This article serves as an introduction to the industrial valve industry and is designed to inform about basics such as valve types, design and history.
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A valve is a device that regulates the flow of gases, liquids or loose materials through an aperture, such as a pipe, by opening, closing or obstructing a port or passageway.
The first valves were made by the Romans more than 2,000 years ago. They were used in domestic plumbing systems (See picture on the right). An interesting fact is that the name of the valve builder was not stamped on the valve or the plumbing components. If a name was stamped on the valves it was the owner of the house where the pimping was installed.
The early roman valve and piping designs were used in variety of water service applications. These initial basic plug valve designs would not change much in the coming 2,000 years. Like other scientific developments, the Dark Ages also inhibited the growth of the flow control technology and it wasn't until the birth of steam-powered Industrial Revolution that valve designs began to change and develop.
Did you know early mass production of ball valves used a process of boring out solid metal bar or rod and machining the internals to accept the seats, ball and retaining rings? Manufacturers were limited to physical dimensions of the diameter of the ball and had to maintain sufficient wall thickness to withstand pressure inside the body of the valve. The size of the ball could not be larger than the inside diameter of the machined bar. Additionally, the ball orifice (the opening in the ball through which the media flows) was also restricted in dimension because it had to keep minimum wall thickness as well.
Used in on/off service, isolating valves can sometimes be called block valves. They are designed to operate in either full open or full closed position, just like light switches are used to turn lights on or off. In critical applications these valves are often motor-operated. The most common type of isolating valve is the gate valve, but other types have made an advance, especially over the past 30 years.
Control valves are usually globe, ball, butterfly, or plug types and are used to regulate the flow in a line. They are designed to operate in the 'less-than-full-open' position. The oldest and still most popular regulating valve design is globe. Gate valves should never be used for throttling or regulating unless the flow velocity is very low. High velocity flow in a gate valve used for regulating can quickly destroy the seating surfaces and result in leakage when fully closed. For effective flow regulation, the flow rate should be proportional to the amount of closure of the stem. However, certain control applications do not need to meet such requirement, such as a quick opening response. The most accurate and reliable flow control is performed by control or regulating valves operated by a controller and an actuator, rather than by manual operation.
Check and non-return valves only allow flow in one direction and block the flow from returning in the opposite direction, as can be shown in picture on the left. Check valves are the 'one-way-street controllers' of the valve industry. They are marked with a flow arrow on the exterior surface to ensure they are installed in the proper orientation. Check valves protect many pieces of process equipment including pumps, turbines and boilers. Except if these valves are equipped with soft, elastomer seating inserts, they will leak to some degree. Therefore, unless soft-seated, check valves should not be counted on for absolute shut-off.
The entire valve industry is very broad shouldered, with very diverse segments that feature valve products that are often totally different from one another. Here is a list of major market segments:
Various types of valves are required in any piping system in order to regulate the fluid flow within that system. Valves can be manually operated or they can have an actuator to change and control the valve opening. The actuator may be pneumatically, hydraulically, or electrically operated. The valves represent a considerable percentage of the overall expenditure of the chemical process industry and, therefore, must be carefully selected.
Valves are installed on equipment/piping to perform any of the following functions,
It is imperative that process piping engineer should be familiar with all types of valves and the advent of any new designs. Thorough knowledge of the operation, maintenance, and adjustment of valves is equally important since the success of the process plant and its total activity will depend on it. In this article we are going to study the overview of few most common valves used for isolation and regulation purpose. These include the gate valve, globe valve, needle valve, butterfly valve, ball valve, plug valve, and check valve.
The gate valve, shown in Fig. 1, consists of a gate-like disc, actuated by a screwed stem and handwheel, which moves up and down at right angles to the path of flow. In the closed position, the disc seats against two faces to shut off the flow. Gate valves are not suitable for throttling service because excessive wear, due to wire drawing (erosion), occurs on the gate and gate seats. They are suitable as stop (or isolating) valves, where conditions require either full flow or no flow. They have the advantage that, when fully opened, the resistance to flow is low with a minimum of pressure drop, as the fluid flow moves in a straight line.
