Northy how can I say this politely...uhm most everything written here is wrong. No offense guys but this was my profession for many years. Like Nate said the difference between a trash pump and a fire pump is the clearance inside the pump.(There is also a general purpose water pump between these two the can vary from cheapo plastic housings to very high quality pumps). As was stated a trash pump is meant to pump solids about half the diameter of the discharge. A 3" trash pump will pass solids a little smaller the 1 1/2" diameter. A "high pressure" pump made for fire fighting needs to pump clear water, not even sand, or it will wear out fast. I put high pressure in quotes because that term has no standard definition outside the fire service. Most consumer and industrial portable pumps are rated in gallons per hour AND head pressure. The two have to be considered together. Gallons per hour or GPH means how much water the pump puts out when drafting (using hard suction hose and with a water source at the same elevation as the pump and at or near sea level). Head pressure means how much force the water stream has to overcome positive elevation change. One foot of elevation equal to .433psi. This can be rounded up to 1/2psi per foot. So if a pump is rated at 80 feet of head pressure then it can put out around 40psi. This is all the pump is capable of, reducing the discharge size can not make the pump produce a higher pressure. Physics and stuff. If you hook the pump to a positive pressure water source like a hydrant that has a higher flow rate than the pump's rating then it may boost the pressure somewhat. Wildland fire nozzles are rated from 50psi to 100psi depending on the nozzle to achieve their rated gallons per minute or GPM and the best performance. A nozzle is matched to the pumps pressure and volume. Still a small fog nozzle made more for industrial use (read plastic nozzle) should work OK with a 1 1/2" pump. Just don't expect much throw.
So there is a trade off between volume (GPH or GPM) and pressure (head pressure or rated pressure). As your need for pressure rises because your fire (and nozzle) is 20' higher than the pump then the volume goes down. There is nothing you can do about this with an inexpensive portable pump. Also if your nozzle has adjustable gallonage and you open it up because the fire is getting bigger, then the pressure, your throw, goes down. This is why you see so many 1 1/2" pumps supplying 1" hose on the back of fire trailers. The operator needs to keep his pressure at the maximum the pump will put out. On the other hand with a water trailer (aka water buffalo) designed for contractor use the hose will be 1 1/2" because the need is for a large volume of water to the construction project.
Purpose built wildland fire pumps are rated up to 450psi. This is because there is a need to overcome friction loss in long hose lays and also to pump uphill. Very long hose lays may have several pumps (and portable tanks) all pumping in tandem. Those pumps are expensive (and finicky). Those are the multi stage pumps that were mentioned above.
So how to simplify this? Buy a Honda WH15 or a WH20. I also think I saw an ad in the latest Loggers' World for someone up your way selling a Honda powered fire pump for about $1,500.00. I hadn't finished reading that issue when I ended up giving to a friend of mine.
Oh and don't buy a trash pump unless it is one of Nate's nuclear powered ones.
