A tank vehicle's off-load rate is a function of time & weight

The performance of any water-on-wheels (WOW) operation is measured in output or gpm, and it's based on a number of operations that involve hardware, people and standard operating procedures (SOPs). The two time segments within a WOW op that have the greatest impact on a tank vehicle's performance are off-load time and fill time. Off-load time equals the time it takes the tank vehicle to off-load the water it carries. Fill time equals the time required to actually fill the tank. These time segments exclude any preparation time prior to off-loading or filling the tank, and the time required to break down (disconnect fill lines, etc.) the off-load or fill operation.

In this month's column, I'll focus on how departments can conduct repeatable off-loading tests on any tank vehicle they use in WOW operations to measure performance and increase output, if needed. Regardless of the types of tank vehicles or modes of operation used, the measured delivery rate is what counts. A WOW operation that produces a higher delivery rate over a given distance is better than one that produces a lower delivery rate.

PHOTOS LARRY DAVIS Figure 1: A tank vehicle off-loads at the off-load point during an ISO® Off-Load Test as a part of a WOW course in Wedowee, Ala. In the background, other tank vehicles wait for their turn at the start point. Traffic cones mark the 200' distance from the start point to the off-load point and to the end point.

THE ISO® OFF-LOAD TEST
The Insurance Services Office® (ISO) uses two basic performance tests to evaluate a rural department's ability to shuttle water with tank vehicles — the Off-Load Test and the Fill Test. For the purposes of this article, I'll focus on the Off-Load Test, which is used to calculate a tank vehicle's off-load rate in terms of gpm.

Figure 2 shows a sketch of the ISO® Off-Load Test arrangement. The purpose of the test: to measure the total (elapsed) time it takes for a full tank vehicle to travel 200 feet from a start point, off-load its water using required SOPs and then travel 200 feet past the end point. The elapsed time and the tank vehicle's capacity are then used to determine the off-loading rate of the vehicle.

To compensate for water lost in transport and water that may not off-load, the ISO® uses a net or effective tank capacity of only 90 percent of the actual tank capacity. For example, a 2,000-gallon tank has an effective carrying capacity of 1,800 gallons (90 percent of 2,000 gallons). However, this isn't the case with vacuum tankers. Because the vacuum tanker is airtight, no water is lost in transport. Therefore, the ISO® uses 100 percent of the vacuum tank's capacity.

If your department would like to conduct its own ISO® Off-Load Test, follow the procedure below.

1. Select a location to conduct the test that won't be problematic when off-loading water. The area should be clear and should measure about 500 feet in length.
2. Determine the start point, and mark it with a traffic cone or other visible marker.
3. Measure a 200' distance from the start point to locate the off-load point, and mark it with a cone or other visible marker.
4. Measure a 200' distance from the off-load point to the end point, and mark it with a traffic cone or other visible marker.
5. Determine the effective capacity of the tank to be tested by multiplying the tank's actual capacity by 0.9 (i.e., the effective capacity of a 1,500-gallon tanker equals 1,350 gallons).
6. Fill the tank until it overflows.
7. Instruct the apparatus driver to move the apparatus to the start point.
8. Assign one individual to time the test. Station them at a location where they can see both the start and end points.
9. Start the timer when the apparatus crosses the start point.
10. Instruct the apparatus driver to proceed to the off-load point and off-load according to their normal SOPs (i.e., backing up to a porta-tank, connecting off-loading hoses or chutes, etc.)
11. Once the tank is empty, instruct the driver to proceed past the end point. The elapsed time ends when the tanker passes the end point.
12. Calculate the elapsed time in minutes (i.e., 5 minutes, 30 seconds = 5.5 minutes).
13. Use the following formula to calculate the off-load rate in gpm: Effective tank capacity ÷ elapsed time = off-load rate. Example: A 2,000-gallon tank (1,800-gallon effective capacity) completes the test in 10 minutes. So, off-load rate = 1,800 gallons ÷ 10 minutes = 180 gpm.

IMPROVING THE OFF-LOAD RATE
Once you've determined your off-load rate, you can focus on improving it. Remember: Because you can't increase tank capacity, the only way to improve your off-load rate is to reduce the elapsed time.

Although the ISO® Off-Load Test is a practical test, the results depend on a number of variables, such as the driver's ability to position the apparatus to off-load, the time required to prepare the tank to off-load and, most importantly, determining when the tank is empty.

When is a tank empty? When the very last drop of water exits the tank? That may take hours. Determining when a tank is empty is a personal judgment call that can significantly increase or decrease elapsed time. The best way to reduce variations in judgment: Conduct an off-load test that only evaluates the tank's ability to off-load, which requires testing the tank's weight.

Figure 3: The South Franklin (Pa.) Volunteer Fire Department’s 1,800-gallon tanker off-loads through its 4 1⁄2" dump valve. At first we thought the tanker could off-load at 500 gpm, but after performing three weight tests, we discovered the actual amount of water offloaded in the first minute equaled only 180 gallons.

WEIGHT TESTING
I first got involved in tanker performance testing in the early 1970s when I lived in Washington County, Pa., where most departments relied on WOW ops to supply firefighting water supplies. In the mid-1970s, I attended the Fire Department Instructor's Conference where instructors from the Midwest taught us that dump valves and porta-tank operations were the two primary ways to move big water.

After hearing this, the South Franklin Township Volunteer Fire Department became the first department in the county to own a tanker equipped with a dump valve. The department purchased a 1,800-gallon/250-gpm tanker outfitted with a 4 1⁄2" rear dump equipped with a butterfly valve. Everyone involved was very proud of this innovation, and based on what we believed, we bragged about the tanker's ability to off-load water at a rate of 500 gpm.

