Liquid pressure gauges, operating principle, advantages. Design of liquid pressure gauges How an open liquid pressure gauge works
In liquid pressure gauges, the measured pressure is balanced by the pressure of the liquid column.
The simplest liquid pressure gauges consist of a U-shaped glass tube and a straight scale with even divisions.
The smallest scale division is 1 mm. The scale is usually double-sided with a zero mark in the middle. Both ends of the tube are filled with liquid to the zero mark.
Operating principle
When pressure is applied to one end of the tube, liquid flows and a difference in liquid levels is visible through the glass. The level difference, expressed in millimeters, gives the measured pressure.
If mercury is poured into the tube, the pressure will be expressed in millimeters mercury. pressure manometer pressure meter
When the tube is filled with water, the pressure will be measured in millimeters of water.
If the tube is filled with other liquids, it is necessary to recalculate based on the specific gravity of the liquid.
So, for example, to convert to millimeters of water column, you need to multiply the readings of a pressure gauge with a given liquid by specific gravity liquid, when converted to millimeters of mercury - multiply by the specific gravity of the liquid and divide by the specific gravity of mercury 13.6.
The difference in the diameters of the left and right parts of the tube does not affect the measurement result. It is also not necessary to fill the tube with liquid to a level that exactly coincides with the zero mark on the scale, since when reading the readings, only the difference in levels by the number of scale divisions is taken into account.
A liquid thermometer is a device for measuring the temperature of technological processes using a liquid that reacts to changes in temperature. Liquid thermometers are well known to everyone in everyday life: for measuring room temperature or human body temperature.
Liquid thermometers consist of five main parts, these are: the thermometer ball, liquid, capillary tube, bypass chamber, and scale.
The thermometer ball is the part where the liquid is placed. The liquid reacts to changes in temperature by rising or falling through the capillary tube. A capillary tube is a narrow cylinder through which liquid moves. Often the capillary tube is equipped with a bypass chamber, which is a cavity into which excess liquid flows. If there is no bypass chamber, once the capillary tube is filled, enough pressure will build up to destroy the tube if the temperature continues to rise. The scale is part liquid thermometer, with which readings are taken. The scale is calibrated in degrees. The scale can be fixed to the capillary tube, or it can be movable. The moving scale makes it possible to adjust it.
Working principle of a liquid thermometer
The operating principle of liquid thermometers is based on the ability of liquids to compress and expand. When a liquid is heated, it usually expands; The liquid in the thermometer bulb expands and moves up the capillary tube, thereby indicating an increase in temperature. Conversely, when a liquid cools, it usually contracts; the liquid in the capillary tube of a liquid thermometer decreases and thereby indicates a decrease in temperature. In the case when there is a change in the measured temperature of a substance, heat transfer occurs: first from the substance whose temperature is measured to the thermometer ball, and then from the ball to the liquid. The liquid reacts to changes in temperature by moving up or down the capillary tube.
The type of liquid used in a liquid thermometer depends on the range of temperatures the thermometer measures.
Mercury, -39-600 °C (-38-1100 °F);
Mercury alloys, -60-120 °C (-76-250 °F);
Alcohol, -80-100 °C (-112-212 °F).
Partial Immersion Liquid Thermometers
Many liquid thermometers are designed to hang on a wall, with the entire surface of the thermometer in contact with the substance whose temperature is being measured. However, some types of industrial and laboratory liquid thermometers are designed and calibrated to be immersed in liquid.
Of the thermometers used in this way, the most widely used are partial immersion thermometers. To obtain an accurate reading with a partial immersion thermometer, immerse the bulb and capillary tube only to this line.
Partial immersion thermometers are immersed to a mark to compensate for changes in ambient temperature that may affect the liquid inside the capillary tube. If changes in ambient temperature (changes in the temperature of the air around the thermometer) are likely, they can cause the liquid inside the capillary tube to expand or contract. As a result, the readings will be affected not only by the temperature of the substance that is being measured, but also by the temperature of the surrounding air. Immersing the capillary tube to the marked line removes the effect of ambient temperature on the accuracy of the readings.
