Analysis of the installation case of spiral wound gaskets
Large diameter spiral wound gaskets are key sealing elements for heat exchanger channel flanges. In order to minimize seal failure at the heat exchanger channel flange, it is important to properly install the spiral wound gasket and apply the correct bolt tightening sequence and proper tightening torque.
1 Question raised
An industrial refrigeration project exported to Kazakhstan, the U-tube heat exchanger, according to GB 151 for the heat exchanger, its model is BIU1500-8.3/2.07-1142-7/19-2 (As shown in Figure 1), the design pressure is 8.3 MPa, the hydraulic pressure test pressure is 10.8 MPa; the full thread bolt is M64×3, the length is L=880 mm, the material is SA-320 L7, the number is 46; The material is SA-194 Gr.4; the gasket used on both sides of the tube sheet is a spiral wound gasket with an inner ring. During the hydraulic test, when the pressure rises to 8.5 MPa, the upper part of the pipe flange is leaked, and after the bolt tightening torque is increased to 9 500 N·m by the torque multiplier, it still leaks. Later, the bolts that loosened the position of the leak were loosened, and then the bolts at the leak and the loose bolts were retightened to re-test the hydraulics. As the test pressure continues to rise, leakage begins to occur at the lowest position of the flange of the pipe box, and there is a tendency to increase gradually, and a small amount of stainless steel metal strip is found to be squeezed from the lowest point of the pipe flange, and As the pressure inside the pipe box rises, the stainless steel metal strip is extruded more and more, and the leakage is also getting larger.
Fig.1 U-tube exchange
2 Removal of the tube box assembly and tube bundle assembly
The leak occurred in the flange of the pipe flange and the spiral wound gasket was damaged, so it was decided to remove the tube box assembly. Since the nominal diameter of the bolts of the pipe flange is M64×3, the tightening torque of some bolts also reaches 9 500 N·m, and the bolt removal torque is about 1.5~2.0 times of the tightening torque, so the pipe box assembly removal belt It has come a lot of difficulty. Because there is no suitable tool available, I have to use the wrench sleeve, the big hammer and the hoist to pull the wrench. After three days of unremitting efforts, there are still two bolts that can’t be removed. Finally, I have to use the gas cutter. The bolts are cut off and the tube box assembly is removed.
After the lower tube box assembly was disassembled, it was found that the wound portion of the spiral wound gasket had all fallen off, only the inner metal ring was left, and the metal strip of the intermediate winding partition of the spiral wound gasket was distorted, as shown in Fig. 2.
Fig.2 Damaged metal spiral wound gaskets of tube sides
During the process of disassembling the tube box assembly, the tube tube bundle assembly and the shell side are not displaced as much as possible, but it is found that the metal-wound gasket on the shell side also exhibits a similar phenomenon that the tube-wound metal-wound gasket is extruded, and The trend is increasing, and for the sake of safety, the metal-wound gasket on the shell side is replaced. With the help of the eyebolt hole reserved on the tube plate and the roller assembly of the tube bundle assembly, the tube bundle assembly weighing about 24 t is finally withdrawn from the barrel with the cooperation of the chain hoist, and it is found that the spiral wound gasket has also been The falling frame falls off, as shown in Figure 3.
Fig.3 Damaged metal spiral wound gaskets of shell sides
3 Leakage analysis
From the perspective of the disassembled spiral wound gasket, the split diaphragm seal of the metal-wound gasket on the pipe side has been twisted and deformed by the splitter partition of the pipe box, indicating that the spiral wound gasket is not installed. Complete positioning between the tube sheet and the flange sealing surface of the tube box is a major cause. Another reason is that the lower bolt is loosened after the leak and the displacement is caused. Once the spiral wound gasket has been displaced, the split diaphragm seal of the spiral wound gasket is also separated from the splitter diaphragm groove of the tube sheet. When the bolt is loosened and the bolt is retightened, the phenomenon that the spiral wound gasket is crushed and the split diaphragm seal is squeezed and twisted appears.
Since the installation of the pipe box seal is not known, it is considered that the leakage is due to the bolt tightening torque not being in place, so the tightening torque is increased, and as the tightening torque of the bolt increases, the spiral wound gasket is pressed. Bad and extruded, thus destroying the seal.
As for the metal-wound gasket on the shell side, the bolt is loosened when the tube box is removed, and the metal-wound gasket is displaced, thereby breaking the seal.
In short, the tube side is also good, the shell side is also good, and there are three reasons for the spiral wound gasket seal failure:
- (1) Because the spiral wound gasket is not completely positioned at the sealing surface position.
- (2) Displacement occurs when the subsequent bolts are loose, causing the spiral wound gasket to disengage from the original sealing surface position.
- (3) If the pre-tightening force of the bolt is too large, the gasket may be crushed or extruded, thereby damaging the seal.
