Backup petrol generator sizing for stormwater pumps

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KiwiME

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#286023 31-May-2021 13:52

I need advice on selecting a backup petrol generator for my rainwater pumps. Power failures during storms have been rare for me but I need to cater for this to avoid property damage.

I have two identical submerged pumps, each has a running draw of 5.2 amps at 230 VAC. These use induction motors but I'm unaware of what the startup current curve looks like. I do know that they seem to start up instantly when triggered. Only one pump would start at any one time but I need to cater for both pumps running as both could be needed in a severe rainfall event.

So, we're looking at about 2400 watts running power. If I assume I'd only run the generator at 80% load which gives me a desired rating of 3kW continuous without allowing for startup.

Between the "generator" type and "inverter" types I'm not fussed about the unit running at synchronous RPM when the pumps are off. I'd like to minimise cost where possible as it might sit unused for years as this is for emergency use only.

Is the conventional type generator more amenable to startup inrush current because of the inertia of the rotating mass or will other limitations kick in? If I follow common advice that I've seen online I get ridiculous estimates of induction motor inrush that lead to absurd overrating. Surely a generator can handle a 10X inrush for a perhaps 50 milliseconds?

Is there an advantage to the inverter type that might cater for inrush better?

Thx for any engineering or practical advice.

Scott3

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#2716224 31-May-2021 14:59

I don't know about it, but my gut feeling is that a non inverter generator would be the better bet. Usually these can start decent load's, but do bog down doing so.

Consider shelling out $80 to your local rental place, and and giving one a try.

https://www.hirepool.co.nz/generator-petrol-3-0kva-to-4-9kva/portable-single-phase-generators

Care to share more about the general setup - are these going to be auto fall over? - If so you need to consider noise every time the power goes out, and refueling every few hours etc.

If you are counting on being home to start the thing, perhaps you would be better served with a petrol pump like a Honda WT20... (not as good for other use though)

PolicyGuy

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#2716229 31-May-2021 15:32

Scott3:

Consider shelling out $80 to your local rental place, and and giving one a try.
https://www.hirepool.co.nz/generator-petrol-3-0kva-to-4-9kva/portable-single-phase-generators

A great idea to rent one to see how it goes

When you do rent one, it will also be a great opportunity for you to try out and refine your regular test plan for when you have your own genset, because the time to find out the #$%^& thing won't start is not when the water is lapping round the tops of your gumboots. :(

Your test plan needs to include a short test every week or fortnight just to make sure everything works, and a less frequent long test - at least several hours running under full load - to make sure that the system doesn't get all hot & bothered if asked to do the job maybe for a day or two. I knew of several places where they did weekly 1/2 hour tests and everything was good, but when they had an extended outage - in summer of course - cooling and where the exhaust fumes went proved to be major issues.
You'll also need to consider how you'll keep the fuel fresh: six-months-old petrol that's been sitting in a drum maybe not so useful

KiwiME

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#2716244 31-May-2021 16:23

Thanks for the replies so far.

This will be a manual setup only. Assuming I'm available and power has failed and it's raining, I'll unplug the pump junction box from its normal 15A outlet, roll the generator to the nearest covered outside area, fill the tank if needed from a small petrol container and connect it to the pump controller with a 5m 15A extension cord. It will run for the duration of the rainfall as long as the power is out, as long as the rainfall is heavy. The pump-out duration is about 1 minute upwards per sump fill, sometimes up to 5 min every 10 minutes if it's really pouring.

If I'm not in town at the time I'll have to live with the results. No doubt I could automate this in the future if I purchased an electric start generator but power has only failed once during rainfall in the last decade. Generally, the lines company are pretty quick about getting power back on because I'm in the CBD.

One pump runs when the tank reaches the first of two high levels, the second pump kicks in if the first can't keep up and the second high level is reached, which only happens in major rainfall events. If the rainfall was light I'd turn the generator off between pumpouts if that time was more than perhaps 20 minutes.

I don't anticipate any issues with exhaust, noise or cooling. I'd have a planned schedule to refresh the gas every few months and test the generator, all expected tasks.

