member 100 is replaced by a fully open period from 3 5 to 5 5 seconds fig7 illustrates the air pressure profile starting from the stack bases 150 and 156 of stacks a and d respectively upwardly to pipe 11 at about 2 5 seconds into the example hereof the air pressure in stack d demonstrates a pressure gradient compatible with the reversed airflow mentioned above the air pressure profile in stack a is typical for a stack carrying an annular water downflow and demonstrates the establishment of a positive backpressure due to the water curtain at the base of the stack a following completion of the discharges of water from discharge sources the airflows will naturally attenuate over a period of time based on the frictional resistance in the system 32 as a minimum or trickle flow is assumed to continue in each stack the rate of attenuation of the entrained airflows is low the initial collapsed volume of the papa 68 installed on pipe 13 was 0 4 liters with a fully expanded volume of 40 liters however due to its relatively small initial volume it may be regarded as collapsed during the phase of the example illustrated in fig7 fig8 shows a surcharge at the base 150 of stack a for pipe 1 at 2 5 to 3 seconds the entrained airflow in pipe 1 reduces to zero at the stack base 150 and a pressure transient is generated within stack a as illustrated in.