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Syria generator sizing for water pumps

Working in a part of Syria to provide water to a population of approximately 150,000 through support of the water units. This is being done by providing generators to power several large powered submersible and horizontal pumps.

We have a pumping station which currently has a 675kVA generator, which is not enough to pump the required amount of water. As such, we want to confirm the size of generator that would be required to provide the required amount of water.

The station has 4 submersible pumps and 4 horizontal pumps, for which their capacities are the following:

All of the four submersible pump capacities are 110kVA. However, their current pulling differs according to the height of water in the level. As such, the currents are 150A, 100A, 150A and 135A.

The capacities of the four horizontal pumps are:

  • 2 x 195kVA, 295A
  • 165kVA, 250A
  • 40kVA, 95A

Calculations have shown that, assuming the generators were worked to their full capacity, that the total power would be 1035kVA, as such, assuming that the generator could be loaded with an average of 70% of its capacity, a generator providing 1035/0.7= 1480kVA would be required.

However, taking the currents into account, a total of 1470A would be required, which, when assuming a power factor of 1.73, a voltage of 380V and 70% loading, gives a required generator power of (1.733801470)/0.7 = 1380kVA.

We are aiming to pump approximately 5000m3/day as such I wanted to be 100% sure that we are getting these calculations right. I am not an electrical engineer so wanted to check if there are any factors which have not been included in these calculations. This are the figures and calculations that we have got from our guys within Syria.

Many thanks for the potential help.

John

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6 Answers

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In general the generator to run a pump should be at least 3 times the nameplate kVA rating of the pump motor. This is because when the motor starts there’s a starting surge which could damage or stall the generator which in turn could damage the pump motor. The corresponding ratings would be

Pump - - - - - - - - - - - - Current - - - - - - - Nameplate kVA - - - - Standard Gen Nameplate kVA
submersible 1 150 110 375
submersible 2 100 110 313
submersible 3 150 110 375
submersible 4 135 110 375
horizontal 1 295 195 750
horizontal 2 250 165 750
horizontal 3 295 195 750
horizontal 4 95 40 ???
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(taken from website: http://www.submersibleborepumps.com.au/generator_size_for_pump.html)


From the information provided, it seems the submerisble pumps are running at below capacity (their running current is below their continuous rating). The 40 kVA horizontal pump is running a higher continuous current than its nameplate rating suggests it should. They should review the suitability of this motor or perhaps there’s a typo.

I would not recommend they procure one generator for all of these loads, but break it down into at least 3 or four generators. That’s because they machines will likely not be all connected at the same location, so there will be a voltage drop running from one generator to too many pumps. In addition, it’ll be easier to get smaller generators than one big one. Finally, they should consider a spare if money allows

Michael E. RedR Expert

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I have consulted with a colleague since I am no expert, but he advises: I would use the motor kVA value rather than current for sizing generators, which summates to the 1035kVA value. The worst case condition for the generator on starting is an on-load value of 840kVA with a 195kVA motor starting. The impact on the generator will be dependent on the method of motor starting (direct-on-line, star–delta etc) which can vary the starting current and resulting voltage drop or dip expected at the generator – under worse cases the generator may fail to start. Usually this is analysed by the generator supplier via appropriate sizing software as a 195kVA motor full load current will be around 370A, which could be multiplied by 8 to reflect starting at direct-on-line ie. nearly 3000Amp.

I think an increased safety margin would be useful and therefore suggest a standby generator size of 2000kVA which is double the 1035kVA load. A 2000kVA unit is usually readily available although this needs checking for the local location, if not then two 1000kVA units may be considered and even preferable if the eight motor loads can be spilt into two separate load centres. In this case the maximum use of the installed capacity is made if the largest motors are started first.

An additional factor is the loading on the diesel engine. If the genset is too large for the running load, there can be serious maintenance problems due to engine 'slobber' (!). I think this percentage value is below 30% loaded for continuous use you can suffer these problems. Ideally a generator should operate at 70% of its rated value.

To minimise the effects of running the engine on low load, it is beneficial to run the engine up to load using an external load for one to two hours, at least once a month. Another measure may be for engines where slightly load or extended idle periods of engine operation are mandatory, that the objectional effects of engine slobber can be avoided by loading the engine to at least 30% load for approximately 10 minutes every 4 hours.

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There isn’t really enough information given to produce a specific sizing for a suitable generator as you would need to know the full system arrangements i.e.

· Type of starting method for each pump · If VSD starting, what type of VSD (i.e. standard 6 pulse, 12 pulse etc) · Starting sequence of pumps i.e. can any of the pumps be started at the same time or are they all staggered start.

