METHOD AND SYSTEM FOR TESTING THE INTEGRITY OF FILTERS

Abstract

A method for testing integrity of a filter can include pressurizing an upstream side of the filter to a test pressure and performing a check step that includes determining a flow rate of fluid from the upstream side to a downstream side of the filter, comparing the determined flow rate with a flow range including a flow threshold, and setting stop criteria based on the comparison.

Claims

1. A method for testing integrity of a filter, the method comprising: pressurizing an upstream side of the filter to a test pressure; performing a check step comprising: determining a flow rate of fluid from the upstream side to a downstream side of the filter; comparing the determined flow rate with a flow range including a flow threshold; setting stop criteria based on the comparison, wherein the stop criteria comprise at least one quantitative constraint indicative of a reliability of the determined flow rate and wherein: if the determined flow rate is within the flow range, the stop criteria are set to first stop criteria; and if the determined flow rate is outside the flow range, the stop criteria are set to second stop criteria; determining whether the stop criteria are satisfied; if the stop criteria are not satisfied, repeating the check step until the stop criteria are satisfied; if the stop criteria are satisfied, comparing the determined flow rate with the flow threshold: if the determined flow rate is greater than or equal to the flow threshold, determining that the filter is non-integral; and if the determined flow rate is less that the flow threshold, determining that the filter is integral.

2. The method of claim 1, wherein: the stop criteria comprise a stability range and a pressure drop threshold; and determining whether the stop criteria are satisfied comprises: determining a stability indicator of the determined flow rate; measuring an instantaneous pressure at the upstream side of the filter to obtain a measured pressure drop as a difference between the test pressure and the instantaneous pressure; comparing the stability indicator with the stability range and comparing the measured pressure drop with the pressure drop threshold; and determining that the stop criteria are not satisfied when the stability indicator is outside the stability range and the measured pressure drop is greater than or equal to the pressure drop threshold.

3. The method of claim 2, wherein the stability range is a numerical interval centered on the determined flow rate corresponding to the c-th performed check step and the stability indicator is an average value of a subset of l determined flow rates corresponding to performed check steps (c?l)-th to (c?1)-th.

4. The method of claim 1, wherein the fluid is a gas and the flow rate is a diffusional flow rate.

5. The method of claim 1, wherein the fluid is water and the flow rate is a bulk flow rate.

6. The method of claim 1, further comprising for stabilization time after pressurizing the upstream side of the filter before performing the check step, wherein: the stabilization time is retrieved from a table based on the test pressure and on a volume of the upstream side of the filter.

7. The method of claim 1, wherein the flow range is a first flow range and the stop criteria are set to the second stop criteria if, further, the determined flow range is within a second flow range, the first flow range being included in the second flow range; and the method further comprises setting the stop criteria to third stop criteria if the determined flow rate is outside the second flow range.

8. A computer program product comprising computer readable instructions, which, when executed on a computer system, cause the computer system to perform operations according to claim 1.

9. A system for testing integrity of a filter, the system comprising: at least one processor; a memory; a communication channel configured to be connected to the filter; wherein the at least one processor is configured to: pressurize an upstream side of the filter to a test pressure; perform a check step comprising: determining a flow rate of fluid from the upstream side to a downstream side of the filter; comparing the determined flow rate with a flow range including a flow threshold; setting stop criteria based on the comparison, wherein the stop criteria comprise at least one quantitative constraint indicative of a reliability of the determined flow rate and wherein if the determined flow rate is within the flow range, the stop criteria are set to first stop criteria, and if the determined flow rate is outside the flow range, the stop criteria are set to second stop criteria; determining whether the stop criteria are satisfied; if the stop criteria are not satisfied, repeat the check step until the stop criteria are satisfied; if the stop criteria are satisfied, compare the determined flow rate with the flow threshold: if the determined flow rate is greater than or equal to the flow threshold, determine that the filter is non-integral; and if the determined flow rate is less that the flow threshold, determine that the filter is integral.

