BY DOUGLAS PIETZ
In the September 2018 issue of Fire Apparatus & Emergency Equipment, I introduced the idea of pump theory driving the pumper design. I believe that pump theory should guide the decisions in the prebuild and spec process as well as having an operator who understands it as part of the committee. I hope to enable you to look at your environment for fire protection resources and ensure you are building the apparatus that best fit those needs.
THE FIRE PUMP
First, the fire pump. There are numerous factors that go into the decision for which pump to put in the apparatus. Many manufacturers have an option for a compact style pump that uses a limited amount of space. These pump choices are an addition to an extensive list of pumps available from various pump manufacturers. Hopefully, this article will give your committee insight into which one fits your needs best, but that, again, means taking a look at pump theory.
The centrifugal pump can move large quantities of water at low pressure or low quantities at high pressure. A 2,250-gallon-per-minute (gpm) pump will move lots of water if paired with a 450- or 500-horsepower engine. This would be a great choice if you need one engine to supply an aerial platform with dual monitors. The weakest link in this system would be the ability to supply the engine with water. This requires a strong hydrant or a very efficient tender operation. Another issue with using one engine to work this hard is fuel consumption. This engine will be able to complete the task, but without a fuel truck available for long-duration greater alarms, using two engines to share the work will prolong the need for fuel.
Most aerials only have a single waterway, and one engine with a 1,500-gpm pump can supply that tower easily. But, there may be a dealer demo platform that is marked down to your price range that is too good of a deal to pass up. Or quite possibly, your department already has one, but supply is an issue. Ensuring that the engine you are purchasing will handle the workload of the aerial is important.
The other instance where it is relevant to have a larger pump and engine combination is for high-rise firefighting. Most new-construction high-rise buildings have fire pumps or boosters that will aid in supplying adequate pressure to higher floors. There are still many standpipes that are dry, which can tax a single supply engine. High-rise tactics incorporate the use of smooth bore nozzles for reach and low-pressure operation, but they can come at the cost of gpm. Multiple lines on multiple floors can overwhelm a single 1,500-gpm pump, especially when having to overcome gravitational loss. A larger pump in this situation will make easier work of pumping to higher floors. One can argue that a second 1,500-gpm pump in tandem pump mode will aid the first in meeting the pressure and volume requirements, which is correct. However, the second attack engine not only is needed to assist the first but also should be considered the backup engine if the other engine in the system has a mechanical or electrical system failure. Either engine should be capable of sustaining the operation, if possible, to provide the highest degree of redundant safety for firefighters working in an extremely hazardous situation. Thus, evaluate the worst-case scenario in a structure your department protects and ensure you can meet the required flows at the pressures desired.
I realize that high-rise pumping operations pertain to only a select number of departments across the country. But, does pump sizing matter as much for rural departments as well? Arguably, yes. Pump ratings are when the pump is working at a draft. From a hydrant, that is work that the pump does not have to do. But when drafting, pump size matters; more importantly, intake size matters too.
The priming system on a pump creates a negative pressure in relation to atmospheric pressure, which causes the atmosphere to “force” water into the pump. Without getting too deep into math class, the maximum lift is 33.9 feet. At this height, the atmosphere is not “forcing” water into the pump anymore. This height is based on a perfect vacuum under ideal conditions, none of which exist in the real world. So, as a general rule, maximum actual lift is around 25 feet. This is because the centrifugal pump is not completely sealed. By this, I mean that the vanes are not tight to the housing, constantly forcing water to the discharge volute. This is great by design as the gap between the vanes and the housing allows the water to be held at pressure, waiting for use when a discharge is opened. This benefit, however, means a loss in vacuum efficiency.
Drafting also plays by the pressure-volume dichotomy of normal operations. The higher the pressure, the less volume one can draft. The majority of the pump work is done to increase the pressure rather than pulling water in. Likewise, a fire pump can pull lots of volume, but it will be at a low pressure. The limiting agents in these systems are the intake sizes and numbers.
A 1,500-gpm pump is rated at a 10-foot lift with a six-inch intake hose. At 20 feet of lift, the gpm is reduced to about 800 gpm and at 24 feet to around 600 gpm. Adding a second six-inch intake hose will increase the gpm of the pump at 150 pounds per square inch (psi) to 1,900 gpm. And at 24 feet, it will add almost 250 gpm, for a total above 800 gpm at 24 feet of draft. Remember, this is on a 1,500-gpm pumper. A 2,250-gpm pumper will produce more gpm at 150 psi.
The most efficient use of pumps today is to have water enter from both sides of the pump, giving access to both sides of the impeller’s eye. This will mean adding additional suction tubes to the apparatus from the opposite side to accomplish the task. The horizontal run of hard suction hose is not governed by lift but rather friction loss and is directly related to the gpm flowing. Thus, try to keep the hard suction runs as short as possible in the horizontal plane as gpm will be lost, affecting the total volume produced.
Much like the high-rise firefighters, rural departments also need to do homework. Evaluate likely water sources and the ability to access them. Establish a worst-case scenario and plan to create an apparatus specification that meets that need. Having an adequate amount of hard suction hose will be a challenge. But if it is available, ensure that your pump can overcome the lift needed or the friction loss present to ensure an adequate water supply.
Evaluate the performance curves of the pumps you might like to use and have conversations with the manufacturer. Ensure that the pump you choose works with the manufacturer’s design requirements. There are numerous style pumps for virtually any application you can think of. Ensure the right one is chosen for your build and your community.
I have spent my career in a large city looking for hydrants at any given fire. Everyone needs to remember that communities are located at bodies of water—especially large cities. Don’t overlook the water potential of natural water in your community. Use pump theory, practice, and know the capabilities of your apparatus.
DOUGLAS PIETZ is a lieutenant with the Milwaukee (WI) Fire Department. He is the vehicle operations training coordinator for the department.