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THE TRIM SYSTEM
 
A. FUNCTIONS
 
2A1. General description. As explained in Chapter 1, unequal distribution of weight in the submarine will upset its balance and stability. The trim system is employed chiefly to correct this condition by regulating the quantity of water in the variable tanks.

Figure 3-9 illustrates the general arrangement of the trim system of a submarine. It shows the trim pump manifold, the main flood and suction lines, the valves, and the connections to the various trim system tanks.

The trim manifold, located on the port side aft in the control room, is considered the center of control for the entire system since it directs the flow of water to the various tanks. It is a casting divided into two longitudinal compartments known as the suction and discharge sides. The discharge side of the manifold contains eight discharge control valves. One of these valves is the trim pump discharge valve which connects the discharge side of the manifold with the discharge side of the trim pump. The suction side of the manifold contains eight suction control valves and is connected to the suction side of the pump through the trim pump suction valve.

The remaining seven discharge and seven suction valves control the flood and suction from the following lines:

1. Trim pump suction from sea and overboard discharge line.

2. Trim line forward flood and suction.

3. Trim line aft flood and suction.

4. Auxiliary ballast tank No, 1 flood and suction.

5. Auxiliary ballast tank No. 2 flood and suction.

6. Negative tank flood and suction.

7. Safety tank flood and suction.

The trim lines forward and aft serve the two trim tanks and the two WRT tanks, while auxiliary ballast tanks No. 1 and No. 2 are served by their own flood and suction lines. These tanks make up the variable ballast tanks

  group. The remaining flood and suction lines are connected to the negative tank and the safety tank. These tanks are called the special ballast tanks.

Cross connection of the trim pump and the drain pump is made by two flanged connections on the after end of the longitudinal axis of the manifold. One connection is on the discharge side, the other on the suction side.

The trim pump, located in the after end of the pump room, provides pumping power for the system. It draws water into its suction side, through the suction side of the manifold, from the tank being pumped, and discharges it through its discharge side, into the discharge side of the manifold, which directs the water to the tank being flooded. When it is desired to pump water into one of the above tanks by means of the trim pump, the discharge valve on the trim pump manifold controlling this particular tank is opened. When water is to be removed from a tank by means of the trim pump, its valve on the suction side of the manifold is opened. Thus, the trim manifold control valves are the means of putting any part of the trim system on suction or discharge. For example, in pumping from forward trim tank to after trim tank, the water is drawn through lines from the forward trim tank through the suction side of the manifold and into the suction side of the trim pump. Then, by pump action, it is forced through the discharge side of the trim pump, through the discharge side of the trim manifold, and finally through lines into the after trim tank.

For a more detailed discussion of the trim pump and the trim pump manifold, see Sections 2B and 2C1.

The functions of the various parts of the trim system are discussed in the following paragraphs. The trim line forward is a three inch line extending from the trim manifold to the forward trim manifold in the forward

 
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torpedo room. The forward trim manifold controls the flooding and pumping of the forward trim tank and the forward WRT tank.

The trim line aft is also a three-inch line, terminating in the after torpedo room at the after trim manifold which controls the flooding and pumping of the after trim tank and the after WRT tank.

Auxiliary ballast tanks No. 1 and No. 2 are piped directly to their suction and discharge valves on the trim pump manifold. Flooding or pumping of these tanks can be accomplished only through the trim manifold. On the other hand, flooding and draining of the safety and the negative tanks can be accomplished in two ways, either by the use of their suction, and discharge valves on the trim manifold or directly from sea by use of their flood valves. In the latter case, the draining is accomplished by opening the flood valves and admitting compressed air into the tanks, thus forcing the water out. The tanks may be flooded by opening both the flood and the vent valves, allowing the sea to enter directly into the tanks.

The trim pump suction from sea and overboard discharge line, connecting the trim manifold with the sea, provides the trim system with an overboard discharge to, or direct flooding from, the sea. In addition to the suction and discharge valves on the trim manifold, this line has also a sea stop valve and a magazine flood valve. The sea stop valve is used to shut off the sea from the trim system and the magazine flood valve. The magazine flood valve guarantees, when the sea stop valve is open, an immediate source of sea water to the ammunition stowage and the pyrotechnic locker.