The main issue is the type of generator and sizing in regards to the inrush current of a sump pump. The rental idea is good, will check that out.

Cheers

compound

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#2716393 31-May-2021 20:38

Generators cant do 10x inrush current but are rated at continuous and intermittent load capacities. As a rough minimum guide 3 x full load current will be required for starting purposes but if the motors only start sequentially then you could say 3 x full load current for motor A and full load current for motor B totaling (5.2 x 4) 20.8 amps or (20.8 x 230V) 4784 watts. Therefore maybe a 5kW (6.8kVA) generator would be a minimum. Yes you might succeed with a smaller generator, but the generator near stalling when the motor starts is not good for either unit. Mains grid power is relatively unlimited and the fuse is normally the limiting factor whereas a generator is limited by the rated power output and the fuse.

Scott3

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#2716960 1-Jun-2021 23:34

Flash option would be to get a LPG generator like the below, use it to back up the whole house (Light use only as 33A limit). Have gasfitter set up a 2x45kg bottle install for emergency use, and have the ability to run on BBQ bottle for testing / maintenance running.

Not quite sure what flow rate's, and head you are dealing with, But if head is less than 4m (not eccentrically to drain it the correct place, but to dump at an emergency point, into a garden or whatever, some cheap options could be worth considering:

- Install a manual Gusher style pump. Fairly Cheap, can do 100L/m. Given you are going to need to be present to start the generator, could well be viable to man a pump. More reliable, and not maintenance running needs like a generator.

https://www.marine-deals.co.nz/whale-gusher-titan-manual-bilge-pump-bulkhead-mount

- Install a 12V system. Get the following:

Lighlty used say 100AH deep cycle battery ($150 - trademe)
Smart charger - $60 - $160
Cira '3500GPH" bilge pump or two (3200L/h per pump at 4m head) $110 each https://www.marine-deals.co.nz/bilge-pumps/seaflo-series-3500gph-submersible-bilge-pump-12v
Float swith or two (cira $50) - set above trigger point of the AC pump as to not wear out the 12v pumps quickly.
Bunch of wire, fuse holders, pipe etc.
Custom, extra long set of jumper leads to reach your driveway.

System would function fully automatically for a few hours untill the battery went flat if power went out during a rainstorm when you weren't home (cira 20A draw per pump on the 100AH battery).

System would be auto re-setting. - would charge it self back up when power came back on.

System would act as a redundant backup for the AC system if it failed for a reason other than a power cut (say the discharge pipe became disconnected). - With run time limited by the ability of the little charger to keep the battery charged.

In the event of a power outage / rainstorm when you are home, you could move a car to the driveway, connect the jumper leads between the deep cycle battery and you car battery, and leave the car to idle. Most cars will produce 20- 50 amps at idle which should be enough to give you continuous running from one big bilge pump at least.

Even with the fuel light on, most cars will idle for several hours, but if you were worried you could keep a jerry can of fuel.

If worried about security (never run car with engine inside garage), you could get a wheel clamp for the car, or see if you can lock the doors with a 2nd key with the engine running.

KiwiME

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#2717059 2-Jun-2021 10:10

compound: As a rough minimum guide 3 x full load current will be required for starting purposes ...

Thanks, yeah, the key is that assumption of the startup current factor and how that fits within the limitations of the genset. Once that's understood the calculation is simple enough. Certainly I'll be looking at a 5kW or larger unit which moves me closer to conventional generator designs rather than inverter types.

Scott3: Not quite sure what flow rate's, and head you are dealing with, But if head is less than 4m (not eccentrically to drain it the correct place, but to dump at an emergency point, into a garden or whatever, some cheap options could be worth considering:

- Install a 12V system.

Thanks, I've been considering the various options for a number of years from all angles including 12V systems and have already concluded that a 230VAC genset is the best solution because the pumps and plumbing are already in place and proven to handle the application. The 400 L/min and 7m head I need would be a stretch for bilge pumps, never mind further limiting the tank volume and increasing cycle frequency, adding custom hardware and dealing with all the added related failure points. I can do a genset with just two off-the-shelf items that will take far less work.

bagheera

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#2717187 2-Jun-2021 11:31

KiwiME:

I'd like to minimise cost where possible as it might sit unused for years as this is for emergency use only.

