The short answer
Lift Station High Level
High level at a lift station is the wet well rising above the point the pumps should have held. It is detected by an independent float placed below the lowest incoming sewer invert, which raises a local and remote alarm and, at most stations, starts the pumps through a hardwired path that does not depend on the transmitter or the controller. The volume between the float and the overflow elevation, divided by inflow, is the time an operator has to respond.
Key points
- The high-level float is independent of the level transmitter and the PLC. That independence is the entire point.
- Place the float below the lowest incoming invert. A surcharged well backs up into the collection system.
- High level should start the pumps on its own path, not only raise an alarm.
- Compute the response time from storage above the float and peak inflow, and set notification to match.
- A high level with pumps running is a capacity or pump problem. Without pumps running it is a control problem.
What high level means
Every lift station has a level the pumps are supposed to hold the well below. When the well rises above it, something in the chain of measure, decide, and pump has failed, or inflow has exceeded what the station can move. High level is the alarm for that state. It is the most important single alarm at the station, because the next state after high level is a sanitary sewer overflow, which is a public health event, an environmental violation, and a reportable incident in most jurisdictions.
That importance is why high level gets its own sensor, its own wiring, and its own path to the pumps. Everything else at the station can share the transmitter and the controller. High level must not.
The high-level float
A high-level float is a mechanical switch in a sealed body hung on its own cable at the elevation of the alarm. It is chosen over a transmitter setpoint for one reason: it fails differently. A transmitter can read low because of fouling, a bad zero, a failed analog card, or a controller that has stopped scanning, and each of those failures looks like a normal, calm well. The float does not know what the transmitter says. When wastewater lifts it, its contact closes.
- Elevation
- Below the invert of the lowest sewer entering the well, with enough margin that a wave in a turbulent well does not trip it. Above the lag-on setpoint by enough that the float is not touched in normal wet weather operation. If those two constraints cannot both be satisfied, the station has a design problem worth documenting.
- Wiring
- Its own circuit, on its own terminals, to a relay that drives the alarm and the pump backup path. Also into a PLC input so the controller knows the float has operated. The PLC input is a copy, never the only path.
- Contact type
- Normally open, closing on rise, is conventional for the pump call. For the alarm circuit, a normally closed contact that opens on rise gives a fail-safe indication if the cable is cut, at the cost of a nuisance alarm when it is. Many panels use a single normally open float and accept the trade-off; either is acceptable if it is documented and tested.
- Mounting
- Tethered from a bracket that can be reached from the hatch without entry, on a cable long enough to swing freely and short enough not to wrap the pump cables or the guide rails. A float that fouls in rags or hangs on the pump cable is the most common reason a high-level alarm fails to sound.
- Testing
- Lift it by hand from the hatch, monthly or on the utility schedule, and confirm the alarm reaches SCADA and the pumps are called. Log the test. A float that has not been tested in a year should be assumed not to work.
What high level must do
Alarm is the minimum. The better answer is alarm and act.
- 01Sound the local alarm: a beacon and, where neighbors allow, a horn. A passerby or an operator driving past is a legitimate detection layer at a remote station.
- 02Call SCADA with a high priority alarm that pages someone. The alarm should be its own point, not derived from the transmitter level, so it survives a transmitter failure.
- 03Call the pumps. Through the backup relay logic, the float closes the pump call path for both pumps, subject only to the low-level cutoff float and the HOA switches in AUTO. If the controller is dead, the pumps run anyway.
- 04Notify by an independent path where one exists. An autodialer or a cellular alarm unit on its own power gives a second route when the SCADA radio or the site power is the thing that failed.
Do not delay the high-level alarm more than a second or two. A short debounce prevents a wave from tripping it; a long delay eats the response time the float was placed to provide.
Response time
The float is an alarm, and an alarm is only useful if someone can act before the consequence. The time available is the storage above the float divided by the inflow rate.
t = V / Q_in
- t = time from high-level alarm to overflow, in minutes
- V = wet well volume between the float elevation and the overflow elevation, in gallons, plus any surcharge storage in the incoming sewer the utility is willing to count
- Q_in = inflow rate, in gallons per minute, using the wet weather peak for the design case
A six-foot well with three feet between the float and the overflow holds about 630 gallons in that band. At a dry weather inflow of 50 gpm that is nearly thirteen minutes. At a wet weather peak of 400 gpm it is a minute and a half. The wet weather number is the one to design notification and response around. If the utility cannot get a crew to the station in that time, the answer is more storage, more pumping capacity, a permanent standby pump, or a generator with automatic transfer, and the high-level alarm history is what tells the utility which stations need it.
