Highway Hydraulics: Main Street to Country Roads, Part 1

Ed Collet posits we're lulled into a false sense of security because water is always there.
Water, whether going to the faucet for a refreshing glass of it or hooking up to the hydrant to obtain it to quell a raging inferno, is something most people take for granted—that is, until it is not there. We are lulled into a false sense of security because water is always there, so it is rare to give any thought to getting water from other sources.

“Why would you ever do that? We have hydrants.” This was the response when drafting from the canal running through downtown was suggested as the second water source should there be a fire downtown. The old city center is full of buildings built in the heyday of the Ohio-Erie Canal nearly two centuries ago. These buildings are large with large cocklofts, few fire breaks, and no sprinkler systems. With only two hydrants in the heart of downtown and the potential to overwhelm the city’s water system, planning to exploit more than a million gallons running behind downtown is sound planning.

Fast forward several years: A building downtown has a fire taking hold in the cockloft, forcing defensive operations. Within 15 minutes of the second aerial going into service, the water department contacted command to say the water tower had already dropped four feet and the system could not sustain the current flow. Luckily, the building was in front of an area often used to train on drafting from the canal. An engine was put in place to supply one of the aerials from the canal. The operation went smoothly because members had trained at this site and plenty of water was available. Familiarity with the location and having the skill to establish drafting operations were key for the success of this alternative water supply. Had the canal not been convenient to the fire scene, tenders would have been needed to haul water to support the municipal system.

 A single-axle 2,000-gallon tender and a tandem-axle 3,000-gallon tender. (Photos by author.)

 An 8,000-gallon semi tender.

 A stainless wrapped poly tank. Note the large black vent hatch and the exposed poly material at the dump chute connection.

 A tender designed with a double side chute to fill two dump tanks at once.

Alternative Water Supply in Urban Environments

The ability to maintain water supply to the fireground is critical in extinguishing any fire. Departments must have a backup plan for when the hydrants stop flowing. Best case scenario is supply lines can be stretched to a functional part of the system with one or two additional engines. Worst case for many urban departments is the need to implement a water shuttle. Components and operations of a water shuttle are the same whether operating in the middle of farm country or in the midst of skyscrapers. The biggest differences are in the planning and training.

For departments in rural areas, the shuttle may be the only means of water supply for any fire, so they train regularly, plan routes, and identify fill sites. These departments have water movement down to a science, highway hydraulics. Departments using a pressurized water system feeding neatly spaced fire hydrants down a block rarely, if ever, give water shuttles a passing thought. The train of thought is often, “These hydrants have always worked and always will; if one breaks, we can just relay down to the next corner.” This works until the main line to the water plant breaks, a massive power outage occurs, or a tornado takes out the water treatment plant and there is no next hydrant to relay from.

These examples have happened and forced departments to call in tenders and water supply officers from surrounding departments. These failures allowed the departments time to prepare, plan, and get resources in place, but what if it happens in the middle of a once-in-a-career fire? Will chaos rule, or will the department have a plan in place to implement an alternative water supply without missing a beat?

The key for urban departments when shifting to a water shuttle is knowing who has the resources needed to get the shuttle running (tenders, drafting engines, auxiliary equipment, and water supply officers), how long will it take them to get to the scene, location of the closest viable water source, and what can be done in the meantime to keep water flowing. This requires urban departments to work closely with neighbors and train on a subject area they normally would not consider.

 A tender with a swivel rear chute

Side chutes

The rear discharge configuration impacts the amount of water available.

 The front bumper tender performance markings.

Highway Hydraulics

Hydraulics is the study of the movement of water. For most departments, this centers around determining pump pressures and water flow for various hose sizes and lengths. Highway hydraulics is the art of determining how to develop needed fire flows from water shuttle operations. There are similarities between the familiar hydraulic calculations and highway hydraulics: Hose size is replaced by tender volume; friction loss is replaced by the time tenders spend at the dump site, at the fill site, and on the road; and hose length is replaced by length of the shuttle loop.

Like the hydraulic formulas many of us are familiar with, there is an equation for estimating the continuous flow capability (CFC) from a water shuttle. And, just like most hydraulic formulas, very few of us will remember the continuous flow capability at 2 a.m. Instead of going through quickly forgotten detail, let us look at the key components to determine how to get the needed amount of water to the scene when there is no water supply close by. A water shuttle is the combination of water sources, apparatus, equipment, routing, management, and time needed to get water from its source to the fire.

