solids in building drainage stacks: a potential cause of ... mechanisms of air entrainment due to...

CIB W062 Symposium 2006

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Solids in building drainage stacks: A potential cause of air pressure transients? Dr. M. Gormley1 1. Drainage Research Group The School of the Built Environment Heriot Watt University Edinburgh EH14 4AS [email protected] +44 (0) 131 451 8262 Abstract The consequences of air pressure transients in building drainage and vent systems (DVS) can be serious as they compromise the last line of defence between the potentially hazardous foul air in the drainage and sewer network and the habitable space within buildings. Pressure transients are caused by rapid changes in flow conditions in fluid systems. Much previous research has defined the nature and effects of air pressure transient generation and propagation within DVS. This paper confirms that solids have an effect on the pressure regime in a vertical stack; that transients are generated and that pressures are modified in a way hitherto undefined. Numerical models have been developed to predict the effects of different configurations and innovative techniques on the air pressure regime in DVS. These models have so far not included the effects of solids in the vertical stack and their influence on the flow and pressure regime within the system. A series of controlled experiments in a 16 storey building with a single stack system are described and a simulation of these events in the numerical model AIRNET confirms the minimal impact, on water trap seals of the short duration transients generated by solids falling in vertical drainage stacks.


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1. Introduction The flows found in DVS are inherently unsteady as a result of appliance discharges throughout a building and are difficult to define. The discharges from appliances generate surge waves and while these are subject to attenuation in the horizontal elements of the network, still emerge into the vertical stack as a rapidly varying flow. Appliance discharges transporting solids will naturally eject these solids into the stack where they will fall within the central air core. Much research has been carried out on the inflow to vertical stacks from branches1,2,3 . The mechanisms of air entrainment due to fluid to fluid traction forces4 has also been defined and numerical models have been developed which can accurately simulate the pressure regime in this two phase flow of air and water. Of great concern to the public health engineer, and the researcher in this field, is the protection of water trap seals from unacceptable pressure fluctuations in the DVS. International codes recognise the importance of protecting water trap seals from undue pressure fluctuations by limiting the allowed variation to 38mm wg. Recent investigations in this area have attributed the spread of the deadly SARS virus in South-East Asia to a possible cross contamination from one housing unit to another in a high rise block as a result of dried-out water traps5,6. This paper considers whether solids falling in a vertical stack can cause the generation of low amplitude air pressure transients of sufficient magnitude and duration to compromise water trap seals in a single stack system. The data gathered for the analysis was obtained from a set of investigative experiments on a 16 storey building in Dundee, Scotland due for demolition. The value of the data lies in its acquisition from a recently used building still connected to the public sewer network and bearing all the marks of a system which had been used for thirty years and having sufficient height to ensure terminal conditions7,8. Little prior research has been carried out on this subject with the exception of a small but significant test series carried out in the 1950s9, when the operational parameters of the single stack system was investigated by the Building Research Station in the UK. These tests showed that flushing wads of newspaper down a WC did not induce siphonage in water trap seals lower in the building. While the outcome of these tests was conclusive, in that they proved that the single stack system was robust enough to deal with heavy flush loads without the need for additional venting arrangements, no pressures were recorded and the mechanisms of transient generation were not defined. It was also assumed that the excessive loads would generate only negative transients. The series of tests described in this paper, and their subsequent analysis, will define the mechanisms of generation and propagation of potential sources of negative and positive air pressure transients in building drainage systems and provide a credible means to predict their effect on system performance via a method of characteristics based computer model.


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2. DVS air pressure Transients: Mechanisms and models Low amplitude air pressure transients in DVS are commonplace, generated by the changing nature of the unsteady water and entrained air flows associated with the random discharge of appliances. These generated transients are suited to the classical elastic analysis of pressure surges associated with waterhammer in bounded systems and are governed by the St. Venant equations of continuity and momentum7,8 . Consider the simple frictionless system shown in Figure 1 consisting of a pipe length terminating in constant pressure reservoirs, with a partially closed valve midway. The pipes on either side of the valve are at different pressures due to the setting of the valve and the ∆p across it (Figure 1a). At some time in the future, as depicted in Figure 1b, the valve is rapidly opened and a positive pressure transient propagates downstream from the valve as the fluid rushes through the open valve. An opposite, negative pressure transient propagates upstream from the valve. These two pressure transients propagate along the two pipes at the local acoustic velocity for the fluid in a rigid bounded conduit. For the case considered in this paper – air in a rigid pipe - the velocity of propagation is approximately 320m/s.

