Application Skills of Liquid Ammonia Bellows Valve Packing
Time: 2020-10-06 12:21:20 Source: Yonglong Valve Reading: zero second
Application Skills of Liquid Ammonia Bellows Valve Packing
The scope of fugitive leakage prevention and control includes not only the leakage of volatile harmful substances - to prevent environmental pollution, but also the leakage of steam and other low-temperature inert media - to prevent production efficiency from decreasing. It is the best choice to prevent leakage at the root. Specifically, it is necessary to carefully design the housing, reasonably select the packing material and structure, and correctly install it. Usually, the stuffing box can accommodate four to six packing rings, while the sealing elements of the valve stem are at least one to two, and most of them may have seven packing rings, as in the rising stem structure. Packing ring can be made by rope weaving or molding. The packing material made by the rope knitting method can be cut into segments according to the circumference length of the spindle/valve stem, and the notch is 45 º (miter) or 90 º (vertical).
Miter joint method is recommended because better end face bonding effect can be obtained under the action of axial load. It must be noted that each packing ring must completely surround the entire circumference of the spindle/valve stem; The best condition is that the tightness between the outer side of the packing ring and the inner wall of the stuffing box is higher than the tightness between the inner side of the packing ring and the valve rod. If the packing rope head at the cut is too long, leading to stacking, or too short, leading to vacancy, it will lead to uneven packing ring density and become a potential leakage path.
Compression and load
When installing packing for liquid ammonia bellows valve, each packing ring must be installed in place in turn until the complete packing set is completed. This can ensure uniform distribution of axial stress, reduce friction and leakage rate, reduce or even avoid the need to replace packing or retighten gland in the future, thus extending the service life of packing set.
On the contrary, if a set of five packing rings is installed at one time, the top packing ring closest to the follower of the gland is subject to the maximum load, but the bottom packing ring closest to the process medium is subject to a small load, even negligible. The reason is that the packing ring will reduce the axial load, and the effect of the bottom packing ring to prevent medium leakage will become worse, whether it is cut and wound or molded packing. Another situation is that the load applied to the top of the packing ring is too large, resulting in excessive friction when the valve rod rotates, or even damaging the packing group. It can be seen from some data that if the installation operation is correct and standardized, the degree of stress relaxation of the packing group after 100 mechanical cycles shall not exceed 20% at most. However, if the installation is not correct, the stress will even be reduced by about 60%, so that the sealing performance is no longer reliable.
Temperature limit
Only within a specific temperature range can packing and sealing materials have the best performance. For example, the temperature range of polytetrafluoroethylene (PTFE) is about - 101 ℃ and 232 ℃. If it is applied to a higher temperature environment, its molecular structure may change, leading to the leakage of harmful gases, which may lead to corrosion of the valve stem, and even lead to the escape of toxic PTFE smoke from the top of the gland follower.
Such fumes may even be carcinogenic. In addition, with the release of flue gas, the packing material will be softened and the weight will be reduced, leading to the leakage of harmful process media or products. If flexible graphite is used, the applicable temperature range is naturally different. In the oxidizing atmosphere, it is generally between minus 196 ℃ and 450 ℃.
Considering the material characteristics, such as the content of ash, chloride, sulfur, fluoride and other halogen compounds, as well as the oxidation quality loss information provided by the manufacturer, it can be judged that the graphite packing can reliably prevent leakage in the environment of 450 ℃ at most, and can withstand 650 ℃ at most in the steam condition where no oxidation reaction occurs. Flexible graphite and polytetrafluoroethylene (PTFE) are widely used in various media with PH value of 0-14; Graphite material is also suitable for fire safety sealing in some cases.
Hazard and risk assessment
Before installing packing, a comprehensive hazard and risk assessment should be carried out. First, properly isolate the valve to ensure safe construction. Operators shall wear protective clothing and goggles as required. Use the packing cleaning tool to clean all the old packing in the stuffing box.
Carefully check the valve rod for pitting and scratches with an inspection mirror, and ensure that the old packing has been completely removed. Refurbish if necessary. Then measure the outer diameter of the valve rod, the inner diameter of the stuffing box and the depth of the stuffing box cavity to accurately calculate the packing size.
