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How to set the valve

Posted by: steel world 2023-10-25 Comments Off on How to set the valve

General Setting Regulations 1.1 Overview These regulations apply to the chemical process system major. The valves mentioned do not include safety valves, steam traps, sampling valves and pressure reducing valves, etc., but include the setting of restricting orifice plates, blind plates and other pipe fittings that have similar functions to valves, with cut-off valves as the components of these valves. General term. The function of the cut-off valve is to isolate the fluid or change the flow direction of the fluid. It should be set according to the requirements of production (including normal production, start-up and shutdown, and special working conditions), maintenance and safety, and economic rationality should also be considered. Valve settings and selection of appropriate category (not model). Valves are an important task for process system professionals when preparing PI diagrams. The content described in this regulation takes into account the general requirements for production and safety. When system majors conduct engineering design with reference to these regulations, they should make choices based on the specific conditions of the project, local meteorological conditions, inter-factory collaboration, device operation requirements, fluid characteristics, user’s special requirements and economy, etc. 1.1.1 Selection of valve categories in engineering design Factors to be considered when selecting valves The selection of valves is based on the rationality of operation, safety and economy, and the empirical results of comprehensive balanced comparisons. The following original conditions must be put forward before selecting a valve:

1. Physical Properties (1) Material state

a. The material state of gas materials includes relevant physical property data, whether it is pure gas or a mixture, whether there are liquid droplets or solid particles, and whether there are components that are easy to condense.

b. The material state of liquid materials includes: relevant physical property data, whether the pure components or mixtures contain volatile components or dissolved gases (which can precipitate out to form a two-phase flow when the pressure is reduced), whether they contain solid suspended matter, and the viscosity of the liquid , freezing point or pour point, etc.

(2) Other properties; including corrosiveness, toxicity, solubility to valve structural materials, whether it is flammable and explosive, etc. These properties sometimes not only affect the material, but also cause special structural requirements or the need to improve the pipeline grade.

2. Working conditions under operating conditions

(1) According to the temperature and pressure under normal working conditions, it is also necessary to combine the working conditions during startup, shutdown or regeneration.

a. The pump outlet valve should consider the maximum closing pressure of the pump, etc.

b. When the system regeneration temperature is much higher than the normal temperature, but the pressure is reduced, for this type of system, the combined effects of temperature and pressure must be considered. c. The degree of continuity of operation: that is, the frequency of opening and closing of the valve, also affects the requirements for wear resistance. For systems with frequent opening and closing, you should consider whether to install double valves.

(2) The allowable pressure drop of the system a. When the system allows a small pressure drop, or allows a large pressure drop but does not require flow adjustment, a valve type with a small pressure drop, such as a gate valve, a straight-through ball valve, etc., should be selected.

b. If you need to adjust the flow rate, you should choose a valve type with better adjustment performance and a certain pressure drop (the proportion of the pressure drop in the entire pipeline pressure drop is related to the sensitivity of the adjustment).

(3) The environment in which the valve is located: outdoors in cold areas, especially for chemical materials, cast iron is generally not available for the valve body material and cast steel (or stainless steel) should be used.

3. Valve function (1) Cut-off: Almost all valves have a cut-off function. If it is simply used for cutting off without adjusting the flow, gate valves, ball valves, etc. can be used. When rapid cutting is required, cocks, ball valves, butterfly valves, etc. are more suitable. The stop valve can both adjust the flow and cut off. Butterfly valves are also suitable for regulating large flows. (2) Change the flow direction: Use a two-way (L-shaped channel) or three-way (L-shaped channel) ball valve or cock to quickly change the flow direction of the material, and since one valve functions as more than two straight-through valves, the operation can be simplified , making switching accurate and reducing the space occupied.

(3) Regulation: Stop valves and plunger valves can meet general flow regulation, and needle valves can be used for fine adjustments; for stable (pressure, flow) regulation in a larger flow range, throttle valves are suitable. .

(4) Check: A check valve can be used when it is necessary to prevent material from flowing back.

(5) Valves with additional functions can be selected for different production processes, such as valves with jackets, valves with drain ports, and valves with bypass, and valves with blow ports to prevent the settling of solid particles, etc. 4. Power to switch valves: Most of the valves operated on-site use handwheels. For those with a certain distance from the operation, sprockets or extension rods can be used. Some large-diameter valves already have motors in their design due to excessive starting torque. In explosion-proof areas, explosion-proof motors of corresponding levels must be used. Remote control valve: The types of power used include pneumatic, hydraulic, electric, etc. Electrical valves can be divided into solenoid valves and motor-driven valves. The choice should be based on needs and the energy available. Characteristics and scope of application of various types of valves 1. Gate valve (1) The fluid does not change the flow direction when flowing through the gate valve. When the gate valve is fully open, the resistance coefficient is almost the smallest among all valves, and it is applicable to the range of caliber, pressure and temperature. The range is very wide. Compared with stop valves of the same caliber, its installation size is smaller, so it is the most commonly used type in chemical production equipment.

(2) The gate valve handle has two distinct stems and concealed stems: the rising stem gate valve is particularly advantageous when used for alternate switching of two or more sets of the same equipment. The rising stem can clearly indicate the switching status of the valve.

