Working principles of various types of valves and pressure testing methods

Thread Content

In chemical production, valves are used wherever there is fluid flow; their importance in the chemical industry is self-evident. Today, the editor will guide you through the pressure testing methods and precautions for commonly used valves, along with information on the working principles and applications of various types of valves. Working principle of globe valves: The mechanism by which a globe valve closes is that, through the pressure exerted on the valve stem, the sealing surface of the valve disc comes into close contact with the sealing surface of the valve seat, thereby preventing the flow of the medium. A stop valve allows fluid to flow in only one direction, and it has a specific orientation when installed. Its structural length is greater than that of gate valves, and it also presents high fluid resistance; moreover, its sealing reliability is not strong when in long-term operation. Stop valves are divided into three categories: straight-through, right-angle, and parallel flow inclined stop valves. Function of a stop valve: A stop valve, also known as a gate valve, plays an important role in shutting off and regulating the flow of the medium in the pipeline it is located in. The globe valve is a type of valve that is widely used. It is popular because it experiences low friction between its sealing surfaces during opening and closing, is quite durable, requires little clearance for full opening, is easy to manufacture, and convenient to maintain. It is suitable not only for medium and low pressures but also for high pressures. Pressure testing method: During strength testing, the assembled valve is usually placed in a pressure testing frame; the valve disc is opened, and the medium is injected to the specified level. It is then checked whether the valve body and cover show signs of sweating or leakage. Strength testing can also be carried out on individual valves. During the sealing test, the valve stem of the stop valve is in a vertical position, the valve disc is open, and the medium is introduced from below the valve disc to a specified level; the gaskets and seals are then inspected ； After it meets the requirements, close the valve disc and open the other end to check for leaks. If both strength and sealing tests of the valve are to be conducted, the strength test can be carried out first; thereafter, the pressure should be reduced to the value specified for the sealing test, in order to check the packing and gaskets ； Close the valve disc again and open the outlet end to check whether the sealing surface is leaking. Gate valve – Working principle: The moving part of a gate valve is the gate, whose movement is perpendicular to the direction of the fluid flow. A gate valve can only be in an fully open or fully closed position; it cannot be used for regulation or throttling. The principle of closure for gate valves is that the sealing surface of the gate and the sealing surface of the valve seat are highly smooth and even, fitting together to prevent the flow of the medium; the sealing effect is enhanced by the use of a top plate, springs, or the shape of the gate itself. Function of gate valves: Gate valves are primarily used in pipelines to shut off flow. They present low fluid resistance, require little effort to operate, and can be used when the medium flows in both directions; they have no directionality. The sealing surfaces are not prone to erosion when the valve is fully open. Their compact structure means they are suitable not only for small valves but also for large ones. Pressure testing method: The strength test for gate valves is the same as that for globe valves. There are two methods for the sealing test of gate valves. The gate is opened, causing the pressure inside the valve to rise to the specified value ； Then close the gate, immediately remove the gate valve, check for any leaks at the sealing areas on both sides of the gate, or directly inject the test medium into the plug on the valve cover to the specified level in order to inspect those sealing areas. The above method is called intermediate pressure testing. This method is not suitable for conducting seal tests on gate valves with a nominal diameter of DN32 mm or less. Another method is to open the gate valve in order to raise the test pressure of the valve to the specified value ； Then close the gate plate, open one end of the blind plate, and check whether there is any leakage at the sealing surface. Lower the head again and repeat the above test until it passes. The sealing test at the packing and gasket of the pneumatic gate valve should be conducted before the seal test of the gate plate. Operating principle of butterfly valves: The butterfly disc is driven by the valve stem; turning it 90° completes one cycle of opening and closing. By changing the deflection angle of the butterfly valve, the flow rate of the medium can be controlled. Function of butterfly valves: Butterfly valves are suitable for use in pipeline systems in various engineering applications such as blast furnaces, gas processing, natural gas, liquefied petroleum gas, urban gas, hot and cold air systems, chemical manufacturing and smelting processes, as well as power generation and environmental protection projects. They are used to regulate and stop the flow of various corrosive and non-corrosive fluid media. Pressure testing method: The strength test for pneumatic butterfly valves is the same as that for globe valves. For the sealing performance test of butterfly valves, the test medium should be introduced from the side where the medium flows in; the butterfly disc should be in the open position while the other end is sealed, and the pressure should be increased to the specified value. After checking that there are no leaks at the packing and other sealing points, close the butterfly valve and open the other end; it is considered