Openai/6944401e-07c8-800d-9890-76b35b1d919d

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gpt-5-2

User: Do duplex septic pumps require both pumps to be operational?

Do duplex septic pumps require both pumps to be operational?

Assistant: Short answer: no—both pumps do not have to be operational for a duplex septic system to function, but it depends on how…

Short answer: no—both pumps do not have to be operational for a duplex septic system to function, but it depends on how the system is intended to operate.

Here’s how duplex septic pump systems are typically set up:

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• Only one pump runs at a time
• Pumps alternate each cycle to equalize wear
• If one pump fails, the other can usually: - Keep the system operating on its own - Trigger an alarm indicating a pump failure

Result: ✔ System still works with one pump operational ⚠ Reduced redundancy until repaired

- One pump is the primary

• The second pump: - Starts only if liquid level rises unusually high - Or if the first pump can’t keep up

Result: ✔ Normal conditions: one pump is sufficient ✔ Peak flow: both pumps may be required

- Both pumps are designed to run together every cycle

• Used in: - Commercial systems - Large or high-head applications

Result: ❌ One pump alone may not meet design flow ⚠ System may alarm or fail to dose properly

- Alternating systems: usually no immediate failure, just loss of redundancy

• Lead/lag systems: alarms during heavy usage
• Simultaneous systems: potential overflow or inadequate dosing

Most systems will:

• Activate a high-water alarm
• Continue operating (if possible) until serviced

Look for:

• Control panel labels (alternating vs lead/lag)
• Float arrangement (separate lead/lag floats vs shared)
• Pump specs (flow/head requirements)
• Local code or engineer’s design documents

If you want, you can describe:

• The number of floats
• Whether the pumps run together or alternate
• Residential vs commercial system

and I can help you identify which configuration you have and whether one pump alone is acceptable.

User: there is an issue with this system overloading a generator on startup, we are trying to find a startup procedure that w…

there is an issue with this system overloading a generator on startup, we are trying to find a startup procedure that will let it run

Assistant: Got it — this helps a lot.

Got it — this helps a lot. Based on the panel photo and what you’re describing, this looks very much like a duplex pump system that can start both pumps together, which is exactly the kind of thing that will slam a generator on startup.

I’ll walk through what’s likely happening, then give you a practical startup procedure that usually works in the field.

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Key observations:

• Two motor contactors labeled M1 and M2 - These are full-voltage across-the-line starters (no soft start, no VFD)
• Two pump breakers labeled PUMP 1 (20A) and PUMP 2 (20A)
• Siemens LOGO! PLC - Almost certainly doing alternation, interlocks, and alarms
• Hand/Off/Auto selector switches for multiple subsystems
• No visible: - Soft starter - VFD - Reduced-voltage starter

Each pump likely pulls:

• 5–7× FLA on startup
• If both contactors close together (even briefly), the generator sees a huge inrush spike

On utility power you’d never notice. On a generator → instant overload / stall / breaker trip.

One (or more) of these is happening:

This can happen if:

• PLC boots and initializes outputs simultaneously
• Lead/lag logic doesn’t delay the lag pump
• High-level float is already made at startup

- Generator struggles

• PLC browns out momentarily
• Outputs re-assert → both contactors pull in again

Even if:

• Running load is fine
• Starting load is not

This is the part you asked for directly.

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• Put both Pump selectors to OFF (or MAN → OFF if that’s how they’re wired)
• Leave generator running unloaded for ~30–60 seconds

- Select Pump 1 → MAN

• Verify: - Generator does not bog - Voltage and frequency stabilize
• Let it run 30–60 seconds

Choose ONE:

• Leave Pump 2 selector in OFF
• Or trip Pump 2 breaker (preferred if selector logic is unclear)

This guarantees no simultaneous start.

