June 2007

Getting The Most Out Of Skid Units
by Gary Handwerk

Over the last few years, we have been seeing more and more flatbed-style trucks equipped with portable pumps, tanks and tool boxes entering the U.S. fire apparatus fleet.

The fact is many of these apparatus equipped with skid units are being used for more than wildland fires. Their missions include grass fires, auto accidents, medical response, trash fires and even initial attack on structural fires.

This is all well and good if the apparatus is designed for all these jobs. The problem is most are not, especially for structural fire fighting, and this is dangerous.

It’s easy to figure out how we got to this state of affairs. First, these apparatus have a tough, get-it-done look. They are easy to drive, even with limited experience. All-wheel drive is readily available without sacrificing drivability and maneuverability. And, of course, they are priced right.

The problem is several concepts have been hammered into firefighters’ heads, but not everyone has all the details down correctly. First, water in the form of gallons-per-minute puts out fires, not pressure. While this is a secure principal, pressure is still needed to deliver the water to the fire.

The nozzle design, hose size, hose length, along with elevation changes, require pressure to push the water through it all to get to the fire.

We also talk a lot about using standard preconnected lines of a specific size, but we do a poor job of teaching how to size up a fire or how different nozzle designs affect operations or how to maximize the available gpm and pressure.

And, depending on which side of the table you’re on, too many of us sell and buy fire apparatus based on money available, not the jobs it will be required to do or the fire loads it will encounter.

Lastly, we’ve come to understand that a full-size pumper with a 1,250 gpm pump means the apparatus has the engine power and pump design to do at least 1,250 gpm at 150 psi from a stringent suction condition and continue deliver 625 gpm all the way up to 250 psi. The fire service has embraced this specification as a generic 1,250 gpm pumper, and rightly so, but without understanding what it all means.

The problem comes when trying to transfer that generic terminology and knowledge to portable pumps on flatbed apparatus. The regulations are not as clear cut for portable pumps. For instance a given portable pump, rated at 300 gpm may only deliver its capacity at 50 psi and that won’t push 300 gpm very far down the hose. Firefighters expect the same kind sustained performance found on their full-size pumper through a range of pressure.

These misperceptions culminate in many departments buying a pump powered by an 18 hp engine, rated at 300 gpm, for $4,800 and later realizing they can’t operate two 1.75-inch preconnected hand lines and fight a structure fire. The reality is, to do that kind of fire fighting, they’d need a pump driven by a 60 hp engine mated to a 300 gpm pump at a price more like $14,000. It’s a basic principal that you need horsepower to get maximum gallons per minute at higher pressure. Unfortunately, it takes money to buy horsepower.

Theoretical Horsepower
There’s a formula to help determine the theoretical horsepower for water flows. It is horsepower equals gallons per minute times pressure, divided by 1,714 (HP=GPM x PSI/1714). This formula assumes 100 percent efficiency and a flooded suction line. Pump efficiency varies based on design, suction conditions and whether one is using the accrual tested efficiency or the non-fire industry commonly accepted textbook calculated efficiency.

It’s worth noting there is nothing wrong with buying an18 hp pump if it satisfies the fire load it will be expected to handle. Keep in mind, I do not advocate buying a big pump just for bragging rights, but I am passionate about safely doing all the jobs the apparatus is intended to do.

To make the correct pump size selection for your department, first start with the regulations required by the city and state along with any governmental agency, such as the United States Department of Agriculture (USDA). You should also be aware if your state has any requirements for an Insurance Service Office (ISO) rating of 8 or 9 or if it requires compliance with National Fire Protection Association regulations.

Outlet Capabilities
We have been talking primarily about flatbed slip-on apparatus, which most commonly are built on a F550 Ford chassis, so there is a limit to size and weight that can be allotted to the pump/engine package.

