Single Stage vs. Two Stage Pumps

[brfoss]

Beginning spec process for a new pumper tanker. Will want at least 1750 gpm. My question is are there any advantages to using a two-stage pump?

In my opinion, single stage means fewer parts (fewer moving parts) to break and no changeover valve to stick.

However, I have heard that pumping lower gpms (i.e. 100gpm, a common occurence) with a big 1750/2000 impeller doesn't move enough water. You can actually cavitate the pump even though you're only flowing 100gpm because the pump wants to move more water. Any truth to this?

Any other factors to be considered?

[ff7134]

OK my first question is, what is the type of response area for this truck??? If it's mostly commercial ok good, but a residential?!?! I think a 1750 is overkill for that. We have a 79' American LaFrance with a 1750 Dual Stage. It is our first out commercial, it also is a mutual aid truck for several other depts. and who ever told that they don't move water good at low pressures is wrong because we use low pressure nozzles and we don't have any problems.

[brfoss]

The response area for this engine will be across the board - municipal, rural, residential, commericial, large industrial. The main reason for the 1750 is to have the large capacity available for the "big ones". Also, a 1750 will match our existing 1750s so we can relay in any order.

The possible damage issue doesn't have anything to do with low pressures. It is pumping a low gpm that supposedly can be problematic. If you're only moving 100gpm through an impeller designed to flow 1750+, is the pump going to overpump the water to the point where you will get cavitation? This was just one possible issue that I heard of. Supposedly a 1750gpm two-stage pump wouldn't have the problem because the impellers are smaller.

Just fishin for info...

[Rayr49]

My personal opinion is that the pump will not cavitate as long as it is full of water.

You might try to contact the pump manufacturers on their web site and ask the question. I'm sure their support people will give you an answer.

Stay Safe
IACOJ

[DRA177]

We use to run all two stage pumps. Now we only have one left, the rest are single stage 1250's. We found that pumping in "volume" the rpm's had to be increased to achieve the same discharge psi. We found there was no difference when drafting with our single stage vs. two stage pumps. So there was no "advantage" in our book. After all it takes pressure to move water through hose.

brfoss you mention large capacity (1750) for the "big ones". Our 1250 single stage pumps have produced 2200gpm from 3000+gpm hydrants. So I'll assume a 1750 could produce around 3000gpm if it is hooked up correctly to a strong hydrant. Now the question is: do you have the means to move that much water? The best we can do on a single lay of Angus 5" is 2000gpm @ 1000'(w/1 engine). That's keeping the discharge pressure at 185psi. I agree with you on "more moving parts" and the change over valve. So I'd say keep it simple and go with a single stage.

You can answer your own question on the cavitation issue. Go out and pump your current 1750 at low gpm's and let us know what happens.

CaptD

[FWDbuff]

There are only two things that can damage the impeller of a pump- Heat, which you solve by circulating water at all times that you are not flowing, or physical matter- such as rocks through the intake, rubber LDH sucked in, (Yes! I have seen it!)

Cavitation should not cause physical damage.

As for the single/two stage.....

With a two stage, in volume mode, you are moving MORE (usually more than 50% capacity) water at LESS pressure, which means less engine RPM's.

Single stage, you're moving less or the same amounts of water at higher pressures.........

[Fire304]

We run a pair of 2000gpm trucks and the only problem we've ever run into is that it's tough to get the dead-headed discharge pressure below 60psi, that's what the pump idles at (you can gate back while flowing). Keep the water moving and you will not cavitate.

Also, don't buy into the overkill argument, the fact is that (at least with Hale) the difference between a 1500gpm and 3000gpm pump is the HP of your engine and number of discharges hooked up, its the exact same pump body. A 1750gpm pump will move roughly 1400gpm at 185psi due to the reduction in capacity at higher pressure (70% at 200psi would be 1225gpm), is this enough to flow your 5"?

[GreggGeske]

Two stage pumps were alot more popular years ago. The usage now is probably about 80% single stage and 20% two stage. Two stage pumps are more efficient across a broader range of performance. High pressure booster lines also were used more years ago. (FWDbuff) Re-circulation cavitation is the cavitation that will cause damage to the pump impeller. It is caused by not moving enough water away from the discharge side of the impeller. It really shows up on pumps that are used for hose testing or for pressure washing. Over time small marks or voids are created on the vane and the edge that joins the vane and the shroud. Given enough time a piece of the vane will break off and will show up in the nozzle. Re-circulation cavitation is observed less in a smaller impeller (1250) and even less in a two stage pump. It may not be a reason to go to a two stage pump but is a reason why my department stuck with 1250 pumps on the pumpers.
Speaking of foreign objects in the pump, I saw a picture of a basketball stuck in an impeller. The officer was demonstrating cavitation to the operators by placing a basketball at the end of the intake hose. Expensive lesson.
Another added problem with a pump rated at 2000 GPM or greater is finding a location that 2-6" intakes can be hooked up to do the annual NFPA service test.

[bendone]

2 stage pumps

I thought the reason for two stage pumps is to be able to pump a given GPM at less engine RPM. For example we have a 2000GPM that will pump 1000GPM in presure mode at about half the RPM it takes to pump 1000GPM in volume mode, and single stage pumps operate the same as a two stage pump in volume mode.

[hazenfd]

Here is what is says on our pump plates

2250 gpm at 150 psi 1500 rpm
2250 gpm at 165 psi 1550 rpm
1570 gpm at 200 psi 1580 rpm
1127 gpm at 250 psi 1750 rpm
1000 gpm at 300 psi 1900 rpm

Same truck with 2 stage

2250 150 1500
2250 165 1570
1577 200 1590
1127 250 1680
1000 300 1825


Not much of a difference for the $2000

[Fire304]

Originally posted by GreggGeske
Another added problem with a pump rated at 2000 GPM or greater is finding a location that 2-6" intakes can be hooked up to do the annual NFPA service test.

This is a problem, although only one 6" intake is needed to run a fireground feed into 5" at 1500gpm and 185psi (where it really matters). At this point we have not addressed annual testing (while the truck is still under warrenty) but are planning to build a dry hydrant specifically for the purpose.

My understanding of the need for 2 stage pumps was that older (like 1940-50's) gas engines did not have the torque required to spin a "large" 750gpm pump. The two stage works like a 2 speed gear box would, allowing a low (volume) gear and a high (pressure) gear. Modern engines have ample power to handle much larger pumps eliminating the need for the complicated two stage rigs.

[Fire304]

Originally posted by hazenfd
Not much of a difference for the $2000

Which one was more expensive?