- Generator is already under load

• PLC sees an already-running system, not a cold start

- Put selector back to AUTO

• Reset breaker if opened

At this point:

• Alternation will resume
• Generator will not see a double inrush

Then one of these is true:

Rule of thumb:

• Generator kVA should be ≥ 3–4× largest motor HP
• Duplex systems often need 5–6× if simultaneous start is possible

Common fix:

• Add time-delay relay on M2
• Or program PLC to: - Enforce 10–30s delay between pump starts

If the wet well is already high:

• PLC may intentionally start both pumps
• That’s fatal on a small generator

From simplest to best:

1. Disable lag pump on generator power - Manual procedure or selector switch
2. Add a start-delay relay to Pump 2
3. Add a soft starter to at least one pump
4. Reprogram PLC to enforce staggered starts
5. Larger generator

- No, both pumps do not need to run for the system to function

• Yes, this panel is capable of starting both, which explains the generator overload
• Manual single-pump startup is the correct workaround
• This is a starting kVA problem, not a running load problem

If you want, tell me:

• Generator kW/kVA rating
• Pump HP (or model numbers)
• Whether this only happens after total power loss

I can sanity-check whether the generator is even theoretically capable of this setup.

User: the user suspects that the pumps are for the sand and dosing pumps

the user suspects that the pumps are for the sand and dosing pumps

Assistant: That suspicion actually makes a lot of sense — and it changes the interpretation of the panel in an important way.

That suspicion actually makes a lot of sense — and it changes the interpretation of the panel in an important way.

Short answer

Those two pumps are very likely sand filter and dosing pumps, not a duplex “same-job” pump pair. If that’s the case, they are not redundant, and the control logic may legitimately start both at the same time, which explains the generator overload.

Evidence from the panel that supports this

Looking closely at the photo:

1. The right-hand selector switches are labeled by process, not pump number

You have switches labeled:

• SAND FILTER
• DOSING TANK
• SPIN FILTER
• SYSTEM FLUSH

That’s already a strong hint this is a treatment train, not a lift station with two identical pumps.

2. The contactors are labeled generically (M1 / M2), not “Lead / Lag”

In true duplex lift stations you usually see:

• “Pump A / Pump B”
• “Lead / Lag”
• Or alternating relays

Here, M1 and M2 are just motors — the PLC decides when each runs.

3. The PLC (Siemens LOGO!) strongly implies sequenced process control

A LOGO! PLC is commonly used in:

• Sand filter backwash systems
• Timed dosing
• Flush cycles

Not just simple alternation.

4. Handwritten conduit labels

At the bottom you have handwritten labels such as:

• Pump (dose)
• Pump discharge
• Controls

That lines up with different functional pumps, not two doing the same job.

What that means operationally

If one pump is:

• Sand filter feed / backwash and the other is:
• Dosing pump to the drainfield

Then:

• ❌ You cannot just disable one indefinitely
• ✅ The system expects both to run during certain cycles
• ⚠️ Startup may intentionally energize both motors

On utility power → fine On generator → inrush disaster

Why the generator is getting hit so hard

Likely startup sequence on power restore:

1. PLC boots
2. Detects system not at setpoint
3. Commands: - Sand filter pump ON - Dosing pump ON (or flush cycle)
4. Both contactors pull in within milliseconds
5. Generator sees 10–14× FLA equivalent
6. Voltage collapses or breaker trips

This is normal behavior for a treatment system — just not generator-friendly.

Safer generator startup procedure (updated, given this new understanding)

Correct field workaround

1. Before starting the generator

• Put SAND FILTER → OFF
• Put DOSING TANK → OFF
• Leave others OFF as well

2. Start generator and let it stabilize

• 1–2 minutes, no load

3. Bring loads online one at a time

1. SAND FILTER → MAN - Let motor fully start and stabilize
2. DOSING TANK → MAN
3. Return both to AUTO

This prevents coincident inrush.

If you want a permanent solution

In order of effectiveness:

Option 1: PLC interlock (best)

Program logic such that:

• Only one motor may start at a time
• Enforce 10–30 second delay between starts
• Especially on power-up

Option 2: Time-delay relay on one contactor

• Cheap
• Very effective
• Common retrofit for generator compatibility

Option 3: Soft starter on the larger motor

• Reduces inrush by 50–70%
• Often enough to make small generators happy

Option 4: Generator load-shed contact

• When on generator, disable non-critical pump cycles

Key takeaway

This is not a duplex redundancy problem — it’s a process sequencing problem.

The system is doing what it was designed to do, but:

• Utility power masked the issue
• Generator power exposes it

If you can provide:

• Pump HP or nameplates
• Generator kW/kVA
• Whether this is septic → sand filter → drainfield

I can tell you exactly which pump should get the delay or soft start.