Eductors are jet-type pumps that contain no moving parts. An eductor moves liquid from one place to another by entraining the pumped liquid in a rapidly flowing stream of water (venturi effect). The eductor can perform low-head dewatering operations at a greater rate of discharge than can be obtained by straight pumping with available emergency pumps. Educators are used to pump liquids that cannot be pumped by other portable pumps. Also, liquids that contain fairly small particles of foreign matter can be pumped by using an eductor.
Since the operating medium of an eductor is water under pressure, portable eductors can be actuated by the ship's firemain pressure. This is accomplished by connecting a 2 1/2-inch
Figure 6-43.-Eductor connected to firemain.
Figure 6-44.-P-250 pump and eductor rigged for dewatering a flooded compartment. 6-34
Figure 6-45-Portable eductors.
hose to the nearest fireplug and to the eductor (fig. 6-43).
You must remember that not all of the eductor discharge comes from the compartment being dewatered. For example, when a 4-inch portable eductor is activated by water from the firemain system at 100 psi, 182 gallons per minute of water is required for the operation of the eductor. Working against a head of 40 feet, the eductor will discharge a TOTAL of 333 gallons per minute. This amount includes the 182 gallons per minute that is being pumped into the compartment to activate the eductor. Therefore, the NET discharge of the eductor is 333 less 182 gallons per minute, or 151 gallons per minute. The compartment's dewatering rate is only 151 gallons per minute.
Eductors can be permanently installed and activated by opening a firemain valve, or they can be rigged as required to dewater a compartment or space. They are often used with the P-250 pump to dewater compartments. Figure 6-44 shows one arrangement that could be used. In this arrangement, both the P-250 pump and the eductor are removing water from the flooded compartment.
When using an eductor for dewatering, remember that the pressure of the water supplied for the operation of the eductor must always be substantially higher than the pressure against which the eductor is required to discharge. If this requirement is not met, the water supplied for operation of the eductor will merely back up through the eductor and assist in the flooding of the compartment. A simple rule to follow is that the pressure of the water supplied for operation of the eductor must be at least three times the static head pressure against which the eductor must discharge. Multiply the discharge head in feet by 0.433, which is the weight of a column of water 1 inch square and 1 foot high. Then multiply the answer by 3 to obtain the pressure required. For example, suppose that the eductor must discharge against a static head of 50 feet. When you multiply 50 by 0.433, you have 21.65 psi. Then multiply 21.65 psi by 3 to get 65.95, or approximately 65 psi. This is the MINIMUM firemain pressure required when the eductor must discharge against a static head of 50 feet.
Two types of eductors (fig. 6-45) are available for shipboard use. One is known as the S-type or single-jet eductor. The other is known as the perijet eductor. Both types of eductors have a 2 1/2-inch inlet and a 4-inch discharge.
The S-type or single-jet eductor is used for dewatering and also for supplying the P-250 pump. The perijet eductor (fig. 6-46) is actuated by six jets around the periphery of the vacuum
Figure 6-46.-Cross-sectional view of perijet eductor. 6-35
chamber. There is a straight bore of approximately 2 1/2 inches through the eductor base into the discharge section. The 2 1/2-inch opening allows debris up to 2 1/2 inches in diameter to pass through the eductor. A 4-inch quick-closing valve (fig. 6-47) is provided at the discharge side to clear the eductor of debris by forcing actuating water back through the intake. Since the perijet has no foot valve, low firemain pressure can result in merely adding water to the compartment instead of removing water from it. The absence of the foot valve also makes the perijet unsuitable for use as a source of supply to the P-250 pump. However, this can be overcome by attaching a P-250 combination foot valve and strainer to the suction side of the perijet with a special 3-to 4-inch adapter. The 3-to 4-inch adapter is required as repair locker equipment. Overall, the perijet eductor is about 7% more efficient than the single-jet eductor.
The following points should be kept in mind when you use an eductor:
• Be sure that the actuating pressure is AT LEAST three times the static head pressure against which the eductor must discharge.
• Keep the supply and the discharge hoses free of kinks.
• If it is necessary to use a suction hose with a perijet eductor, ensure that you use a hard rubber hose.
• Charge the eductor only when it is fully submerged.
• Keep the eductor fully submerged while operating it.
In this chapter, you were introduced to various protective equipment and atmosphere testing
Figure 6-47.-External view of periject eductor.
devices. The basic fire-fighting procedures and the equipment involved were also discussed. As you participate as an active member of the duty in-port fire party and as a member of a repair party at sea, you will become more familiar with the equipment, systems, and procedures used to combat fire and flooding. Refer to the Repair Party Manual, the NWP 62-1, the Naval Ships' Technical Manual, chapter 074, volume 3, the Naval Ships' Technical Manual, chapter 079, volume 2, and the Naval Ships' Technical Manual, chapter 9930, for more information on damage control and fire-fighting equipment and procedures.