The History of Valves
Prior to Greek and Roman times, little is known about the methods used to control the flow of fluids. Some form of sluice gate was obviously used to hold and retain water in irrigation channels and we know there was some knowledge of the principles of flow because of the water clocks made by the early Egyptians.

The Greek and Roman periods saw the development of many mechanical and hydraulic machines and the first use of valves of sophisticated design. In the case of the plug cock valve, the design remained virtually unchanged until the 19th century.
Flap valves and coin valves were the forebears of the present swing and lift check valves and were used in the water force pumps. Bronze and brass plug cocks were in common use as stop valves on water mains and supply pipes to public and domestic buildings during the Roman period. A large bronze cock-valve was found on Capri among the ruins of the Palace of Tiberius, built around AD25.
The early years of the 18th century marked the start of the Industrial Revolution and the arrival of the steam engine as a practical and commercial proposition. In 1698, Thomas Savery had patented his engine for the 'raising of water' and in 1705 Thomas Newcombe introduced his advanced version of Savery's engine, the atmospheric beam engine. James Watt provided the decisive step forward in the development of the steam engine when he patented the separate condenser in 1769.

During the Nineteenth Century, a number of eminent engineers directed their attention to valves, notably Timothy Hackworth, who introduced adjustable springs instead of weights to the steam safety valve. This valve is preserved in the Science Museum in London.

Another major innovation was the introduction of the groove-packed plug-cock by Dewrance & Co. in 1875. This made the valve easier to operate and more suitable for use with steam.

In 1886, Joseph Hopkinson introduced the parallel slide valve, in which the sealing of the valve was effected by the line pressure on the disc - a development which is still being manufactured today.

During the past 60 years, many other types of valves have been designed to cater for the new and hazardous processes which have been developed. Traditional valve types such as Gate and Globe valves have been reappraised as improvements have taken place in materials and the availability of new plastics and synthetic rubbers. This has also led to the development of the lubricated taper plug, diaphragm, ball and butterfly valves which have all been developed and engineered into practical and industrially acceptable products
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The lubricated taper plug valve was developed during World War 1 by Sven Nordstrom, a Swedish engineer, who was trying to overcome the excessive leakage and sticking of ordinary plug valves. The diaphragm valve was developed by a South African engineer named Saunders, who, working in the gold mines, was faced with excessive leakage of compressed air at the glands of the valves being used. In 1929 he developed a valve using a diaphragm both to isolate the valve operating mechanism and also to act as the closing member, which proved a great success.

The ball, or spherical plug valve, is a relative newcomer to the valve family. Initially developed for fuel systems on aircraft during World War II, the valve was further developed in the post-war years to produce the first industrial range of ball valves. During the last 30 years, many valve manufacturers have directed their attention to the ball valve and a variety of new, improved designs have been introduced. This has led to a much wider diversification and expansion of the capabilities of the ball valve for duties in practically all sections of the valve market.
Reference had been made to James Watt, who made use of a butterfly valve in his steam engine, and the first Mercedes car built around 1901 introduced a butterfly valve in the fuel intake linked to the accelerator pedal. The first butterfly valves used metal-to-metal seats but, after World War II, improvements in modern synthetic rubbers for the sealing members extended the application of the butterfly valve into many industrial fields. In the last ten years, the butterfly valve has been developed further to handle much higher pressures and temperatures than previously envisaged. This design is known as the high performance butterfly valve.

Summary
The Industrial Revolution of the 18th and 19th centuries increased the pace of development of valves in terms of design and manufacture to meet the new demands from industry. These design have been fine-tuned in the latter half of the 20th century with the advent of more sophisticated manufacturing methods and the availability of more exotic materials capable of handling the extreme pressures and temperatures of the new and superior fluids which have been developed.

Valve Terminology
The definition of a valve is a piping component which influences the fluid flow in systems comprising of pipes, vessels, apparatus, and machines, by opening, closing, diverting, mixing, or partially obstructing the passage through itself.
There are five basic designs of valves, which are distinguished by the operating motion of their closure device (actuator) and the direction of flow in the seating area. The valve types are:-
1. Gate valve - a valve in which the closure device moves in a straight line and in the seating area, across (at right angles to) the direction of flow.
2. Globe valve - a valve in which the closure device moves in a straight line and, in the seating area, longitudinally (parallel) to the direction of flow.
3. Plug and ball valves - valves in which the closure device rotates about an axis at right angles to the direction of flow and in the open position the flow passes through it.
4. Butterfly valve - a valve in which the closure device rotates about an axis at right angles to the direction of flow and in the open position, the flow passes around it.
5. Diaphragm (pinch) valve - a valve in which the closure device is provided by the deformation of a flexible diaphragm or tube.

