Valves are an important part of the pipeline system, and metal valves are the most widely used in chemical plants. The function of the valve is mainly used for opening and closing, throttling and ensuring the safe operation of pipelines and equipment. Therefore, the correct and reasonable selection of metal valves plays an important role in plant safety and fluid control systems.
1. Types and uses of valves
There are many types of valves in engineering. Due to the difference in fluid pressure, temperature and physical and chemical properties, the control requirements for fluid systems are also different, including gate valves, stop valves (throttle valves, needle valves), check valves, and plugs. Valves, ball valves, butterfly valves and diaphragm valves are the most widely used in chemical plants.
1.1 Gate Valve
is generally used to control the opening and closing of fluids, with small fluid resistance, good sealing performance, unrestricted flow direction of the medium, small external force required for opening and closing, and short structure length.
The valve stem is divided into a bright stem and a concealed stem. The exposed stem gate valve is suitable for corrosive media, and the exposed stem gate valve is basically used in chemical engineering. Concealed stem gate valves are mainly used in waterways, and are mostly used in low-pressure, non-corrosive medium occasions, such as some cast iron and copper valves. The structure of the gate includes wedge gate and parallel gate.
Wedge gates are divided into single gate and double gate. Parallel rams are mostly used in oil and gas transportation systems and are not commonly used in chemical plants.
1.2 Stop valve
is mainly used for cutting off. The stop valve has large fluid resistance, large opening and closing torque, and has flow direction requirements. Compared with gate valves, globe valves have the following advantages:
(1) The friction force of the sealing surface is smaller than that of the gate valve during the opening and closing process, and it is wear-resistant.
(2) The opening height is smaller than the gate valve.
(3) The globe valve usually has only one sealing surface, and the manufacturing process is good, which is convenient for maintenance.
Globe valve, like gate valve, also has a bright rod and a dark rod, so I won’t repeat them here. According to the different valve body structure, the stop valve has straight-through, angle and Y-type. The straight-through type is the most widely used, and the angle type is used where the fluid flow direction changes 90°.
In addition, the throttle valve and the needle valve are also a kind of stop valve, which has a stronger regulating function than the ordinary stop valve.
1.3 Chevk valve
Check valve is also called one-way valve, which is used to prevent the reverse flow of fluid. Therefore, when installing the check valve, pay attention to the flow direction of the medium should be consistent with the direction of the arrow on the check valve. There are many types of check valves, and various manufacturers have different products, but they are mainly divided into swing type and lift type from the structure. Swing check valves mainly include single valve type and double valve type.
1.4 Butterfly valve
Butterfly valve can be used for opening and closing and throttling of liquid medium with suspended solids. It has small fluid resistance, light weight, small structure size, and rapid opening and closing. It is suitable for large-diameter pipelines. The butterfly valve has a certain adjustment function and can transport slurry. Due to the backward processing technology in the past, butterfly valves have been used in water systems, but rarely in process systems. With the improvement of materials, design and processing, butterfly valves have been increasingly used in process systems.
Butterfly valves have two types: soft seal and hard seal. The choice of soft seal and hard seal mainly depends on the temperature of the fluid medium. Relatively speaking, the sealing performance of a soft seal is better than that of a hard seal.
There are two types of soft seals: rubber and PTFE (polytetrafluoroethylene) valve seats. Rubber seat butterfly valves (rubber-lined valve bodies) are mostly used in water systems and have a centerline structure. This kind of butterfly valve can be installed without gaskets because the flange of the rubber lining can serve as a gasket. PTFE seat butterfly valves are mostly used in process systems, generally single eccentric or double eccentric structure.
There are many varieties of hard seals, such as hard fixed seal rings, multilayer seals (Laminated seals), etc. Because the manufacturer’s design is often different, the leakage rate is also different. The structure of the hard seal butterfly valve is preferably triple eccentric, which solves the problems of thermal expansion compensation and wear compensation. The double eccentric or triple eccentric structure hard seal butterfly valve also has a two-way sealing function, and its reverse (low pressure side to high pressure side) sealing pressure should not be less than 80% of the positive direction (high pressure side to low pressure side). The design and selection should be negotiated with the manufacturer.
