Two days ago, a friend who made thermal pipes asked me to spit, saying that the flue expansion joint in their factory leaked after less than a year. When I took it apart, the bottom of the bellows was rusty. When I asked, I found out that I didn't consider drainage at all when I installed it. Today, let's talk about this easily overlooked but particularly critical accessory — —Flue expansion joint drainage joint。 How to choose, how to install, and how to avoid pits, I'll tell you thoroughly at once.
1. Why must the flue expansion joint be equipped with drainage joint? — Find out what this thing is for first
The smoke in the flue looks dry, but as soon as the temperature drops, the water vapor becomes condensed water. Especially when the machine is started or shut down in winter, the temperature changes greatly and the condensed water flows down. If you think about it, this water is accumulated at the bottom of the expansion joint, and if you useUniversal corrugated expansion jointOrMetal rectangular expansion joint, soaked in water for a long time, the stress concentration point inside the bellows begins to corrode first. IfNon-metallic expansion joint (fabric fiber expansion joint)That was even worse — the fabric layer was softened by water, and its strength collapsed directly.
So, the function of the drain joint is to drain away the condensed water accumulated at the low point. Not pretending? That's like planting a ticking time bomb in the expansion joint.
Can I install a plug? Tsk, the plug can only be temporarily sealed, not draining. A drain outlet with ball valve or inner wire connection must be installed, drained manually regularly, or connected with a pipe to the floor drain.
Second, how to choose the drainage joint? — — Choose according to the type of expansion joint used in your home
There are many materials for drainage joints on the market, but don't buy them with your eyes closed. Find out what material the expansion joint body you are using is made of first.
- Metal corrugated expansion joint(e.g.Corrugated expansion joint for power station industry、Metal Corrugated Expansion Joints in Cement Industry、High temperature axial expansion joint): These casings are metal, and the drain joint is best made of 304 stainless steel or 316L, which is corrosion resistant and can be welded together with the main body. The size is generally DN15 and DN20, which are selected according to the discharge amount.
- Non-metallic expansion joint（Non-metallic expansion joint (fabric fiber expansion joint)、rubber compensator、Rubber PTFE compensator): This type is generally lined with plastic joints (PPR or PVC-U) or stainless steel with PTFE. Because the non-metallic body cannot be welded directly, it is usually necessary to flange or clamp the joint.
- Large diameter thick wall expansion jointOrSleeve type pipe expansion joint: These usually have reserved sewage outlets at the bottom, just buy the corresponding wire plug or ball valve directly. However, note that the reserved mouth thread may be imperial or metric, so measure it first.
There's another key point —Temperature resistance of drainage joints。 Plastic joints melt in minutes where flue temperatures are high, such as near boiler outlets. In this case, metal joints must be used, and when welding, it is necessary to avoid excessive damage to the expansion joint body in the heat affected zone.
TipsIf you're not sure, buy it directlyPTFE-lined hoseThe same material is used as a joint, which is corrosion-resistant and temperature-resistant, so it's right.
3. What should I prepare before installation? — — Don't get started in a hurry. Let's see if these things are all ready.
- Movable wrench (one size)
- Raw tape (preferably thickened)
- Anaerobic adhesive or high temperature resistant sealant (temperature resistance above 250℃)
- Pistol drill + hole opener (if the expansion joint you bought doesn't have a reserved drain)
- M Tap and tapper (if self tapping thread)
- Welding machine + stainless steel welding electrode (required for welded joints)
- Drain connector body (ball valve, inner wire direct, outer wire plug)
- A section of hose or steel pipe (leading to the gutter)
- Pipe clamp or support frame (to fix drain pipe)
Verify that the expansion joint has been properly installed in place, that the pipe has been secured, and that the tie rod nut (if any) of the expansion joint has been adjusted in place as per the instructions. Don't wait until the joint is installed to find out that the expansion joint is in the wrong position-then you have to remove it and start over.
4. Practical steps: from opening to fixing, teach you how to install it step by step
All right, the tools and materials are all ready, let's get to work.
