Low Pressure vs. High Pressure: Which Is Right for You?

By Steve Green

In the summer of 2015, the Fire Instructors Association of North Texas commissioned a study to evaluate the differences between low-pressure fog nozzles.

The purpose was to compare and contrast 50-, 75-, and 100-pound-per-square-inch (psi) fog nozzles at various gallon-per-minute (gpm) flows. The results of this study are summarized in this article. The merits and differences of smooth bore nozzles are not in debate and were not used during these tests. As always, your fireground results and observations may be different. Also, your practical experiences will differ depending on building construction, weather, and fire load. Base all your tactical decisions on near and expectant fire conditions.

The Scope and Mission

Numerous discussions determined that there is not a “set standard” for nozzle selection in the Metroplex (the region that comprises Dallas, Fort Worth, and Arlington, Texas). That can be said of most of the country, but in the South, there is a large usage of fog nozzles. That is not to say that we use fog nozzles only but that there is a proclivity to attach them to our most frequently used lines. In the Metroplex, it is common to see a dual crosslay with 1¾-inch hose varying in lengths from 150 to 250 feet with some type of fog nozzle. What has been of interest is the move toward lower pressure nozzles over the past five years. Numerous departments have made a decision to go to a 50- or 75-psi nozzle using roughly the same diameter line.

Along with this, there seems to be some debate among firefighters as to which setup is the most effective. So, this study put some cold facts on the table for firefighters to discuss. For the purpose of the study, we stayed consistent with our 1¾-inch attack line and used 200 feet. We thought this was a good compromise and the average for the departments in our region.

The group evaluated the nozzles based on the following:

  1. Reach and penetration.
  2. Hose kinks and deployment issues.
  3. Nozzle reaction and ease of use.
  4. Overall performance in suppression activities.

Again, this article does not cover the merits of smooth bore for reach and penetration, and we did not use them as a comparative factor.

As a setup for what follows, let us do a quick review. Our goal in the fire service has always remained the same: put the fire out. How you do it and what works best for you and your jurisdiction will always differ from others. How you best accomplish this usually depends on how well your people are trained, staffing, equipment, and the hard lessons learned by those who came before you. Whether you use smooth bore, fog, compressed-air-foam systems, positive pressure attack, etc., it all comes down to one simple truth: Do what works, and repeat it every time.

If you attempt to recreate what we have done here, be realistic. Do not base your results on four- to five-person staffing if your department does not use that. If you have two- to three-person engines, factor that into your evaluations. Otherwise, you are going to have some very unrealistic expectations for your crews on scene.

1 This portable monitor stand allowed evaluators to move nozzles around into different environmental conditions on different days. (Photos by author.)
1 This portable monitor stand allowed evaluators to move nozzles around into different environmental conditions on different days. (Photos by author.)

Initial Evolutions

To validate some prevailing theories, our group had a portable monitor stand built. This allowed us to move our nozzles around into different environmental conditions on different days. Some days the flows were done with 20-mile-per-hour crosswinds-not uncommon for our area at all. Other days we used a headwind and some no wind. We were evaluating the streams and nozzles for the following:

  1. Stream pattern at 25 to 30 feet.
  2. Stream pattern at 100 feet.
  3. Stream pattern at critical velocity.

We observed or concluded the following:

  • Stream pattern of 25 to 30 feet: There was no discernable difference in pattern or overall performance noted. This does include the windy days with headwinds and crosswinds.
  • Stream pattern of 100 feet: This is where we did notice differences in performance. The 100-psi nozzle had a significantly better pattern than the two lower pressure nozzles. It held together at the 100-foot breakover much better.
  • Stream pattern at critical velocity: Let’s review quickly what we mean when we say critical velocity and its importance. In IFSTA’s Essentials of Firefighting, critical velocity is described as the “practical limit at which water can be flown through a line.” If the velocity is increased beyond this point, the friction is so great that the water in the line will be agitated by that resistance. And, the stream coming out of the nozzle will be ineffective and fall apart. Also worth mentioning is those pressures will have reached the upper threshold of what a two-person crew can handle as the nozzle reaction (NR) will be tremendous. At critical velocity, we observed the 100-psi nozzle still “attempting” to maintain a manageable pattern. The 75-psi nozzle stream started to dramatically fall apart, and there was a visible difference in distance. The 50-psi nozzle was affected even greater than that. So at higher pressures, the effectiveness of these streams is more influenced using the 50- and 75-psi nozzles.
Test Fires and Study Group

Our next task was to pull together a study group to evaluate the nozzles in real time using actual fire. Rather than attempt to use film footage from fires or talk to crews using different setups, we settled on a series of test fires. To attempt to have more than 35 departments record observations in real time seemed impractical and also allowed for possible personal bias.

As we had done for a study in December 2014, we went back to the Johnson County (TX) ESD 1 Training Facility. This is a Class A fuel burn facility. We were able to control the amount of fire load to closely monitor each burn and regulate the conditions. While no fires are identical, these conditions were as close as we could get them.

2 At higher pressures, the effectiveness of these streams is more influenced using the 50- and 75-psi nozzles
2 At higher pressures, the effectiveness of these streams is more influenced using the 50- and 75-psi nozzles.

Our study group was actually the Weatherford College Fire Academy, Class 44. The reason we chose them is very simple-they had no preconceived notions about what is the best nozzle. Very few had any real applicable experience outside of the classroom, and very few had habits that they had formed when using attack lines. The members ranged from 21 to 45 years old and ranged from no experience to two years of experience as volunteers. Since the candidates had been schooled on basics and understood those expectations, it made it very easy to record observations.

