Measurement of air pressure fluctuations at lower levels in a high-rise drainage stack

Transcription

1 Measurement of air ressure fluctuations at lower levels in a high-rise drainage stack W.L. Woo(), K.W. Mui(2), C.L. Cheng(3), L.T. Wong(4), W.J. Liao(5) (4) beltw@olyu.edu.hk (),(2),(4) Deartment of Building Services Engineering, The Hong Kong Polytechnic University, Hong Kong, China. (3),(5) Deartment of Architecture, National Taiwan University of Science and Technology, 43 Keelung Road Section 4, Taiei, Taiwan, ROC. Abstract Drainage system is a basic and essential rovision for all buildings. The maximum air ressure in a discharging drainage stack can delete the aliance water tra seals that revent the ingress of foul gases and odors into a habitable sace. The control of air ressures in discharging drainage stacks has been identified as an imortant factor in ensuring the erformance of a building drainage system. This study reorts air ressure fluctuations at a stack at the lowest floor of a high-rise building induced by a consecutive oeration of water closet flushes. Mathematical exressions roosed earlier for steady flow conditions are examined to correlate the maximum air ressure with the robability density function of the data measured. The average rediction of the absolute eak ressure is determined. The results showed that the maximum air ressure at the discharging stack with unsteady flows could be described by the robability density function of the measured data using earlier roosed mathematical exressions with a modified constant. Such outcome enhances the understanding of the characteristics of air ressure fluctuations at a drainage stack at lower floors of high-rise buildings and it is useful for further develoment of stochastic models for air ressure fluctuations at a drainage stack. Keywords Drainage and ventilation system, discharging stack, air ressure fluctuations 93

2 . Introduction Prevention of the ingress of contaminated air into a habitable sace is a matter of concern in the design of building drainage and vent systems. It can be achieved by rovision of water tra seals at various oints in drainage networks []. The control of air ressure fluctuations in vertical drainage stacks is an imortant factor in ensuring the erformance of a building drainage system [2]. Aart from external influences to the drainage and ventilation systems, such as wind blowing across the drainage stack oening, oeration of a mechanical fan, and the oening and closing of a washroom window or door, air ressure fluctuations in a drainage stack dominantly source from a change in fluid flow conditions of the stack. Current design ractice rotects tra seals from ressure fluctuations u to 375 Nm 2. However, in a oorly designed system, air ressure contributes to the robable deletion of aliance water tra seals which revent the ingress of foul gases into a habitable sace. Air ressure transient roagation in building drainage systems from some identified sources was described by a mathematical model [3]. An unsteady friction model was develoed to identify the raid flow changes under transient conditions and determine the tra seal deletion with raid changes of local air ressure. Air ressure fluctuations in artially filled ie flow were investigated through laboratory tests [4]. Mathematical exressions were also roosed to determine the maximum air ressure in drainage stacks due to some exerimental flushing oerations [5]. Measurements of air ressure fluctuations in drainage systems were addressed; however, their characteristics at unsteady flow conditions have not yet been studied intensively. To enhance the understanding of the effects of air ressure fluctuations in a drainage stack in a high-rise building, this study reorts the reliminary results for determining the characteristics of air ressure fluctuations in discharging drainage stacks. The results would be useful for evaluating the maximum air ressure in drainage systems of highrise buildings. Figure - Drainage stack. 94

