Gas purging plug, gas purging system, method for characterization of a gas purging plug and method for purging a metal melt

Abstract

Various embodiments provide for a gas purging plug (10) with a ceramic refractory body (10k) with a first end (10u) and a second end (10o); the second end (10o) is in the mounted position of the gas purging plug (10) in contact with a metal melt (41); the first end (10u) is at least partially covered with a metal cover (12.1), the metal cover (12.1) comprises an opening (16) to which optionally a gas supply adapter (20) is connected; the gas purging plug (10) is designed in such a way, that a purging gas which is supplied via the gas supply pipe (30) to the opening (16) flows through the body (10k) and exits the body (10k) at the second end (10o); and wherein at least one electronic sensor (70, 70.1, 70.2, 70.3, 70.4) is in contact with the gas purging plug (10), to detect a mechanical vibration (81).

Claims

1. Gas purging plug (10) for metallurgic applications comprising a.) a ceramic refractory body (10k) with a first end (10u) and a second end (10o); b.) the second end (10o) is in a mounted position of the gas purging plug (10) that is in contact with a metal melt (41); c.) the first end (10u) is at least partially covered with a metal cover (12.1), the metal cover (12.1) comprises an opening (16) to which optionally a gas supply adapter (20) is connected; d.) the gas purging plug (10) is designed in such a way, that a purging gas, which is supplied via the opening (16), flows through the body (10k) and exits the body (10k) at the second end (10o); e.) and at least one electronic sensor (70, 70.1, 70.2, 70.3, 70.4) in direct contact with the gas purging plug (10), to detect an oscillation waveform of a mechanical vibration (81), whereas the electronic sensor (70, 70.1, 70.2, 70.3, 70.4) is an acceleration sensor.

2. Gas purging plug (10) for metallurgic applications according to claim 1, whereas the at least one electronic sensor (70, 70.1, 70.2, 70.3, 70.4) is mounted on the metal cover (12.1) or on the gas supply adapter (20) of the gas purging plug (10).

3. Gas purging plug (10) for metallurgic applications according to claim 1, whereas the electronic sensor (70, 70.1, 70.2, 70.3, 70.4) is a piezoelectric acceleration sensor (70, 70.1, 70.2, 70.3, 70.4).

4. Gas purging system comprising a gas purging plug (10) for metallurgic applications and a gas supply pipe (30) connected to the gas purging plug (10), the gas purging plug (10) comprising: a.) a ceramic refractory body (10k) with a first end (10u) and a second end (10o); b.) the second end (10o) is in a mounted position of the gas purging plug that is in contact with a metal melt; c.) the first end (10u) is at least partially covered with a metal cover (12.1), the metal cover (12.1) comprises an opening (16) to which optionally a gas supply adapter (20) is connected; d.) the gas purging plug (10) is designed in such a way, that a purging gas which is supplied via the gas supply pipe (30) to the opening (16) flows through the body (10k) and exits the body (10k) at the second end (10o); e.) and wherein at least one electronic sensor (70, 70.1, 70.2, 70.3, 70.4) is in direct contact with the gas purging plug (10), to detect an oscillation waveform of a mechanical vibration (81), whereas the electronic sensor (70, 70.1, 70.2, 70.3, 70.4) is an acceleration sensor; the gas purging system further comprises: f.) a data processing unit (80) for acquiring the oscillation waveform of a mechanical vibration (81) detected by the electronic sensor (70, 70.1, 70.2, 70.3, 70.4) of the gas purging plug (10) and for calculating a bubble index-signal (83) from the oscillation waveform of a mechanical vibration (81) detected; g.) a control unit (100); wherein the control unit (100) is configured to: display the bubble index-signal (83); and/or vary the volume flow (102) through the gas supply pipe (30) depending on the bubble index signal (83); and/or generate a warning signal (101) when the bubble index signal (83) lies outside a defined range.

5. Gas purging system according to claim 4, further comprising at least one of the following components, connected to the control unit (100): a control valve (100a) to control the volume flow (102) through the gas supply pipe (30); a flow meter (100b) to measure the volume flow (102) through the gas supply pipe (30); optionally a pressure gauge (100c) to measure the pressure in the gas supply pipe (30).

6. Gas purging system according to claim 4, whereas the data processing unit (80) determines at least one bubble index component (86.1, 86.2) by summing frequency amplitude values (82a) from the frequency spectrum (82) over a defined frequency range.

