Bypass Pipeline for flushing out. 1

[shenoy]

I have a problem. A colloing water ipe line 2.2 m dia. supplies water from the sea to a power station. The water, hot , after cooling the machinery is pumped back into the sea through a similar parallel line.

When the shutdown takes place for the plant, silting takes place inside the incoming line, though we have all sorts of filters, screeners, silt chambers along the line.

To flush out the dirty water, before starting the plant, we want to flush out the silt/ dirt collected in the 2.2 m dia pile line. This will involve having a bypass line from the inlet line to outlet line short-circuiting the plant, in U shape.

There is an opening at the blind end of the inlet pipe, 550 mm dia, now used for cleaning the pipe. We intend to use the opening and use a 550 dia bypass flush out.

Will some body be able to tell me whether it will work? have some of you done such designs earlier? If you feel that it can work, What are the Design considerations?

Thanks in advance.

Shenoy.

 

[cvg]

wouldn't it be easier to install some kind of a plug or bulkhead on the inlet end of the pipe to keep the silt out of the pipe in the first place? If silt is able to get in while the pumps are shut down, likely it is also getting in while the pumps are running. Maybe the screens aren't working? Even if you bypass, will you also be pumping more sediment in?

In order to show that the bypass line will work, you will need to calculate the flow velocity in the inlet pipe during the flushing. This will be controlled by the capacity of the bypass line. It seems that the bypass line should be a bit larger - maybe 1000 mm or larger. Then the velocity in the larger line will be more able to move the sediment.
Example: If you pump 40 cfs through the bypass line at a velocity of 16 feet per second, the velocity in the large pipe will only be 1 foot per second. This is quite slow and may not clean it out. However, 16 feet per second is quite high for the bypass line.

 

[shenoy]

Thank you cvg.
Yes it would have been easier just to have a plug. But we have some pollution related problems here, letting in salt water to a sweet water lake.

The Velocity in the larger pipe is app 2m/s. This will force a velocity of approx . 32 m/s in the bypass line.( It would be some thing like a jet!!) and correspondingly, pressure will be less for the same discharge.

But will an eccentric connection work? with a sudden reduction? ( Even if i am to accept your suggestion to make it 1000 mm?

Thank you once again for your reply.

Shenoy.

 

[cvg]

working in english units again for my convenience, that is nearly 105 feet per second through a 21 inch bypass pipeline.

The friction loss will be incredible, both in the bypass pipe and in the connections. You should do some hydraulic analysis (especially estimating the losses through the tee or eccentric connections) to confirm that your pump can actually pump at 8 m3/sec with this configuration. I would guess that cavitation and thrust restraint might also be concerns.