Flow direction Valve

∆P

Constant P Constant P

+x

Negative transient travels in the –x direction

Positive transient travels in the +x direction

-x

+x -x

Figure 1 Simple Transient Analysis

Figure 1(a) Valve closed condition

Figure 1(b) Transient generated following momentary opening of valve

∆

∆


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The above simple model of system operation can be applied to the flows associated with building drainage systems. Figure 2 shows the relevant components of the system. The air flow in the system is initiated as a result of the shear between the annular water film and the air core. In the case of Figure 2 the water at the branch/stack junction forms a partial occlusion to the air and is therefore equivalent to the valve in Figure 1 with a pressure difference across it, although it is not wholly closed. If the make up of the water curtain at the branch/stack junction is changed so that a momentary opening appears then a positive transient will be generated and propagate down towards the base of the stack and conversely, a negative transient will be generated and propagated upwards towards the top of the stack. Effectively an enlarged passage reduces the obstruction and results in an increased entrained airflow. This can only be maintained if the entrained airflow in the upper stack rises in response to the propagation of a negative transient. This opening could be created by a solid falling in the centre of the stack pipe as depicted in Figure 2. The mechanisms described in this section can explain how falling solids could generate air pressure transients in a vertical building drainage stack. The task of proving whether this phenomenon would compromise water trap seals in practice will be evaluated by a combination of experimental investigation in a real building stack and simulation of their effect using numerical techniques described below. 3. Numerical modelling of air pressures in DVS The mechanisms of air pressure transient propagation in DVS described above are well understood. Modelling air pressure wave propagation in a bounded conduit is suited to numerical techniques applicable to full bore fluid flow. Previous research3 has developed the simulation techniques used extensively in this research to evaluate the risk associated with the generation and propagation of air pressure transients resulting from the presence of falling solids in drainage flows in vertical stacks. The program simulates the propagation of low amplitude air pressure transients using the fundamental St. Venant equations of momentum and continuity and by the numerical solution

∆P

Figure 2 Solid piercing water curtain at a branch to stack junction

Discharging branch

Partial occlusion to air by water curtain

Vai

r


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of these equations, via the method of characteristics. The method is used to yield air pressure and velocity within a bounded duct system subjected to air pressure transient propagation1,2,4,10. 4. Experimental set up 4.1 General The data upon which this paper is based was obtained from an extensive set of site investigations on a 16 storey local authority residential building in Dundee, Scotland which was due for demolition. The building contained 90 local authority housing units and the drainage system was sub-divided into 6 different sections, each one served by a 150mm cast iron vertical drainage stack which ran the height of the building. Each of the 6 single stack systems served a total of 15 housing units, and also served to drain the roof area of run-off rain water. For modelling purposes, pressure and airflow measurements taken on the stack must be attributable to specific appliance discharges, which is an extremely difficult task in an inhabited building. Discharges from different appliances, WCs, baths, showers, washing appliances are difficult to control and predict, due to their random usage in reality. It was therefore necessary to control flows in the stack in order to cross reference discharges with pressure and airflow measurements. The single stack system, shown in Figure 3, represents the test installation in the building. In order to ensure that all the air was entering the system through the roof termination, all water trap seals were disconnected from the system and the connecting pipes sealed. Metered water pumping stations were set up on 4 different floors, Figure 3 shows the arrangement. Airflow was measured using a pitot-static tube on the top floor of the building mounted in a 50mm connecting pipe. Pressure transducers were installed along the height of the stack. 4.2 Data Acquisition and possible error The transducers were located just above the branch connections and were routed back to the Data Acquisition station on floor 10 of the building. The transducer outputs were connected to a data acquisition system based around a Keithley Instruments KPCMCIA 3116 Analogue to digital converter (A/D) with 16 bit resolution. The scan rate of the system was set to 200Hz. The scan rate was chosen as a compromise between the need for high speed resolution and the file size on long test runs (typically 2 minutes). As the system was designed to record transient propagation, it was possible to miss a small amount of information using a scan rate of 200Hz. The error associated with the scan rate can be expressed either in time (seconds) or in distance (metres). A 200Hz sample rate will record a measurement every 0.005 seconds. This was considered appropriate for measuring pressure transients in this experimental scenario.


