Analysis of Leakages of Steam Flange Sealing

Therefore, the leakage of sealing of the steam flange is the research object, and the concept of flange seal, the form of leakage and the analysis of the reasons for the leakage of steam flange seal are expounded. According to the analysis results, a solution to steam flange leakage is proposed combined with on-site and actual production.
 
When the steam pipeline is cold and the steam is introduced to generate a liquid hammer.
Combined with the characteristics of the company's steam pressure fluctuations, a further detailed analysis of the leakage of the steam flange sealing is made. The reasons are as follows:
(1) During the non-heating period, the average pressure of the company's saturated steam pipeline is 0.9MPa. When the steam pipeline introduces steam in a cold state, because the temperature of the pipeline is lower than that of the steam, part of the energy of the steam is consumed when the pipeline and other auxiliary facilities are heated in the heating process, causing the steam to release heat and change from saturated steam to unsaturated steam. The temperature and pressure of the steam medium are reduced while condensed water is precipitated. Water is carried in the steam due to the precipitation of condensed water, and the flow rate is fast; a large amount of flow rate in the steam pipeline results in the liquid hammer. At this time, the impact force of the liquid hammer on the partial pipeline is relatively great, especially at the flange seal, and a great tensile force is generated at the fastening bolt of the flange. At this time, the bolts are stretched and deformed, causing the tightening force of the bolts to the flange gasket to be lower than the required compression force for sealing, resulting in instantaneous failure of the flange seal.
 
(2) During the heating period, the average pressure of the company's saturated steam pipeline is lower than 0.4MPa; the water content of the steam is relatively high, and the steam flow rate and pressure are relatively low. When the steam pipeline introduces steam in a cold state, the liquid impact force in the pipeline cannot reach the deformation of the stretched bolt, but reduces its fastening force. After the instantaneous impact force is released, the bolt’s rebound force after stretching under the action, and the compression amount of the sealing gasket increases instantly. Under the alternating load of frequent hydraulic impact stretching and rebound compression, the bolts and metal sealing gaskets undergo plastic deformation and the rebound ability reduces, resulting in the relative separation of the sealing surface of the flange and the sealing gasket surface and leakages.
 
Stable delivery of steam by steam pipelines
Leakages caused by flange sealing can also occur during the normal operation of steam pipes. This is because during the heating stage of the steam pipeline, the gaskets and flanges are heated and expanded, and the compression force between them is increased; the flanges and bolts are also heated and expanded, and the thermal expansion of the flanges is greater than that of the bolts. The extra temperature load intensifies the tightening force of the bolt, which increases the compression force at the gasket. In the stable operation stage of the steam pipeline, since the pipeline and auxiliary facilities are always in a high-temperature, saturated steam environment, the elastic modulus and yield limit of the metal will be greatly reduced, and the gasket will creep and the resilience will decrease, failing. Because the flange is much thicker than the gasket and much shorter than the bolt, elastic deformation mainly occurs and there is less plastic deformation. When the number of steam increases or decreases, causing the steam pipe to follow its fluctuations, leakage will occur where the flange gasket has failed.
 
Decreased temperature of the steam pipe
there will inevitably be partial blind ends for some steam pipelines, dead ends and difficult drainage due to the influence of equipment, process and other factors during laying, where steam with poor fluidity accumulates for a long time, and its heat is gradually transferred to the external environment through the pipelines. Although Some pipelines are covered with a thick insulation layer, as long as there is a temperature difference, it cannot guarantee that the heat will not be lost; the heat will dissipate at a slower rate, resulting in the gradual cooling of the steam there to form condensed water. In this case, the temperature of the pipeline will drop from saturated steam to condensate temperature below 100°C. In the heating stage, because the flange is in contact with steam, and the bolts are subjected to the heat conducted through the flange; the temperature difference between the two makes their thermal expansion inconsistent, so they are mutually restrained and cannot expand freely; additional temperature will inevitably be generated. Because they are under high temperature and high pressure for a long time, the elastic modulus and yield limit of metals is greatly reduced, resulting in plastic deformation and decreased resilience. When the temperature drops, the bolts often do not rebound with the same shrinkage of the flange, resulting in a decrease in the tightening force, and a gap appears on the flange sealing surface, failure of flange sealing and leakages.
 
As a result, the leakage of the flange of the steam pipeline is mainly due to the plastic deformation of the gasket and the reduction of the rebound ability under high temperature and alternating load; the great tensile deformation of the bolt occurs and the fastening force decreases, which cannot compensate for the deformation of the gasket when the amount of clearance generated or the compressive force on the gasket is less than the required compressive force for sealing, and the seal will fail.
 
The solution to the leakage of the steam flange sealing
Increasing the sealing contact surface between the flange and the gasket
Apply high-temperature sealing grease for steam turbine sealing on the seal, and the sealing grease will fill all the tiny gaps and defect surfaces between the flange sealing surface and the gasket; its contact area is much larger than the area between the gasket surface and the flange sealing surface. It will start to solidify as the temperature gradually rises, and will maintain a certain degree of elasticity for a long time.
 
Strengthening the control of the stable state of the bolt-tightening force
(1) Add a piece of disc spring to each bolt. The selection of the disc spring is determined according to the working temperature and pressure of the flange and the material, specification and quantity of the bolt.
(2) When installing, use a manual wrench to tighten the bolts, and ensure that the disc springs are tightened until they are completely flattened.
(3) In order to achieve a better sealing effect, tighten the bolts evenly.
 
During the normal and stable operation of the equipment, under the action of high temperature and alternating load, the gasket is deformed, but the high-temperature sealing grease cured on its surface still maintains good elasticity; the bolts extend due to plastic deformation. The pressure on the disc spring is reduced, and elastic recovery occurs, so that the bolt remains in the state of the original compression force, thereby eliminating the leakage caused by the failure of the flange seal.
 
Application examples of solutions to leakages of steam flange sealing
(1) Solution effect of leakages of flange seal of No. 2 blast furnace blast steam humidification valves
In December 2011, after the overhaul of the blast steam humidification pipeline, the first manual gate valve was installed during the introduction of steam.
 
Leakage occurs at the seal of the outer flange, and the reasons for the leakage of the flange seal are as follows:
(1) After the overhaul is completed, steam is introduced when the steam pipeline is in a cold state, and a large amount of condensed water is precipitated and not removed in time, resulting in frequent liquid hammering and great alternating loads at the flange seal, and the flange gasket undergoes plastic deformation and has low resilience.
(2) Since the distance from valve A to pipeline A of the company is only 21.5m, alternating loads are generated at the outer port of valve A. In addition, at the beginning of the introduction of steam by the humidification steam pipeline, the flow and pressure adjustments of the humidification system are frequent, and alternating loads are also generated at the inner port of the valve A flange.
 
In October 2011, the part was carefully dealt with through maintenance. The treatment is as follows: when reassembling, apply high-temperature sealing grease between the sealing gasket and the sealing surface. Add a pre-tightened disc spring to each bolt. 
 
Conclusion
It can be seen that the bolts and gaskets of the steam pipeline are affected by many factors such as static load, impact load and alternating load based on the above analysis and actual field application. No leaks cannot be guaranteed by the traditional tightening methods under the effect of comprehensive factors. The method of coating the sealing surface with high-temperature sealing grease and bolts with disc springs is simple, and the construction quality can be quantitatively controlled (by visually observing the disc springs to be tightened and flattened), which effectively solves the flange leakage of the steam system. At the same time, it is necessary to manage the equipment scientifically and reasonably and implement management regularly and effectively to ensure the long-term stable operation of the equipment.