New Advances in Automatic Discharging of Ammonia Valve Li Haijiao (Hunan Anchun High-tech Co., Ltd., Changsha 410015, China) Abstract: The service life of a new type of automatic ammonia discharging valve has reached over one and a half years. Pay the liquid level control difficult problems. Introduce the ammonia valve research process and the new structure, working principle and application. Key words: put ammonia valve; two-phase flow; spool; sleeve valve; caliber CLC number: P23 Document code; B Article ID: 1000-3932 (2001) 05-0047-04 1, Cold and ammonia are required ammonia level control, liquid ammonia seal to ensure that the high-pressure gas system does not string into the low pressure system. In general this is an easy to implement general level adjustment program. However, this simple level adjustment does not work because the ammonia valve has a very short life (about a month out of control). Nearly 40 years, the fertilizer industry, instrumentation and valve manufacturers and research institutes have done a lot of work, developed a variety of high pressure differential flow control valve. The actual results vary, mostly in less than six months. Compared with the universal valve has made great progress, but did not solve the fundamental problem. Starting from the mechanism of ammonia valve damage, I studied other kinds of ammonia release, adopted comprehensive measures from the aspects of structure and materials, and developed a new valve for ammonia release. At present, Hunan Youxian Xiangdong Chemical Fertilizer Plant, Fujian Yong'an Chemical Industry Co., Ltd. joint venture chemical fertilizer plant, the longest period of use has been more than a year and a half. From the valve opening situation, there is still a long service life. 2, let ammonia valve damage mechanism 2.1 The existence of two-phase fluid is to put ammonia-specific physical phenomenon Two-phase fluid is produced by two completely different factors. One is under high pressure liquid ammonia through the small gap valve when its static pressure drops below the saturated vapor pressure, the occurrence of a flash. However, evaporation is incomplete due to the short throttle area. Showing the coexistence of two-phase liquid state, in which the volume of gas exceeds the volume of liquid; another reason is that under high pressure liquid ammonia produced by the solution of a portion of the hydrogen-nitrogen gas, the release of ammonia decompression process of ammonia nitrogen Gas resolved. According to relevant information, 1m³ ammonia decompression, the resolution of 48m³ hydrogen and nitrogen gas. It is clear that the volume of gas in the intermediate phase of the two-phase fluid produced significantly exceeds the volume of the liquid. As a result, droplets of unvaporized liquid ammonia are suspended in the ammonia and hydrogen-nitrogen gas and are surrounded by them. 2.2 High velocity ammonia droplet impact so that the valve core wear a single gas of its molecular mass is small, the flow rate can be high. A single liquid, its molecular mass is small, the flow rate can be high. A single liquid of its larger molecular mass, but the flow rate is high up. Therefore, a single fluid can not form a destructive force for the spool. The two-phase fluid suspended in the liquid ammonia in the liquid droplets have a liquid quality, at the same time with the gas velocity. Mass multiplied by the speed, also has far more than a single gas or a single liquid kinetic energy, so there is a valve core, the seat of the extraordinary destruction of power. This damage process is similar to the sandblasting process, where the sand is propelled by the air as a carrier and the abrasive is approaching the speed of the carrier. The droplet during flashing corresponds to the abrasive, and the carrier is hydrogen and ammonia. Its movement is exactly the same, although less destructive, but serious still can damage a valve between January and January. Field observations Damaged spools have a very smooth profile compared to cavitationally damaged, honeycomb-like rough surfaces, suggesting that both chemical corrosion and cavitation are insignificant. The main reason for the damage is the physical damage caused by the two-phase flow generated by flash evaporation. Gas decompression exacerbates this damage. And the irreversible two-phase flow exists in the whole process of putting ammonia. 3, a variety of put ammonia valve structural characteristics and solutions to the problem of life problems For many years in order to solve the service life of high pressure valve, the valve manufacturer has produced a variety of different valves, the use of ammonia units are also used to do A lot of improvement attempts. The main measures are nothing more than a few: that is, step-down step by step; to change the liquid flow Shan way; the sealing surface and worn surface staggered; using high-strength materials. Practice shows that some measures have no effect or little effect. We can also see that adopting only one measure is far less effective than using multiple measures together. The crux of the problem lies in the right valve damage mechanism for the right medicine. Here are some typical structures for some discussion. 