1. What is work stress? Find out what kind of pressure the non-metallic compensator carries first
"How many kilograms of pressure can your non-metallic compensator withstand?" — the question itself is a pit. Non-metallic compensators (also called non-metallic expansion joints, fabric fiber expansion joints) are completely different from metal compensators. Metal parts carry internal pressure and axial force, relying on metal wall thickness and corrugated structure; The non-metallic compensator, to put it bluntly, is a flexible connector, which mainly carries the thermal displacement, vibration and installation error of the pipeline system, and pressure is only a "sideline" that it copes with by the way.
What is work stress? For non-metallic compensators, it refers to the continuous static pressure exerted on the inner wall of the compensator by the internal media of the pipeline (flue gas, air, corrosive gas, etc.) under normal operating conditions. This pressure is usually not large, generally between several kPa and tens of kPa, which is the so-called "micro positive pressure" or "negative pressure" working condition. If you take the design idea of a few megapascals of a metal compensator, something will happen. The reason is simple: the belt (skin) of non-metallic compensator is made of soft materials such as rubber, fluororubber, polytetrafluoroethylene, glass fiber cloth, etc. Its strength is far inferior to that of metal, but it is superior in softness, corrosion resistance, high temperature resistance and absorption of multi-dimensional displacement.
Therefore, we have to correct our mentality first: the working pressure of non-metallic compensators is not the core indicator, but the bottom line that must be stuck. The pressure is high, and the cost soars; If you choose small, the circle will bulge, tear, or even burst directly. Then how exactly? Look down.
2. Several major factors that affect pressure resistance: materials, structure and temperature, none of them can be missed
The pressure bearing capacity of non-metallic compensator is not determined by the manufacturer, but by three variables:Material, Structure, Temperature。
Let's talk about the materials first.The band of a non-metallic compensator is typically composed of multiple layers of composite material. For example, our common rubber compensator, the inner layer is made of corrosion-resistant fluororubber or EPDM rubber, and the outer layer is coated with high-strength glass fiber cloth and silicone rubber. Each material has its own pressure limit. Fluorine rubber has good temperature resistance but average tear strength, while silicone rubber is soft but easy to deform when the pressure is high. Polytetrafluoroethylene compensator is invincible in corrosion resistance, but it is hard, and its pressure-bearing capacity is limited by the interlayer bond strength. Therefore, when selecting the type, we should keep an eye on the number of layers and the thickness of each layer of composite material, not only the total thickness.
Let's talk about structure.The structural forces of rectangular non-metallic expansion joints and circular expansion joints are completely different for the same non-metallic compensator. Stress concentration exists in four corners of rectangular structure, and rectangular shape is more likely to bulge than circular shape under the same pressure. There are also guided tubes with and without guided tubes-the guided tubes can reduce the direct scouring of the air flow to the ring belt and indirectly raise the upper limit of the actual pressure. In addition, the flange connection mode (internal pressure type or external pressure type) also affects the pressure resistance of the seal.
Finally, there's the temperature.This is often overlooked, and it is also a high accident area. The mechanical properties of non-metallic materials decrease sharply with the increase of temperature. For example, silicone rubber can still maintain a certain strength at 200℃, but when it reaches above 250℃, its strength will be halved. Many working pressure data given by national standards are based on the reference value of normal temperature (20℃). Under actual working conditions, for every 100℃ increase in temperature, the allowable pressure may be discounted by 40%. Don't just look at the "rated pressure" on the nameplate, be sure to check the temperature correction coefficient in combination with the medium temperature.
3. Turning over the national standard: What does JB/T 12235-2015 say about work pressure?
The domestic standard for non-metallic expansion joints is JB/T 12235-2015, the full name of which is Technical Specifications for Non-metallic Expansion Joints. There is a clear description of work pressure in this standard, but many people only look at the parameter table and not the text.
Design pressure, test pressure, maximum working pressure。 The design pressure is the maximum value given when the pipeline system is designed; The test pressure is usually 1.5 times the design pressure (hold the pressure for 15 minutes without air leakage); The maximum working pressure is a continuous pressure value that cannot be exceeded during normal operation. Note that instantaneous stress is not listed separately here, but I will talk about it in the next section.
