Safety valve

Locomotive of the York Newcastle & Berwick Railway after the boiler burst, 1850

Safety valves ( pressure relief valves ) belonging to the safety valves in accordance with DIN EN 806-1 and DIN 3211 and protect pressurized spaces or pressure vessel (eg. B. heating and steam boilers , pipelines , reactors ) in front of a for printing apparatus are prohibited or printing system pressure increase. If the response pressure is exceeded, gases , vapors or liquids are discharged into the atmosphere or into collecting pipes.

The safety valve is a pressure relief device that starts to respond when the pressure exceeds the maximum permissible operating pressure in the event of foreseeable faults. When an overpressure of 10% of the max. permissible operating pressure, the safety valve must be fully open. If the safety valve is dimensioned correctly, the pressure build-up remains manageable. After the overpressure has been released by blowing into the environment, the valve closes again and the system can continue to be operated. In order to ensure permanent tightness of a safety valve, the permanent operating pressure should be 90% of the max. Operating pressure do not exceed. One example is the boiler safety valve on a steam locomotive .

Every closed circuit that is heated requires a safety valve, e.g. B. hot water generators , water heating systems , solar collector circuits . The safety valve prevents a system from bursting if thermal fluid expansion occurs due to heat transfer and there is no defined gas cushion. Even a temperature increase due to solar radiation from around 10 to 30 ° C can cause the pressure vessel to deform. Organic liquids (e.g. antifreeze agents such as ethylene glycol ) are more critical than water because of their high volume expansion coefficient .

In diaphragm safety valves, the flexible diaphragm prevents the medium from coming into contact with the spring and the bearing of the axis of the poppet valve when the valve is triggered, which means that reliable operation can be achieved over a longer period of time.

The response pressure of the safety valve on hot water heating boilers is often 3 bar. If the membrane expansion vessel is leaky , the nitrogen gas cushion is not required . The thermal expansion when the heating water is heated, like the refilling of the heating system from the water network, can cause an impermissible pressure (the pressure in the drinking water pipe is often higher than the permissible pressure of the heating boiler). The nominal diameter of the safety valve must be designed for the maximum blow-off mass flow or volume flow.

Safety valves for heating boilers are selected according to the nominal heat output. Safety valves on air compressors are designed according to the delivery volume of the compressor . Safety valves on containers for the storage of cryogenic gases (e.g. liquid oxygen) must be designed for the evaporation mass flow if the insulation of the container is omitted due to the loss of vacuum in the thermal insulation .

In R & I-flow schemes according to international standards, the marking is a pressure safety valve with PSV ( P ressure S afety V alve) with a following count number.

Classification

With regard to their response behavior, safety valves are classified according to:

Proportional valves . The opening behavior between the response pressure p _set and full opening (1.1 p _set ) is proportional to the pressure,
Full-lift safety valves open suddenly and with full lift when responding,
Normal safety valves have no special response behavior.

Furthermore, safety valves are differentiated according to the type of counterforce to the pressure to be protected:

spring-loaded safety valves,
weight-loaded safety valves,
medium-loaded safety valves.

The weight and the lever arm transmission ratio or the preload of the spring are in equilibrium with the force that acts on the valve disk due to the internal pressure.

In the case of medium-loaded safety valves, the same pressures act on both sides of the sealing plate; due to differently sized surfaces, a greater force acts in the closing direction. The valve is controlled by an additional valve unit. If the response pressure is exceeded, the area above the sealing plate is vented via the additional valve unit, the closing force is removed and the valve opens suddenly.

A distinction is also made between direct-acting and controlled safety valves. Controlled safety valves correspond to the direct acting except for an additional closing and opening force. This can be a pneumatic or hydraulic cylinder drive which, within certain limits, causes an additional load force and, if the response pressure is exceeded, an additional lifting force. This achieves a much more precise response. Controlled safety valves are z. B. used in larger power plant boilers. Due to the re-evaporation with larger water volumes, there are longer times until the safety valve closes again after the pressure is exceeded if the safety valves are not controlled. A particular class constitutes the flame arrestor (Engl. Flashback arrestor), as incorporated in decades in welding by the pressure control valve and before the welding hose pair. If a flame front creeps or an explosion front snaps in the hose against the regular gas flow, these components close off the path to the gas cylinder. These components (about 12 cm long, 2.5 cm in diameter) contain spring-loaded pistons and a porous barrier made of sintered metal , are controlled thermally and by the incoming pressure shock and prevent bottle fires.

