Pressure.docx

Pressure Pressure (symbol: p or P) is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled gage pressure) is the pressure relative to the ambient pressure. Various units are used to express pressure. Some of these derive from a unit of force divided by a unit of area; the SI unit of pressure, the pascal (Pa), for example, is one newton per square metre; similarly, the pound-force per square inch (psi) is the traditional unit of pressure in the imperial and US customary systems. Pressure may also be expressed in terms of standard atmospheric pressure; the atmosphere (atm) is equal to this pressure, and the torr is defined as 1⁄760 of this. Manometric units such as the centimetre of water, millimetre of mercury, and inch of mercury are used to express pressures in terms of the height of column of a particular fluid in a manometer. Pressure gage in steam boiler Steam boiler use pressure gage to measure pressure at some places either for water fluid or steam fluid. Pressure gauge is useful for the operator or personnel who working in steam boiler area to see working pressure and try to control the pressure under MAWP (Maximum Allowable Working Pressure). The pressure gauge is place in area which easy to see or visible for the operator. Whenever liquids or gases are moved, stored or processed, values such as (gauge or absolute) pres-sure, differential pressure, level and flow must be measured precisely and reliably. Pressure measurement devices are therefore subject to high requirements. Steam is involved in different industrial applications, for example in the chemical and pharmaceutical industry, food industry for sterilisation, for heating, or to drive generator turbines in power plants. All these applications use steam boilers where filling levels have to be measured. Level metering is the basis for control and process control in many industries using process technology. The amount of energy required to generate steam with a certain pressure in a closed vessel can be calculated from the enthalpy of water plus the vaporisation heat at the respective pressure. The enthalpy is the difference between boiler water temperature and boiling temperature, multiplied with the specific heat and density. Since steam boilers are under pressure, differential pressure transmitters (e.g. ABB type 266MST) must be used for level measurement in order to compensate for the vessel’s overpressure.

To make sure during this differential pressure management that hot steam does not condensate at the colder offset side of the transmitter in an undefined way, leading to a changing water head, a condensate offset vessel is usually added to the measurement system. Due to the condensate vessel, the water seal/water head on the offset side of the transmitter always has a constant level. This water seal also serves as a reliable temperature barrier which protects the transmitter against the high steam temperature. In the condensate vessel, the water vaporises and accumulates up to a defined, constant level. Excessive condensate will flow back to the vessel. To make sure a transmitter can be de-installed for inspection without any problems, a triple valve block is installed directly before the transmitter for differential pressure measurement. This allows the pressure, which may vary between the individual connecting pipes, to achieve identical pressure values through the counterbalance valve while the process is reliably shut off by means of the other two valves. At the same time, the transmitter can be removed from the measuring point without interrupting the process. When measuring steam, a quintuple valve block combination, i. e. a triple valve block with two separate exhaust valves, is preferably connected in front of the transmitter. In contrast to a compact quintuple valve block, the sepa-ration of the exhaust valves prevents hot steam from getting in contact with the transmitter when exhausting the pipes, thus causing damage to the transmitter. ABB pressure and differential pressure transmitters are used for standard applications as well as for demanding applications in automation environments, for monitoring and for safety-related measurements with SIL2 and SIL3 TÜV certification. Hart 4…20mA, Profibus PA, Foundation Fieldbus or Modbus communication are available. The measuring error of the transmitters is 0.025, 0.04, 0.06, 0.075 or 0.1 % of rate, depending on the requirement and type. Measurement parameters can easily and individually be entered via buttons and LCD (optionally also using the TTG (through the glass) technology, without having to open the cover). It is also possible to correct or configure the zero point and span without any specified pressure. Examples for diagnostic functions are self test, limit value monitoring with event counter, drag indicator functions and simulation functions.

