DE2751239A1 - Inflammable gas anti-explosion system - uses flame chamber with throttle opening and air lock interconnected with inlet line - Google Patents

Abstract

The system for preventing lighting back of inflammable gases resulting from an industrial process, uses a combustion chamber (4) for the gases. A branch line (1) from the main supply line (2) for the gases is connected by a liquid air lock (3) to the combustion chamber. Opposite the flame (7) at one side of the chamber is a throttle opening (8) of constant size. An outlet at the bottom of the chamber is connected to a further liquid air lock. This has a vessel (13) which holds liquid supplied from a tank (14) alongside, with a pipe (15) between them. The tank has a coil (17) connecting it to the supply line (2) and also a vertical outlet pipe (11). This has an outlet valve (20) controlled by a level monitoring system (21) for the tank.

Description

Bayer Aktiengesellschaft ²

Central Patents Department. Trademarks and licenses

5090 Leverkusen, Bayerwerk Ki / eb

Nov 15, 1977

Method and device for preventing re-ignition in

Exhaust ducts

The invention relates to a method and a device for preventing backfiring in exhaust air lines, which at times Lead flammable exhaust air by automatically shutting off the exhaust air lines as soon as the concentration of flammable substances above the lower ignition limit and the oxygen concentration is sufficiently high for ignition.

In the case of chemical production processes, especially those that work discontinuously, it cannot always be avoided that the resulting exhaust air flows are loaded with combustible waste materials beyond the lower explosion limit at unforeseeable times. If such exhaust air cleaned by being fed to a combustion chamber, then ^fc is the risk of re-ignition, the through aerated ie the flame from the combustion chamber to return to the operation. But even without an open flame there can be a risk of re-ignition, for example due to sparks as a result of static charging or on surfaces with a catalytic effect. The task here is to reliably prevent back-ignition through lines that temporarily carry ignitable air-gas mixtures, air-vapor mixtures or air-dust mixtures right into operation.

In addition to the known method, one that runs back in a line Catching the flame, e.g. with so-called flame sieves or by immersing in water or by shooting in extinguishing agents,

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There are also processes in which the exhaust air line quickly moves between the ignition source (e.g. flame) and the plant at the moment is blocked where the exhaust air becomes ignitable. For a reliable process it is necessary that

1) a measuring or analytical process to determine the loading of the exhaust air with flammable substances up to above the lower ignition limit or up to a safety distance from the lower ignition limit in a sufficiently short safety distance Measuring time and

2) the signal from the analyzer serves as a control pulse for the shut-off device in the exhaust air line, the shut-off of the exhaust air must also take place in a sufficiently short time in terms of safety.

In order to analyze whether the ignitability has been reached or a proportion of the load in the exhaust air required for ignition are next to The IR or UV absorption measuring devices that have proven themselves for individual substances also know the increased flame ionization or the temperature increase that occurs on oxidation catalysts. The latter two analytical methods have the advantage that they can also be used with mixtures of flammable substances and, in particular, with those with strongly fluctuating composition can still be used.

However, all known methods have the following basic disadvantages:

a) The total concentration that is decisive for the reignition capability to flammable substances with sufficient oxygen content for ignition at the same time cannot be used directly can be detected. It just becomes a related benchmark measured, such as the concentration of a single component or, at best, the cumulative effect of several Components, e.g. in the last-mentioned method of flame ionization or oxidation catalysts

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b) The commercially available analyzers are fed with relatively small amounts of exhaust air, so that from the time of A certain time elapses until the measured value is available.

c) Analyzers require maintenance and are not an absolutely safe link in a safety circuit. First when installing two devices working in parallel, whose puncture is monitored in a suitable manner by measuring technology the output measured value can be used in a safety circuit.

Because of the great dangers for operation when a flame is re-ignited, the shut-off must be carried out reliably and quickly. As a result, the monitoring of the analysis devices is of great importance and for this reason also great effort is involved. Because of the mutual dependencies, the error probability increases with the number The control loops quickly become so large that the cost of risk reduction is hardly justifiable from an economic point of view.

The invention is therefore based on the object of providing a reliable method for determining the lower ignition limit of a To develop gas-vapor mixture, its operational reliability It can be checked at any time using tried and tested means and that a safe and fast shut-off of the exhaust air line guaranteed by the main combustion chamber.

