Safety valve

Safety valves protect pressurized spaces or pressure vessel (such as boilers, pressure vessels, piping, transport containers ) against an excessive increase in pressure, which can lead to damage of the attached printing device. Safety valves derive when exceeding the set pressure gases, vapors or liquids in the atmosphere or in collecting pipes.

General

The safety valve is a pressure relief device that secures a printing system against excessive pressures. In addition to the safety valve include the following facilities to the pressure relief devices:

• Rupture Disks
• Fusible
• Water templates

Rupture disks are used when large mass flows must be removed and / or losses on the seat seal must be avoided to a safety valve. Since, in contrast to the safety valves, the pressure relief opening is not closed again automatically, rupture discs can only be used if the high mass flows from the bursting no hazard exists (eg blowing into a closed system, following a blow-off pipe or use of nontoxic gases ). Bursting discs are used as a pressure relief device explosions and detonations. Rupture discs are also used in conjunction with a safety valve to the one to ensure the tightness of the safe system during normal operation and on the other to ensure the re-closing of the related hedged plant according to the safety device. Because of their better cleanability compared with the safety valve bursting discs are (, Food Pharma ) used in the hygiene sector.

Can fusible secure ( fire) in plant parts impermissible pressure due to heat.

The safety valve is triggered if the pressure in a pressure vessel exceeds the allowable operating pressure under foreseeable accidental by more than 10 %. With proper dimensioning of the safety valve of the pressure build-up remains manageable. After activation of the safety valve and reduction of excessive pressure by blowing into the environment or a pipe, the valve closes again and the system can continue to operate. One example is the boiler safety valve of a steam locomotive.

Further, a safety valve must be installed in each closed circuit, that is heated. This is the case for water heating systems, collector circuits and all direct or indirect heating circuits. The safety valve prevents the bursting of a plant, when a thermal fluid expansion occurs by heat transfer and no defined gas cushion is available. When a gas overlay is missing, the expansion of the fluid leads to a deformation of the walls. Even small increases in temperature - just by sunlight - can already lead to undue pressures with an increase in temperature in the range of 10 to 30 ° C. Organic liquids (for example ethylene glycol ) are more critical than the water because of the high coefficient of volume expansion.

The safety valve on hot water heating boiler speaks at bar at 3. It protects the system against unauthorized printing, if an error in the pressure maintenance occurs. If a defect of the bladder in the diaphragm expansion vessel, the nitrogen gas cushion is omitted in the system. The heating of the heating water and the resulting thermal thermal expansion of the liquid may have an unacceptable pressure result. Furthermore, can cause an unacceptable pressure by refilling the heating system from the water supply, since the pressure of the water network is generally higher than the permissible pressure of the boiler. The safety valve must be designed for the maximum Abblasemassestrom or volume flow. Safety valves for heating boilers are reported according to the rated output.

Another example are safety valves on air compressors that are designed for air delivery of the compressor. Safety valves on tanks for the storage of cryogenic gases (eg liquid oxygen ) must be designed for the evaporation mass flow when the vacuum is broken inside the insulation.

In many assets are marking a pressure safety valve with PSV (Pressure Safety Valve ) with a subsequent sequential number.

Classification

Safety valves are in terms classified according to their response:

• Proportional valves. The opening behavior between the area of ​​the set pressure pset up to fully open ( 1.1 pset ) is proportional to the pressure,
• Vollhubsicherheitsventile open in response abruptly and with full stroke,
• Normal safety valves have no specific response.

Furthermore, safety valves are distinguished according to the type of counter-force to pressure hedged:

• Spring-loaded safety valves,
• Weight-loaded safety valves,
• Medium -loaded safety valves.

The weight and Hebelarmübersetzungsverhältnis or the bias of the spring are in the response pressure in equilibrium with the force acting on the valve plate due to the internal pressure.

For medium -loaded safety valves act on both sides of the sealing plate, the same pressures, by differently large areas, a greater force acts in the closing direction. The valve is controlled via an additional valve unit. When exceeding the set pressure of the overlying the sealing plate area is on the supplementary venting valve unit, the closing force drops off and the valve opens abruptly.

Furthermore, a distinction is made between direct-acting and controlled safety valves. Controlled safety pressure corresponding to the direct acting up on an additionally applied closing and opening force. This can be a pneumatic or hydraulic cylinders drive, which causes a certain extent, an additional loading force and when the set pressure is exceeded, an additional lifting force. This is a much more accurate response is achieved. Operated Safety Relief Valves are used eg for larger power plant steam boilers. Due to the re-evaporation with larger volumes of water longer times result in non-controlled safety valves to re- close the safety valve of a pressure surge.

Characteristics

The characteristic values ​​for the design of safety valves are:

• Pset: pressure in bar (gauge pressure),
• D0: minimum aperture diameter of the valve seat in mm,
• Or KDR: awarded reduced discharge coefficient,
• H: height of the relief valve ( between the seat position of the valve disc, and the maximum aperture ) in millimeters.

