Devices that ensure the safe operation of machinery and equipment by limiting speed, pressure, temperature, electrical voltage, mechanical load and other factors that contribute to the occurrence of dangerous situations are called safety devices. They should operate automatically with a minimum inertial delay when the controlled parameter goes beyond the allowable limits.
Safety devices against mechanical overloads are shear pins and pins, spring-cam, friction and gear-friction clutches, centrifugal, pneumatic and electronic regulators.
A pulley, sprocket or gear located on the drive shaft is connected to the drive (driven) shaft with shear pins or pins designed for a certain load. If the latter exceeds the allowable value, then the pin is destroyed and the drive shaft starts to rotate idle. After eliminating the cause of such loads, the sheared stud is replaced with a new one.
Pin diameter, mm, safety clutch, which is usually made of steel 45 or 65 G,
where Mp is the design moment, N*m; R is the distance between the center lines of the transmission shafts and the pin, m; τav is the ultimate shear strength, MPa (for steel 45 and 65 G depending on the type of heat treatment under static load τav = 145...185 MPa; under pulsating load τav = 105...125 MPa; under symmetric alternating load τav = 80...95 MPa); for calculations, it is recommended to take smaller values.
Usually, the calculated moment Mp is taken 10 ... 20% higher than the maximum allowable moment Mp, i.e.
Mp = (1.1...1.2) Mpr.
Friction-type clutches automatically operate in case of exceeding the torque to which they are pre-adjusted. Switch-off condition, for example, for a gear-friction safety clutch:
where Mp is the design torque, N m; Mpred - maximum permissible torque, N * m; a is the angle of inclination of the side surface of the cam (α = 25...35°); β is the friction angle of the side surface of the cam (β = 3...5°); D is the diameter of the circumference of the points of application of the circumferential force to the cams, m; d is the shaft diameter, m; f1 is the coefficient of friction in the keyed connection of the movable bushing (f1 = 0.1...0.15).
Safety clutches for chain and belt drives of agricultural machines with toothed friction washers are standardized.
Diesels, steam and gas turbines, expanders are supplied with speed controllers, mainly of the centrifugal type. To prevent an increase in the crankshaft speed, which is dangerous for the machine and maintenance personnel, by limiting the supply of fuel or steam, a regulator is used.
Limit switches are necessary to prevent equipment breakdowns that occur when moving parts go beyond the established limits, limit the movement of the caliper on metal-cutting machines, for the movement of cargo in the vertical and horizontal planes during the operation of hoisting mechanisms, etc.
Catchers are used on hoisting and transporting machines, in elevators to hold the lifted load in a stationary state, even in the presence of self-braking brake systems, which, if worn or improperly maintained, may lose their performance. There are ratchet, friction, roller, wedge and eccentric catchers.
Safety valves and diaphragms are used to avoid excess steam or gas pressure. Safety valves are by type cargo (lever), spring and special; hull structures - open and closed; placement method - single and double; lifting height - low-lift and full-lift.
Lever valves (Fig. 7.3, a) have a relatively small capacity and, when the pressure exceeds the permissible value, they release working gas or steam into the environment.
Rice. 7.3. Schemes of safety lever (o), spring (b) valves and membranes (c and d):
1 - tension screw; 2 - spring; 3 - valve disc
Therefore, in vessels operating under pressure of toxic or explosive substances, closed-type spring valves are usually installed (Fig. 7.3, b), which discharge the substance into a special pipeline connected to the emergency tank. The lever valve is adjusted to the maximum allowable value on the pressure gauge by changing the mass of the load m or the distance b from the valve axis to the load. The spring valve is adjusted with the help of a tension screw 1, which changes the pressing force of the valve plate 3 with a spring 2. The main disadvantage of safety valves is their inertia, i.e. providing a protective effect only with a gradual increase in pressure in the vessel on which they are installed.
To determine the flow area of safety valves, the theory of gas outflow from the hole is used. Consider the following dependency:
where Q is the throughput of the valve, kg/h; μ is the outflow coefficient (for round holes μ = 0.85); SK is the cross-sectional area of the valve, cm2; p is the pressure under the valve, Pa; g = 9.81 cm/s2 is the free fall acceleration; M is the molecular weight of gases or vapors passing through the valve; k = cpcv is the ratio of heat capacities at constant pressure and constant volume (for water vapor k= 1.3; for air k = 1.4); L is the gas constant, kJ/(kg*K), for steam R = 461.5 kJ/(kg*K); for air R = 287 kJ/(kg*K); T is the absolute temperature of the medium in the protected vessel, K.
