Plumbing Engineering Services Course

Question raised by a participant on the number of SWV discharge stack to a single manhole.

Dear Ir Hanafi,
You asked me a question about the number of SWV discharge pipes ONE manhole can handled, I had done my research and my reply as follows however I must make a disclaimer that I am not an expert in this area and that it my personal evaluation.

Each Discharge stack has a rated flow rate based on the number of discharge units or what the discharge volume example 100mm dia flow rate of 6.9 to 7.2lit/sec at 1/4 full. If you have say another discharge 150mm dia, flow rate of 20.60 lit/sec at 1/4 full. Therefore if you add up ALL the discharge stacks into this manhole, you will get a total EXPECTED flow rate. Theoretically, it is limited to the size of the manhole, more discharge stack into the manhole the bigger it gets.

The discharge FROM the manhole to the sewer plant will be via Open Channel concept. An open channel is a duct through which a liquid flows with a free surface. At all points along its length the pressure on this free surface will be the same, usually atmospheric pressure, it is not running full, a partly filled pipe in this case is treated as open channel.

Now you have to use Chezy formula to determine the Q (discharge from the manhole in the open channel concept), factors like gradient, C coefficient of resistance of the channel, the hydraulic mean depth, etc.

Answer to your questions how many discharge stack, it will be dependent on the design Q of the Open Channel, it must be greater than the combined discharge of all the stacks, a safety factor for the future as the resistance in the open channel increases with time.

I hope I answered your question, in giving you this answer, I learned from it too.

Please help me to promote my courses, next is Fire Engineering on the 22 and 23 June 2010.

Regards,

Gary Lim