Typically, the apparatus in a water shuttle are tenders specifically designed to efficiently move water. When they are not available for a shuttle, engines can fill the gap but with some modifications to the standard shuttle setup. The capacity of each tender is the base for the CFC of the shuttle. Bigger tanks and more apparatus increase the CFC.

When a shuttle is needed, having many tenders is the first step in having the CFC meet the needed fire flow. Just because a tender is rated at 3,000 gallons does not mean it is bringing 3,000 gallons to the fire. Some water will be lost through the venting of the tender when it is filled, and not all the water can be efficiently offloaded at the fire. For conventional tenders, those using gravity to dump and requiring venting to fill, a capacity factor of 0.90 is assigned to its base capacity. A 3,000-gallon tender should only be expected to deliver 2,700 gallons. If a vacuum tender is used, 100 percent of its capacity can be counted on because the tank is sealed and it uses pressure to offload all the water in the tank.

The biggest reduction in CFC is time: time standing still and time on the road. When a tender is being filled or offloading, time is reducing its contribution to the CFC. To combat this, efficiently laid out fill sites and dumps sites are needed, along with training so time is not wasted by firefighters being inefficient in their assignments. Time on the road is based on the distance between the dump and fill sites. The closer the two, the less travel time is needed to make the trip. Yes, speed also contributes to travel time; while pushing the accelerator seems the easiest way to reduce travel time, it is also the easiest way to get in big trouble. An average shuttle travel speed of 35 miles per hour (mph) is recommended as reasonable and safe, according to National Fire Protection Association (NFPA) 1142,< em> Standard on Water Supplies for Suburban and Rural Fire Fighting.1

More tenders with larger tanks will increase the CFC, and fill, dump, and travel time will decrease the CFC. Getting as many tenders as practical early in the water shuttle is critical for maximizing the CFC. Training reduces wasted motion at the fill and dump sites, helping to reduce the time a tender is standing still. Planning numerous water sources allows routes to be optimized and multiple fill sites to be quickly established when needed. Planning and training are needed to maximize the continuous flow capability of a water shuttle.

 A tender with the capacity marked on the rear.

10  A vacuum tender.

In a profession where it is said that there is “never a never and never an always,” this is the exception: Never consider speed a means to increase shuttle flow. There are too many cases of injury and death each year from tender accidents. This must stop.

Let’s look at strategies and tactics to safely optimize water shuttles whether in the wide open country or bustling urban environment. Examining the four main aspects of a shuttle (tender design, fill site, dump site, and routes) will provide insight into ways to increase flow through efficiency of operation and design without sacrificing safety.

Tender Design

Without a network of mains to bring water from a source to the fire scene, tenders are needed to carry water across the network of streets, roads, and highways. Tenders can range in size from a single rear axle apparatus carrying 1,500 gallons to semi-trucks hauling 8,000 gallons and everything in between. The performance of any tender is dictated by its areas of operation. A 3,000-gallon tandem-axle tender may haul more water than a 2,000-gallon single-axle tender but is not well suited for winding roads found in hilly or mountainous areas. In this case, the travel time is extended, reducing the flow of the 3,000-gallon tender.

Fire departments need to consider the operational environment tenders will be used in when they are designed and when calling for mutual aid. Getting several large tenders to operate in a tight urban environment will hamper operations compared to using several smaller tenders. If the environment has multilane roads with little congestion, bigger tenders may be best. Consider the lane width and potential congestion of the road when determining the best size of tender to integrate into the shuttle.

Flexible tactical operations are important to consider when designing a tender. This apparatus has the potential to be used extensively outside your jurisdiction and should be able to integrate into different supply operations. Not only will this apparatus assist neighboring jurisdictions, it will be in the department for a long time. So plan for future applications and not just how it might be used in today’s setting.

Tank. Tanks come in a variety of shapes and sizes. Tanks on tenders with extensive cabinet space normally are made of polyethylene plastic (“poly”) construction to allow for customized shapes. Tenders with exposed tanks and minimal cabinets can have tanks made from poly or stainless steel. In the past, tenders had stainless steel elliptical tanks with storage under the tank. With advances in material technology, poly tanks are able to be as large as steel and even be formed into ellipses.