1.5 Cock valve
The plug valve has small fluid resistance, good sealing performance, long service life, and can be sealed in both directions, so it is often used on highly or extremely hazardous materials, but the opening and closing torque is relatively large, and the price is relatively high. The plug valve cavity does not accumulate liquid, especially the material in the intermittent device will not cause pollution, so the plug valve must be used in some occasions.
The flow passage of the plug valve can be divided into straight, three-way and four-way, which is suitable for multi-directional distribution of gas and liquid fluid.
Cock valves can be divided into two types: non-lubricated and lubricated. The oil-sealed plug valve with forced lubrication forms an oil film between the plug and the sealing surface of the plug due to forced lubrication. In this way, the sealing performance is better, the opening and closing is labor-saving, and the sealing surface is prevented from being damaged, but it must be considered whether the lubrication pollutes the material, and the non-lubricated type is preferred for regular maintenance.
Sleeve seal of the plug valve is continuous and surrounds the entire plug, so the fluid will not contact the shaft. In addition, the plug valve has a layer of metal composite diaphragm as the second seal, so the plug valve can strictly control external leakage. Plug valves generally have no packing. When there are special requirements (such as external leakage is not allowed, etc.), packing is required as the third seal.
The design structure of the plug valve allows the plug valve to adjust the sealing valve seat online. Due to long-term operation, the sealing surface will be worn. Because the plug is tapered, the plug can be pressed down by the bolt of the valve cover to make it tightly fit with the valve seat to achieve a sealing effect.
1.6 ball valve
The function of the ball valve is similar to the plug valve (the ball valve is a derivative of the plug valve). The ball valve has good sealing effect, so it is widely used. The ball valve opens and closes quickly, the opening and closing torque is smaller than that of the plug valve, the resistance is very small, and the maintenance is convenient. It is suitable for slurry, viscous fluid and medium pipelines with high sealing requirements. And because of its low price, ball valves are more widely used than plug valves. Ball valves can generally be classified from the structure of the ball, the structure of the valve body, the flow channel and the seat material.
According to the spherical structure, there are floating ball valves and fixed ball valves. The former is mostly used for small diameters, the latter is used for large diameters, generally DN200 (CLASS 150), DN150 (CLASS 300 and CLASS 600) as the boundary.
According to the structure of the valve body, there are three types: one-piece type, two-piece type and three-piece type. There are two types of one-piece type: top-mounted type and side-mounted type.
According to the runner form, there are full diameter and reduced diameter. Reduced-diameter ball valves use less materials than full-diameter ball valves and are cheaper. If the process conditions permit, they can be considered preferentially. Ball valve flow channels can be divided into straight, three-way and four-way, which are suitable for multi-directional distribution of gas and liquid fluids. According to the seat material, there are soft seal and hard seal. When used in combustible media or the external environment is likely to burn, the soft-seal ball valve should have an anti-static and fire-proof design, and the manufacturer’s products should pass anti-static and fire-proof tests, such as in accordance with API607. The same applies to soft-sealed butterfly valves and plug valves (plug valves can only meet the external fire protection requirements in the fire test).
1.7 diaphragm valve
Diaphragm valve can be sealed in both directions, suitable for low pressure, corrosive slurry or suspended viscous fluid medium. And because the operating mechanism is separated from the medium channel, the fluid is cut off by the elastic diaphragm, which is especially suitable for the medium in the food and medical and health industries. The operating temperature of the diaphragm valve depends on the temperature resistance of the diaphragm material. From the structure, it can be divided into straight-through type and weir type.
2. Selection of end connection form
The commonly used connection forms of valve ends include flange connection, threaded connection, butt welding connection and socket welding connection.
2.1 flange connection
Flange connection is conducive to valve installation and disassembly. The valve end flange sealing surface forms mainly include full surface (FF), raised surface (RF), concave surface (FM), tongue and groove surface (TG) and ring connection surface (RJ). The flange standards adopted by API valves are series such as ASMEB16.5. Sometimes you can see Class 125 and Class 250 grades on flanged valves. This is the pressure grade of cast iron flanges. It is the same as the connection size of Class 150 and Class 300, except that the sealing surfaces of the first two are full plane ( FF).
Wafer and Lug valves are also flanged.
2.2 Butt welding connection
Due to the high strength of the butt-welded joint and good sealing, the valves connected by the butt-welded in the chemical system are mostly used in some high temperature, high pressure, highly toxic media, flammable and explosive occasions.
