Going from pressure flow to gravity flow... 1

[cdale]

Can anyone tell me what the implications are of tying into a gravity flow line with a pressure line?

I have a main pressure line and I want to put in a bypass to the gravity sewer in case I need to divert some of the flow from the main line...

Thanks for your help...

 

[Montemayor]

cdale:

You haven’t revealed what fluid you are purging and its thermodynamic properties. Presumably, you’re dealing with a relatively high pressure water main and wish to purge some of its contents into a water sewer gravity system.

The only implication I can detect is that the capacity of the gravity sewer system be able to handle the maximum liquid purge flow rate coming from the water main. If the high pressure water in the main is supercooled, there will be no flashing through the let-down control valve that is doing the main purging. However, if you failed to tell us that the high pressure water is saturated, then we have another story.

The resultant product from purging a saturated liquid under pressure will be a 2-phase flow and this could cause you considerable capacity problems within your gravity sewer system.

Since your post is devoid of a lot of basic data, that is all I can offer.
Hope this helps.

 

[cdale]

To respond to Montemayor...

The fluid is clean water that would otherwise be going to an infiltration gallery. The infiltration gallery will accept 160 gpm. If our flow increased to 180, for example, I want to be able to divert 20 gpm to the gravity sewer. The forcemain is 4 inch HDPE, the gravity sewer is 8 inch PVC. The total head for my primary forcemain is less that 50 ft of water.

Hopefully that helps clarify the problem some, thanks...

 

[JStephen]

If you exceed the downstream capacity of the sewer line, you could be blowing a lot of sewage out of every available opening upstream of your injection point. Not good.

 

[cdale]

JStephen...

Good observation, but right now we currently discharge 160 gpm to the gravity sewer. Any discharges in the future will only be a portion of that, if any...

My question is more along the lines of, is it possible to discharge from a pressure line into a gravity line? And is it as simple as tying into the sewer line and controlling flow from my pressure line with a valve?

thanks...
cdale

 

[JStephen]

I would think you'd want to release the pressure and let the product flow in. In other words, make sure you never pressurize the sewer line in any way. Some sort of an airgap arrangement.

Also make sure the pressure line is not used for or connected to potable water lines anywhere. If so, you definitely want some sort of an air gap and drain, and not a direct connection to a sewer.

 

[PEDARRIN2]

Sewer piping accepts pressurized flow all the time. It happens when you have a source (floor drain or sump pit in elevator) which is below the invert of the outside sewer pipe.

Then you have a pumped system to evacuate the flow. Typically, the force main enters the gravity main at the top of the pipe.

The gravity system must have the capacity to accept the flow. It must also, at least for a sanitary system, have an area for air or else you will get a plug of water which will push the downstream air with it and you could have things happen downstream which you do not want.

If you want to divert 20 gpm to a 6" and later to a 8", you are probably safe, but you have to account for what is already in the system.