Nginring Dsign rjt - Lab Report Example

NGINЕЕRING DЕSIGN РRОJЕСT Student’s Name Class Instructor Institution of Affiliation City Date Table of Contents 1RSET 5 1.1Calculation of RSET 5 1.1.1The travel time 6 1.1.2Flow time(time taken on exits). 7 1.1.3Reaction time, pre-movement time 9 1.1.4Detection Time 11 1.1.5Fire growth parameters 12 2ASET 14 2.1Endpoints 15 2.1.1Tenability conditions of smoke 15 2.1.2Tenability of fire and heat 15 2.1.3Tenability toxic gases conditions 17 2.2ASET calculations. 18 2.2.1Computational fluid dynamics models 18 2.2.2Zone model 18 2.2.3Simple calculations 19 References 20 Figure 1 – Ground floor plan of warehouse *Note: Arrow on stairs indicates travel upwards Figure 2 – Proposed mezzanine arrangement 1 RSET RSET refers to the time it takes for occupants within a building to escape or exit a building safely when a fire ignites. It depends on detection, warnings by alarms and the reaction of occupants take in their movement to evacuation points. 1.1 Calculation of RSET RSET contains various components which are used in its calculation. They are: pre-movement time (time before occupants react to a fire alarm after it has been raised), detection time (time that a fire takes from when it ignites to when it is sensed) and movement time (time an occupant takes to the exits of the building.) RSET can be illustrated diagrammatically as shown below by figure 1 The figure can be illustrated as an equation to express the various elements of RSET[Bri04]. RSET is given by the following equation Where Tdet - time taken for the alarm system to sense a fire after ignition ta - time taken from detection to the sounding of an alarm tpr -time taken by building occupants to react to a fire alarm. ttravel -time taken to travel to safe abode or fully evacuate the building by individuals. 1.1.1 The travel time The longest travel distance is a sum of the longest distance travelled to the stairs on the mezzanine and the longest distance travelled on the ground floor exit doors. This are obtained using the Pythagoras theorem, where the hypotenuse is the distance in question. The door farthest from the stair case is used since it represents the worst case scenario, longest travel time. The findings are as expressed =61.23m Walking speed is taken as the horizontal travel speed. Nelson and Mowrer proposed a walking speed of 1.2 m/s which is widely used in most case scenarios. The walking speed is 1.2 m/s as specified by PD7974-6 for unobstructed flow, however, the warehouse has goods stocked in the space which obstructs free movement. =51 seconds 1.1.2 Flow time(time taken on exits). The flow on exits/doors is expressed with the following equation Number of people is calculated from area of the building and the occupancy density of the warehouse according to specifications. The Formula s as shown below. Number of persons = Area of the warehouse = = =3120 m2 The warehouse standard occupancy is 1 persons per 30 square metre. Therefore, number of people is: = =104 persons Total Number of persons = = =124Persons (Mezzanine floor occupants in total are 20 persons as specified.) Door factors as specified by PD 7974 part 6 is 1.3 persons/m/s Therefore, time on exits = =53s The time taken to move through the staircase is as shown T=0.68+0.081p0.75 Where p- population per metre of effective stair width (Effective stair width is given as actual width minus 0.3m) T- Minimum time required to evacuate the stairs in minutes = 47s Total travel time is given as =Flow time + travel time Flow time is the summation of the time it takes to go through exits doors and stairs = = =100s Evacuation time: Post-alertness is given by = = =151s 1.1.3 Reaction time, pre-movement time The occupants of the building are awake and familiar with the building surroundings and the obstacles within since no member of public is allowed access. The Pre-movement time can therefore be obtained from table 1 and table 2C of PD7974 part 6. Parts of the tables are as given below. Equation 4 of the PD7974 part 6 gives the pre-movement time as T= Where t1st percentile- time taken for the first percentage lot of occupants to respond to an alarm t99 percentile- time taken for the other 99 percent of occupants to respond to an alarm From the table t=0.5+1 t=1.5 minutes or 90s NOTE: The warehouse occupants are trained staff, therefore category A of table C-1 which outlines the different pre-times for different designs is chosen with the Automatic fire detection (B1-B2 A1-A2)system and high levels of fire management system (M1). The tabular data is indicated for first occupants and the Whole occupant distribution.[Bri04] Table 1 extract from PD7974 part six showing behavioural scenarios Table C1 extract from PD7974 part 6 Total Evacuation time can be expressed as summation of travel times (inclusive of flow) and the pre movement time = =241s 1.1.4 Detection Time Formulas are obtained from PD 7974-2, 2002. Ceiling case scenario is adopted in this calculation (Cl 9.2.6).[Bri021] CIBSE GUIDE E expresses the total heat release rate using the following equation: Where: : is the height of ceiling above of fire (m). : is the temperature of the jet (K). : is the (absolute) ambient air temperature (K). : is the total heat release rate (kW). Where: Horizontal distance from fire. Height of ceiling above base of fire (m), assume that the fire point is close to the mezzanine floor. Height is taken as that of the spacing of the mezzanine from the ground floor, which happens to be 9m.[Cha03] Horizontal distance is assumed to be 3.0m. This is because the warehouse stores inflammable goods and the fire spread and risk is also higher. Therefore, the following equation is taken to calculate the total heat release rate: Rearranging the formula given into a form where the total heat release rate can be obtained 1.1.5 Fire growth parameters According to (PD 7974-1:2003) the fire growth parameter is ultra-fast because it’s industrial storage. Therefore a value of 0.188 should be used to acquire the time from ignition as mentioned in table 2.[Bri02] Where: : Total heat release rate from the fire during the growth phase (KW). : Time from ignition (s). : Time of ignition (s). Fire growth parameter. The equation above is rearranged into the following equation Detection time Required Safe Escape Time can be obtained since the detection time has been calculated The required safe escape time from the mezzanine to fully vacate the premise is taken as 317.7 seconds. 