To relieve excess pressure into the atmosphere, spring safety valves are used, which are special pipeline fittings that provide reliable protection pipeline from malfunctions and mechanical damage. The device is responsible for automatically discharging excess liquids, steam and gas from vessels and systems until the pressure is normalized.
Purpose of a spring valve
Dangerous excess pressure in the system occurs as a result of external and internal factors. An increase is caused by both incorrect assembly of thermal-mechanical circuits, which causes malfunctions in the functioning of equipment, heat entering the system from extraneous sources, and intra-system physical processes that are not provided for by standard operating conditions that periodically occur in the system.
Safety products are an essential part of any household or industrial system, working under pressure. Installation of safety mechanisms is carried out on pipelines in compressor stations, in autoclaves, in boiler rooms. Valves perform protective functions on pipelines through which not only gaseous but also liquid substances are transported.
Design and principle of operation of spring valves
The valve consists of a steel body, the lower fitting of which is used as a connecting element between it and the pipeline. If the pressure in the system increases, the medium is discharged through the side fitting. A spring adjusted depending on the pressure in the system ensures that the spool is pressed against the seat. The spring is adjusted using a special bushing, which is screwed into the top cover located on the body of the device. The cap located in the upper part is designed to protect the bushing from destruction as a result of mechanical influences. The presence of a special ear for sealing allows you to protect the system from outside interference.
For valves in which a spring acts as a balancing mechanism, the force of the working element is selected. If the parameters are selected correctly, when in good condition system, the spool responsible for releasing excess pressure from the pipeline must be pressed against the seat. When performance increases to a critical level, depending on the type of spring device, the spool moves up to a certain height.
The safety spring valve, which ensures timely pressure relief, is made of different materials:
- Carbon steel. Such devices are suitable for systems in which the pressure is in the range of 0.1-70 MPa.
- Stainless steel. Valves from stainless steel designed for systems in which the pressure does not exceed 0.25-2.3 MPa.
Classification and characteristics of spring valves
The spring safety valve is available in three versions:
- Low lift devices suitable for gas and steam pipeline systems, the pressure in which does not exceed 0.6 MPa. The lifting height of such a valve does not reach more than 1/20 of the seat diameter
- Mid-lift devices, in which the lifting height of the spool is from 1/6 to 1/10 of the nozzle diameter.
- Full lift devices, in which the valve lift height reaches up to ¼ of the seat diameter.
There is a known classification of valves based on the method of opening them:
- Non-return spring valve. To control spring check valves, an indirect external pressure source is used. Reverse spring valves, which are called impulse safety devices, can operate through the influence of electricity.
- Straight valve. In devices direct type working pressure The environment has a direct effect on the spool, which rises as the pressure increases.
Highlight valves open And closed type . In the case of using a direct type device, when the valve is opened, the medium is discharged directly into the atmosphere. Closed type valves remain completely sealed against environment, releasing pressure into a special pipeline.
Advantages
Highlight various types equipment that relieves excess pressure from the system, but spring safety valves are popular due to the presence of important advantages:
- Simplicity and reliability of design.
- Ease of setting operating parameters and ease of installation.
- Variety of sizes, types and designs.
- Installation of the safety product is possible in both horizontal and vertical positions.
- Relatively small overall dimensions.
- Large flow area.
On to the cons safety valves include the presence of restrictions in the lifting height of the spool, increased requirements for the quality of manufacturing of the spring for safety valves, which can fail when operating in an aggressive environment or constant exposure to high temperatures.
How to choose a spring valve?
When choosing a fuse, you should be based on several important principles, the consideration of which determines the uninterrupted operation of the system and the ability of the fuse to perform the necessary functions:
- Spring safety valves have the smallest dimensions compared to other types of safety relief valves, so they should be chosen in cases where there is not enough free space.
- Features of the use of valves are associated with the presence of increased vibrations, which negatively affect performance characteristics device and can quickly render it unusable. For example, lever-load type devices are more susceptible to breakdowns due to exposure to vibrations due to the presence of a long lever with weight and hinges in the design. Therefore, for systems in which significant vibration effects are observed, it is worth choosing a spring safety valve.