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4.3 Methodology The hypothesis tested in this paper is that when solids are discharged from an appliance such as a WC, they fall in the centre of the pipe, driven by a combination of forces from the WC discharge and gravity. If such a solid were to strike the water from a discharging branch then the rapid change in airflow conditions as the water curtain opens would cause a pressure transient which would then propagate throughout the system. In order to test this hypothesis it was necessary to simulate a WC flush containing a solid and to pump water into the stack at some point below to simulate a discharging branch, as shown in Figure 3. The WC discharge was simulated by pumping water into a WC containing a 250mm long maternity pad. The water pumping profile used represented a standard drop valve flow rate against time (Q/t) curve for a WC. The flow rate from floor 6 was set to 1.5 l/s steady flow.

15 13 11 9 7 5 3 1

14 12 10 8 4 2

Airflow measurement by Pitot-Static tube and transducer

Ground

Pumping Station

Valve Meter Pump

Pumping Station

Building Water

supply tank

Data Acquisition

System Laptop with PCMIA 16 bit DAQ

card Power

Floor 10

= Transducer Installed above junctions

To public sewer

Pumping Station

Figure 3. On-site Investigation schematic

Valve Meter Pump

Valve Meter Pump

Valve Meter Pump

Pumping Station


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Test number Solid

discharge Steady flow discharge

Type of solid used

Termination

1 15 2 No solid Open 2 15 2 200g pad

Open 3 15 6 No solid Open 4 15 6 200g pad Open 5 15 None 200g pad Open 6 15 10 200g pad Open 7 15 6 200g pad Closed 8 15 6 2 x 200g pads Open 9 15 10 & 6 2 x 200g pads Open 10 15 None 2 x 200g pads Open Table 1. Test schedule The test schedule given in Table 1 represented the range of investigations possible in the on-site part of this research. The tests formed a mixture of possible scenarios with different solids discharged from different heights and interacting with branch flows at various distances along the stack. 5. Results In order to establish base conditions at the test site a series of experiments were executed to determine expected pressure levels when no solids were present. The methodology employed was to establish a steady flow from floor 10 of 1.5 l/s and then to simulate a WC flush from floor 15. The resultant pressure trace is shown in Figure 4. It can be seen that when the flow from floor 10 is turned on the pressure in the stack begins to go negative, eventually steadying when flow conditions steady. As expected there is a pressure regain towards the base of the stack and a slight positive pressure on the lower floors. Once this flow has been established the simulated flush flow was initiated. The pressure profile is modified by the unsteady flow from floor 15. By introducing a solid into the discharge, it is possible to see the generated pressure transient (Figure 5b.). The pressure traces follow a similar pattern to those shown in Figure 4 until the solid pierces the water curtain formed at the branch to stack junction on floor 6. At this point a pressure surge can clearly be seen. Upon closer inspection (Figure 5a) it is clear that the sense of the transients generated are: positive down toward the stack base and negative up toward the roof termination. This is caused by the solid piercing the curtain, creating a momentary orifice in the water curtain allowing a pulse of air, in the form of an air pressure wave, down the stack. The negative air pressure transient propagating up the stack is a necessary consequence of the opening in the water curtain, as the system communicates its need for air to the open termination at the top of the stack. The


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0

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80 80.5 81 81.5 82 82.5 83 83.5 84 84.5 85

Time (seconds)

Pres

sure

(mm

wg)

floor 15floor 13floor 10floor 2

-170

-120

-70

-20

30

80

130

82.4 82.45 82.5 82.55 82.6 82.65 82.7

Time(s)

Pres

sure

(mm

wg)

Floor 15Floor 6Floor 2

t1t1 = 0.035secs

peak positive pressure = 106mmpeak negative = -144mm

Fl 15 - solid in

Fl 61.5l/s

Fl 2

Transient duration t2 = 0.045sec

t2

t3 =0.083 sec

mechanism involved in this process is identical to that described above and illustrated in Figure 1. The water curtain formed at the junction is analogous to the partially closed valve in Figure 1, and the pressure surge is generated by the same mechanism as that described by the momentary opening of the valve which has a pressure difference across it.

