Why do you have to use metal expansion joints for fire pipes? — Talk about thermal expansion and cold contraction
Fire pipes usually look quiet, but when there is a fire, the temperature soars instantly as soon as the water runs. Every 100℃ increase in the temperature of the metal pipe itself will expand by about 1.2mm per meter of length-it doesn't sound much, but if the pipe is 100m long, the thermal elongation will directly rush to 120mm. Without metal expansion joints to absorb this displacement, pipe joints, valves, and pump flanges would all suffer. From water leakage to pipeline cracking.
Some people say, can you use a rubber compensator? We have to admit that the rubber compensator is indeed cheap, but the fire pipe is filled with high-pressure water (usually 1.6MPa or even higher), the temperature changes greatly, and the rubber ages quickly. Especially in the case of oil and chemical solvents, the rubber expansion joint will be brittle in two years. Fire protection systems require high reliability, so metal expansion joints (also called corrugated expansion joints) become standard. It uses thin-walled stainless steel bellows to absorb displacement, pressure-resistant, temperature-resistant and fatigue-resistant, and its life can reach more than 15 years.
In addition, metal expansion joints have a hidden skill: vibration reduction. The impact of the fire pump starting instantly and the vibration of the water hammer in the pipe can all be eaten by the bellows. This thing is not an optional accessory, it is a key link to ensure the safety of the fire protection system.
Before choosing, find out: Who should go on the general type, axial type and pressure balance type?
When it comes to choosing expansion joints for fire pipes, many novices ask, "Give me a general one". Universal corrugated expansion joints are really versatile, but when they really arrive at the scene, they are often pitted. Why? There are three kinds of fire pipe directions: straight pipe, elbow and equipment connection. Different positions require completely different directions of compensation.
Axial expansion joint(Like the one in our stationExternal pressure single axial expansion joint) Only absorbs the expansion and contraction of the length of the tube, and is suitable for installation on a straight tube section between two fixed brackets. If the pipe has lateral displacement (such as expansion and contraction at the corner), you hard-mount the axial type, and the bellows are stressed laterally, and it will crack fatigue in less than two years.
Pressure balance type(includingStraight pipe pressure balanced expansion jointAndCurved tube pressure balance type expansion joint) is much more advanced. It has a balancing cavity inside, which can offset the blind plate force generated by internal pressure, so that the fixing bracket can be made lighter. It is especially suitable for large diameter and high pressure fire trunk pipes-such as DN400 or above, with a pressure of 1.6MPa. But the price is expensive, the installation space is large, and small pipes can't be used.
Besides, some sites are "Z" or "U" pipe sections, which requireCompound hinge transverse expansion jointOrLarge tie rod expansion jointTo absorb both horizontal and vertical bidirectional displacements. If you choose the universal type, you are betting that it will match exactly — the probability is not much higher than that of you winning the lottery.
3 details that are most prone to rollover at the installation site: deflector, tie rod, arrow direction
Choose the right model, install a fork, the same is useless. After stepping on the scene for a few years, I put out the three most common pits-you remember, pay less tuition in the future.
1. Install the guide tube backwards, and the flow rate will be doubled directly
Internal to the expansion jointguide tube(also called lining cylinder) has two functions: to guide the medium to flow smoothly through the bellows and prevent high-speed fluid from washing the corrugations. But this thing has a direction! The fixed end of the guide tube must be directed in the direction in which the medium flows. If you install it backwards, the medium directly impacts the corrugation, and the flow velocity suddenly changes at the corrugation, resulting in vortex, and the stress at the root of the corrugation will be concentrated, which will shorten the life by half. I once saw a fire pump room where the deflector tube was installed backwards and it leaked in three months. Remember: What is the role of the deflector? Is to let the medium "contact the fixed end first, and then smoothly transition to the corrugated end". Look at the arrow on the body of the tube before installing it-yes, that's the arrow.
2. The tie rod nut is not adjusted, which is equivalent to white installation
For expansion joints with tie rods (e.g.Large tie rod expansion joint), the role of the tie rod is to restrain the bellows from excessive stretching or compression during installation and pressure testing. Many masters get to the scene and screw the nut to death directly-this is a big taboo! The tie rod nut of the expansion joint should be adjusted to the "pre-tension/pre-compression" position, depending on the design. After installation, the pipe is in place, and the nut 1-2 buckle is loosened to leave a compensation margin. If you screw it to death, it is equivalent to turning the expansion joint into a rigid tube, which will not work when it expands and contracts thermally. The correct adjustment method of the expansion joint tie rod is: first set the pre-stretching amount according to the drawing, and then loosen the locking nut back half a turn to one turn after the pipeline connection is qualified.
