram The instrumentation department of an engineering firm is responsible for the selection of field devices that best matches the process design requirements. This includes the selection of the transmitters that fit the operating conditions, the type and sizing of valves, and other implementation details. An instrumentation engineer selects field devices that are designed to work under the normal operating conditions specified in the process flow diagram. Tag numbers are assigned to the field devices so they may be easily identified when ordering and shipping, as well as installing in the plant. Tag number Unique identifier that is assigned to a field device. The decisions that are made concerning field instrumentation, the assignment of device tags, and piping details are documented using a piping and instrumentation diagram (P&ID). A piping and instrumentation diagram is similar to a process flow diagram in that it includes an illustration of the major equipment. However, the P&ID includes much more detail about the piping associated with the process, to include manually operated blocking valves. • Closed loop control - Automatic regulation of a process inputs based on a measurement of process output. • Manual control – Plant operator adjustment of a process input.
- 26. Piping and Instrumentation Diagram P & ID Drawing that shows the instrumentation and piping details for plant equipment. The P&ID acts as a directory to all field instrumentation and control that will be installed on a process and thus is a key document to the control engineer. Since the instrument tag (tag number) assigned to field devices is shown on this document, the instrument tag associated with, for example, a measurement device or actuator of interest may be quickly found.
- 27. Loop Diagram The piping and instrumentation diagram identifies, but does not describe in detail, the field instrumentation that is used by the process control system, as well as field devices such as manual blocking valves that are needed in plant operations. Many of the installation details associated with field instrumentation, such as the field devices, measurement elements, wiring, junction block termination, and other installation details are documented using a loop diagram. ISA has defined the ISA-5.4 standard for Instrument Loop Diagrams. This standard does not mandate the style and content of instrument loop diagrams, but rather it is a consensus concerning their generation. A loop diagram, also commonly known as a loop sheet, is created for each field device that has been given a unique tag number. The loop diagrams for a process area are normally bound into a book and are used to install and support checkout of newly installed field devices. After plant commissioning, the loop Diagrams provide the wiring details that a maintenance person needs to find and troubleshoot wiring to the control system. • Loop Diagram – Drawing that shows field device installation details including wiring and the junction box (if one is used) that connects the field device to the control system. The loop diagram is a critical piece of documentation associated with the installation of the control system. As has been mentioned, the engineering company that is designing a process normally has standards that they follow in the creation of a loop diagram. These standards may be documented by the creation of a master template that illustrates how field devices and nomenclature are used on the drawing.
- 28. LOOP DIAGRAM
- 30. Tagging Conventions The ISA-5.1 standard defines a standard tag number convention for valves, transmitters, other field devices, and calculation and control functions. In addition, the standard defines expanded tag number conventions that may better meet the documentation requirements of large installations that consist of many process areas or multiple measurements that serve similar functions. TYPICAL TAG NUMBER- TIC 103 - Instrument Identification or Tag Number T 103 - Loop Identifier 103 - Loop Number TIC - Function Identification T - First-letter IC - Succeeding-Letters EXPANDED TAG NUMBER- 10-PAH-5A - Tag Number 10 - Optional Prefix A - Optional Suffix
- 32. Line and Function Symbols Different types of lines are used in process flow diagrams, piping and instrumentation diagrams, and loop diagrams to indicate the type of connection between field devices and the control system. The ISA-5.1 standard defines the instrument line symbols that are commonly used in control system documentation. As illustrated in Figure 7-10, a solid line is used to represent a physical connection to the process. Two slashes shown as points along a line are used to indicate a pneumatic signal. One of the most common ways to indicate an electric signal is a dashed line as defined in ISA-5.1. Communication links between devices and functions of a distributed control system are indicated by small bubbles along the line as in figure.. 1. Instrument supply- 2. Pneumatic Signal- 3. Electric Variable or Binary - - - - - - - - - - - - - - - - - - 4. Communication Link
- 33. • The previous examples of the process flow diagram, piping and instrumentation diagram, and loop diagram contained one or more circle symbols. In these drawings, a circle is used to indicate a discrete instrumentation or control function. A horizontal line drawn through the middle of the circle indicates the function may be accessed by the plant operator. There are many functions, such as those performed by an I/P transducer or valve positioner, that are typically not directly accessible by the operator. Also, some field devices for measurement and actuation may only be accessed by control or calculation functions in the control system and thus would not be shown in the documentation as being directly accessed by the operator. However, the associated control or calculation function that is accessed by the operator would include a horizontal line. one of the conventions advocated in ISA-5.1 is to include a square around the circle if the associated function is accessed by an operator through a video display of a distributed control system (DCS). However, in practice, this convention is often not followed.
- 35. TAIWAC GAUGES • DRY GAUGE SPECIFICATION-