Diagnosing a high level
Start with one question: were the pumps running?
| Condition | Likely causes | What to look at |
|---|---|---|
| Pumps running, level still rising | Inflow above station capacity from a storm or an upstream station; a rag-bound or worn pump moving little; a partially closed discharge valve; a stuck check valve; high force main pressure from a downstream problem | Motor current per pump against its normal value; drawdown rate on the trend; discharge pressure; valve positions; the other stations on the same force main |
| Pumps not running, level rising | Transmitter reading low; controller stopped or faulted; pumps not in AUTO; phase monitor or overload lockout; control power lost; starter or drive faulted | The transmitter reading against a tape measure; controller status; HOA positions; fault indications on the starters or drives; control circuit voltage |
| One pump running, the other will not start | Lag call not made; lag pump failed to prove; alternation or availability logic holding it out | Lag call bit and setpoint; run confirmation and fault history for the idle pump |
| High level with the well visibly normal | Float hung up, fouled, or wired wrong; float tripped by turbulence; alarm point mapped to the wrong input | Lift and lower the float from the hatch; watch the input in the controller; check the cable for wrap |
Inflow and infiltration
A station that reaches high level only in rain is telling the utility that the collection system is leaking in. Stormwater enters through cracked pipes, leaky manholes, and illegal roof and yard drain connections, and the station sees the whole of it. Trending level and pump run time against rainfall makes the pattern obvious. The fix is in the pipes, not the panel, but the station is where the evidence is collected, and a rate-of-rise lag call and an early high-level page are what get the utility through the storm in the meantime.
Frequently asked questions
- Can the high-level alarm come from the level transmitter?
- A high-level setpoint in the controller is useful and should exist, but it is not the high-level alarm. It shares every failure mode of the transmitter and the controller. The float is the alarm. If the transmitter and the float disagree, that disagreement is its own alarm and usually means the transmitter is wrong.
- Should high level start the pumps or only alarm?
- Start the pumps. The most common reason a station reaches high level is that the pumps were not called when they should have been. A float that only alarms leaves the pumps idle while someone drives to the site. The exception is a station where starting the pumps on a float could be unsafe, such as one with a known discharge problem, and that is a temporary condition, not a design.
- How high can I let the well go before it counts as an overflow?
- The overflow elevation is physical: the lowest point wastewater can leave the system, which may be a manhole lid upstream rather than the station itself. Surcharging the incoming sewer is not an overflow, but it backs up into laterals and basements, and in many utilities it is reportable on its own. Set the float so the well never surcharges the incoming sewer in normal failure cases.
- How often should the high-level float be tested?
- Monthly is common, and at least at every station visit. The test is a lift by hand from the hatch while someone confirms the alarm at SCADA and the pump call. Record it. The float is the one device in the station whose failure is invisible until the day it is needed.
Related topics
- Lift Station Backup ControlHow a lift station keeps pumping when the transmitter, the PLC, or the SCADA link is gone: float backup logic, the control transfer relay, redundant-off protection, and how to test it with the controller actually dead.
- Float SwitchesThe oldest level instrument and still the one that saves a station: how tilt and stem floats work, the wiring conventions, mounting and spacing in a wet well, classified-area ratings, and how to use them alongside a continuous instrument.
- Duplex Lift Station ControlsThe two-pump station that makes up most of a collection system: what is in the panel, how the sequence works, what is monitored, and where these stations actually fail.
- Lift Station Lead/Lag ControlHow a duplex or triplex station decides when one pump is enough and when it is not: the setpoint ladder, cycle volume, the lag call, parallel pumping, and what to do when the lead pump fails.
- The Alarm PhilosophyThe document that decides what is allowed to be an alarm: the definition, the criteria, the priorities and their meaning, the performance targets, the handling rules, and who owns it. Under ISA-18.2 it comes first.
- Triplex Lift Station ControlsWhat changes when a station has three pumps: the lead, lag, and standby roles, rotation with three positions, the diminishing return of the third pump on the system curve, availability logic, and the power and panel arrangements that three pumps require.
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