Metal expansion joint for gas pipeline, wrong type may cause major accident
Two days ago, I chatted with a friend in a steel mill. The expansion joint on a coke oven gas pipeline in his factory cracked in less than half a year. Fortunately, the inspection found it in time and did not cause a disaster. After removing it, the selection is simply wrong-an ordinary general-purpose corrugated expansion joint is used, and the medium temperature of that pipeline often soars above 400℃. Anyone has to be afraid of this.
The gas pipeline thing, it's no joke. A leak hits an open flame, that's a bomb. So in today's article, let's find out how to choose and install gas metal expansion joints.
What exactly is the gas metal expansion joint piping? Not just thermal expansion and contraction
Many people think that expansion joints are used to absorb thermal expansion and contraction. Yeah, not entirely. In the gas pipeline system, besides the axial expansion and contraction caused by temperature, there are also the sagging of the weight of the pipeline itself, uneven settlement of the foundation, and vibration caused by fans or compressors. These displacements are superimposed together. If there is no good expansion joint to carry it for you, the stress of the pipeline will be concentrated, the weld will crack and the flange will leak.
The gas metal expansion joint also has to undertake the sealing function. In addition to the main components of carbon monoxide, hydrogen and methane, there are also messy things such as hydrogen sulfide, naphthalene and tar in gas. Once the seal fails, toxic gas leaks out, which is no joke. Therefore, choosing an expansion joint is equivalent to insuring the pipeline. It is a small matter to insure it well, but it is a big matter to insure it poorly.
First look at these three hard indicators: pressure, temperature and media corrosiveness
If any of these three parameters is wrong, the life of the equipment will be directly discounted by 50%, or even scrapped on the spot.
pressure
Gas pipelines are divided into low pressure, medium pressure and high pressure. For example, blast furnace gas pipelines are usually low pressure (tens of kPa), while the pipe network coming out of the pressure delivery station may reach more than 0.6MPa. The nominal pressure of the expansion joint must be greater than the design pressure of the pipeline, which is not negotiable. Choose small bellows bulge or even burst, choose big waste of money, but safety is always the first priority.
Temperature
Temperature has a great influence on the material of corrugated pipe. It is no problem to use 304 stainless steel for normal temperature gas, but the temperature of coke oven gas and high-temperature waste gas can reach above 500℃. Ordinary 304 is directly oxidized and peeled, so heat-resistant alloy or high-temperature axial expansion joint must be applied. This site'sHigh temperature axial expansion jointIt is specially designed for high-temperature working conditions. The corrugated pipe is made of high-temperature resistant materials, and heat insulation measures have been made in the structure.
Medium corrosivity
The gas contains hydrogen sulfide, ammonia, chloride, etc., which poses corrosion risk to stainless steel. If the medium is highly corrosive, you can consider choosing 316L or duplex steel, or even lined with PTFE. But note that this site'sPTFE-lined hoseAndPTFE compensatorMainly used in strong corrosive fluids, gas pipelines are generally based on metal bellows, special working conditions can consult manufacturers.
To put it bluntly, these three indicators determine the "physique" of the expansion joint. Physique is wrong, how to pretend is a hidden danger.
Universal corrugated expansion joint, high temperature axial type, direct burial type... Which kind should I use for gas pipeline?
So many product models, a headache to choose? In fact, just grasp the core scene.
- Universal corrugated expansion joint: It is suitable for conventional gas pipelines, with low temperature, low pressure and axial displacement. Has the advantages of simple structure, low cost and convenient maintenance.
- High temperature axial expansion joint: As mentioned above, it must be used in high-temperature occasions such as coke oven gas and hot blast stove gas. Thermal insulation layer is added inside the bellows, and cooling structure is added outside to prevent heat radiation from damaging the corrugations.
- Direct buried (fully buried) type expansion joint: Used for underground directly buried gas pipelines. The exterior is anti-corrosion and waterproof, and the internal bellows bear soil pressure and displacement. Many chemical parks bury the gas pipe network, and this model is common.
- External pressure single axial expansion joint: Suitable for large diameter, low pressure gas pipeline, strong anti-instability ability.
- Compound hinge transverse expansion joint: This model is more flexible if the pipe has many turns and needs to absorb lateral displacement.
Don't expect one model to conquer the world. A while ago, a customer asked about the working conditions of high-temperature pipelines with the parameters of direct burial type. Isn't this a joke? Selection must communicate with the manufacturer the actual operating parameters, don't just look at the drawings.
Should the tie rod nut be removed during installation? Don't reverse the direction of the guide tube arrow
If the installation details are wrong, no matter how good the expansion joint is, it will be useless. Two of the most common questions:
Do you remove the tie rod nut or not?
Many expansion joints leave the factory with transport tie rods to prevent bellows from deforming during transportation. After installation, these tie rod nutsMust be loosened or removedOtherwise, the expansion joint cannot be freely displaced, which is equivalent to welding a dead frame. How exactly to adjust? The FAQ of this site specifically includes "How to adjust the tie rod nut of the expansion joint" and "Does the screw of the expansion joint need to be disassembled". It is recommended that the installer read it carefully.
Direction of guide tube arrow
There is generally a guide tube inside the expansion joint, which is used to guide the flow direction of the medium and protect the bellows from being washed. The guide tube is marked with an arrow and the direction of the arrow must bePointing to the flow direction of the medium。 Install backwards, the guide tube becomes a throttle, and the bellows is directly exposed to high-speed airflow, which intensifies the wear and leaks in less than a year. Remember: arrows are lifelines.
Why are rubber compensators or non-metallic expansion joints not recommended for gas lines?
Some people want to use it cheaplyrubber compensatorOrNon-metallic expansion joint(such as fabric fiber expansion joints) instead of metal expansion joints. Let's just say the conclusion: not recommended.
- Insufficient temperature resistance: Ordinary rubber compensator can withstand 100℃, but the temperature of gas pipeline often exceeds 150℃. At high temperature, rubber ages, becomes brittle and even burns. Although the temperature resistance of non-metallic expansion joints is a little higher than that of rubber, it usually does not exceed 350℃, and it is easy to delaminate and fall off.
- Poor corrosion resistance: Hydrogen sulfide and aromatic hydrocarbons in the gas will directly swell the rubber, resulting in seal failure. The fiber layer of the non-metallic compensator is also afraid of chemical corrosion and has a short life.
- High risk of leak: The metal expansion joint bellows is a multi-layer structure, even if the outer layer leaks slightly, the inner layer can still stick and have redundancy. Once a small hole appears in rubber and non-metal, the leakage amount increases instantly, and it is difficult to be detected in time.
So, the gas pipeline is honestGas metal expansion jointDon't joke about safety. Spend thousands more dollars for more than ten years of peace, and anyone will settle this account.
Selection, installation and use, every step should not be fooled. In the event of an accident, not only equipment will be lost, but human lives will also be lost. People who know the business naturally understand this truth.