3 2. Measurement of air ressure Full-scale exeriments were held in a camus building of The Hong Kong Polytechnic University. A 52.5 m high drainage stack in the Lee Shau Kee Building (Core Y) was referred. The stack collected an effluent of 2 water closets (WC) from 4/F to 4/F and turned at 2/F level as shown in Figure. It was a 2-ie system with a drainage stack of 50 mm diameter and a 50 mm diameter ventilation ie connected to each WC; this vent ie connected to a main ventilation ie of 00 mm diameter. A WC at 3/F which searately drained to another drainage stack of 00 mm diameter was ventilated by the same main ventilation ie. For the stack being monitored in this study, the nearest water seal of WC from the osition where maximum ositive ressure likely generated in the system stack was at 4/F, and it located at a height of 6 m from the stack base. Air ressure fluctuations at the drainage ie acting on the WC water seal were measured with a non-destructive testing method mentioned in revious studies for air ressure fluctuation measurements at drainage stack was adoted in this study [6,7]. In brief, one end of a ressure-hose was connected to an air ressure sensing system and the other end was ut into the water tra of the WC at the 4th floor (Location 6 in Figure ). The ressure sensing system (Uniulse UL84, Jaan) consisted of a ressure sensor, a data receiver and an amlifier with factory calibrated data logger which can measure air ressure ±5,000 Pa with the resolution of Pa and a frequency resonse u to 000Hz. This device set to gauge 200 data er second (200Hz) in this study. The exeriments were conducted in the unoccuied eriods. Air ressure fluctuations at location 6 were measured under the following flushing conditions. (a) A flush of WC at locations, 2, 5 as shown in Figure ; (b) 3 consecutive flushes, where the next flush was made 5s after the last flush, by a WC on the same floor; and (c) same as case (b) excet the next flush was made 30s after the last one. The air ressure fluctuation (m, water gauge ressure or denoted as m wg. ) in the vertical drainage stack can be characterized by the instantaneous air ressure at time t, or (t) (m wg.), within the drainage stack, relative to the atmosheric ressure outside the water seals connected to the stack. For unsteady discharging flow rates, a short averaging time of s for characterizing the ressure fluctuation was used to determine the arameters in the equations below. The instantaneous air ressure i (t) can be exressed by an equation below, where θ (m wg.) is the average air ressure over the eriod and θ i (m wg.) is the instantaneous air ressure fluctuation at time t, i t i ; i 0 () Taking the mean ressure θ (m wg.) and standard deviation (m wg.) as the redictors of a normal distribution function N, the instantaneous ressure density function ~ θ (m wg.) over the eriod is aroximated by, ~ N, (2) 95

4 Finally, a roosed eak ressure model exressing the eak air ressure (m wg.) in the stack is exounded in an equation below, where a 0 is a constant indicating the sign of the eak ressure ( for negative ressure and + for ositive ressure), a is a coefficient characterizing the air ressure fluctuation in the discharge stack, min and max are resectively the minimum and maximum air ressures over the measurement eriod [5], a 0a ; a 0 ; ; min min max max (3) It is noted that the robability of excessive eak ressure beyond (m wg.) with a from the above model can be exressed by a robability density function of the air ressure θ ~ (m wg.), a 0 a 2 0 ~ d f,, a (4) 3. Results and discussion The exerimental results of the characteristics of air ressure fluctuation were resented in terms of the average ressure (m wg.), standard deviation (m wg.) and eak ressure (m wg.) measured at a normalized stack height H for a WC flush(es) discharged at a normalized height H d. Given by the maximum absolute ressure measured, is maxmin, max. Stack air ressures were measured at the water closet on the 4th floors, i.e., H =6 m. The water flush(es) oints were made on the 8 th, 0 th, th, 2 th and 3 th floors, corresonding to discharge height H d = m, m, m, m and m. Determined by the stack height H s, height of ressure measurement H (m) and discharge height H (m), the normalized heights are, H H H s ; d H Hd H Figure 2 exhibits the samle measurement results of air ressure in a stack with a WC flush, 3 consecutive flushes by WC at time intervals of 5s and 30s, at discharging heights H d =22.02 m, m and m resectively. Preliminary observations showed that flush(es) at higher level would associate with a higher maximum absolute ressure. The absolute eak ressure was increased when the discharged height increased. Air ressure fluctuation by a flushing WC on the 2th floor was found to be higher as comared with other cases. More flushes could enhance or demote the maximum absolute ressure. Comared with the cases of single flush, the air ressure fluctuations due to a flush would be affected by the revious flush if consecutive flushes were made in a short time interval. Pressure fluctuations sustained for a longer time was shown in case (c). d s (5) 96

5 Air ressures were measured reeatedly for each case. Figure 3 shows the average eak air ressure and the average air ressure at H =6 m of the stack, with WC flush(es) at discharging heights, 0.42, 0.56, 0.64, 0.72 and 0.8 resectively. All, excet one case, showed ositive eak air ressures due to consecutive flushes were higher than the ones induced by a single flush. In the excetion case, it was noted that a very different negative average air ressure was found. The results showed that air ressure fluctuations would also be due to interactions between flushes and the time intervals between 2 consecutive flushes would be exlanatory arameters. The resent exerimental results from reeated 0 observations from each case showed oortunistic nature of air ressure fluctuations. Additional data is required for ossible robabilistic model formulation. Figure 2 - Instantaneous air ressure fluctuations at a discharging stack for (a) single flush; (b) 3 consecutive flushes each 5s aart; (c) 3 consecutive flushes each 30s aart; at a discharging height H d () 22.02m; (2) m; (3) m. 97