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This object is achieved in that from the main exhaust air flow is branched off a partial flow, which through an ignition chamber is conducted, in which a constant lure flame burns and when reaching the count in the chamber occurring pressure or temperature increase is used to to operate a shut-off valve located in the main exhaust air line. The method according to the invention thus uses as an indicator, the ignitability itself, in that a partial flow of the exhaust air is brought to the ignition temperature in the presence of a flame. The barrier can thus be reliable and quickly through the measurement of the pressure or the temperature and thus through the transmission of an electrical or pneumatic impulse to the shut-off device to be triggered. The measuring and control devices required for this are known to be reliable members of a safety circuit.

A further development of the invention is that the shut-off valve as a hydraulic closure according to the principle the communicating tubes is formed and the pressure increase that occurs is used directly to the mirror to raise a barrier liquid in the main exhaust air line to such an extent that the main exhaust air flow is blocked.

In this case, when reaching the lower Combustion energy released from the ignition limit is used directly to close the shut-off valve in the main exhaust air line actuate.

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A constant flow of combustible gas is advantageously added to the branched-off partial flow before it comes into contact with the attracting flame. Natural gas is preferably used for this purpose, which is mixed in with a volume fraction of 2-5 volumes ^. In this way, the shut-off process can be triggered when a fraction of the exhaust air load with combustible substances is reached. In the case of a natural gas concentration of, for example, 3.75 % , the shut-off device responds when only 40 % of the ignitable load has been reached.

The ignition chamber is advantageously provided with a throttle through which the partial flow flows out. When given Pressure in the ignition chamber behind the throttle maintained such a pressure and the throttle cross-section selected so that the critical flow velocity is established in the throttle. In practice, this is achieved by going behind the throttle is about half of that prevailing in the Zühdkammer Pressure setting. This method has the advantage that there is no feed pump between the ignition chamber and the main air line is needed.

A device suitable for carrying out the method described above is characterized according to the invention:

a) that from the main exhaust line a branch line

leads to the ignition chamber, in which there is a constant lure flame burns and which has a throttle of constant cross-section as an outlet opening for the partial exhaust air flow,

b) that the ignition chamber to one leg of a communicating Line system with a barrier liquid is connected and the other leg is designed as a shut-off vessel in the main exhaust air line,

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c) and that the outgoing exhaust air line protrudes from above into the shut-off vessel so that its lower end is closed by the rising barrier liquid.

For reasons of safety, the main exhaust air line must also pass through if an ignitable mixture is present a conventional valve can be shut off. This can be achieved in a simple manner with a shut-off valve in the main exhaust air line via a level indicator on the The shut-off vessel is actuated.

The shut-off vessel is in the normal case, i.e. in the open state so far filled with sealing liquid that the distance a) between the lower end of the outgoing exhaust air line and the The liquid level is 0.2 d to 1 d, preferably 0.3-0.4 d, where d is the inside diameter of the outgoing exhaust air line.

To keep the liquid level in the shut-off vessel constant and to compensate for evaporation losses, the shut-off vessel is expediently provided with a barrier fluid flow regulator.

An important further development of the invention is that a dwell time pipe is installed in the main exhaust air line between the location of the partial flow branch and the shut-off vessel, which is dimensioned so that its dwell time for the flowing exhaust air is at least equal to the setting time of the communicating pipe system. Assuming flow velocities of a maximum of 8 m / sec, the length of the residence time tube should be at least JO m. This compensates for the inertia of the actuator, so that in no case ignitable exhaust air can get into the line coming from the shut-off vessel.

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Further improvements and refinements of the invention are described in the subclaims.

The advantages achieved with the invention consist primarily in the fact that the same physical effect is used as the measured variable is used that is to be prevented during operation; i.e. the ignitability itself. In this way, the safety for monitoring can be significantly improved. Another advantage is that there are no complex measuring and control circuits are needed. The energy required for the shut-off process comes from the hydraulic version Closure directly from the probe. Due to this construction, there is a high level of operational safety with minimal maintenance guaranteed.

In the following an embodiment of the invention is explained with reference to a drawing.