As stated before, the main function of the trim system is to shift and adjust the distribution of weight throughout the submarine. This is done by transferring water ballast

  from one variable tank to another, adding water to the variable tanks or discharging excess water from the tanks overboard. The water handled by the trim system is measured in pounds; and a gage, graduated in pounds to show the amount of water transferred by the trim pump, is located above the trim manifold where the operator can observe its readings.

Because the trim pump used on the fleet type submarine is of the centrifugal type, it must be primed before beginning the operation. A priming pump is used for this purpose. It primes the trim pump by removing all air from the trim pump casing, the trim manifold, and the lines leading to it, thus allowing water to replace the air in this equipment and fill it completely. (See Section 2B2 on the priming pump for a more detailed discussion of its operation.)

NOTE: The previously installed reciprocating-type trim pump will be replaced by the centrifugal model on all fleet-type submarines. A number of vessels have a "Deepwell" type pump. This pump is similar to the centrifugal unit although the priming arrangement is different.

The trim system can also be used to supply or drain water from the torpedo tubes. Water for torpedo tube flooding is normally taken from the WRT tanks through the torpedo tube flood and drain lines. These lines are controlled by the torpedo tube flood and drain valves.

The trim line forward and the trim line aft are provided with hose connections, one in each compartment of the submarine. These connections can be used for fire fighting, or for bilge suctions in those compartments without bilge suction facilities. Of course, if the connections are used for bilge suction, the trim line must be on SUCTION, and if for fire fighting, the line must be on DISCHARGE.

 
B. TRIM PUMP
 
2B1. Source of power. The trim pump (see Figure 2-1), located on the port side of the pump room just forward of the after bulkhead, is driven by a 10-to-25-horsepower motor   directly connected to the drive shaft of the trim pump by means of a flexible coupling.

The controller relay panel for the motor is mounted on the after bulkhead of the pump

 
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Figure 2-1. Trim pump.
Figure 2-1. Trim pump.
 
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Figure 2-2. Trim pump controls.
Figure 2-2. Trim pump controls.
room. However, the motor is started or stopped by push-button controls in the control room. Once started by these controls, the speed of the pump, and thereby the rate at which water is moved in the system, is regulated by a rheostat control also located in the control room just below the push-button switches (see Figure 2-2). Although the trim pump is driven by an electric motor, the starting of the motor does not guarantee that the trim pump will pump water, for since the trim pump is of the centrifugal type, it cannot pump air. Therefore, it cannot be operated until the system is free of air.

2B2. Priming pump. Freeing the system of air is the function of the priming pump, located outboard of the trim pump. Since any appreciable amount of air entering the inlet side of the trim pump will cause it to lose suction and thereafter run without pumping, it is necessary to use the priming pump to eliminate the air. A vacuum gage, mounted

  in the control room, provides a check on the satisfactory operation of the priming pump. If the trim pump is started and there is no indication of flow, the priming pump should be started at once to insure that the trim pump is fully primed, before other sources of trouble are investigated.

The priming pump, like the trim pump, is started or stopped by push-button controls in the control room. The priming pump is a vacuum pump with a float valve in the line running from the priming pump to the trim manifold and the trim casing. The valve consists of a float with a ball-ended stem. The purpose of the float is to permit the passage of air and to prevent the passage of sea water into the priming pump. As the water rises in the float valve, the upper part of the ball-ended stem is automatically forced against the valve seat, thus preventing sea water from entering the priming pump. When the float valve is filled with water, the vacuum gage

 
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will read about 20 inches of vacuum and the system is fully primed.

The priming pump is of the water-piston type and consists of three major parts 1) rotor, 2) lobe, and 3) port plate. The rotor is made up of a series of curved plates projecting radially from the hub. The lobe is elliptical in shape and forms the outer casing for the rotor. The port plate consists of-two inlet and two outlet ports corresponding to the inlet and outlet ports on the rotor. The pump is end-mounted on the direct driving electric motor as shown in Figure 2-3.

