GSizing an Overhead Service Reservoir: Capacity and Staging Height
An overhead service reservoir does two jobs — it balances the mismatch between steady inflow and peaky demand, and it provides the head that drives the distribution. Both fix its size.
Capacity from balancing storage
The reservoir’s capacity is driven mainly by balancing storage: the pumps or the gravity main fill it at a fairly steady rate over the pumping hours, while the village draws from it in sharp morning and evening peaks. The storage must hold the difference so that supply continues through the peak even though inflow is steady. For many rural schemes this balancing requirement works out to roughly a third to a half of the maximum daily demand, but it should be derived from the actual supply and demand pattern rather than assumed.
To that balancing volume a designer may add an allowance for breakdown or emergency reserve where the manual requires it. The sum, rounded to a practical tank size, is the design capacity.
Staging height from residual head
The staging — the height of the tank above ground — is fixed not by the tank volume but by the pressure the distribution network needs. Working back from the minimum residual head required at the most remote and highest node, and adding the friction loss from the reservoir to that node, gives the water level the reservoir must maintain, and hence the staging height.
This is why staging height is a distribution decision as much as a structural one: too low a staging starves the far end no matter how large the tank, while needlessly high staging adds structural cost for pressure nobody uses. Fixing the staging from the critical node keeps both in check.
What mainly decides OHSR capacity?
Balancing storage — the volume needed to bridge steady inflow against peaky demand — plus any emergency reserve the governing manual requires. It is derived from the actual supply and demand pattern, not assumed.
How is the staging height decided?
By working back from the minimum residual head needed at the most remote, highest node and adding the friction loss to that node. Staging is a distribution (pressure) decision, not a structural one.