Poly tanks have many advantages in terms of maintenance, corrosion, and flexibility, but they do have one key limitation: fill rates. For poly tanks with less than a 1,000-gallon capacity, fill rates should not exceed the capacity of the tank per minute. This limits a 500-gallon tank to a fill rate of 500 gallons per minute (gpm).2 For tanks 1,000 gallons and over, the maximum fill rate is 1,000 gpm. In either case, the maximum pressure at the tank inlet is limited to 100 psig.

These limits are in place to protect tanks from being pressurized during filling. When water enters the tank, an equal amount of air must vent from the tank to prevent pressure buildup in the tank, especially as water pushes out when the tank is full. Going over these limits has the potential to pressurize and damage the structure of the tank. Stainless steel tanks generally do not have these limits. Just because a tank looks like stainless steel does not mean it is. Some elliptical poly tanks are wrapped to give the look of a stainless steel tank. Take the time at the fill site to determine the construction if fill limits are not posted near the inlets. One tip to identify a wrapped poly tank is a large vent tower in the middle of the tank with a poly door. It is best practice to label the maximum pressure at the fill connections large enough that it can be easily read. When in doubt, limit the rate of fill to 1,000 gpm and pressure to 100 psig to avoid inadvertent damage to poly tanks on mutual-aid water tenders.

Pumps. Tenders can be equipped with a fire pump or without. Having a fire pump adds the ability to nurse an attack engine at the fire scene. A pump on a tender is faced with the same limitations as a pump on an engine. If the tank-to-pump plumbing is sized only to meet the minimum requirements of NFPA 1901, < em>Standard for Automotive Fire Apparatus (2015),3 the flow rate will be limited nominally to 500 gpm. When tenders are equipped with a pump, the tank-to-pump capacity should be specified to meet a minimum flow of 750 gpm.

Tenders with pumps should be equipped with a true large-diameter hose (LDH) discharge. This discharge must be on the officer side to comply with NFPA 19013 requirements prohibiting the placement of discharges larger than 2½ inches at the pump panel. For a top-mount pump, the LDH discharge could be on either side, but keeping it on the officer’s side helps to keep the LDH lines out of the road. At a minimum, 3-inch plumbing should be used from the pump casing to the discharge outlet, with 4- or 5-inch plumbing being ideal. This limits friction loss in the plumbing and hydraulic losses across the adapter needed to increase the discharge to the size of the LDH. To add flexibility, the LDH cap can be an adapter with a-inch male adapter allowing – or 3-inch lines to be used without searching for the correct adapter.

Tenders with fire pumps can be equipped with preconnected attack lines and deck guns. This provides tactical and operational flexibility. If needed, a tender can fill as an engine while a first-line engine is out of service for maintenance. A tender could be used as the first-out apparatus for fires on limited-access highways for the extra water volume and to act as a substantial blocking apparatus. During initial operations on defensive fires, the tender can use its deck gun while dump site operations are being set up. The addition of a fire pump adds flexibility and operational capacity to a tender. Without a pump, all a tender is capable of doing is hauling water, with dump operations being the only tactic available at the fire scene.

Rapid Discharges. The number and style of discharges will determine the offload speed and the time needed to maneuver at the dump site. The goal of the discharge is to rapidly offload water from the tender into portable tanks with minimal maneuvering. Tenders must be capable of offloading from the rear and both sides at a rate of 1,000 gpm for 90 percent of the tank’s volume.3 The force to dump water from a conventional tender is the static head of water above the dump chute opening in the tank. The higher the head, the greater the driving force and consequently the greater the flows. The static head of the tank drops as water is offloaded, decreasing the flow rate. Initial dump rates can be as high as 3,000 gpm for the initial few seconds, tapering off to less than 250 gpm when the tank level gets low.

Placement, size, and design of the dump chute impact the dump rate and handling time. Side dumps allow a tender to pull along dump tanks with minimal maneuvering. A tender using the rear dump must back up to line the chute up with the dump tank, increasing maneuvering time at the dump site. Depending on the dump site layout, this can add significant handling time at the dump site.