Substituting the values of μ, g, R and the average value of k with a known value of Q into the last formula, we can determine the cross-sectional area of the safety valve, cm2,
SK=Q/(216p√M/T).
The number and total cross section of safety valves are found from the expression
ndkhk = kkQk / pk,
where n is the number of valves (on boilers with a steam capacity of ≤ 100 kg/h, it is allowed to install one safety valve, with a boiler steam capacity of more than 100 kg/h, it is equipped with at least two safety valves); dk is the inner diameter of the valve disc, cm (dk = 2.5...12.5 cm); hк - valve lift height, cm; kk - coefficient (for valves with a small lift height at hk≤ 0.05dk kk = 0.0075; for full-lift valves at 0.05dk< hк≤ 0,25dк kк = = 0,015); Qк — производительность котла по пару при максимальной нагрузке, кг/ч; рк — абсолютное давление пара в котле, Па.
To protect vessels and apparatuses from a very rapid and even instantaneous increase in pressure, safety membranes are used (Fig. 7.3, c and d), which, depending on the nature of their destruction during operation, are divided into bursting, shear, breaking, popping, tear-off and special. The most common bursting discs, which collapse under the action of pressure, the value of which exceeds the tensile strength of the membrane material.
Membrane safety devices are made from various materials: cast iron, glass, graphite, aluminum, steel, bronze, etc. The type and material of the membrane is selected taking into account the operating conditions of the vessels and apparatus on which they are installed: pressure, temperature, phase state and aggressiveness of the medium, rate of pressure rise, overpressure release time, etc.
To ensure the operation of the membrane, it is necessary to determine the thickness of the membrane plates depending on the value of the breaking pressure. Throughput, kg/s, of membrane safety devices with increasing pressure in the protected vessel:
Qm=0.06Sworksppr√ M/Tg,
where Swork is the working (flow) section, cm2; ppr - absolute pressure in front of the safety device, Pa; Tg is the absolute temperature of gases or vapors, K.
Required thickness of the working part of the breaking membrane, mm,
Rice. 7.4. Scheme of operation of the low pressure water lock:
a - during normal operation; b - during reverse impact; 1—shut-off valve; 2— gas outlet tube; 3 - funnel; 4— safety tube; 5— body; 6 - control valve
b = ppdplkop(4[σcp]),
where pp is the pressure at which the plate must collapse, Pa; dm is the working diameter of the plate, cm; kon is a scale factor determined empirically (at d/b - 0.32 k - = 10...15); [σav] is the shear strength, MPa.
The thickness of membranes made from brittle materials
b = 1.1rpl√pp/[σout]
where rpl is the radius of the plate, cm; [σiz] is the bending strength of the plate material, Pa.
The safety devices that prevent the explosion of an acetylene generator include water locks (Fig. 7.4), which do not allow the flame to pass into the generator. In the event of a reverse flame strike, which occurs, for example, when a gas burner is ignited, the explosive mixture enters the seal and displaces some of the water through the gas outlet tube 2. Then the end of the tube 4 will receive a connection with the atmosphere, the excess gas will come out, the pressure will return to normal and the device will start working again according to the scheme shown in Figure 7.4, a. To protect electrical installations from an excessive increase in current strength, which can cause a short circuit, fire and human injury, automatic switches and fuses are used.
According to existing safety requirements, no machine, machine tool or equipment can be considered suitable for work if they do not have safety protective devices in case of emergency conditions. The safety devices are based on the principle of shutting down equipment when a controlled parameter (pressure, temperature, force, movement, etc.) goes beyond the permissible limits.
The principal solutions and design of safety devices are varied and depend on the characteristics of the given equipment and the technological process.
Depending on the nature of the occurrence of a hazardous production factor, all safety devices can be divided into four groups:
Fuses against mechanical overloads;
Guards against movement of parts of the machine beyond the established dimensions;
Fuses against excess pressure and temperature;
Fuses from increasing the strength of the electric current above the permissible limits.