- Depending on the design features of the device, the spring may change the pressure force over time. This is due to the fact that the constant rise of the spool causes changes in the structure of the metal.
Installation nuances
Safety valve spring type installed at any point in the system that is subject to increased pressure and is at risk of mechanical damage. The device does not require a lot of free space, which is a significant advantage compared to other types safety devices.
To avoid operational problems, do not install any shut-off valves. To reset gaseous medium special devices are installed or discharge occurs directly into the atmosphere. To alert personnel, a special whistle is mounted along with the spring valves, which is placed on the discharge pipe. When the valve is activated, a whistle will sound, indicating that the pressure in the system has increased and the valve has opened to release the medium.
Possible causes of safety valve failures
Safety valves are durable and reliable devices that provide constant protection of systems from overpressure. A direct or reverse spring valve fails for several reasons:
- The presence of increased vibrations;
- Constant exposure to an aggressive environment on the safety choke.
- Incorrect installation of safety spring throttle or valve.
In order to avoid accidents and malfunctions in the functioning of systems, safety valves undergo periodic checks for malfunctions. Valves are tested for strength and tightness before being put into operation. Periodic checks are also carried out to determine the tightness of sealing surfaces and gland connections.
At making the right choice safety devices taking into account system parameters, periodic inspections and timely troubleshooting, spring safety valves will ensure reliable operation of the system and trouble-free protection against overpressure for a long time.
Spring safety valve (PPV)– a type of pipeline fittings intended for automatic protection equipment and pipelines from exceeding pressure above a predetermined value by releasing excess working fluid and ensuring that the discharge stops when the closing pressure is restored and the operating pressure is restored.
Main valve assemblies and parts:
1 - body, 2 - seat, 3 - spool, 4 - cover, 5 - rod, 6 - nut, 7 - pin, 8 - spring, 9 - bellows (installed in bellows valves), 10 - locking screw, 11 - adjusting bushing, 12 - guide bushing, 13 - partition, 14 - adjusting screw, 15 - cap, 16 - threaded flange.
Operating principle. At normal operating pressure, the force of the compressed spring presses the spool against the seat (the passage for relieving the working medium is closed). When the pressure increases above the set value, an oppositely directed force begins to act on the spool, which compresses the spring, and the spool rises, opening a passage for discharging the working medium. After the pressure in front of the valve decreases to the closing pressure, the spool under the action of the spring is again pressed against the seat, stopping the discharge of the medium.
Installation position – vertical, cap up.
Shutter tightness– class “B” GOST R 54808. At the customer’s request, it is possible to manufacture with other classes of tightness.
Possible valve designs:
- A sealed cap with a forced opening unit, and without one.
- Balancing bellows.
- Thermal barrier.
- "Open" lid.
- A locking element that prevents the valve from operating.
Pipeline connection:
- flanged;
- for lens gasket (flange according to GOST 9399);
- fitting;
- tsapkovoe.
Valves with bellows.
The bellows is a mechanism that compensates for the effect of back pressure at the outlet of the valve. The bellows is designed to protect the valve spring from harmful effects aggressive working environment at high or low temperatures. Bellows valves are made of steel grades 12Х18Н9ТЛ and 12Х18Н12МЗТЛ and are intended for working environments with temperatures from minus 60 °С and below. Designation of bellows valves: KPP4S, KPPS.
The design of the sealing surfaces and the connecting dimensions of the valve flanges are in accordance with GOST 12815-80, row 2, face-to-face lengths are in accordance with GOST 16587-71.
Valves DN 25 PN 100 kgf/cm2 can be manufactured with union ends for connection to a pipeline in accordance with GOST 2822-78, as well as with a flange connection in accordance with GOST 12815-80, row 2.
Safety valves with nominal pressure PN 250 kgf/cm2 and PN 320 kgf/cm2, like other models, are designed to protect equipment from unacceptable excess pressure by automatically releasing excess working fluid. Used on equipment with liquid and gaseous working media that do not cause corrosion of body parts greater than 0.1 mm.