Show it

Figure 1 is a flow diagram for a backfire protection, in which the temperature or occurring in the ignition chamber Pressure increase converted into an electrical signal and thus an actuator in the main exhaust air line is operated and

Figure 2 is a flow diagram for a backfire protection with hydraulic Closure of the exhaust air line.

In the backfire protection according to FIG. 1, a sample (0.5-3% » preferably 1 %) of the main flow is continuously taken from the exhaust line 2 with the help of a constant volume feed pump la in a branch line 1 and through a level-monitored immersion fuse 3 into a heat-insulated ignition chamber 4 headed. The ignition chamber also flows a constant amount of ignition gas 5 and amount of fresh air 6, so that a

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Lockflanune 7 burns constantly. Natural gas is used as the ignition gas. The exhaust air / gas mixture flows through a throttle 8 into the open, so that under normal conditions there is a constant overpressure in the ignition chamber 4. The throttle cross-section and the pressure ratio - ^ are chosen so that the critical flow velocity is established in the throttle 8. Here, p _{a means} the external pressure and ρ the pressure in the additional chamber. "Critical

Flow rate "means that the gas at the in the throttle prevailing state (pressure and temperature) flows at the speed of sound. That is in practice Pressure ratio p _ft / p approximately at 0.5 (critical pressure ratio). With a given flow rate, the associated throttle cross-section can then be calculated (see, for example, Ulimann's Encyclopedia of Technical Chemistry, 3rd edition, 1st volume, Verlag Urban-Schwarzenberg, Munich-Berlin, 1951 »p. 24, formula 45.) Increases in. Exhaust air flow 2 increases the load of combustible substances so strongly that the lower ignition limit is exceeded, then combustion of the combustible exhaust air substances also begins in the ignition chamber 4 at the attracting flame.

The associated rise in temperature causes an increase in the volume flow through the throttle 8 and thus an increase in pressure in the ignition chamber 4. This increase in pressure is converted into an electrical one by a pressure transmitter 9 Signal converted that actuates an electromagnetic shut-off valve 10 in the exhaust air line 11. Above the Shut-off valve, an emergency outlet 12 is provided which can be opened when the valve 10 is shut off. Instead of the pressure increase, the pressure in the ignition chamber 4 The temperature increase occurring during the ignition process can be used to actuate the shut-off element 10. In place of the Pressure transmitter 9 is then an amplifier, e.g. the converts the temperature increase measured with a thermocouple into a sufficiently large electrical signal. In the When selecting the shut-off element 10 and the measuring transducer 9, a fast response time is in the foreground.

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In the embodiment according to FIG. 2, when reaching the Zundbegrenzer in the chamber 4 occurring pressure increase used directly to a hydraulic closure in the exhaust air line 11 to be pressed. The hydraulic lock consists of a communicating line system with a hydraulic one Chamber 13, and a shut-off chamber 14 through the Line 15 are interconnected. The communicating line system 13, 14, 15 is with water as a barrier liquid filled. The one that normally prevails in the ignition chamber 4 Overpressure causes the sealing water level in the hydraulic Chamber 13 and the shut-off chamber 14 on different but are constant in height. When an incapacitant occurs Mixture and the then onset of strong combustion in the ignition chamber 4, an increase in pressure occurs and a decrease the water level in the hydraulic chamber 13 for Consequence. The water level in the shut-off vessel thus rises above the lower edge of the vertically opening pipe 11 for the outgoing main exhaust air flow, so that this line is shut off when the exhaust air is above the ZUnd limit is loaded with flammable substances. In order to ensure safe operation, the shut-off vessel 14 is filled with sealing liquid filled that the distance a between the lower pipe end of the exhaust line 11 and the liquid level 0.3 d - 0.4 d (d = inner diameter of the line 11).

In order to further increase the safety, in a preferred embodiment of the device by a level control

16 evaporation losses in the shut-off chamber 14 compensated. Furthermore, there is a dwell time loop in the main exhaust air line 2

17 between the removal of the partial exhaust air flow and shut-off chamber 14 of such a length that the shut-off time is shorter than the dwell time of the exhaust air between the point of extraction and the shut-off vessel 14. Furthermore, the Avoidance of overpressure in case of blocking the main exhaust air flow drained by the anti-immersion device 18, which may also be monitored by level, via the supply line 19 the branched off partial flow before contact with the lure flame

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a constant natural gas flow can be added. For example, if the natural gas content is 3.75 vol # (corresponding to 60 # of the lower ignition limit for natural gas) «, the shut-off device is activated when the exhaust air load with combustible substances reaches 40% of the lower ignition limit.