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0 10 20 30 40 50 60 70 80 90

Time (secs)

Pres

sure

(mm

wg)

Flow on Steady flow from Floor 10 stabilized Flush initiated

Small increase in pressure as flush flow reaches floor 10 reaches

Small positive pressure on floor 1

Floor 15

Floor 10

Figure 4. Baseline pressure trace – no solids discharged Figure 5. Trace showing positive and negative transients Due to the predictable nature of transient propagation, it is possible to confirm that the pressure ‘spikes’ observed from the pressure traces are actually air pressure transients and not merely stray electro- magnetic forces (EMF) from pumps or other electrical noise. From Figure 5a the propagation time between floor 6 and floor 2 (t1 at 0.035 s), floor 6 and floor

Figure 5b – Trace showing entire test run

Figure 5a Definition of generated transients


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15 (t3 at 0.075 s) can be measured. The distances between the transducers on floor 6 and floor 2 was approximately 12 m and between floor 6 and floor 15 was approximately 27 m - the velocity of propagation (VOP) is 320 m/s. It is therefore possible to confirm the distances travelled by multiplying the VOP by the travel time. The calculated distances between floor 6 and floor 2 was 320 x 0.035 = 11.2 m - and between floor 6 and floor 15 was 0.083 x 320 = 26.5 m – thus confirming the presence of air pressure transients as a result of falling solids in the stack. The solid used in this experiment was a maternity pad 250mm x 65mm x 10mm with a wet weight of 200g. The experiment was repeated for the combination of solids and experimental configurations given in Table 1. From the analysis of the data recorded from these experiments Table 2 was produced and an assessment of a peak positive and negative pressure transient was obtained for use in the simulations. As can be seen from Table 2 the peak positive pressure recorded was 106mm wg and the peak negative pressure recorded was -195mm wg. Solid weight

Solid discharge

Steady flow discharge

Peak pressure

rise recorded (mm wg)

Peak positive pressure recorded(mm wg)

Peak negative pressure recorded (mm wg)

Transient duration (seconds)

200g 15 2 51 90 -90 0.060 400g 15 2 40 27 -80 0.055 200g 15 6 110 106 -150 0.045 400g 15 6 120 53 -130 0.040 200g 15 10 87 27 -100 0.050 400g 15 10 123 77 -140 0.065 200g 15 10 & 6 120 98 -195 0.050 Table 2 Test Results 7. Solid Velocity The average velocity of a solid can be measured if the time a solid passes two known points along the stack can be logged. Since the distance is known then the solid velocity can be derived from V = l/t where l is the distance between the points and t is the time taken to travel between the points. In the experimental set-up described in this paper no special arrangements were adopted to measure a passing solid and it was therefore necessary to calculate solid velocity by using the principles of transient propagation. To do this, a steady flow was pumped into the stack at 2 locations – floor 10 and floor 6 and a solid was discharged from floor 15 in a flush volume as described above. As the solid pierces both water curtains, transients are


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generated – it should therefore be possible to measure the time it takes the solid to travel between the water curtains by analysing the pressure traces. Figure 6 shows such a pressure trace. By measuring the time between the 2 distinct peak pressure transients, a solid velocity of 4.27 m/s was established. This is an average velocity over the time taken to travel between the two transducers. Since the solid falls down the centre of a pipe there will be acceleration due to gravity and a significant drag factor. The solid also travels through the water curtain which inevitably causes some deceleration – the solid is therefore not in free fall. This investigation has established an average velocity for a solid in a vertical drainage stack, however more work is required in this area to establish the influence of parameters such as: solid characteristics, water and air flow rates and the pressure regime in the stack on solid velocity since these will all influence the terminal velocity and drag co-efficient of solids in practice.

Falling Solid Velocity

-300

-200

-100

0

100

200

300

76 76.5 77 77.5 78 78.5 79 79.5 80

Time (secs)

Pres

sure

(mm

wg)

15106

1

1.5l/s from 101.5l/s from 6Solid from 15 in a flush

floor 1

floor 6

floor 10(transducer above inlet)

Floor 15

Solid pierces water curtain on floor 10 (-ve transient up +ve transient down)

Solid pierces water curtain on floor 6(+ve transient evident on floor 1) less definition up due to water curtain on floor 10

Floor 1Time between solid piercing water curtain on floor 10 and water curtain on floor 6 = 2.81 Sec. Separation distance = 12 m V = 4.27 m/s

Figure 6. Assessing Solid Velocity 8. Numerical Simulation The mechanism underlying the generation of pressure transients has been established as a modification to the pressure regime at a branch to stack junction by a solid piercing the water curtain, allowing a passage for air movement for a short time. The pressure regime at this junction can be modelled by introducing a concentrated loss on the entrained airflow – the result of a restriction to air movement caused by combining branch and stack water flows. The effect of the solid piercing the water curtain can therefore be achieved by reducing the concentrated loss to simulate an opening in the water curtain. 8.1 Effects on water trap seals The simulation of the effects of solids falling in a vertical stack can be achieved by modelling the simple pipe configuration shown in Figure 7. By introducing a positive air pressure transient with the characteristics of those observed in the site investigations it is possible to establish whether such transients would be problematic in real installations.


