Principles of naval engineering - Historic Naval Ships Association

Chapter 14. -PIPING, FITTINGS, AND VALVESin the body, the solid part of the plug blocks theports and thus prevents the flow of fluid.Plug valves are quite commonly used inconnection with auxiliary machinery. The petcocksthat are used as vents on lubricating oilcoolers for auxiliary machinery are usuallyplug valves. The three-way and four-way cocksthat allow selective routing of various fluidsare usually variations of the plug valve. Theshut off device that allows fuel oil or lubricatingoil to be diverted from one basket to anotherof a duplex strainer is often a modified plugvalve.Piston ValvesA piston valve is a stop valve that may bethought of as a combination of a gate valve anda plug valve. The piston valve consists basicallyof a cylindrical piston operating in a hollowcylinder. The piston is attached to the valvestem, and the valve stem is attached to a handwheel.When the handwheel is turned, the pistonis raised or lowered within the hollow cylinder.The cylinder has ports in its walls. When thepiston is raised, the ports are uncovered andfluid is allowed to pass through the valve.Needle ValvesNeedle valves are stop valves that are usedfor making relatively fine adjustments in theamount of fluid that is allowed to pass throughan opening. The distinguishing characteristicof a needle valve is the long, tapering, needlelikepoint on the end of the valve stem. This"needle" acts as the valve disk. The longerpart of the needle is smaller than the orificein the valve seat, and therefore passes throughit before the needle seats. This arrangementpermits a very gradual increase or decreasein the size of the opening and thus allows amore precise control of flow than could be obtainedwith an ordinary globe valve.Needle valves are used as overload nozzleson some auxiliary turbines. Needle valves areoften used as component parts of other morecomplicated valves. For example, they areused in some types of reducing valves. Mostconstant-pressure pump governors3 have needlevalves to minimize the effects of fluctuationsin pump discharge pressure. Needle valves^Constant -pressure pump governors are discussed inchapter 16 of this text.are also used in some components of automaticboiler control systems.Butterfly ValvesThe butterfly valve, shown in figure 14-15,is being used increasingly in naval ships. Thebutterfly valve has some definite advantagesfor certain services. It is light in weight, ittakes up less space than a gate valve or a globevalve of the same capacity, and it is relativelyquick acting. The butterfly valve provides apositive shutoff and may be used as a throttlingvalve set in any position from full open to fullclosed.Butterfly valves vary somewhat in designand construction. However, a butterfly-type diskand a positive means of sealing are common toall butterfly valves.The butterfly valve described and illustratedhere consists of a body, a resilient seat, abutterfly-type disk, a stem, packing, a notchedpositioning plate, and a handle. The resilientseat is under compression when it is mountedin the valve body, thus making a seal aroundthe periphery of the disk and both upper andlower points where the stem passes throughthe seat. Packing is provided to form a positiveseal around the stem if the seal formed by theseat should become damaged.To close a butterfly valve, it is only necessaryto turn the handle a quarter of a turn inorder to rotate the disk 90 degrees. The resilientseat exerts positive pressure against thedisk, ensuring a tight shutoff.Butterfly valves may be designed to meeta variety of requirements. The shipboard systemsin which these valves are now being usedinclude fresh water, salt water, JP-5, Navyspecial fuel, diesel oil, and lubricating oil.Check ValvesCheck valves are designed to permit flowthrough a line in one direction only. There arealmost innumerable examples of check valvesthroughout the engineering plant. Check valvesare used in open funnel drains, in fuel oil heaterdrains, and in various other drains. They areused in connection with many pumps, and inany line in which it is important to prevent theback flow of fluid.The port in a check valve may be closed bya disk, a ball, or a plunger. The valve openswhen the pressure on the inlet side is greater.375