After the hydraulic shut-off, a final separation of the exhaust air line 11 is usually necessary. For this purpose there is a shut-off valve 20 with an electromagnetic drive in the exhaust air line 11, which is operated by a Level sensor 21 in the shut-off chamber 14 is switched. The level sensor 21 responds when the water level in the shut-off vessel 14 has risen a little above the lower end of the exhaust air line 11. As in the embodiment according to FIG. 1, an emergency outlet 12 is arranged above the shut-off valve 18. At the The valve 12 must be closed and the shut-off element 20 and the hydraulic lock in the shut-off chamber must be closed 14 can be opened again. Before operation can be resumed, the dwell time loop 17 and the ignition chamber 4 flushed with fresh air to ensure that no ignitable residual exhaust air is present.

If the flammable exhaust air substances are soluble in water, another suitable anti-kickback device is used instead of the immersion protection 3, ζ.B. a commercially available tape lock is used. Prepares the conveyance of a constant partial flow of exhaust air Difficulties, e.g. due to the risk of corrosion, according to an alternative embodiment, there is one behind the throttle 8 Maintain negative pressure (e.g. with the help of a water jet or a water ring pump) that in the throttle 8 the critical Speed is achieved. The pump 1 is then omitted.

The method and the device are explained using an example: The amount of exhaust air is 3600 nr / h in the standard state (1013 mbar and 0 ° C) with changing loads of flammable substances. This main flow becomes constant as a partial flow taken via the pump 1 40 nr / h and into the cylindrical

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heat-insulated ignition chamber of 150 mm 0 and 250 mm length 4 introduced tangentially. 0.1 1117 h of natural gas and 1 nr / h of fresh air are also fed to the chamber centrally to the UV-monitored lure burner 7. The gas mixture leaves the ignition chamber through the central discharge nozzle 8 (19 * 3 mm 0) opposite the attracting flame at a minimum of 59 ° C., with an overpressure of 20 mbar being established in the chamber 4. The chamber I3 with the ignition chamber 4 is connected through a pipe of 40 mm ~ 0 has a 0 of 500 mm and is almost completely filled with sealing water.

The main exhaust line 2 has 357 mm 0. The dwell time loop 17 is approx. 35 m long. The shut-off vessel 14 has a 0 of 564 mm, the central exhaust duct also 357 now 0. The The water level is measured via the controller 16 134 mm below the The end edge of the outlet pipe 11 is held, a check valve preventing any backflow into the feed line will. If the loading of the exhaust air with flammable substances reaches the lower ignition limit, 4 rises in the ignition chamber the overpressure δ ρ according to the relationship

* 2

_A _ VTR

Δ ρ =

2 A ² . S. ρ. / A at. Thereby means

V is the constant volume flow supplied to the ignition chamber

of exhaust air + natural gas + fresh air

T is the absolute temperature

R is the general gas constant

A is the cross section of the throttle 8

£ is the contraction number of the flow in the throttle 8

ρ is the total pressure in the chamber 4

μ. the molar mass of the gas mixture

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Since all other quantities remain constant with sufficient precision « Consequently

Δ ρ = constant T.

When the lower ignition limit is reached, the temperature in the ignition chamber rises to at least 600 ° C. and thus the overpressure to 52.6 mbar, so that the water level in the shut-off chamber 14 also rises by 326 mm and thus closes the exhaust line 11. The time from reaching the ignition limit in the extracted partial flow 1 to the closure of the exhaust air line 11 is 3 s with the selected dimensions, so that ignitable exhaust air does not get into the pipe 11 under any circumstances. If the load in the exhaust air falls below the ignition limit, the temperature and overpressure in the ignition chamber 4 drop to the initial values. The sealing water can thus also flow back into the chamber 1J> so that the exhaust air line 11 is released again.

