Sequential or random switching

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Sequential or random switching

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Paul2011

I have a rainwater harvesting system with three tanks each with a separate pump. I will have a pressure vessel and pressure switch to automatically start pumping when water is required. However, I do not want to run all three pumps together but one at a time. I need a switch which will ensure all three pumps are used in roughly equal proportions. It seems to me this could be sequential, switching to each pump in turn or completely random i.e. starting a different pump randomly each time.

Does such a switch exist and if so what is its technical name? Any suggestions - other than changing configuration of tanks and pumps which cannot be done!

thanks
 
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Why have the tanks got 3 pumps? Surely the tanks are linked so they self-level. I'd just use one pump all the time and keep the other two as redundancy in the case of failure.
 
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Marvo said:
Why have the tanks got 3 pumps? Surely the tanks are linked so they self-level. I'd just use one pump all the time and keep the other two as redundancy in the case of failure.
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Sorry. Without going into detail the tank/pump configuration has to stay the same so I need an electrical switching solution
 
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Marvo said:
I still don't understand why there's 3 different pumps.
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Okay. The tanks have to stay separate and unlinked. One surface mounted pump could be used but that would mean that I have to be sure of the one pump drawing equal amounts of water from the three tanks. I have considered this as an option but on balance I think the three submersible pumps with an electrical switching system is the better solution.
 
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Paul2011 said:
Okay. The tanks have to stay separate and unlinked. One surface mounted pump could be used but that would mean that I have to be sure of the one pump drawing equal amounts of water from the three tanks. I have considered this as an option but on balance I think the three submersible pumps with an electrical switching system is the better solution.
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What Marvo is asking, ...is ''WHY'', because basically it makes no sense??

If the the 3 tanks are linked, they will self level themselves (the larger the linking pipes, the faster the self leveling will be), and only one pump will be required rather than three pumps. Oh, and motorised pumps, be they submersible or otherwise, are very reliable bits of kit today and not something you need to constantly watch over....
 
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If asked id build a small panel PLC controlled, water level, flow switches and alarm status... Pumps would work as requested if flow switch failed to monitor water flow the pump in question would be locked out and other pump takes over. Alarm would be raised.

A little bit more money but you won't get a call out in the middle of the night saying its flooded because one pump failed.
 
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With electronics moving so fast now Darkwood adding features and fail safes ontop of fail safes, does not really add any more cost in development, it seems to be the final sale price where it is bumped up.
 
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Know what your saying was just referring to the concept of having a design fail-safe set-up is going to cost more than a cheaper sequencing device thats switching existing controls.

Enclosure, Selv - DC power source, door interlock, float level relay etc etc just the basic will ramp cost up and like when i design systems from scratch i do charge for design and commissioning 'if complex' so it soon can mount up although in the OP case id probably russle up the panel without much thinking.
 
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darkwood said:
If asked id build a small panel PLC controlled, water level, flow switches and alarm status... Pumps would work as requested if flow switch failed to monitor water flow the pump in question would be locked out and other pump takes over. Alarm would be raised.

A little bit more money but you won't get a call out in the middle of the night saying its flooded because one pump failed.
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If were talking about rainwater harvesting, the tanks will be outside, so i can't see a problem with flooding to be honest.

Seen the system using three or four tanks being used for this very purpose all over Southeast Asia, and none of them needed a PLC controlling 3 pumps. If anything, you stand more chance of the PLC going ---'s up, than a pump... Bit different if this system was for an industrial process, i'd certainly agree with you in that instance... I'd be keeping this rainwater system as simple as possible...
 
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Engineer54 said:
If were talking about rainwater harvesting, the tanks will be outside, so i can't see a problem with flooding to be honest.

Seen the system using three or four tanks being used for this very purpose all over Southeast Asia, and none of them needed a PLC controlling 3 pumps. If anything, you stand more chance of the PLC going ---'s up, than a pump... Bit different if this system was for an industrial process, i'd certainly agree with you in that instance... I'd be keeping this rainwater system as simple as possible...
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You make a good point, im blinkered a bit here wasn't thinking too much about the general set-up and what its actually doing and was just looking at control options available.
 
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Can see with external sensing were the cost can come in......from a circuit side, now its just a case 16 inputs and 16 outputs, and a good programmer sorts out all the requirements, to control it actions, and user input and safety inputs...with less components i did a the quick design for op......life is a lot easier now...lol
 
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Engineer54 said:
What Marvo is asking, ...is ''WHY'', because basically it makes no sense??