The effluent mass flow is calculated from the conversion of the pressure energy into kinetic energy ( Bernoulli equation). The discharge coefficient describes the integrated over the flow cross -section A0 actual flow rate to the at uniform velocity distribution ( Pfropfenprofil ).

The coefficient of discharge Kdr is identified in the component test in experiments. This value is multiplied by a safety factor of 0.9 and can then be used as conferred reduced coefficient of discharge Kdr in the bill for the Abblasemassenstrom. The discharge coefficients of gases and liquids are different.

The flowing out of a safety valve flow rate for the gases can be calculated from the following equation:

• : Resting pressure (absolute) in the tank and set pressure of the safety valve
• : Specific volume of the gas in the container
• : Pressure ( absolute) in the outflow
• : Isentropic (air :; Ne, Ar:, water, CO2: )

For print all pressures are given in gauge pressures (eg, the operating pressure of pressure vessels, the set pressure of safety valves ). In the thermodynamic equations always the absolute pressure must be used.

At subcritical or supercritical pressure conditions ( pressure in the internal pressure to pressure in the discharge space ) gas flow at sonic speed ( critical pressure ratio of air: ). The mass flow rate is then independent of the pressure in the volute. However, it is to be noted that with increasing back pressure, the opening force on the valve seat decreases, the valve does not open completely, and thus, the maximum stroke is no longer reached. This changes the discharge code, and ultimately reduces the dissipated mass flow. In the calculation rules for safety valves, this is observed in that the coefficient of discharge for certain discharge pressures are valid ( proportional valves. Maximum 15 % back pressure, safety valves with bellows: max 30 % back pressure. ).

Rated

The design of safety valves must the load assumptions (causes that lead to the activation of the safety valve, payable volumes, type of medium ) to a particular extent, define, calculate the cross sections of the safety valves and take into account the type of Abblasesysteme ( construction of a possible counter- pressure). Detailed instructions and algorithms for computing program: The use of safety valves on the limitation of very rapid pressure changes ( water hammer ) is discussed controversially in the literature. While bursting discs are recommended for the relief at extremely rapid increases in pressure such as explosions, safety valves is at a great uncertainty as to whether they can free the needed cross-sectional flow fast enough. New investigations and numerical calculations show, however, that safety valves can be used generally to surge limit. Here, however, a careful assessment of the safety valves is necessary because of the wrong size valves can be made ​​to vibrate and can thus even increase the pressure pulse amplitude.

Standardize

Before implementation of the Pressure Equipment Directive 2002 Safety valves were designed ( pressure vessels, equipment Working Group ) to the AD - leaflet A2. The safety valves had to be type tested. The tests were carried out by TÜV or similar institutions. Method and extent of component testing were listed in the VdTÜV " safety valve 100 "

After the introduction of the Pressure Equipment Directive 97/23/EC AD 2000 Data Sheet A2 can be used as a test basis. Alternatively, other regulations, such as the harmonized European standard series EN 4126 may be used. The standard has seven parts:

• EN 4126 Part 1: Safety valves
• EN 4126 Part 2: Bursting disc
• EN 4126 Part 3: Safety valves and bursting disc safety devices in combination
• EN 4126 Part 4: Pilot operated safety valves
• EN 4126 Part 5: Controlled safety pressure relief systems (CSPRS )
• EN 4126 Part 6: Bursting disc safety devices - Selection, use and installation
• EN 4126 Part 7: Common data

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

• EN 13136 Refrigerating systems and heat pumps, pressure relief devices and their associated piping - Calculation method

Safety valves are according to the Pressure Equipment Directive as equipment with safety function. They are assigned to the highest category IV and must be manufactured according to the specified manufacturing and testing requirements under power on a notified body.

By appropriate tests, the discharge coefficient is determined. Due to the different aerodynamic characteristics, the discharge coefficient for gases and liquids is different. In addition, it must be shown that the pressure increase from the lifting of the valve disk to the complete opening is not more than 10 %. Safety valves shall operate at a pressure drop of 10% ( gas) and 20% (liquids) close below the response again.

Depending on the interpretation of the spring set pressure of safety valves can be adjusted within a certain range. The predetermined opening pressure is achieved by the spring preload is affected by a threaded insert. The position of the bias is secured for example via a lock nut against loosening. The Adjustable fitting is sealed with a cap. As a measure against unauthorized change of attitude this hood is secured with a lead seal. For smaller safety valves and caps are used as cover. The unauthorized adjustment of safety valves can cause a significant hazard and represents a dangerous manipulation

Safety valves for non-toxic substances must be liftable. Acts with the aid of a lever or a cap, which can be loosened, and against the spring force, the valve must be caused to open at the latest on reaching a pressure of 85 % of the response. Safety valves are to be ventilated, in order to counteract sticking or Festkorrodieren of the valve disk. With the use of hazardous or environmentally hazardous fluids dispensing with the Anlüftbarkeit. Instead, two safety valves are connected to the two output sides of a shuttle valve and the input side is connected to the pressure chamber being hedged. This arrangement allows the development and testing of a safety valve when the other valve is in use. The continued operation of the connected pressure chamber is thus possible. For larger LPG storage tanks, ammonia tanks for cryogenic gases or containers in refrigeration systems that can only be emptied consuming, this arrangement is prescribed in the standards.

