Leakage analysis and treatment of large flange in s-zorb reactor
The first s-zorb unit of a refinery was built and put into operation in 2009, with a designscale of 1.5 MT / A. the clean gasoline is produced by adsorption desulfurization technology, which has the characteristics of high desulfurization rate (sulfur content is reduced from 1000 μ g / g to less than 10 μ g / g) andsmall octane number loss. Since the firststart-up in November 2009, the reactor flange of the unit has been leaking, and it has been on fire for many times, which has become an important bottleneck affecting thesafe production and normal operation of the unit.
What is a s-zorb device?
S-zorb unit mainly consists of four parts: feed and desulfurization reaction, adsorbent regeneration, adsorbent circulation and productstability. The process overview is as follows:
Feed and desulfurization reaction part
Thesulfur-containing gasoline from the catalytic unit or tank farm enters the raw material buffer tank, mixes with hydrogen and goes to the feed heating furnace for heating, then enters the bottom of the desulfurization reactor and conducts adsorption desulfurization reaction in the reactor.
Adsorbent regeneration part
The regeneration process is the oxidation reaction with air as oxidant. The compressed air is heated by air dryer, regeneration air preheater and regeneration gas electric heater in turn, and thensent to the bottom of regenerator, where the regeneration reaction occurs with the waiting adsorbent from regeneration feed tank.
Adsorbent circulation part
The regeneration and waiting of adsorbent are realized through the lock hopper to realize the mutual transportation of reactionsystem and regenerationsystem, and the isolation of hydrogen and oxygen environment, high and low pressure environment. Thesteps and operation are completed by the lock hopper controlsystem.
Productstability
Thestabilizer is used to treat the desulfurized gasoline products to make themstable.
Basic situation of reactor large flange
The reactor medium is hydrogen, gasoline and adsorbent, the operating pressure is about 2.6MPa, and the operating temperature is about 416 ℃. The fluidized bed process is adopted, with bottom feeding and top discharging, and filter is set at the top. When the gas-phase medium in the reactor passes through the filter, the adsorbent particles entrained in the gas phase will gather on the surface of the filter element and form the filter cake. With the gradual increase of the cake layer thickness, the pressure drop of the filter also gradually increases. When the pressure drop reaches the preset value, SIS system starts the automatic back blowing program to remove the filter cake on the filter element surface.
Leakage cause analysis
Cycle of reverse blowing changes with the start-up cycle
The filter has 6 back blow valves in total, which are respectively back blown in sequence. All 6 back blow valves are blown into a back blow cycle. The duration of each blowback cycle is 6 min, and the interval between the two blowback cycles changes with the start-up cycle of the unit. The initial period of start-up is generally 2-3h, and the final period is generally 0.5 h or even shorter.
In the back blowing cycle, the opening time of each valve is controlled within 0.5s, the back blowing time is controlled within 1.3s, and the valve closing time is controlled within 1s. SIS system controls the opening and closing of the valve, combined with the strengthening nozzle on the back blowing manifold, so that the low temperature and high pressure hydrogen can pass through instantaneously, forming a strong blasting force, so as to remove the filter cake on the surface of the filter element.
The six zones are periodically and clockwise blowed back, and the pressure and temperature fluctuate frequently, which has a serious impact on the flange sealing: the pressure fluctuates frequently, which has a fatigue impact on the bolts and gaskets in the connection joint; because of the material and structure differences of the components in the connection joint, including the bolts, nuts, flanges, sealing gaskets and the connecting cylinder, the operating temperature changes In the process of transformation, there will be deformation and incongruity, and such deformation incongruity will continue to deteriorate with the increase of alternating times; the sequential switch of six valves will produce incongruity on the stress level in the whole flange circumference direction. Finally, it will lead to the partial or overall loss of working seal stress and seal leakage failure.
Gasket selection can not adapt to alternative working conditions
Tooth shaped gasket is selected as gasket of sealing surface. The manufacturing standard Hg 20632-2009 has no specific requirements for compression and rebound rate, and there is no inspection item for rebound performance in gasket quality evaluation report.