(3) When the gate valve is half-open, the valve core is prone to vibration, so the gate valve is only suitable for fully open or fully closed situations, and is not suitable for occasions where flow needs to be adjusted.

(4) The gate valve body has grooves, so it is not suitable for fluids containing solid particles. In recent years, there are gate valves with blowing ports that can be used in this situation.

2. Stop valve (1) Stop valve is a valve type widely used in chemical equipment. Its sealing performance is reliable and it is also suitable for adjusting flow. It is usually installed at the pump outlet, upstream of the regulating valve bypass flow meter, etc. where the flow needs to be adjusted.

(2) The fluid changes flow direction when flowing through the valve core, resulting in a large pressure drop. At the same time, solid matter is easily deposited on the valve seat, so it is not suitable for suspensions.

(3) Compared with gate valves of the same caliber, the stop valve is larger in size, thus limiting its maximum caliber (maximum DNl50~200).

(4) Compared with ordinary straight-way valves, Y-type stop valves and angle stop valves have smaller pressure drops, and angle valves also have the function of changing the flow direction.

(5) The needle valve is also a type of stop valve. Its valve core is tapered and can be used for small flow fine-tuning or as a sampling valve.

3. Plugs, plunger valves, and ball valves (1) have similar functions and are all valves that can be opened and closed quickly. The valve core has transverse openings, allowing liquid to pass through directly with small pressure drop, making it suitable for suspensions or viscous liquids. The valve core can be made into an upper-shaped or T-shaped channel to become a three-way or four-way valve. The appearance is regular and easy to make a jacket valve for situations where heat preservation is required. These types of valves can be easily made into pneumatic or electric valves for remote control.

(2) The difference between the three is that the working pressure of the plunger valve and the ball valve is slightly higher. 4. The butterfly valve has a certain adjustment function and is especially suitable for large flow adjustment. The operating temperature is limited by the sealing material.

5. Check valve (1) Check valve is a valve used to prevent reverse flow of fluid. Generally used to prevent contamination, temperature rise or mechanical damage caused by fluid backflow. (2) There are three commonly used types: swing type, lift type and ball type. The swing type has a larger diameter than the latter two and can be installed on horizontal or vertical pipes. When installed on vertical pipes, the fluid should flow from bottom to top. The lifting type and ball type have smaller diameters and can only be installed on horizontal pipelines.

(3) The check valve can only be used to prevent sudden backflow but has poor sealing performance. Therefore, other measures should be taken for materials that are strictly prohibited from mixing.

(4) When the inlet of the centrifugal pump is in the suction state, the bottom valve installed at the end of the inlet pipe to keep the liquid in the pump is also a check valve. When the container is open, the bottom valve can be equipped with a strainer.

6. When diaphragm valves and pipe pinch valves are used, the fluid only contacts the diaphragm or hose and does not touch other parts of the valve body. They are especially suitable for corrosive fluids or viscous liquids, suspensions, etc. However, the range of use is limited by the material of the diaphragm or hose.

1.2 Valve settings at the boundary

1.2.1 Process material and public material pipelines should be equipped with cut-off valves at the device boundary (usually inside the device boundary), with the following exceptions:

(1) Exhaust system.

(2) The discharge pipe when the emergency discharge tank is located outside the boundary; in these two cases, if a valve must be installed, the seal must be opened (C.S.O).

(3) Material pipes that will not cause material leakage or accidents.

(4) Material pipes that do not require measurement. 1.2.2 The valve settings at the boundary are shown in several ways as shown in Figure 1.2. Among them (1) is suitable for cutting off general materials; when the cross-flow of materials may cause safety accidents such as explosions and fires or important product quality accidents, in order to prevent internal leakage of the valve, use (2), (4), (5) in Figure 1.2 ) Add a blind plate; (3) and (5) in Figure 1.2 are suitable for situations where the line needs to be swept upstream or downstream after feeding. Valve a can be used for purging, draining, and checking leaks. It can also be used to detect measuring instruments. Installed between two valves in series. (5) in Figure 1.2 is suitable for places where pressure changes may be large, and the check valve can act as an instant cut-off.

1.3 Setting of root valve

1.3.1 When a medium needs to be transported to multiple users, in order to facilitate maintenance or save energy and prevent freezing, in addition to installing a cut-off valve near the equipment, a cut-off valve called a root valve is installed on the branch pipe close to the main pipe. Usually used in public material systems (such as steam, compressed air, nitrogen, etc.). When a process material is routed to multiple users (e.g. solvent), the same settings are required. The valve shown in Figure 1.3 is the root valve. When there are requirements such as energy saving and antifreeze, the distance between the root valve and the main pipe should be as small as possible. Figure 1.3 Schematic diagram of root valve setting

1.3.2 All branch pipes of public material pipelines in chemical installations should be equipped with root valves to prevent the installation or the entire plant from shutting down due to damage to individual valves.

1.3.3 Steam and overhead water pipelines, even if they only lead to one device or piece of equipment, need to be installed when the branch pipe exceeds a certain length.

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