7. Gas purging system according to claim 4, whereas the data processing unit (80) determines the bubble index signal (83) from the summation of the differences or quotients between at least one of actual bubble index components (86.2) and at least one of reference bubble index components (86.1).

8. Method for characterization of a gas purging plug (10), comprising the following steps: Setting an actual volume flow (300) of a gas through the purging plug (10); Acquiring an oscillation waveform of a mechanical vibration (81) at the actual volume flow (102) by at least one electronic sensor (70, 70.1, 70.2, 70.3, 70.4) in direct contact with the gas purging plug (10), whereas the electronic sensor (70, 70.1, 70.2, 70.3, 70.4) is a piezoelectric acceleration sensor; Calculating at least one bubble index component (301) from the acquired oscillation waveform of a mechanical vibration (81) at the actual volume flow (102); Storing at least one bubble index component (302).

9. Method for purging a metal melt (41) in a metallurgical vessel (40) with a gas, comprising the steps of: Setting an actual volume flow (401) of a gas through a purging plug (10) to a pre-determined value of an initial volume flow (102); Acquiring an oscillation waveform of a mechanical vibration (81) at the actual volume flow (102) by at least one electronic sensor (70, 70.1, 70.2, 70.3, 70.4) in direct contact with the gas purging plug (10), whereas the electronic sensor (70, 70.1, 70.2, 70.3, 70.4) is an acceleration sensor; and: Varying the volume flow (404) through the gas supply pipe (30) depending on the acquired oscillation waveform of the mechanical vibration (81); and/or Generating a warning signal (403) depending on the acquired oscillation waveform of the mechanical vibration (81).

10. Method for purging a metal melt (41) in a metallurgical vessel (40) with a gas according to claim 9, comprising the steps of: Calculating a bubble index signal (402) from the acquired oscillation waveform of a mechanical vibration (81) at the actual volume flow (102); and: Generating a warning signal (403) if the bubble index signal (83) lies outside a predefined bubble index range (85), and/or Varying the volume flow (404) through the gas supply pipe (30) as a function of the bubble index signal (83).

11. Method for purging a metal melt (41) in a metallurgical vessel (40) with a gas according to claim 9, whereas before the step of setting the volume flow (401), a step of determining predetermined values (400) for at least one of the values of the following groups is performed: a reference bubble index component (86.1), the initial volume flow (102) through the gas supply pipe (30), a bubble index range (85), a target gas volume (103).

12. Method for purging a metal melt (41) in a metallurgical vessel (40) with a gas according to claim 10, whereas the step of calculating a bubble index signal (402) comprises that the bubble index signal (83) is calculated from the weighted summation of the differences or quotients between the actual bubble index components (86.2) and the reference bubble index components (86.1).

13. Method for purging a metal melt (41) in a metallurgical vessel (40) with a gas according to claim 10, whereas the step of varying the volume flow (404) comprises: increasing or keeping constant the volume flow (404a) through the gas supply pipe (30) in case the bubble index signal (83) lies within a predefined bubble index range (85); and decreasing the volume flow (404b) through the gas supply pipe (30) in case the bubble index signal (83) lies outside a predefined bubble index range (85).

14. Method for purging a metal melt (41) in a metallurgical vessel (40) with a gas according to claim 9, wherein the gas purging plug comprises: a.) a ceramic refractory body (10k) with a first end (10u) and a second end (10o); b.) the second end (10o) is in the mounted position of the gas purging plug (10) in contact with the metal melt (41); c.) the first end (10u) is at least partially covered with a metal cover (12.1), the metal cover (12.1) comprises an opening (16) to which optionally a gas supply adapter (20) is connected; d.) the gas purging plug (10) is designed in such a way, that the gas, which is supplied via the opening (16), flows through the body (10k) and exits the body (10k) at the second end (10o); e.) and at least one electronic sensor (70, 70.1, 70.2, 70.3, 70.4) in direct contact with the gas purging plug (10), to detect an oscillation waveform of a mechanical vibration (81), whereas the electronic sensor (70, 70.1, 70.2, 70.3, 70.4) is an acceleration sensor.

