The earliest fire pumps were piston-type, which evolved from manpower to steam power to eventually being driven by an internal combustion engine like the Ahrens Fox. The rotary pump showed up in the 1800s and was popular in the early 1900s. Both of these pump designs are positive-displacement, so they have the advantage of being power-efficient and, to some degree, self-priming. By the end of the 1930s, most of the piston and rotary pumps were no longer offered. Why did the market go away from these designs? They are expensive to make, they are very susceptible to wear from dirt, and there is no boost in performance being passed through the pump when operating from a pressurized hydrant source. All of these problems are addressed with the centrifugal pump design but at a price-less horsepower (hp) efficiency.
One of the big gains of centrifugal pumps is passed-through pressure boost, which allows a centrifugal pump to operate at a lower engine rpm at small fires and provide higher performance at big fires. Positive-displacement pumps can’t take advantage of pass-through pressure.
A centrifugal fire pump has some very special characteristics. To create the vacuum at the impeller eye, which allows drafting performance, and to produce the range of flows and pressures a fire pump requires, a special hydraulic design is necessary, which yields lower power efficiency than the typical process/industrial pump and even more so when compared with a positive-displacement pump. Industrial/process pumps do not draft water at high lifts and at big flows like a fire pump. They also tend to cavitate quickly when not operating at near full design flow rates.
What Size Engine?
What is the typical hp required to drive a fire pump? A 1,000- to 2,000-gallon-per-minute-(gpm) rated fire pump will have a best efficiency point of 68 to 75 percent normally at or close to the pump’s maximum rated flow offered for that basic pump model, according to National Fire Protection Association (NFPA) 1901, Standard for Automotive Fire Apparatus. For example, a 1,500-gpm pump at its best requires 186 hp. The reality is that most pumps take a little more hp than this example, and all test points need to be checked for power usage. The 165-psi NFPA overload test will have a lower efficiency and will usually be a high-power test point. So will the 250-psi test because as you move to a lower flow, the power efficiency goes down. The farther away from the specific best efficiency point, the lower the actual efficiency will be. It is not uncommon to be at 50 to 60 percent efficiency at 50 percent capacity at the 250-psi test point. As a frame of reference, an industrial process centrifugal pump will have an efficiency of 80 to 90 percent and a positive-displacement pump will be 85 to 90 percent efficient. Either of these types of designs would theoretically save 30 to 40 hp at 1,500 gpm, but the apparatus would be difficult to use and would cost even more to buy. There are no bargains out there.
Historically we looked at the chassis engine as the limiting factor when looking at a big pump rating. When the common engines were 230-hp gasoline engines or 210-hp diesel engines, 1,000-gpm-rated pumps were very popular. By the mid 1980s, we started to see 225- to 240-hp diesel engines become common. Guess what-the pump size grew to the 1,250-gpm rating. Today we commonly see 330-hp diesel engines, and the most common pump rating is now 1,500 gpm. Today, having 330 to 450 hp is so common that we don’t get concerned about engine size until we get to a 2,000-gpm rating or higher.
Today the bigger issue is not the size of the engine but the drive-through rating of the power takeoffs (PTOs), which are becoming more popular. Rear-engine (REPTOs) or transmission PTOs are not full-engine-power-capable. They are limited to the size of the internal drive gear supplying power to the PTO itself. For a very long time, the available transmission PTOs were very small-usually not capable of more than a 500-gpm rating and many times not even that big. This is one of the key reasons the midship split-driveline pump became so popular-you could benefit from all the engine’s power with very few losses and no limitations.
Today’s excitement over PTO pump drives is being driven in part by the availability of newer, bigger PTO options and by the perception that PTO drives will make the final apparatus packaging more space-efficient. The other factors are the potential for pump and roll for no extra money, a high-capacity single-stage pump, and the possibility of saving big money and preventing apparatus runaway. So, are these perceptions correct?
The basic pump doesn’t take up any more space whether it is split-drive or PTO-driven other than the size of the gear box needed for the drive type. What makes the traditional midship pump look so big is the suction and discharge cast manifolding system. With an equal manifolding setup and a well-thought-out layout, either a PTO or split-driveline pump can be an efficient package. Weight is where the PTO pump does have an advantage of about 250 pounds. When buying a standard pumper chassis and a big single-stage pump, the pump-and-roll performance, for most departments, will be unacceptable and difficult to operate. Typical performance of 100 gpm at 100 psi will be at approximately 1,100 engine rpm, which is going to be at about seven miles per hour (mph) and will require using the throttle and brakes together.
Some people in the industry are using the term “pump and move” to describe this performance-maybe it should be called pump and drag. All apparatus built in the past 11 years have interlocks that prevent runaways if you shift the pump into pump gear. If the apparatus is left in road mode and is in drive, a PTO pump is going to run away also in the same set of conditions. If the truck is in drive and the pump is not shifted, it will run away-maybe this is the pump and roll we hear about! The list price comparison between equal PTO vs. a split-driveline pump with the 870 transmission PTO factored in is about $1,000 list price savings. So there it is, there is a small savings in weight and money when opting for a PTO pump.
