Editor’s Note: This is the first of a three-part series on fire apparatus water delivery system design. The first part covers the water’s route from the fire pump volute outlet through the pump discharge manifold, apparatus plumbing, discharge valve and fire hose to the nozzle or other fire appliance. The second part will cover the suction inlet side of the system, and the third will look at the pump drive system.What is a pumper? It’s a delivery system that transports equipment and a fire crew to the fire scene, but what it really does is put wet stuff on the red stuff. The water delivery system in most apparatus being purchased are specified by the chief, based on past experiences, a sales representative or someone at the apparatus builder, usually with input from each of these people.
The objective should be to design the fire pump system output for the flow and pressure required at the final outlet while operating the pump system at the lowest overall operating pressure.
Low system operating pressure allows the apparatus to operate at a lower engine rpm, because lower pump pressure requires less engine speed. This means less noise, and a lower overall pressure is safer and causes less wear and tear on the pump and engine system. The process also guarantees the performance capability of the apparatus at the end of the hose.
In the discharge system design process analysis, we need to determine the outlet performance required at each outlet connection that is not preconnected; at the outlet of preconnected hose lines; and at the outlet of fixed appliances, like deck guns. The first place for this information is the specific fire department’s standard operating procedures and the model and operating pressure setting of the department’s nozzles and appliances. On any discharge where a department’s SOPs do not dictate a required outlet flow and pressure, the accompanying chart sets commonly expected performances from most discharge setups.
If a special outlet performance application is required, such as a high-rise stand-pipe feed or an apartment feed line, select the required flow and calculate the pressure loss. All the pump and nozzle manufacturers have cards, charts or calculators of some kind to help in this process.
What this process achieves is assigning a required flow and pressure to each apparatus pump discharge outlet. This is the time to look at all the outlets and decide which will be operated simultaneously. If possible, keep all the outlets used simultaneously operating at approximately the same pressure. This allows the operator to keep the master pressure set as low as possible.
When looking at the various outlets that are going to be used with a nozzle, keep in mind that as the hose pressure decreases, the hose will kink easier, but as the tip pressure goes down so does the reaction force the nozzle person has to handle. On outlets with preconnected hose, this is a good time to consider adding a marker arrow on the individual line gauge indicating the required operating pressure for that individual line.
Once this performance is identified it is time to look at the plumbing and manifolding on the discharge side of the pump system.
To minimize pressure loss for a given flow in a manifold and plumbing system, a few basic guidelines:
- Try to avoid changing the water direction. If you have to, change it as little as possible and keep the passageway larger than normal. This will slow the water down, which lowers the pressure loss and turbulence caused by the turns.
- Avoid sharp edges, blobs of welding and uneven transitions between different sections of the waterway, all of which lead to turbulence.
- Use full-flow valves with full-size ends.
- As a general rule, a 3inch opening in a manifold or a 3 inch pipe is good for flows up to 1,000 gpm, and a four-inch opening in a manifold or a 4 inch pipe is good for flows up to 1,500 gpm.
- Do not try to tie too many discharges to one pipe or manifold opening. If more than one outlet is fed by a single pipe or opening, add up the individual outlet flows and follow the guideline above for 3 and 4 inch openings.
- Ask the question. Pump manufacturers know the loss involved at each discharge opening for a given flow.
To do the flow test, attach the extension to the outlet, open the discharge valve fully, attach test hose and monitor to the valve end of the extension pipe and gate the extension pipe valve. While flowing the required gpm, read the two master pressure gauges and subtract the extension pressure from the pump master pressure to get the loss at this outlet.
Why should you do all of this? So you can specify the performance you expect at each discharge and maximize the performance of your next pumper.
Editor’s Note: Gary Handwerk is global pump product manager for Hale Products. He has been involved with the fire service industry for more than 36 years, working for various fire apparatus and pump manufacturers. He has been a member of the National Fire Protection Association (NFPA) Fire Apparatus Standards Committee for more than 15 years.