Description

(1) Exemplary embodiments of the invention are explained in more detail by means of illustrations:

(2) FIG. 1 shows a schematic representation of an embodiment of the gas purging plug according to the invention,

(3) FIG. 2 shows a schematic representation of an embodiment of the gas purging system according to the invention,

(4) FIG. 3 shows a schematic sequence of an embodiment of the method according to the invention,

(5) FIG. 4 shows a schematic sequence of an embodiment of the method according to the invention,

(6) FIGS. 5 and 6 show an exemplary diagram of bubble index components.

(7) FIG. 1 shows a first embodiment of the invention, namely a purging plug (10) for metallurgic applications comprising a ceramic refractory body (10k) with a first end (10u) and a second end (10o), the second end (10o) is in the mounted position of the gas purging plug (10) in contact with a metal melt (41, not shown in FIG. 1), the first end (10u) is covered with a metal cover (12.1), the metal cover (12.1) comprises an opening (16) to which a gas supply adapter (20) is connected, the gas purging plug (10) is designed in such a way, that a purging (treatment) gas, which is supplied via the gas supply adapter (20) to the opening (16), flows through the body (10k) and exits the body at the second end (10o), and at least one electronic sensor (70, 70.1, 70.2, 70.3) in mechanical contact with the gas purging plug (10), to detect an oscillation of a mechanical vibration (here a piezoelectric acceleration sensor is used: ICP accelerometer, Model Number 352C33). Between the metal cover (12.1) and the first end (10u) of the body (10k) an optional hollow space (14) allows for a distribution of the purging (treatment) gas before the purging (treatment) gas enters the body (10k) via its first end (10u). An optional metal jacket (12.2) surrounds (at least partially) the body (10k), the metal jacket is connected to the metal cover (12.1) in a gas-tight way, e.g. by welding the metal jacket (12.2) and the metal cover (12.1) together.

(8) In a first alternative embodiment the sensor (70, 70.1) is mounted on the outside of the metal cover (12.1). The sensor (70, 70.1) is configured to detect oscillations/accelerations of a mechanical vibration in a direction normal to the second end (10o) of the body (10k).

(9) In a second alternative embodiment the sensor (70, 70.2) is mounted on the outside of the gas supply adapter (20). The sensor is integrated into a removable clamp (not shown) which can be attached to the gas supply adapter (20). The sensor (70, 70.2) is configured to detect oscillations/accelerations of a mechanical vibration in a direction normal to the second end (10o) of the body (10k).

(10) In a third alternative embodiment the sensor (70, 70.3) is mounted on the inside of the gas supply adapter (20). The sensor (70, 70.3) is configured to detect oscillations/accelerations of a mechanical vibration in a direction normal to the second end (10o) of the body (10k).

(11) In a fourth alternative embodiment the sensor (70, 70.4) is mounted on the inside of the metal cover (12.1). The sensor (70, 70.4) is configured to detect oscillations/accelerations of a mechanical vibration in a direction normal to the second end (10o) of the body (10k).

(12) FIG. 2 shows a second embodiment of the invention, namely a gas purging system comprising a gas purging plug (10) for metallurgic applications and a gas supply pipe (30) connected to the gas purging plug (10) via the gas supply adapter (20). The gas purging plug (10) comprises a ceramic refractory body (10k) with a first end (10u) and a second end (10o), the second end (10o) is in the mounted position of the gas purging plug (10) in contact with a metal melt (41), the first end (10u) is covered with a metal cover (12.1), the metal cover (12.1) comprises an opening (16) to which a gas supply adapter (20) is connected, the gas purging plug (10) is designed in such a way, that a purging gas which is supplied via the gas supply pipe (30), via the gas supply adapter (20) to the opening (10) flows through the body (10k) and exits the body (10k) at the second end (10o), and with at least one electronic sensor (70, 70.1, 70.2, 70.3). The gas purging system further comprises a data processing unit (80) for acquiring the oscillation waveform of a mechanical vibration (81) detected by the electronic sensor (70, 70.1, 70.2, 70.3) of the gas purging plug (10) and for calculating a bubble index signal (83) from the oscillation waveform of a mechanical vibration (81). The gas purging system further comprises a control unit (100), wherein the control unit (100) is configured to display the bubble index signal (83) and to vary the volume flow (102) through the gas supply pipe (30) (and thereby through the body (10k) of the gas purging plug (10)), depending on the bubble index signal BI(t) (83).