10. The system of claim 9, wherein: the stop criteria comprise a stability range and a pressure drop threshold; and the at least one processor is further configured to: determine a stability indicator of the determined flow rate; measure an instantaneous pressure at the upstream side of the filter to obtain a measured pressure drop as a difference between the test pressure and the instantaneous pressure; compare the stability indicator with the stability range and comparing the measured pressure drop with the pressure drop threshold; and determine the stop criteria are not satisfied when the stability indicator is outside the stability range and/or the measured pressure drop is less than the pressure drop threshold; and determine the stop criteria are satisfied when the stability indicator is within the stability range and the measured pressure drop is greater than or equal to the pressure drop threshold.

11. The system of claim 10, wherein the stability range is a numerical interval centered on the determined flow rate corresponding to the c-th performed check step and the stability indicator is an average value of a subset of l determined flow rates corresponding to performed check steps (c?l)-th to (c?1)-th.

12. The system of claim 9, wherein the fluid is a gas and the flow rate is a diffusional flow rate.

13. The system of claim 9, wherein the fluid is water and the flow rate is a bulk flow rate.

14. The system of claim 9, wherein the at least one processor is further configured to: retrieve a stabilization time from a table based on the test pressure and on a volume of the upstream side of the filter; and wait for a stabilization time after pressurization of the upstream side of the filter before execution of the check step.

15. The system of claim 9, wherein the flow range is a first flow range and the stop criteria are set to the second stop criteria if, further, the determined flow range is within a second flow range, the first flow range being included in the second flow range; and the least one processor is further configured to set the stop criteria to third stop criteria if the determined flow rate is outside the second flow range.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0111] Details of exemplary embodiments are set forth below with reference to the exemplary drawings. Other features will be apparent from the description, the drawings, and from the claims. It should be understood, however, that even though embodiments are separately described, single features of different embodiments may be combined to further embodiments.

[0112] FIG. 1 shows a schematic representation of an integrity tester connected to a filter.

[0113] FIG. 2 shows a flow chart of an exemplary integrity test.

[0114] FIG. 3 shows a plot of pressure versus time over the different phases of the integrity test.

[0115] FIG. 4 shows an exemplary table with stabilization time values as a function of test pressure and upstream volume.

[0116] FIG. 5 shows a flow chart of an exemplary check phase of the integrity test.

[0117] FIG. 6 shows a flow chart of an exemplary adaptive assessment of the duration of the check phase.

[0118] FIG. 7 shows a representation of the flow threshold and the flow range.

[0119] FIG. 8 shows a plot of pressure drop and diffusionary (or diffusion-based) flow rate versus time during the check phase.

[0120] FIG. 9 shows an exemplary table with pressure drop threshold values as a function of upstream volume.

[0121] FIG. 10 shows a plot of a stability range versus time during the check phase.

DETAILED DESCRIPTION

[0122] In the following, a detailed description of examples will be given with reference to the drawings. It should be understood that various modifications to the examples may be made. Unless explicitly indicated otherwise, elements of one example may be combined and used in other examples to form new examples.

[0123] FIG. 1 shows a schematic representation of an integrity tester 10 connected to a filter 30 by means of a tube 15. The integrity tester 10 is a system comprising at least one processor and a memory (not shown), two output units (a printer and a display) and an input unit (a keyboard and/or a barcode scanner). Alternatively, the integrity tester 10 may comprise a touch screen acting both as output and input unit.

[0124] The filter (or filter assembly) 30 comprises a housing 32 and a membrane 34. The filter 30 is connected via an upstream pipe 36 to a draining setup and via a downstream pipe 38 to the downstream filtrate side.

[0125] The function of the communication channel, the tube 15, between the integrity tester 10 and the filter 30 is to provide gas to the filter 30 during the pressurization phase.

[0126] The setup may additionally have more external valves or sensors connected to the filter under test 30. These external valves could be utilized for additional safety and protect against backflow. The external sensors can be utilized to supplement or replace the internal sensors of the integrity tester 10.

[0127] FIG. 2 shows a flow chart of an exemplary integrity test 100 carried out by the integrity tester 10. Prior to the test 100, a preparation of the filter may be carried out, e.g. hydrophilic filter may be wetted with wetting liquid while a hydrophobic filter may blinded with water.