Adding a swivel chute to the rear discharge of a tender allows it to meet the requirement of dumping from both sides and the rear with a single tank opening. While the rear swivel discharge chute reduces the costs by eliminating the need for separate side discharges, it may reduce the offloading rate of the tank. Just like putting several kinks in an attack line will reduce the flow, the bends in a swivel chute will reduce the flow from the tank. The chute must be well-designed and not restrict the offloading rate of the tank.

Some departments have designed tenders with double side discharge chutes on each side of the apparatus. Not only does this increase the speed of offloading, but it allows two dump tanks to be filled at once. For larger tenders with a capacity exceeding the capacity of a single dump tank, this eliminates time needed to move to a second dump tank. One way to achieve this is by installing a rear chute with a swivel in addition to a single side chute on each side. In this configuration, the side chute needs to be far enough forward to allow offloading into separate dump tanks. This design maintains the ability to dump off the rear.

The location of the discharge chute relative to the apparatus tank bottom impacts the speed and amount of water available when offloading. Ideally, the dump chute opening is installed in a sump lower than the tank floor, allowing the maximum amount of water to be offloaded under the greatest head pressure. Photo 7 shows a T-tank with the rear discharge low on the vertical leg of the T and the side discharges higher on the leg. The rear discharge is capable of offloading the full 3,000 gallons, while the side discharges can only offload 2,500 gallons. The extra flow contribution available by using the rear chute may be offset by the maneuvering time. Side chutes provide less volume in this case but reduce the maneuvering time to almost zero. It is important for a water supply officer to realize the impact of chute location when determining dump site layout and potential flow capacity of the shuttle.

Fills. There are any number of configurations for tank fills, depending on the age of the tender, department preferences, and local conventions. For example, historically in my county, all tenders have double threaded 2½-inch inlets. A county on the other side of the state uses double 3-inch Storz connections on the tenders. A single LDH connection is gaining popularity since it eliminates connecting two hoses. Whatever the configuration, it is always a good practice to carry adapters on the tenders to allow various hose connections. Having a clappered siamese on an LDH fill provides connection flexibility and less flow restrictions than placing a LDH adapter on a 2½-inch fill.

Valves on fills are commonly ball or butterfly valves. It is very easy to create water hammer when shutting down the fill once the tank is full. This has the potential to damage the hose, engine, hydrant, and water main as well as injure personnel. In any case, water hammer has the potential to put the fill site out of commission. Proactive departments with LDH fills have installed the same ball intake valves as used on engine intakes. These valves are slow acting to prevent water hammer and provide an integrated pressure relief valve to protect the tank against overpressure. Once the valve is closed, the vent valve can be used to relieve pressure on the fill line. The pressure bleed is especially helpful when disconnecting LDH fill lines.

Venting. When water enters or leaves the tank, air must be pushed out or allowed to enter. Air moves through the tank’s vent. Vents are sized to prevent the tank from experiencing positive or negative pressure. This is one reason fill rates are limited; 1,000 gallons of water are equal to 7,480 cubic feet. When filling a tank at the target flow of 1,000 gpm, 7,480 cubic feet of air must be vented every minute to prevent an increase in internal pressure. Venting is something many do not recognize as a potential limitation in dump rate. The same volume of air must be able to enter the tank as the water exiting to prevent negative pressure developing in the tank. With today’s well-engineered tenders, venting is not the issue it once was with homegrown tenders and repurposed tankers.


You have just been assigned as the water supply officer for a quickly organized shuttle because of a major failure in the city water system. Dispatch is sending tenders from neighboring departments, but you know nothing about them. Tenders must carry markings that concisely convey key configuration information. The is especially true when an urban department must put a shuttle together in an emergency.

Ideally, tender markings will be standardized in placement and information. Markings must be visible as the tender pulls into the staging, dump, or fill site. Many departments put the tender’s capacity on the rear, which does little to inform the water supply officer facing the front of the tender. Placing the markings on the driver’s side front bumper allows the water supply officer to see the information as the tender pulls up. Marking should convey information such as capacity, dump configuration (rear, sides, or both), and whether or not it has a pump .

For example, a 2,000-gallon tender with a pump (p) and side (s) and rear (r) dump would be marked PSR 2000. Confusion occurs when markings are not standardized. The tender in photo 8 indicates a rear (r) and side (s) dump, but 175 does not make sense as a capacity. This department chose to put the offloading time for the tender in seconds. Markings are a great help to the water supply officer for optimizing a shuttle, especially in an urban environment where apparatus may be unfamiliar.

