Control of Three Pumps Using Ladder Logic

I agree. Also, no three pumps will have an even wear considering the piping and curves into and out the pumps.

Assuming there is no connection between time of day and demand, your timer scheme wouldn't produce even wear - A never runs as standby and C never runs as the duty pump. A would have more running time than B or C, and C would have less than A or B.

It does produce an even wear as the system will be designed in such a way to produce pump alternation between the two pumps.

For example if Switching sequence 1 is active i.e Pump A and Pump B

1st start signal from PT1- Pump A runs (Pump B Standby) until stop signal is received from PT1 itself

2nd start signal from PT1- Pump B runs (Pump A Standby) until stop signal is received from PT1 itself

If PT2 is energized- Both pump A and Pump B shall run.

In case any one pump is faulty. switching sequence is switched to the remaining 2 pumps irrespective of the time of the day. For example- if pump B is faulty-Switching sequence PA and PC is active until fault on PB is reset.

In case two pumps fail- only remaining one pump shall run irrespective of time of the day- No switching sequence.

Because it's yours, then do as you will.

That thing you're planning can be strongly relevant to be termed as optimisation.

or may be do some linear programming

First. Why are there three pumps when only two will run at the same time?

Second. If one pump is running when the demand (most likely) has increased. How/why can/do you have/insist both motors start at the same time?

Third.

Who marked me OT? And why? These questions are 100℅ related to the OP question?

That's the trouble with opinions. Best avoided.

I didn't infer that my questions were 100% valid, just 100% related.

Because of a defect in the rating system, the only way to down-vote an unsuitable or wrong answer is to call it off-topic.

If you're saying the OP's questions were well defined enough for my questions (not answers) to be unsuitable or wrong? So be it.

While pumps are not my area of expertise. I do have relevant knowledge of control circuits. I know I can set up a system with a number of motors using all manner of ladder diagrams controls and sensors. I've even been trained and paid to do so. So where's the harm? I was trying to get some clarification.

As for the 'defect' in the rating system? It's says 'off topic' not 'wrong'. the difference is obvious and intentional.

..don't forget to mark this OT.. just in case it's not obvious.

That's exactly the defect: allowing only OT rather than wrong.

1. The third pump can be standby in case either of the others fails or is out for service.

2. There is no good reason to start the first two pumps at the same time, though there could be reason to run two at the same time.

1. completely understandable. I was expecting the OP to respond with the same.

2. That's what I was thinking. I just didn't get how or why two would start at the same time if one was running.

perhaps an error or two

Re. 2: I think the OP was just using messy language (or thinking).

"... PT2 shall give signals to start 2 pumps at the same time ..." should've been

"... PT2 shall give signals to start a second pump to run at the same time as the first pump (which is already running)...", or words to that effect.

Pumps always have a back up. We've built dozens of 2 pump skids, where I work, where only one runs at a time.

Directing to this current post, we have never been asked to try to create even wear on our 2 pump skids. I find that concern to be a bit odd.

In your timing scheme you don't seem to consider whether PT1 or PT2 is activated. Because from your original statement it looks like you want to only operate one pump if PT1 is giving the signal.

If you want to schedule the pump operation based on time, using timers is the best option.

It is customary to advance the run hours across a 3-pump installation so that all three do not become due for maintenance at the same time.

You wanted to run all the pumps equally in number of hours.

Log hours of operation of each pump into PLC.

For every start sequence of start operation, make the lowest run pump to start.

By this method more or less at given point of time all the three pumps would have run equally.

This logic can be achieved with PLC.

If you require can develop Ladder for this.

Have done similarly.

What you describe is typical control for larger sewer pumpstations.

Terminology may change, but "duty, standby, spare" or "lead, lag, standby" would be typical of the descriptions.

The logic is in the ladder program and not the sensors.

Usually they are run on "round robbin" cycle so that each pump takes turns at being duty to balance the runtime and more importantly the number of starts and also the delay between successive starts.

Three pumps would typify a larger or more critical pumpstation with logic to sense pump failure and energise the spare. The configuration also enables one pump to be removed from service and the station still able to run in two pump configuration, but at higher risk if there is failure.

The only accurate way to ensure even wear is to attach an accumulating timer to each pump. When the duty pump timer reaches a preset value advance the duty schedule next time all the pumps are off.

Think about how you would control the pumps using just the leading and trailing edges of PT1. Start the duty pump when you see a leading edge. If you don't see a trailing edge within a preset time the duty pump is failing to cope so bring in the standby pump. (Not within the brief but if you still don't see a trailing edge bring in the second pump. This caters for a blocked or tripped pump) When you see a trailing edge drop out the last pump that was started. If you don't see a leading edge after a preset time drop out the next pump. We used to do this with banks of up to seven compressors, as the normal way of bringing in compressors at progressively lower system pressures (as your PT1 and PT2) leaves you seriously under working system pressure by the time you have cascaded seven pressure switches each with 3-5psi. differentials. This would leave PT2 to alarm fault conditions.

Are you using VFD. If not then use runtime logic instead of clock. I would set 10-8-6 so that i can have standby motor if one is down later.

If using VFD you can program it in VFD. Vfd will also consider affinity law and can have reliability and energy saving.

Best of luck