The short answer
The Tank Level Trend Tells the Whole Story
A storage tank level trend over several days shows the diurnal demand as the daily sawtooth, the source capacity as the slope of the fills, a leak or a main break as a fall that does not match the time of day, pump wear as fills that take longer each month, water age as how much of the tank actually cycles, and the control bands as the turning points. It is the single most informative trend in a water system, and it should be visible on the main screen every day.
Key points
- The daily sawtooth is demand; its depth is how much of the tank turns over, which is water age.
- The fill slope is source capacity minus demand; a slope that flattens over months is a pump or a well losing capacity.
- A fall at the wrong time of day, or a floor that keeps dropping, is a leak, a break, or an open hydrant.
- The turning points are the control bands; a tank that never reaches the top has a source problem, one that never reaches the bottom is not turning over.
- Put the seven-day tank trend on the main screen, with fixed scales and the bands drawn on it.
Ask an experienced water operator how the system is doing and they will look at one thing before they answer: the tank. Not the number, the trend. A week of storage tank level on one chart is a record of everything the system did, and reading it is a skill that takes an hour to learn and a career to refine. It is also the best argument for putting a trend, rather than a number, in the center of the main screen.
The daily sawtooth
Over a normal day the tank falls through the morning as people wake, shower, and irrigate, flattens or rises in the middle of the day, falls again in the evening, and fills overnight when demand is low and the sources catch up. That sawtooth is the demand curve of the community drawn by the tank. Its depth, from the overnight peak to the evening trough, is the fraction of the tank that actually turns over each day, and that fraction is the water age. A tank that swings a fifth of its depth turns over in about five days; one that swings a twentieth is holding water for weeks, losing its disinfectant residual and warming in the sun. The control bands decide the swing, and the sawtooth is the evidence of whether they are set for turnover or for a comfortably full tank.
The fill slope
When the sources are running and the tank is rising, the slope of the rise is the source capacity minus the demand at that hour. Overnight, with demand low, the slope is close to the source capacity itself. Watch that overnight slope month to month. A well pump that is wearing, a screen that is plugging, a booster whose impeller is eroding, or a source whose static level is falling all show up as a fill that takes a little longer than it did last season, long before any single night looks wrong. The comparison is the diagnosis, and a historian that keeps a year of tank level makes the comparison a matter of overlaying two weeks.
| Shape | Reading | Look at |
|---|---|---|
| Regular sawtooth, tank reaches the top every night | Normal; sources adequate | Turnover depth for water age |
| Sawtooth deepening in summer, tank still reaches the top | Seasonal demand within capacity | Nothing; note the margin |
| Tank no longer reaches the top on hot days | Sources at their limit | Source capacity, the schedule, a pump losing flow |
| Fill slope flatter than last year at the same demand | A source losing capacity | Pump curve, well level, screens, valves |
| Fall at two in the morning | Leak, main break, open hydrant, or a large night user | Night flow, pressure zones, the streets |
| Floor lower each day with normal fills | A leak growing, or a demand the sources cannot meet | Night demand trend |
| Flat line | Transmitter frozen, or nothing flowing either way | Instrument, then the valves |
| Sawtooth with a jagged edge | Pumps cycling too often, or an altitude valve hunting | Control bands, starts per hour, the valve |
| Level rising faster than any source could fill | Transmitter fault | Instrument |
Leaks and breaks
The most valuable thing the tank trend does is find water that is leaving the system when nobody is using it. A fall in the small hours, when demand should be at its minimum, is water going somewhere: a main break, a hydrant left open, a service line failure, a large customer running at night, or an interconnection valve that is not closed. A floor that drops a little further each night while the fills look normal is a leak growing. Both are visible days before the complaint call, and both are the reason the trend deserves a fixed scale and an operator who looks at it with coffee every morning.
Pump health
A fill that takes longer is the early sign; the trend also shows the pumps in the shape of the fill. A fill that starts fast and flattens is a well pulling down its water level. A fill that stops short with the pump still running is a pump that has lost its head, a valve that is not open, or a break between the pump and the tank. A fill that starts late every night is a level band that is too low or a pump that takes several tries to start. Each of those, matched against the pump run status drawn on the same chart, is a diagnosis without a site visit.
The control bands
The turning points on the trend are the control settings made visible. The level at which the sources start and stop, the pressure a booster holds, the schedule that fills overnight: they draw the top and bottom of the sawtooth. A tank that never reaches the off level is a system where the sources cannot keep up or the schedule is too short; a tank that never falls to the lead-on level is one that is not turning over. Drawing the bands on the trend as horizontal lines makes the comparison instant, and an operator who sees the level bumping against a band knows which setting to look at.
What the number cannot tell you
A tank level of 24.6 feet says the tank is fairly full. It does not say whether it was 26 feet at this time yesterday, whether it took an hour longer to fill last night, whether it fell at three in the morning, or whether it has been within a foot of full for a month and the water is old. The trend says all of that at a glance. A system that shows operators numbers and hides trends behind a menu is a system that finds its leaks by phone call.
Frequently asked questions
- What if the system has several tanks?
- A trend per tank, and one chart with all of them on the same scale for the overview. Tanks in the same pressure zone should move together; one that does not has a valve or a pump problem between it and the others.
- How deep should the daily swing be?
- Deep enough to turn the tank over in a few days while keeping the fire reserve and the outage reserve. The residual at the tank outlet is the check; if it is falling through the week, the swing is too shallow.
- The trend is smooth but the water age is still high.
- A smooth sawtooth that only uses the top few feet is the sign: the fill starts before much has left. Lower the lead-on level, deepen the swing, and consider a mixer if the tank stratifies.
- Can the controller read the trend for me?
- Partly. A rate-of-change alarm catches a fast fall and a transmitter fault, a night flow calculation catches a growing leak, and a fill-time calculation catches a source losing capacity. Those alarms are worth building; they still do not replace an operator who looks at the trend.
Related topics
- Ground Storage TanksControls for ground storage tanks: level measurement and its datum, filling from wells and treatment by pump control or altitude valve, and the telemetry that ties the tank to the system.
- Tank Level ControlHow a storage tank is filled and drawn on level setpoints, how the setpoints are chosen to keep the water turning over, and what the station does when the tank stops talking.
- Pump Sequencing for StorageSequencing the pumps that fill and draw from storage: staging on level bands, rotating wells and sources for even use and water quality, time-of-day and demand-based scheduling, and the records that show the sequence is doing its job.
- How to Trend DataBuild trends that operators use: choose the tags that tell one story per trend, set scales and time spans that show the behavior, and make ad hoc trending fast enough that an operator reaches for it before a phone call.
- Trends on the HMITrend displays that answer questions: embedded sparklines on process displays, pre-configured trend groups, the axes and time spans that make a trend readable, and the difference between a trend for operating and a trend for investigating.
- Well Pump ControlControlling a groundwater well: submersible and line-shaft turbine pumps, the pump-to-waste start, drawdown and the low-water cutoff, cycling limits from the well and the aquifer, and what the controller monitors to protect a pump that cannot be seen.
Direct contact
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