6 (a) Peak (b) Average Figure 3 - Air ressure fluctuations at H =6 m for WC flush(es) at discharging heights Figure 4 - Air ressure fluctuations as a function of average ressure for (a) single flush; (b) 3 consecutive flushes each 5s aart; (c) 3 consecutive flushes each 30s aart. Figure 4 shows the correlation between the fluctuation ratio and the ressure ratio with a determined via least squares fitting (R>0.8, < 0.000, t-test), where ε is an error term, a ; ; Average values of coefficient for characterizing air ressure fluctuations a were 3.23 for both cases (a) and (c) and 2.93 for case (b) resectively. Comared with the coefficient a =3.684 determined for steady flow conditions, this study reorted a lower ; (6) 98

7 a for unsteady flow. An even lower a would be associated with larger air ressure fluctuations. Figure 5 - Comarison between the redicted and measured absolute maximum air ressure in a discharging stack. The goodness-of-fit of Eq. (6) to the measured data in the least-squares analysis is grahed in Figure 5. Significant correlations between the redicted eaks and measured eaks were reorted for (R>0.9, 0.000, t-test). Over-redictions in the eak ressure were made in few cases for (b), consecutive flushes of time intervals of 5s. Predictions made by the model with a derived for steady flow conditions were determined with unsteady flow conditions for comarison. The model with the constants obtained from steady flow exeriments would over redict the eak ressures in a stack with unsteady discharge flow rates. It was noted that higher air ressure fluctuation values were reorted for a discharging stack with unsteady flow rates. The alication of the constant a derived from steady flow conditions accounted for lower air ressure fluctuation values was therefore over-redicted the eak ressure. 4. Conclusion This study examined the characteristics of air ressure fluctuations, including the maximum and average ressures with their resective standard deviations, along drainage stacks for buildings. Mathematical exressions were roosed to correlate the maximum air ressure with the robability density function of the measured data. The average rediction and the maximum under-rediction of the absolute eak ressure were determined with the margin of error taken within certain confidence levels. The results would be useful for understanding the effects of air ressure on tra seals connected to a drainage stack in a high-rise building and for measuring the maximum air ressure in high-rise drainage systems. 99

8 5. Acknowledgements The work described in this aer was artially suorted by a grant from the Research Grants Council of the HKSAR, China (PolyU533709E). The authors would like to thank the Architecture & Building Research Institute of the Ministry of the Interior of Taiwan (ABRI) and the National Science Council of the Reublic of China (NSC E-0-22) for financially suorting this research. 6. References. Wise A.F.E., Swaffield J.A. (2002). Water, sanitary and waste services for buildings. Butterworth Heinemann, Oxford. 2. Jack L. (2006). Drainage design: factors contributing to SARS transmission. Proceedings of the Institution of Civil Engineers-Municial Engineer, 59(), Swaffield J.A., Cambell D.P. (992). Numerical modeling of air-ressure transient roagation in building drainage systems, including the influence of mechanical boundary conditions. Building and Environment, 27(4), Cheng C.L., Yen C.J., Lu W.H., He K.C. (2008). An emirical aroach to determine eak air ressure within the 2-ie vertical drainage stack. Journal of the Chinese Institute of Engineers, 3(2), Cheng C.L., Mui K.W., Wong L.T., Yen C.J., He K.C. (200). Characteristics of air ressure fluctuations in high-rise drainage stacks. Building and Environment, 45(3), Liao W.J., Cheng C.L., He K.C., Yen C.J. (2008). Testing Method and verification of ortable measurement and Analysis for Air Pressure Distribution in the Stacks of Building Drainage System. Proceedings of the 34th W062 The 34th International Symosium on CIB W062 Water Suly and Drainage for Buildings, 8-0 Setember, The Hong Kong Polytechnic University, Hong Kong, Wong L. T., Mui K. W., Hui P. S. (2008). Air ressure variations at drainage stacks of high-rise residential buildings. Facilities, 26(/2), Presentation of authors Dr. L. T. Wong is an associate rofessor at the Deartment of Building Services Engineering, the Hong Kong Polytechnic University. Dr. K. W. Mui is an associate rofessor at the Deartment of Building Services Engineering, the Hong Kong Polytechnic University. 200