By Gary Handwerk
In “The Ins and Outs of Fire Pumps: Intakes” (March 2016), we looked at the “ins” of fire pumps. This month, we will look at the “outs,” the discharge side of the pump system. This includes the pump body, the attached manifolds, piping, and valves.
Meeting the minimum National Fire Protection Association (NFPA) 1901, Standard for Automotive Fire Apparatus, or NFPA 1906, Standard for Wildland Fire Apparatus, performance does not guarantee optimum pump performance. Actually, it is somewhat easy to meet the NFPA performance requirements with almost any discharge combination. Not only is there performance to gain by optimizing your discharge manifold, piping, and valves, but there are noise and safety aspects that can be improved on. When pumping, there must be enough pressure supplied by the pump to overcome the discharge manifold, piping, and valve losses along with the discharge hose friction loss and any elevation changes to provide the correct pressure at the nozzle while supplying the required flow. Based on that, if we need to create more pressure to overcome the losses, that equals more core pump operating pressure, which leads to higher engine operating speed. More pressure needed generates more noise from the apparatus engine, which is not a good thing on the fireground-especially when the noisiest part of any apparatus is the engine. Additionally, operating the pump system at an overall lower pressure makes controlling everything easier and safer. In the past, we relied on using engine speed and power to create additional pressure to overcome manifold and piping losses.
Obtaining the maximum available performance is critical on high-flow applications such as industrial fires, where flowing through the pump is important. It is also important at the extreme opposite end of the market on slip-on wildland/grass apparatus, where the engine driving the pump is very small with limited power to overcome added discharge side pressure losses.
As a pump designer, I look at the velocity of the water traveling in the pipe, waterway, or hose as a reference point in any evaluation of this type. The water speed is commonly measured in feet per second. To calculate this, I use the following formula: Velocity in feet/second = (0.32 × gpm)/the area of the waterway in square inches.
There are two basic types of discharges, one where the outlet is feeding a hoseline-this can be a preconnect for directly fighting fires or a feeder/supply line-and a hard-piped, directly connected device, such as monitor.
NFPA 1901 has stipulated that the safest optimum velocity, while keeping the hose losses workable, in a given discharge hose is 16.33 feet/second. So, the discharge piping and valve must accommodate this velocity at sufficient working pressure. While doing this, we still need to keep the losses to a minimum.
Part of NFPA 1901 requires two 2½-inch discharges and enough additional 2½-inch or larger hoseline connections, based on this 16.33 feet/second velocity, to equal the pump’s rated capacity. The requirement is for the first fixed hose connection only, and there are no standards for the manifold, piping, or valves feeding these hose connections. Hose-to-hose connection adapters are not counted. NFPA 1901 doesn’t require a 2½-inch outlet connection for every 250-gallon-per-minute (gpm) increment of the pump rating. A 1,500-gpm pump can be rated with two 2½-inch connections and one five-inch connection.
The second discharge type is not dictated by any specific standard, but anecdotal evidence does give some guidance that has calculated it to a velocity of about 45 feet/second for very direct short-coupled outlets, like the top of a pump monitor connection. A more practicable velocity, if piping and elbows are involved, is somewhere around 23 feet/second.
There are rules to follow to achieve these performance levels without creating excessive pressure loss through the system. This is especially true for the direct-piped discharges (45 feet/second).
The following rules apply:
- The transition from one inside diameter to another must be made with a concentric-type adapter for either increasing or decreasing changes. Never create an abrupt change in size.
- Avoid using a square cross section manifold-the corners will never see the correct velocities and can cause added flow losses or turbulence.
- Weld seams and pipe joints must be aligned and smooth. Ridges and edges will cause turbulence and disrupt the laminar flow of the water.
- Elbows are your enemy. Use as few as possible. Increase pipe inside diameter when practical.
- 45 feet/second is the maximum output velocity acceptable and should normally only be applied to direct-mounted very short discharge connections, similar to a top-mounted pump monitor discharge, and will limit the maximum potential output or require more main pump pressure.
- On a fully manifolded pump, do not connect more discharges to a pump port than Chart 1 indicates. Add all discharge outputs that will be fed by the pump body port to determine if the port can support the discharges if all are activated simultaneously. On a three-inch port, never exceed a total flow of 1,000 gpm. On a four-inch port, do not exceed 1,800-gpm total flow. Do not assume that extension add-on manifolds bolted to the main pump body have the flow loss characteristics of a port that is directly off the main body. Added piping to a port will add losses.