2.3 Socket welding and threaded connection
is generally used in piping systems whose nominal size does not exceed DN40, but cannot be used for fluid media with crevice corrosion.
Threaded connection shall not be used on pipelines with highly toxic and combustible media, and at the same time, it shall be avoided to be used in cyclic loading conditions. At present, it is used in the occasions where the pressure is not high in the project. The thread form on the pipeline is mainly tapered pipe thread. There are two specifications of tapered pipe thread. The cone apex angles are 55° and 60° respectively. The two cannot be interchanged. On pipelines with flammable or highly hazardous media, if installation requires threaded connection, the nominal size should not exceed DN20 at this time, and seal welding should be performed after threaded connection.
3. Material
Valve materials include valve housing, internals, gaskets, packing and fastener materials. Because there are many valve materials, and due to space limitations, this article only briefly introduces typical valve housing materials. Ferrous metal shell materials include cast iron, carbon steel, stainless steel, alloy steel.
3.1 cast iron
Gray cast iron (A1262B) is generally used on low pressure valves and is not recommended for use on process pipelines. The performance (strength and toughness) of ductile iron (A395) is better than gray cast iron.
3.2 Carbon steel
The most common carbon steel materials in valve manufacturing are A2162WCB (casting) and A105 (forging). Special attention should be paid to carbon steel working above 400℃ for a long time, which will affect the life of the valve. For low temperature valves, commonly used are A3522LCB (casting) and A3502LF2 (forging).
3.3 Austenitic stainless steel
Austenitic stainless steel materials are usually used in corrosive conditions or ultra-low temperature conditions. The commonly used castings are A351-CF8, A351-CF8M, A351-CF3 and A351-CF3M; the commonly used forgings are A182-F304, A182-F316, A182-F304L and A182-F316L.
3.4 alloy steel material
For low-temperature valves, A352-LC3 (castings) and A350-LF3 (forgings) are commonly used.
For high temperature valves, commonly used are A217-WC6 (casting), A182-F11 (forging) and A217-WC9 (casting), A182-F22 (forging). Since WC9 and F22 belong to the 2-1/4Cr-1Mo series, they contain higher Cr and Mo than the WC6 and F11 belonging to the 1-1/4Cr-1/2Mo series, so they have better high temperature creep resistance.
4. Drive mode
The valve operation usually adopts manual mode. When the valve has a higher nominal pressure or a larger nominal size, it is difficult to manually operate the valve, gear transmission and other operation methods can be used. The selection of the valve drive mode should be determined according to the type, nominal pressure and nominal size of the valve. Table 1 shows the conditions under which gear drives should be considered for different valves. For different manufacturers, these conditions may change slightly, which can be determined through negotiation.
5. Principles of valve selection
5.1 Main parameters to be considered in valve selection
(1) The nature of the fluid delivered will affect the choice of valve type and valve structure material.
(2) Function requirements (regulation or cut-off), which mainly affects the choice of valve type.
(3) Operating conditions (whether frequent), which will affect the selection of valve type and valve material.
(4) Flow characteristics and friction loss.
(5) The nominal size of the valve (valves with a large nominal size can only be found in a limited range of valve types).
(6) Other special requirements, such as automatic closing, pressure balance, etc.
5.2 Material selection
(1) Forgings are generally used for small diameters (DN≤40), and castings are generally used for large diameters (DN>40). For the end flange of the forging valve body, the integral forged valve body should be preferred. If the flange is welded to the valve body, 100% radiographic inspection should be carried out on the weld.
(2) The carbon content of butt-welded and socket-welded carbon steel valve bodies should not be more than 0.25%, and the carbon equivalent should not be more than 0.45%
Note: When the working temperature of austenitic stainless steel exceeds 425°C, the carbon content should not be less than 0.04%, and the heat treatment state is greater than 1040°C fast cooling (CF8) and 1100°C fast cooling (CF8M).
(4) When the fluid is corrosive and ordinary austenitic stainless steel cannot be used, some special materials should be considered, such as 904L, duplex steel (such as S31803, etc.), Monel and Hastelloy.
5.3 The selection of gate valve
(1) Rigid single gate is generally used when DN≤50; elastic single gate is generally used when DN>50.
(2) For the flexible single gate valve of the cryogenic system, a vent hole should be opened on the gate on the high pressure side.