Under the action of bolt load, the flange surface contacts with the tooth gasket and produces certain plastic deformation to form a seal. The graphite covered on the surface of the tooth gasket can enhance the meshing between the sealing surfaces, but does not change its compression and rebound characteristics. When the bolt load changes, the working seal stress will decrease rapidly, which will easily lead to seal failure. Therefore, the toothed gasket is not suitable for the sealing requirements under the condition of alternating working conditions.
Few bolts are arranged
During the construction and operation of the S Zorb device, there are 7 sets of S Zorb devices started construction at the same time in China. The 7 sets of devices are all filters of the same manufacturer, and the fasteners are designed and supplied by the filter manufacturer. After the start of construction, the 7 sets of equipment all had different degrees of leakage. According to the investigation, the diameter of flange bolt center circle of s zorb device of enterprise a is the largest, but the screw thread specification of bolt is the smallest, and the number of bolts is the least, which will result in the poor uniformity of bolt preload in the whole flange circumference direction. Even if it is considered to replace other gaskets with better rebound performance in the future, it is still possible to over pre tighten the gasket locally, resulting in the disappearance of gasket elasticity and thus leakage. Therefore, the number of bolts should be increased as much as possible in the design to make the pre tightening force of bolts and the stress of gasket more uniform.
There is room for improvement of flange type
In view of the complex pressure and temperature fluctuations in the reactor of s zorb unit, it is more reasonable to change the flange sealing type to the concave convex surface (FM / M) type with reference to the design of the shell flange sealing surface of the shell and tube heat exchanger. That is to say, two concave large flanges are used to hold the tube plate to better fix the gasket and reduce the impact of pressure fluctuation on the sealing.
In conclusion, considering that all leakage occurs after a certain period of operation, the fluctuation of operating pressure and temperature should be the main reason. Because the technological process cannot be changed, and the reactor has not been replaced in a short period of time, the optimization of bolt layout and the improvement of flange type cannot be implemented, so the use of gasket with good rebound performance is an effective measure.
Discussion on treatment plan
Repair treatment of flange surface
The field repair mainly uses the portable flange processing equipment for processing. This equipment adopts modular design, which is easy to assemble and disassemble on site. It can be used for all types of flange surface, sealing groove, welding groove processing and heat exchanger flange processing. The surface roughness can reach Ra1.6 μ m, and the flatness error can be controlled within ± 0.05 mm.
In the field construction, select the installation chassis of appropriate size, install the inner card on the inner diameter of the flange, and install the rotating part and the cross beam part on the base. Take the flange surface as the benchmark, use the dial indicator to adjust the equipment to be parallel to the flange surface; take the flange center as the benchmark, adjust the equipment to be concentric with the flange; measure and estimate the thickness of the material to be removed by turning, and the feed amount each time is 0.1-0.2mm; at the final stage of processing, the feed amount each time is about 0.1mm, until the whole sealing surface is processed to the required size and roughness.
Gasket modification
Use bimetal wave tooth pad instead
In May 2012, when it was shut down for maintenance, bimetal corrugated gasket was used as gasket. In this kind of gasket, first the corrugated groove is machined on one side of the two annular metal plates, then the smooth surface of the two annular metal plates is jointed, the inner circumference is not treated, and the outer circumference is fused together by fusion welding, forming a metal framework with open inner circumference and closed outer circumference. Finally, a certain thickness of flexible graphite material is added to the outer surface of the metal framework
In the normal production of the device, because the inner circumference of the gasket metal framework is open and the outer circumference is closed, the medium will penetrate between the two circular metal plates through the inner circumference, act on the inner surface of the two metal plates, and push the two metal plates to press outward to the corresponding flange surface respectively. Compared with the common gasket under the same working condition, this kind of gasket always has an additional stress equivalent to the medium pressure. When the bolt load drops, the additional stress can effectively compensate the gasket stress. Therefore, this principle of gasket is particularly suitable for fluctuating conditions.