In conditions industrial production It is often necessary to measure the temperatures of substances passing through pipes or contained in containers. Measuring temperature under these conditions creates two problems for instrument technicians: how to measure the temperature of a substance if there is no direct access to this substance or liquid, and how to remove a liquid thermometer for inspection, verification or replacement without stopping the process. Both of these problems are eliminated if measuring channels are used to insert thermometers.
The measuring channel for inserting the thermometer is a pipe-shaped channel that is closed at one end and open at the other. The measuring channel is designed to accommodate the ball of a liquid thermometer and thus protect it from substances that can cause corrosion, toxic substances, or under high pressure. When measuring channels are used to insert thermometers, heat exchange occurs in the form of indirect contact (through the measuring channel) of the substance whose temperature is measured and the thermometer ball. The measuring channels are a seal for high blood pressure and prevent the liquid, the temperature by which is measured, from escaping.
Measuring channels are made standard sizes, so they can be used with various types thermometers. When the thermometer is installed in the measuring channel, its ball is inserted into the channel, and a nut is screwed on top of the thermometer to secure the thermometer.
The pressure gauge is a compact mechanical device for measuring pressure. Depending on the modification, it can work with air, gas, steam or liquid. There are many types of pressure gauges, based on the principle of taking pressure readings in the medium being measured, each of which has its own application.
Scope of use
Pressure gauges are one of the most common instruments that can be found in various systems:
- Heating boilers.
- Gas pipelines.
- Water pipelines.
- Compressors.
- Autoclaves.
- Cylinders.
- Balloon air rifles, etc.
Externally, the pressure gauge resembles a low cylinder various diameters, most often 50 mm, which consists of a metal body with glass lid. Through the glass part you can see a scale with marks in pressure units (Bar or Pa). A tube with external thread for screwing into the hole of the system in which it is necessary to measure pressure.
When pressure is injected into the medium being measured, the gas or liquid through the tube presses the internal mechanism of the pressure gauge, which leads to a deflection of the angle of the arrow that points to the scale. The higher the pressure created, the more the needle deflects. The number on the scale where the pointer stops will correspond to the pressure in the system being measured.
Pressure that a pressure gauge can measure
Pressure gauges are universal mechanisms that can be used to measure various values:
- Excess pressure.
- Vacuum pressure.
- Pressure differences.
- Atmospheric pressure.
The use of these devices allows you to control various technological processes and prevent emergency situations. Pressure gauges intended for use in special conditions may have additional housing modifications. This may be explosion protection, resistance to corrosion or increased vibration.
Types of pressure gauges
Pressure gauges are used in many systems where there is pressure, which must be at a clearly defined level. The use of the device allows you to monitor it, since insufficient or excessive exposure can harm various technological processes. In addition, excess pressure causes rupture of containers and pipes. In this regard, several types of pressure gauges designed for specific operating conditions have been created.
They are:
- Exemplary.
- General technical.
- Electric contact.
- Special.
- Self-recording.
- Ship's.
- Railway.
Exemplary pressure gauge intended for verification of other similar measuring equipment. Such devices determine the level of excess pressure in various environments. Such devices are equipped with a particularly precise mechanism that gives minimal error. Their accuracy class ranges from 0.05 to 0.2.
General technical used in shared environments that do not freeze into ice. Such devices have an accuracy class from 1.0 to 2.5. They are resistant to vibration, so they can be installed on transport and heating systems.
Electric contact are designed specifically for monitoring and warning of reaching the upper limit of a dangerous load that can destroy the system. Such devices are used with various media such as liquids, gases and vapors. This equipment has a built-in electrical circuit control mechanism. When excess pressure appears, the pressure gauge gives a signal or mechanically turns off the supply equipment that pumps pressure. Also, electric contact pressure gauges may include a special valve that relieves pressure to a safe level. Such devices prevent accidents and explosions in boiler rooms.
