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Does high discharge pressures hurt the engine/pump?
One characteristic that always gets brought up in low pressure hose(low fricition loss characteristics)and low pressure nozzle discussions is the fact that they save wear and tear on the apparatus engine and pump. I don't think this is an issue. Any thoughts?
Replies to This Discussion
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Permalink Reply by Paul Spencer on
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Hey Brother it’s great seeing you here on FFN. You’re absolutely correct… pressure has no direct effect on pump wear. Diesel fuel use is higher at higher pressure (RPM) but pump wear is attributable to only a couple of things.
Impeller damage is most commonly caused by cavitation, which happens to be easier to do at higher pressures. The farther needles are spread apart (net pump pressure) the easier it is to cavitate. Intake pressure of 100 psi and discharge of 250 psi not a big deal. Same discharge at draft has a much higher likelihood of cavitation.
In all fire service pumps there is a component that is called a “wear ring”. This is the main wear point in the pump. It creates the seal between the intake and discharge sides of the impeller. Water slips from the discharge side to the intake side through the ring. As the clearance increases due to wear then less water goes to the discharge. Wear of the wear ring is created by debris in the water or running the pump dry.
For me the main purpose of low friction loss hose and low pressure nozzles is to get the pump discharge pressure into a better place on the “pump curve”. A 1,000 gpm pump is only 1,000 gpm at 150 psi. At 250 psi it can only pump 50% of that or 500 gpm. The pump curve is the slide of efficiency between 150-250 psi and 100-50% of efficiency.
The example is an awesome big nozzle that requires 250 psi to flow 500 gpm. That nozzle will tap the full capacity of a 1,000 gpm pump and only deliver 500 gpm into the fire. On the other hand a really cool low pressure nozzle that flows 500 gpm at 150 psi discharge pressure changes everything. The same 1,000 gpm pump can supply two separate 500 gpm nozzles at 150 psi discharge pressure for 1,000 gpm into the fire. Ultimately it’s all about the pump curve and discharge pressure.
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Permalink Reply by Ben Waller on
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For comparable GPMs and hose layouts, a low-pressure nozzle will result in lower engine and pump RPMs to get the neccessary PDP.
Fewer RPMs equals less wear on the pump since there's less friction. Friction is the primary thing that damages the wear rings. (There's a reason that the word "wear" is in their name.)
Less friction also equals less heat. More heat equals more damage to the pump over time, so the higher RPMs from high-pressure nozzles actually hurt the pump in two ways.
Yes, foreign objects and cavitation hurt the pump, but you can wear a pump out more quickly without ever cavitating it or having the pump ingest a single foreign object.
The lower pressure nozzles also put less stress on the fire hose, since it takes less pressure to move the same amount of water through the same hose and nozzle.
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Permalink Reply by Don Catenacci on
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Less wear and tear never entered our minds when we made the change to 2 inch hose and low pressure combo nozzles and smoothbores. We did it to get higher flows with less nozzle reaction.
Modern fire apparatus have no problems pumping 250 psi all day long so I don't understand the thinking behind your question.
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The point is that while the apparatus are capable of pumping higher pressures, why make them do it if you don't have to.
Higher PSI equals higher pump and engine RPMs, which demonstrably puts more wear on the pump and the engine. That's true for anything mechanical.
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Permalink Reply by Paul Shapiro on
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I can't speak about the specifics for the pumps other than to say that the moderate increase of pressure/rpms is not much. As far as hose is concern most discharge hose in service today have a annual service test pressure of 400 psi which means that it can be used up to 365 psi. I don't think you can come up with any proof that the higher pressure operations(200 to 250 psi) will have a negative result on the hose. For the most part high pressure operations for handlines are done to make deployment easier on the guys by using lighter hose that is easier to handle. When in doubt ask the manufacturers about there products.
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I have a different reason for pumping at higher pressures. I travel around putting a digital flow meter on engines to help engine companies find out what pressure to pump. For many, to actually deliver the amount of water they thought they were, the pump operator needs to throttle up 50psi or more. I cringe thinking about an attack crew making entry with a 85 gpm hand line into today’s fires thinking it is 150 gpm or more.
http://www.firefighternation.com/article/engine-co-operations/deliv...
I also have done failure analysis on fire pumps as I have rebuilt them over 20 years as a master emergency vehicle technician. I know that the impeller shaft is supported by caged bearings and are not affected by pressure. In regular operations bronze wear rings are kept cool by the water in the pump (keep it moving please) and normally have clearance from the impeller so there is no direct wear. Anything in the water changes this as it travels between the impeller and wear rings. It is my belief as a master technician of fire pumps that there is no wear attributable to pressure. You need to be a much better pump operator however at higher pressures... Respectfully, Paul S. (wow, we have the same initials ;-)
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I find it beyond disturbing how many FDs don't have a clue how much water they are flowing. We know less about the tool that we count on to kill the fire than the average mechanic knows about the tools in his toolbox...
When my POC FD wanted to change nozzles to increase our flow, and reduce the nozzle reaction, we borrowed a flow meter and new EXACTLY what are flows were. When we received our new hose and nozzles we used a flow meter again to set our discharge pressures. Guessing, and formulas, don't get you the results you believe you are getting.
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Right on Don. I think the problem is that we don't really know what we don't know. We tend to take passed on information as the truth without really verifing. The differences between brands of hose and plumbing design can make a huge difference in the PDP. People don't realize this. I think the answer is more education with new facts, not the traditional book facts.
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One of my favorites is going into a fire department and look at either a department wide pump chart OR all preconnect discharges marked the same despite manufacturer and age differences. What are the odds that the pping is EXACTLY the same for different ages of the same brand, let alone different brands of trucks?
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Absolutely! Even the amount of rust on the inside of pipe over time changes friction loss. Friends ask me if it really matters? My answer is always "only if you are trying to flow high gpm to put out a really hot fire"
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Our apparatus have panel-mounted flow meters, so we know what we're flowing.
There is wear from higher RPMs, as there is no such thing as a frictionless pump and friction causes wear. That wear may not be as pronounced as with the old-school pump systems, but it does occur over time. The apparatus motor wears out faster at higher RPMs, too.
For the friction loss, it helps if you test your hose and determine your actual friction loss. In our case, it meant several weeks of experimentation and tabulating data to determine that our actual 1.75 inch coefficient was closer to 10 than the IFSTA-published 15.5.
The standardized 50 PSI nozzles (fog/straight tip breakaparts) make it easy to pump the correct pressure and they reduce the PDP by 50 from our old nozzles. We are fortunate to have standardized apparatus with standardized hose loads, discharges, and color-coding from the pump to the nozzle.
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Ben, the next time you are driving the truck at 50 mph, check your tach. Most high pressure pump operations will probably be in that range and are not done that frequently. Everything that moves in the apparatus will eventually cause wear. The fact is that it is a work truck and has to be able to perform certain operations including high pressure operations. The Stratosphere hotel in Vegas requires a 600 psi operation and is done with two pumpers in series each pumping 300 psi. If I remember correctly the RPMs are around 1750 to 1800.
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