If the the 3 tanks are linked, they will self level themselves (the larger the linking pipes, the faster the self leveling will be), and only one pump will be required rather than three pumps. Oh, and motorised pumps, be they submersible or otherwise, are very reliable bits of kit today and not something you need to constantly watch over....
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The answer to this is that my understanding is that for this to work the tanks have to be linked near the base. As they are buried this would involve excavating a 2 metre plus deep trench between them. As I also want a way of isolating the tanks so that for example one could be cleaned out whilst still using the others, isolating valves would need to be fitted and some means of access provided. None of this is particularly practical.

Thanks to everyone for their contribution. Really interesting - I was hoping for a very easy and cheap solution. Might well go with the tazz design, but a bit beyond me so would need to get someone to make it up for me.

One idea that has just occurred to me since starting to write this is that I could just have three cheap programmable timers - one for each pump- set up so that only one pump was in use at any one time. Not exactly what I asked for but close.
 
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See, that now explains WHY!! It would have been better if you had explained the situation in your opening post. Though why anyone would bury rainwater tanks is a bit obscure to me ...lol!! I had a feeling that Tazz's drawing was going to be a bit beyond most electricians comfort zone to sit down and make. A simple exercise, is now going to be well and truly over complicated.
 
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Engineer54 said:
See, that now explains WHY!! It would have been better if you had explained the situation in your opening post. Though why anyone would bury rainwater tanks is a bit obscure to me ...lol!! I had a feeling that Tazz's drawing was going to be a bit beyond most electricians comfort zone to sit down and make. A simple exercise, is now going to be well and truly over complicated.
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I did say in my opening post that changing the configuration of the tanks was not possible, but if I post again I will try and give more detail if that is what is required.

As for burying the tanks two good reasons are it stops them freezing up in the winter, and it's aesthetically much better in a small garden not to be looking at large tanks.
 
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Lol.... my over complex idea now seems to be a consideration as the OP has slowly drip fed us the set - up well i'd be inclined to go with one pump up top (2 if you want redundancy) and a valve system which works off 3 separate 4-20mA level sensors so if one tank gets drawn off too quickly a percentage marker will be exceeded in comparison to the other tanks and the valve will swap over. PLC wise simple operating principle but will be more complex on the number crunching side.

Where are you based?
 
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darkwood said:
Lol.... my over complex idea now seems to be a consideration as the OP has slowly drip fed us the set - up well i'd be inclined to go with one pump up top (2 if you want redundancy) and a valve system which works off 3 separate 4-20mA level sensors so if one tank gets drawn off too quickly a percentage marker will be exceeded in comparison to the other tanks and the valve will swap over. PLC wise simple operating principle but will be more complex on the number crunching side.

Where are you based?
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Suffolk, England.
 
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Well out of my catchment area sorry to say but maybe we have a more localised Engineer within the forum... and i personally would be one to go the whole hog ....cheaper systems may be fine if consequences are low in a fault or failure. I've done a similar set-up although not to the same requirements where the operator had full LCD screen overview of tank levels it was fully automated and as it was 24hr monitoring it had multiple redundancy with auto fault clearance. It cost a bit but has already excelled in its use and earned its money by avoiding 2 plant shutdowns.

Its down to really asking what your customer needs, what would happen if say the only pump fitted failed... what this affects and what the inconvenience and cost to anyone, then and only then can you recommend the best set-up for the job.

As Tazz brought up before a lot of the extra's can be done at low cost i.e. because you would have multiple tank monitoring of levels it doesn't take much to write in a monitor watch on fill rates so if one filling grate/channel got blocked to one tank then it would fill up unequally and be left low while other tanks filled normally - this can all be set to alarm up just by having a simple logic model written.

Again we can easily over-complicate this but we lack info and requirements.
 
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Paul2011 said:
I did say in my opening post that changing the configuration of the tanks was not possible, but if I post again I will try and give more detail if that is what is required.

As for burying the tanks two good reasons are it stops them freezing up in the winter, and it's aesthetically much better in a small garden not to be looking at large tanks.
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So what are these buried rainwater tank being used for, watering the garden or for something else?? If watering the garden you wouldn't need the tanks operational during the winter, plenty of rainwater in the UK during winter. Though aesthetics i can understand...lol!! Though if as you say, it's a small garden, why do you need 3 large rainwater tanks??

This control system is going to cost in terms of both materials and time, more than it's worth. Why not just pump the water as and when required with the flick of a switch, rather than trying to keep all three tanks at the same level?? I still don't quite understand the thinking behind that stipulation??
 
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