3. The direction of the arrow is not randomly drawn
Does the direction of the arrow of the expansion joint represent the direction of the flow of the medium? Not just. Some expansion joints (such asStraight pipe pressure balanced expansion joint) also indicates the working direction of the bellows-that is, the allowable main displacement direction. If you install it against the arrow, the force analysis of the bellows is completely reversed. Especially for expansion joints with balance rings, wrong pointing of arrows can lead to destabilization of bellows under internal pressure. So,The direction of the arrow of the expansion joint refers toThe media flow direction must be consistent with the arrows during installation, while ensuring that the bellows can expand and contract freely in the direction indicated by the arrows. During on-site acceptance, be sure to check one by one.
Let's talk about materials and pressure levels-don't let bargains fall off the chain when it matters
The core material of metal expansion joint of fire pipe is corrugated pipe. Common materials on the market are austenitic stainless steel 304 or 316L. Is 304 enough? Most indoor fire pipe environments (temperature-20℃ ~150℃) use 304 no problem. However, if the pipeline is near the seaside, chemical plant, or there is a risk of chloride ion corrosion, it must be 316L or even duplex stainless steel. You save hundreds of dollars, stress corrosion cracks appear after three years, and the labor cost of replacing them once is worth ten expansion joints.
Pressure level, fire protection system design pressure is usually 1.0MPa, 1.6MPa, 2.5MPa three grades. When selecting the expansion joint, the nominal pressure (PN) should be at least equal to the design pressure of the pipeline. For example, for a 1.6MPa pipeline, if you buy a PN1.0 expansion joint, you will surrender your life to luck. Remember, the pressure resistance of the expansion joint mainly depends on the wall thickness and number of layers of the bellows-single-layer bellows have lower pressure resistance, while double-layer or multi-layer bellows have higher pressure resistance and greater compensation. From our stationLarge diameter thick wall expansion jointIt is for high-pressure large pipelines.
Stiffness. The rigidity of the bellows directly affects the force of the pipe support. The greater the stiffness, the greater the thrust of the pipe against the fixed bracket. When selecting the model, you can ask the manufacturer to provideStiffness and Calculation Formula of BellowsCheck whether the total thrust is within the range of the bracket.
From Type Selection to Maintenance: A List of Practical Experience of an Old Engineer
Finally, I condensed the lessons I had learned over the years into a few lists suitable for printing and posting in the equipment room.
- First Three Questions for Model Selection: What is the direction of displacement? What is the pipe pressure temperature? Is there enough installation space for tie rods? If these three items are not clear, don't place an order.
- Five Looks Before Installation: Look at the product nameplate model (confirm that it is consistent with the design), look at the direction of the guide tube (the arrow points to the incoming flow), look at the position of the tie rod nut (pre-stretched in place), look at the surface of the bellows (whether there is any transportation bump), and look at the flange sealing surface (repair it quickly if there are scratches).
- Contraindications during pressure testing: When pressing, a temporary limiting device (such as locking with a screw) must be added to the expansion joint to prevent excessive extension or compression of the bellows. Remove the temporary limit after the pressure test is completed.
- Look at three points in daily inspection: Look at whether there is liquid accumulation, rust and crack on the outer surface of the bellows; Second, see whether the flange seal is leaking; Third, see if the tie rod nut is loose. The expansion joint of fire pipe shall be inspected at least once a year, and the air tightness test shall be done if conditions permit.
- Don't wait until it's broken before replacing: The design life of metal expansion joints is generally 15 years, but the actual life will be shortened when the pipeline system vibrates heavily and the water hammer is frequent. It is recommended to do a comprehensive assessment every 10 years or so, and change it when it is necessary. Don't be reluctant to give up that little money. Once the fire system needs to act, you can't afford the cost of dropping the chain.
Okay, everything from model selection to maintenance is said. Final question: Is the expanding excerpt on your fire pipe correct? If you are unsure, welcome to talk to me with the parameters. In our business, the details make or break-after all, firefighting is no joke.