In actual operation, the gasket only used for 3 months (2012-05-2012-08) had leakage. This type of gasket is also used in the two high-pressure heat exchangers of the device. From May 2012 to April 2016, no leakage was found. In April 2016, when the maintenance was stopped, dye penetrant inspection was carried out on the four gaskets removed from the two sets of high replacement, and surface micro cracks were found on the outer circumference of one gasket. It can be concluded that the welding seam of this kind of gasket is its weak point, and the adaptability of the welding seam to the pressure fluctuation becomes worse in the case of occasional manufacturing defect and installation damage. Therefore, the large flange of the reactor is in severe condition, and local leakage occurs. After single point injection, although the serious leakage of the medium is temporarily prevented, the crack is still expanding, which leads to leakage in other parts, and finally only the whole flange surface can be injected.
Change to constant stress pad
When it was shut down in May 2016, the gasket type was changed to constant stress gasket. The gasket is composed of a first stage C-type seal ring, a second stage graphite seal ring, a third stage auxiliary seal and a leak detection component.
After the gasket is installed and the bolt is tightened, the C ring will be compressed to be flush with the limit outer ring, and the flange surface on both sides of the gasket will directly contact with the limit outer ring of the gasket, which is called “metal to metal”. The gasket manufactured based on this design concept has the following characteristics: the pressure of C-ring is far lower than the elastic limit of metal, and it still has good resilience; the outer ring is limited to never collapse within the compressive strength range of metal to ensure that C-ring will never collapse; the outer ring is limited to store enough bolt force, which acts on the limit when thermal shock, pressure fluctuation and other factors cause bolt relaxation The load of the outer ring is released preferentially to ensure that the C ring has enough stress; as the metal collides with the metal, the overall rigidity of the connecting parts is increased.
C-ring is the main seal (seal tightness 1 × 10-9 PA · m3 / s), which is composed of C-type metal spring in the inner layer, metal in the middle layer and flexible seal in the outer layer. The spring adopts Inconel x750 nickel base alloy with high temperature resistance and creep resistance, the middle layer adopts Inconel 600 alloy with good corrosion resistance, and the sealing layer is soft silver. The density grade of graphite seal ring is 1 × 10-6 PA · m3 / s, which can improve the back pressure and reduce the pressure difference of sealing surface of inner C ring. The two are coordinated to ensure the continuous and effective sealing. The auxiliary seal ring adopts a very low seal stress, which is mainly used to collect the leaked medium and collect and monitor it through the leakage pipe.
However, this gasket also has obvious disadvantages: due to the narrow sealing surface of C ring, the texture of blue surface can be eliminated by the flexible metal plastic deformation on the surface, which is closer to the metal wire sealing structure than the general non-metal gasket. Therefore, the flange surface in contact with C ring needs higher roughness requirements, which is generally required to reach at least ra3.2 μ M. Considering that the design values of the roughness of the filter shell flange, the upper and lower surfaces of the tube plate, and the large flange surface of the reactor are Ra6.4 μ m, it is necessary to arrange the field repair of the filter shell flange and the reactor flange before each gasket replacement, and coordinate the filter manufacturer to improve the roughness of the upper and lower surfaces of the tube plate.
When the plant was shut down in May 2016, constant stress pads were used for the first time. In September 2016, it was found that there was condensate at the leakage pipe, 3 drops / min. in addition, no leakage point was detected on the whole flange surface. The reason may be the lack of experience in the first use and the low bolt load. Considering that the leakage medium from the leakage pipe is the overall leakage medium of the seal, the leakage is not large, so no treatment has been made. In May 2018, when the maintenance was stopped, the tightening torque of gasket was increased from 14000 n · m to 19300 n · m, and no leakage has occurred since the operation.
Summary
- (1) There are many reasons for the leakage of reactor flange, such as process operation, gasket selection, bolt arrangement, flange type and so on. Combined with the actual situation of the device, the use of gasket with good rebound performance is an effective measure.
- (2) Constant stress pad, based on the design concept of “metal against metal”, combined with multi-level line and face combination sealing form, forms multi-level gradient seal, which can adapt to the pressure and temperature fluctuation of process medium, effectively solves the leakage problem of reactor large flange, but also has the disadvantage of high requirements for sealing face.
Source: China Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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