Safety valves with a stamped-welded body can be manufactured with individual construction length(L and L1), height (H) and connecting dimensions of the flanges, which allows them to be used as substitutes for imported fittings without changing already installed equipment and pipelines.
Calculation of valve capacity - according to GOST 12.2.085-2002.
Setting pressure, pH– greatest overpressure at the entrance to the safety valve, at which the valve is closed and the specified tightness of the valve is ensured.
Opening start pressure, Рн.о.(starting pressure; set pressure) – excess pressure at the inlet to the safety valve, at which the force tending to open the valve is balanced by the forces holding the locking element on the seat. When the opening pressure begins, the specified tightness in the valve shutter is broken and the locking element begins to rise.
Full opening pressure, Рп.о.– excess pressure at the inlet to the safety valve, at which the valve moves and the maximum throughput.
Closing pressure, Рз(reseating pressure) – excess pressure at the inlet to the safety valve, at which, after the working medium is discharged, the locking element is seated on the seat, ensuring the specified tightness of the valve. Valve closing pressure, Рз – not less than 0.8 Рн.
Back pressure– excess pressure at the outlet of the fittings (in particular, from the safety valve).
Back pressure is the sum of the static pressure in the exhaust system (if closed system) and the pressure arising from its resistance during the flow of the working medium.
Mandatory minimum order information.
When ordering valves, you must fill out a questionnaire (Appendix B):
- product type, designation, type designation (according to the table of figures);
- nominal diameter inlet pipe, DN, mm;
- nominal pressure, PN, kgf/cm2;
- setting pressure (Рн, kgf/cm2) or spring number (when only the spring number is specified, the valve is adjusted to the minimum value from the range of the specified spring);
- body material;
- the presence of a manual detonation unit in the valve design;
- the presence of a bellows in the valve design.
Example of designation when ordering a spring safety valve:
An example of designation when ordering a spring safety valve DN 50 PN 16 kgf/cm2 made of steel 12Х18Н9ТЛ with a manual detonation unit, setting pressure – Рн=16 kgf/cm2, model KPP4R according to TU 3742-005-64164940-2013:
Safety valve KPP4R 50-16 DN 50 PN 16 kgf/cm2, pH=16 kgf/cm2, 17nzh17nzh. When placing an order, the need to complete the valves with matching parts (matching flanges, gaskets, studs, nuts; for valves DN 25 PN 100 - nipples with union nuts and gaskets) is specifically stated.
Check valve - element pipeline system, providing for the movement of the working medium in only one direction. Its use is mandatory for autonomous pumping stations and other equipment that may fail when the fluid flow moves in the opposite direction.
A spring check valve is one of the types of shut-off elements. It belongs to the category of direct-acting valves and is activated automatically by the energy of the working environment, which prevents equipment failure in the event of a power outage or other malfunctions.
Design features
The spring valve is structurally composed of three elements:
- A body usually made of brass and equipped with elements for fastening to the pipeline (coupling, thread). The body is also made of steel, cast iron and polypropylene. The choice of material is determined by the parameters of the working environment and the diameter of the pipeline.
- The working element, which represents a movable spool, includes two plates with a special sealed gasket between them and a rod.
- An actuator representing a spring located between the work element plates and the seat. Provides automatic shutoff of fluid flow when pressure decreases or changes its direction. It depends on the spring stiffness minimum pressure working environment at which the valve automatically opens.
Advantages of spring check valves:
- Possibility of installation in any position;
- simplicity of design;
- versatility.
At the same time, the valve is sensitive to contaminants in the water, which lead to wear of the sealing plates, so it is advisable to install a filter in front of it. It is also recommended to mount the valve in easily accessible places to simplify installation. maintenance and replacements.
It is advisable to install the valve in a vertical position so that the blocking force of the spring coincides with the force of gravity. For correct installation it is necessary to focus on the arrow marked on the valve body, which shows the direction of flow of the working medium through.
