What exactly is 704 silicone used in non-metallic expansion joints?
Sealing and bonding. 704 silica gel is a single-component room temperature vulcanized silicone rubber, which can be cured by contacting air moisture after extrusion, and has good elasticity and good adhesive strength. On non-metallic expansion joints, it is usually used to fill gaps between layers of fabric fibers, or to adhere the flange surface of the expansion joint to the pipe flange to prevent smoke from seeping out of bolt holes or between layers.
But here's the problem-in many working conditions, 704 silicone is not standard at all. You flipped through the national standard of JB/T 12235-2015 "Non-metallic Expansion Joints", and there was no mention of silicone seal in the whole article. Then why does anyone else have to use it? Because it is cheap, easy to buy and simple to construct. But can this thing really withstand the harsh environment of power stations and cement industries? We have to figure out the three hard indicators of temperature, medium and pressure.
Icing on the cake or superfluous? Real Cases of Power Stations and Cement Industries
Look at the power station first. The medium temperature of flue gas pipelines of coal-fired units is usually 250℃ ~400℃, and some denitrification inlets can even soar to 420℃. Non-metallic expansion joints (fabric fiber expansion joints) can meet the sealing requirements by themselves by relying on a multi-layer composite structure-the outermost temperature-resistant coating, the middle insulating cotton, and the inner anti-corrosion film. At this time, you brush a layer of 704 silicone gel, which is equivalent to double insurance for the seal, but the premise is that the temperature does not exceed the limit. A power plant in Shandong used a rectangular non-metallic expansion joint with 704 silica gel at the outlet of induced draft fan. After half a year of operation, all the silica gel was carbonized and cracked, and the air leakage rate increased by 8%. Check the record: The exhaust temperature is 320℃ all the year round, and the upper temperature resistance limit of 704 silica gel is only 250℃ (it may be higher in a short time). How can this be held?
Look at the cement industry again. The temperature of the exhaust gas pipeline at the end of the kiln is not high, often hovering at 150℃ ~200℃, but the medium contains a large amount of SO₂ and alkaline dust. 704 silica gel has weak acid resistance. When it encounters sulfur-containing flue gas, it begins to become sticky and bubbly in two weeks. A cement factory in Zhejiang coated 704 on the flange surface of the circular baffle door exit. As a result, the seal failed three months later, and the on-site feedback was that "the glue layer is like plasticine". What's the icing on the cake? It was clearly superfluous.
What if the temperature resistance limit does not match? What sealing scheme to change?
The typical operating temperature of 704 silica gel is-60℃ ~ +250℃, and the short-term peak may reach 280℃. However, non-metallic expansion joints are often 300℃ + on flue gas pipelines, especially in places such as power station boilers and steel hot blast stoves. 400℃ is a common occurrence. What if it doesn't match? Hard use is gambling with longevity.
First, replace it with high-temperature resistant silicone rubber sealing strip, and the working temperature can reach 350℃, but the cost is doubled.
Second, directly use ceramic fiber rope or graphite packing as flange seal, rely on mechanical compaction, temperature resistance 600℃ +, but the elasticity is poor, so the bolts need to be compressed regularly.
Third, for ultra-high temperature working conditions (above 500℃), metal corrugated expansion joints should be used instead of non-metal expansion joints, such as this station'sHigh temperature axial expansion jointOrCorrugated expansion joint for power station industryCompletely avoid organic materials such as silica gel.
When selecting a model, don't just look at the temperature, but also the medium. Don't touch 704 silicone for sulfur-containing flue gas. It is more reliable to replace it with fluororubber or polytetrafluoroethylene seal-this station'sPTFE compensatorAndRubber PTFE compensatorThat's what it does.
Installation link: When must it be painted? When should you never apply it?
A client fell on this two days ago. He's installingRectangular non-metallic expansion jointWhen, 704 silicone was coated all over the flange surface, and then the bolts were tightened firmly. As a result, the silicone was squeezed everywhere, which blocked the displacement gap of the expansion joint. After two weeks of running, the expansion joint was torn because it could not stretch and contract freely, and the fabric layer was torn. You say it was wrong or not?
When must it be applied? There are only two scenarios:
① The pipe flange surface is uneven, with local depressions or scratches, and silicone is used to fill and level.
② The medium has a slight positive pressure (≤0.1MPa), and the temperature is lower than 200℃. Coating silica gel can prevent leakage by micro-seaming.
When should you never apply it?
① When the temperature is> 250℃, the coating is equal to white coating.
② When the expansion joint needs to absorb large displacement (axial> 50mm or transverse> 30mm), the silica gel will limit the sliding between layers after curing, which will affect the compensation ability.
③ The medium contains acid and alkali or organic solvent, and 704 silica gel has poor chemical resistance and fails in minutes.
PTFE gasket or graphite wound gasket is used for flange sealing, and the expansion joint body is not additionally coated. If the customer insists on painting, confirm the displacement and mark "bolt preload not more than 80% of the design value" in the installation instructions.
The national standard doesn't mention silicone. Is the industry's tacit understanding reliable?
JB/T 12235-2015 does not have a clause for "silicone seal". The requirement of sealing in the standard is "no leakage in airtight test". As for what means to achieve it, the manufacturer decides itself. So all kinds of "local methods" popped up on the scene: some people painted 704, some people painted butter, and some people wrapped raw tape-guess what? The raw material belt ages quickly above 150℃, and the butter flows when it encounters high temperature. These local methods are effective in the short term, but they are all hidden dangers in the long term.
Select the sealing mode according to the working condition. Temperature Non-metallic expansion joint (fabric fiber expansion joint)The product has been equipped with temperature-resistant and corrosion-resistant lining as standard when it leaves the factory, so no additional glue is required for normal installation. If you have to seal it, it is recommended to specify the temperature resistance grade (for example, label "300℃ silica gel") instead of shouting 704 in general.
Selection suggestion: How to match 704 silicone specified by customers?
If the customer specifically indicates "704 silicone should be matched on the rectangular non-metallic expansion joint or round baffle door" when placing an order, don't rush to agree. Three questions first:
① What is the sulfur content of the medium? 704 Silica gel is afraid of acid and alkali, and the wet flue gas (pH ② Is the working temperature stable below 250℃? Notice the peak volatility.
③ What are the displacement parameters of the expansion joint? If the amount of compensation is large, it is recommended to use it insteadDouble-sealed single-axis circular baffle doorOrElectric plug-in insulation doorThey use mechanical seals instead of gluing, which is more reliable.
In actual matching, a layer of 704 silicone can be pre-coated on the flange surface as an installation aid, but not as the only sealing means. Attach a warning to the customer before delivery: when the medium temperature exceeds 200℃, check the silica gel status every quarter, and the carbonization crack must be cleaned in time and the sealing method must be replaced. As for this site'sFlue gas baffle doorAndDesulfurization flue gas baffle doorIt is recommended to directly match PTFE sealing rings to avoid subsequent tearing.
704 silicone is suitable for "playing auxiliary" of non-metallic expansion joints, so don't let it be the main force. When the three conditions of temperature, medium and displacement are satisfied, it is no harm to use it; Whenever one exceeds the standard, change the plan quickly. After all, the loss of on-site air leakage shutdown can be much more expensive than a tube of 704 glue.