Level measurement in a vacuum In order to measure the level in a vessel under vacuum, the entire measurement system must be able to operate under vacuum conditions. Suitable measurement systems include: Hydrostatic measurement with instilling into the condensate/offset vessel Hydrostatic measurement with instilling into a T-piece below the condensate vessel Hydrostatic measurement with a vacuum-tight pressure sensor The measurement with instilling into the condensate/offset vessel is a proven, hydrostatic measurement system using a differential pressure transmitter. The measurement system is simple, but it relies on high mechanical requirements since the valves also have to be vacuum-tight. Furthermore, liquid has to instill into the condensate vessel since the required water column standing before the transmitter and in the condensate vessel vaporises due to the vacuum. This loss of water must be offset. However, the instilling of liquid directly into the condensate vessel causes unrest/pulsations as the instilled water abruptly vaporises. A permanent feed is therefore required to ensure correct measurements. Completely desalinated water must be available at the tapping point, the feed of which should be monitored. Otherwise the measurement value would slowly start to drift away if instilling should fail. In contrast to the first hydrostatic measurement system, water is not instilled into the condensate vessel, but into the impulse pipe below the condensate vessel using a T-piece in case of the second type of hydrostatic measurement. This prevents the pulsation problems which occur when instilling directly. Otherwise, the other properties are identical to the first type of hydrostatic measurement. Instrument isolators/pressure sensors When measuring pressures and differential pressures in aggressive media, special isolating diaphragms/pressure sensors, with diaphragm material adapted to the media to be monitored, are used to protect the measurement cell. Integral capillary tubes filled with silicon oil are used for connection to the measurement cell of the differential pressure transmitter. Such pressure sensors can also be used to implement a stable, hydrostatic measurement system in the vacuum range. The benefit is that the complete mechanism and the monitoring of the previously described measurement systems are not required; on the other hand this assembly with increasing length involves increasing temperature dependence of the capillary tube between transmitter and pressure sensor. It is therefore recommended to

make sure that no significant variations of environmental temperature can occur. Furthermore, the pressure sensors should be in a vacuum-resistant version. While this is not really a problem, this should not be forgotten. The amount of instrumentation required as such is comparably low. Only large flanges (standard: DN80/DN50) must be made available on the vessel side. To ensure a secure measurement especially for applications in- volving pressure sensors in the high vacuum range, the transmitter should always be installed on the level of the lower pressure sensor connecting port, or preferably below the lower connecting port. This is recommended to make sure that in case of a higher level mounting the vacuum generated by the oil filling in the capillary tube does not add up to the vacuum in the vessel, which may lead to the steam pressure of the oil filling being undercut. In this case, the oil would outgas, generating gas bubbles and causing the measurement to drift away. All three hydrostatic measurements have a small disadvantage: they detect the pressure of the water column above the transmitter which with reference to the real level depends on the density of the medium. Pressure measurement Pressure measurement is the analysis of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure and display pressure in an integral unit are called pressure gauges or vacuum gauges. A manometer (not to be confused with nanometer) is a good example, as it uses a column of liquid to both measure and indicate pressure. Likewise the widely used Bourdon gauge is a mechanical device, which both measures and indicates and is probably the best known type of gauge. Everyday pressure measurements, such as for vehicle tire pressure, are usually made relative to ambient air pressure. In other cases measurements are made relative to a vacuum or to some other specific reference. When distinguishing between these zero references, the following terms are used: Absolute pressure is zero-referenced against a perfect vacuum, using an absolute scale, so it is equal to gauge pressure plus atmospheric pressure. Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted.

To distinguish a negative pressure, the value may be appended with the word "vacuum" or the gauge may be labeled a "vacuum gauge". These are further divided into two subcategories: high and low vacuum (and sometimes ultra-high vacuum). The applicable pressure ranges of many of the techniques used to measure vacuums have an overlap. Hence, by combining several different types of gauge, it is possible to measure system pressure continuously from 10 mbar down to 10−11 mbar. Differential pressure is the difference in pressure between two points. The zero reference in use is usually implied by context, and these words are added only when clarification is needed. Tire pressure and blood pressure are gauge pressures by convention, while atmospheric pressures, deep vacuum pressures, and altimeter pressures must be absolute. For most working fluids where a fluid exists in a closed system, gauge pressure measurement prevails. Pressure instruments connected to the system will indicate pressures relative to the current atmospheric pressure. The situation changes when extreme vacuum pressures are measured, then absolute pressures are typically used instead. Differential pressures are commonly used in industrial process systems. Differential pressure gauges have two inlet ports, each connected to one of the volumes whose pressure is to be monitored. In effect, such a gauge performs the mathematical operation of subtraction through mechanical means, obviating the need for an operator or control system to watch two separate gauges and determine the difference in readings. Moderate vacuum pressure readings can be ambiguous without the proper context, as they may represent absolute pressure or gauge pressure without a negative sign. Thus a vacuum of 26 inHg gauge is equivalent to an absolute pressure of 30 inHg (typical atmospheric pressure) − 26 inHg = 4 inHg. Atmospheric pressure is typically about 100 kPa at sea level, but is variable with altitude and weather. If the absolute pressure of a fluid stays constant, the gauge pressure of the same fluid will vary as atmospheric pressure changes. For example, when a car drives up a mountain, the (gauge) tire pressure goes up because

