It’s important when supplying aerial master streams to understand the key factors involved, and understand the limitations of the hose and the aerial device you are using.
First, many departments opt to use 4" or 5" Large Diameter Hose (LDH) to handle all of their water supply needs. There are two general categories of this type of hose, supply or standard grade, which has a service pressure of 200 PSI and attack grade or high pressure which has a maximum service pressure of 300 PSI. Some brands may offer higher service pressures too. For the sake of safety, we always say to deduct 10% from the maximum service pressure of the hose. That means that if you are using supply grade, your max pump pressure is 180 PSI, and if you are using attack grade your max pump pressure is 270 PSI.
Aerials have inherently high pressure loss, as the waterway has some sharp bends in it to allow the piping to make its way up the center of the turntable. It’s not uncommon to see four 90 degree elbows. These bends and elbows represent extreme inefficiency, and will require higher pump pressures to overcome them.
Aerials also suffer from the effects of head pressure loss, as the weight of the water in the aerial exerts back pressure downward and requires higher pump pressures to overcome it. Elevation loss is .434 PSI per foot. It’s easier to represent that as .5 PSI per foot. This means that for every 10 feet of elevation you are losing 5 PSI due to head pressure loss.
It is hard to say exactly what the pressure loss in the piping of an aerial is, until you run actual flow tests. It is, however, safe to say that the loss is typically no less than 50 PSI, and in some cases as high as 100 PSI. Remember, pressure loss (due to flow of water) rises almost exponentially as flow increases, so an increase in flow of every 500 GPM doesn’t mean the pressure loss value is an equal value for each flow increase. For example, the pressure loss in 5" hose for 500 GPM is 1.7, for 1000 GPM it is 6.0, for 1500 GPM it is 15 and for 2000 GPM it is 27. So you can see that the difference in pressure loss from 500 GPM to 1000 GPM was only 4.3, but the difference between 1500 GPM and 2000 GPM was 12.
The last issue at hand is the nozzle. Remember, a nozzle represents restriction. The higher the nozzle pressure must be, the more restriction/backpressure and the harder your pump must work to overcome it. Using a smooth bore tip on an aerial monitor will lower your supply pump pressure 20 PSI compared to using a 100 PSI automatic nozzle, because the smooth bore only requires 80 PSI at the tip for its "rated flow". So, for an equal flow the smooth bore requires a lower pump pressure. Don’t forget that all the other restrictions in flow in the aerial and the elevation are still in play though.
What if the aerial has a pump? Whenever possible, supplying an aerial that is equipped with a pump, it is hydraulically sensible to use the pump to boost the incoming pressure so that you can overcome the restrictions in the rig itself. The one instance where this is not desirable is if the aerial rig is a Quint and it is operating handlines at the same time. The volume of water and required pressure flowing for the aerial master stream, coupled with handlines being used can cause the risk of overpressurizing the handlines if the master stream is shut off and the pressure governor or relief valve isn’t capable of managing the extra water surge. It is recommended that if the aerial is operating handlines off its pump, to use a separate supply pumper to feed the aerial directly through its rear inlet.
Let’s look at the factors that come into play again, using the pump discharge formula;
PDP=NP+FL+DL+EL
Where;
- PDP=Pump discharge pressure
- NP= Aerial nozzle TIP pressure – 80 PSI for Smoot Bore, 80 or 100 for Fog, check tip for exact pressure
- FL=Friction/pressure loss in the HOSE
- DL=Device loss, in the aerial piping-unknown exact value unless tested
- EL=Elevation loss - .5 PSI per foot
Using some numbers taken from an actual aerial flow operation I recently participated in, we were able to prove this theory. A 100 foot aerial tower, with a 100 PSI automatic nozzle was only able to flow 900 GPM through two 100 foot lengths of supply grade 5” hose. The supply pumper was operating at 185 PSI. The aerial was only off the ground approximately 25 feet, for most of the operation. We were able to deduct the pressure loss in the aerial piping at around 75 PSI, because the nozzle pressure was 100 PSI and the pressure loss in the hose was about 10 PSI. With a pump pressure of 185, subtracting the nozzle pressure (100) and the pressure loss in the hose (5 PSI per 100ft, for a total of about 10 PSI) the remaining 75 PSI represents pressure loss in the aerial waterway. The waterway on this particular rig is capable of 1500 GPM. The hose limited us to 900 GPM. The investment into better hose or use of multiple 3” lines would allow maximum water flow in this particular evolution.