(13) Alternatively (shown in FIG. 4) a warning signal (101) can be generated when the bubble index signal BI(t) (83) lies outside a defined range ΔBI (85). During operation, the gas purging plug (10) is installed in a wall of a metallurgical vessel (40). A purging (treatment) gas is supplied from a gas reservoir (not shown) via the gas supply pipe (30), through the control valve (100a), the flow meter (100b) and the pressure gauge (100c) of the control unit (100) to the gas supply adapter (20) through the opening (16) to the gas purging plug (10), where the gas passes from the first end (10u) to the second end (10o) of the body (10k) into the metal melt (41). The gas bubbles inside the metal melt constitute the purging gas treatment (42). The sensor (70) detects oscillations of a mechanical vibration at the gas purging plug (10) by recording the structure borne vibrations generated when gas bubbles leave the body (10k) at its second end (10o) into the metal melt (41). As shown in FIG. 3 the sensor transmits the detected oscillation values (as an electronic signal) of a mechanical vibration to the data processing unit (80). The detected oscillation values of a mechanical vibration are digitalized by the data processing unit (80) and constitute the oscillation waveform g(t) of a mechanical vibration (81). A Fourier Transformation is performed, which transforms the oscillation waveform g(t) of a mechanical vibration (81) into a frequency spectrum (82) comprising frequency amplitude values G(f) (82a). Bubble index components BI.sub.n(t) can be calculated from the frequency amplitude values G(f) (82a) of the frequency spectrum (82), e.g. by summing frequency amplitude values (82a) over a certain frequency range, at a specific time. Thus the data processing unit (80) determines the bubble index components (86.1, 86.2) by summing frequency amplitude values (82a) from the frequency spectrum (82) over a defined frequency range.

(14) In another embodiment the system can be used to perform the following method for characterization of a gas purging plug (10), comprising the following steps:

(15) Setting the volume flow (300) of a gas through the purging plug (10), e.g. to a pre-determined value of the initial volume flow (102);

(16) Acquiring an oscillation waveform of a mechanical vibration (81) at the actual volume flow (102);

(17) Calculating at least one bubble index component (301) from the measured oscillation waveform of a mechanical vibration (81) at the actual volume flow (102);

(18) Storing at least one value of the bubble index component (302) as a reference bubble index component (86.1).

(19) In this way several values for the bubble index components (86.1) can be stored, e.g. as a function of the volume flow (102) through the gas purging plug (10). These values can be used later for reference. The values can be recorded e.g. during operation of the gas purging plug (10) in a water bath (not shown) or during operation in a metallurgical vessel (40) in a trial run/calibration run (in a setup as shown exemplary in FIG. 2).

(20) In another embodiment shown in FIG. 4 the system can be used to perform the following method for purging a metal melt (41) in a metallurgical vessel (40) with a gas, comprising the steps of:

(21) Loading predetermined values (400) for: reference bubble component BI.sub.n(0) (86.1), an initial volume flow Q.sub.0 (102) through the gas supply pipe (30), a bubble index range ΔBI (85), a target gas volume V.sub.MAX (103).

(22) Setting the volume flow (401) of a gas through the purging plug (10) to a pre-determined value of the initial volume flow Q(t)=Q.sub.0 (102);

(23) Calculating a bubble index signal (402) BI(t) (83) from the measured oscillation waveform g(t) of a mechanical vibration (81) at the actual volume flow Q(t) (102) by determining the bubble index-signal BI(t) (83), whereas the bubble index-signal BI(t) (83) is calculated from the weighted summation of the differences or quotients between the actual bubble index components BI.sub.n(t) (86.2) and the reference bubble index-components BI.sub.n(0) (86.1) and

(24) Variation of the volume flow (404) Q(t) (102) through the gas supply pipe (30) as a function of the bubble index signal BI(t) (83).

(25) The variation of the volume flow (404) Q(t) (102) comprises:

(26) increasing or keeping constant the volume flow (404a) Q(t) (102) through the gas supply pipe (30) in case the bubble index signal BI(t) (83) lies within a predefined bubble index range ΔBI (85), so when |BI(t)|≤ΔBI;

(27) decreasing the volume flow (404b) QM (102) through the gas supply pipe (30) in case the bubble index signal BI(t) (83) lies outside a predefined bubble index range ΔBI (85), so when |BI(t)|>ΔBI.