Therefore, how to ensure that the metal-wound gasket does not shift during the installation process and the bolt pre-tightening force control is the key to avoid seal leakage.
4 Program development
The structure of the flat-top tube box of a manufacturer’s dry evaporator consists of two elastic cylindrical pins, which are composed of a cylinder section, a gasket and a flat cover. The two elastic cylindrical pins are used to place the gasket and the barrel and the tube. The plate is positioned and the tube box assembly is secured to the tube sheet by bolts. Therefore, the solution is to use the elastic cylindrical pin to fix the spiral wound gasket on the flange or the tube plate, so that the spiral wound gasket will not be misaligned and displaced. The specific method is as follows:
First, drill four uniform positioning holes with a diameter of 3.2 mm on the inner ring of the spiral wound gasket. The opening has no effect on the strength. Secondly, the four positioning holes are matched with the pipe flange. This reduces the adverse effect of the vibration on the spiral wound gasket during drilling, and the drilling also avoids drilling errors and leads to inaccurate assembly.
5 Program implementation
First, clean the flange sealing surface, and pre-assemble the winding gasket of the unwrapped plastic packaging tape with the pipe flange to ensure that the gasket is properly seated, and the stainless steel inner ring and tube of the spiral wound gasket The box flange is equipped with the above four uniformly distributed positioning holes. Then, carefully remove the plastic wrap from the metal-wound gasket and inspect the gasket surface for defects such as radial scratches. Finally, the spiral wound gasket is fixed to the pipe flange by the elastic cylindrical pin, and then the pipe box assembly and the pipe plate are assembled.
6 Tube box installation
After the tube box assembly is assembled with the tube sheet, the following key processes play a key role in sealing.
- (1) Bolt tightening sequence requirements. The threads of the bolts and nuts must be clean and not allowed to adhere to mechanical impurities. The force applied when tightening the nut is uniform. The tightening of the bolts should be performed symmetrically and symmetrical, and the bolts should be gradually and evenly divided into three rounds. In one round, the bolts should be tightened to 1/3 of the final tightening torque. The starting point of each wheel should be offset by 120° from each other to make the gasket evenly pressed.
- (2) Bolt tightening torque requirements. According to the tightening rules of threaded fasteners and the actual operating experience at home and abroad, the recommended torque of M64×3 bolts is 8 500 N·m. When tightening the bolt torque, the torque needs to be gradually increased to three rounds. The specified torque value cannot be tightened into place at one time. Relevant statistics show that about 80% of the domestic pre-tightening of the seal is operated according to experience, and only 22.3% of the sealing reliability can be guaranteed. Therefore, according to the tightening torque of the bolt and the selection of the appropriate torque wrench, it is an important factor to ensure the sealing. .
- (3) Select a reasonable amount of spiral wound gasket compression. At present, there are two main methods for controlling the pre-tightening force of the metal-wound gasket: one is to use a torque wrench; the other is to control the compression amount of the metal-wound gasket by measuring the gap of the flange sealing surface. For a 4.5 mm thick spiral wound gasket, the compression is about 1.2 mm. When pre-tightening the bolt, care should be taken to make it evenly and symmetrically to prevent tight deviation.
In combination with the above assembly requirements, the tightening order of the bolts to the tightening torque of the bolts and the compression amount of the spiral wound gaskets are respectively controlled (mainly tightening torque of the bolts, supplemented by the amount of compression), and the torque of the bolts From 9 500 N·m to 8 000 to 8 500 N·m; in the hydrostatic test, the pressure test passes once and no leakage occurs at the flange seal; these experiences are successfully replicated to another identical heat transfer The pressure test was also passed once.
7 After-sales maintenance
If there is a positioning hole drilled in the inner ring of the metal-wound gasket at the flange of the pipe box, if the equipment is to be repaired at the customer site or the customer overhauls and the metal-wound gasket needs to be replaced, the original elastic pin and the method can be adopted. The gasket is removed and the locating holes are drilled in the new spiral wound gasket, which has no effect on the installation of the customer site.
Proper and reliable installation of spiral wound gaskets is the key to good sealing. For non-basic spiral wound gaskets, usually with inner ring or with inner ring and centering ring type, the case of burring locating holes in spiral wound gaskets described in this paper is for spiral wound gaskets. The installation provides new ideas, especially large-diameter metal-wound gaskets. At the same time, the tightening sequence and tightening torque of the control bolts are important factors to ensure the sealing of the metal-wound gasket. It should be pointed out that for large-diameter metal-wound gaskets, the influence of internal pressure on the stress of the sealed metal-wound gasket may cause the bolt to stretch. Properly increasing the bolt tightening torque is an important factor for achieving long-term reliable sealing.
Source: China Spiral Wound Gaskets Manufacturer – Yaang Pipe Industry (www.steeljrv.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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