August 2007

Pump Talk

by Gary Handwerk

High-Rise Buildings Require Big Pumps

Many major cities are planning new high-rise buildings. Each one seems to be higher than the next. Because of the horrors of the attacks of September 11, many city departments are searching for the best tools to combat unthinkable fires in these buildings.

Buildings that are 10 to 20 stories can easily be handled with garden variety city pumpers with a typical single-stage pump. For the purposes of this article, we’re talking about buildings in excess of 50 stories.

The fact is, to fight a high-rise fire, you need sufficient water at usable pressures on the top floor of the building. You cannot count on the building systems jockey pumps – the ones connected to the fire sprinkler system, to maintain sufficient pressure. The jockey pumps may not work because the power is out, or they may not supply sufficient water for the fire load, or they may be damaged.

It’s ideal if the fire department can be involved with the original planning for the building, but if that’s not possible, department officials ought to do a pre-plan of all high-rise buildings in their district so they’ll know what to do should disaster strike.

In this pre-planning stage, there is data to collect and calculations to do. First, identify the department’s standard operating procedures (SOPs) and determine what hose packs and nozzles will be put in place on the top floor. Determine the flow rates from each line and how many lines will be deployed simultaneously and at what pressure.

That information, plus the length and size of the hose, will determine the flow rate and provide the information to calculate the pressure. If your department has no SOPs to govern high-rise responses, assume a minimum flow of at least 300 gpm – 500 gpm has become more common. The formula for calculating the requirement is: length in feet multiplied by the width in feet divided by three. From this information, departments can calculate pressure at the flow required at the top of the building – that is the worst hose line and the highest nozzle pressure required.

You will need to know how high the building is in feet. If you take that height and divide it by 2.31, that will equal the loss due to elevation. Another important factor, one that’s a little more difficult to determine, is the friction loss for the building’s standpipe system. Building officials in city hall may be able to find it for you, but if you have no luck, at least find out what the pipe riser size is and use a hose loss chart to find an estimate of the friction loss and add a small factor of five percent for safety. You should also assume, and calculate for, hose being deployed in the stairways.

By adding the pressure needed at the top of the building, plus the amount lost because of the height of the building plus the friction loss, plus the amount of pressure needed for hose deployed in stairways, that will equal the pressure needed at the determined flow at the base of the building.

To that number, you need to factor in the amount of pressure needed between the building and the apparatus and that is determined by the size of the hose and the length. When that number has been figured, add that to the pressure needed at the base of the building.

At this point, a couple of questions must be asked. Can the piping in the building and the connecting supply hose handle the required pressure? Can the hose deployed in the stairways handle the pressure?

Available Water Supply

When all the calculating is done, you’ll know what the flow and pressure needs to be at the apparatus.

Next, it is important to determine available water supply. To determine this, you will need to have the following information: the hydrants used and what pressure and flows you can, realistically, expect from these hydrants. You’ll need to determine what size hose and length will connect the hydrant to the apparatus and calculate its friction loss. To determine the pressure delivered to the apparatus you’ll need hydrant pressure minus the supply hose friction loss. That will give you the pressure at the apparatus.

Then, you’ll need to determine the net pump pressure required at the needed flow. To do this, take the required output pump pressure and subtract the incoming hydrant pressure and that will give you the net pump pressure.

At this point, you have two options, one is to call your favorite pump manufacturer and let them do the work or second, do the calculations yourself. Either way, you must decide on the chassis and engine make or model you plan to use.

Are we good so far? Well, here is the rest of the story.

You’ll need to figure out if you need a single-stage or a two-stage pump. You’ll also have to consider whether a third or fourth stage needs to be added to the two-stage pump. Then, there’s the question about the apparatus being big enough to deliver the kind of water needed or are two trucks in order?

A Few Guidelines

Assuming the engine power of a single apparatus is sufficient, here are a few guidelines to help in the decision process.

If less than 300 psi net pressure is needed at a flow of less than 1,000 gpm and no more than 600 psi output pressure, a single-stage pump should handle the requirement.

If you need no more than 500 psi net pressure, less than 500 gpm and no more than 600 psi output pressure, a standard two-stage pump should do the trick.

If you need no more than 550 psi net pressure, less than 1,000 gpm and no more than 600 psi output pressure, you can use two apparatus with standard single-stage pumps.

If you need no more than 600 psi net pressure, less than 500 gpm and no more than 800 psi output pressure, you can use a special high-pressure rated two-stage pump or a two-stage pump and an added 3rd or 4th auxiliary stage system.

If you need no more than 700 psi net pressure, more than 500 gpm and no more than 800 psi output pressure, you will need the 3rd or 4th stages or two apparatus.

Engine horsepower will be a big limitation. In some cases, two apparatus in series may be your only choice.

Supply hose or stand pipe pressure limitations can also restrict your application.

Remember that these are guidelines and each applications must be looked at specifically and individual calculations done.

If you are planning to do the calculations yourself, be aware that it gets even more complicated now. But, for those souls bent on doing even more complicated math, here are the steps.

First, you’ll need the power curve of the apparatus engine and the performance curve of the specific model fire pump in the apparatus. You will also need to know the pump gear ratio.

Look on the pump performance curve at the net pump pressure line and find the point on that line that you find the required flow. This will allow you to read the pump horsepower needed and the required impeller speed measured in revolutions per minute (RPM). Take the impeller RPM and divide it by the pump gear ratio and you now have the engine RPM.

Now look at the engine curve, follow the required RPM line up to the horsepower curve and over to read the horsepower available.

To find out if you have enough power, take the engine available power and subtract 10 percent off of it. Now take the pump required horsepower and add 10 percent to it. Compare the two, and if you have at least 10 hp reserve on the engine, you have a good application.

What you see is a complicated process. Your department’s SOPs, what type of apparatus you buy and what your shops can efficiently service will affect which answer to your high-rise problems makes the most sense.

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