Differences between manual stop valves, gate valves, and plug valves

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This post was last edited by Refinery Operator on 2015-10-16 08:26. Differences between manual stop valves, gate valves, and plug valves – Li Shaopeng, Technical Department, Hangzhou Watt Energy Saving Engineering Co., Ltd. Since stop valves are often used to prevent fluids from entering a certain area, they are very important in fluid systems. But sometimes it is also used to manually control the flow rate of fluids. Stop valves can be used in various applications that require on/off control: to change the flow direction of the process medium ； The purpose of shutting off the fluid is: equipment maintenance ； Dismantle equipment ； Turn off the device. Stop valves in various forms and designs can meet the requirements of different applications and operating conditions. Based on the manner in which their closing elements move, valves can generally be divided into two categories. Linear-motion valves – the closing element moves in a straight line; this category includes gate valves, ball valves, diaphragm valves, and throttle valves. These valves will be described in more detail in this section. Rotary valves – The closing element moves around an axis that is perpendicular to the direction of fluid flow. Butterfly valves and ball valves are the two most important types of rotary valves in steam systems. Linear-motion valves evolved from early water gates used to control water flow in irrigation canals; since then, many different types of valves have been developed for fluid control purposes. Although linear-motion valves are characterized by the linear movement of their closing elements, the direction of fluid flow may be either perpendicular to this movement (as in gate valves) or parallel to it (as in ball valves). The main feature of a linear motion valve is that it closes tightly thanks to the closing element on the valve stem. Gate valves are widely used in domestic water systems and are by far the most commonly used type of valve; however, their use in industry has been declining in recent years. However, gate valves are still used in places where the flow properties of the fluid need to remain undisturbed. Because once the gate valve is opened, its gate can fully retract into the valve cap, resulting in minimal pressure; this makes the gate valve particularly suitable for applications such as isolation and shut-off. A gate valve consists of 4 main components: the valve body, the valve cap (cover), the gate, and the valve stem. The gate slides between the valve seats, and the valve rises perpendicular to the direction of fluid flow until it is fully open; when the gate is completely retracted into the valve cap, the pressure drop across the valve is minimized. Based on the design of the valve and its seat surface, it can be divided into various types: solid wedge gate valves. These valves have a wedge-shaped gate, with the seat located on the valve body; the valve stem is connected to the wedge-shaped gate, and operation is achieved through mechanical threading. Due to the presence of the wedge angle, it is not necessary to apply excessive force when turning the handwheel to close the valve. Sometimes, the valve seat can be lined with PTFE to improve the sealing performance when the valve is closed. Although there are various types of elastic wedge gate valves, most of them use elastic dual disc assemblies, similar to two wheels on a short shaft; the elasticity of these discs enables the valve to close more tightly over a wide range of temperature and pressure conditions. The most commonly used elastic wedge gate valve in steam systems is the parallel slide valve; its gate consists of two discs that are pressed against the valve seat under the action of a spring. The pressure of the upstream fluid acts on the upstream disc, causing it to move away from the valve seat; this thrust is transmitted to the downstream disc, forcing it to press tightly against the valve seat and thus ensuring a tight seal. Due to its excellent elasticity, the gate can expand and contract when the temperature changes, making it suitable for use in steam systems. Spherical valves are the main type of valve with linear motion; they are used more extensively than gate valves, come in a greater variety of structural designs, and can be applied in most situations. The flow direction of the fluid is the same as the movement direction of the closing element, which indicates that the inlet and outlet of the valve are horizontally opposite to each other; the fluid must change direction here before continuing onward. This arrangement has one major advantage: it allows for faster opening and closing of the valve compared to gate valves. This is because the distance between the spherical valve element and the seat is very short. This is a significant advantage when the valve needs to be opened and closed frequently. The downside is that the fluid must change its direction at the valve element, which increases flow resistance and generates turbulent vibrations. As a result, the pressure drop across this type of valve is higher than that across gate valves. Spherical valves provide better sealing than gate valves, which means they can be used at higher temperatures and pressures, such as in steam systems or systems involving hazardous or expensive fluids. Although ball valves are more expensive than gate valves, they are safer and result in less fluid loss. Due to the pressure of the upstream fluid acting on the valve core and transmitting this force to the valve stem, it becomes difficult to close the valve, which limits the diameter of ball valves to DN250 only. In high-pressure differential closed systems, a balance disc can be used to reduce resistance, allowing the nominal diameter of the valve to reach 500 mm. The balance disc includes a pre-actuation valve that functions as a pilot valve; when the valve opens, this