JB/T 12235-2015 also requires that the ring belt of non-metallic compensator must undergo air pressure strength test and tightness test before leaving the factory. There shall be no leakage, bulging or abnormal deformation of the ring belt under the test pressure. In addition, the recommended pressure range for common working conditions (flue gas, hot air, dusty gas) is given in the standard appendix, but it is only recommended, and the details must be determined according to the actual material combination.
It did not give a dead count to the work pressure, but emphasized that "it should be agreed by both supply and demand sides". Why? Because the non-standard properties of non-metallic compensators are too strong. The same rectangular non-metallic expansion joint, with two layers of silicone cloth and three layers, can be twice the pressure bearing capacity. So don't take the standard as the Bible. The standard is just the bottom line. What really determines the pressure is your working condition parameters and the design ability of the manufacturer.
4. The two easiest pits to step on during model selection: regard low pressure as high pressure and instantaneous pressure as long-term pressure
After doing this for a long time, I found that 90% of the selection mistakes lie in these two pits.The first pit: count low pressure as high pressure.When many customers look at the flue gas pipeline system, they feel that the pressure is only 3-5kPa, so they choose the cheapest model. As a result, the temperature difference is large in winter, the thermal expansion and contraction of the pipeline produce additional displacement, the ring belt is elongated, and the actual pressure bears increases instantly, and it cracks after two years. Under low pressure conditions, the displacement compensation amount becomes the main contradiction, but the lower pressure limit should not be too low, otherwise the ring belt will be too thin and too soft, and the resistance to wind pressure will be insufficient, so the wind will bulge up. Generally, it is recommended that when selecting the type of flue gas pipeline, the working pressure should be designed at least above 5kPa, and the margin should be allowed.
The second pit: treat instantaneous stress as long-term stress.Some systems have valves that suddenly close, which can produce a water or air hammer effect, and the pressure spikes to 2-3 times the design value in an instant. For example, the non-metallic compensator connected behind the desulfurization flue gas baffle door will suddenly change the pressure in the pipeline when the baffle is switched. This instantaneous pressure, the non-metallic ring belt can withstand once or twice, but it can't withstand repeated impacts. Over the long term, the interlayer peels off, the loop tape blisters, and eventually leaks. The correct approach is to multiply the instantaneous peak pressure by a safety factor (such as 1.25) when selecting the model, and then confirm it with the manufacturer as a calibration condition. Don't think that "it just happens occasionally" and ignore it. The most fearful thing for non-metallic materials is fatigue.
On another note, rubber compensators and rubber PTFE compensators are slightly better resistant to this impact than pure fiber fabric bands, but also limited. The best way is to install pressure relief valves or buffer devices, and don't let the compensator carry hard.
5. Actual case: How to determine the working pressure in the flue gas pipeline? Give a practical advice
Not long ago, a power plant customer consulted. Their flue gas pipeline has a diameter of 1.5 meters, a medium temperature of 180℃ and a design pressure of 8kPa. However, the actual steady-state pressure at the fan outlet is only 4-5kPa, and occasionally there will be a peak of 12kPa when the gate is closed. The customer looked at the rectangular non-metallic expansion joint products on our official website and asked if it could be used.
- The working pressure is firstly determined according to the steady state of 4-5kPa, but the number of belt layers is one more than the conventional one (6 layers become 7 layers), because the strength of silicone rubber and glass fiber cloth will decrease by about 15% at the temperature of 180 ℃.
- The instantaneous pressure of 12kPa is used as the calibration condition, and the ring belt is required to undergo a withstand pressure test of 1.5 times 12kPa before leaving the factory (i.e., 18kPa holding pressure) to ensure that the ring belt has no leakage.
- Structurally, the stainless steel guide tube is added to avoid the smoke gas directly washing the inner wall of the ring belt, and also can reduce the pressure fluctuation caused by turbulence.
- The flange sealing surface adopts the combination of labyrinth + rubber sealing strip to prevent trace leakage under low pressure.
Finally, the customer customized the non-metallic expansion joint according to this scheme, and it took more than two years without any problems. The core is:Don't just look at design pressure, look at the combination of actual steady state + temperature + instantaneous peak。 The working pressure in a flue gas pipe is never an isolated number, it is linked to temperature, wind speed, dust concentration, and the rigidity of the pipe. Next time I ask "the working pressure of non-metallic compensators", remember to bring these dimensions together.
If the pressure does exceed 50kPa, it is recommended to go directly to the metal compensator and pin it on the hard roll of the inappropriate track.