Scope of application
Spring check valves are widely used in systems autonomous water supply, intra-house networks of multi-apartment buildings. They are installed on the suction lines of pumps, before DHW water heaters, boilers, water meters and other equipment.
All vessels operating under increased pressure must be equipped with safety devices against increased pressure. For this we use:
lever-load PCs;
safety devices with collapsible membranes;
Lever-load PCs are not allowed for use on mobile vessels.
Schematic diagrams of the main types of PCs are shown in Figures 6.1 and 6.2. Weight on lever-weight valves (see fig. 6.1,6) must be securely fixed in the specified position on the lever after calibration of the valve. The design of the spring PC (see Fig. 6.1, c) must exclude the possibility of tightening the spring beyond the established value and provide a device for
Rice. 6.1. Schematic diagrams of the main types of safety valves:
checking the proper operation of the valve in working condition by forcing it to open during operation. The design of the spring safety valve is shown in Fig. 6.3. The number of PCs, their sizes and bandwidth must be calculated so that in Fig. 6.2. The bursting safety membrane did not exceed more than 0.05 MPa for vessels with pressure up to 0.3 MPa, at
15% - for vessels with pressure from 0.3 to 6.0 MPa, by 10% - for vessels with pressure more than 6.0 MPa. When operating PCs, it is allowed to exceed the pressure in the vessel by no more than 25%, provided that this excess is provided for by the design and is reflected in the vessel passport.
PC throughput is determined according to GOST 12.2.085.
When determining the size of the flow sections and the number of safety valves, it is important to calculate the valve capacity per G (in kg/h). It is performed according to the methodology outlined in the SSBT. For water vapor, the value is calculated using the formula:
G=10B 1 B 2 α 1 F(P 1 +0.1)
Rice. 6.3. Spring device
safety valve:
1 - body; 2 - spool; 3 - spring;
4 - outlet pipeline;
5 - protected vessel
Where bi - a coefficient that takes into account the physicochemical properties of water vapor at operating parameters in front of the safety valve; can be determined by expression (6-7); varies from 0.35 to 0.65; coefficient taking into account the pressure ratio in front of and behind the safety valve, depends on the adiabatic index k and indicator β, with β<β кр =(2-(k+1)) k/(k-1) коэффициент B 2 = 1, показатель β вычисляют по фор муле (6.8); коэффициент B 2 varies from 0.62 to 1.00; α 1 - flow coefficient indicated in the safety valve data sheets, for modern designs of low-lift valves α 1 = 0.06-0.07, high-lift valves - α 1 = 0.16-0.17, F- valve flow area, mm 2; R 1 - maximum excess pressure in front of the valve, MPa;B 1 =0.503(2/(k+1) k/(k-1) * 
Where V\ - specific volume of steam in front of the valve at parameters P 1 and T 1, ) m 3 /kg - temperature of the medium in front of the valve at pressure Pb °C.
(6.7)β = (P 2 + 0.1)/(P 1 +0.1), (6.8)
Where P2 - maximum excess pressure behind the valve, MPa.
Adiabatic exponent k depends on the temperature of the water vapor. At a steam temperature of 100 °C k = 1.324, at 200 "C k = 1.310, at 300 °C k= 1.304, at 400 "C k= 1.301, at 500 ° Ck= 1,296.
The total throughput of all installed safety valves must be no less than the maximum possible emergency influx of medium into the protected vessel or apparatus.
Safety diaphragms (see Figures 6.2 and 6.4) are specially weakened devices with a precisely calculated pressure failure threshold. They are simple in design and at the same time provide high reliability of equipment protection. The membranes completely seal the discharge hole of the protected vessel (before actuation), are cheap and easy to manufacture. Their disadvantages include the need for replacement after each actuation, the inability to accurately determine the actuation pressure of the membrane, which makes it necessary to increase the safety margin of the protected equipment.
