Tank Level Control

What the tank is for

An elevated tank or a ground storage tank with a booster does three things. It stores water so that the source and the treatment can run steadily while demand swings. It holds pressure on the system by its elevation, so that the pumps do not have to. And it holds a reserve for fire flow and for outages. The control system has to keep it in the band that serves all three without letting the water sit long enough to go stale.

Fill and draw

The control is a level comparison. When the tank level falls to the call-on setpoint the supply pumps start, or the well or the plant is asked for water. When it rises to the call-off setpoint they stop. Between the two the tank drains on demand. With more than one pump the same lead and lag structure used in a lift station applies, with the second pump called on at a lower level or after the first has run for a time without gaining.

Call-off, the top of the band

Set below the overflow with a margin for the pump to stop and the check valve to close. Near the top of the tank in winter for reserve, lower in summer for turnover.

Call-on, the bottom of the band

Set above the level that keeps fire reserve and keeps the pressure on the system acceptable. The gap to the call-off setpoint is the turnover each cycle.

Lag call-on

A level below the lead call-on at which a second pump joins, for days when one pump cannot keep up with demand.

Low level alarm and high level alarm

Outside the control band, alarmed regardless of what the control is doing.

Turnover and water quality

A tank that is kept full is a tank that is not used. If the band is a foot at the top of a forty foot tank, most of the water in it has been there for days or weeks, and chlorine residual decays with time and temperature. In summer, in a tank in the sun, the residual at the bottom can be gone. The remedy is to use the tank: set the band deep enough that a meaningful fraction of the volume is exchanged every cycle, and let the level cycle through it daily.

This is a deliberate trade against reserve. A deeper band means less water in the tank at the bottom of the cycle, and the low point has to stay above the fire reserve and the pressure floor. Many utilities run a summer band and a winter band and switch between them on a schedule or on water temperature, and some run a periodic deep drawdown to turn the tank over completely. The setpoints are an operational decision the control system should make easy to change from the HMI and log when changed.

Measuring the level

A pressure transducer at the base of the tank, on the inlet piping or in a tap on the shell, reads the hydrostatic head and is the most common tank level instrument. It is accessible, cheap, and unaffected by what is happening at the surface. Its reference is the transducer elevation, and it reads the static head only when there is no flow through the tap it is on, which is why it goes on a dead leg. A radar unit on the roof looking down at the surface is the alternative, immune to piping effects and accessible without draining anything, at more cost and more exposure to weather and ice.

Either way the overflow alarm should be a float or a conductivity probe at the overflow level, wired to the telemetry unit directly. A transducer that has drifted or a radar that has iced over will not report an overflow, and an overflow at an elevated tank is a lot of water on the ground and a headline.

Telemetry and what happens without it

The tank and the pumps are rarely in the same place. The level is measured at the tank, sent by radio, cellular, or a leased circuit to the pump station or the SCADA host, and used there. The signal is late by the polling interval, and it goes missing when the radio path fades, the modem drops, or the tank site loses power. The control has to be designed for that.

1. 1

   Timestamp and validate the value.

   Treat a value that has not updated within a few polling intervals as stale, and a value outside the physical range of the tank as bad. Do not control on a frozen number.

2. 2

   Define the fallback.

   On a stale or bad level the station runs a defined program: pump on a schedule sized to typical demand, pump on local discharge pressure, or hold the last state for a limited time and then alarm. The control narrative says which, and the choice depends on whether an overflow or a dry tank is the worse outcome at that site.

3. 3

   Alarm the communication failure separately.

   The operator needs to know the tank is not reporting before the tank is empty or overflowing, which is why the communication alarm is its own alarm with its own delay.

4. 4

   Recover cleanly.

   When the signal returns, resume level control from the live value rather than acting on the whole history of missed updates at once.

Altitude valves and hydraulics

Where the tank floats on the system without a dedicated fill pump, an altitude valve on the tank inlet closes hydraulically when the tank is full and opens as it draws down, and the tank level is set by the system pressure rather than by a controller. The control system then monitors rather than controls, and the setpoints above become the alarm limits and the valve adjustment. Some systems combine the two, with a valve for overflow protection and a pump station controlled on level.