pre-actuation valve opens first, enabling the medium to flow at a certain velocity and thereby reducing the pressure difference, which makes it easier for the main valve to open. To assist in closing the valve, globe valves equipped with balance discs should be installed upside down, so that the pressure from upstream can act on the top of the valve stem. The main drawback of plunger valves, which are valves that move in a linear manner, is that they are susceptible to the influence of impurities and prone to thread formation; therefore, regular maintenance is required. Although the valve seat can be replaced in theory, this usually involves time and cost. Piston valves were developed to solve this problem. A plunger valve differs from traditional ball valves in that the conventional seat and conical plug are replaced by a piston and a piston cylinder. The plunger is connected to the valve stem and handwheel. There are two sets of sealing rings at the top and bottom of the piston cylinder; the piston passes through these rings, which are pressed tightly by a valve cap. The upper set of sealing rings serves the same purpose as the conventional gland packing, while the lower set functions similarly to the seat. Moreover, the sealing area between the piston and the sealing rings is large, ensuring a tighter seal. Piston valves are not designed for throttling control; they must be either fully open or fully closed. When the valve is fully open, only the bottom surface of the plunger comes into contact with the fluid, while the entire cylindrical surface is in close contact with the seal ring; this means that the sealing surface (the side surface of the plunger) is not subjected to erosion by the fluid. During maintenance, all internal components can be easily removed; by taking off the valve cap, the plunger can be pulled out, and the seal ring and piston cylinder can be extracted using an extractor. It is simple to operate; there is no need to remove the valve from the pipeline. Generally, the plunger does not need to be replaced, and the sealing ring can also be used for a long time if the valves are not opened and closed frequently. Diaphragm valves are the third main type of linear-motion valve; the valve stem pushes down on the diaphragm to block the flow path of the fluid. Depending on the geometric shape of the valve body, there are two different types of diaphragm valves. — The diaphragm-type cofferdam is cast together with the valve body; when closed, the diaphragm sits on the cofferdam to prevent fluid flow. — The DC-type flow orifice runs transversely through the valve body; when closed, the diaphragm assumes a wedge shape to block fluid flow. The greatest advantage of diaphragm valves is that the diaphragm prevents direct contact between the valve’s moving parts and the fluid, allowing them to be used with corrosive fluids as well as those containing solid particles. Additionally, since the valve cover does not come into contact with the fluid, it can be made from inexpensive materials such as cast iron, thereby reducing the overall cost. With the development of new diaphragm materials, it can be applied to most fluids. However, the temperature is generally limited to 175°C for use with process fluids. Linear motion valves come in various configurations regarding the valve stem: lift/non-lift stem. If the valve stem is lift-type, it moves vertically upward when the valve is opened. It is different from a non-lifting valve stem that only rotates without lifting. The degree to which the valve stem is raised indicates the degree of opening of the valve, as well as the flow rate passing through it. However, raising the valve stem requires more space on top of the valve cover to accommodate the space occupied by the stem when the valve is fully open. Non-lifting valve stems are recommended for valves with gland packing seals, as this reduces wear on the packing. Internal/external valve stem threads – Valve stems with external threads, whose drive threads are located outside the valve body and do not come into contact with the process fluid. Since these threads are susceptible to corrosion, external threads are often used in applications involving corrosive or erosive fluid media. If the temperature changes significantly, it is also suitable to use a valve stem with external threads, because if the threads are inside the valve body, the expansion and contraction of the valve stem will cause the threads to lock. To prevent the process medium from leaking along the valve stem between the fluid and the external environment, a barrier is necessary; there are two types of valve stem sealing: gland packing sealing and bellows sealing. The gland packing is made of polymeric materials, such as the typical PTFE, and is tightly filled between the valve stem and the gland to prevent leakage of the process medium. A telescopic metal bellows is used in the diaphragm seal valve; one end of the bellows is integrated with the valve stem, while the other end is pressed tightly against the valve cover, thereby preventing fluid leakage. The bellows expand and contract as the valve stem moves up and down, providing an extremely effective seal with zero leakage. The bellows is equipped with a anti-twist device that prevents the bellows from twisting together with the valve stem; this device is very important, as continuous twisting of the bellows would lead to failure and leakage. Although they are cheaper than diaphragm-sealed valves, gland packing-sealed valves do not provide as good a sealing performance as diaphragm seals. If a gland packing-sealed valve is not used for a certain period of time, the packing will harden, resulting in leaks when the valve is used again; this problem does not occur with diaphragm seals. Additionally, gland packing-sealed valves require regular replacement of the packing, whereas diaphragm-sealed valves generally do not need any maintenance even after 100,000 cycles of opening and closing.