3.1 multi-stage spool relief shown in Figure 1, this valve uses a multi-stage spool structure. Spool valve seat and the valve with the total pressure difference into several small pressure, step-down step by step. The design intent is to keep the differential pressure at each level above the critical pressure. The purpose is to prevent liquid ammonia flashes over the critical pressure to produce gas, resulting in two-phase flow. Send pictures to the phone, the subject related pictures are as follows: Figure 1 Multi-stage valve structure This structure and method has some effect, but can not solve the fundamental problem. The reason for this is that the amount of gas that is produced as a result of the ammonia release process is too large. It does not result in no resolution because the pressure drop does not exceed the critical pressure. Therefore, two-phase flow can not be avoided even if flashing is avoided. At best, it reduces only a small amount of flash gas and the conflict is moderated. Similar to the valve before and after the same throttle throttling method, the principles and effects are similar. 3.2 Roadway valve-specific valve shown in Figure 2, Southwest plant produced a valve of this type of design is still the main idea of ​​decentralized pressure drop, it is to make the spool and valve seat between the throttle area as possible Stretched to form a step-down roadway, along the length of the roadway dispersed pressure drop. Second, since the taper of the spool can be made small, the angle between the direction of fluid flow and the axial direction of the spool is also small. In this way, the fluid ejected from the orifice inlet naturally reduces the component force acting on the vertical surface of the spool. In addition, the valve core material to improve the hardness after treatment. Therefore, this valve has become a better valve. Send the picture to the mobile phone, the related picture of this subject is as follows: Figure 2 Roadway valve special valve However, the same reason: decentralized pressure drop can not avoid two-phase flow, so the fundamental problem is still difficult to solve. The general service life is about six months. At the same time because of the increased hardness of the spool, has brought new problems. Ammonia valve diameter are generally small, the use of long after the valve spool diameter is smaller, the hardness becomes more vulnerable after the increase. And the long roadway structure makes the valve core slide into the valve seat, if there is different heart, the radial shear force becomes large, as long as the concentricity of the valve core and seat a slight deviation, it will break the spool. This situation is not uncommon. 3.3 The structure of the sealing surface to avoid the impact of two-phase flow Figure 3 shows an inverted conical valve core, the designer's intention is to minimize the component angle of attack in the direction of fluid advance to avoid fluid from perpendicular to Component surface impact direction. At the same time the wear position and sealing surface staggered. In fact, this structure is more suitable for manual valve. Automatic adjustment is not easy to meet the requirement when the flow characteristics need to be equal percentage. Used, this valve life is not long. Send pictures to the phone, the picture is as follows: Figure 3 inverted cone 3.4 valve seat hard surface surfacing or the use of new materials Widely used at home and abroad Stellite alloy (Rc45) hardened tool steel (Rc70) manufacturing Anti-cavitation valve seat. Some of these materials are very hard and brittle. In order to achieve better overall mechanical properties, these materials were then deposited or sprayed on some stainless steel substrates (see Figure 4). In general, this method accommodates larger valve seat sizes. Ammonia plant in China most of the devices with an annual output of 100,000 tons below, put ammonia valve diameter is small, the spool is also small, inconvenient processing. At the same time, small diameter, high hardness, it is easy to break. Send pictures to the phone, the subject related pictures are as follows: Figure 4 valve seat hard surface surfacing or spray welding I have used pre-thin-walled zirconium valve material valve seat, on the one hand the high cost, on the other hand the effect is not as good as Intentions. Large-size spool oxide film well controlled, long service life. Small spool more than six months difficult. In fact, a variety of improved programs have improved dietary materials. But only consider the material, do not change the structure, the service life can not be greatly improved. 