To protect against mechanical overloads and prevent accidents associated with this, couplings, load limiters, speed controllers, shear pins and studs are used. Friction clutches are widely used in agricultural machines, in which the pressure between the friction surfaces is created by springs adjusted to transmit the limiting moment. The clutch is activated when the working body is overloaded. Lifting mechanisms are equipped with load limiters, which eliminates dangerous overload during lifting and moving the load.
The simplest type of cargo lever limiter is shown in figure 3.4.
Figure 3.4 - Scheme of operation of the cargo lever load limiter
When the crane is overloaded, the force P from pressing the branches of the cargo rope 1 will exceed the value of the balancing moment from the load 4. The lever 3 will turn, and its right end will press the limit switch lever 5 and open the control circuit of the electric motor. The actuation moment is regulated by moving the load G along the lever. The mechanism is triggered if the following condition is met:
If the pulleys and gears are fixed on the drive shaft with safety pins or studs, then when the permissible loads are exceeded, they are cut off and the pulley (gear) rotates idle on the shaft. To resume the operation of the mechanism, it is necessary to replace the sheared pin (pin).
The speed controllers work on the principle of automatically limiting the supply of fuel to the engine cylinder and prevent a dangerous increase in speed in the event of malfunctions of the fuel supply devices in internal combustion engines.
To protect against the transition of moving parts of the machine beyond the established limits and to prevent the associated breakdowns of machines, limit switches (stops), stops, grips and stops are used. Limit switches are widely used in hoisting mechanisms to limit the path of movement of the load in both horizontal and vertical planes, on metal-cutting machines - to turn off the movement of the caliper, to change the direction of movement of the working body, etc.
To prevent accidents (explosions), mechanisms operating under pressure of vapor, gas or liquid above atmospheric are equipped with safety devices in the form of valves and membranes. All steam boilers, hydraulic and pneumatic systems are equipped with safety valves, which, when the pressure exceeds the established norms, open and thereby relieve excess pressure of steam, liquid or gas (air).
Valve designs are different, but they have the same purpose - to prevent an accident and prevent an accident with maintenance personnel.
If we neglect the masses of the valve levers, then the condition under which the lever valve starts to open will look like:
(3.7)
and for a spring valve:
(3.8)
where a is the coefficient of steam flow through the valve; H is the maximum working pressure in the vessel, Pa; G is the mass of the movable load, kg; T is the force of the spring, N; , – lever arms, m; d is the hole diameter, m.
Relief valves can effectively protect equipment only if the pressure builds up relatively slowly, no greater degree of tightness is required, and there is no corrosive effect of the medium. In conditions where the performance of the safety valve is insufficient, safety diaphragms are used. A thin metal plate is used to manufacture the membrane, the thickness of which must be such that, at a pressure above the permissible limits, it breaks and the rupture wave enters the atmosphere.
Safety devices of this design are installed on some models of foam fire extinguishers. For boiler installations, the membrane is made from sheet asbestos. The design and dimensions of the membrane must be such that, after its rupture, the possibility of a further increase in pressure in the vessel is excluded.
Figure 3.5 shows a diagram of the operation of a low pressure safety water seal installed on acetylene generators to prevent their explosion. During the reverse impact, the explosive mixture enters the shutter, while part of the water is displaced through the gas outlet tube 4. When the end of the tube 5 is found, the gas will begin to escape into the atmosphere. After the excess gas exits through tube 5 and the pressure drops, the valve will start to work normally.
A great danger is the appearance of electric current on parts of the equipment that are not energized under normal conditions. To prevent the current from rising to dangerous values, fuses are used on parts of the equipment. When the current strength exceeds the established limits, the fuse melts and interrupts the electrical circuit. In more critical installations, circuit breakers are used for protection.
Figure 3.5 - Scheme of operation of a low pressure water seal: a) during normal operation; b) with a reverse impact; 1 - body; 2 - funnel; 3 - shut-off valve; 4 - gas outlet tube; 5 - safety tube; 6 - control valve
Safety devices are divided into shut-off and relief. Safety shut-off devices (shut-off valves) - devices that ensure the shutdown of gas supply, in which the speed of bringing the working body to the closed position is not more than 1 second. Safety relief devices (relief valves) are devices that protect gas equipment from an unacceptable increase in gas pressure in the network.
