In the beginning, Valves have been critical to channel water into farmland and waste away from cities. It acts as a control by placing physical blocks, or “obtuators”, within the flow stream. While originally made with wood and stones, advancement in material have allowed valves to become stronger and more sophisticated. Today, we make valves with exotic material for high corrosion resistance and it’s not uncommon to see multiple NPS 48 valves on a pipeline or side of a tank. As modern life is not possible without valves, this page will introduce you to some valve basic knowledge.
Who should read this article: If you are new to the valve world, and need a basic understanding of the different type of valves, how they operate, and some reason why one valve is selected over another, read on. However, if you have been in the industry for a year or more, save your time and move unto more advanced information.
Valve Basic Structure
The three main components of a valve is the obtuator, the shell and the shaft. the designs of the obtuator and the method of encapsulating the media ultimately is what creates the variation of valves in the market today.
The obtuator located inside the valve in-line of the media flow is the most critical portion of the valve and is often where a valve gets its name. The most common shapes an obtuator takes is variations of rectangle, circle, cylinder, or ball. The obtuator is the main device which allows controls of the media and are always in contact with the flow media.
A Shell acts to contain the flow media inside the valve while allowing access to the obtuator. Traditionally, this is accomplished with two pieces of material. The larger piece of material, which typically houses the obtuator, is called the Body of the valve. The shell pieces which encapsulates the body can have different names based on the valve type. For a shaft which comes out from the top, it is referred to as the Bonnet of the valve. For a valve which does not have external access, it is often referred to as Caps. Finally, if the secondary pieces are located horizontally from the obtuator, and especially if it’s part of a connecting flange, it’s referred to as End–Caps.
A shaft, or stem, is the last major component of valves. A shaft of the valve must penetrate through the shell to allow access and manipulation of the obtuator without allowing leakage or damage to the valve or itself. This shaft may be threaded or not, depending on method of manipulation and can either rise, rotate, or both rise and rotate to control the obtuator. The shaft of a valve is an important piece which often gets overlooked and causes valve failure, damage to equipment, and/or even lost of life.
Basic Valve Types
For a novice, the vast variety of valves can seem intimidating at first, However, the basic way to differentiate valves are by the shape of the obtuator, the portion of the valve which controls the flow. Typical valve types are gate, globe, check, plug, ball, and butterfly valves. Each type of valves have different pros and cons which must be evaluated with the required application to make the best valve selection.
Gate valves are the most common type of valve and are typically used for on-off isolation services. The obtuator of a gate valve can be of various designs, single piece solid, single piece flexible, single piece slab, double piece expanding, and other variations. Gate valves are usually the “de-facto” valve whenever it can be used due to the simplicity of the valve and the lower price point.
The gate valve should never be used for throttling service as a rule of thumb, as the valve can suffer from vibration, erosion, stem damage, and many other problem in addition to the non-linear flow characteristics. Despite that, gate valves are very versatile, with sizes ranging from 1/4 NPS to up to 48 NPS being regularly produced and commissioned world wide.
Globe valves are often the most critical valve in any systems and are often used for throttling or pressure drop services. The obtuator of a globe valve are of various designs ranging from parabolic, flat disc, ball, caged, and other variations. Specialized globe valves, also called controlled valves, are custom designed to provide perfect flow control for a designed condition, and can cost well over 50,000 USD for a single valve. With how much system shutdowns have been attributed to globe valve failures, along with the smaller quantity of globe valves, this is one valve which paying for a premium for a better product is worth the increase in reliability.
Check valves are the fail safe valve for system and necessary to prevent unintended reverse flow to any piping system The obtuator of a check valve can be a flat disc, ball, plug, or other variations and usually hinged to allow pressure to act upon the obtuator.
Check valves are often over-sized by piping engineers who are hungry for the highest CV for their piping system, only to find out that the valve is constantly at a half open position, or worse yet, fail to open. Thus, It is paramount to request the minimum cracking pressure for any check valve, as well as have an idea of minimum flow require to keep disc at full open position. Finally, It is often that check valves need to be specially modified to allow access from the outside to control the disc for special occasions, or damper installed to reduce water-hammer effect.
Plug valves are quarter turn valve mainly for isolation shut off. The obtuator of a check valve is a cylindrical shaped object with a slot within it to allow passage of media. This cylinder can often be tapered, and be lubricated or un-lubricated variety. A plug valve is have a very large seating surface, making positive sealing of a new valve much easier than other quarter turn valves, however, plug valves tend to have a shorter lifespan in tougher conditions due to the larger seating surface. An improperly designed plug valve or one which have been compromised can also have operation difficulty due to pressure locking of the plug and/or a massive increase in friction.
Ball valves are the primary quarter turn valves used specifically for their superior sealing capability and low pressure drop across the valve. As the valve name implies, the obtuator is a round ball with an opening in the ball to allow media to pass through. Additionally, ball valves are typically further categorize by the method of ball retention: floating or secured (trunnion mounted), as well as if the seating surface is soft material or hard material.
As long as the temperature and media cleanliness permit, a ball valve can easily be the “go to” for all isolation needs. However, as valve size increase, the cost of ball valve starts to rise when compared against other valves due to the cost of the large ball and the body enclosure necessary for the valve. This, is why there exist reduced port ball valve, to sacrifice some pressure drop for a significant reduction in cost.
Butterfly valves are the lightest quarter turn valves and heavily utilized when weight and space is a premium. The obtuator for butterfly valves are always in a disc shape, and is unique that the obtuator protrudes into the piping system. Thus, designers need to accommodate for this protrusion by requesting for the butterfly valve chord length and the preferred shaft orientation of the valve.
The three primary variations of butterfly valves are resilient seated concentric butterfly valve, single or double offset resilient seated butterfly valve, or triple offset butterfly valve. The increase in offset generally reduce rubbing between the valve disc and valve body seat, extending the life of the valve. When compared to a ball valve, butterfly valve is unattractive in smaller sizes due to the lower CV value and higher cost, however, as size increases, the butterfly valve sees tremendous advantage in cost as it takes less material to make.
Next Article, we will learn about differentiating valves through their actuation method, as well as basic material and standards associated with valves.
- Valve Basics (2 of 2)
- Valve Material – Where does Steel come from?
- What is a Gate Valve?
- What is a Ball Valve?
- What is Valve Trim?
- What is a Butterfly Valve?
- Common Valve Standards