acceptable if there are no leaks at the sealing area of the butterfly valve. Butterfly valves used for flow regulation do not need to undergo sealing performance tests. Ball valve working principle: A ball valve is a type of valve that uses a sphere with a circular passage as the closing element; the sphere rotates along with the valve stem to enable opening and closing. The operating element of a ball valve is a perforated sphere that rotates around an axis perpendicular to the channel, thereby enabling the opening and closing of the channel. Function of ball valves: Ball valves are primarily used to open and close the flow of media in pipes and equipment, serving mainly to quickly turn fluids on and off. Since the degree of switching cannot be precisely adjusted and controlled, it is not suitable for flow regulation. Pressure testing method: The strength test of pneumatic ball valves should be conducted with the ball in a partially open position. Sealing test for floating ball valve: Place the valve in a semi-open position, introduce the test medium at one end, and seal the other end ； Rotate the sphere several times; with the valve in the closed position, open the closed end for inspection, and at the same time check the sealing performance at the packing and gaskets to ensure there is no leakage. Then, the test medium is introduced from the other end, and the above experiment is repeated. Sealing test for fixed ball valves: Before the test, rotate the ball several times with no load; the fixed ball valve should be in the closed position, and the test medium is introduced from one end to reach the specified level ； Check the sealing performance of the inlet using a pressure gauge; the gauge should have an accuracy of 0.5–1 grade, with a range that is 1.5 times the test pressure. No drop in pressure within the specified time is considered acceptable ； Introduce the test medium from the other end and repeat the above experiment. Then, set the valve in a semi-open position with both ends sealed, fill the internal cavity with the medium, and under the test pressure, check the packing and gaskets to ensure there is no leakage. The three-way ball valve should be tested for leakiness at various positions. Diaphragm valve operating principle: The moving part of a diaphragm valve is a diaphragm made of a soft material, which separates the interior volume of the valve body from the interior volume of the valve cover. A diaphragm valve is essentially just a \"clamping\" valve – an elastic, deformable diaphragm that is connected to a compression element via a screw. This compression element is moved up and down by a valve stem; when the compression element rises, the diaphragm lifts, creating an open passage, while when the compression element descends, the diaphragm presses against the valve body’s weir (in the case of a weir-type valve) or against the bottom of the contour (in the case of a straight-through type valve). Function of diaphragm valves: Diaphragm valves are suitable for switching and throttling purposes. They have a simple structure, low fluid resistance, and a higher flow capacity compared to other types of valves of the same specification ； Leak-free, and can be used for regulating high-viscosity media as well as those containing suspended particles. The diaphragm separates the medium from the chamber on the valve stem, so there is no leakage of the medium even in the absence of packing. However, due to the limitations of the diaphragm and lining materials, their pressure and temperature resistance are poor, and they are generally only suitable for a nominal pressure of 1.6 MPa and temperatures below 150°C. Pressure testing method: For the strength test of diaphragm valves, the medium is introduced from one end, the valve disc is opened, while the other end is sealed. Once the test pressure reaches the specified value, it is considered successful if there are no leaks in the valve body or cover. Then reduce the pressure to the sealing test pressure, close the valve disc, and open the other end for inspection; no leakage indicates success. Working principle of check valves: A check valve is a valve that relies on the flow of the medium itself to automatically open and close its valve disc, thereby preventing the backflow of the medium. It is also known as a non-return valve, one-way valve, reverse flow valve, and backpressure valve. This type of valve operates automatically; the valve disc opens under the pressure of fluid flowing in one direction ； When the fluid flows in the opposite direction, the fluid pressure together with the weight of the valve disc causes the valve disc to act on the valve seat, thereby stopping the flow. Function of a check valve: A check valve is a type of automatic valve whose main function is to prevent the backflow of fluid, to stop the pump and its driving motor from rotating in reverse, and to prevent the leakage of fluid from containers. Check valves can also be used in pipelines that supply auxiliary systems whose pressure may rise above the system pressure. Check valves can be mainly divided into swing check valves (rotating around their center of gravity) and lift check valves (moving along their axis). Pressure testing method – Test condition for check valves: The axis of the valve disc in a lift-type check valve is in a position perpendicular to the horizontal ； In a swing check valve, the axis of the passage and the axis of the valve disc are in a position that is approximately parallel to the horizontal line. During the strength test, the test medium is introduced from the inlet side to the specified level, while the other end is sealed; the test is considered successful if there is no leakage in the valve body or valve cover. In the sealing test, the test medium is introduced at the outlet end, and compliance is achieved if no leakage is detected at the sealing surfaces, as well as at the packing and gaskets, at the inlet end.