When it comes to vehicle-mounted fire pumps in North America, the fire service buys 1,500-gpm rated pumps. Why do we do that?
A little history shows the U.S. fire service has purchased progressively bigger fire pumps, a trend that corresponds to an ongoing rise in available reasonably-priced truck engine power.
The defunct National Board of Fire Underwriters wrote the original vehicle-mounted fire pump rating system, which required 100 percent pump capacity at 120 psi, 50 percent at 200 psi and 33.3 percent at 250 psi. By the late 1930s, some apparatus were available with pumps capable of higher performance.
The rating system officially changed in 1947 when separate Class A and Class B ratings were established. Class B had the original requirements, and the new Class A system was based on delivering 100 percent pump capacity at 150 psi, 70 percent at 200 psi and 50 percent at 250 psi. This change came about because newer truck chassis had bigger, more powerful engines that allowed fire pumps to be operated at higher flows and higher pressures.
By 1957 Class B ratings were discontinued, and all pumps were rated at Class A levels, as they are today. Why? Because we had the engine power. Class B served as a transition. Records from Hale and American Fire Pump (Barton American) show the pumps did not change, even though the ratings changed. It was all about available engine power.
From the 1920s to the early 1940s, the market was dominated by 400- to 600-gpm Class B fire pumps. By the late 1940s and into the 1950s, the common pump size was Class A 500-gpm. Commercial truck engines were commonly 110 to 150 horsepower. By the 1960s into the early 1970s, the 750-gpm pump size was the big seller, and engine power jumped to 160 to 190 horsepower.
The 1970s saw the transition from gasoline engines to diesel and an ongoing growth of practical, reasonably priced available horsepower. In the early 1970s, a 350-hp diesel engine was the big engine in a custom chassis. The custom chassis could handle a 1,500-gpm pump while the commercial might have a maximum 1,000-gpm pump because it had a 220-hp gasoline engine.
By the end of the 1970s, the big engine in a custom chassis was 450 horsepower, and the commercial chassis had a 210- to 240-hp diesel. Guess what? The normal size fire pump also went up – from 750 gpm in the early 1970s to 1,000 gpm as the big seller, with 1,250 gpm growing in popularity.
The 1980s saw the 1,250-gpm pump become the norm. The size of the engine on a commercial chassis increased to 240 to 280 horsepower. In the mid-1990s, we saw the 300-plus-hp engine become the norm, and we saw a corresponding increase to 1,500-gpm rated pumps. For most of the last decade, the 1,500-gpm rating has been the big seller.
In addition to the rise in engine power availability, the way fire pumps and valves are priced and sold changed over the years and helped fire departments afford the larger pumps. Historically the pump came with 2 1/2-inch discharge valves, one for each 250 gpm of rating. The price of the pump went up with each 250 gpm of rating in addition to the added price of the extra 2 1/2-inch valves.
By the 1990s, all the pump manufacturers had pulled the discharge valves out of their base pricing and priced pumps by model family, not specific rating size. All pump manufacturers offer multiple National Fire Protection Association gpm ratings for each model pump, mostly at no change in price for the pump itself. For instance, a Waterous CSU or Hale Qmax has the same base pump price whether the pump is rated at 1,000 or 2,000 gpm. Why? Because the pump is the same, regardless of the rating.
The NFPA does require a valve count based on the pump rating. This outlet connection count is intended to allow an apparatus to supply water 1,000 to 1,200 feet at rated pump capacity down the road using outlet connections available without using excessive pressure, which would be unsafe. This system counts the first permanent outlet connection, 2 ½-inches or larger. Two 2 ½-inch connections are required, and any number of 2 ½-inch connections or larger which add up to the rating of the pump.
The outlet connections have a rating based on water velocity in the same size hose. A 2 ½-inch connection is rated at 250 gpm, a 3-inch connection is rated at 375 gpm, a 4-inch connection is 625 gpm and a 5-inch connection is 1,000 gpm. With large diameter hose outlet discharges, most departments have more than enough outlet connections to exceed the pump rating needed.
Another factor in the popularity of 1,500-gpm pumps is their ability to maximize water delivery while drafting from long-run or high-lift dry hydrants or drafting sites. As the flow is reduced, a fire pump’s ability to create vacuum at the impeller eye increases, allowing pumping performance from long- or high-lift conditions. A 1,500-gpm pump will outperform a pump that is only available up to a 1,250-gpm rating by about 150 to 250 gpm depending on pump models. A typical 1,500-gpm pump will do about 750 gpm at a 26-foot lift, maybe more depending on the hose and strainer being used.
Why Not 1,750-gpm?
If the increase in engine horsepower, pump price and outlet count alone drove the pump size, we could expect the market to already be migrating to 1,750-gpm pumps. It is not. Most 330-hp engines will easily power a 1,750-gpm pump. So why not?
The 1,750-gpm pump, in many cases, has a selling price for the base pump that is the same as a 1,500-gpm pump. I suspect there are two driving forces keeping most departments from buying a 1,750-gpm or larger pump. One is the need to use two sets of suction hose to get the performance while drafting and more importantly to do the yearly service test. Finding a pump-drafting site which easily adapts to a dual set of suction hose is difficult. A single 6-inch hose can handle no more than a 1,500-gpm rating test. A 1,750-gpm rating test requires a second set of hose. We have hit a plateau with the suction hose limit.
The other factor is many departments know they are really buying a 1,750- or 2,000-gpm pump even when it has only a 1,500-gpm rating, which allows for years of wear while continuing to pass the 1,500-gpm test. The fact is they have 1,750- or 2,000-gpm performance with or without the rating.
Today’s 1,500-gpm fire pumps are efficiently matched to the current chassis engines and provide a full performance range from a jump line to well over 2,000-gpm from a sufficient hydrant. At this time, it looks like we will see the 1,500-gpm pump continue to be the dominant size.
Editor’s Note: Gary Handwerk is engineering manager for Hale Products. He has been involved with the fire service industry for more than 38 years, working for various fire apparatus and pump manufacturers. He has been a member of the National Fire Protection Association (NFPA) Fire Apparatus Standards Committee for more than 18 years.