(28) Alternatively/additionally it is possible to generate a warning signal (403) if the bubble index signal BI(t) (83) lies outside a predefined bubble index range ΔBI (85) (not shown in the figure), so when |BI(t)|>ΔBI.

(29) Additionally gas purging can be stopped (405), once the total volume flow (Q.sub.total=ΣQ(t) or ∫Q(t)) reaches a predefined target gas volume V.sub.MAX.

(30) Exemplary results obtained from a purging plug with a porous body of 20 cm diameter in a water bath model are shown in FIG. 5. In this example, the following bubble index components BI.sub.n are calculated by a summation in a frequency range starting from a to b according to BI.sub.n=G.sub.n(t)=Σ.sub.j=a.sup.bG(t, f.sub.j):

(31) TABLE-US-00001 BI.sub.0: a = 20 Hz . . . b = 1000 Hz BI.sub.1: a = 1000 Hz . . . b = 6000 Hz BI.sub.2: a = 6000 Hz . . . b = 8000 Hz

(32) FIG. 5 shows the bubble index components BI.sub.0, BI.sub.1, BI.sub.2 as a function of the volume flow Q (measured in liter per minute (l/min)). BI.sub.0 relates to large sized bubble, BI.sub.1 relates to medium sized bubbles and BI.sub.2 relates to small sized bubbles. The y-axis shows the relative contribution of the respective bubble index component BI.sub.n to the overall analyzed signal (in percent). Thus it can be seen that up to approximately a volume flow of 80 liters per minute the signal BI.sub.0 is close to 0, thus the amount of large bubbles up to this volume flow is very low. Starting around 80 liters per minute volume flow the signal BI.sub.0 rises, showing that from 80 liters per minute and above the contribution of large bubbles increases. For example the signal BI.sub.0 reaches a contribution of around 20% at 120 liters per minute. From the signal BI.sub.2 it can be seen that the signal relating to small bubbles is relatively constant and high in a range starting from around 50 liters per minute up to around 120 liters per minute. The signal BI.sub.1 shows the contribution of medium sized bubbles, which is slightly and constantly decreasing in the range between 50 to 120 liters per minute. Overall it can be seen that this purging plug shows a good bubble distribution in the range between 50 to around 120 liters per minute of volume flow of a purging gas flowing through the body.

(33) FIG. 6 shows a comparison of the signal BI.sub.0 (a=20 Hz . . . b=1000 Hz) relating to different purging plugs. BI.sub.0-20 shows the purging plug of FIG. 5, BI.sub.0-12 shows a purging plug with a porous body of 12 cm diameter and BI.sub.0-12b shows a purging plug with a porous body of 12 cm diameter with a less porous body (e.g. many blocked pores). As discussed for FIG. 5, the purging plug with the signal BI.sub.0-20 shows a low signal arising from large bubbles up to around 120 liters per minute, where the signal BI.sub.0-20 arising from large bubbles reaches 20% contribution. The purging plug with the signal BI.sub.0-12 already reaches the same 20% contribution (arising from large bubbles) to the signal at a volume flow of around 85 liters per minute. Therefore, for this plug the range of volume flow for a good bubble distribution is reduced to 85 liter per minute compared to the purging plug of FIG. 5 with a range of up to 120 liter per minute. The purging plug with the signal BI.sub.0-12b (less porosity/blocked pores) shows a high contribution arising from large bubbles already at very low volume flows (e.g. at a 5 liters per minute the contribution the signal arising from large bubbles already shows a contribution of about 40%). Therefore this plug does not show a good bubble distribution for any volume flow, the method will issue a warning signal (101), e.g. requiring replacement of the purging plug (10).

(34) A simple implementation of the method according to the invention could be as shown in the following example:

(35) Loading predetermined values (400) for: reference bubble component BI.sub.0(0)=0 (86.1) (e.g. the target is to have no or at least a low contribution of large sized bubbles, BI.sub.0: a=20 Hz . . . b=1000 Hz), an initial volume flow Q.sub.0=80 liters per minute (102) through the gas supply pipe (30), a bubble index range ΔBI=20% (85), a target gas volume V.sub.MAX=1200 liter (103).