Safety locking devices are installed in front of the gas pressure regulator. Their diaphragm head is connected to the final pressure gas pipeline through a impulse tube. When the final pressure increases above the established norms, the shut-off valves automatically cut off the gas supply to the regulator.
Safety and relief devices used in hydraulic fracturing ensure the release of excess gas in the event of a loose closure of the slam-shut valve or regulator. They are mounted on the outlet pipe of the final pressure gas pipeline, and the outlet fitting is connected to a separate candle. If the technological process of gas consumers provides for the continuous operation of gas burners, then the PZK is not installed, but only the PSK is mounted. In this case, it is necessary to install gas pressure alarms that notify of an increase in gas pressure in excess of the permissible value. If the hydraulic fracturing (GRU) supplies dead-end facilities with gas, then the installation of a shut-off valve is necessary.
Consider the most common types of locking and safety devices.
PZK low (PKI) and high pressure (PKV) control the upper and lower limits of the outlet gas pressure; are issued with conditional passes of 50, 80, 100 and 200 mm. The PKV valve differs from the PKN valve in that it has a smaller active area of the membrane due to the imposition of a steel ring on it.
The schematic diagram of these valves is shown in the figure below.
Safety shut-off valves PKN and PKV
1 - fitting; 2, 4 - levers; 3, 10 - pins; 5 - nut; 6 - plate; 7, 8 - springs; 9 - drummer; 11 - rocker; 12- membrane
In the open position, the valve is held by a lever, which is fixed in the upper position by the pin of the anchor lever; the drummer with the help of a pin rests against the rocker and is held in a vertical position.
The pulse of the final gas pressure through the fitting is fed into the under-membrane space of the valve and exerts counterpressure on the membrane. The upward movement of the membrane is prevented by a spring. If the gas pressure rises above the norm, the diaphragm will move up and the nut will move up accordingly. As a result, the left end of the rocker will move up, and the right end will drop and disengage from the pin. The striker, freed from engagement, will fall and hit the end of the anchor arm. As a result, the lever is disengaged from the pin, and the valve closes the gas passage. If the gas pressure drops below the permissible norm, then the gas pressure in the sub-membrane space of the valve becomes less than the force created by the spring resting on the protrusion of the diaphragm rod. As a result, the diaphragm and the stem with the nut will move down, dragging the end of the rocker arm down. The right end of the rocker arm will rise, disengage from the pin, and cause the striker to fall.
The following setting order is recommended. First, the valve is adjusted to the lower limit of operation. During adjustment, the pressure after the regulator should be maintained slightly above the set limit, then, slowly reducing the pressure, make sure that the valve operates at the set lower limit. When setting the upper limit, it is necessary to keep the pressure slightly above the set lower limit. At the end of the adjustment, you need to increase the pressure to make sure that the valve operates exactly at the specified upper limit of the allowable gas pressure.
Safety shut-off valve PKK-40M.
In the cabinet GRU (figure below), a small-sized PZK PKK-40M is installed. This valve is designed for an inlet pressure of 0.6 MPa.
Scheme of piping cabinet GRU with PZK PKK-40M
a - schematic diagram: 1 - inlet fitting; 2 - inlet valve; 3 - filter; 4 - fitting for a pressure gauge; 5 - valve PKK-40M; 6 - regulator RD-32M (RD-50M); 7 - fitting for measuring the final pressure; 8 - outlet valve; 9 - discharge line of safety valves built into the regulators; 10 - impulse line of final pressure; 11 - impulse line; 12 - fitting with a tee; 13 - pressure gauge; b - section of the PKK-40M valve: 1, 13 - valves; 2 - fitting; 3, 11 - springs; 4 - rubber seal; 5, 7 - holes; 6, 10 - membranes; 8 - starting plug; 9 - impulse chamber; 12 - stock
To open the valve, the starting plug is unscrewed, after which the valve's impulse chamber communicates with the atmosphere through the hole. Under the action of gas pressure, the membrane, stem and valve move upwards, while when the membrane is in its uppermost position, the hole in the valve stem is covered with a rubber seal and the flow of gas from the housing to the impulse chamber stops. Then the starting plug is screwed. Through the open valve, the gas enters the pressure regulators and through the impulse tube into the chamber. If the gas pressure behind the regulators rises beyond the established limits, then the membrane, overcoming the elasticity of the spring, will move up, as a result of which the hole, previously covered with a rubber seal, will open. The upper membrane, rising, rests with its disk against the cover, and the lower one, under the action of the spring and the mass of the valve with the stem, goes down, and the valve closes the gas passage.
