DEVELOPMENT OF AN ENHANCED PROTECTION POSITIVE PRESSURE RESPIRATOR (E3PR) PROTOTYPE CANISTER

Transcription

1 DEVELOPMENT OF AN ENHANCED PROTECTION POSITIVE PRESSURE RESPIRATOR (E3PR) PROTOTYPE CANISTER Dr Mark Summers and Claire Hemmings Dstl Porton Down Salisbury Wiltshire SP4 0JQ UK INTRODUCTION During a CBR incident, there may be a need for operators to enter areas where they require guaranteed high levels of respiratory protection throughout the entire period of exposure. In some scenarios, operators may require a higher level of protection than that offered by current in-service Air Purifying Respirators (APRs) and may need to employ Powered Air Purifying Respirator (PAPR) devices. Although these PAPR systems offer high levels of protection, they are bulky and heavy, and therefore may impinge on the performance of the operator. In addition, they demand significant amounts of power and additional filtration capacity owing to the large amounts of airflow required. In response, Dstl has developed the Dual Cavity Respirator (or Enhanced Protection Positive Pressure Respirator (E3PR)) from the Avon FM53, which isolates the positive/negative pressure transients within an oro-nasal mask (nose cup) and creates a positive pressure within the eye space, using a self contained blower unit. This is illustrated in Figure 1. Figure 1 Dual cavity concept This E3PR has been shown to offer a step change increase in the level of protection offered by a standard APR, without the recognized shortcomings of a PAPR device (i.e. weight, power, bulk, etc).

2 ENHANCED PROTECTION POSITIVE PRESSURE RESPIRATOR (E3PR) CANISTER This work builds upon the dual cavity concept, where the aim is to design and produce a novel, ergonomic, dual flow filtration canister incorporating a separate filter element to allow independent filtration for the blower unit (Figure 2). Airflows will be directed into the respirator via a specially designed canister mount. Figure 2 E3PR canister concept CANISTER REQUIREMENTS No test standards exist for this type of novel, dual flow canister concept. The NIOSH Statement of Standard for Full Face Piece Air Purifying Respirators was therefore used as a reference guide for the canister development, where parameters such as breathing resistance, pressure drop, size and weight are defined. DESIGN CONCEPT STAGE Dstl worked with Emcel Filters Ltd in the design of two dual flow canister concepts. A basic, Dstl canister breakthrough model was used to help inform the design of each prototype by predicting canister breakthrough times for the two separate sections of the canister. DESIGN 1 The first design (conceived by Dstl and modified by Emcel Filters Ltd.) was based upon a dual, axial flow canister, shown in Figure 3, where the inner section of the canister (red arrow) provides filtered air to the inner oro-nasal mask and the outer section (blue arrows) provides filtered air to the blower in the ocular cavity of the respirator.

3 Figure 3 Initial dual flow canister design (supplied by Emcel Filters Ltd.) However, preliminary work demonstrated that it was difficult to achieve a reliable seal between the two sections. Therefore this design was not taken any further. DESIGN 2 Emcel Filters Ltd designed and produced the second design, in which the outer section of the canister (that provides filtered air for the blower) was a radial filter positioned at the base of the canister, as illustrated in Figure 4. The two sections in this configuration now act completely independent of each other and no longer rely upon an o-ring seal to separate them. This design was taken forward for prototype development. Figure 4 CAD drawings of the second design concept (supplied by Emcel Filters Ltd.) PROTOTYPE DEVELOPMENT The canister was manufactured by Emcel Filters Ltd. from acetal material and filled with ASZM/T 12 x 30 US MESH carbon under environmentally controlled conditions (20.5 C

4 and 36% RH). The prototype dual flow canister is shown in Figure 5, where Bed A refers to the primary carbon bed that provides filtered air during respiration and Bed B provides filtered air for the blower. Figure 5 Prototype dual flow canister A total of nine prototype canisters were manufactured. The outer diameter of the prototype canister was 12.0 cm (compliant with the NIOSH standard) with a body height of 6.9 cm (excluding the screw tread). The carbon filling procedure produced consistent results and the loadings for Beds A and B are displayed in Table 1. The average carbon loading for Bed A was 153 g ± 1 g and for Bed B was 47 g ± 1 g. The total weight of the prototype canister was 625 g, which is 125 g heavier than the NIOSH limit of 500 g. CANISTER TESTING The particle filtration efficiency was tested against a sub-micron DOP aerosol challenge. The integrity of the carbon beds was examined using a standard bromobutane challenge. The results of the testing are displayed in Table 1. Bed A was tested at 85 l min -1 continuous flow and Bed B was tested at 10 l min -1 continuous flow (consistent with the blower flow rate for an E3PR). Tests were performed at 21 C and 37% RH.

