Chapter 7 Control Valves

Introduction Cavitation has been a familiar phenomenon for a long time particularly in shipping. In 1917, the British physicist Lord Rayleigh was asked to investigate what caused fast-rotating ship propellers to erode so quickly. He discovered that the effect of cavitation, already proved in experiments by Reynolds in 1894, was the source of the problem. Despite numerous investigations into the subject of cavitation in the years that followed, many of the accompanying effects have still not yet been completely explained. This is no wonder considering the complexity of the process involving the areas of acoustics, hydrodynamics, thermodynamics, optics, plasma physics and chemistry. Cavitation can be caused in a fluid by energy input. For example, a laser beam creates a plasma in liquids which causes the liquid to evaporate creating a cavity. Ultrasonic waves can be used to induce complex high-frequency alternating compression and rarefaction phases in liquids which cause cavitation. In this way, cavitation effects can be applied usefully for cleaning surfaces, for non-invasive operations in the field of medicine and for breaking down agglomerates in the textile finishing industry. In sewage treatment plants, cavitation is used to break down molecules and bacteria cell walls, break up pollutants and dissolve out minerals from organic material Furthermore, cavitation can arise in hydrodynamic flows when the pressure drops. This effect is, however, regarded to be a destructive phenomenon for the most part. In addition to pump rotors, control valves are particularly exposed to this problem since the static pressure at the vena contracta even at moderate operating conditions can reach levels sufficient for cavitation to start occurring in liquids.

DEFENITION THE CONTROL VALE IS THE EARLIEST KNOWN FINALCONTROL ELEMENT WHICH DOES EITHER OF THE FOLLOWING THREE FUNCTIONS: 1. DISPENSING APPLICATION (REGULATE FLOW) 2. DISSIPATING APPLICATION (PRESSURE LET DOWN) 3. DISTRIBUTING APPLICATION (DIVIDE PROCESS FLOW)

Introduction   Valves were mentioned in the earlier module Pipework, where they were listed with other pipe fittings. Because they are the most important part of any piping system, they are described more fully in this module. The gas liquefaction process uses a lot of valves. ADGAS uses many different types of valves in a large range of sizes.   Valves control the flow of fluids through pipes by:  

• starting and stopping the flow—to control the process or isolate part of a pipeline
• changing the flow rate—allowing more or less fluid to flow
• re-directing flow from one line to another at a pipeline junction
• allowing flow in one direction only
• reducing fluid pressure
• keeping the pressure in a container or pipeline below a fixed maximum
• preventing accidents by relieving overpressure in a container or pipeline.

It is very important to use the correct:  

• valve type—to suit the task it performs, as described above
• size—to suit the pipe size and the flow rate required
• material—to suit the fluid passing through it and to avoid corrosion The ADGAS Piping Specifications list the valves used for all applications on the plant.