So far in my explanation of water-on-wheels (WOW) operations, I’ve covered three methods of offloading vehicular water tanks: using a gravity dump valve to dump the water, pumping the water off, and pressurizing the tank on a vacuum tanker with air to force the water through a dump valve. But there is yet another method used to offload tank vehicles-the jet dump.
A jet dump is a piping arrangement that uses the Venturi effect to increase the flow from a dump valve. Jet dumps can be internal or external, although the internal jet dump is most common.
The characteristic internal jet dump (Figure 2) consists of a nozzle aimed at the inside of a dump valve, supplied by a pump capable of pumping at least 250 gpm at 150 psi. To operate the jet dump, you must: 1) open the dump valve, 2) engage the pump, 3) open the jet dump discharge valve and 4) increase the pump pressure to about 150 psi to provide the jet nozzle with at least 50 psi. Figure 3 shows this setup at work.
In Figure 4 we see a comparison of two very similar tank apparatus. Although both are equipped with 6″ rear dump valves, the one on the right is equipped with an internal jet dump. You can clearly see the effects of this difference in the two discharges.
Why Use Jet Dumps?
Firefighters who don’t use tank vehicles equipped with jet dumps often ask, why would a department use a 5″ jet dump when a 10″ gravity dump valve could achieve the same or better results without an apparatus pump? There isn’t a simple answer to this question. In many cases, fire departments do what they do today simply because it’s what they did yesterday and the day before. As I’ve said before, how a department gets the job done is irrelevant so long as they get the job done-and the job in this case is delivering the gpm required by the incident.
In many cases, the equipment and methods used by departments are regional. Although I’ve seen tank vehicles with internal jet dumps throughout North America, they are most common in the Southeast United States. By the same token, reel trucks carrying up to a mile of 4″ or 5″ hose are common in the Northeast but are rare in other areas, and vacuum tankers are more common in Ohio, where Firovac makes them.
I just returned from the Fire Department Instructors Conference (FDIC), the biggest fire service tradeshow in North America. I’ve attended FDIC for all but 1 of 30 continuous years. It was at FDIC in the mid-1970s that I had my first real rural-water-supply learning experience. Instructors from the Midwest were explaining why tankers with large gravity dump valves were best, while instructors from the Southeast were explaining why jet dumps were best. Following these sessions, at informal meetings, attendees argued which tank-offloading method was superior. Ironically, I heard the same arguments at FDIC 2007.
Performance Tested
The major benefit of the jet dump when it was first introduced was that it significantly increased the flow from the 4 à‚«”, 5″ or 6″ dump valves used at that time. That was before 8″, 10″, 12″ and larger gravity dump valves became commonly available for tank vehicles.
Discussions at FDIC all those years back led to the use of weight tests to document the performance of tank-offloading methods, including jet dumps. The first tanker with a jet dump I had the opportunity to test was a demo unit built by the now-defunct FMC Corporation. This unit (shown in Figure 4) was a very simple and inexpensive 1,200-gallon tanker equipped with a 250-gpm power-take-off-driven pump and a 5″ jet dump. The gravity dump-testing procedure follows:
- Fill the tank until it overflowed.
- Weigh the tank to obtain full weight.
- Offload the tank by gravity for 1 minute.
- Reweigh the tank to obtain test weight.
- Subtract the test weight from the full weight to determine the net weight.
- Divide the net weight by 8.34 lbs. to determine gallons offloaded.
The jet dump test was the same as the procedure above, but water was offloaded using the jet dump.
In 1 minute, the gravity dump test resulted in 346 gallons offloaded. In the same period, the jet dump offloaded 1,012 gallons. As these tests show, the jet dump allowed the tank to offload 81 percent of the 1,200-gallon tank capacity in 1 minute, as opposed to only 28 percent of tank capacity by gravity. The jet dump increased the tank discharge by 196 percent over gravity.
Looking at it another way, at 50 psi, the 1 1?8″ jet flowed about 250 gpm. This discharge caused an additional 762 gallons to be offloaded. Surely, one can see the benefit of this particular jet dump.
Not Created Equal
Although the jet dump test results of the FMC tanker are impressive, not all jet dumps show such dramatic increases in efficiency for a number of reasons, including jet diameter, piping arrangement and pump discharge pressure.
For instance, tests of a 1,900-gallon elliptical tanker equipped with 6″ jet dumps on each side and rear showed less dramatic results. In 1 minute, the rear gravity dump offloaded 754 gallons. In the same period, the rear jet dump offloaded 1,122 gallons, an increase of only 49 percent. In a side test, the tank offloaded 567 gallons using gravity and 889 using a jet dump, to increase the side discharge by only 57 percent.
As you can see from the results of these two tanker tests, the jet dump on the FMC tanker provided a much greater increase in tank discharge than did the jet dump on the second tanker. Just because a tanker is equipped with a jet dump doesn’t mean it will perform as well as you might hope. Weight testing of individual apparatus and their dumps is the only way to know for sure what advantage a jet dump might provide over gravity. Tip: When ordering a new jet dump on an apparatus, specify with the manufacturer the offloading performance, in gpm, you expect.
External Jet Dumps
After reviewing the test results, you can see why most tankers are equipped with 10″ or 12″ dump valves on each side and the rear. Internal jet dumps work well so long as we understand their properties and strengths. But, overall, I have found external jet dumps to be most beneficial in improving the performance of an existing dump valve without cutting the tank.
The East Haddam (Conn.) Fire Department (EHFD) provides a classic case of an external jet improving the offloading performance of a tanker. Its 750-gpm/1,200-gallon pumper/tanker had been intended to serve as a pumper, with a 4 à‚«” dump valve installed in the left rear compartment (Figure 5). During a WOW class, we tested the dump valve and found the 1-minute discharge to be 505 gallons.
Convinced they could do better, members built their own 4 à‚«” jet dump using a set of hard suction couplings, a 1 à‚«” female coupling and 1″ tubing (Figure 6). They then installed a 1 à‚«” discharge to supply the external jet. A section of 4 à‚«” suction hose was then shortened to serve as an extension (Figure 7). Using this setup, weight tests of the external jet dump resulted in a 1-minute discharge of 1,159 gallons-an increase of 654 gallons over the gravity discharge. The work of the EHFD and these test results led to Kochek’s manufacturing of the external jet as shown in Figure 8.
Conclusion
In closing, I want to emphasize that jet dumps can be a valuable water supply tool-so long as they are designed to perform as you, the customer, demand. Next month, we’ll take a look at more pros and cons related to jet dumps.
‘Til then, stay safe.