Characteristic properties

The characteristic values for the design of safety valves are:

p _set : response pressure in bar (overpressure),
d ₀ : smallest opening diameter at the valve seat in mm,
K _Dr or : assigned reduced coefficient of discharge, ${\ displaystyle \ alpha}$
h : Travel of the safety valve (position of the valve disc between seat and maximum opening) in mm.

The outflowing mass flow is calculated from the conversion of the pressure energy into kinetic energy ( Bernoulli equation ). The coefficient of discharge describes the actual flow velocity integrated over the flow cross section A ₀ in relation to that with a uniform velocity distribution (plug profile).

The coefficient of discharge K _Dr is determined in tests as part of the component test. This value is then multiplied by a safety factor of 0.9 and can then be used as an assigned reduced coefficient of discharge K _Dr in the calculation for the blow-off mass flow. The discharge coefficients of gases and liquids are different.

The mass flow flowing out of a safety valve can be calculated for gases up to the critical pressure ratio p ₂ ≤ p _crit from the following equation:

{\ displaystyle {\ dot {m}} = K _ {\ mathrm {Dr}} \ cdot A_ {0} \ cdot {\ sqrt {\ frac {p_ {1}} {v_ {1}}}} \ quad { \ sqrt {2 \ cdot {\ frac {\ kappa} {(\ kappa -1)}} \ cdot \ left \ lbrack {\ left ({\ frac {p_ {2}} {p_ {1}}} \ right ) ^ {\ frac {2} {\ kappa}}} - \ left ({\ frac {p_ {2}} {p_ {1}}} \ right) ^ {\ frac {\ kappa +1} {\ kappa }} \ right \ rbrack}}}

In the case of a supercritical pressure ratio, the critical pressure p _crit is to be used for p ₂ .

{\ displaystyle p_ {1}}

: Rest pressure (absolute) in the container and response pressure of the safety valve

{\ displaystyle v_ {1}}

: specific volume of the gas in the container

{\ displaystyle p_ {2}}

: Pressure (absolute) in the discharge space

{\ displaystyle p _ {\ mathrm {krit}} = p _ {\ mathrm {L}}}

: critical or laval pressure

${\ displaystyle \ kappa}$ : Isentropic exponent (air: Ne, Ar: , water, CO ₂ : ) ${\ displaystyle \ kappa = 1 {,} 4}$ ${\ displaystyle \ kappa = 1 {,} 66}$ ${\ displaystyle \ kappa = 1 {,} 3}$

In pressure technology, all pressures are specified in excess pressures (e.g. the operating pressure of pressure vessels, the set pressure of safety valves). The absolute pressure must always be used in the thermodynamic equations.

In the case of critical or supercritical pressure conditions (pressure in the internal pressure to pressure in the outflow space), gases flow out at the speed of sound (critical pressure ratio of air:) . The mass flow is then independent of the pressure in the outflow space. It should be noted, however, that as the counter pressure increases, the opening force on the valve seat decreases, the valve no longer opens completely and thus the maximum stroke is no longer reached. This changes the outflow coefficient, and thus ultimately the mass flow that can be discharged is reduced. This is taken into account in the calculation rules for safety valves insofar as the discharge coefficient is only valid for certain counterpressures ( proportional valves : max. 15% counterpressure, safety valves with bellows : max. 30% counterpressure). ${\ displaystyle p_ {2} / p_ {1} <0 {,} 528}$

Design of a safety valve

A spring-loaded safety valve in a power plant

The set pressure or response pressure is the pressure at which the safety valve begins to open in order to release the pressure. This is the pressure that is permanently set at the safety valve. The set pressure is subject to a tolerance (according to DIN EN ISO 4126-1) of (± 3%) or for valves below 3 bar of (± 0.1 bar).

Proportional safety valve

The closing pressure is the pressure value at which the safety valve is completely closed again after it has blown off the excess pressure. For liquid media, this value is 20% below the set pressure (at a set pressure <3 bar, the value is 0.6 bar below).

For gaseous media the value according to AD2000 is 10% below the set pressure and according to DIN EN ISO 4126-1 15% below (at a set pressure <3 bar the value is 0.3 bar below the set pressure).

The opening pressure is also called the blow-off pressure and is the point at which the safety valve reaches the maximum stroke for the required mass flow to be discharged. With normal-lift safety valves, this is max. 10% above the set pressure and with full-lift safety valves at max. 5% above the set pressure.

The operating pressure is the working pressure of a plant or system under normal circumstances. Ideally, the operating pressure is 5% below the closing pressure of a safety valve.

Rating

Depending on the load assumption (causes that lead to the response of the safety valve, volume flows to be discharged, type of medium), the nominal size of the safety valve can be calculated and the type of blow-off systems (build-up of a possible counter pressure) can be determined.