Overall, the new method has the advantage that the physical property of the exhaust air, which in the presence of a Ignition source also allows back ignition, namely reaching or exceeding a load equal to the lower ignition limit and the simultaneous presence of a sufficient oxygen concentration for ignition (generally depending on the type of fuel) between 8 and 12 VoIJi) immediately triggers the shut-off of the exhaust air line 11. The ignition capability is still for the unfavorable case found, since the ignition source is an open flame with temperatures> 1200 ° C with the release of radicals that accelerate the ignition.

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Claims (1)

I claims: 1.Methods for preventing reignition in exhaust ducts, the temporarily ζundable exhaust air through automatic Shutting off the exhaust air lines as soon as the concentration of combustible substances is above the lower ignition limit and the oxygen concentration is sufficiently high for ignition, characterized in that a partial flow from the main exhaust air flow is branched off, which is passed through an ignition chamber in which a lure flame is constantly burning and the increase in pressure or temperature that occurs in the chamber when it is ignitable is used to actuate a shut-off device in the main exhaust air line » 2. The method according to claim 1, characterized in that the shut-off device as a hydraulic closure according to the principle the communicating tubes is formed and the pressure increase that occurs is used directly to the mirror a Raise the barrier liquid in the main exhaust air line so that the main exhaust air flow is blocked. 3. The method according to claim 1, characterized in that the A constant flow of flammable gas is added to the branched off partial flow before it comes into contact with the lure flame. 4. The method according to claim 5, characterized in that as flammable gas natural gas is used. 5. The method according to claim 4, characterized in that a Volume fraction of 2-5 Vol £ natural gas is added. 6. The method according to claim 1 to 5 »characterized in that the partial flow in the ignition chamber flows out through a throttle and at a given pressure ρ in the ignition chamber, the throttle cross-section and the pressure p _a downstream of the throttle are selected so that el sets the critical flow velocity in the throttle. Le A 18 066 - 13 - 909820/0485 7 · Device for carrying out the method according to claims 1 to 6, characterized in that from the main exhaust air line (2) a branch line (1) leads to an ignition camera (4), in which a lure flame (7) is constantly burning and one throttle (8) constant cross-section as an outlet opening for the partial exhaust air flow that the ignition chamber (4) at the one Leg (13) of a communicating pipe is connected to a barrier fluid and the other leg is used as a shut-off vessel (14) is formed in the main exhaust air line (2) and that the outgoing exhaust air line (11) from above into the Shut-off vessel (14) protrudes so that its lower end is closed by the rising barrier liquid. 8. Apparatus according to claim 7 * characterized in that a level indicator (21) is attached in the shut-off vessel (14), which actuates a shut-off valve (20) in the exhaust air line (11) when the liquid rises. 9. Apparatus according to claim 6 to 8, characterized in that the shut-off vessel (14) in the normal case so far with sealing liquid is filled so that the distance a between the lower end of the outgoing exhaust air line (11) and the liquid level is 0.2-1 * d, preferably 0.3 to 0.4 * d, wherein d is the inside diameter of the outgoing exhaust air line (11). 10. The device according to claim 9, characterized in that the shut-off vessel (14) with a barrier liquid itszuflußregeler (l6) is provided in order to keep the liquid level constant. 11. The device according to claim 6 to 10, characterized in that in the branch line (1) a pump (la) with a constant Delivery volume is built in. 12. Apparatus according to claim 6 to 11, characterized in that between the pump (la) and the ignition chamber (4) an immersion fuse (3) is arranged. Le A 18 066 - 14 - 909820/0485 Ij5 · Device according to claim 6 to 12, characterized in that « that between the location of the partial flow branch and the shut-off vessel (14) a residence time pipe (17) in the main exhaust air line (2) is installed, the dwell time of which is at least equal to the setting time of the communicating pipe system (13 * 14, 15) or the setting time of the measuring and control system (9,10). 14. Apparatus according to claim 13 »characterized in that the length of the dwell tube (17) at a flow velocity of a maximum of 8 m / sec is at least JO m. 15 · Device according to claim 6 to 13, characterized in that that an overpressure safety device (18) is installed in the main exhaust air line (2), which in the event of the outlet line being shut off (11) opens. Le A 18 066 - 15 - 009820/0485