The valves most commonly used throughout industry can be generally categorised as linear (multi-turn) or rotary (quarter-turn). A major feature of the linear valve is that tighter shut-off within the limits of the materials and designs may be achieved by tightening down on the threaded stem. Rotary valves, on the other hand, are usually simple, lightweight, easy to automate, and easy to maintain; they are available in multiport configurations, are quick opening and can be adapted to a broad range of applications.
There are also many unique valve designs which are intended for a particular application, or that combine the features of other valves for improved performance. These are known as 'hybrid' valves.

The types of valves related to their function are as follows:
1. Isolating valve - A valve designed for use in the open or closed position.
2. Regulating valve - A valve designed for use in all positions between fully open and fully closed.
3. Control valve - A power operated device which changes the fluid flow rate in a process control system.It consists of a valve connected to a power operated actuator that is capable of changing the position of the closure device in the valve in response to a signal from the controlling system.
4. Safety valve - A valve which automatically, without the assistance of any energy other than that of the fluid concerned, discharges a certified quantity of the fluid so as to prevent a pre-determined safe pressure being exceeded, and is designed to re-close and prevent the further flow of fluid after normal pressure conditions of service have been restored.
5. Check (non-return) valve - A valve which automatically opens by fluid flow in a defined direction and automatically closes by fluid flow in the reverse direction.
6. Diverting/mixing valve. A valve having more than two body end parts
7. Multi-function valve. A valve which can be used for more than one function.

Basic Valve Design
A process fluid must be fully contained in a properly designed piping system to avoid endangering personnel and the environment, and contamination of itself. A pipeline can have many potential leak paths, such as pipe joints, seams, equipment connections and finally valves. Valves can cause the biggest headache on a plant if, for example, wrongly selected, poorly designed, of poor quality or non-fire-safe in a fire-safe environment.. The maxim 'You pay for what you get' certainly applies in the valve industry and cost cutting on such important items as valves could have disastrous implications for the future operation of a plant.

A valve, selected correctly for the application, should last at least the life of the plant, with the minimum of maintenance. Therefore, understanding the basic design elements of valves should be an important factor in the training of any plant engineer and piping engineer.

Materials of construction
A wide range of materials is available to meet the severest service conditions. Components are usually obtainable in alternative materials, and in selecting a valve it is advisable to consider each part of the valve separately, that is, body, bonnet, trim (comprising closing member, stem, mating seats), and so on, in order to arrive at the optimum materials specification to meet the service conditions. The materials most frequently used for valve construction include cast iron, bronze, nickel alloys, copper alloys, steel, stainless steel, and, to a lesser extent, aluminium and titanium. Cast iron and bronze valves are generally used for applications involving comparatively low temperatures i.e. up to approximately 200-260C (392-500F). Carbon steels or alloy steels are employed for higher temperatures. If the fluids being handled are highly corrosive, or chemical reaction with the metal surfaces must be avoided to prevent contamination of the fluid, the valve may be lined with ebonite, plastics, glass, or ceramics.

All plastic valves are being specified increasingly as an alternative to stainless steel or alloy valves. Plastic valves are made from a variety of materials and have proven satisfactory for handling difficult chemicals, such as weak acids and for corrosive systems. Some of the materials employed are unplasticised polyvinyl chloride (UPVC) acrylonitrile butadienestyrene (ABS), polypropylene(PP), and polyethylene (PE). Plastic valves may be used for low pressure service, but for higher pressure plastic lined steel valves are used.

Valve integrity
The valve body is the main pressure containing element and serves as the basic element for all the other components. It must be strong enough to withstand the pressure and temperature of the process fluid inside the system and the loads generated by the piping connections and the actuator on the outside. Valve type, pressure rating, manufacturing method, material and cost - all must be considered and there may be a variety of potential leak paths through the body, such as bonnet attachment, body seals, stem seal, and the pipe/valve connection area. Pressure boundary integrity requires basically sound pressure parts, pressure tight assembly joints and an effective dynamic seal between the moving stem and valve bonnet.

Full port or reduced port
Certain categories of valves, notably ball valves, can be designed with a full port or reduced port through the valve. Full port is the term used to describe the port diameter which is similar to the adjacent paper work and valves of this design are used where low pressure drops are required in the piping system or line 'pigging' is required.
Reduced port valves are designed with a port diameter usually reduced to the next pipe diameter down and are used to save weight, material and hence cost. The use of reduced port valves will, however, increase the pressure drop in the piping system and increase the velocity of fluid through the valves. This can lead to excessive wear on seats, noise in the system, and difficulty in closing hand operated valves.

