Difference between angular stop valves and angular throttle valves

This post was last edited by ylb913 on 2015-7-26 at 16:53. Globe valves and throttle valves have a similar structure; the only difference lies in the shape of their valve discs: the valve disc of a globe valve is disc-shaped, while that of a throttle valve is usually conical and streamlined, which makes it particularly suitable for throttling – it allows the cross-sectional area of the passage to be changed in order to regulate the flow rate and pressure of the fluid.

The closing element of a globe valve is a plug-shaped disc, with a sealing surface that is either flat or conical in shape； Globe valves are forced-seal type valves; therefore, when the valve is closed, it is necessary to ； The commonly used globe valves include the following types: ； 1) Direct-acting globe valve ； 2) Direct-flow globe valve: In direct-flow or Y-type globe valves, the valve body’s ； 3) Angle-type globe valve: In an angle-type globe valve, the fluid only needs to change direction once ； 4) Plunger-type globe valve: This type of globe valve is a conventional globe valve ； Globe valves have the following advantages: their closing element is a plug-shaped valve disc, the sealing surface is flat or conical, and the valve disc moves in a straight line along the centerline of the fluid. The movement pattern of the valve stem can be of the lift-type (where the valve stem moves up and down while the handwheel does not move), or it can be of the lift-rotate type (where both the handwheel and the valve stem rotate and move up and down, with the nut located on the valve body). Stop valves are only suitable for full open and full closed positions; they cannot be used for regulation or throttling. Globe valves are considered to be valves with forced sealing; therefore, when the valve is closed, pressure must be applied to the valve disc in order to ensure that the sealing surfaces do not leak. When the medium enters the valve from below the valve disc, the resistance that the operating force must overcome consists of the friction between the valve stem and the packing, as well as the thrust generated by the pressure of the medium. The force required to close the valve is greater than that needed to open it; therefore, the diameter of the valve stem must be large, otherwise the valve stem may bend. In recent years, since the emergence of self-sealing valves, the flow direction of the medium in globe valves has changed to enter the valve chamber from above the valve disc. As a result, under the pressure of the medium, the force required to close the valve is smaller, while the force required to open it is greater; consequently, the diameter of the valve stem can be reduced accordingly. At the same time, under the influence of the medium, this type of valve is also more airtight. China’s regulations on valve design specified that for globe valves, the flow direction should always be from top to bottom. When the stop valve is opened, the flow rate reaches its maximum when the opening height of the valve disc is 25% to 30% of the nominal diameter, indicating that the valve has reached its fully open position. Therefore, the fully open position of the stop valve should be determined by the travel of the valve disc. Classification of globe valves: The commonly used types of globe valves include the following: 1) Straight-through globe valve. 2) Angle-type globe valve: In angle-type or Y-shaped globe valves, the flow channel within the valve body is at an angle to the main flow direction; as a result, the disruption to the flow pattern is less severe compared to conventional globe valves, and consequently the pressure loss across the valve is also lower. 3) Angle-type globe valve: In an angle-type globe valve, the fluid only needs to change direction once, resulting in a lower pressure drop across this valve compared to globe valves with conventional designs. 4) Plunger-type globe valve: This type of globe valve is a variant of the conventional globe valve. In this valve, the valve disc and seat are typically designed based on the plunger principle. The valve disc is polished to form a plunger that connects with the valve stem, and sealing is achieved through two elastic sealing rings fitted over the plunger. The two elastic sealing rings are separated by a collar, and the sealing rings around the plunger are pressed firmly in place by the load applied to the valve cover by the valve cover nut. The elastic sealing ring can be replaced and is made from a variety of materials. This valve is primarily used for opening or closing, but it also comes with special types of plungers or rings that allow for flow regulation. Globe valves have the following advantages: 1. They have a simple structure, making them easy to manufacture and maintain. 2. The working stroke is small, and the opening and closing time is short. 3. It has good sealing performance, low friction between the sealing surfaces, and a long service life. The disadvantages of globe valves are as follows: 1. High fluid resistance, and significant force is required to open and close them. 2. Not suitable for media containing particles, with high viscosity, or prone to coking. 3. The regulation performance is poor. According to the position of the valve stem thread, stop valves are classified into external-thread type and internal-thread type. Classified by the flow direction of the medium, there are straight-through type, direct-current type, and angular type. Based on the sealing type, globe valves are divided into gland-sealed globe valves and diaphragm-sealed globe valves. Structural forms of globe valves: The structural forms of the globe valve body include straight-through type, parallel flow type, and right-angled type. The straight-through type is the most common structure, but it has the greatest fluid resistance. DC flow has lower fluid resistance and is often used for fluids containing solid particles or those with high viscosity. Right-angled valve bodies are often forged and are suitable for globe valves with smaller diameters and higher pressures. The following points should be noted for the installation and maintenance of globe valves: 1. Globe valves operated by handwheels or handles can be installed at any position in the pipeline. 