Safety valves for use in solar systems are selected in accordance with DIN 4757, Part 2.

While bursting disks are also recommended for pressure relief in the event of rapid pressure increases such as explosions, safety valves cannot release the required flow cross-section quickly enough in exceptional cases. If sudden pressure surges are to be expected, the safety valves must be carefully dimensioned. Improperly dimensioned valves can start to vibrate, which can increase the pressure surge amplitude.

Norms

Compressed air safety valve

Standardized representation of a spring-loaded safety valve
1.
Straight shape 2. Corner shape

Before the 2002 Pressure Equipment Directive was implemented , safety valves were designed in accordance with AD leaflet A2 (Working Group on Pressure Vessels , Equipment). The safety valves had to be component tested. The tests were carried out by TÜV or similar institutions. The procedure and scope of the component test were listed in the VdTÜV information sheet “Safety valve 100”.

Since the Pressure Equipment Directive 97/23 / EC and Directive 2014/68 / EU came into force, the AD-2000 data sheet A2 can be used as a test basis. Alternatively, other regulations, e.g. B. the harmonized European series of standards EN ISO 4126 are used. The standard comprises the following parts:

EN ISO 4126 Part 1: Safety valves
EN ISO 4126 Part 2: Rupture disc devices
EN ISO 4126 Part 3: Combination of safety valves and bursting disc devices
EN ISO 4126 Part 4: Pilot operated safety valves
EN ISO 4126 Part 5: Controlled safety valves (CSPRS)
EN ISO 4126 Part 6: Rupture disc devices - Selection, application and installation
EN ISO 4126 Part 7: General data
EN ISO 4126 Part 9: Use and installation
EN ISO 4126 Part 10: Two-phase flow
EN ISO 4126 part 11: Function and flow test

A separate standard for pressure relief devices has been developed for refrigeration systems:

EN 13136 refrigeration systems and heat pumps , pressure relief devices and associated lines - calculation method

According to the Pressure Equipment Directive, safety valves are considered equipment parts with a safety function . They are assigned to the highest category IV and must be manufactured in accordance with the specified manufacturing and testing requirements with the involvement of a notified body.

The discharge coefficient is determined through appropriate tests. Due to the different flow characteristics, the coefficient of discharge for gases and liquids is different. In addition, it must be proven that the pressure increase from the lifting of the valve disc until it opens completely does not exceed 10%. Safety valves must close again when the pressure drops by 10% (gases) or 20% (liquids) below the response pressure.

Depending on the spring design, the response pressure of safety valves can be set within a certain range. The specified response pressure is achieved by influencing the spring preload via a threaded insert. The position of the bias is z. B. secured against loosening by a lock nut. The adjusting screw connection is closed with a hood. As a measure to prevent unauthorized changes to the setting, this cover is secured with a seal. Caps are also used as covers for smaller safety valves. The unauthorized adjustment of safety valves can cause a considerable hazard and represents dangerous manipulation .

Oxygen safety valve

Safety valves for non-toxic substances must be ventable. With the help of a lever or a cap, which can be loosened and which acts against the spring force , the valve must be opened when a pressure of 85% of the response value is reached at the latest. Safety valves should be opened in order to prevent the valve disk from sticking or corroding. When using dangerous or environmentally hazardous fluids, there is no need for venting. Instead, 2 safety valves are connected to the two output sides of a shuttle valve and the input side is connected to the pressure chamber to be protected. This arrangement allows one safety valve to be removed and checked while the other valve is in use. It is thus possible to continue operating the connected pressure chamber. This arrangement is specified in the standards for larger liquid gas storage containers , ammonia containers, containers for cryogenic gases or containers in refrigeration systems that can only be emptied with great effort .

When using a blow-off line, it must be ensured that the counter pressure remains in the permissible range and thus the maximum stroke and thus the outflow mass flow is guaranteed. Using the Bernoulli equation , the pressure loss determined from the flow resistances (pipeline, bends, constrictions) must be within the permissible range. In addition, there is a requirement that the pressure loss on the inflow side must not be more than 3% of the response pressure.

Operators of printing systems must set deadlines for testing safety valves as part of a risk assessment in accordance with the Industrial Safety Ordinance.

Installation instructions

When using safety valves, the operating instructions must be observed, as the types are only suitable for certain applications. The following factors must be taken into account when choosing:

Dimensioning in relation to possible pressure generators,
Corrosion (medium-material compatibility),
Resistance of the seat seal (when using soft material seals ),
Drainage of condensates from the valve body on the downstream side,
Impairments from dust or products with a sticky effect.

