Performance Testing Your Fire Tanker

How much water does your tanker/tender actually hold? At fires in areas without hydrants, you can’t use water that doesn’t efficiently come out of your tankers.

TRAINING NOTEBOOK ❘ By CHARLES D. CLARK

How much water does your tanker/tender actually hold? Measuring a tanker’s water capacity can be quick and simple: Take your tanker to a certified truck scale and weigh it while it is completely full of water. Next, drain all the water from the truck into a dump tank. Now, weigh the empty tanker. You can convert the weight difference to gallons by dividing the weight difference by 8.345 pounds per gallon of water.

You now have a precise measurement of the actual capacity in gallons for your tanker’s onboard water tank. Don’t be surprised if your water tank falls 5% to 10% short of its nominal (advertised) size.

At fires in areas without hydrants, you can’t use water that doesn’t efficiently come out of your tankers. When dumping from conventional gravity dump tankers with a large dump valve, the water flow is very strong during the first minute; dumping that last 10% of the tanker’s water is much slower.

To chart how much water is involved, use the weight test again. Measure the water dumped after the first minute and at 30-second intervals thereafter until the tanker is empty. You can calculate flow rates at each point. Now you can chart at what point in time, called the “critical dump time,” is more efficient to stop dumping and leave to get more water. Make a mark on the dump chute where the water flow has reached this critical dump point (photo 1).

(1) Has this tanker reached its critical dump point? Is it more efficient to stop dumping and go after more water? How much water is actually delivered to the fire scene? Is there room to lower the dump valve to create more head pressure? (Photos by author.)

(2) More than 300 gallons (12 inches) of water remain on this 2,000-gallon tanker after water stops flowing from the side dump. This is not a homemade tanker; it was built by a major manufacturer.

Several experts in the field of water supply recommend that a 10-inch square dump valve (unobstructed 100 square inches of cross-sectional area) should be the smallest dump valve used on a gravity dump tanker. In fact, a leading dump valve company stopped selling 10-inch-diameter round dump valves because of a 15% to 20% drop in performance from 10- × 10-inch square dump valves. Smaller dump valves (Table 1) may meet the National Fire Protection Association’s (NFPA’s) minimum dumping standard of 1,000 gallons per minute (gpm). However, they would not fit into an efficient tanker shuttle where tankers side dump at 1,700-plus gpm as they top off several dump tanks down a single traffic lane.

At the dump site, lower performing tankers can be pulled over to allow more efficient tankers to pass by and dump with no delay. If you issue vague specifications such as, “Tanker shall have side dumps,” instead of detailed performance requirements, you may get very disappointing production. You’ll have to live with that underperforming tanker for the next 25 to 30 years (photo 2). Clearly, the tanker shown in photo 2 was built with no performance specifications.

No matter the size of the dump valve used, getting the last 10% of the tanker’s water efficiently into the dump tank is a significant challenge. The dump rate declines rapidly because of a loss of head pressure within the tank. ISO acknowledges this problem of declining head pressure when it deducts 10% from the usable capacity of your gravity dump tanker during its evaluation. This performance penalty is often quite generous; many conventional gravity dump tankers struggle to efficiently dump 90% of their water.

Vacuum tankers are not penalized partially because they are designed to compensate for this loss of head pressure. This is done by using a PTO vacuum/pressure air pump to slightly pressurize the water tank (up to 15 pounds) to force water from its onboard water tank more efficiently. This pressurized water tank has consistent head pressure throughout the dump equivalent to a water tank up to 30 feet in height (photo 3).

In 1991, William Eckman reported in Fire Engineering that a 2,000-gallon gravity dump tanker/tender he designed and tested could dump 99% of its water in 67 seconds using a 10-inch square dump valve (the average dump rate is 1,773 gpm). The key to this tanker’s efficient performance was how it dealt with declining head pressure while dumping. The tank featured a 12- × 12-inch trough at its bottom that ran the full length of the tank. The dump valve ran full until water remained only in the trough. A downside of this tank design might be that there would be a higher center of gravity for the apparatus (Figure 1).

