Pumps. New Ideas?

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Permalink Reply by Bob Franklin on December 4, 2008 at 2:06am

Hi Mike,

As a pump mechanic for over 35 years and a firefighter with 39 years experience, maybe I can shed some light on your project. Since the 1970s we have been in the single stage pump mode. This was an idea that a single stage would do the same job as a two-stage pump. This eliminated the transfer valve, which for some reason seemed to be a major problem for the new pump operator coming up in the fire service. The biggest comment I heard was our drivers do not know when to switch from pressure (series) to volume (paraell). This was a training problem not an operator problem. You don't
mention how many years you have in the fire service. Some firefighters have never operated a two-stage pump and have a lack in how it functions. For all practical purposes, the two-stage pump is the most practical pump to use. It gives you the option of a pressure pump ( low flows @ high pressure with less engine RPM) and a volume pump ( large flows at less pressure but more RPM.)

Lets start with a two-stage pump. We will use a 1000 G.P.M. rated pump for our example. First we have two 500 g.p.m.rated impellers placed back to back on the impeller shaft. We have a transfer vale that is an internal valve that directs water from the first impeller (1st stage) to the second impeller (2nd stage). Which impeller is which? Easy!
Facing the pump panel (left side) the impeller to your right is the first stage, the impeller to your left is the second stage.
We will use a fog nozzle that will flow 125 gpm at 150 pump discharge pressure (PDP).

We are going to produce a 125 gallon fire stream at 150 pounds pump discharge pressure. We turn up our RPM. Water enters the 1st. impeller and produces the 125 gallons at 75 pound pressure. The water is routed to the second inpeller eye at 125 gallons at 75 psi. The second impeller only adds and additional 75 psi that brings our flow to 125 gallons per minute at 150 pounds pressure. This was done at 1300 R.P.M. The second impeller doesnot add gallons only pressure.
Remember; Pressure stage 1/2 the gpm at twice to pressure

The next big statement was,"When do you change from pressure to volume?" The answer, " When you exceed to 1/2 the capacity of the pump rating." If the pump operator knows the flow of the lines they are suppling, they will know when to make the changeover, if the incoming supplyline can furnish the increase volume water required.

Now in the volume position (which is what as single stage pump is) water enter both eyes of the each impeller at the same time. The transfer valve is closed. At 1300 R.P.M. our discharge pressure gauge will show 75 psi, but you should be producing 250 gallons per minute. Each impeller is now producing independant of each other. This sounds great but there are several drawbacks to be discussed. First, you are flowing 125 gallons more that the nozzle is designed for so you will have to gate the valve down ot adjust the RPM lower, which will again drop the reach of the line even more.
REMEMBER: In volume stage you get TWICE the gallons at 1/2 the pressure.

At the lower pressure 75 you lose distance or reach of the stream. To get your reach you would automatically increase your R.P.M. When you increase your RPM you are increasing the gallons also. Knowing that you would need a nozzle pressure of 100 plus the friction loss of 20 psi per 100 (200 ft. of 1-3/4) 20+20+100= 140, you will actually be flowing around 300 gallons that the nozzle was not designed for damaging the strem deflector and placing the nozzle operator in danger.

The single stage pump is best suited for large capacity fire streams, as it is a volume pump, not a pressure pump.
They have proven to be ineffective for lower flow operations. Using a flowmeter and inline gauges will help you to determine which pump will deliver the best performace for you department.

The other big complaint I have heard is the single stage is much cheaper than the two-stage pump. Initial cost yes! But you have to look a down the road 20 years and the life of the apparatus. We have found that apparatus with single stage pumps have a higher cost due to wear and tear on the engine, transmission, pump gear case and impeller and shafts.
I guess what I am trying to say here is, don't buy what the salesman wants you to have. Get the apparatus and pump that will meet your needs. It makes since when you can produce fire streams at 1300 RPM rather than 2300 RPM.

I hope this helps, have a safe holiday and merry christmas.

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Permalink Reply by Frozenhosehead on December 4, 2008 at 5:40am

Good luck with you project Haefalo! If I come up with anything that will help I'll send it your way brother.

Thanks Bob that was a great post and a great refresher for us older brothers who were around when single stage pumps were the norm and now it's the opposite.

Will the two stage pump make a comeback in the future? With the price of fuel and uncertainty in the economy departments are going to try and squeeze every bit they can out of a piece of apparatus to get more bang for their buck that will help them stretch a few more years of service out of them.

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Permalink Reply by Kirk Allen on December 4, 2008 at 10:08pm

Great Info Bob.

Mike, besides all the pump issues already outlined, another major factor that can reduce the expenses your referencing is focusing on correcting all the Friction Loss problems of the internal plumbing. Having tested thousands of handline flows and pressures, with flow and pressure measuring equipment I can tell you I see 20-100 PSI of Friction loss in internal preconnect plumbing for flows in the 200 gpm range. That FL means higher pump pressures, thus more fuel.

The next problem area is in the brand of hose. I have seen some hose that has double the FL as another brand for the same flow.

The last part of the problem is Nozzle Pressures. Utilizing a low pressure nozzle can reduce engine pressures!

After addressing these three problems in my department we were able to reduce our engine pressure by 60 PSI for 200 GPM. We now pump 100 PSI through 200 Feet of Ponn Conquest hose and by using low pressure nozzles we are getting 200 GPM.

Kirk Allen
Chief
Kansas, Fire Protection District

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