The safety equipment of pressure equipment must be verified by a qualified person or an approved ZÜS monitoring body as part of a "test before commissioning" in accordance with the Industrial Safety Ordinance.

The outlet openings of safety valves that protect harmless substances ( water , air ) can usually be guided in a room. If necessary, the opening should be arranged so that people are not endangered (e.g. hot water escaping from the boiler ).

For all other substances that z. B. are poisonous, displacing air or flammable , must be diverted into areas in which no hazard is to be expected. The safety valves are connected to blow-off lines which, for. B. opens on the roof. Possibly. escaping fluids are fed to washers to absorb hazardous substances.

When safeguarding media with dangerous properties, system-related I&C protective measures must be provided in order to keep the probability of a response low. For the dispersion behavior of dangerous substances in larger systems, which are emitted when a safety valve is triggered, the approval authorities may require dispersion calculations. It is z. B. to plants in the chemical industry that fall under the Federal Immission Control Act (BImSchG). It must be proven that no hazards can occur. In the past, there was considerable personal injury when safety valves were triggered, e.g. B. uncontrolled spread of liquefied gas in the basement (accident Hotel Riessersee ) or ammonia emissions from refrigeration systems .

Note: In the hotel on the Riessersee there was a serious liquefied gas explosion, which resulted in 11 deaths and 14 injuries. The hotel had a buried liquid gas storage container in which a heater was installed in order to vaporize sufficient liquid gas during peak demand. On December 27, 1986 the control failed and the heating was not switched off after the target pressure was reached. As a result of the pressure increase, a safety valve in the dome shaft opened. Since the terrain to the hotel was sloping, the liquid gas flowed into the basement rooms and ignited there.

Since the tightness of the seat of the safety valve is limited, rupture discs are installed upstream for dangerous substances, which are considered technically tight. The space between the rupture disc and safety valve must be monitored in order to be able to detect a ruptured rupture disc .

Safety valve with bellows , which compensates for the influence of any back pressure that may be present (back pressure compensated overflow valve )

Further pressure relief devices

Bursting disc

Rupture discs are used when large mass flows have to be removed and / or losses via the seat seal on a safety valve have to be avoided. Since, in contrast to the safety valves, the pressure relief opening is not automatically closed again, rupture discs can only be used if the high mass flows do not pose a risk when bursting (e.g. blowing off into a closed system, connection to a blow-off line or the use of non-toxic gases ). Rupture discs are used as a pressure relief device in the event of an explosion or deflagration . Rupture discs are also used in connection with a safety valve, on the one hand to ensure the tightness of the secured system in normal operation and on the other hand to ensure that the secured system is reclosed after the safety device has responded. Because they are easier to clean compared with the safety valve, bursting discs are also used in the hygienic area (pharmaceuticals, foodstuffs). Also compare the function of sprinkler valves .

Vacuum breaker

Vacuum breakers are ventilation valves to protect the container against impermissible vacuum. Vacuum could arise if a steam supply is interrupted while cold water is being supplied at the same time due to condensation of the residual steam.

Melt plug

Melt plugs can protect against impermissible pressures from the effects of heat (fire) in system parts.

Web links

Commons : Safety valves - collection of pictures, videos and audio files

Individual evidence

↑ Peter Schott: Sanitary technology - safety and safety fittings in drinking water installations , In: IKZ-Haustechnik, edition 10/2000, page 56 ff.

↑ Pressure Energy - Lexicon of Physics. Spektrum Verlag, accessed on May 28, 2017 .

↑ Bernd Glück: Safety valves . Load assumptions, cross-section design, blow-off systems, algorithms for a computer program and examples.

↑ Andreas Ismaier: Investigation of the fluid dynamic interaction between pressure surges and system components in centrifugal pump systems. Dissertation, 2010, ISBN 978-3-8322-9779-4 .

↑ munichre.com ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (PDF).@1@ 2Template: Dead Link / www.munichre.com

[Schott-1] Peter Schott: Sanitary technology - safety and safety fittings in drinking water installations , In: IKZ-Haustechnik, edition 10/2000, page 56 ff.

[druckenergie-2] Pressure Energy - Lexicon of Physics. Spektrum Verlag, accessed on May 28, 2017 .

[3] Bernd Glück: Safety valves . Load assumptions, cross-section design, blow-off systems, algorithms for a computer program and examples.

[4] Andreas Ismaier: Investigation of the fluid dynamic interaction between pressure surges and system components in centrifugal pump systems. Dissertation, 2010, ISBN 978-3-8322-9779-4 .