(3) Low-leakage gate valves should be used in working conditions that require low-leakage. Low-leakage gate valves have a variety of structures, among which bellows-type gate valves are generally used in chemical plants
(4) Although the gate valve is the most used type in petrochemical production equipment. However, gate valves should not be used in the following situations:
① Because the opening height is high and the space required for operation is large, it is not suitable for occasions with small operating space.
② The opening and closing time is long, so it is not suitable for fast opening and closing occasions.
③ It is not suitable for fluids with solid sedimentation. Because the sealing surface will wear out, the gate will not close.
④ Not suitable for flow adjustment. Because when the gate valve is partially opened, the medium will produce eddy current on the back of the gate, which is easy to cause erosion and vibration of the gate, and the sealing surface of the valve seat is also easily damaged.
⑤ Frequent operation of the valve will cause excessive wear on the surface of the valve seat, so it is usually only suitable for infrequent operations
5.4 The selection of globe valve
(1) Compared with the gate valve of the same specification, the shut-off valve has a larger structure length. It is generally used on pipelines with DN≤250, because the processing and manufacturing of the large-diameter shut-off valve is more troublesome, and the sealing performance is not as good as that of the small-diameter shut-off valve .
(2) Due to the large fluid resistance of the shut-off valve, it is not suitable for suspended solids and fluid media with high viscosity.
(3) The needle valve is a shut-off valve with a fine tapered plug, which can be used for small flow fine adjustment or as a sampling valve. It is usually used for small diameters. If the caliber is large, the adjustment function is also required, and a throttle valve can be used. At this time, the valve clack has a shape such as a parabola.
(4) For working conditions requiring low leakage, a low leakage stop valve should be used. Low-leakage shut-off valves have many structures, among which bellows-type shut-off valves are generally used in chemical plants
Bellows type globe valves are more widely used than bellows type gate valves, because the bellows type globe valves have shorter bellows and longer cycle life. However, bellows valves are expensive, and the quality of the bellows (such as materials, cycle times, etc.) and welding directly affect the service life and performance of the valve, so special attention should be paid when selecting them.
5.5 The selection of check valve
(1) Horizontal lift check valves are generally used in occasions with DN≤50 and can only be installed on horizontal pipelines. Vertical lift check valves are usually used in occasions with DN≤100 and are installed on vertical pipelines.
(2) The lift check valve can be selected with a spring form, and the sealing performance at this time is better than that without a spring.
(3) The minimum diameter of swing check valve is generally DN>50. It can be used on horizontal pipes or vertical pipes (the fluid must be from bottom to top), but it is easy to cause water hammer. The double disc check valve (Double Disc) is often a wafer type, which is the most space-saving check valve, which is convenient for pipeline layout, and is especially widely used on large diameters. Since the disc of the ordinary swing check valve (single disc type) cannot be fully opened to 90°, there is a certain flow resistance, so when the process requires it, special requirements (requires full opening of the disc) or Y type Lift check valve.
(4) In the case of possible water hammer, a check valve with slow closing device and damping mechanism can be considered. This kind of valve uses the medium in the pipeline for buffering, and at the moment when the check valve is closed, it can eliminate or reduce the water hammer, protect the pipeline and prevent the pump from flowing backward.
5.6 The selection of plug valve
(1) Due to manufacturing problems, non-lubricated plug valves DN>250 should not be used.
(2) When it is required that the valve cavity does not accumulate liquid, the plug valve should be selected.
(3) When the sealing of the soft-seal ball valve cannot meet the requirements, if internal leakage occurs, a plug valve can be used instead.
(4) For some working conditions, the temperature changes frequently, the ordinary plug valve cannot be used. Because temperature changes cause different expansion and contraction of valve components and sealing elements, long-term shrinkage of the packing will cause leakage along the valve stem during thermal cycling. At this time, it is necessary to consider special plug valves, such as the Severe service series of XOMOX, which cannot be produced in China.
5.7 The selection of ball valve
(1) The top-mounted ball valve can be repaired online. Three-piece ball valves are generally used for threaded and socket-welded connection.
(2) When the pipeline has a ball-through system, only full-bore ball valves can be used.
(3) The sealing effect of soft seal is better than hard seal, but it cannot be used at high temperature (the temperature resistance of various non-metallic sealing materials is not the same).
(4) shall not be used in occasions where fluid accumulation in the valve cavity is not allowed.