Before starting the priming pump, it is necessary first to provide sealing fresh water to it. This water is needed to fill the lobe partially and provide a water seal. Fresh water should be added until the seal water gage shows 2/3 full (see Figure 2-3). Serious damage may result if the pump is allowed to run in a dry condition. The motor is then started by the push-button control in the control room.

In operation, the rotor revolves in the lobe, which has been partially filled with water, at a speed high enough to throw the water out from the hub by centrifugal force. This results in a solid elliptical-shaped ring of water revolving at the same speed as the rotor. Referring to Figure 2-3, it will be seen that a ring of water for a given rotor section, guided by the lobe, will move in and out from the hub, forming a liquid piston. As the rotor passes the inlet port, the water ring is farthest from the hub and air is permitted to enter. As the rotor advances to the discharge port, the air space becomes less and air is forced out the discharge port. This cycle is repeated twice for each revolution of the rotor.

2B3. Operation of the trim pump. A brief review of the general principles of the centrifugal pump will be helpful in understanding the operation of the trim pump. A centrifugal pump, as the name implies, employs centrifugal force to move a liquid from a lower to a higher level. In its simplest form, this type of pump consists of an impeller rotating in a watertight casing which is provided with inlet and outlet ports.

  The impeller consists of two parallel disks with curved vanes, or bulkheads, radiating from the hub and between the disks. One of these disks (upper or lower, depending upon where the water is brought in) has an inlet port, or circular opening, called the eye, which is concentric with the hub of the impeller. Actually then, one disk holds the impeller to the shaft while the other admits the water. The periphery of the impeller is open, as shown in Figure 2-1.

In operation, water enters the eye of the impeller, is picked up by the vanes and accelerated to a high velocity by the rotation of the impeller, and then discharged by centrifugal force into the casing and out the discharge port. When water is forced away from the eye of the impeller, pressure in this area is lowered ("suction" is created), and more water flows in. Consequently there is a constant flow of water through the pump. Considerable air in the inlet port of the pump will interrupt the action of the pump since, upon entering the impeller, it will break the suction which is dependent on the presence of water at the eye. For this reason, the pump casing and the system served by the pump must be completely filled with water before starting to pump.

The centrifugal pump just described has only one impeller and is known as a single-stage pump. A pump with four impellers may be known as a four-stage pump; with six impellers, a six-stage pump; and so forth. In actual practice, however, any pump with more than one stage is referred to as a multi-stage pump.

The mechanical details of the trim pump are shown in Figure 2-1. It will be seen that the pump is a six-stage centrifugal pump. The valve on the forward end permits either parallel or series operation and is manually operated. The schematic diagram in the upper right corner of the illustration shows the flow of the water being pumped, for both series and parallel operation. With the manually operated series-parallel valve in the SERIES position, the incoming water enters the first stage, proceeds through the second and third stages, and then back through the series-

 
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Figure 2-3. Priming pump.
Figure 2-3. Priming pump.
 
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parallel valve to the fourth, fifth, and sixth stages. With the series-parallel valve in the PARALLEL position, half of the inlet water proceeds through the first, second, and third stages, and is then discharged through the series-parallel valve. Simultaneously, the other half of the inlet water is directed by the series-parallel valve to the fourth, fifth, and sixth stages, and is then discharged directly. Series operation of the pump produces twice the discharge pressure, but only half the volume produced by parallel operation. The pump is operated in series only when the submarine is at a depth of approximately 250 feet or more and discharging to the sea; the higher pressure is necessary to overcome the greater sea pressure encountered at that depth.

In summary, it must be remembered that before starting the trim pump after installation or reassembly, it is necessary to make certain that the trim system lines and the pump casing are free of air. After the trim pump has been used, the casing should remain primed, because of the location of the pump in relation to the trim manifold. But if flow does not commence after starting the trim pump, the priming pump should be used to

  eliminate the air before restarting the trim pump.