Diaphragm safety devices can be installed instead of lever-load and spring safety valves if these valves cannot be used in a particular environment due to their inertia or other reasons. They are also installed in front of the PC in cases where the PC cannot operate reliably due to the peculiarities of the influence of the working environment in the vessel (corrosion, crystallization, sticking, freezing). The membranes are also installed in parallel with the PC to increase the capacity of pressure relief systems. The membranes are also installed in parallel with the PC to increase the throughput of pressure relief systems. Membranes can be bursting (see Fig. 6.2), breaking, tearing (Fig. 6.4), shearing, snapping out. The thickness of bursting discs A (in mm) is calculated by the formula:
P.D./(8σ vr K t )((1+(δ/100))/(1+((δ/100)-1)) 1/2
Where D - working diameter; R- membrane response pressure, σ BP - tensile strength of the membrane material (nickel, copper, aluminum, etc.); TO 1 - temperature coefficient varying from 0.5 to 1.8; δ is the relative elongation of the membrane material at rupture, %.
For tear-off membranes, the value determining the response pressure is
is the diameter D H (see Fig. 6.4), which is calculated as
D n =D(1+P/σ time) 1/2
It is not allowed to install any shut-off valves between the vessel and the safety device, as well as behind it. In addition, safety devices must be located in places convenient for their maintenance.
Safety devices. Safety devices (valves) must automatically prevent pressure from increasing above the permissible level by releasing the working fluid into the atmosphere or disposal system. At least two safety devices must be installed.
On steam boilers with a pressure of 4 MPa, only pulse safety valves should be installed.
Passage diameter (conditional) installed on lever-type boilers; load and spring valves must be at least 20 mm. The tolerance is to reduce this passage to 15 mm for boilers with a steam capacity of up to 0.2 t/h and a pressure of up to 0.8 MPa when installing two valves.
The total capacity of safety devices installed on steam boilers must be no less than the rated capacity of the boiler. Calculation of the capacity of limiting devices of steam and hot water boilers must be carried out according to 14570 “Safety valves of steam and hot water boilers. Technical requirements".
The installation locations of safety devices are determined. In particular, in hot water boilers they are installed on the outlet manifolds or drum.
The method and frequency of regulation of safety valves on boilers is indicated in the installation instructions and instructions. Valves must protect vessels from exceeding the pressure in them by more than 10% of the calculated (permitted) pressure.
Short answer: All vessels operating under increased pressure must be equipped with safety devices against increased pressure. For this we use:
spring safety valves (SC);
lever-load PCs;
pulse safety devices consisting of a main PC and a direct-acting pulse control valve;
safety devices with rupture membranes;
other safety devices, the use of which has been approved by the Gosgortekhnadzor of Russia.
With pressure exceeding the established one. The valve must also ensure that the release of the medium ceases when the operating pressure is restored. The safety valve is a fitting direct action, operating directly from the working medium, along with most designs of protective fittings and direct-acting pressure regulators.
Dangerous overpressure can arise in the system both as a result of third-party factors (improper operation of equipment, heat transfer from third-party sources, incorrectly assembled thermo-mechanical circuit, etc.), and as a result of internal physical processes caused by some initial event not provided for by normal exploitation. PC are installed wherever this can happen, that is, on almost any equipment, but they are especially important in the field of operation of industrial and household pressure vessels.
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Why a safety valve in a hot water supply system?
Safety valve design (in stereo anaglyph format)
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Operating principle
When the safety valve is closed, a force from working pressure in the protected system, tending to open the valve and the force from the set pointer, preventing the opening. With the occurrence of disturbances in the system, causing an increase in pressure above the operating pressure, the magnitude of the force pressing the spool against the seat decreases. At the moment when this force becomes equal to zero, equilibrium occurs between the active forces from the influence of pressure in the system and the setpoint on the sensitive element of the valve. The shut-off element begins to open, if the pressure in the system does not stop increasing, the working medium is discharged through the valve.
With a decrease in pressure in the protected system caused by the release of the medium, disturbing influences disappear. The shut-off element of the valve closes under the force of the adjuster.