5.8 The selection of butterfly valve
(1) When both ends of the butterfly valve need to be disassembled, a threaded lug or flange butterfly valve should be selected.
(2) The minimum diameter of the centerline butterfly valve is generally DN50; the minimum diameter of the eccentric butterfly valve is generally DN80.
(3) When using triple eccentric PTFE seat butterfly valve, U-shaped seat is recommended.
5.9 Selection of Diaphragm Valve
(1) The straight-through type has low fluid resistance, long opening and closing stroke of the diaphragm, and the service life of the diaphragm is not as good as that of the weir type.
(2) The weir type has large fluid resistance, short opening and closing stroke of the diaphragm, and the service life of the diaphragm is better than that of the straight-through type.
5.10 the influence of other factors on valve selection
(1) When the allowable pressure drop of the system is small, a valve type with less fluid resistance should be selected, such as a gate valve, a straight-through ball valve, etc.
(2) When quick shut-off is required, plug valves, ball valves, and butterfly valves should be used. For small diameters, ball valves should be preferred.
(3) Most of the valves operated on-site have handwheels. If there is a certain distance from the operating point, a sprocket or an extension rod can be used.
(4) For viscous fluids, slurries and media with solid particles, plug valves, ball valves or butterfly valves should be used.
(5) For clean systems, plug valves, ball valves, diaphragm valves and butterfly valves are generally selected (additional requirements are required, such as polishing requirements, seal requirements, etc.).
(6) Under normal circumstances, valves with pressure ratings exceeding (including) Class 900 and DN≥50 use pressure seal bonnets (Pressure Seal Bonnet); valves with pressure ratings lower than (including) Class 600 use bolted valves Cover (Bolted Bonnet), for some working conditions that require strict leakage prevention, a welded bonnet can be considered. In some low-pressure and normal-temperature public projects, union bonnets (Union Bonnet) can be used, but this structure is generally not commonly used.
(7) If the valve needs to be kept warm or cold, the handles of the ball valve and the plug valve need to be lengthened at the connection with the valve stem to avoid the valve’s insulation layer, generally no more than 150mm.
(8) When the caliber is small, if the valve seat is deformed during welding and heat treatment, a valve with a long valve body or a short pipe at the end should be used.
(9) Valves (except check valves) for cryogenic systems (below -46°C) should use an extended bonnet neck structure. The valve stem should be treated with corresponding surface treatment to increase the surface hardness to prevent the valve stem and the packing and packing gland from scratching and affecting the seal.
In addition to considering the above factors when selecting the model, the process requirements, safety and economic factors should also be considered comprehensively to make the final choice of the valve form. And it is necessary to write a valve data sheet, the general valve data sheet should contain the following content:
(1) The name, nominal pressure, and nominal size of the valve.
(2) Design and inspection standards.
(3) Valve code.
(4) Valve structure, bonnet structure and valve end connection.
(5) Valve housing materials, valve seat and valve plate sealing surface materials, valve stems and other internal parts materials, packing, valve cover gaskets and fastener materials, etc.
(6) Drive mode.
(7) Packaging and transportation requirements.
(8) Internal and external anti-corrosion requirements.
(9) Quality requirements and spare parts requirements.
(10) Owner’s requirements and other special requirements (such as marking, etc.).
6. Concluding remarks
Valve occupies an important position in the chemical system. The selection of pipeline valves should be based on many aspects such as the phase state (liquid, vapor), solid content, pressure, temperature, and corrosion properties of the fluid being transported in the pipeline. In addition, the operation is reliable and trouble-free, the cost is reasonable and the manufacturing cycle is also an important consideration.
In the past, when selecting valve materials in engineering design, generally only the shell material was considered, and the selection of materials such as internal parts was ignored. Inappropriate selection of internal materials will often lead to failure of the internal sealing of the valve, the valve stem packing and the valve cover gasket, which will affect the service life, which will not achieve the originally expected use effect and easily cause accidents.
Judging from the current situation, API valves do not have a unified identification code, and although the national standard valve has a set of identification methods, it cannot clearly display the internal parts and other materials, as well as other special requirements. Therefore, in the engineering project, the required valve should be described in detail by compiling the valve data sheet. This provides convenience for valve selection, procurement, installation, commissioning and spare parts, improves work efficiency, and reduces the probability of errors.
Post time: Nov-13-2021