The trim pump should not be operated at speeds greater than are necessary to produce the rate of flow specified for a given depth.

The following table lists the proper valve position and pump output in pounds of water per minute, recommended for different depths.

DepthPump OutputValve Position
On surface 1500-2500 lbs. per min. Parallel
0-200 ft. 1500 lbs. per min. Parallel
Trimming- tank to tank 1500 lbs. per min. Parallel
200-250 ft. 1250 lbs. per min. Parallel
250-400 ft. 1000 lbs. per min. Series
400 ft. or more 1000 lbs. per min. Series

The pump should not be operated at a motor speed greater than 2400 revolutions per minute. Excess speeds place an overload on the bearing and mechanical parts of the pump and motor and may cause breakdown.

 
C. MANIFOLDS
 
2C1. Trim manifold. In Section 2A, the trim manifold is referred to as the center of distribution for the trim system. It acts as a switchboard between the trim pump and the lines of the system, providing a centralized station to direct the flow of water to and from the variable tanks. Used in connection with the trim manifold, but connected to each variable tank, is a measuring gage, or liquidometer. These gages record the amount of water in each tank and provide the diving officer with an indication of the amount of water ballast being redistributed by the trim manifold through the trim system. The trim manifold is mounted hip-high on the port side of the control room just forward of the after bulkhead. The gage board is mounted directly above it.

Figure 2-4 shows the mechanical construction of the trim manifold, with the proper nomenclature of its parts, as used in this

  manual. The manifold is a boxlike, two-piece casting, divided internally into two longitudinal compartments known respectively as the suction and discharge sides. The suction side contains eight suction control valves, while the discharge side has eight discharge, or flood, control valves. Each of these sixteen valves is of the disk and seat type, with rising stems and individual bolted-on bonnets. Name plates, attached to each bonnet, indicate the function of that particular valve.

Starting from the after end outboard of the manifold, the valves control the functions indicated in the table on page 11.

The discharge valves are all on the starboard side of the manifold, with the corresponding suction valves opposite them on the port side. A special wrench for operating the valves is provided.

Flanged outlets are cast integral with the manifold to connect with the lines of the

 
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Figure 2-4. Trim manifold.
Figure 2-4. Trim manifold.
 
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Outboard-Suction Inboard-Discharge
1. Trim pump suction 9. Trim pump discharge
2. Auxiliary ballast tank No. 2 suction 10. Auxiliary ballast tank No. 2 discharge
3. Auxiliary ballast tank No. 1 suction 11. Auxiliary ballast tank No. 1 discharge
4.Safety tank suction 12. Safety tank discharge
5.Negative tank suction 13. Negative tank discharge
6. After trim line suction 14. After trim line discharge
7. Forward trim line suction 15. Forward trim line discharge
8. Sea suction 16. Discharge to sea

system. Two outlets on the after end lead to the drain line cross connection and to the drain pump discharge, to permit emergency use of the drain pump for actuating the trim system.

In all pumping operations, the trim pump suction and the trim pump discharge valves on the manifold must be opened to permit flaw within the system. To flood a tank, the discharge valve for that tank must be opened at the trim manifold; to pump a tank, its suction valve must be opened. This should be done before the trim pump is started. All valves on the manifold should be shut immediately after the pumping operation is complete. Figure 2-4 shows the direction of flow when flooding or pumping auxiliary ballast tank No. 2.

Fully detailed instructions for specific trimming operations are given in Chapter 4. They explain the exact procedure to be followed in operating the trim manifold in conjunction with the other units of the trim system.

2C2. Forward and after WRT and trim tank manifold. The WRT and trim tank manifolds are used in conjunction with the trim manifold to control the flooding and pumping of the WRT tanks and the trim tanks, both fore and aft.

The forward trim manifold (Figure 2-5) is located in the forward torpedo room, port

  side, aft of the torpedo tubes. The after trim manifold is located in the after torpedo room, port side, forward of the torpedo tubes (see Figure 3-9).

The forward and the after WRT and trim tank manifolds are identical in operation and construction, differing only in the fact that they serve different tanks.

