MICE HYDROGEN SYSTEM PUMP ENCLOSURE VENTILATION REQUIREMENTS

Transcription

1 MICE HYDROGEN SYSTEM PUMP ENCLOSURE VENTILATION REQUIREMENTS Author: M Hills Document Number: MICEH2-RQ Rutherford Appleton Laboratory Harwell Science and Innovation Campus DIDCOT OX11 0QX

2 Change Record Issue Change Person Draft MH 1 Updated to reflect final enclosure size MH Page 2 of 7

3 Table of Contents 1. INTRODUCTION 4 Reference Documents... 4 External References CALCULATION OF RELEASE RATE 4 Assumptions VENTILATION REQUIREMENTS 6 Assumptions CONCLUSIONS 7 List of Tables Table 1: Source of release data and calculated values (highlighted red)... 5 Table 2: Ventilation rate data and calculated values (highlighted red)... 6 Table 3: Influence of independent ventilation on type of zone (from RD1 section B7)... 7 Page 3 of 7

4 1. Introduction The proposed pumping enclosure for the MICE Hall will house the vacuum pumps for the Hydrogen Delivery System and has been deemed a zone 2 under DSEAR 1. This technical note details the calculation of the ventilation rates required in the enclosure to meet this classification. It follows the procedure outlined in BS EN :2009 (RD1). Reference Documents RD Title Ref 1 Explosive atmospheres Part 10-1: Classification of BS EN :2009 areas Explosive gas atmospheres 2 Pump Enclosure Installation Drawing 'D' Fume stack support AND Vacuum pump.pdf External References 1..xls (N:\100 MICE - Hydrogen Delivery System - M Hills\Safety\DSEAR Compliance\Hydrogen Zone Classification) 2. Calculation of Release Rate Catastrophic failures do not need to be accounted for in the assessment of release rates under DSEAR. As such, the release from a pipe rupture is not considered in the following calculations; instead the assessment is based on the typical release that may occur from a leak in the rotary pump seals. Industry guidance on this 2 is that the leak area, S, is calculated as πld, where l is the seal clearance and d is the shaft diameter. The values used for these and the other relevant data (with references) are listed below: 1 The Dangerous Substances and Explosive Atmospheres Regulations 2 Classification of Hazardous Locations Appendix 7 Page 4 of 7

5 Source of Release Data Units Value Reference Notes External pressure, p 0 Pa 1.01E+05 Ambient Polytropic index,γ Baseefa data for H2 Critical pressure for choked release, p c Pa 1.92E+05 Calculated according to BS EN A.3.2 Pressure in H2 space, p Pa 1.30E+05 Pump shaft diameter, d m Pump shaft clearance, l m 5.00E-05 Cross section of opening, S m E-06 For pumps, Baseefa guidance ("Classification of Hazardous Locations - Appendix 7") on leak size is that S = πld, where d is shaft diameter and l is clearance Universal gas constant, R J/kmolK 8314 Molecular Mass, M kg/kmol Air Liquide web site: Absolute temperature of release, T K 293 Maximum release rate, (dg/dt) max kg/s For formulae refer to BS EN A and A Safety Factor, k 0.5 Usual safety factor applied to secondary grades of release. See BS EN B Lower Explosive Limit by % volume, 4 RAL Safety Code No.1 LEL v Lower Explosive Limit by mass, LEL m kg/m Calculated according to BS EN Minimum volumetric flow rate of fresh m 3 /s Calculated according to BS EN air required, (dv/dt) min Table 1: Source of release data and calculated values (highlighted red) Assumptions i. A typical pump inside the enclosure will be a Leybold two-stage rotary vane vacuum pump (e.g. TRIVAC D65B). A shaft diameter of 25mm and a seal clearance of 50 microns have been assumed ii. The maximum pressure inside the pump has been taken as 1.3bara. iii. The polytropic index of H2 is taken as 1.41 Note that as p<p c, the release is not choked and therefore the expression in section A of RD1 is used to calculate the release rate. The calculated release rate is kg/s requiring a volumetric flow rate of fresh air of m 3 /s to dilute it below the LEL. Page 5 of 7

6 3. Ventilation Requirements To assess the degree of ventilation required inside the pump enclosure, the hypothetical volume of the vapour cloud and time of persistence are calculated according to RD1. m to BS EN Description Units Value Reference Notes Minimum volumetric flow rate of fresh air 3 /s Calculated according required, (dv/dt) min Safety Factor, k 0.5 Usual safety factor applied to secondary grades of release. See BS EN B Number of fans serving volume 1 Flow Rate per fan m 3 /s 0.5 Total flow rate of fresh air through the volume under consideration, dv 0 /dt Entire volume served by ventilation (i.e. m 3 Pump enclosure volume), V 0 m 3 /s 0.5 Air changes per second, C s Calculated according to BS EN Efficiency of ventilation in terms of dilluted explosive atmosphere, f [Typically between 1 (ideal) and 5 (impeded)] - 3 Initial concentration of flammable % vol. 100 NOTE: Upper Limit of flammability substance by volume, X 0 in air = 77% Ref: PD IEC :2000 Initial concentration of flammable kg/m 3 substance by mass, X Calculated as x 10-3 x M x [Initial concentration by % vol] Lower Explosive Limit by mass, LEL m kg/m Calculated according to BS EN Hypothetical Volume of the vapour cloud, V z m Calculated according to BS EN Time of Persistence, t s Calculated according to BS EN Time of Persistence in minutes, t min 9.4 Calculated according to BS EN Air changes per hour hr Table 2: Ventilation rate data and calculated values (highlighted red) Assumptions i. The pump enclosure is 3.5m long x 3m wide x 2.3m high 3 ii. The efficiency of the ventilation (f) is 3 as the ventilation is considered to be moderately unobstructed. 3 See RD2 Page 6 of 7

7 4. Conclusions To claim a zone 2 for an area where secondary releases ( release which is not expected to occur in normal operation and, if it does occur, is likely to do so only infrequently and for short periods 4 ) are expected, the degree of ventilation must be medium (see table 3). Ventilation can be considered medium when the hypothetical volume of the vapour cloud (V z ) is less than the volume under consideration (V 0 ). Additionally, the time of persistence in a zone 2 should not exceed 30 minutes. As shown in table 2 above, a ventilation rate of 75 air changes/hr satisfies both these requirements. Table 3: Influence of independent ventilation on type of zone (from RD1 section B7) As with Gas Panel Enclosure, it may be advisable to increase this ventilation rate when hydrogen is detected. 4 See RD1 section 3.10 Page 7 of 7