Special Pressure gauges are designed to work with a specific gas. Such devices usually have colored cases rather than the classic black ones. The color corresponds to the gas with which this device can work. Also, special markings are used on the scale. For example, pressure gauges for measuring ammonia pressure, which are usually installed in industrial refrigeration units, painted in yellow. Such equipment has an accuracy class from 1.0 to 2.5.
Self-recording are used in areas where it is required not only to visually monitor the system pressure, but also to record indicators. They write a chart that can be used to view pressure dynamics over any period of time. Such devices can be found in laboratories, as well as at thermal power plants, canneries and other food enterprises.
Ship's include a wide model range pressure gauges that have a weatherproof housing. They can work with liquid, gas or steam. Their names can be found on street gas distributors.
Railway pressure gauges are designed to monitor excess pressure in mechanisms that serve electric rail vehicles. In particular, they are used on hydraulic systems, moving the rails when extending the boom. Such devices have increased resistance to vibration. Not only do they withstand shock, but the indicator on the scale does not react to mechanical stress on the body, accurately displaying the pressure level in the system.
Types of pressure gauges based on the mechanism for taking readings of pressure in the medium
Pressure gauges also differ in the internal mechanism that results in taking pressure readings in the system to which they are connected. Depending on the device they are:
- Liquid.
- Spring.
- Membrane.
- Electric contact.
- Differential.
Liquid The pressure gauge is designed to measure the pressure of a liquid column. Such devices operate on the physical principle of communicating vessels. Most devices have a visible level working fluid, from which they take readings. These devices are one of the rarely used. Due to contact with liquid, they inner part gets dirty, so transparency is gradually lost, and it becomes difficult to visually determine the readings. Liquid pressure gauges were among the very first to be invented, but are still found.
Spring pressure gauges are the most common. They have simple design which is suitable for repair. Their measurement limits usually range from 0.1 to 4000 Bar. The sensitive element of such a mechanism itself is an oval tube, which contracts under pressure. The force pressing on the tube is transmitted through a special mechanism to a pointer, which rotates at a certain angle, pointing to a scale with markings.
Membrane The pressure gauge operates on the physical principle of pneumatic compensation. Inside the device there is a special membrane, the level of deflection of which depends on the effect of the pressure created. Typically, two membranes are soldered together to form a box. As the volume of the box changes, the sensitive mechanism deflects the arrow.
Electric contact Pressure gauges can be found in systems that automatically monitor pressure and adjust it or signal when a critical level has been reached. The device has two arrows that can be moved. One is installed on minimum pressure, and the second to the maximum. The electrical circuit contacts are mounted inside the device. When the pressure reaches one of the critical levels, the electrical circuit is closed. As a result, a signal is generated on the control panel or triggered automatic mechanism for emergency reset.
Differential pressure gauges are one of the most complex mechanisms. They work on the principle of measuring deformation inside special blocks. These pressure gauge elements are pressure sensitive. As the block deforms special mechanism transmits changes to the arrow pointing to the scale. The pointer moves until the changes in the system stop and stop at a certain level.
Accuracy class and measurement range
Any pressure gauge has technical passport, which indicates its accuracy class. The indicator has a numerical expression. The lower the number, the more accurate the device. For most instruments, the norm is an accuracy class of 1.0 to 2.5. They are used in cases where a small deviation is not of particular importance. The biggest error is usually caused by the devices that motorists use to measure air pressure in tires. Their class often drops to 4.0. Best class Exemplary pressure gauges have precision, the most advanced of which operate with an error of 0.05.
Each pressure gauge is designed to operate over a specific pressure range. Massive models that are too powerful will not be able to record minimal fluctuations. Very sensitive devices, when exposed to excess, fail or are destroyed, leading to depressurization of the system. In this regard, when choosing a pressure gauge, you should pay attention to this indicator. Typically, you can find models on the market that are capable of recording pressure differences ranging from 0.06 to 1000 mPa. There are also special modifications, so-called draft meters, which are designed to measure vacuum pressure down to a level of -40 kPa.