The calculation formula of packing ring radius is ((OD-ID)/2), and then calculate how many layers of packing ring thickness can match the stuffing box cavity depth. The packing material can be vertically cut (radially) around the mandrel with the same diameter as the valve stem; The special cutting tool for packing can also be used for oblique cutting. Care should be taken to avoid stretching packing materials along the length direction, and also to avoid cross section deformation caused by extrusion. It should also be noted that the cutting surface must be neat and tidy, and there should be no impurities or debris on the cutting surface.
Separate packing ring
Never coil the packing material to fill the stuffing box, but use a special cutting tool to cut the packing braided material and make an independent packing ring with a cut of 45 º (miter), so that the load can be used to obtain a better end face bonding effect.
Check whether the size of each packing ring just matches the stuffing box; The interface of each ring shall be properly butted and closely fitted with the inner wall of the stuffing box to the maximum extent. It is not allowed to stretch the braided material to match the inner diameter. All packing rings installed later must have the same size and cutting method as the first one.
Prefabricated packing rings are very common and do not need to be cut on site, so they are easy to install. First, check the valve gland bolts, nuts and gaskets, and replace them if damaged. Bolts and nuts shall be coated with grease. In order to effectively control fugitive leakage, a torque wrench must be used and the operating instructions provided by the packing supplier must be followed.
Rotate the valve rod
The stuffing box shall also be clean and free from impurities and grease. Put the first packing ring into the stuffing box, pay attention to the position of the interface, and first insert the interface part into the stuffing box, then insert the rest of the packing ring. If it is a flexible graphite molded packing ring, each ring should be stretched into a spiral shape before being installed in place to avoid damage to the packing ring. Install one packing ring at a time, and ensure that it has been compressed and seated firmly in the stuffing box before installing the next one. If the length of the gland follower is not enough to reach the bottom of the stuffing box, temporary tools or an extension ring are required for installing the packing ring.
Unless otherwise specified, sealing elements shall not be lubricated. Apply load to the first packing ring installed according to the operating specifications provided by the manufacturer. Repeat the same steps for the next packing ring, and install only one ring at a time. The interfaces of any two adjacent packing rings must be at 90 º and 180 º respectively. After installation, the valve rod must be operated to make the valve cycle three to ten times in the open and close positions, so as to ensure that the packing set is properly seated. During this process, the packing set will have a certain degree of relaxation, so after the switch operation is completed, it should be retightened according to the parameters provided by the manufacturer, so that there is sufficient margin to offset the future relaxation. Similarly, after the valve is put into use under normal conditions, the packing set shall be tightened for the last time to reserve enough margin for future stress relaxation.
Avoid leakage
If the packing is correctly installed, the attenuation when the axial stress is converted to the radial stress will be reduced. This helps the packing maintain reliable sealing performance for a longer period of time, and reasonable friction load distribution. On this basis, the leakage sealing performance of the valve and even the production efficiency will be significantly improved.
In the 1970s, if the valve packing leaked several drops of liquid every minute, it was acceptable. In the current era, the government's legislative orientation is to eliminate leakage, and has introduced relevant regulatory regulations and procedures to find, reduce and control valve leakage. In addition, both manufacturers and end users require valves to successfully pass the tests of various standards, such as ISO15848-1, API 624 and API 641.
Correct packing installation procedure is one of the factors that determine whether the sealing performance is qualified or not. The sealing performance is an important sign to judge whether the value of valve suppliers exceeds that of peers.
Copyright and Disclaimer
1、 This article is reproduced for the purpose of conveying information, which does not mean that we agree with the article's views and are responsible for its authenticity. The content is for reference only.
2、 The copyright of the article on this site belongs to the original author and the original source. If you have any questions about the content, copyright and other issues, please contact us, and we will delete the content at the first time!
Shanghai cut-off valve manufacturer: emergency cut-off valve - electric cut-off valve - liquid ammonia cut-off valve - ammonia cut-off valve - pneumatic cut-off valve - pneumatic ball valve - pneumatic emergency cut-off valve - Shanghai Yonglong Valve Factory
[Recommended reading] / TECHNICAL ARTICLES
