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Anonymous Poster #1

Control of Three Pumps Using Ladder Logic

03/27/2017 8:10 AM

Hi Guys,

am currently doing a project consisting of the following

1. Three Pumps

2. Two Pressure Transmitter (PT1 and PT2) with dry contacts to control start and stop of pumps.

Control Philosophy shall be as follows:

Only one pump is required to run at normal conditions- Start and stop signal is given by PT1

In case demand is high, PT2 shall give signals to start 2 pumps at the same time

My issue is i am unable to find a correct way to do the pump alternation to ensure an even wear in all three motors.

I have tried to use clocks to break down 24hours into three equal parts

00 00 - 08 00 - Pump A and B operates on duty/standby

08 00- 16 00 - Pump A and C operates on duty/standby

16 00 - 00 00 - Pump B and C operates on duty/standby

Can you guys please suggest any other ways in achieving the above controls instead of using clocks.

Thanks and Best Regards

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#1

Re: Control of three pumps using ladder logic

03/27/2017 8:34 AM

Why do you want to have even wear on all three motors? If one fails, then the other two are both about to fail also. This even-wear philosophy isn't necessarily all that smart.

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#2
In reply to #1

Re: Control of three pumps using ladder logic

03/27/2017 9:04 AM

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

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#3

Re: Control of three pumps using ladder logic

03/27/2017 9:13 AM

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.

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Anonymous Poster #1
#25
In reply to #3

Re: Control of three pumps using ladder logic

03/30/2017 12:46 AM

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.

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#4

Re: Control of Three Pumps Using Ladder Logic

03/27/2017 9:18 AM

If (after reading Tornado's comment) you still wanted to "share the wear", you could do it with a mod-3 counter.

Count 0 : A duty, B standby

Count 1 : B duty, C standby

Count 2 : C duty, A standby.

The counter could be incremented by e.g. a one-hour clock.

Implementing a mod-3 counter in ladder logic is left as an exercise for the reader.

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#5

Re: Control of Three Pumps Using Ladder Logic

03/27/2017 9:21 AM

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

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#6

Re: Control of Three Pumps Using Ladder Logic

03/27/2017 9:40 AM

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?

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#8
In reply to #6

Re: Control of Three Pumps Using Ladder Logic

03/27/2017 9:49 AM

Third.

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

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#11
In reply to #8

Re: Control of Three Pumps Using Ladder Logic

03/27/2017 12:42 PM

That's the trouble with opinions. Best avoided.

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#12
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Re: Control of Three Pumps Using Ladder Logic

03/27/2017 1:16 PM

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

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#13
In reply to #12

Re: Control of Three Pumps Using Ladder Logic

03/27/2017 9:04 PM

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.

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#14
In reply to #13

Re: Control of Three Pumps Using Ladder Logic

03/27/2017 10:47 PM

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.

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#15
In reply to #14

Re: Control of Three Pumps Using Ladder Logic

03/27/2017 11:13 PM

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

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#17
In reply to #6

Re: Control of Three Pumps Using Ladder Logic

03/28/2017 12:19 AM

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.

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#23
In reply to #17

Re: Control of Three Pumps Using Ladder Logic

03/28/2017 8:08 PM

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

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#24
In reply to #23

Re: Control of Three Pumps Using Ladder Logic

03/29/2017 4:59 AM

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.

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#20
In reply to #6

Re: Control of Three Pumps Using Ladder Logic

03/28/2017 10:08 AM

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.

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#7

Re: Control of Three Pumps Using Ladder Logic

03/27/2017 9:42 AM

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.

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#9

Re: Control of Three Pumps Using Ladder Logic

03/27/2017 11:26 AM

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.

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#10

Re: Control of Three Pumps Using Ladder Logic

03/27/2017 12:11 PM

What you are describing is an industry standard called a "Triplex pumping system". It's not "rocket science", this has been implemented in relay logic for decades and there are now standardized triplexing alternator relays that do this. But if you want to implement it in PLC logic, it's easy once you break it down to this:

With all three pumps in operating condition, your triplex logic is "Lead-Lag-Standby". Each pump control circuit has logic for it to operate as the Lead pump, or the Lag pump, or the Standby. Then your pressure switches have a logic block that simply triggers a Lead, then a Lag if the Lead pump can't keep up, and whichever is not being used as Lead or Lag is left as Standby. That logic block uses a shift register to move the three selections in a stack and when both Lead and Lag turn off, the shift register shifts the three pump circuits used.

Then you have to deal with the failure of one pump, taking it out of the sequence. So you need failure sensing in each pump circuit that knows it's not functioning. If any one of those is triggered, you jump to a different shift register that creates a "duplex" pump control scheme using the two remaining pumps.

Then if two are failed, you have a third circuit that operates as a "simplex" pump scheme.

If you don't understand what I was referring to in logic programing, you should probably just go buy a triplex alternating relay...

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#16

Re: Control of Three Pumps Using Ladder Logic

03/27/2017 11:54 PM

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.

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#18

Re: Control of Three Pumps Using Ladder Logic

03/28/2017 12:43 AM

It depends your application. You can switch pumps below ways

  • Use time control as you already suggested
  • Use time control & pump operating time
  • Alternating pump on each start and stop

But this depend on the application. Please select best method as per application.

Eg: If pump usually does not run in 00 00 - 08 00, your purpose will not be achievable.

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#19

Re: Control of Three Pumps Using Ladder Logic

03/28/2017 5:55 AM

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.

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#21

Re: Control of Three Pumps Using Ladder Logic

03/28/2017 5:26 PM

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.

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#22

Re: Control of Three Pumps Using Ladder Logic

03/28/2017 6:02 PM

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

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