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0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

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sure

(mm

WG

)

Appliance sideSystem side


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0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

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sure

(mm

WG

)


Appliance System side

The magnitude and duration of the positive air pressure transient to be simulated was based on the maximum figures obtained from Table 2. The results of the simulations are shown in Figures 8 and 9. and shows the effect of the simulated positive and negative air pressure transients on the water trap at the end of the 2m branch. It can be seen that while the magnitude of the pressure transients were 106 mm wg (positive) and -195 (negative) they do not ‘blow the trap’ despite there only being a trap depth of 50mm. The reason for this is that there is insufficient energy in the pressure transient, due to its very short duration, to overcome the inertia of the water in the trap seal. This results in reduction in the effective pressure experienced by the water in the trap seal such that the water fluctuates by approximately 10mm, a phenomenon which will not compromise the integrity of the seal between the drainage system and the habitable space. Figure 7. Simulated Configuration.

Water trap seal depth, h = 50mm

Transient propagates at local acoustic velocity

Vertical Stack

branch length = 2m Diameter = 50mm

Figure 8. Effect of simulated positive transient on water trap seal.

Figure 9. Effect of simulated negative transient on water trap seal.


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9. Conclusions This research has been undertaken in order to attempt to quantify a significant possible source of air pressure transients in building drainage systems. It has long been known that air pressure transients can cause problems with water trap seals. Anecdotal evidence and accepted mainstream thinking by professionals in the field, has often attributed the generation of such air pressure transients to solids falling down a vertical stack. This paper has shown that such pressure transients are generated by solids falling in a vertical drainage stack. This paper has identified the mechanisms of this transient generation and has confirmed that their effect on water trap seals can be assessed using a method of characteristics based numerical model. While it is clear from this paper that air pressure transients are generated in this manner it has also been shown that their duration is too short to have any real impact on the integrity of the water seal between the building drain/sewer network and the habitable spaces inside the building envelope. 10. References [1] Swaffield J. A. and Campbell D. P., 1992a, “Air Pressure Transient Propagation in Building Drainage Vent Systems, an Application of Unsteady Flow Analysis”, Building and Environment, Vol 27, no. 3, pp 357-365 [2] Swaffield J. A. and Campbell D. P., 1992b, “Numerical modelling of air pressure transient propagation in building drainage vent systems, including the influence of mechanical boundary conditions”, Building and Environment, Vol 27, no. 4, pp 455-467 [3] Swaffield, J.A., Jack, L.B. Campbell, D.P., & Gormley M. (2004) ‘ Positive air pressure propagation in building drainage and vent systems.’ Building Services Engineering Research and Technology, 25(2) (2004) pp 77-88 [4] Jack L.B., (2000). Developments in the definition of fluid traction forces within building drainage vent systems, Building Services Engineering Research & Technology, Vol 21, No 4, pp266-273, 2000. [5] World Health Organisation (WHO) (2003) ‘Inadequate plumbing systems likely contributed to SARS transmission’ WHO Press Release WHO/780 pp1-2, 26 Sept. [6] Hung, H et al , (2006) ‘Industrial experience and research into the causes of SARS virus transmission in high-rise residential housing estate in Hong Kong’ Building Services Engineering Research & Technology, Vol 27.2 pp 91-102, 2006. [7] Pink, B.J. (1973) ‘ A study of stack length on the air flow in drainage stacks’ Building Research Establishment, Current Paper 38/73. [8] Fox, J.A. (1989) ‘Transient Flow in Pipes, Open Channels and Sewers’. Ellis Horwood Ltd. Chichester UK. [9] Wise A.F.E, 1957 ‘Drainage pipework in buildings: Hydraulic design and performance’ HMSO, London [10] Swaffield J. A. and Boldy A.P., 1993, ‘Pressure surge in pipe and duct systems’, Avebury Technical, England 11. Author

Dr. Michael Gormley is a Research Associate and has been a member of the Drainage Research Group since 2000. He specializes in air pressure transient analysis in building drainage systems, solid transport and flushability criteria for commercial products in horizontal drains and the effects of surfactants on drainage systems.

solids in building drainage stacks: a potential cause of ... mechanisms of air entrainment due to...

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