[0128] During the (optional) sensor verification 110 the tester 10 makes sure that the internal sensors are calibrated and operating properly. Then, in response to a user input, it is set whether to perform a volume measurement 120 of the upstream volume of the filter 30. The volume measurement 120 may be performed e.g. using Boyle's law. If no volume measurement is made, the value of the volume may be input by the user.

[0129] Directly after the sensor verification 110 or after the volume measurement 120 the pressurization 130 takes place. In the pressurization phase the pressure at the upstream side of the filter 30 is brought to a test pressure, whose value may be input by a user or may be pre-programmed in the integrity tester 10 (e.g. for a specific filter model).

[0130] After pressurizing the upstream volume to the test pressure, the tester 10 ensures that the pressure is maintained for a certain amount of time. This is the phase of stabilization 140 and its duration (or stabilization time) is set based on the test pressure and the upstream volume. The table of FIG. 4 shows exemplary stabilization time values associated to different ranges of test pressure and upstream volume. Accordingly, a suitable stabilization time is automatically set using a look-up table like the one in FIG. 4.

[0131] Following the stabilization 140, the test 100 comprises the check 150, during which the actual integrity checking takes place. The check 150 will be described in more detail below with reference to FIG. 5. At the end of the test 100, the filter is vented at 160.

[0132] FIG. 3 shows a plot of pressure versus time over the different phases of the integrity test 100. It can be seen that the pressure increases to finally reach the test pressure during the pressurization phase, then stays at the test pressure during the stabilization phase, starts decreasing during the check phase and finally drops to zero after venting.

[0133] FIG. 5 shows a flow chart of an exemplary check phase of the integrity test. The check phase 150 of the integrity test 100 comprises determining 240 a flow rate of fluid from the upstream side to the downstream side of the filter 30. The flow rate is determined as proxy for the actual retention capability of the filter.

[0134] In order to determine the flow rate, a measurement of the pressure drop at the upstream side of the filter may exemplarily be needed. Accordingly, the check phase may optionally comprise first measuring 210 the effective test pressure, i.e. measuring the starting pressure at the beginning of the check phase, which may be slightly different from the nominal test pressure. The measurement of the effective test pressure at 210 is performed only once at the beginning of the check phase 150 and not as part of the check step, which may be repeated multiple times.

[0135] Then at 220 the check step begins and the instantaneous pressure at the upstream side of the filter is measured in order to obtain, at 230, the pressure drop. Using the pressure drop, the flow rate can be determined at 240, e.g., for a diffusion test, as a diffusionary flow rate that depends logarithmically on the pressure drop. Alternatively, for a water flow test, the bulk/volumetric flow rate depends linearly on the pressure drop.

[0136] As mentioned, the determined flow rate is the quantity used to assess the integrity of the filter and, thus, it is important that the determined flow rate is accurate. Conventionally, the flow rate is determined at each of a succession of time points and after a fixed, relatively long time it is assumed that the determined flow rate is sufficiently accurate for the purpose of integrity assessment. In other words, the last data point in a time series of determined flow rates is considered reliable.

[0137] While waiting for a sufficiently long amount of time ensures that the determined flow rate is sufficiently accurate, the temporal efficiency of the test 100 is negatively affected by this. According to the invention, instead, the duration of the check phase is adaptively determined at 250. This adaptive assessment will be described in detail with reference to FIG. 6 below.

[0138] The adaptive assessment determines whether the determined flow rate corresponding to a given time point is sufficiently reliable for the test aim. If not, the check phase continues by repeating the check step and the flow rate is determined anew at a subsequent time point; if so, the check phase is terminated and the (last) determined flow rate is compared with a flow threshold.

[0139] The flow threshold is a test parameter whose value may be input by a user or may be pre-programmed in the integrity tester 10 (e.g. for a specific filter model). If the determined flow rate is greater than or equal to the flow threshold, the filter 30 does not pass the test 100, meaning that its integrity has been compromised. If the determined flow rate is less than the flow threshold, the filter 30 passes the test 100, meaning that its integrity is confirmed.