So what does all this mean? Using standard grade supply hose is not the best choice for supplying an aerial master stream unless you are supplying the rig through its pump, if it is so equipped. 3” or 3 ½” hose is a good option if you still carry it, but the better long term solution is to switch over to attack grade LDH. If your budget is limited, having a few lengths of attack grade supply hose on the aerial rig is a good interim solution. Make sure your apparatus operators know that they need to use this hose when the rig is being supplied through its rear inlet.
The bottom line is that if you want to flow an aerial at its maximum capable flow, you must forecast the approximate required pump pressure ahead of time and have hose that is capable of delivering the required pressure at the inlet, to get the desired flow out the tip. To break it down into simple terms, pump through the shortest required length of hose, and use the highest pressure rated hose possible to supply aerials. If your LDH is supply grade, assure you have an appliance to allow at least three 3” or 3 ½” lines to be used to feed the inlet of the aerial.
If you only carry supply grade 5" Hose, (rated to 200 PSI), consider supplying aerials with no pump using multiple 3" lines so you can provide the proper pressure to the inlet. In this instance, each 3" line is only flowing about 350 GPM, as the tip flow is around 1100. Because this hose is rated to flow at pressures up to 400 PSI, it makes it a no brainer to supply this aerial. Without knowing the pressure loss in this rig, we know that at an elevation of 100 feet, the head pressure loss is 50 PSI, the nozzle pressure is 80 PSI and the pressure loss in the hoselines is 10 PSI. That is a pump pressure of 140 PSI, without factoring in the aerial piping pressure loss, which is often at least 50 PSI. The potential pressure for this aerial to flow 1100 GPM is AT LEAST 190 PSI if it is 100 feet away from the supply pumper using three 3" lines.
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| This rig has 1100 feet of 5" in each bed, the left bed is supply grade hose the right bed is attack grade hose. |
If youy only carry supply grade 5" Hose, (rated to 200 PSI), consider supplying aerials with no pump using multiple 3" lines so you can provide the proper pressure to the inlet. In this instance, each 3" line is only flowing about 350 GPM, as the tip flow is around 1100. Because this hose is rated to flow at pressures up to 400 PSI, it makes it a no brainer to supply this aerial. Without knowing the pressure loss in this rig, we know that at an elevation of 100 feet, the head pressure loss is 50 PSI, the nozzle pressure is 80 PSI and the pressure loss in the hoselines is 10 PSI. That is a pump pressure of 140 PSI, without factoring in the aerial piping pressure loss, which is often at least 50 PSI. The potential pressure for this aerial to flow 1100 GPM is AT LEAST 190 PSI if it is 100 feet away from the supply pumper using three 3" lines.
Because supply grade 5" hose was used to feed this tower, a maximum flow of around only 900 GPM was available, even though the waterway can flow up to 1500 GPM. This was only a 200ft lay. The supply pumper was providing 185 PSI at its discharge. Doing the math, we can deduct that the pressure loss int he hose is about 10 PSI, at the flow of 100 GPM, and the nozzle is 100 PSI at the tip. That means the pressure loss in the aerial is about 75 PSI.
Using 2 100ft lengths of standard supply grade 5" hose, we could not provide more than 900 GPM to this aerial. It has a 1500 GPM waterway and the hose was maxed out at 185 PSI, at low elevation.
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| This 1250 GPM single stage pumper easily supplied a 1400 GPM tower ladder which is not equipped with a pump. The tower is equipped with stacked tips and is being fed utilizing attack grade supply hose. In addition, the rig flows 500 GPM from its wagon pipe, for a total of 1900 GPM |
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| Pump pressures on the panel of the 1250 GPM engine show high loss between the master discharge and the line gauge for the 3" LDH discharge. Specing larger valves will improve efficiency |
The summary of all this is that if all you care about is having water come out the nozzle tip, then carry on as you were, if you want to deliver the aerials capacity, or understand how to achieve that, you must conduct flow tests, then be equipped with the proper hose, nozzle and appliances.