2 ASET ASET, Available safe egress time, refers to the time elapsed between the development of hazardous conditions for human occupancy after a fire starts/ ignites. ASET can only be predicted by using estimated intensity curves for smoke and heatin a fire, toxic gases such as carbon monoxide to establish the endpoints of ASET. End points are the times when the conditions in the building become untenable/unbearable for anyone in the building to evacuate due to the smoke, heat and toxic gases exposure.[Bri04] For any building to be fire safe, ASET has to be greater than RSET. A factor of safety which is referred to as a margin is usually the difference between the ASET and RSET. ASET can be expressed in form of RSET using the following equation: ASET is usually computed using models which are difficult to combine all the endpoint conditions, therefore computer programs are used to run this models. Popular computer models include Computational fluid dynamics (CFD) model and zone model. Simple calculations of ASET can be expressed in the form of the following equation Where: : Detection time. : Notification time. : Time for the onset of hazardous conditions 2.1 Endpoints 2.1.1 Tenability conditions of smoke Smoke effects are irritation and visual obscuration making it hard to move through spaces and to locate exits within a building. Normal visibility is affected in smoke irritant areas with people moving as if they are in darkness with a visibility of 5m (D-1=0.2). Most designs adopt a tenability limit of 10m visibility (D.m-1=0.08) since smoke from fires contain a variety of irritant chemicals. Irritancy of smoke depends on the composition of the burning fuel, with oil have little smoke while solid substances such as wood emitting a lot of smoke. Exposure to smoke also affects the walking speed of the occupants. The figure below is borrowed from PD7974-6:2004 shows the tenability levels for smoke. 2.1.2 Tenability of fire and heat Occurrence of a flaming fire along an escape route will discourage its use especially when the height of the flame is more than 300. Exposure to either radiant or convected heat to individuals especially at high doses causes burns on his/her skin. This happens when one passes close to a fire or under or over a hot effluent layer. The tenability limit for exposure of unprotected skin to fire/heat has been proposed as that that causes severe pain on the victim. This occurs above a heat flux of 2.5kW per square metre, however, lower fluxes can be tolerated for few minutes or seconds depending on the flux level. Severe pain occurs when a victim is exposed to fluxes of 80-100 kW per square metre. Exposure of greater than 2000kW per square metre can lead to death while 240-300 kW can cause second degree burns. For convected heat exposure to temperatures greater than 121 degrees Celsius can cause skin pain and burns. Water use to extinguish fires should be with caution since when water vapour of temperatures greater than 60 degrees Celsius is breathed in it causes burns on the respiratory system. However, the tenability heat conditions of respiratory organs is usually higher than that of exposed skin surfaces. The skin is assumed to be dosed when exposed to heat over a certain period of time. Fractional Effective dose (FED) are used to calculate the summed up effect of radiant and convected heat which is then compared with predetermined FEDs which are deemed acceptable.[Bri04] Represented in the figure below are the tenability limits for both convected and radiant heat. It is borrowed from PD7974-6:2004 annex G 2.1.3 Tenability toxic gases conditions Combustion of some fuels by fire leads to the production of harmful gases which are classified as asphyxiants or irritants depending on their effect on an individual. Asphyxiants are only lethal, affecting movement of the victim and reasoning, when they exceed certain thresh-hold levels. Examples of Asphyxiants include hydrogen cyanide, carbon monoxide, low oxygen levels and higher levels of carbon dioxide. This gases can cause collapse at certain levels of inhalation. The tenability criteria is usually calculated for every gas using the intensity curves. FEDs are usually calculated and then compared with the predetermined FED. Irritants cause pain to the eyes, irritation on the respiratory system, lungs, nose and throat affecting movement speeds of the victim resulting in lower escape efficiency. Example of irritant gases include nitrogen oxides and hydrogen chlorides which are prevalent components of most combustible elements. The tenability of this gases varies depending on the fuel composition and combustion environment, therefore irritant wood smoke levels is used to calculate the tenable limits. The tenability limit is usually set at a maximum value of FED of 0.3 which is usually considered safe to safely exit the building. Calculated FEDs should be less than 0.3. FED of 0.3 is considered safe for a larger proportion of the population, however, a certain proportion within the population (approximately 10%) can be affected by such levels. A tenability limit FED of 0.1 is recommended. The figure below reflects the tenability limits expressed in concentration and time for irritant gases. It is borrowed from Annex G of PD7974-6:2004.[Bri04] 2.2 ASET calculations. The models used to calculate ASET are as explained 2.2.1 Computational fluid dynamics models They are a branch of fluid mechanics. They are used by engineers to determine smoke and fire behaviours and thermal behaviour. They use very limited data to achieve the computations for ASET or fire growths.[Yeo09] Various companies have come up with programs which are based on this models. Among the Programs in the market are FDS (fire dynamics simulator). 2.2.2 Zone model They are used to calculate smoke layers heights, temperatures and fire gases using simple geometry calculations. Among the programs which run this model in the market is CFAST, a two zone program. 2.2.3 Simple calculations Where: And: H: Room height S: Area of the floor Α: growth rate factor pg: upper layer density ρa: air density g: Gravitational force Cp-: specific heat[Hur16] References Bri04: , (British Standard Institute, 2004), Bri021: , (British Standard Institute, 2002), Cha03: , (Chartered Institute of Building Services Engineers, 2003), Bri02: , (British Standards Institute, 2003), Yeo09: , (Yeoh, 2009), Hur16: , (M.J., 2016), Read More