[0140] FIG. 6 shows a flow chart of an exemplary adaptive assessment 250 of the duration of the check phase. The decision on whether to terminate the check phase is made dependent on stop criteria, which, in turn, depend on how close the determined flow rate F(t.sub.c) is to the critical value of the flow threshold.

[0141] A flow range can be defined around the flow threshold as the numerical range between (1?x)F.sub.th and (1+x)F.sub.th, with x<1. For example, x<0.15, such as x=0.08. In other words, the flow range is exemplarily the interval ?8% around the flow threshold. FIG. 7 shows a representation of the flow threshold and the flow range.

[0142] The determined flow rate is compared with the flow range at 310. If the determined flow rate is within the flow range (see cross in FIG. 7), the stop criteria are set to be strict criteria or first stop criteria at 320. If the determined flow rate is outside the flow range (see circle in FIG. 7), the stop criteria are set to be lenient criteria or second stop criteria at 330.

[0143] Then, at 340, it is checked whether the stop criteria are satisfied. The stop criteria include a constraint on the value of the pressure drop at the time at which the flow rate was determined. The reason is that the pressure drop is correlated with the error on the determined flow rate, as shown in the plot of FIG. 8: the higher the pressure drop, the lower the error bar. It should be noted that FIG. 8 shows a diffusionary flow rate as a particular example of flow rate.

[0144] Thus, in order to ensure that the determined flow rate is sufficiently reliable, the constraint is that the pressure drop has to be at least equal to a pressure drop threshold. The pressure drop threshold is a test parameter whose value may be input by a user or may be pre-programmed in the integrity tester 10 (e.g. fora specific filter model). The pressure drop threshold may vary based on the upstream volume.

[0145] FIG. 9 shows an exemplary table with strict pressure drop threshold values as a function of upstream volume. For example, for a volume of 5 L, if first stop criteria have been selected, the pressure drop has to be at least 40 mbar. For second stop criteria, the pressure drop threshold for 5 L may be e.g. 15 mbar.

[0146] Accordingly, at 340 the measured pressure drop is compared with the pressure drop threshold. Further, another check is made to verify whether the stop criteria are met, namely using a stability indicator. Indeed, the stop criteria comprise a further constraint on this stability indicator.

[0147] The stability indicator is obtained by computing the average (arithmetic mean) over a subset of the determined flow rate values preceding the value under consideration, F(t.sub.c). In particular, the 10 values before F(t.sub.c) are averaged, i.e. F(t.sub.c-10) . . . F(t.sub.c-1). The constraint is that this average, A.sup.10(t.sub.c-1), has to be within an interval around F(t.sub.c), the stability range.

[0148] Exemplarily, for first stop criteria, 0.998F(t.sub.c)?A.sup.10(t.sub.c-1)?1.002F(t.sub.c) and, for second stop criteria, 0.989F(t.sub.c)?A.sup.10(t.sub.c-1)?1.011F(t.sub.c). The stability range is a test parameter whose values may be input by a user or may be pre-programmed in the integrity tester 10 (e.g. for a specific filter model). FIG. 10 shows a plot of a stability range versus time during the check phase for a diffusionary flow rate.

[0149] Accordingly, at 340 the measured pressure drop is compared with the pressure drop threshold and the stability indicator is compared with the stability range. If both constraints of the stop criteria are satisfied, i.e. the pressure drop is equal to or higher than the pressure drop threshold and the stability indicator is within the stability range, it is determined that the check phase can be ended at 260. As discussed above, in this case, the determined flow rate is compared with the flow threshold.

[0150] If one or both constraints is/are not satisfied, it is determined that the check phase should be continued and the check step should be repeated at 350. Accordingly, a new flow rate is determined and then it is again evaluated whether the new determined flow rate is reliable enough to be compared with the flow threshold and, thus, provide the test result. The check step is repeated until the stop criteria are met.

[0151] Therefore, the duration of the check phase is adaptively determined, thereby making the check phase as fast as possible while maintaining a desired accuracy.