The body of each trim manifold is a two-chambered casting containing two valves which control flood and suction of the WRT tank and the trim tank, respectively. The after valve in the after torpedo room and the forward valve in the forward torpedo room control the trim tank. The valves are of the disk and seat type with bolted bonnets. The connecting passage between chambers of the integrally cast valve casting allows either valve to be operated independently. The handwheels carry name plates designating the uses of the individual valves.

When open, the manifold valve marked TRIM TANK FLOOD AND SUCTION permits the flooding or pumping of the trim tank from, or into, the trim system when the trim line is on service.

The other valve, marked WRT TANK FLOOD AND SUCTION, permits the flooding or pumping of the WRT tank from, or into, the trim system when the torpedo tube drain stop valve to the WRT tank is open.

2C3. Torpedo tube drain manifold. In Section 2A the flooding and draining of the torpedo tubes were mentioned as one of the functions of the trim system. This function is controlled by the torpedo tube drain manifolds. Two of these manifolds are located in the forward torpedo room, each servicing three torpedo tubes; two are located in the after torpedo room, each servicing two torpedo tubes. Figure 3-9 shows the location of these manifolds (the forward manifold servicing the three starboard tubes is not shown in the illustration). In each case, the control levers are adjacent to the manifold.

The body of the torpedo tube drain manifold is a three-chambered casting, housing three cam-actuated plunger-type valves, and provided with flanged outlets for connection to the trim system and to the torpedo tube

 
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Figure 2-5. Forward WRT and trim tank manifold.
Figure 2-5. Forward WRT and trim tank manifold.
 
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Figure 2-6. Torpedo tube drain manifold.
Figure 2-6. Torpedo tube drain manifold.
 
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drains. The cam mechanisms are attached to the back of the casting. Separate control levers and connections are provided for each of the valves (see Figure 2-6).

Each hand lever operates one cam, through the action of its connecting rod and cam lever. In Figure 2-6 the cam and valve are shown in

  the open position, permitting water to flow to or from the torpedo tube drains. In draining or flooding the tubes, the manifold valves are used in conjunction with the torpedo tube drain stop valve to the WRT tank, which must be open when draining from the tubes to that tank.
 
D. VALVES
 
2D1. Trim pump sea stop valve. In discharging water ballast from any part of the trim system to sea, the trim pump sea stop valve must be opened, thus providing a passage from the trim manifold through the pressure and outer hulls to the sea. The same line is used to permit water to enter the system from the sea when additional water ballast is to be added. This valve is located on the port side of the control room, directly below the trim manifold (see Figure 3-9).

The sea stop valve is of the rising stern disk and seat type, with a bolted bonnet. Flanged connections are provided to the sea discharge line, the trim manifold, and the magazine flood line. A guide which extends below the valve disk serves to center and seat the valve disk. The mechanical construction is shown in Figure 2-7.

The connection to the pressure hull is a flange, cast integral with the valve body below the valve seat to insure a pressure-tight connection to the pressure hull. The lower part of the valve body, with the screwed flange, projects through the pressure hull and connects to the line overboard.

Turning the handwheel counterclockwise to the OPEN position raises the, valve disk and permits the suction of sea water into the trim system, the direct flooding of the magazine, or the overboard discharge of water from the trim system.

Turning the handwheel clockwise seats the valve disk and cuts off the suction from, or discharge to, the sea. In Figure 2-7 it will be seen that the handwheel controls only the up-and-down movement of the stem; therefore, only the suction from, or discharge to, sea is affected by this handwheel. The side outlets remain open irrespective of the valve

  stem position, thereby providing a means of supplying water to flood the magazine and the trim system simultaneously.

2D2. Torpedo tube drain stop valve to the WRT tank. The torpedo tube drain stop valve to the WRT tank serves as a stop valve between that tank and the individual torpedo tube drain valves.

There is a torpedo tube drain stop valve to the WRT tank in both the forward and the after torpedo rooms (see Figure 3-9). Both of these valves are identical in function and construction (see Figure 2-8).

