Valves certainly go back to the era of steam and the industrial revolution. In those days steam valves with the exception of safety valves would have been manually operated. Automatic control valves today manipulate a flowing fluid, including gas, steam, water, slurries and chemical compounds to compensate for load disturbances and to keep regulated process variables as close as possible to desired set points.
The control valve is a critical part of the control loops of process plants where these loops are designed to keep important process variables such as pressure, temperature, flow, level etc within a required operating range to ensure consistent quality of the final end product. The control valve is not just a simple mechanical device but consists of the valve body, internal trim parts, an actuator to operate the valve and a number of other accessories including controllers, positioners, transducers, regulators and limit switches, a far cry away from the early valves of the steam era.
Originally used in the steam and water industries, today control valves are used across the whole gamut of the process industry including such critical applications as nuclear power generation where failsafe devices are essential. Control valves are widely used in the mining, oil and gas, chemical and petrochemical, pulp and paper, food and beverage and power generation industries.
Some control valve companies have been in existence for more than 100 years, one example being Fisher Controls (part of the Emerson Group) that was formed in 1880 to manufacture pump governors followed by back pressure steam valves in 1906. Another early entrant was Asco, well known for its solenoid valves. Asco started business in 1888 manufacturing switching controls for elevators and using the magnetic concepts involved here it went on to introduce the first solenoid valve in 1910. Asco sold its elevator control patents to the Otis Elevator Company in 1907 to focus on valves, adding packless valve technology in 1930 and pneumatic valves in 1988. Outside of North America one of the oldest valve manufacturers is the Swedish company Stafsjo with over 100 years experience.
Over time these companies expanded their range of valves, controls, actuators, and electronics mainly through aggressive acquisitions. The control valve business is however a dynamic one - and many new entrants were incorporated in the twentieth century such as Larox, which introduced pinch valve technology (1983) and our own manufacturers Mitech (established 1987) and DLM. Mitech focuses its R&D in solving customers' application specific applications and specialises in globe control valves.
Today, a vast array of devices are available. These include ball, butterfly, solenoid, globe, check, knife gate, plug and pinch valves. In all of these, modern design techniques, including CAD and proprietary software, have allowed optimisation of the valve characteristics to reduce cavitation, improve flow characteristics and reduce dead time, backlash and to optimise size. Today these valves can be obtained in an equally impressive range of sizes from 12,7 mm up to over 2 m. When considering control valves the automated on-off valve is often excluded from the hierarchy but recent developments from NAF in Sweden have seen this distinction become less clear. With a new intelligent on-off valve controller NAF have introduced a ramping feature to avoid rapid valve closure causing severe pressure shocks and possible pipeline rupture. This ramping feature can also be used to limit the valve opening, a function for which a true control valve is normally used.
In terms of manufacture the use of NC machining techniques from the 1960s resulted in breakthroughs in both machining efficiency and quality and this has been improved even further with more modern CNC capabilities. More effective welding technologies (such as TIG and plasma arc) have also made an important contribution to quality and reliability. Another improvement was the introduction of forging of some valve bodies rather than the traditional casting technique. This has resulted in minimised leaks, reduced maintenance and increased service life, in certain applications.
Other improvements in the basic mechanics of valves has come about through improved testing techniques that include radiography, ultrasonic, magnetic particle and dye penetrant technologies. Most manufacturers today will test valves under load with propane, steam, etc, cycling typically 1000 times or more. Valves for nuclear plant applications obviously are subjected to more extensive testing.
The quality and consistency of valves and associated products has also improved dramatically following the introduction over the last 30 years of the certified ISO 9000 QMS system by virtually all manufacturers, this covering both design and manufacture. This system itself has been reinforced through the progressive introduction of standards that products had to adhere to. These standards include a range of both North American (ANSI, API, ASME, ASTM etc) and European (DIN, BS, TÜV, etc) protocols.