Step 1: Determine the location of the outlet
The drain port must be opened at the lowest point of the expansion joint. You take a level ruler or visually inspect to find which part of the bottom of the expansion joint is the most concave. Generally, there will be a slightly raised sewage trough at the bottom of the designed expansion joint, which is where the joint is installed. LikeDirect buried (fully buried) type expansion jointIt may be buried in a ditch, and the operating space should be reserved in advance.
Step 2: Open holes (if there is no reserved opening)
With an electric drill and a hole opener, it is safest to punch holes from the inside to the outside. Why? Because there is a guide tube inside the bellows (refer to FAQ Article 7), the guide tube itself is very thin, and drilling from the outside to the inside is easy to penetrate the corrugations. The opening diameter is 1-2mm smaller than the outer diameter of the joint, and the subsequent tapping or welding is performed.
attention: After opening the hole, the burrs should be cleaned up with a file, and iron chips should not be left inside the expansion joint, otherwise the bellows will be worn out during operation.
Step 3: Install the connector
- welding type: Align the stainless steel joint with the hole position, fix it by spot welding first, and then fully weld it symmetrically. After welding, check whether there are pores, and repair welding if there are. Apply anti-rust paint after the weld is cooled (if used in a non-corrosive environment).
- Threaded: Tap out the internal thread with the tap, wrap the raw material tape (at least 5 turns), and screw into the outer wire joint. Be careful to use two wrenches, one to catch the joint and the other to tighten to prevent torque from transmitting to the expansion joint and causing deformation.
I suggest that welded type is more reliable, especially for high temperature and high pressure flues. But if it isrubber compensatorOrNon-metallic expansion jointIt can't be welded, so it can only be clamped by threads and flanges. The specific method is to open a slightly larger hole at the bottom of the expansion joint, use a joint with flanges, put a rubber gasket inside and outside, and clamp it with bolts.
Step 4: Connect the drain
Install a ball valve (normally closed state) on the outside of the joint, and connect a pipe behind the ball valve to lead to the safe discharge point. Note that the pipe should have a downward slope and no U-bend, otherwise the accumulated water will not be discharged. Fix it to the nearby pipe bracket with a pipe clamp. Don't let the drain press the expansion joint.
Key Reminders: If you are usingUniversal corrugated expansion jointAnd the pipeline has axial displacement, the drainage pipe must use a hose (such asMetal hoseOrPTFE-lined hose) to connect to avoid the rigid tube obstructing the normal displacement of the expansion joint.
5. How to try after installation? — — Water leakage, blockage and poor drainage, these pits should be prevented in advance
After finally installing, don't rush to seal and insulate, do two things first:
1. Pressure leak test
Turn off the ball valve, pump 0.2-0.3MPa compressed air into the expansion joint, or fill it with water (note that the water pressure does not exceed the design pressure of the expansion joint). Brush the joint welds and threads with soapy water to see if there is any bubbling. If there is a leak, repair weld or rewrap the raw tape. This step can't be saved, wait for the insulation packet to leak again and wait to cry.
2. Drainage test
Open the ball valve and pour a little water into the expansion joint to see if the water can flow out smoothly. If it is not smooth, it may be that the opening position is on the high side or the drain pipe is blocked. Dredge it, or adjust the pipe slope.
In addition, during usual inspection, attention should be paid to the ball valve of the drainage joint not to be open for a long time, otherwise the negative pressure of the flue will suck cold air and affect the flow of flue gas. It is recommended to open the ball valve regularly (such as once a month) to drain water, and close it immediately after draining.
There is another situation – freezing in winter. Friends in the north pay attention. If the ambient temperature is lower than 0 degrees, the water in the drain will freeze and expand, freezing and cracking the joint. What to do? Either electrically trace the drain pipe or install an automatic drain trap (such as thermodynamic) at the lowest point.
Although the drainage joint is small, it is related to the safety of the whole flue gas pipeline. Use more snacks when you install, and you won't have to bother for the next few years. If the picture saves trouble and doesn't install it, one day the expansion joint leaks, and the cost of replacing one is enough to buy dozens of joints. Is that the truth?