atmospheric pressure goes down. The absolute pressure in the tire is essentially unchanged. Using atmospheric pressure as reference is usually signified by a "g" for gauge after the pressure unit, e.g. 70 psig, which means that the pressure measured is the total pressure minus atmospheric pressure. There are two types of gauge reference pressure: vented gauge (vg) and sealed gauge (sg). A vented-gauge pressure transmitter, for example, allows the outside air pressure to be exposed to the negative side of the pressure-sensing diaphragm, through a vented cable or a hole on the side of the device, so that it always measures the pressure referred to ambient barometric pressure. Thus a vented-gauge reference pressure sensor should always read zero pressure when the process pressure connection is held open to the air. A sealed gauge reference is very similar, except that atmospheric pressure is sealed on the negative side of the diaphragm. This is usually adopted on high pressure ranges, such as hydraulics, where atmospheric pressure changes will have a negligible effect on the accuracy of the reading, so venting is not necessary. This also allows some manufacturers to provide secondary pressure containment as an extra precaution for pressure equipment safety if the burst pressure of the primary pressure sensing diaphragm is exceeded. There is another way of creating a sealed gauge reference, and this is to seal a high vacuum on the reverse side of the sensing diaphragm. Then the output signal is offset, so the pressure sensor reads close to zero when measuring atmospheric pressure. A sealed gauge reference pressure transducer will never read exactly zero because atmospheric pressure is always changing and the reference in this case is fixed at 1 bar. To produce an absolute pressure sensor, the manufacturer seals a high vacuum behind the sensing diaphragm. If the process-pressure connection of an absolutepressure transmitter is open to the air, it will read the actual barometric pressure.

A vacuum gauge is a pressure gauge used to measure pressures lower than the ambient atmospheric pressure, which is set as the zero point, in negative values (e.g.: −15 psig or −760 mmHg equals total vacuum). Most gauges measure pressure relative to atmospheric pressure as the zero point, so this form of reading is simply referred to as "gauge pressure". However, anything greater than total vacuum is technically a form of pressure. For very accurate readings, especially at very low pressures, a gauge that uses total vacuum as the zero point may be used, giving pressure readings in an absolute scale.  Bourdon pressure gage The measuring element is a curved tube with a circular, spiral or coiled shape, commonly called a bourdon tube. This tube moves outward when the pressure inside the tube is higher than the external pressure, and inward when the internal pressure is lower. This motion is proportional to the pressure to be measured, and it is coupled to the pointer mechanism. Bourdon tube pressure gauges are used for the measurement of relative pressures from 0.6 ... 7,000 bar. They are classified as mechanical pressure measuring instruments, and thus operate without any electrical power. Bourdon tubes are radially formed tubes with an oval cross-section. The pressure of the measuring medium acts on the inside of the tube and produces a motion in the non-clamped end of the tube. This motion is the measure of the pressure and is indicated via the movement. The C-shaped Bourdon tubes, formed into an angle of approx. 250°, can be used for pressures up to 60 bar. For higher pressures, Bourdon tubes with several superimposed windings of the same angular diameter (helical tubes) or with a spiral coil in the one plane (spiral tubes) are used. Bourdon tube pressure gauges are developed, qualified and manufactured to the EN 837-1 standard by WIKA themselves. We produce Bourdon tube pressure gauges with various common worldwide measuring ranges, process connections, approvals and nominal sizes. For critical applications there are Bourdon tube pressure gauges with liquid filling. Through the case liquid, a precise readability is ensured, even with

high dynamic pressure loads. It damps the moving parts within the case and thus prevents damage and increased wear. Safety pressure gauges complete the wide portfolio. In addition to a solid baffle wall between the dial and the measuring system, these Bourdon tube pressure gauges feature a blow-out back. Thus, any persons standing in front of the pressure gauge are protected. Equally important for a reliable measured value display is the movement. Our “Swiss movement” mechanisms are not only as precise as a Swiss watch movement, but they are also especially robust and durable.

 Bellow pressure gage The bellows are used in two forms. In one arrangement, pressure is applied to one side of the bellows and the resulting deflection is counter balanced by a spring. This arrangement indicates the gauge pressure. In the second arrangement, the differential pressure is also indicated. In this device, one pressure is applied to the inside of one sealed bellow while the other pressure is applied to the inside of another sealed bellow. By suitable linkage and calibration of the scale, the pressure difference is indicated by a pointer on the scale.

 Spiral pressure gage A pressure sensor is a device for pressure measurement of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually acts as a transducer; it generates a signal as a function of the pressure imposed.