Chapter 14. -PIPING, FITTINGS, AND VALVES 



in the body, the solid part of the plug blocks the 

 ports and thus prevents the flow of fluid. 

 Plug valves are quite commonly used in 

 connection with auxiliary machinery. The pet- 

 cocks that are used as vents on lubricating oil 

 coolers for auxiliary machinery are usually 

 plug valves. The three-way and four-way cocks 

 that allow selective routing of various fluids 

 are usually variations of the plug valve. The 

 shut off device that allows fuel oil or lubricating 

 oil to be diverted from one basket to another 

 of a duplex strainer is often a modified plug 

 valve. 



Piston Valves 



A piston valve is a stop valve that may be 

 thought of as a combination of a gate valve and 

 a plug valve. The piston valve consists basically 

 of a cylindrical piston operating in a hollow 

 cylinder. The piston is attached to the valve 

 stem, and the valve stem is attached to a hand- 

 wheel. When the handwheel is turned, the piston 

 is raised or lowered within the hollow cylinder. 

 The cylinder has ports in its walls. When the 

 piston is raised, the ports are uncovered and 

 fluid is allowed to pass through the valve. 



Needle Valves 



Needle valves are stop valves that are used 

 for making relatively fine adjustments in the 

 amount of fluid that is allowed to pass through 

 an opening. The distinguishing characteristic 

 of a needle valve is the long, tapering, needle- 

 like point on the end of the valve stem. This 

 "needle" acts as the valve disk. The longer 

 part of the needle is smaller than the orifice 

 in the valve seat, and therefore passes through 

 it before the needle seats. This arrangement 

 permits a very gradual increase or decrease 

 in the size of the opening and thus allows a 

 more precise control of flow than could be ob- 

 tained with an ordinary globe valve. 



Needle valves are used as overload nozzles 

 on some auxiliary turbines. Needle valves are 

 often used as component parts of other more 

 complicated valves. For example, they are 

 used in some types of reducing valves. Most 

 constant-pressure pump governors3 have needle 

 valves to minimize the effects of fluctuations 

 in pump discharge pressure. Needle valves 



■^Constant -pressure pump governors are discussed in 

 chapter 16 of this text. 



are also used in some components of automatic 

 boiler control systems. 



Butterfly Valves 



The butterfly valve, shown in figure 14-15, 

 is being used increasingly in naval ships. The 

 butterfly valve has some definite advantages 

 for certain services. It is light in weight, it 

 takes up less space than a gate valve or a globe 

 valve of the same capacity, and it is relatively 

 quick acting. The butterfly valve provides a 

 positive shutoff and may be used as a throttling 

 valve set in any position from full open to full 

 closed. 



Butterfly valves vary somewhat in design 

 and construction. However, a butterfly-type disk 

 and a positive means of sealing are common to 

 all butterfly valves. 



The butterfly valve described and illustrated 

 here consists of a body, a resilient seat, a 

 butterfly-type disk, a stem, packing, a notched 

 positioning plate, and a handle. The resilient 

 seat is under compression when it is mounted 

 in the valve body, thus making a seal around 

 the periphery of the disk and both upper and 

 lower points where the stem passes through 

 the seat. Packing is provided to form a positive 

 seal around the stem if the seal formed by the 

 seat should become damaged. 



To close a butterfly valve, it is only neces- 

 sary to turn the handle a quarter of a turn in 

 order to rotate the disk 90 degrees. The resi- 

 lient seat exerts positive pressure against the 

 disk, ensuring a tight shutoff. 



Butterfly valves may be designed to meet 

 a variety of requirements. The shipboard sys- 

 tems in which these valves are now being used 

 include fresh water, salt water, JP-5, Navy 

 special fuel, diesel oil, and lubricating oil. 



Check Valves 



Check valves are designed to permit flow 

 through a line in one direction only. There are 

 almost innumerable examples of check valves 

 throughout the engineering plant. Check valves 

 are used in open funnel drains, in fuel oil heater 

 drains, and in various other drains. They are 

 used in connection with many pumps, and in 

 any line in which it is important to prevent the 

 back flow of fluid. 



The port in a check valve may be closed by 

 a disk, a ball, or a plunger. The valve opens 

 when the pressure on the inlet side is greater. 



375 