Vacuum Tenders

Vacuum (vac) tenders have been around since the early 1980s. They have the ability to self-load at rates of more than 1,000 gpm with minimal staffing needed at the water source. Often the driver can set up fill operations at a static source alone. The principle of operation is similar to priming a centrifugal pump. A large vacuum pump is used to evacuate air from the tank, allowing atmospheric pressure to push water from the source to the tank. Since atmospheric pressure is used to move the water to the tank, it has the same limitation on lift as an engine would while drafting. Because of the large volume of air moved by the vacuum pump, the vac tender can achieve closer to theoretical lift.

Compared to conventional tenders with diminishing offload rates, a vacuum tender has a constant discharge rate. When dumping, the vacuum pump is used to pressurize the tank, allowing for a constant pressure in the tank to force water out.


For an apparatus to be defined as a tender, it must have a minimum tank volume of 1,000 gallons.3 However, when water needs to be hauled, anything with a tank will do the job. A majority of urban engines have water tanks ranging from 500 to 1,000 gallons. While these will not contribute as much to the CFC as a purposed designed 3,000-gallon tender, they are quickly available when the need arises for an emergency shuttle. Every fire scene usually has engines where the crew is committed to a fireground task while the apparatus is not used. When a water system failure occurs, command must remember there is usable water in these rigs, and they can comprise the initial shuttle. I was told of a fire where the water system failed, and command was frantically working to get tenders to the scene. All the while seven engines each with 1,000 gallons sat on scene unused.

Table 1: Continuous flow contribution for an engine with a 1,000-gallon tank.

Table 2: Continuous flow contribution for an engine with a 750-gallon tank.

Table 3: Continuous flow contribution for an engine with a 500-gallon tank.

Not having dumps requires engines to pump water to dump tanks or nurse the attack engine. Pumping increases the time to offload water because of the standard flow rate from tank to pump being 500 gpm and the additional time to connect hoses. For a five-mile shuttle loop, a 500-gallon engine can contribute roughly 36 gpm to the shuttle. This is based on a travel speed of 35 mph, a 500-gpm fill rate, and a one-minute handling time at dump and fill sites. A total of 10 engines are needed to provide a 360-gpm water delivery rate. An engine with a 1,000-gallon tank can contribute 68 gpm to a shuttle using the same parameters as the previous calculation but with a fill rate of 1,000 gpm provided by the 1,000-gallon tank. It may take many engines to get the needed water delivery rate without tenders but it might be easier to get a large number of engines in the early stages of establishing an urban water shuttle.

When the water system fails, the official designation of an apparatus means very little. If it has a tank and can safely be used to haul water from point A to point B, it must be pressed into service. In some areas, this may require thinking outside the box for resources, like using tankers from pool water hauling companies. Knowing the attributes of a tender better prepares firefighters and officers to use apparatus for water hauling when the time comes.

Setting up water shuttles is something every firefighter should understand even if the department operates primarily from hydrants. It is a matter of when, not if, there will be some level of failure occurring in the water system. When this happens, having trained in shuttle tactics and with neighboring departments means the water supply will experience minor-to-moderate hiccups instead of total failure.


1. National Fire Protection Association. (2016). Standard 1142, Standard on Water Supplies for Suburban and Rural Fire Fighting, Edition 2017. Quincy, Ma.

2. Fill Rates. (n.d.). Pro Poly America. Retrieved August 1, 2019, from http://www.propolyamerica.com/wp-content/uploads/2016/04/FillRates.pdf.

3. National Fire Protection Association. (2015). Standard 1901, Standard for Automotive Fire Apparatus, Edition 2016. Quincy, Ma.

Ed Collet has been in the fire service for 19 years and is a firefighter/AEMT on the Jackson Township and Canal Fulton (OH) Fire Departments. He is the lead instructor for the Bowling Green State Fire School Pump Operation and Water Supply class. He is a co-chair of the Ohio Fire Chiefs Association Water Supply Technical Advisory Committee, which helps to develop and spread best practices for alternative water supply throughout the state. He has a bachelor’s degree in mechanical engineering from the University of Akron and a master’s degree in engineering management from Ohio University.

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