5 FILTER NO. DOP EFFICIENCY % BROMOBUTANE TEST / ppm CARBON WEIGHT /g PRESSURE DROP / mmh 2 O BED A BED B BED A BED B BED A BED B BED A BED B Table 1 - Carbon loadings, particle filtration efficiencies and bromobutane test results for Beds A and B of the prototype E3PR dual flow canister The results of the particle filtration efficiency test superseded the NIOSH requirement of a P100 particulate filter (i.e %) for all prototype canisters, apart from Bed B of prototypes 4 and 9. The pressure drop across Bed A for all canisters was fairly consistent and ranged between 36 mmh 2 O and 39 mmh 2 O, which is compliant with the NIOSH requirement of less than or equal to 50 mm H 2 O at 85 l min -1 continuous flow. The pressure drop across Bed B for all canisters was also fairly consistent and ranged between 18 mmh 2 O and 21 mmh 2 O. The results of the bromobutane tests showed zero deflection across both beds for all the canisters, demonstrating efficient carbon packing without any voids that would have lead to instantaneous breakthrough. CYCLOHEXANE BREAKTHROUGH TESTING Cyclohexane breakthrough testing was performed on Dstl s small scale canister test line (Figure 6). The test conditions were compliant with the NIOSH standard at: 25 ⁰C and 25% RH and 25 ⁰C and 80% RH. The cyclohexane challenge concentration used was 2600 ppm,

6 with a breakthrough concentration of 10 ppm. The flow rate through Bed A was 64 l min -1 (compliant with NIOSH), whereas 20 l min -1 (approximately a factor a two higher than the blower flow rate for an E3PR) was used for Bed B. The final breakthrough test carried out was the Service Life test. This states that the canister must provide a minimum breakthrough time of 5 minutes when tested at 50% RH and 25 ⁰C, using a flow rate of 100 l min -1. The results of the breakthrough testing are displayed in Table 2. Figure 6 Dstl small scale canister line

7 Canister Test Bed A / min Bed B / min 1 Dry (25% RH) Dry (25% RH) Dry (25% RH) Mean Wet (80% RH) Wet (80% RH) Wet (80% RH) Mean Service Life (100 l min -1, 50% RH) > Service Life (100 l min -1, 50% RH) Table 2 Cyclohexane breakthrough test results (all tests performed at 25 ⁰C) Tested against cyclohexane under dry conditions (i.e. 25% RH), a consistent breakthrough time of 27 minutes was measured for Bed A. Under NIOSH, canisters are categorized into breakthrough times of 15 minute intervals, therefore based upon this result the prototype canister passed the lowest NIOSH interval. Under dry conditions (i.e. 25% RH), the results for Bed B were also fairly consistent, with an average breakthrough time of 17 minutes. It should be noted that a continuous flow of 20 l min -1 is expected to be at least a factor of two higher than required for the E3PR. This flow rate was used to minimise the testing time. Tested against cyclohexane under wet conditions (i.e. 80% RH), an average breakthrough time of 23 minutes was measured for Bed A. This was 4 minutes shorter than the average breakthrough recorded at 25% RH; however, it is still within the NIOSH limit. The results at 85% RH for Bed B were reasonably consistent, with an average breakthrough time of 12 minutes, which was 5 minutes shorter than the average breakthrough recorded at 25% RH. The prototype canister passed the NIOSH Service Life test successfully. In the interest of testing time, the Service Life test for Canister 7 was halted after 11.5 minutes.

8 SUMMARY A prototype dual flow E3PR canister has been manufactured by Emcel Filters and tested according to the NIOSH Statement of Standard for Full Face piece Air Purifying Respirators. Based upon the tests completed, the prototype canister was found to meet all the NIOSH requirements, apart from weight. This was not considered a major problem as this can be addressed through design and material selection. Future work is planned to integrate the dual flow canister into an E3PR. In addition to this, a real-time fit indicator will be developed to provide information relating to the outer cavity pressure and fit, and to provide control over the blower flow rate, via a feedback loop. This will ensure a high level of protection is maintained in the event that any leaks occur in the outer face seal. The performance of the final E3PR demonstrator (combined respirator, canister and real time fit indicator) will be assessed against commercially available PAPRs. Acknowledgements: Dstl wishes to thank TSWG for funding this work and Emcel Filters Ltd for advice and guidance with the canister design and production.