- On all of the calculations, we have assumed that we are dealing with a full flow condition. When looking at a valve system greater than four inches, we do not have a full flow situation, and that changes everything. Assume a wafer valve (butterfly valve) to be the equivalent of the next size down from its nominal size.
- Non full flow conditions are more common than you may realize. There are 3.5-inch valves that are really three- or 31⁄16-inch inside diameter ball valves. There are also pump body ports that are not what they seem to be and do not efficiently provide 1,000 gpm from a three-inch port or 1,800 gpm from a four-inch port without having a very high pressure loss.
- Check with the fire pump manufacturer to determine the output of each main body port and the loss in pressure at that rated flow. If that information is not forthcoming, do not buy that pump. The loss through any port at the rated performance should not be greater than 10 pounds per square inch (psi). The lower the loss, the better.
- The loss in a discharge outlet can be easily measured. The procedure is:
- Make a test setup with a female hose thread connection with a two-foot pipe extension and a pressure-reading tap on top of the extension. A gate-type valve is then added to the extension pipe. The outlet of the valve has a male hose thread connection.
- The above test setup is connected to the outlet being tested.
- Open the outlet’s main control valve completely; all gating will be done using the gate valve on the test setup.
- Attach a master pressure gauge to the pump and another master gauge to the pipe extension on the test setup.
- Connect the hoses going to a testing monitor and pitot reading arrangement. At the correct flow, compare the two master gauge readings to find the pressure loss in the system.
Monitors and Hose Reels
In this evaluation, we have assumed the device being fed is efficient. This is a misnomer. Nothing could be further from the truth. The culprits include monitors and hose reels. Hose reels, in particular, have a history of very high loss even at modest flows. This has left hose reels for low-performance applications. But, maybe this will change, and the hose reel may once again gain favor if performance can be improved. The swivel, axle, and neck of most reels are very restrictive. As flows go up, the losses become very high. The losses in the hose reel are particularly significant on a slip-on grass/wildland rig, where the pump is driven by a small engine. The minimum power from the engine limits the ability to create added pressure to overcome the pressure losses in the reel itself.
Optimizing Discharge Performance
What steps should fire departments take to optimize their outlet performance when buying their next apparatus?
- Assign each discharge a required operational flow rating. If that outlet is connected to fire hose, assign the size hose and maximum normal operating length of it to that outlet. Verify the required operating pressure and flow of any device connected to the outlet, including monitors and nozzles. Calculate the outlet pressure and flow required for each outlet by adding hose friction losses to the pressure required by the device (nozzle, monitor, etc.). Some discharges may have two or more performance requirements. Any discharge that cannot be assigned a specific performance requirement is not needed and should be eliminated. Most apparatus today are built with more discharges than are required by NFPA 1901 or 1906. Do not buy more discharges than you need.
- Ask the apparatus builder to verify which outlets will be attached to which port on the pump’s main body.
- Have the potential pump manufacturer verify the pressure loss at the required flow for each main body port assigned to the specific outlets.
- When practical, try to match all your discharges that can be used simultaneously and see if their operating pressures are approximately the same. This lowers your overall operating pressure. This, in turn, makes for a quieter operating environment and adds to the overall safety of the operation.
- Assign the largest practical hose size for each outlet hoseline, and verify the hose loss numbers for the lengths that will be used on this apparatus.
- Select the lowest-loss monitors for the required flows.
- Select the handline nozzles to be used, and identify their line locations.
- Carefully evaluate the use of hose reels.
- Adjust department standard operating procedures to match the changes in apparatus design.
What is clear is that for most departments, there will be a big fire or tactical situation where the optimum pumping performance will pay dividends. Specify this article’s suggestions that match the department’s potential responses, environment, water source capabilities, and training level.
GARY HANDWERK has been in the fire equipment industry for more than 43 years and has worked for various fire apparatus or fire pump manufacturers, holding positions in engineering or product management. He is president of US Fire Pump Co. He has been active with NFPA 1901, 1906, 1911, 1912, and 1925 for more than 25 years.