(36) Setting the volume flow (401) of a gas through the purging plug (10) to a pre-determined value of the initial volume flow Q(t)=Q.sub.0=80 liter per minute (102);

(37) Calculating a bubble index signal (402) according to BI(t)=BI.sub.0(t)−BI.sub.0(0)=BI.sub.0(t) (83) from the measured oscillation waveform g(t) of a mechanical vibration (81) at the actual volume flow Q(t) (102) by determining the bubble index-signal BI(t) (83), whereas the bubble index-signal BI(t) (83) is calculated from the weighted summation of the differences or quotients between the actual bubble index components BI.sub.0(t) (86.2) and the reference bubble index-components BI.sub.0(0)=0 (86.1) and

(38) Variation of the volume flow (404) Q(t) (102) through the gas supply pipe (30) as a function of the bubble index signal BI(t) (83).

(39) The variation of the volume flow (404) Q(t) (102) comprises:

(40) increasing the volume flow (404a) Q(t) (102) through the gas supply pipe (30) up to Q(t)=120 liter per minute where the bubble index signal BI(t) (83) lies within a predefined bubble index range ΔBI=20%, so until |BI(t)|≤ΔBI (85) is fulfilled and

(41) stopping the gas purging (405), when the total volume flow Q.sub.total=ΣQ(t) (102) through the pipe (30) reaches a predefined target gas volume V.sub.MAX=1200 liters (102), which is achieved at a little more than 10 minutes of gas purging. In a second example the same values are used as in the previous example, with the exception that the initial volume flow is loaded to be Q.sub.0=150 liters per minute (102). Now the variation of the volume flow (404) Q(t) (102) comprises:

(42) decreasing the volume flow (404b) Q(t) (102) through the gas supply pipe (30) as long as the bubble index signal BI(t) (83) lies outside a predefined bubble index range ΔBI=20% (85), so as long as |BI(t)|>ΔBI, which is until the volume flow is reduced to Q(t)=120 liter per minute.

(43) stopping the gas purging (405), when the total volume flow Q.sub.total=ΣQ(t) (102) through the pipe (30) reaches a predefined target gas volume V.sub.MAX=1200 liters (102), which is achieved at a little less than 10 minutes.

(44) In case the purging plug used in the examples degrades during purging, e.g. in a case where the signal BI.sub.0 increases at an actual volume flow (e.g. at 120 liters per minute as in the examples), the method according to the invention will reduce the volume flow until the same contribution of BI.sub.0 is reached again, but at a lower volume flow. In such a case the purging time will be increased until the target gas volume is reached. Thereby the method allows to maintain constant gas bubble distributions over the whole duration of the purging process with a pre-defined overall target gas volume.

(45) List of reference numerals and factors (German translation in parenthesis): Gas purging plug (Gasspül-Element) 10k Ceramic refractory body (keramischer feuerfester Körper) 10u First end of ceramic refractory body 10o Second end of ceramic refractory body 12.1 Metal cover (Metalldeckel) 12.2 Metal jacket (Metallmantel) 14 Hollow space (Hohlraum) 16 Opening (Öffnung) 20 Gas supply adapter (Gasanschlussstutzen) 30 Gas supply pipe (Gaszuführ-Leitung) 40 Metallurgical vessel 41 Metal melt 42 Purging gas treatment 70 Sensor (Sensor) 70.1 Sensor mounted outside of metal coat 70.2 Sensor mounted outside of gas supply adapter 70.3 Sensor mounted inside of gas supply adapter 70.4 Sensor mounted inside of metal coat 80 Data processing unit 81 Oscillation waveform g(t) of a mechanical vibration 82 Frequency spectrum 82a Frequency amplitude values G(t, f) 83 Bubble index signal BI(t) 85 Bubble index range ΔBI 86.1 Reference bubble index components BI.sub.n(0) 86.2 Actual bubble index components BI.sub.n(t) 100 Control unit 100a Control valve 100b Flow meter 100c Pressure gauge 101 Warning signal 102 Volume flow Q(t) 103 Target gas volume VMAX 300 Setting the volume flow 301 Calculating at least one bubble index component (86.1) 302 Storing at least one value of the bubble index component (86.1) 400 Determining predetermined values 401 Setting the volume flow (102) 402 Calculating a bubble index signal (83) 403 Generating a warning signal (101) 404 Variation of the volume flow (102) 404a Increasing or keeping constant the volume flow (102) 404b Decreasing the volume flow (102) 405 Stopping the gas purging