The closing pressure in some cases turns out to be 10-15% lower than the operating pressure, this is due to the fact that to create a tightness of the shut-off valve after operation, a force is required that is significantly greater than that which was sufficient to maintain the tightness of the valve before opening. This is explained by the need to overcome, during landing, the adhesion force of the molecules of the medium passing through the gap between the sealing surfaces of the spool and the seat, to displace this medium. Also, the decrease in pressure is facilitated by the delay in closing the shut-off organ, associated with the impact on it of dynamic forces from the passing flow of the medium, and the presence of friction forces, requiring additional force for its complete closure.
Classification of safety valves
According to the operating principle
- direct acting valves - usually these are the devices that are meant when the phrase is used safety valve, they open directly under the influence of pressure from the working environment;
- indirect-acting valves - valves controlled by using an external source of pressure or electricity, the generally accepted name for such devices is pulsed safety devices;
- proportional action valves (used on incompressible media)
- on/off valves
- low-lift
- mid-lift
- full lift
- cargo or lever-load
- spring
- lever-spring
- magnetic spring
Design Differences
Safety valves usually have an angular body, but they can also have a straight body; regardless of this, the valves are installed vertically so that the stem goes down when closing.
Most safety valves are manufactured with one seat in the body, but designs with two seats installed in parallel are also available.
Low-lift safety valves are those in which the lifting height of the locking element (spool, plate) does not exceed 1/20 of the seat diameter; full-lift are valves in which the lifting height is 1/4 of the seat diameter or more. There are also valves with a poppet lift from 1/20 to 1/4, these are usually called mid-lift. In low-lift and medium-lift valves, the lift of the spool above the seat depends on the pressure of the medium, therefore they are conventionally called valves proportional action, although the rise is not proportional to the pressure of the working medium. Such valves are generally used for liquids where large flow capacities are not required. In full-lift valves, the opening occurs immediately at the full stroke of the disc, which is why they are called valves on/off action. Such valves are high-performance and are used for both liquid and gaseous media.
The greatest differences in safety valve designs lie in the type of load on the spool.
Spring valves
In them, the pressure of the medium on the spool is counteracted by the compression force of the spring. The same spring valve can be used for different response pressure settings by equipping it with different springs. Many valves are manufactured with a special mechanism (lever, fungus, etc.) for manual detonation for control purging of the valve. This is done to check the functionality of the valve, since various problems may arise during operation, such as sticking, freezing, or sticking of the spool to the seat. However, in some industries under conditions of aggressive and toxic environments, high temperatures and pressures, control blowing can be very dangerous, therefore, for such valves, the possibility of manual blowing is not provided and is even prohibited.
Most often, springs are exposed to the working environment, which is discharged from a pipeline or container when triggered; special spring coatings are used to protect them from mildly aggressive environments. There is no stem seal in these valves. In cases of working with aggressive media in chemical and some other installations, the spring is isolated from the working environment using a seal along the rod with a stuffing box, bellows or elastic membrane. Bellows seals are also used in cases where leakage of the medium into the atmosphere is not allowed, for example at nuclear power plants.
Lever-weight valves
In such valves, the force on the spool from the pressure of the working medium is counteracted by the force from the load, transmitted through the lever to the valve stem. Setting such valves to the opening pressure is done by fixing a load of a certain mass on the lever arm. Levers are also used to manually purge the valve. Such devices are prohibited from being used on mobile vessels.
To seal large-diameter seats, significant masses of weights on long levers are required, which can cause strong vibration of the device; in these cases, housings are used, inside of which the medium discharge cross-section is formed by two parallel seats, which are overlapped by two spools using two levers with weights. Thus, two parallel operating valves are mounted in one body, which makes it possible to reduce the mass of the load and the length of the levers, ensuring normal operation of the valve.
Magnetic spring valves
These devices use an electromagnetic drive, that is, they are not direct-acting valves. The electromagnets in them can provide additional pressing of the spool to the seat, in this case, when the response pressure is reached based on a signal from the sensors, the electromagnet is turned off and only the spring counteracts the pressure, the valve begins to operate like a regular spring one. Also, the electromagnet can create an opening force, that is, counteract the spring and force the valve to open. There are valves in which the electromagnetic drive provides both additional pressing and opening force, in this case the spring serves as a safety net in case of interruption