When using a blow-off pipe is to be noted that the back pressure remains within the permissible range, and thus, the maximum stroke and thus discharge mass flow is ensured. Using the Bernoulli equation must from the flow resistances ( piping, bends, constrictions ) calculated pressure loss in the allowed range. There is also the requirement that the pressure loss on upstream side must not be more than 3% of the set pressure.

Operators of printing equipment need to set time limits for consideration of safety valves as part of a risk assessment according to the Ordinance on Industrial Safety.

Installation instructions

With the use of safety valves, the operating instructions must be observed because the types are only suitable for certain applications. The following factors must be considered in the selection:

• Dimensioning with respect to possible pressure generator,
• Corrosion ( medium - material compatibility )
• Resistance of the seat seal (if using soft gaskets )
• Discharge of condensate from the valve body on the downstream side,
• Impairments caused by dust or sticky -acting products.

The safety equipment of pressure equipment must be proven by a competent person or an authorized body ZÜS as part of a " Preoperative Tests " according to regulation.

The outlet openings of safety valves that protect dangerous substances (water, air) can be usually done in a room. The orifice opening is necessary, to arrange it so that people are not at risk (eg, discharge of hot water from boilers ).

For all other substances that are toxic, flammable or luftverdrängend for example, need to be derived in areas where no danger is to be expected. The safety valves are connected to discharge pipes, which opens on the roof, for example. If necessary. escaping fluids are supplied to scrubbers for absorbing hazardous substances.

For hedges of media with hazardous characteristics plant engineering MCR protection measures must be taken to keep the probability of a response low. For the propagation behavior of hazardous substances in larger systems that are emitted in response of a safety valve, request for admission authorities if necessary dispersion calculations. It covers, for example, by chemical industry plants, which fall under the Bundesimmissionschutzgesetz ( Federal Pollution Control Act ). It is to prove that no hazards can occur. In the past there have been substantial personal injury upon actuation of safety valves, for example, uncontrolled spreading of liquefied gas in the basement (Accident Hotel Riessersee ) or ammonia emissions from refrigeration systems.

Note: In the hotel at the Riessersee a momentous LPG explosion, which had 11 deaths and 14 injuries result occurred. The hotel had a erdgedeckten LPG storage tank in which a heater was installed to sufficiently evaporate liquid at peak demand. On December 27, 1986 denied the regulation and the heating was not switched off after reaching the required pressure. As a result of the pressure rise opened a safety valve in the manhole. Since the terrain was steep to the hotel, poured the liquid into the basement and ignited there.

Since the tightness of the seat of the safety valve is finite, bursting discs are upstream for hazardous substances that are considered technically tight. The space between the bursting disc and safety valve shall be monitored to detect a ruptured burst disk can.

Overflow

Relief valves relieve pressure chambers upon the occurrence of unacceptable pressures in enclosed spaces with lower pressures. Thus it is ensured that no medium penetrate the atmosphere and thus no danger occur.

Safety valves can be used in principle as relief valves. This implies that the upper part of the safety valve, the spring housing and the cover are made ​​sufficiently close to the earth's atmosphere. The valve stem guide for the spring housing is not tight, so that there is in the upper part of the outflow side pressure, and thus it acts as a counter force on the valve disk. As a result, the set pressure is dependent on the back pressure.

By installing a bellows (see picture), the back pressure can be compensated. A bellows to the valve plate is welded, the inner shafts have exactly the diameter D0 of the valve opening. The bellows encloses the stem guide and is gas -tightly connected to the housing. In this manner, rests on the valve disk from the top of the cross-sectional direction A0 of the ambient pressure. Thus, the set pressure of the relief valve is independent of the pressure on the downstream side. The overflow valve is back pressure independent or pressure balanced.

However, it is to be noted that with increasing back pressure of the pulse of the flow is reduced. This has the consequence that the valve disc is raised not to the full stroke h. The discharge coefficient does not reach the value that was determined in pressure-less outflow. The dissipated mass flow is correspondingly lower. Furthermore, it should be noted that due to the manufacturing tolerance of the bellows and the effective contact surface of the valve head on the seat there be any major Ansprechtoleranzen.

Vacuum breakers are air valves to protect the tank against unacceptable vacuum. Vacuum could result from disruption of a supply of steam at the same time cold water supply by condensation of the residual steam.