Due to the size and weight of an engine capable of driving a 750 gpm NFPA 1901 pump, this can normally be ruled out of the selection list unless water is expendable or you move up to a bigger chassis. While the bigger pumps are typically not on the table when talking about these kind of apparatus, I have, however, seen several successful apparatus built on F550 Fords using a 60 hp engine pump package.

Once local requirements have been satisfied, it’s a good idea to look at nationally recognized practices. There are two that should be considered for this class of apparatus. The first is a flow rate of 125 gpm, as a minimum, for vehicle fires and having 300 gpm for interior structural fire hand lines.

No mater whether you select a regulation size pump or some thing different, I recommend doing an analysis of your outlet performance capabilities of each line.

To do this, look at each line independently as follows: determine the output flow required for the line. This could be based on the nationally recognized norms or determined by sizing the potential fire loads or by department standard operating guidelines; determine the pressure required at the end of the hose or monitor. That pressure is commonly based on the nozzle you choose. Not all nozzles are set at 100 psi, as some are set up for 70 or 75 psi all the way down to 50 psi. Most nozzles can be ordered with one of several design pressures. Some nozzles can be converted to operate at a lower pressure. Straight tip nozzle performance is normally rated at 50psi.

Losses In Hose Lay
Departments should also determine the loss in the hose lay for the required flow, as hose size goes up, losses will drop for a given flow – as hose length increases for a given flow, the losses will go up.

It will also be important to look at internal apparatus piping losses. Here, you will need to estimate a little. The rules are, if you are connected directly to a valve on a discharge manifold, losses will typically only be 1 to 2 psi. If the hose connection is remote piped on the truck, but the distance is short, a few feet, the losses will be less than 10 psi. If you want to minimize the losses, specify an 1.5-inch valve and piping for a 1-inch hose reel and 2-inch valve and piping on a 1.75-inch hose line.

Now look at any elevation changes you can anticipate. A good estimate is you will lose a half pound psi for each foot of elevation. For those who want to be perfectly accurate, the actual conversion is 1 psi = 2.31 feet of elevation.

To achieve the desired flow, add nozzle pressure plus hose loss, plus piping loss, plus loss due to elevation and that will equal the pump pressure required.

Add together any line which must operate simultaneously and that sum will equal the pump performance required for this combination.

Look at the combined line usage and your single line requirements and compare them to various pump manufactures’ data to make final pump selection decisions. Keep in mind that most pump companies will do this for departments and so will many of the apparatus builders.

There is another thing to consider when making decisions. Not all portable pump manufactures do their performance curves/charts the same way. Some are done from draft while others are done from a flooded suction. If drafting performance is important to the department, look at the data carefully.

If you select a big pump/engine package, in the 60 hp range, and are planning to operate two 1.75-inch hand lines at the same time, a discharge pressure control system meeting NFPA 1906 ought to be strongly considered. Since most of these small engines may not adapt easily to a governor, a relief valve system will likely be the best answer.

What’s Selling Today
Some of you might be wondering how does all this stack up against what’s being sold in the fire service today. Well, the biggest selling unit now is the 18 hp pump, which is good for ISO 9, NFPA 1906 and the National Wildfire Coordinating Group (NWCG) 4 requirements. Departments with that size pump are ready for wildland fires (but limited in long relays), normal small car fires, trash fires, and exterior structural fire support.

If a department wants true structural initial attack capabilities, consider giving up 60 gallons of water on the apparatus, or some loose equipment to accommodate the weight of the bigger engine, and find some extra funds to buy the big 60 hp diesel engine/pump package.

That will give the department the capability of meeting or exceeding ISO 8 and 9, NFPA 1906, NWCG 3 and get credit for an NFPA 1901 initial attack unit.

Editor’s Note: Gary Handwerk has been involved with the fire service industry for 36 years working for various fire apparatus or fire pump manufacturers. He is currently global pump product manager for Hale Products and has been a member of the National Fire Protection Association (NFPA) Fire Apparatus Standards Committee for 15 years.