Figure 1. Tank Trough

A 12- × 12-inch trough below the bottom of the tank creates 0.41 pound of additional head pressure, which increases dump rate and significantly increases percentage of tank volume actually dumped.

A fire tanker shuttle can be compared to a NASCAR race with two pitstops per lap (dump site and fill site). If you spend a lot of time in the pits, you’ll not “win the race.” To avoid a big lineup of tankers at the fill site, set a limit of three to five tankers per fill site engine. No one in NASCAR would wait in line for 12 minutes for four other “racers” to be reloaded. A tanker shuttle can only deliver to the fire scene the volume of water you can load on your fire tankers each minute. Your goal should be to fill tankers at up to 1,000 gpm. Interrupting the flow to switch tankers at the fill site cuts your net fill rate by 25% (from 1,000 gpm to 750 gpm). Set up the fill site to allow for hooking up one tanker while another tanker is being loaded. Loading at more than 1,000 gpm can void poly water tank warranties. Run the load test several times to see how different loading setups compare in efficiency (three-inch large-diameter hose, multiple hoselines, and fill inlets) (photo 4). Also, do not exceed 100 pounds per square inch (psi) pump pressure on your fill site engines; this prevents exceeding the water pressure limit on the poly water tank warranties (85 to 100 psi).

Your water supply officer can benefit from a list of mutual-aid tankers with more accurate information on their water delivery capabilities. For example, our gravity dump tanker, with a nominal (advertised) capacity of “3,000” gallons, is equipped with two four-inch fill inlets and 14- × 14-inch custom dump valves. It can dump 2,500-plus gpm (one minute of critical dump time). On paper, we have a super tanker. However, a weight test shows that this truck delivers only 2,400 gallons of water each trip to a dump tank at a fire scene; that’s only 80% of its nominal (advertised) size. Crediting that truck with 3,000 gallons of water per trip will leave the water supply officer scratching his head as to why 600 gallons of water “disappear” from each load delivered by our “3,000”-gallon tanker/tender.

(3) The vacuum tanker has the advantages of being able to self-fill closer to the fire scene and unload a complete load of water (“zero loss”). This earns 100% credit for its tank capacity from ISO.

(4) Multiple tanker fill inlets allow for safer and more efficient filling. Why do many tankers come with only one fill inlet? How many people are needed to fill this truck?

Delivering just 80% of a tanker’s stated capacity is equivalent to one tanker out of five running around empty. Can you afford to buy five trucks to do the work of four? Why are many conventional gravity dump tankers built with less-than-stellar levels of performance? Fire tankers are built to your specifications. There are tankers that deliver more water to a fire scene with less expense, less staffing, less travel time, and more flexibility.

Vacuum fire tankers can do everything a conventional gravity dump tanker can do plus much more, while super (vacuum) tankers can self-fill from alternative sources of water just six inches deep. Such water sources may be just across the road from your next fire scene.

After testing your tanker, what performance specifications should you spell out the next time you order a tanker? Following are the performance specifications met by the most recent tanker purchase by the Colerain Township (OH) Fire Department, based on their checklist.

Several tankers from your area can be performance tested at one time with just a little bit of organization. You can now report an accurate tank volume, dump time, and fill time to ISO. Should ISO require a one-time performance test of tanker/tenders? Our constituents deserve the most “bang for their buck”-efficient tankers.

CHARLES D. CLARK is the water supply officer for and a charter member (1983) of the Colerain Township (OH) Fire Department (CTFD). He is also a member of the Ohio Fire Chiefs Association’s Advisory Committee on water supply. In 1994, Clark introduced the single-lane water supply setup to the CTFD and now teaches water supply classes across the Ohio and West Virginia regions. He retired after 34 years as a teacher with the Pickaway-Ross Career & Technology Center and after 20 years as a Colerain Township fiscal officer. In 2013, he was recruited to serve on the Ohio Fire Chiefs Advisory Committee on water supply for firefighting.