The operating principle is based on balancing the measured pressure or pressure difference with the pressure of a liquid column. They have a simple design and high measurement accuracy, and are widely used as laboratory and calibration instruments. Liquid pressure gauges are divided into: U-shaped, bell and ring.
U-shaped. The principle of operation is based on the law of communicating vessels. They come in two-pipe (1) and single-pipe cups (2).
1) represent glass tube 1, mounted on a board 3 with a scale and filled with sealing fluid 2. The difference in levels in the elbows is proportional to the measured pressure difference. “-” 1. series of errors: due to inaccuracy in measuring the position of the meniscus, changes in T surrounding. environment, capillarity phenomena (eliminates by introducing corrections). 2. the need for two readings, which leads to an increase in error.
2) rep. is a modification of two-pipe ones, but one elbow is replaced with a wide vessel (cup). Under the influence of excess pressure, the liquid level in the vessel decreases and in the tube increases.
Float U-shaped Differential pressure gauges are similar in principle to cup gauges, but to measure pressure they use the movement of a float placed in a cup when the liquid level changes. By means of a transmission device, the movement of the float is converted into the movement of the indicating arrow. “+” wide measurement range. Operating principle liquid pressure gauges are based on Pascal's law - the measured pressure is balanced by the weight of the column of working fluid: P = ρgh. Consist of a reservoir and a capillary. The working fluids used are distilled water, mercury, ethanol. They are used for measuring small excess pressures and vacuum, barometric pressure. They are simple in design, but there is no remote data transmission.
Sometimes, to increase sensitivity, the capillary is placed at a certain angle to the horizon. Then: P = ρgL Sinα.
IN deformation pressure gauges are used to counter the elastic deformation of the sensing element (SE) or the force developed by it. There are three main forms of SE that have become widespread in measurement practice: tubular springs, bellows and membranes.
Tubular spring(gauge spring, Bourdon tube) - an elastic metal tube, one of the ends of which is sealed and has the ability to move, and the other is rigidly fixed. Tubular springs are mainly used to convert the measured pressure applied to interior space spring, into proportional movement of its free end.
The most common is a single-turn tubular spring, which is a 270° bent tube with an oval or elliptical cross section. Under the influence of the supplied excess pressure, the tube unwinds, and under the influence of vacuum it twists. This direction of movement of the tube is explained by the fact that, under the influence of internal excess pressure, the minor axis of the ellipse increases, while the length of the tube remains constant.
The main disadvantage of the springs considered is their small angle of rotation, which requires the use of transmission mechanisms. With their help, moving the free end of a tubular spring by several degrees or millimeters is converted into an angular movement of the arrow by 270 - 300°.
The advantage is a static characteristic close to linear. The main application is indicating instruments. Measurement ranges of pressure gauges from 0 to 10 3 MPa; vacuum gauges - from 0.1 to 0 MPa. Instrument accuracy classes: from 0.15 (exemplary) to 4.
Tubular springs are made of brass, bronze, stainless steel.
Bellows. Bellows is a thin-walled metal cup with transverse corrugations. The bottom of the glass moves under pressure or force.
Within the linearity of the static characteristics of the bellows, the ratio of the force acting on it to the deformation caused by it remains constant. and is called the rigidity of the bellows. Bellows are made of bronze various brands, carbon steel, stainless steel, aluminum alloys, etc. Bellows with a diameter of 8–10 to 80–100 mm and a wall thickness of 0.1–0.3 mm are mass-produced.
Membranes. There are elastic and elastic membranes. An elastic membrane is a flexible round flat or corrugated plate that can bend under pressure.
The static characteristic of flat membranes changes nonlinearly with increasing pressure, therefore a small part of the possible stroke is used as the working area. Corrugated membranes can be used for larger deflections than flat ones, since they have significantly less nonlinearity of the characteristic. Membranes are made from various grades of steel: bronze, brass, etc.
