Water Tank Location and head

[154500]

A finished water storage tank is to be constructed for an existing system. Water is to flow from an existing 600,000 gallon reservoir to this finished water storage tank by gravity. The location of the tank should be on such a place where it can be filled by gravity and will have enough head towards supply. Another rstriction is that the tank has to be placed on an abandoned reservoir site (after draining and filling) in order to avoid clearing a whole new area in the middli of a state forest. Existing reservoir is 10'deep & proposed tank is 42' hig. The elev. difference between reservoir and tank is 11.5'. The two structures are to be coonected by a mile long, 30 year old CIP (C=90), and the tank level is to be controlled by one way altitude valve. We shall have a minimum of 230gpm flowing in to the tank. After allowing for minor losses and calculating major loss by Haze-Williams, the remaining head will be < 1'.I didn't fill comfortable with the limited head that we have. Does anyone has a similar encounter and can share experience?

 

[BobPE]

154500:

Solving for the head is one way to look at it. A better way is to solve for the flow. I assume 230 gpm is your design flow, but using the information you provided, this flow may vary. What are the acceptable ranges for this flow. As time goes on and that CIP degrades, the flow will decrease. You can project the degradation of the CIP and show the client what the future flows will be in the system. By looking at the project in this way, its a lot easier to say your design will fall into the specified flow range rather than nailing down a head that the design will maintain. After all, the tank will fill as long as its below the gradient, but the rate it fills may be a lot smaller than anyone thought.

I am assuming you are designing a two pipe tank with an altitude valve on the fill line and the distribution system floating on the effluent line, again, from the info in your post. Elaborate a bit more on that if you could.

BobPE

 

[154500]

Thank you for the quick and helpful response. My concern was that of the real flow being less than the calculated and weather or not a c value of 90 was fair.
The 436000 gallon proposed tank is to be fed from the reservoir by means of 8&quot; CIP. There will be a 6&quot; one way altitude valve just before the tank. There will be a 10&quot; pipe drawing water out to supply. The tank is to be filled up to 40' of its height leaving 2' of freeboard. When the tank reaches 40', altitude valve shuts off. When water in the tank drops down to 4' of the full tank level, altitude valve opens. In case the altitude valve malfunctions, a high & low water alarms are set at 41' and 27' respectively. The existing average and maximum water demand for the system are 110gpm & 165gpm respectively. It was inorder to allow for further deterioration of the pipe and the uncertainity of the c value I used that a minimum of 230gpm was used to calculate friction loss when the reservoir is almost empty and the the new tank is full. (A senario which almost never occurs considering the way the reservoir operation is set up). Actually there will be 10' to 4' of water in the reservoir which makes the elevetion difference 21' to 15'. I have checked flows for heads of 21',15' and 11' and it ranges from 215gpm to 340gpm. 230gpm is the demand the system mightg reach in approximately 17 years.
The main concern of fill up rate will be during fire flow situations which for this system is taken as 1250gpm for 2hrs. I think Even then the tank may have enough water to sustain the system as long as the amount of water from the reservoir at least equals the daily peak demand. Don't you think?

Thank You