Flow coefficient values can be calculated to determine the relative pressure drop across a valve and in the case of liquid, flow is represented by a basic formula taken from ISA S39 standard.
The formula assumes the flow to be neither viscous, cavitating or flashing.
The flow efficient values for full port valves are higher than those for reduced port.
Flow coefficients for control valves tend to be calculated against the IEC-534/ISA S75 standard which takes the pressure drop between two points, one to two diameters upstream and one to six diameters downstream of the valve. These points allow for cavitation in the fluid caused by the control valve and give a more accurate reading. However the 'C', values are lower and dP correspondingly higher, owing to the pressure drop in the pipe also being included.

Valve end connections
The choice of end connections for connecting a valve to its associated pipework is dependent upon the pressure and temperature of the working fluid and the frequency of dismantling the pipeline or removing the valve from the line. The types of end connection in general use are as follows:
1. Screwed. - Male threads of various forms may be used for special purposes, but as a rule screwed end valves have female pipe threads, wither tapered for assembly to taper threaded pipe, or parallel for assembly to taper or parallel threaded pipe. In taper-to-taper and in taper-to-parallel connections, the pressure-tight joint is made on the threads. In parallel-to-parallel connections, the pressure tight joint is made by compressing a grummet or gasket against the end face of a valve. Screwed ends, usually confined to pipe sizes of 150mm and smaller, are widely used for bronze valves and to a lesser extent in iron and steel valves.
2. Flanged. - Flanged-end valves are easy to install or remove from a pipeline, being bolted to the mating pipe flanges. To ensure a tight seal, a gasket is usually fitted between the machined facing of the flanges. The type of gasket, which can be non-metallic, metallic or a combination of both, depends upon service conditions and upon the type of flange facing employed. Bronze and iron valves are normally supplied with plain (flat) facings, and steel valves with plain (flat), raised or male facings, although female, tongue and groove, or ring-joint types are available. Flanged end valves are made in sizes from 15mm upwards.
3. Socket-weld. - In this type, the ends of a valve are socketed to receive plain-end pipe. A circumferential weld is made on the outside of the pipe so that 'icicles' and weld spatter are unable to enter the pipeline. Socket-weld ends are used only on steel valves, and as a rule they are limited to sizes of 50mm and smaller for higher pressure/temperature applications in pipelines not requiring frequent dismantling.
4. Butt-weld. - In this case, the ends of the valve are bevelled to match wall thickness and machined bevel at the end of a mating pipe. A circumferential weld is made at the abutted mating bevels. 'Backing rings' which are basically sleeves fitting inside the pipe, are sometimes used to align the pipe and valve bores also to prevent 'icicles' and weld spatter from entering the pipeline. Butt-weld ends are used only on steel valves, normally in sizes 50mm and upwards, for the higher pressure/temperature applications in pipelines which do not require frequent dismantling.
5. Compression. - This type of valve end has a socket to receive the pipe and is fitted with a screwed union nut. The joint is made by the compression of a ring or sleeve on to the outside of a plain-end pipe, or by compressing a preformed portion of the pipe end. As a rule compression ends are used with copper tubing and steel tubing up to 65mm diameter and are used for low pressures or where pipes may require frequent dismantling.
6. Capillary. - these valves are soldered to the mating pipe. The ends of the valve have a socket, machined to close tolerances to receive the plain-end pipe. The joint is made by the flow of solder by capillarity along the annular space between the socket and the outside of the pipe. Capillary ends are commonly used with copper tubing and confined to sizes 65mm and smaller. The high temperature use of capillary end valves is limited due to the comparatively low melting point of the solder.
7. Socket - the ends of the valve are socketed to receive the plain spigot end of the pipe, the seal being made by the insertion of a yarn ring joint, corked with lead. Other forms of socket ends use a rubber scaling ring. These ends are either in the style of flange and socket adaptors for bolting to flanged end valves, or incorporated in the valves. Socket ends are normally associated with cast iron valves for water services in sizes 50mm and upwards.
8. Spigot - The type of socket used in the coupling or on the pipe end determines the form of the spigot ends. For cast iron pipes with lead joints the spigot end is provided with a raised band, for screwed and bolted gland and other forms of mechanical joint the spigot end is prepared to suit the joint. For asbestos cement connections, the spigot end is finished plain in the same way as the pipe. These spigot ends are normally associated with cast iron valves for water services in sizes 50mm and upwards.

Author - The late Rod Whitehouse