The torpedo tube drain stop valve to the WRT tank is a globe-type valve with a bolted yoke-type bonnet and a rising stem. The mechanical construction is shown in Figure 2-8. The inscription plate carries the valve designation. In the OPEN position, the valve permits water to be blown into the torpedo tubes from the WRT tank, or to be drained back into the tank from the torpedo tubes, provided the individual torpedo tube drain valves are open. The valve is also opened to flood or drain the WRT tank by the trim line. All flow of water to and from that tank is cut off by closing the valve.

2D3. Magazine flood valve and testing casting. The magazine flood valve and testing casting (see Figure 2-9) provide an emergency method of flooding the magazine compartment. This is another secondary function of the trim system.

The magazine flood valve is used to control this emergency flooding system. The testing casting is used in checking the magazine flood valve to make certain that it is ready for immediate use. Both the magazine flood valve and the testing casting are located in the control room on the magazine flood line

 
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Figure 2-7. Trim pump sea stop valve.
Figure 2-7. Trim pump sea stop valve.
 
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Figure 2-8. Torpedo tube drain stop valve to WRT tank.
Figure 2-8. Torpedo tube drain stop valve to WRT tank.
 
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Figure 2-9. Magazine flood valve and testing casting.
Figure 2-9. Magazine flood valve and testing casting.
 
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of the trim system. The accessory box containing the operating plug and wrench is mounted directly above the testing casting.

The magazine flood valve is a disk and seat type globe stop valve with a bolted bonnet, rising stem, and flanges for connection to the testing casting and the sea stop valve. A cylindrical box bolted to the bonnet encloses the stem. The end of the protecting box has a hinged glass door equipped with a hasp-and-chain padlock. Protecting the glass cover, is a smaller, hinged metal cover. Inside the box is a crank that fits the valve stem.

The magazine flood valve is used only in emergency to flood the magazine compartment. The outer metal cover and the glass door are unlocked and opened during periods of possible emergency or when testing. In cases of emergency, the glass door should be broken. The valve is opened, using the crank, to flood the magazine.

  The testing casting is used for periodic testing of the magazine flood valve to make certain that the flood valve is in operating condition and ready for use when needed.

The testing casting is T-shaped, with the long leg and one arm flanged for connection to the magazine flood line and the flood valve. The other arm is threaded to receive the protecting cap. The operating plug, in the accessory box, fits the inside threads in the testing casting. The wrench fits the cap and the plug.

In testing the magazine flood valve, the cap is removed from the testing casting and the plug is screwed in tightly. After the valve is opened, the vent in the magazine line is opened to see if the line contains water. If the valve is operating correctly, the line will contain water. The flood valve is closed and the line through the testing casting drain is drained before the plug is removed. Next, the plug is removed and the cap replaced, making

Figure 2-10. Trim line hose connection.
Figure 2-10. Trim line hose connection.
 
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the testing connection watertight. The plug and wrench are then replaced in the box, and the door to the valve stem enclosure is locked.

2D4. Trim line hose connection. The trim line hose connections (Figure 2-10) may be used as fire main outlets or as additional bilge suction. They may be used in pumping a compartment, or any area not covered by the drain system, when there is suction in the trim lines.

The forward trim line has three hose connections; one in the forward torpedo room, one in the forward battery compartment, and one in the control room. The after trim line has five hose connections: one each in the after battery compartment, the forward engine room, the after engine room, the maneuvering room, and the after torpedo room.

  The hose connection is a globe valve with a rising stem. It has one end flanged for connection to the trim line and the other end threaded for a hose coupling. A locking cap attached to the valve body by a chain, and secured with a padlock, fits onto the hose coupling end.

In using the hose connection either for flooding or pumping, the cover is unlocked and removed. The hose is attached by coupling to the threaded end of the connection. The valve is opened by turning the handwheel counterclockwise, thus providing suction or flooding as required. When sufficient water has been obtained or removed, the valve is shut, the hose removed, and the cover replaced and locked. When not in use, the valve should be kept shut and the cover locked to prevent possible leakage.

 
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