Safety shut-off valve KPZ(figure below) is installed in front of the gas pressure regulator. Its upper actuation limit must not exceed the rated working pressure after the regulator by more than 25%, and the lower actuation limit is not set in the rules, since this value depends on pressure losses in the supply gas pipeline and on the control range.
Safety shut-off valve KPZ
1 - body; 2 - valve with rubber seal; 3 - axis; 4, 5 - springs; 6 - lever; 7 - control mechanism; 8 - membrane; 9 - stock; 10, 11 - tuning springs; 12 - emphasis; 13, 14 - bushings; 15 - tip; 16 - lever
The principle of operation of the CPP is as follows:
- in the working position, the valve levers are engaged and in rest with the tip of the diaphragm head rod, and the KPZ valve is open;
- when the gas pressure changes above or below the allowable one, the membrane bends and moves the rod, respectively, to the change in pressure to the right or left, together with the tip;
- the lever comes out of contact with the tip , in this case, the engagement of the levers is disturbed and, under the action of the springs, the axis closes the valve;
- the inlet gas pressure enters the valve and presses it more tightly against the seat.
Relief safety devices, unlike shut-off valves, do not shut off the gas supply, but discharge part of it into the atmosphere, thereby reducing the pressure in the gas pipeline.
There are several types of relief devices, different in design, principle of operation and scope: hydraulic, lever-load, spring and membrane-spring. Some of them are used only for low pressure (hydraulic), others - for both low and medium pressure (membrane-spring).
Safety relief valve PSK. Membrane-spring ISC (figure below) is installed on low and medium pressure gas pipelines. PSK-25 and PSK-50 valves differ from each other only in dimensions and throughput.
Safety relief valve PSK
1 - adjusting screw; 2 - spring; 3 - membrane; 4 - seal; 5 - spool; 6 - saddle
Gas from the gas pipeline after the regulator enters the valve membrane. If the gas pressure is greater than the spring pressure from below, then the membrane moves down, the valve opens and the gas goes to the discharge. As soon as the gas pressure becomes less than the force of the spring, the valve closes. The compression of the spring is adjusted by a screw at the bottom of the housing. To install PSK on low or high pressure gas pipelines, appropriate springs are selected.
The spool of the PSK-25 relief valve has the shape of a cross and moves inside the seat. In the PSK-50, the valve spool is equipped with profiled windows. The reliability of the PSK valve largely depends on the quality of the assembly.
When assembling you need:
- having cleaned the valve device from mechanical particles, make sure that there are no scratches or dents on the edge of the seat and the sealing rubber of the spool;
- achieve alignment of the relief valve spool with the central hole of the membrane;
- to check alignment, loosen or remove the spring and, while pushing the spool through the reset hole, make sure that it moves freely inside the seat.
Safety relief valve PPK-4.
The spring-loaded safety valve of medium and high pressure PPK-4 (figure below) is produced by the industry with conditional passages of 50, 80, 100 and 150 mm. Depending on the diameter of the spring 3, it can be adjusted to a pressure of 0.05-2.2 MPa.
Safety relief valve PPK-4
1 - valve seat; 2 - spool; 3 - spring; 4 - adjusting screw; 5 cam
Gas filters.
In the GRU with a conditional passage of up to 50 mm, angular mesh filters are installed (figure below), in which the filter element is a clip covered with a fine mesh. In hydraulic fracturing with regulators with a nominal bore of more than 50 mm, cast-iron hair filters are used (figure below). The filter consists of a housing, a cover and a cassette. The cassette holder is covered with a metal mesh on both sides, which traps large particles of mechanical impurities. Finer dust settles inside the cassette on a pressed fiber, which is lubricated with a special oil.