Like all petrol motors - if left unused for years, then it will unlikely to start when you want it - so add in test runs to test that it still works, and service it every year if you want to use it in an emergency and have it start.

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Zeon

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#2717308 2-Jun-2021 14:13

Question on generator power output - as with many motor applications the in-rush current seems to be the killing factor. Once the motor is spinning, current draw is much less. Would it be feasible to use something like nitrous oxide or perhaps a battery/capacitor powered turbocharger to boost the generator capability for a short period of time needed for the in-rush current?

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compound

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#2717414 2-Jun-2021 19:37

Zeon:

Question on generator power output - as with many motor applications the in-rush current seems to be the killing factor. Once the motor is spinning, current draw is much less. Would it be feasible to use something like nitrous oxide or perhaps a battery/capacitor powered turbocharger to boost the generator capability for a short period of time needed for the in-rush current?

Unfortunately the speed of the generator regulates the AC frequency so you cant just rev it higher. inverter units change revs as the power is "corrected" by electronics to make a near perfect 230v 50hz waveform. So yes, those do rev up to make more power but the power is limited by the electronics capacity.

Soft start electronic starters to help start the motor will be more effective for reducing the inrush start current but the generator can be cheaper than the starter for that kind of stuff.

notesgnome

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#2717492 2-Jun-2021 23:32

I know you were thinking about existing infrastructure, etc. But, with the issues of inrush current, would a petrol water pump be an option? They can usually cope with 10 metres of suction. It then comes down to throughput.

You don't have the advantage of easy turn off/on, but it could be an alternative (although probably one you've already considered).

KiwiME

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#2717816 3-Jun-2021 17:58

notesgnome:.. would a petrol water pump be an option? They can usually cope with 10 metres of suction. It then comes down to throughput. ...

Prefacing the question I posed at the top, a few nights ago as I was attempting to fall asleep about midnight I did think about all the various options I have, including that one, because it happened to be raining lightly and I'm always concerned that my good luck might not be assured. However, I was kept awake by a faint beeping, which after quite some time realised that it was the UPS on my iMac complaining about this dreaded subject, a power failure.

Mild panic set in because this was the first time that this had happened in the 14 years that my rainwater pump station had been installed. But I had a half-baked plan, get dressed, grab my battery-powered lantern with a bucket and head down to the sump tank in the courtyard. I lifted off the steel cover to see the 300 L tank about 1/3 full and withdrew 5 litres with the bucket. Then realised that the only place to dump the water productively was past my electric sliding gate. Ran back up the stairs, grabbed the actuator key and unlocked the gate to dispose of said water down driveway, noting that some of it flowed right back. After a few buckets realised that the gully trap was much closer so dumped the water down there instead. After another 5 bucketloads the tank was nearing empty and the rain had subsided so I went back to bed.

I figured I can dispose of about 20 litres per minute manually if I'm willing to work in the rain and possible darkness. But the pumps can do roughly 450 litres per minute at full tilt, required during the highest rainfall events that happen about once a year or two. I think a petrol-powered pump might be able to do that rate but I don't have a place nearby to store it out of direct weather and I'd have to run the outflow to the same driveway, about 10m away and work in the rain. Given that the existing pumps and drainage outflow to the street (30m away) are already in place with the required capacity, it just makes sense to use them.

I can store a genset in my kitchen (I'm in a 1st floor flat) and wheel it out the nearby door onto a covered part of the 1st floor patio. A 5m extension cord is long enough to chuck over the railing to the pump electrical box below at ground level. Other than buying the genset and storing a few litres of petrol somewhere outside, all I need to do is rewire the existing hard-wired pumps to use a 15A plug and outdoor receptacle combo so that I can swap it over to the generator conveniently and safely. I'll drain the petrol from the genset when not in use so that it's safe to store indoors.

I think it's the best solution and I'll be looking at a 6kW+ conventional generator based on advice from a vendor.

Thanks for the suggestion however, appreciated.