I know what you are thinking now: “These are not ‘my people.’ ” No they are not, but this study is based on an underlying theme-the worst case scenario. If you have limited staffing with limited experience, welcome to today’s fire service. There is a huge turnover going on in our area that is only getting worse with the folks leaving who came here when the Metroplex exploded 25 to 30 years ago. As with anything, experience is a force multiplier. The fact is, we are running one half of the fires we were 20 years ago-down from two million to around 1.2 million. And with an aging population, our EMS calls are only going up. Train your people in fire science, tactics, and building construction. Do not let those retirees get away without sharing those lessons. We can’t afford it.

Burn Day

We performed multiple burns under each of the following parameters: 125-gpm flow, 150-gpm flow, and 175-gpm flow. We used a standard length of 200 feet of 1¾-inch hose for all and ensured multiple rotations at each pressure. After the evolutions were over, students recorded their observations and made comments on a form. We compiled all data after four days of actual live fire. We used a variety of scenarios: single-family, commercial, multi- and single-story, etc. We added all the compiled data and individual comments to the final draft. We had many comments on performance, ease of handling, and controllability.

Test Facts and Figures

It is notable that previous studies have discussed the effects of NR on effectiveness. Refer to “Firefighting Nozzle Reaction Parameters” by Paul Grimwood and the London Fire Brigade for more information. It can be found in the Fire Engineering Training Community Blog archives.

I have included the NR table for what we used in our test fires. As a review, here is the formula for nozzle reaction:

NR = 0.0505 x Q √NP

Q is the quantity (gpm). NP is nozzle pressure.

As you can see, the lower psi nozzles do have a pronounced difference in the amount of nozzle reaction. As noted in previous studies, the effectiveness of firefighters does start to decrease at nozzle reaction pressures of greater than 60 pounds. This would also be a factor for two- to three-person companies more than four- to five-person companies as well-what some friends of mine call “battle fatigue.”

Observations

Here is a summation of our work over the past seven to eight months. Note that operating hoselines on the interior of ranch-style homes in the South and Southwest is common. This does not mean fire departments in this region do not fight fires in high-rise or multifamily occupancies.

  1. We noted no appreciable differences in nozzle reach or penetration during single-story interior direct attack scenarios.
  2. There were multiple noticeable issues with hose kinks at 125 and 150 gpm using the 50-psi nozzle.
  3. On two-story interior attacks, there were noticeable differences in nozzle reach using the 50-psi nozzle.
  4. In exterior or transitional attack, there were no appreciable differences at close range (less than 20 feet). At greater than 25 to 30 feet, there were noticeable differences in nozzle reach and stream pattern regardless of environmental factors between the 50-psi nozzle and the 75- and 100-psi nozzles.
  5. In exterior testing, the 75- and 100-psi nozzles had superior range at all gpm settings. The stream effectiveness, pattern, and ability to hold together approaching critical velocity was very evident.
  6. Overall, it was an overwhelming decision of our test group that they preferred a 75-psi nozzle.

Author Comments

  1. If you are following the mantra of “cooling from a safe location,” and the area you are operating in is less than 30 × 30, it really does not matter what nozzle you are using. The reach and stream effectiveness is going to be the same across the board.
  2. 125 to 250 gpm: I don’t care what nozzle you use. If it’s big fire, use a big line. Do not use the same size line for every fire. That is a critical mistake for new firefighters-do not automatically use the same line you used on the last fire. Each fire is different.
  3. Hose kinks are everyone’s responsibility. If you walk by a hose, kick the kink out. If you are inside and see one while crawling, help a brother or sister out. Getting water to the fire is job one!
  4. Know how to throw water. We spend a lot of time on a variety of topics, but physically moving water out of a nozzle seems to be left out frequently. The problem, and this is a fire instructor speaking now, is our teaching new firefighters in live training fire to “just wet it down.” This is my fault-I am making an effort to stop it. Inconveniencing our instructors to build a fire backup is better than having a firefighter throw five to six seconds of water and assume it’s out in a real life-and-death situation. It’s called muscle memory for a reason. Let’s prepare our people to fight like we train-throw that water! Your people deserve realistic training, and they need to know how much water is realistic too.
  5. Every tailboard firefighter needs to do his own size-up on arrival. If you are relying on your company officer to tell you what line to pull, we need to train a little more. You should be able to size up the length of the potential stretch, look at the amount of fire, and anticipate how big it’s going to get before you arrive at the area of suppression. Learn building construction and fire behavior-that’s what will make the difference for you and your crew.
  6. Please do not buy cheap hose. I am not here to endorse any brand of nozzle or hose. I will say that if you are buying bargain hose, your results on the fireground over time will not be as efficient. Our testing did show that quality hose makes a difference.

Whatever nozzles you choose, any weapon is only as good as the hand that wields it. Take time to practice your technique. Our job is life safety, incident stabilization, and property conservation. Take it seriously and be safe. Strive for perfection, settle for excellence.

STEVE GREEN has more than 30 years of combined service in the fire/EMS service. He is a TCFP Master Firefighter, Instructor III-Master, former FTO, and former training officer. Green retired from the Southlake (TX) Fire Department and is an adjunct instructor for Tarrant County (TX) College Fire Service Training Center and is the fire CE coordinator for Weatherford College. He also volunteers for Parker County (TX) ESD1 and holds numerous SFFMA certifications.

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