Gas filters
a - angular mesh; b - hair: 1 - body; 2 - cover; 3 - grid; 4 - pressed fiber; 5 - cassette
The filter cassette resists the flow of gas, which causes a pressure difference before and after the filter. Increasing the gas pressure drop in the filter to more than 10,000 Pa is not allowed, as this may cause the fiber to be carried away from the cassette.
To reduce pressure drops, it is recommended to clean the filter cassettes periodically (outside the hydraulic fracturing building). The internal cavity of the filter should be wiped with a cloth soaked in kerosene.
Depending on the type of regulators and gas pressure, various filter designs are used.
The figure below shows the design of a filter designed for hydraulic fracturing, equipped with RDUK regulators. The filter consists of a welded body with connecting pipes for gas inlet and outlet, a cover and a plug. On the side of the gas inlet, a metal sheet is welded inside the housing, which protects the grid from direct ingress of solid particles. Solid particles coming with the gas, hitting the metal sheet, are collected at the bottom of the filter, from where they are periodically removed through the hatch. Inside the case there is a mesh cassette filled with nylon thread.
Welded filters
a - filter for RDUK regulators: 1 - welded housing; 2 - top cover; 3 - cassette; 4 - hatch for cleaning; 5 - breaker sheet; b - filter revision: 1 - outlet pipe; 2 - grid; 3 - body; 4 - cover
The solid particles remaining in the gas stream are filtered in the cassette, which is cleaned as necessary. The filter top cover can be removed to clean and rinse the cassette. Differential pressure gauges are used to measure pressure drop. In front of the rotary counters, additional filtering devices are installed - a revision filter (figure above).
Safety devices include impulse safety devices (IPU) and direct acting safety valves. Safety devices are designed to ensure the safe operation of equipment and systems of power plants by protecting against excess pressure of the working medium (saturated or superheated water vapor) above the permissible value.
Safety devices operate automatically and, when opened, discharge the excess of the working medium from the protected vessel or system into the atmosphere. IPU are designed for installation on drums and outlet manifolds of boiler units with a nominal steam pressure of 10.0, 14.0 and 25.5 MPa, on "cold" and "hot" lines of pipelines for reheating steam, as well as on pipelines of reduced and cooled steam ( behind reduction-cooling units) with a nominal pressure of 6.3 MPa.
The main difference between the pulse valves (IC), which are part of the IPU, supplied for the protection of cogeneration units, and those supplied for reheat pipelines, as well as reduced and cooled steam, is their equipping with an electromagnetic drive, which ensures high accuracy of operation (opening and closing ) of these valves and IPU as a whole. Such an electromagnetic drive is based on two electromagnets or one double-acting electromagnet, which ensure the timely opening and closing of the device.
Setting the IPU to a given opening and closing pressure is done only by a pulse valve. This is achieved by placing a weight on the IK lever in a position that allows the valve to open at set pressure. The IC and IPU as a whole are closed at a pressure lower than the nominal one. In the event of a loss of electrical power in the control circuit, the safety device is activated by the action of a weight on the lever of the impulse valve.
GPK are equipped with a hydraulic damper in order to mitigate the impact of the undercarriage parts when the valve is actuated to open and close. The brake fluid is technical water, the constant supply of which to the damper is provided by the device shown in the wiring diagram.
The choice of one or another direct-acting valve or IPU from the nomenclature given in this catalog is carried out depending on the parameters of the working medium in the protected vessel or system, as well as on the required throughput, i.e. steam flow through the valve per unit time.
The number of safety valves and their throughput for general-purpose power plants must be selected according to the calculation in accordance with the NTD agreed with the technical supervision of the Republic of Belarus.
Technical requirements include structural strength requirements for given energy parameters of the working medium (pressure, temperature); corrosion resistance of the material against the chemical effects of the working environment; compliance with the nominal diameter of the passage and connecting (main) branch pipes with the corresponding dimensions of the pipeline; compliance of the device design with its functional purpose; ensuring the required hydraulic parameters and characteristics (capacity); ensuring the required speed; compliance of the type of energy used to control the device with the available energy sources (electric, compressed air, mineral oil under pressure, working medium transported through the pipeline). The overall dimensions of the device are also evaluated, which determine the dimensions of the room or space required for its placement, the convenience and method of control, and reliability parameters.