PRECHAMBER BURNER
Prechamber burner - a device consisting of a gas manifold with holes for gas outlet, a monoblock with channels and a ceramic refractory prechamber, placed above the manifold, in which mixing of gas with air and combustion occurs gas-air mixture. The prechamber burner is designed to burn natural gas in the furnaces of sectional cast-iron boilers, dryers and other thermal installations operating with a vacuum of 10-30 Pa. Prechamber burners are located on the firebox floor, thereby creating good conditions for uniform distribution of heat flows along the length of the firebox. Prechamber burners can operate at low and medium gas pressure. The prechamber burner consists of a gas manifold ( steel pipe) with one row of holes for gas outlet. Depending on the heat output, the burner can have 1, 2 or 3 collectors. A ceramic monoblock is installed above the gas manifold on a steel frame, forming a series of channels (mixers). Each gas outlet has its own ceramic mixer. Gas streams flowing from the manifold holes eject 50-70% of the air required for combustion, the rest of the air comes due to rarefaction in the firebox. As a result of ejection, mixture formation is intensified. The mixture is heated in the channels, and upon exiting it begins to burn. From the channels, the burning mixture enters the prechamber, in which 90-95% of the gas is burned. The prechamber is made from fireclay bricks; it looks like a slit. Gas combustion occurs in the furnace. The height of the torch is 0.6-0.9 m, the coefficient of excess air is 1.1...1.15.
Compensators are designed to mitigate (compensate) temperature expansion of gas pipelines, to avoid pipe rupture, for ease of installation and dismantling of fittings (flange, valves).
A gas pipeline 1 km long with an average diameter when heated by 1 °C lengthens by 12 mm.
Compensators are:
· Lens;
· U-shaped;
· Lyre-shaped.
Lens compensatorhas a wavy surface that changes its length depending on the temperature of the gas pipeline. The lens compensator is made from stamped half-lenses by welding.
To reduce hydraulic resistance and prevent clogging, a guide pipe is installed inside the compensator, welded to inner surface compensator on the gas inlet side.
The lower part of the half lenses is filled with bitumen to prevent water accumulation.
When installing the compensator in winter time, it needs to be stretched a little, and in the summer, on the contrary, compressed with coupling nuts.
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U-shapedLyre-shaped
compensator.compensator.
Changes in the temperature of the environment surrounding the gas pipeline cause changes in the length of the gas pipeline. For a straight section of a steel gas pipeline 100 m long, the lengthening or shortening with a temperature change of 1° is about 1.2 mm. Therefore, on all gas pipelines after the valves, counting along the gas flow, lens compensators must be installed (Fig. 3). In addition, during operation, the presence of a lens compensator facilitates the installation and dismantling of valves.
When designing and constructing gas pipelines, they strive to reduce the number of installed compensators by maximizing the use of self-compensation by changing the direction of the route both in plan and in profile.
Rice. 3. Lens compensator 1 - flange; 2-pipe; 3 - shirt; 4 - half lens; 5 - paw; 6 - rib; 7 - traction; 8 - nut
Operating principle of a liquid pressure gauge
In the initial position, the water in the tubes will be at the same level. If pressure is applied to the rubber film, the liquid level in one elbow of the pressure gauge will decrease, and in the other, therefore, it will increase.
This is shown in the picture above. We press on the film with our finger.
When we press on the film, the pressure of the air in the box increases. Pressure is transmitted through the tube and reaches the liquid, displacing it. As the level in this elbow decreases, the fluid level in the other elbow of the tube will increase.
Based on the difference in liquid levels, it will be possible to judge the difference atmospheric pressure and the pressure that is exerted on the film.
The following figure shows how to use a liquid pressure gauge to measure the pressure in a liquid at various depths.
Diaphragm pressure gauge
In a membrane pressure gauge, the elastic element is a membrane, which is a corrugated metal plate. The deflection of the plate under liquid pressure is transmitted through a transmission mechanism to the instrument pointer sliding along the scale. Membrane instruments are used to measure pressure up to 2.5 MPa, as well as to measure vacuum. Sometimes devices with an electrical output are used, in which an electrical signal is sent to the output, proportional to the pressure at the input of the pressure gauge.