However, after a few months of operation, some of us started to suspect that the tanker's off-loading rate was less than 500 gpm. We then tried to figure out how we could accurately measure this. We first considered marking 500- and 1,000-gallon increments on the inside of a 2,000-gallon porta-tank and measuring the time required for the water off-loaded to reach those levels. But that idea was scratched because porta-tanks are not very accurate measuring containers.

We decided that since water weighs about 8.34 lbs./gallon, we could use the weight of water to measure the tanker's off-load capability. All we needed was a set of truck scales and a stopwatch. We took the following steps during our test:

1. Fill the tank until it overflows.
2. Weigh the full tanker with the driver in the cab.
3. Off-load the tank for one minute and then shut the dump valve.
4. Weigh the tanker again with the driver in the cab.
5. Subtract the off-load weight from the full weight.
6. Divide the weight difference by 8.34 lbs./gallon to determine the gallons off-loaded in one minute.

Figure 4: A tank vehicle is weighed during a WOW course in Columbus, Mont. Each of the vehicles used were weighed full and empty to ensure they carried the amount of water they were rated for. They were also weighed after off-loading for one minute to determine the number of gallons they off-loaded in the first minute.

We then found a set of truck scales at a gravel yard and conducted the weight test, but we didn't believe the results. So we refilled the tank and conducted the test a second time. And just to make sure the results were accurate, we conducted a third test. The result of each test found that we were off-loading only 180 gallons in the first minute, well below the 500-gpm off-load rate we had hoped for. But the scales and the tests were accurate.
We discovered that the low off-load rate was caused by the baffles within the tank.

Eventually, after working with a torch to open up the baffles and adding an external jet assist to the dump valve, we increased the off-load rate in the first minute to 850 gallons. In the years since that first weight test, I've learned a great deal about tank vehicle performance based on weight testing. One of the most interesting lessons I learned came from a WOW course in Wedowee, Ala.

THE WEDOWEE WOW TESTS
During a WOW course in Randolph County, Ala., I discovered some fascinating information that can significantly alter the results of ISO® off-load tests. We conducted ISO® off-load tests on two tankers, Woodland (Ala.) Volunteer Fire Department's T-12 and Morrison Crossroads (Ala.) Fire Department's T-16, which were built by the same manufacturer and featured identical 1,800-gallon tanks with 10" square rear dump valves.

The elapsed time of T-12's off-load test equaled 5.9 minutes with an off-load rate of 275 gpm; the elapsed time of T-16 equaled only 3.9 minutes with an off-load rate of 415 gpm.

Why the difference in elapsed times? Because in the ISO® test, someone must visually determine when they think the tank is empty. In this case, the difference in judgment equaled two minutes; therefore, the problem with the ISO® test surrounds the person who determines when the tank is empty. In other words, the department should determine when the tank is empty, but that determination can't be left to someone's eyesight and personal judgment.

In discussing these test results, I pointed out why first-minute weight tests are so important. One of the attendees then said we could access a set of truck scales at a lumberyard. As a result, we conducted first-minute weight tests on T-12 and T-16, the results of which proved most interesting. 

Figure 5 & 6: Woodland (Ala.) Volunteer Fire Department’s T-12 (top) and Morrison Crossroads (Ala.) Fire Department’s T-16 (bottom) have virtually identical 1,800-gallon tanks. Weight tests showed that opening the tank hatch cover allowed the first-minute off-load to increase by 326 gallons. Based on first-minute weight tests of more than 200 tank vehicles, it’s very rare that opening the hatch would improve an operation. However, as this test proves, weight testing is the only way to determine whether opening a tank vehicle’s hatch does or does not help.

T-12 off-loaded 1,254 gallons (77 percent of its net capacity) in the first minute while T-16 off-loaded 1,580 gallons (98 percent of its net capacity) in the first minute. Why the 326-gallon difference? In discussing the test results, we learned that T-16's hatch cover had been opened during the weight test while T-12's hatch had been closed. So we tested T-12 again with the hatch open. As a result, T-12 off-loaded 1,566 gallons (97 percent of its net capacity), virtually the same as T-16.

The most important information gained from these weight tests: Both tankers were about 98 percent empty in the first minute they off-loaded. This vital information must be used in conducting the ISO® off-loading rate test.

Instead of an individual deciding when T-12 or T-16 was "empty" during the ISO® test, the dump valve should be opened for only one minute. At that point, the tank is "empty enough" and the vehicle should head for the end point. This reduces the elapsed time and increases the ISO® off-loading rate significantly.

A FINAL NOTE
Before a department conducts an ISO® Off-Load Test on a tank vehicle, it should conduct first-minute weight tests, one with the hatch open and one with it closed. In addition, if the tank vehicle is equipped with multiple dump valves, the department should conduct a test using each valve to see which provides the highest off-load rate.
In future columns, I'll talk more about the lessons I've learned by conducting tank vehicle weight tests.
 
Larry Davis is a full member of the Society of Fire Protection Engineers, a certified Fire-Protection Specialist and a certified Fire-Service Instructor II, with more than 30 years of experience as a fire-service instructor. He is vice president of GBW Associates, LLC, and the chairman of the newly created Rural Firefighting Institute. Davis has conducted more than 400 Rural Firefighting Tactics and Rural Water Supply Operations seminars throughout the United States and Canada. In addition, he has written numerous fire service texts, including "Rural Firefighting Operations," books I, II and III. Most recently, Davis co-wrote the "Rural Firefighting Handbook" and "Foam Fighting Operations," book I with Dominic Colletti. Reach Davis at ldavis@gotbigwater.com or ldavis@RFI411.org.