The economic requirements include: the cost of the structure; the cost of operation, repair, replacement of worn parts, maintenance, the cost of the required premises; the cost of products (working medium) lost through possible leaks in the shut-off organ, stuffing box and through the destroyed membrane after its rupture; the cost of equipment downtime caused by the need to repair or replace an installed device.
If several designs can satisfy the requirements, the final decision is made on the basis of a comparative assessment of competing options. The first step is to establish the possibility of using a design that is mass-produced by the industry, and only in the absence of the required one, data are prepared for its design and manufacture by special order.
On pipelines transporting combustible, flammable oil products or active gases and liquids with toxic properties, it is recommended to use designs specially designed for these media under specified operating conditions. The use of general-purpose structures is allowed only if they, as well as the materials of the parts, comply with the requirements for reliable and safe operation. For aggressive environments, it is allowed to use parts with corrosion-resistant metallic and non-metallic coatings deposited on their surfaces. The nominal diameter of the passage in the vast majority of cases is equal to the diameter of the pipeline passage.
For explosive and flammable, toxic or extremely clean environments, designs with a bellows seal of the rods are used, it is also provided for when the system is evacuated. On mobile installations (tanks), safety valves and loading limiters (level regulators) of general technical structures are not recommended, since they are not designed to work in vibration conditions. For devices operating on lines with toxic, flammable and explosive environments, there are increased requirements for the tightness of the shut-off body, stuffing box (bellows) and detachable connections of the cover to the body and connecting pipes.
The attachment of protective and safety devices to the pipeline is most often provided by flanged connections, which allow for quick replacement or removal for repair. The type of flange connection and gasket material are selected depending on the operating conditions of the product, pressure, temperature and corrosive properties of the working medium. In pipelines of small diameter passage (DN<80 p="">
However, the scope of its application is limited by a number of its inherent disadvantages, which include the following: the difficulty of mounting the product on the pipeline due to the need to screw the pipe section, fitting or the product itself; the possibility of forming a permanent connection as a result of corrosion of the surfaces in contact in the thread; the complexity of manufacturing a large diameter thread and the large torque required when assembling a large diameter threaded connection. A threaded connection is chosen only when the dismantling of the product is unlikely. Flange connection is universal and is often used if it is expected that the product will need to be removed for repair or replacement. The most reliable and hermetic connection is achieved by welding, and it is widely used for steel in all cases where this is permissible.
In flange connections with ru<2,5 300="">2.5 MPa (regardless of temperature) and at temperatures above 300 °C (regardless of pressure), studs with nuts are used.
To assess the operating conditions of protective and safety devices, the physical and chemical properties of the working environment are important. The viscosity of liquid petroleum products can be in a wide range of values. The dynamic viscosity of a fluid is measured in pascal seconds (Pax). To assess the viscosity of oil products, the values of conditional viscosity are used, which are defined as the ratio of the time of expiration of 200 ml of the tested oil product from the Engler viscometer at the test temperature (viscosity of oil products depends on temperature) to the time of expiration of the same volume of distilled water at 20 °C, which is the water number of the device. This ratio, expressed in arbitrary degrees, is denoted by the WU.
Of great importance for mechanisms operating in oil products is the lubricity of the liquid, which helps to reduce friction. Gasoline, as a solvent for mineral lubricants, exposes metal and creates conditions for friction without lubrication in movable mates. Increased viscosity creates difficulties in transporting oil products through pipes and through safety valves due to high internal fluid friction and, as a result, high hydraulic resistance of local hydraulic obstacles. Very viscous petroleum products are transported heated. The viscosity of petroleum products determines paraffin, the content of which in oil ranges from tenths to 15%. According to the content of paraffin, oils are divided into three types: low paraffin (up to 1.5%); paraffinic (1.51-6.0%) and highly paraffinic (more than 6%).
The operating conditions of protective and safety devices, such as the SMDC valve, are affected by the corrosive effect of petroleum products associated with the content of acids, water, sulfur and hydrogen sulfide in them. The acidity of oil products is estimated by the acid number, which is determined by the number of milligrams of KOH required to neutralize 1 ml of the oil product. Usually it does not exceed 0.02-0.07.
