Oily water Drainage design - Aramco Standard

[Amit_ugcivil]

4.5 Line Sizing
4.5.1 Gravity flow piping shall be sized to maintain a minimum flow velocity of 0.9 meters per second (3 feet per second) at the peak flow rate under full flow condition. In no case, the size shall be less than the general requirement stated in para 4.2.29.

Above clause stated In Aramco Design Standard:
1. 0.9m/s velocity shall be maintained considering pipe is flowing full.
2. Actual flow from upstream catchment area generates very much less flow.
3. Hence the for actual flow corresponding actual velocity is much less than velocity @ flowing full.
4.In case of actual velocity is used to satisfy for Vmin=0.9m/s, the pipe slope shall be 1:43
5. Where as for same pipe for Vfull = 0.9m/s the slope is 1:134

My Interpretation is to achieve economy in design the Sewer is designed considering Full Flow(ref 4.5.1) for Vfull = 0.9m/s minimum.

Theoretically the actual flow shall be flowing below 0.9m/s.

please reply is my interpretation is correct:

a) Sewer shall be designed for flowing full Vmin =0.9m/s
or
b) Sewer shall be designed for Actual flow velocity to be at Minimum = 0.9m/s

regards

 

[bimr]

Regulations usually have a sentence similar to "All sewers shall be designed and constructed to give mean velocities, when flowing full, of not less than "x" feet per second, based on Manning's formula using an "n" value of "x".

The purpose is to specify a pipe slope that will provide a minimum velocity when the sewer is flowing full.

For removing the impurities present in sewage i.e., sand up to 1 mm diameter with specific gravity 2.65 and organic particles up to 5 mm diameter with specific gravity of 1.2, it is necessary that a minimum velocity of about 0.45 m/sec and an average velocity of about 0.9 m/sec should be developed.

The flow velocity should be such that the suspended materials do not silt up in the drainage pipes; i.e. the velocity should be such as to cause automatic self-cleansing effect. The generation of such a minimum self cleansing velocity, at least once a day, is important, because if certain deposition takes place and is not removed, it will obstruct free flow, causing further deposition and finally leading to the complete blocking of the sewer.

Due to variation in discharge, the depth of flow varies, and hence the hydraulic mean depth (r) varies. Due to the change in the hydraulic mean depth, the flow velocity (which depends directly on r2/3) gets affected from time to time. It is necessary to check the sewer for maintaining a minimum velocity of about 0.45 m/s at the time of minimum flow (assumed to be 1/3rd of average flow). The designer should also ensure that a velocity of 0.9 m/s is developed at least at the time of maximum flow and preferably during the average flow periods also.

For industrial drainage applications where the variations between peak hourly flow and daily average flow are normally much less than for municipal sewage applications, where the solids loadings may be greater, and where fluids contain oils, one would be interested in designing for actual flow velocity to be at minimum = 0.9m/s.

 

[nabeel3]

Amit,

Oil water sewer is different from sewage( normal draingae) . U cannot deploy sanitary drainge formulas into OWS.

 

[cvg]

You may need to evaluate the sediment and oil volume and properties for your design. Recommend you trap and remove as much oil and sediment as possible before it goes into the drain. Drainage sumps, grates and oil/water separator may be needed. If you don't remove it before it gets in, then you will need to either manually flush or clean the pipes, or you must have periodic flushing flows adequate to clear it out.