In the early 1990s, Allison launched its world series transmission family and it had a 10-bolt PTO opening for which Chelsea and Muncie quickly supplied a big PTO package to maximize the potential. These 859 class PTOs could power bigger fire pumps-1,000 gpm on a 3000 transmission and 1,250 gpm on a 4000 transmission. Under some conditions even higher ratings were offered, but with some departments the PTO life between rebuild of the PTO was shorter than many liked. However, some departments never saw a problem, even after many years of service.
Why? The full continuous power operating design life of these 859 class PTOs was 100 hours. But if you didn’t use the pump to its full rated output, the PTO life was considerably longer. Exact load life data have never been released by Allison or the PTO manufacturers, but as a general engineering rule of thumb, halving the load gives you five times the life. So as an example, a 1,000-gpm pump/PTO combination on a 3000 transmission will run at 1,000 gpm for 100 hours without too many PTO issues. The same pump operating at 500 gpm continuously will operate for close to 500 hours and theoretically last 2,500 hours at 250 gpm. This is why we saw only a handful of departments with wear issues. If you pump deck guns, supply LDH, and feed aerials on a regular basis, the PTO issues are more likely to show up.
But, many apparatus only see these activities once every few years. Day-to-day operations are for a jump line or maybe twin crosslays and the flows are 30 to 300 gpm. But, the big question is: When you have the big one, how old is the apparatus, and will it fail because of high loads for hours on an apparatus with high hours of low-performance handline usage?
The new Chelsea 870 class PTO for the EVS Allison transmissions has a higher torque rating and longer duty cycle than the older 859 class of PTOs and can drive up to a 1,500-gpm-rated pump. What that improved duty cycle is has not been quantified yet, but Chelsea is looking at listing a rating.
No matter how you look at it, you still need to design the pump system PTO drive so all the normal pump performance applications are at or below the continuous-duty PTO rating. That includes overperformance that is achievable by the pump system.
It is not uncommon to see departments with high-flow applications and only modest hydrant pressure. These conditions can easily cause an excess of 800 ft-lbs. of power through the pump drive system. The shortcoming of all engine or transmission PTOs is they are limited in power transmission capabilities to considerably less than the pump’s potential when connected to a hydrant. Anyone’s 1,500-gpm pump will do 2,000 gpm or higher at up to 200 psi from a hydrant and be using 700 to 850 ft-lbs. of torque. This is beyond any transmission’s or engine PTO’s continuous or even intermittent duty ratings. So, the questions departments need to ask if they are looking at PTO driving the pump on their next apparatus include the following: Will I be flowing more than the basic pump rating? If so, will I be using a hydrant that gives me sufficient pressure at my desired flow that keeps the power consumption below the PTO’s continuous-duty rating? How many times a year will I pump these big flows and for how long? How much additional performance can the pump I picked out provide, and can the PTO handle it?
We need to ask these questions because for decades we have bought apparatus that would easily outperform the NFPA pump rating. You can buy more than a few 2,000-gpm pumps with a 1,250-gpm rating that from hydrant, if you have the power, will deliver upward of 3,000 gpm. This is the kind of application best served by a split-driveline system. Because of this, pumps used for PTO drive applications are not 2,000-gpm pumps derated to 1,250 or 1,500 gpm. This helps limit some of the overpowering the PTO, but not all the time. The 2,000-gpm pump rated at 1,500 gpm has years of reserve performance to counter usage and wear-you will not have that on any of the 1,500-gpm PTO drive pumps. Another way to limit some potential overpowering situations is by setting up the pump to operate at the highest engine speed possible and still meet the NFPA tests. This limits some of the pump’s potential to outperform the PTO, and it also lowers the torque the pump consumes. The standard engineering formula for torque shows that as the speed goes up the torque will go down. The drawback is the PTO-driven applications normally operate at 200- to 400-rpm engine speed, faster than a split-driveline application.
There are about 90,000 split driveline fire pumps in service in North America. They are very reliable products. Most are designed to allow full engine power delivery continuously, if needed. In Europe, most pumps are PTO-driven, but pump sizes are also normally smaller. A 1,000-gpm pump is big by European standards; many apparatus are only the equivalent of a 500-gpm pump.
If the apparatus is not going to normally use the pump, except for a couple of handlines, then a PTO/REPTO is a good option. Today, the best place for the PTO pump drive is in rescue pumpers, interface apparatus, or tanker pumpers.
In North America, four pump manufacturers offer ranges of pump products that can be driven by an engine or a transmission PTO system to NFPA ratings from 500 to 1,250 or even 1,500 gpm and larger. All of them will get the ratings promised, and all can be matched with a PTO drive system of some kind. Three of these manufacturers offer a full range of split-driveline-driven pumps with or without full cast manifold systems attached with ratings as high at 3,000 gpm. All four pump producers want you to have the performance and reliability you deserve and need. The key is knowing your performance requirements, expectations, and potentials. This will allow you to make the best selection for your department with the help of the apparatus builders and pump suppliers.
GARY HANDWERK is the product engineering manager for Hale Products. He has been involved with the fire service industry for more than 39 years. He has been a member of the NFPA Fire Apparatus Standards Committee since 1991.