2 tanks under gravity flow

[plumberet]

I have 2 waste water tanks. Feed from these tanks to the biotreater is via gravity. Problem is, we cannot control the tank levels in example given below. When tank A level is 10 m with tank B at 5 m, opening both tanks to biotreater under gravity will result in the 2 tank levels equalising.

Thinking of adding a pump to the tamk - biotreater lineup.
a. With the pump, is it possible to empty tank B while both tanks are line up to the biotreater ? Or will it pump more from tank A due to its higher level (thus head) ?
b. Current discharge pressure (tank head under gravity) to the biotreater is about 1 barg. What is the recommended discharge pressure of the new pump ? 0.5 bar higher ? My concern here is water splashing into the biotreater if the pressure is too high. How high is too high ?

Thanks.

 

[katmar]

Why is it a problem if the tank levels equalize?
If it is a problem, why do you open both discharge valves at the same time?

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[iainuts]

Flow from either tank is determined by the overall flow restriction and the head forcing flow through the pipes. For tank A, the restriction is due to flow from the tank outlet to a common line, plus the restriction in the common line due to the flow from both tanks. Similarly for tank B, the restriction is due to flow from the tank to the common line plus the common line restriction.

If the tanks levels are equalizing, then there must not be a lot of restriction between the tank and the common line. So if you increase the restriction in one line and not the other, the tanks won't equalize.

You must have some criteria and a method of measuring how fast each tank empties. I assume you have instrumentation set up so that criteria can be measured. Using that instrumentation, use it to control flow in one or both lines from the tank to the common line. Adding a control valve to one or both lines is fairly easy. You may even be able to convert an existing manual valve to a control valve by adding a positioner or simply swapping it out.

Note that putting a pump into one line or the other essentially does the same thing but with much added complexity and added cost.

 

[plumberet]

a. One tank is for normal use and the other one is for emergency - collect off spec water and then slowly mix the water with the other water into the biotreater. Problme arises when trying to run both tanks.

b. Thanks for the feedback on the FM side. Proposal is to line up both tanks to the new pump. Come to think of it, may be it is better and cheaper to have separate lines to the biotreater, one for each tank. No need for the pump, no maintenance, no electrical power, better for the environment.

Thanks !

 

[cvg]

put a check valve in the drain lines from each tank

 

[plumberet]

Was thinking of a check valve to prevent backflowing from higher level tank to the lower level tank. BUT then I will not be able to drain the lower level tank and at the same time draining from the higher level tank.

A separate line from each tank should do the job. But any smarter way with just one line instead of two ?

Thanks.

 

[iainuts]

Use one pipe from the tanks, but between each tank and this common line, use a control valve to allow flow out of the tank at whatever prescribed rate you determine.