Figure 1
Gate Valve
The globe valve, shown in Fig. 2, is constructed in such a way as to cause the flow of the fluid passing through it to change direction twice. The disc and the seat are parallel to the main flow path, and the disc is moved toward, or away from, the seat by means of a threaded stem. Due to its construction, the globe valve is ideal for throttling or regulating flow with a minimum of wire drawing and seat erosion. Another advantage of the globe valve compared to the gate valve is that it is cheaper to manufacture. On the other hand, the globe valve offers much more resistance to flow than does the gate valve. The unbalanced single disc type of globe valve shown in Fig. 2 is seldom used in sizes larger than 305 mm (12 in.) due to difficulty in opening and closing against fluid pressure.
Figure 2
Globe Valve
Needle valves, shown in Fig. 3, are designed to allow precise flow control. Its name is derived from the sharp pointed disc and matching seat. They are extensively used for continuous blowoff or chemical feed control services. The stem threads are finer than usual so that considerable movement of the hand wheel is required to increase or decrease the opening through the seat. Usually, these valves have a reduced seat diameter in relation to the pipe size.
Figure 3
Needle Valve
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A butterfly valve, shown in Fig. 4, consists of the valve body, disc, shaft, and the necessary packing and bushings for shaft support. The body is frequently a solid ring type, which is mounted between pipe flanges. The disc is generally cast in one piece. Correct alignment of this valve is required to prevent binding of the swing-through disc. The thickness of the disc is determined by the pressure drop across the valve (throttling or closed position). Butterfly valves come in sizes from 25 to mm (1 to 152 inches) and are designed for pressures up to 13 800 kPa and temperatures up to ºC. The flat disc can be rotated through 90º from the wide open to the fully closed position. The valve is fitted with a lever for manual operation. A power actuator is required to position the disc for bigger sizes because large pressure differentials can exist across the disc. The valve shown in Fig. 5 can be manually or electrically operated. The butterfly valve is commonly used in thermal and hydroelectric power stations, oil and gas processing industries, oil and gas transmission, and in water and sewage plants. They have the following advantages: relatively light weight, ease of operation, self cleaning, and negligible pressure drop across the valve when it is fully open.
Figure 4
Butterfly Valves
Figure 5
Power Operated Butterfly Valve
The ball valve, shown in Fig. 6, contains a spherical plug with a passage bored through it which controls the fluid flow through the valve body. The basic type of ball valve requires a quarter turn from the fully open to the fully closed position. The valve can be operated by means of a lever, which also serves as an open or shut indicator, or by the use of an automatic actuator.
Figure 6
Ball Valve
The spherical plug not only gives precise control of the flow through the valve, but also gives a tight shutoff when in the closed position. The valves are designed so that no internal lubrication is required and the torque required to rotate the ball is negligible. The ball and stem are generally machined from one piece.
For larger sizes and higher pressure ratings, the ball is constructed with a double stem and is supported by bearings. This construction requires a seal for one end and a packing box for the opposite end. Fig. 7 shows such an arrangement.
Figure 7
V Ball Valve
Fig. 8 illustrates the alternative manual-automatic operation for this particular valve. The ball valves are manufactured in sizes from 3 to mm (1/8 to 40 in.) and for pressures up to kPa with service temperatures from -185ºC to 550ºC.
Figure 8
Manual-Automatic Positioning
Ball valves are suitable for handling slurries and fluids with a high solid content, and for this reason have found wide applications in the paper industry, chemical plants, and sewage treatment plants.
The plug valve, shown in Fig. 9, is a quarter turn valve, as are butterfly and ball valves. The plug valve consists of a tapered or straight cylinder containing a hole, which is inserted into the cavity of the valve body. The hole in the plug lines up with the axis and opening in the valve body. The valve has a tapered plug which is secured in the valve body by the valve cover. A packing box is recessed in this cover, with packing held in place by the gland, thus preventing leakage along the valve stem.
Figure 9
Plug Valve
The tapered plug has a tendency to jam in the tapered seat and can cause scoring if forced to turn. Most plug valves are lubricated to eliminate this problem. The lubricant is supplied through the center of the stem and is distributed through channels to the seating surfaces. Other valves are equipped with a flexible, smooth liner which eliminates the need for
lubrication. The pressure drop across this valve when it is in the wide open position is very low. This type of valve is self-cleaning. Plug valves are used as quick opening valves in gas supply lines, low pressure steam lines, water treatment plants, pulp and paper, and chemical industries.
The check valve, shown in Fig. 10 and Fig. 11, is a valve which prevents reversal of flow in piping. The flow of fluid keeps the check valve open while gravity and reversal of flow will cause the valve to close. The two basic types of check valves are the swing check and the lift check.
Figure 10
Swing Check Valve
Figure 11
Lift Check Valve
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