New materials have had a major impact since the 1950s, one of the major ones being the accidental discovery of an ideal lubricant material, 'Teflon' (PTFE), by a Du Pont scientist in 1938. World War 2 delayed the implementation of this new material, but it was widely used from the early 1950s. Two of the earlier adopters to reduce friction were Fisher (1946) and Garlock (1947). Other synthetic materials that have contributed to valve effectiveness include nitrile, PVD and UHMWPE. Recently, Fisher has brought out valves using an impregnated low friction type of graphite that has better properties than Teflon under certain conditions. Metal and alloy developments (eg, titanium, Monel, Stellite) have also played a major role, particularly in the control of corrosive substances.
In their earliest application, control valves were merely open and close devices operated manually often using radio or shouted messages. This was still the situation in South African industry even in the '60s and '70s. The problem of using humans for such tedious operations on long shifts is that they forgot to operate the valves at the correct time or fell asleep, resulting in ruined batches of product. With the availability of compressed air, pneumatic actuators were soon used to replace human muscle and these were closely followed by analog (4 to 20 mA) electrically operated devices, which became the first choice of the power generation industry, in particular.
With analog controls things moved rapidly with feedback loops being introduced and then onboard memory being added, to store, for example, calibration data. This allows either remote or laptop interrogation of a valve to determine whether it is still operating within specification. This allows companies with thousands of valves in their plant to only replace those that are going wrong and saves the costs associated with manual inspection of all valves on an ongoing basis or removal of perfectly operational devices after the expected lifetime has been exceeded.
Developments in technology in the latter part of the 20th Century were rapid and can best be based on the example of one of the industry leaders - Fisher Controls. Fisher had a first with the release of its Control Valve Handbook in 1965 with this currently being available in its third edition (1999). The globe style valve was the most commonly used type up until about 1960 when the rotary V notch ball type rotary valve appeared. As flow velocities and pressures encountered in the industry increased so did problems with vibration noise and cavitation. As a result of this, Fisher radically redesigned the globe valve in 1970 with a new cage design that allowed amelioration of these problems. The butterfly valve as originally designed with a conventional disk suffered from instability at opening angles greater than about 60°, a problem eased somewhat by the introduction of the so-called 'fish-tail' disk. In the early 1980s a significant change in design to incorporate the eccentric disk eliminated the stability and sealing problems.
By the late '80s the first computer-based sizing programme was released easing a formerly tedious task (up until then performed with a slide rule). 1990 saw the company starting to get involved in valve diagnostics with the objective being to be able to determine the internal health of the valve (preventative maintenance) without unnecessarily pulling it out and stripping it. With pressure from environmentalists, Fisher, also in 1990, introduced the first environmental package, reducing emissions through much lower leakage levels.
A major change came during 1994 when the ISP Foundation completed phase one of the world's first fully digital fieldbus field trial at a Monsanto chocolate plant in Bayou. Fisher itself in 1996 introduced field-based architecture, intelligent field devices, digital communications and open systems, the latter at that time being seen as a major market differentiator.
While open architecture and digital control loops are the in thing today many operating plants have a significant legacy investment in closed protocol systems. Having spent large amounts of money in installing their control systems these companies are hesitant to go the open route and effectively throw away an optimally operating system. Many smaller companies are also still using analog electro-pneumatic control systems. The reality therefore is that the ideal of open systems in all industrial plants still has a long way to go.
As for the future, valve manufacturers are still working flat out to find materials that will create a truly frictionless valve. Much more effort will definitely go into the digital type of controllers and more intelligence will be pushed out into the field (migration from DCS to field level). Overall the valve manufacturing industry's objective is to reduce process variation in the plant, increasing efficiency and lowering cost in the industry. Another interesting development is the use of the valve manufacturer's engineers over the Internet to interrogate the status of valves in a process plant and advise plant engineers timeously of the necessity for valve replacement. As for the valves themselves it is expected that we will see further improvements in sound level attenuation and reduced cavitation. There is also a move towards predictive (rather than preventative) maintenance.
The author would like to thank Ron Nel (Valve & Automation) and Peter Underhill (Alpret Control Specialists) for their assistance in the preparation of this article.
Dr Maurice McDowell has many years' experience as a technical journalist, editor, business manager and research scientist. His third party analyses of world-class companies and processes, as well as his insight into industry and technology trends are well respected.
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