3.5 The main improvement of the new ammonia valve 3.5.1 The use of sleeve valve structure Sleeve-type structure has long been one of the basic structure of the control valve, but the current series of high pressure valve is not available for the product. There is no product even in the series of small and medium-sized sleeve low pressure valves. In high differential pressure, small flow valve sleeve structure is an innovation. The designer is based on this consideration (see Figure 5): send the picture to the phone, the picture of the subject is as follows: Figure 5 Sleeve Structure The fluid injected from the symmetrical holes on the sleeve wall becomes two-phase fluid, on the one hand The fluid collides with each other in the center of the sleeve to create a high degree of perturbation. A large number of disturbances cause most of the high-velocity droplets to lose their velocity, lose their destructive force and are converted into thermal energy by the frictional resistance of the fluid. On the other hand, the jet of droplets in the center of the sleeve to achieve high speed, only destructive power. In this case, even if it is possible to pass through the center droplet and the droplet which still has the second velocity after the impact, reaching the opposite tube wall will be blocked by the liquid in the stagnation barrel cavity. Liquid self-buffer energy dissipation. This avoids the valve plug (equivalent to the spool) and the wall of the direct damage, protecting the sealing surface. A further structural advantage is that the valve plug always moves up and down in the sleeve and the sleeve acts as a guide. There is no ordinary angle valve in the opening and closing process, because the valve core and seat of the different heart, leading to the spool of the radial shear force and break the spool phenomenon. Obviously, the use of sleeve structure can solve many other structural problems can not be solved. However, it is not perfect. Valve plug movement in the sleeve, there must be a gap between the two, so the leak always exists. To completely solve the leak, the structure can get complicated. Fortunately, ammonia discharge process fluid is long-term continuous, regardless of whether there is a leak, but whether the leak is small enough. Leakage is as small as the minimum ammonia production, the valve plug is closed, the liquid level can rise, and the amount of leakage remained unchanged, which will meet the production requirements. 3.5.2 Adoption of "side-in-bottom-out" flow to general high-pressure angle type valve with "side-in and bottom-out" to reduce valve core wear. This approach is adopted here for this purpose. As a result of "side-in and bottom-out", the bottom surface of the valve plug is in a low pressure area. After unbalanced operation, the upward thrust of the valve plug is small and the operating force of the membrane head can also be reduced. More importantly, the valve plug is also above the low-pressure zone, high-pressure packing only need to make low-pressure seal, the sealing problem can be easily solved. 3.5.3 The use of high strength, high hardness alloy material droplets in two-phase fluid, although it has a great impact kinetic energy, but after all it is a liquid, do not have the solid quality and hardness. As long as the valve structure, especially the throttle valve sleeve, valve plug with high strength and hardness of the material; as long as the strength and hardness of these materials to a sufficient extent that can withstand the destruction of two-phase flow. The new valve has found and adopted this material. 4, put some new problems in the application of ammonia valve 4.1 put ammonia valve caliber and the ability to choose the right choice to put ammonia valve design is a matter of routine design, there is no new problem to be discussed. However, caliber and liquidity problems often become controversial issues between designers and contractors. Therefore, it is especially clarified here. According to the annual production capacity of ammonia 10000t / a calculated under the conditions of data: Q1 = 2.48m3 / η (flow) Ï = 0.58g / cm3 (density) P1 = 29000kPa (valve pressure) p2 = 1600kpa = 30 ° C (liquid temperature before valve) D1 = 1600kpf (pipe diameter) Fl = 0.8 (pressure recovery factor) PC = 11378kpa (thermodynamic critical pressure) Pv = 1216kpa (saturated vapor pressure at valve inlet temperature) The related pictures of this subject are as follows: According to â…¢ J type supporting process, 75% of ammonia is separated by cold treatment, and the other part is borne by ammonia. Therefore: Kv = 0.145 × 0.75 = 0.108 for cold junction Ammonia is divided into Kv = 0.145 × 0.25 = 0.036 According to this calculation result, it is easy to calculate the size of the flow capacity of the valve required for devices of various capacities. Dn15 valve maximum flow capacity can reach 4.0, to meet the domestic production requirements of any manufacturer. Therefore, re-election on the caliber is completely unnecessary. 