Nonmetallic Compensator Leak Rate: How Important Is This Metric Really?
Peers who do pipeline design all know that non-metallic compensators (that is, we often call fabric fiber expansion joints and rubber compensators) are widely used in flue gas, hot air and dust removal systems. But one parameter is often overlooked – leakage rate. To put it bluntly, the leakage rate is how much media the compensator leaks per unit time under the rated pressure. This figure looks inconspicuous, and if something goes wrong, it can range from soaring energy consumption and unstable system pressure to polluting the environment and even causing safety accidents.
For example, two days ago, a customer of a cement factory reported to us that the non-metallic expansion joint on their flue gas pipeline took less than half a year, and the surrounding area was full of dust. The inspectors felt choked when they wore masks. After checking, the leakage rate exceeded the standard by a full three times. How do you think this loss is calculated? Not only the waste of materials, but also the cost of environmental fines and shutdown maintenance. Therefore, the leakage rate requirement of non-metallic compensator is by no means an optional index, but a hard lever directly related to the safety and economy of the system.
Then how to determine this leakage rate? Are there any standards to follow? Let's break it up one by one.
Where do leak rate requirements come from? Standard of JB/T 12235-2015
The standard implemented by domestic non-metallic expansion joints is JB/T 12235-2015 "Non-metallic Compensator". This standard clearly stipulates the test method and qualified index of leakage rate. Simply put, for low-pressure systems with nominal pressure PN ≤0.1MPa, the leakage rate usually requires no more than 0.5% ~1% (depending on the caliber and structure); For the working condition of PN> 0.1MPa, the requirements will be stricter, generally controlled within 0.2% ~0.5%. Note that what is said here is a percentage, which refers to the proportion of air leakage to the design flow.
However, standards are standards, and there are often more demanding additional requirements in actual projects. For example, the non-metallic compensator behind the desulfurization flue gas baffle door, because the medium is highly corrosive and the temperature fluctuates greatly, the owner often requires the leakage rate to be less than 0.2%, or even directly designs it according to zero leakage. At this time, the rubber compensator or PTFE compensator we choose has to make a fuss about the material and sealing structure.
In addition, JB/T 12235-2015 also stipulates that the pressure of airtightness test is generally 1.5 times of the design pressure, the pressure holding time is not less than 5 minutes, and the pressure drop does not exceed the specified value before it is considered qualified. These details must be thoroughly understood by procurement and quality inspection personnel.
Several key factors affecting the leakage rate: material, structure, installation
Leakage rate is not something that can be controlled by just looking at standards. In practice, there are so many influencing factors that people have a headache. Let's pick three core ones:
Materials
The loop belt of the non-metallic compensator is generally made of multi-layer fabric fibers, rubber, fluoroplastics, etc. For example, the one in our stationRubber PTFE compensatorIt combines the flexibility of rubber with the corrosion resistance of PTFE, but if the interlayer bonding strength is not enough, it will be easy to delaminate under long-term high temperature, forming micro-leakage. AndNon-metallic expansion joint (fabric fiber expansion joint)The selection of silicone cloth or fluorine cloth directly affects the sealing performance after aging. If the material is selected wrong, the leakage rate will start high.
Structure
It is also a rectangular non-metallic expansion joint, some with multiple sealing layers, and some with only a single layer of ring belt. With double seal structure, the leakage rate can be controlled below 0.1%; Monolayer may reach 0.5% or even higher. In addition, the compression method of flange connection is also very critical-the bolt preloading force is uneven, the ring belt is not compressed firmly, and air leakage is inevitable.
install
Installation is a link that can easily roll over. The butting deviation of the pipeline, the flatness of the flange surface and the tightening torque of the bolt can each increase the leakage rate. There was a power plant project before, usingCorrugated expansion joint for power station industry(Metal) were all well installed. As a result, in the non-metallic compensator section, the on-site workers directly used ordinary gaskets to save trouble, but didn't screw them according to the required torque, and they leaked a mess during the pressure test. Therefore, the torque value in the mounting instructions is not a display.
Practical Case: Control Points of Leakage Rate under Different Working Conditions
Talk about something grounded. TakeDesulfurization flue gas baffle doorFor the non-metallic compensator behind, the flue gas contains sulfur, has high humidity, and the temperature fluctuates between 80 and 180℃. Under this working condition, the life of ordinary rubber compensator is less than one year, and the leakage rate rises linearly. The solution is to chooseRubber PTFE compensatorOrPTFE compensatorThe inner layer is added with tetrafluorine film to block corrosion, and the outer layer is reinforced with high-strength fiber. At the same time, the flange sealing surface should be processed into grooves, and with acid-resistant sealing strips, the leakage rate can be stabilized below 0.3%.
Look againMetal Corrugated Expansion Joints in Cement IndustryThe non-metallic compensator connected in parallel next to it is used in the high-temperature flue gas pipeline at the head and tail of the kiln. The temperature often rushes above 350℃. At this time, the band material has to be made of high-temperature resistant silicon-titanium composite cloth, and a heat insulation layer should be added inside. There was once a project in which ordinary fabric fiber expansion joints were selected. As a result, the fibers were carbonized at high temperature, and the leakage rate soared from 0.5% to 3%. Later, all of them were replaced with models with ceramic fiber insulation to solve the problem. You see, how important it is to match the working conditions when selecting the model.
AndDouble hinge expansion joint for air-cooled island vacuum pipelineThe non-metallic compensator used in the system requires high vacuum degree, and the leakage rate must be controlled within 0.1%. At this time, it is not enough to rely on the ring belt alone. It is necessary to design a double seal at the interface, and add sealing grease in the middle. We have encountered customer feedback that the sealing grease dried up and caused leakage. Later, we added regular grease injection ports, and the problem was solved.
How to verify that the leakage rate is up to standard? Testing Procedure and Precautions
It's useless to talk on paper, you have to measure it in practice. Leakage rate detection is generally divided into two steps:
The first step is the airtightness test.Install the compensator on the test bench, close both ends, and fill it with compressed air to 1.5 times the design pressure (be careful not to exceed the upper pressure limit of the ring belt material). Apply all welds and flange joints with soapy water and observe for air bubbles. This step can quickly identify obvious leaks.
The second step, quantitative leakage rate measurement.At steady pressure, the leakage is calculated by flowmeter or pressure drop method. Specific operation: Hold the pressure for 5 minutes to record the initial pressure P1, then hold the pressure for 10 minutes to record P2, and calculate the leakage rate according to the pressure difference and system volume. JB/T 12235-2015 stipulates that the pressure drop shall not exceed the specified value during the pressure holding period. Note that nitrogen or dry air is best used as the test medium to avoid the influence of moisture on the ring belt.
The ambient temperature of the test has a great influence on the results. The leakage rate in winter and summer can be more than 30% different because the elastic modulus of rubber and fluoroplastics varies with temperature. Therefore, the standard requires that the test temperature should be within the range of 20±5℃. If the conditions are not met at the site, at least the actual temperature should be recorded, and the correction should be made later.
The leakage rate requirement of non-metallic compensator is not to choose a broad "qualified" one, but to reverse the required leakage level according to the specific working conditions, medium, pressure and temperature, and then guarantee it from three dimensions: material, structure and detection. If you choose the right one, the system will run steadily for ten years without trouble; If you choose the wrong choice, you will make up the leak every three days, and spend money to delay production. This matter really can't be vague.