Vent lines for Oily Water Sewer system

[nabeel3]

I am working in a Refinery project . The project specification for underground drainage for Oily water sewer system says that " Vent shall be installed in the following locations (1) upstream of the first sealed Manhole in a Oily water drainage system ( 2) Down stream of the farthest catch basin (3) Those section of the drainage line located between liquid seals.
Can anyone explain why the vent should be installed in the upstream of the first sealed Manhole. The Oily water sewer system consist of a number of catch basin connected into a main branch which drains into a manhole .Like that Manholes are connected one by one and draining out of battery limit. My question is that if the downstream of catch basin have a vent then why again on the upstream of the Manhole. Is it to protect the water seal of the Manhole?

 

[ashtree]

Nabeel3

As the liquid naturally moves from upstream to downstream manhole, it will tend to induce a small draft thereby potentially inducing some ventilation when there is flow.
Likewise any gas that is released that is lighter than air will tend to move up the slope of the drain to the upstream manhole and can then be vented if necessary.


Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[bimr]

Gravity sewers are designed to be partially full, not full pipe flow. Should the sewer be surcharged (meaning full pipe flow), then a siphon will occur. You would need to vent the upstream manhole.

If the sewer pipes were oversized so that the sewer would never surcharge, you could probably do away with the upstream vent.