4.2 The configuration of the valve pipeline and the degree of valve control requirements as shown in Figure 6 put ammonia valve into the bottom of the way out of the way. All piping and supporting valve diameter are Dn15. Because the synthesis section of the pipeline catalyst particles and dirt, the control valve aperture is small, very easy to plug. Therefore, set up a dedicated high-pressure filter. Send the picture to the mobile phone, the picture of this subject is as follows: Figure 6 Ammonia release valve with side-in and out-way piping Under normal conditions of use, the two stop valves before and after the control valve must be fully open to ensure that all the pressure drops on the control valve. The cut-off valve almost no pressure drop, the cut-off valve is not easy to damage. Once the need for maintenance, can be closed without leakage due to inability to overhaul the regulator. The cut-off valve on the by-pass must be completely shut-off because the normal shut-off valve has no design to prevent cavitation at all and no material strengthening. The use of the bypass valve will soon be damaged, even if the closure can not avoid leakage. At this time the level gauge is still in the lower limit of the liquid level can not be controlled, often make people think that the control valve leakage, it is not. Of course, the fundamental solution to the problem is to follow the principle of ammonia regulator valve design, the design of a new globe valve. 4.3 Common Problems and Solutions 4.3.1 Blockage and Blocking Because of the oil generated by the synthesis system, solvent particles and powder, slag and other metals are always present, filters are provided. However, the capacity of the filter is limited, so the maximum pore size of the filter through the solid material is set to Ф2mm. A large number of particles and powder that can pass through the regulating valve less than Ф2mm let it vent, and only block objects larger than Ф2mm. However, it is possible for individual small particles (most typically amorphous catalyst particles) that are smaller than Ф2mm in one direction and again larger than Ф2mm in other directions to cross the filter. Such small particles may catch in the through-hole of the sleeve and extend into the sleeve so that the plug can not be closed. Although this happens rarely, but the occurrence of stop the use of the valve, it is necessary to open the clean-up to troubleshoot. Therefore, ammonia, cold should be more ways to find ways to prevent dirt as much as possible through the automatic control valve of this pipeline. 4.3.2 self-excited oscillation Self-excited oscillation exists in the control system of the circuit which, the other is present in the control valve and valve positioner formed by the small circuit. No matter what kind of self-excited shock will cause the valve packing rapid wear and tear, it must be avoided. The reasons for the former situation is more complex, depending on which part of the system has problems, specific problems to be solved. This situation I encountered in the fertilizer plant in Yueyang City, due to improper handling, resulting in early wear packing. Two months or so filler leak ammonia, had to stop using. In fact, the valve is still intact. Self-oscillating oscillations also occur in the small closed loop formed by the control valve and the valve positioner. The reason is that the phases of the positive path of this closed loop are phase shifted. When the valve membrane is very small, the system itself may have a resistance-capacitance correction can not overcome enough phase shift, resulting in negative feedback into a positive feedback, resulting in self-oscillation. For this reason, the design of the small diaphragm valve and the electric valve positioner (especially the electric type П) is not suitable for supporting, and the problem of the electric converter should be adopted. 5, Conclusion One idea is to prevent the occurrence of two-phase fluid, the truth is beyond reproach. However, two-phase flow to the lake but only one attention to the two. Therefore, to solve the problem fundamentally can not solve the problem at all. Another idea is to admit that the occurrence of two-phase flow is inevitable and take advantage of the characteristics of the fluid to take measures to avoid the consequences of the two-phase flow. The effect is quite obvious. The life span of the ammonia valve is greatly extended based on the correct diagnosis of the cause of the damage. The fortification measures taken are comprehensive and relatively complete, thus achieving good results.
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