2. Handwheels, handles, and actuating mechanisms shall not be used for lifting purposes. 3. The flow direction of the medium should be consistent with the arrow indicated on the valve body. A throttle valve is a valve that controls the flow rate of fluid by changing the throttling cross-section or throttling length. Connecting a throttle valve and a check valve in parallel can form a one-way throttle valve. Throttle valves and check throttle valves are simple flow control valves. In hydraulic systems with fixed-displacement pumps, throttle valves, in combination with relief valves, can form three types of throttle speed control systems: the inlet circuit throttle speed control system, the return circuit throttle speed control system, and the bypass throttle speed control system. Throttle valves do not have a flow negative feedback function, and thus cannot compensate for speed instability caused by changes in load; they are generally used only in situations where the load changes little or where high speed stability is not required. The performance requirements for the throttle valve are: · a large flow regulation range, with smooth changes in flow in response to pressure differences ； ·The internal leakage is low; if there is an external oil leakage port, the external leakage amount is also low ； ·It has a low adjustment torque and is sensitive in operation. The external structure of a throttle valve is identical to that of a globe valve; the only difference lies in the shape of their closing elements. The moving parts of throttle valves are mostly of conical streamline shape; by changing the cross-sectional area of the passage, flow rate and pressure can be regulated. Throttle valves are used to reduce the pressure of a fluid under conditions of extremely high pressure drops. The flow velocity of the medium between the throttle valve disc and the valve seat is very high, causing rapid damage to the surfaces of these components – a phenomenon known as cavitation. To minimize the effects of cavitation, the valve disc is made of a cavitation-resistant material (alloy steel) and shaped into a streamlined cone with a tip angle of 140–180 degrees; this also allows the valve disc to achieve a greater opening height. Throttling at small gaps is generally not recommended. Throttle valves have the following characteristics: 1. They have a simple structure, which facilitates manufacturing and maintenance, and their cost is low. 2. The adjustment accuracy is not high, so it cannot be used for adjustment purposes. 3. The sealing surface is prone to erosion, and it cannot be used for cutting media. 4. The sealing performance is poor. Throttle valves can be divided into straight-through type and angle type based on their channel configuration ； Based on the shape of the opening and closing element, there are three types: needle-shaped, groove-shaped, and window-shaped. The following points should be considered regarding the installation and maintenance of the throttle valve: Since this valve needs to be operated frequently, it should be installed in a location where it is easy to operate. During installation, make sure the direction of the medium is consistent with the arrow indicated on the valve body. Causes of clogged throttle orifice: 1. Mechanical impurities in the oil, along with gums, asphalts, carbon residues and other contaminants resulting from oxidation, accumulate in the throttle gap. 2. Due to the aging of the oil or the formation of charged polarized molecules as a result of compression, and because there is a potential difference on the metal surfaces of the throttle gap, these polarized molecules are attracted to the surface of the gap, forming a stable adsorption layer. The thickness of this adsorption layer is generally 5 to 8 micrometers, which in turn affects the size of the throttle gap. When the aforementioned deposits and adsorbed substances reach a certain thickness, they are washed away by the liquid flow, only to attach themselves to the valve opening again. Repeating this process over and over creates fluctuations in traffic flow. 3. When the pressure difference at the valve opening is high, the high temperature at the valve opening increases the degree of compression of the liquid, and the metal surface is more susceptible to frictional effects that lead to the formation of a potential difference; as a result, blockages are likely to occur under high pressure differences. Measures to reduce throttle port clogging: 1. Choose a thin-blade throttle port with a large hydraulic radius. 2. Perform precise filtration and replace the oil regularly. 3. Appropriately reduce the pressure difference before and after the throttle orifice. 4. Use metal materials with a small potential difference, select oils with good oxidation stability, and reduce the surface roughness of the throttle orifice. Applications of throttle valves: Since the flow rate through a throttle valve depends not only on the area of the throttle opening but also on the pressure difference before and after it, and because the valve has low stiffness, it is suitable only for applications where the load on the actuator changes little and high speed stability is not required. For throttle speed control systems where the load on the actuator varies greatly and high speed stability is required, pressure compensation of the throttle valve is necessary to maintain a constant pressure difference before and after the throttle valve, thereby ensuring stable flow rate