Caterpillar have a good web based generator sizing calculator (requires registration) which gives their recommended generator set (engine / alternator) combinations for a given loading scenario which can be found here:

http://www.cat.com/en_US/articles/solutions/power-systems/electric-power-specsizer.html

This calculator produced a caterpillar alternator sizing of 1750kVA based of the pumps being VSD (6 pulse) controlled and started in groups of two at a time i.e. 4 steps.

Although the website will give an idea of the size of the required generator I would definitely recommend that the advice of the generator supplier is sort as to the sizing of the generator as they will know the most suitable generator set for any given scenario.

Regards

John C

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There is an important rule to observe with this kind of problem: always start the largest motor first and also always make sure the full load rating of the generator is never exceeded, especially upon starting a new motor load. Remember that squirel cage induction electric motors take up to 6 times their full load current upon starting. This is why when these pump motors are fed from a "limited capacity" generator they are always started by means of assisted device like an autotransformer or a star-delta motor starter.

Another rule: never run the generator on full load permanently. Aim at 80% FL if the load is permanent. Never above 80% of generator capacity.

The type of starter is not mentionned. But I would guess they are star-delta starters, quite suitable for centrifugal pumps motors. With this kind of starter the starting current of the motor is reduced to 2 times full load current.

The method for appraising the generator size is as follows:

Starting with largest motor first:

1) 195 kVA horizontal pump, 296A FL, load on generator is 592A

2) 195 kVa pump, 296A FL, load on generator: 296 + 592A (2X296) = 888 A

3) 165 kVA horizontal pump, 250A FL, load on generator: 592 + 500A = 1092A

4) 110 kVa pump, 162A FL, load on generator: 296 + 296 + 250 + 324 = 1166 A

5) 110 kVA pump, 162 A FL, load on generator: 296+296+250+162+ 324= 1328A

6) 110 kVA pump, 162A FL, load on generator = 296+296+250+162+162+ 324 = 1490A

7) 110 kVA , 162A FL, load on generator: 296+296+250+162+162+162+324= 1652A

8) 40 kVA , 62A FL, load on generator: 296+296+250+162+162+162+162+124 (2X62)= 1614 A upon starting of last generator.

Thus generator minimum size required is : 1.732X380X1614= 1062 KVA minimum.

So you would need another generator of 1062 kVA size. Preferably 1200 kVA. Or a second one of circa 500 kVA to operate in paralell with the existing one.

This calculation above is based on the assumption that all electric motors are star-delta started and that all pumps and motors run on full load.

Daniel FAYOLLE

RedR Expert

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The CAT and other manufacturers generator sizing guides and software are excellent. Having worked in Iraq for a number of years ensure you take note and apply the appropriate de-rating factor for high summer temperatures, altitude and poor quality diesel fuel.

Regards,

Tony

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Definitely add a second generator, this will give some redundancy if either is down for maintenance or other problems, you can still pump some water. Yes, you will need to split the loads, the cost for that should be minor compared to the cost of the generator. Once you have the pump motor started it will not need the extra starting current until it is turned off and restarted, so you don't need to size genset based on starting all at once, that just drives up cost of generator. So Daniel's answer is good. Second since generators are made in standard sizes, when you figure what the pump split you want to put on each genset (mix of submersibles and horizontal pumps in the same piping path - you want to be able to deliver water even if only one genset is running) use the 70% loading and buy the next larger standard available size unit (with allowances for high ambient temperature and altitude).
Third, watch where the crankcase oil vent line is located. When I was with a telecom company we had issues in Iraq with crankcase vent line blowing oil mist where air flow took it into the radiator which then clogged with the fine dust sticking to the oil on the radiator fins. This caused the genset overheating since air flow through fins was limited. Yes, wet stacking would be a problem if the units were loaded UNDER about 30% but I don't see that as much of a risk in your case. BUY and install fuel/water separators and keep spare fuel (and oil filters) on hand and sealed. Most generator engine manufacturers allow a limited mix of waste engine oil back into diesel fuel. Check on that and you can solve the problem of getting rid of waste engine oil, you probably will need to filter it. If you can easily split the loads don't bother paralleling the generators. Unless you can match the winding pitch of the existing unit (often hard to find out what it is) they will have cross current problems if you try to parallel them. Save the headaches! Make sure folks check and drain any water in those fuel/water separators regularly.
It can be a wise option to buy the engine set with dual oil filter banks and valves so one bank can be switched out of service to allow for then to be serviced, while the second bank is filtering engine oil, this reduces engine downtime and get more water pumped.

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