Valves

Valves Valves are devices that control the flow of fluids or gases through a piping system Valves can provide on-off service only, allow different flow rates, relieve excess pressure , or prevent reversal of flow. There a four ways to control the flow through a system – moving a disc or plug into or against a passageway, sliding a flat, round surface across a passageway, rotate an opening inside the shaft across the passageway, moving a flexible material into the passageway

Valves that start and stop flow Gate valves – start and stop liquid flow, but do not regulate or throttle flow A disc is turned into the flow stream Little drop in pressure when the valve is open because there is little resistance to flow This valve should only be used in the fully open or fully closed position because partially open can cause disc and seat wear

Knife gate valve – this valve is used in slurry and waste lines and other low pressure applications The sharp edge of the disc is forced closed in contact with a metal or elastomeric seat The disc can cut through deposits and flow-stream solids like resin slurry or pulp

Ball valves – use a ball to start or stop flow They are rotary action valves The ball has a hole in it – when open the ball is turned to where the hole is in line with the inlet and outlet – when closed the hole is perpendicular to the openings Most ball valves are quick acting and only require a 90 degree turn to open or close They are generally not good for throttling because it erodes from the flow

Plug valves – rotary action valves – the stop disc is on the shape of a plug The disc is solid with a bored passage at a right angle to the longitudinal axis of the plug The openings of the valve may be different shapes and sizes to regulate flow

Three-way valves – installed at the intersection of three lines They direct flow between two lines and block off the third line

Valves that regulate flow Control and provide accurate flow through a system Globe Butterfly Diaphragm

Globe valve – disc can be totally removed or can completely close the flow path The disc moves perpendicular to the seat which allows good throttling ability The disc can close in the same direction as fluid flow or against direction of fluid flow

Butterfly valve A quarter turn valve with a plate like disc that stops flow when the outside area of the disc seals against the inside of the valve body They have a arrow stamped on it to tell the direction of the flow through the valve They must be installed in the proper flow direction or it will not seal

Diaphragm valve Flow is controlled by a flexible disc that is connected to a compressor by a stud molded into the disc The valve stem moves the compressor up and down to control flow

Needle valves Make fine adjustments in the amount of fluid allowed to pass through an opening

Valves that relieve pressure Safety valves – used in steam, air, or other gas pipelines – they operate in the closed position until the pressure in the line rises above the preset pressure limit – it will remain open until the pressure drops, then the valve snaps shut Pressure relief valves – used to relieve pressure in liquid services – when the pressure gets too much, the disc that is attached to a spring rises to release the pressure – when the pressure is released the spring pushes the disc back into place There is also a pressure relief valve that is called a rupture disc – the disc ruptures when the pressure gets too high – can not be reused

Valves that regulate direction of flow Check valves prevent backflow – only permit flow in one direction Swing check valves – disc that swings open or closed Ball check valve – uses a ball to allow fluid to pass through the valve – not spring loaded Butterfly check valve – two vanes that fold back from a central hinge to open but fold back to prevent backflow

Valve actuators Provide automatic control of the valve or to reduce the effort required to manually operate a valve Gear operators, chain operators, pneumatic and hydraulic actuators, electric or air motor driven actuators

Installing valves Location is very important – direction of flow, height of the valve, ability to work on the valve