阀的种类 - 请对照学习

以下是我在做实习生的时候摘录的一些基本知识，大家可以结合中文教材学习
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Types of valves
the cargo piping system of a typical tanker incorporates valves of various designs. these include the following:
1. gate valves.
as the name implies, this type of valve employs a metal gate, fitted in grooves, which slides across the valve opening, the gate is fitted with a threaded spindle which connects to the valve stem. when turned by means of a handwheel, the spindle edges the gate slowly upward or downwrd. it is necessary to turn the handwheel many times--perhaps 30 or more --to move the gate from fully open to fully closed. gate valves are therefore too slow and cumbersome to be easily adapted to automatic control.
but they have advantages. they are dependable and durable. when fully open, the gate retracts into the valve body and offers no resistance to the flow of oil--a definite advantage over valves with swinging gates.
2. butterfly valves
butterfly valves are fitted with a swinging gate, which remains in contact with cargo even when the valve is fully open. butterfly valves offer several advantages:
a. they operate easily and quickly (1/4 turn swings the gate from fully open to fully closed).
b. they adapt readily to automatic control.
c. they are more compact and less expensive than gate valves.
they also have several disadvantages:
a. it is generally more difficult to adjust cargo flow precisely with a butterfly valve than with a gate valve.
b. the valve gate remains in the center of flow, even when fully open, thus offering resistance and slowing the rate.
c. butterfly valves tend to develop leaks more readily and need more frequent maintenance than gate valves.
3. globe valves
globe valves are not as common in cargo systems as gate and butterfly valves, but they are favored on some vessels. they operate as follows: a round disc is fitted on the end of a threaded stem and as the stem is turned with the handwheel, the disc wedges into the valve aperture, directly against the flow of oil. this action is a little like stopping a bottle with a cork. globe valves tend to be difficult to operate in large sizes and at high pressures, and because oil must change direction as it flows through the valve, an undesirable pressure drop may occur on the outlet side.nevertheless, golbe valves are valuable whenever a precise throttling, or control of pressure, is desired.
4. angle valves
angle valves are basically globe valves in which the inlet and outlet flanges are turned at a 90-degree angle to each other, forming an elbow. angle valves produce less resistance to flow than a globe-valve/elbow combination; they are therefore the economical choice when a valve is needed at a bend in a pipeline. Like globe valves, angle valves provide excellent throttling control in the partially open position.

special valves
certain valves perform special functions in the cargo system:
1. check valves
to prevent oil from backflowing into the tanks, a check valve is normally installed on the discharge side of each centrifugal pump.
each valve contains a weighted or spring-loaded gate that swings up under pressure, allowing oil to flow out the discharge line. when the pump stops and pressure ceases, the gate swings shut against the valve seat, preventing oil from running back into the tanks.
because check valves operate atomatically, many officers tend to forget them or take them for granted. They are foolish to do so because check valves somtimes stick, causing spills when least expected.
you should never assume a check valve is working properly. whenever the pumps are stopped, even temporarily, close the block valves at the manifold for sure protection against cargo backflow.
2. relief valves
Each cargo pump is equipped with a relief valve and a short recirculating line. Whenever the pressure becomes excessive on the discharge side of the pump, the relief valve opens and allows oil to recirculate to the suction side, thereby relieving the pressure.

valve markings
on most tankers a system of one sort or another is used for marking valves and main-deck handwheels that operate valves inside the tanks. This practice reduces confusion and diminishes the risk of serious mistakes.
pumproom valves are often numbered; numbers are stencilled on each valve and are also indicated on a plan kept in the ship's office. Manifold valves are sometimes marked to indicate which pumpimg system each line nermally ties into(for example, No 1 pump-1,2,3,4, across).
the marking of handwheels on the main deck is especially helpful because they operate valves remotely (a situation where crew members are most likely to become confused). Tank suction handwheels are commonly painted a characteristic color, such as bright green. In addition, the tank number may be stencilled on deck next to the handwheel(1C, 3P, 5S, etc.).
In the same manner, crossovers and master valves are distinguished by the color of their handwheels(for example, master painted yellow; crossovers, red and white). Stripping valves may also be color coded.
 
Valves used on tanker
on NEW WORLD, bottom valves of main cargo pipes inside of tanks use B(bravo) to indicate, e.g. B01--B27; cross valves beween cargo pipes inside tanks use C(charli), e.g. C01--C25; drain valves use D(delta), e.g. D01-D05 & D18(starboard side), D10--D14 & D19(port side), D20--D22; valves of slop tanks use SL, e.g. SL1--SL8; sea water pipes use W(whiskey), e.g. W01--W27; drain valves conneting cross valves on deck(113,213 and 313) use A(alfa), e.g. A10,A11,A12. On main cargo lines on deck, drop valves are 110, 210 and 310; and master valves are 109, 209 and 309.
    a. Gate valve:like a door,turn the screw connected to the gate,to let it up for openig,or let it down to colse.Normally,clockwise turning is for closing,and anticlockwise is for opening.It can endure a high pressure,usually used for foam monitor valves, or on fire line.
    b. Butterfly valve: widely used on board. It can be closed or opened very easily: turn the screw or handle  on it ,let the butterfly to turn 90 degrees for opening or closing. It can endure as high pressure as 20kg/ cm2.They are widely used on cargo line, ballast line, COW line and IGS.
    c. Globe valve: can be used as a non-return valve.

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2nd Officer
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