What exactly is a GIS metal expansion joint? Where is it used?
GIS metal expansion joints, to put it bluntly, are bellows compensators used on gas insulated switchgear (GIS) pipes. GIS equipment is filled with sulfur hexafluoride gas, and once the pipeline system is displaced due to temperature change or foundation settlement, the seal failure will be troublesome. At this time, we have to rely on GIS metal expansion joints to absorb these displacements, so as to ensure that the gas does not leak and the equipment runs normally.
What is the difference between this thing and our common corrugated expansion joints and high-temperature axial expansion joints used in power station industry? Structurally all corrugated pipes, but GIS metal expansion joints have much higher requirements for tightness, cleanliness, material corrosion resistance – after all, sulfur hexafluoride leaks are no joke. It is mainly used in GIS bus barrel, circuit breaker connection, pipeline between transformer and GIS. To put it bluntly, as long as there is SF6 gas flowing and the displacement needs to be compensated, it can't be done without it.
Several Parameters Must Be Known in Type Selection
This matter of selection looks simple, but it is actually a lot of pits. There are four core parameters: nominal diameter, design pressure, design temperature and compensation amount. The nominal diameter is easy to understand, that is, it matches the pipe diameter. The pressure level is generally from 0.25MPa to 6.4MPa, which depends on the pressure level of the GIS equipment itself. In terms of temperature, the working temperature of GIS metal expansion joint is usually between-40℃ and 80℃, but if it is close to the circuit breaker or bus bar heating area, the local temperature may be higher, so you have to ask clearly.
The amount of compensation is the easiest to underestimate. It is divided into three types: axial, transverse and angular. Many engineers only give axial displacement, and ignore the lateral offset caused by thermal expansion and contraction of pipe system. As a result, after installing the bellows, the life of the bellows is greatly reduced.It is recommended that the design institute provide a complete pipe system stress analysis report during type selectionOr take the pipeline layout diagram to find an experienced expansion joint manufacturer for accounting. The general corrugated expansion joint and the double hinge transverse expansion joint in our station can refer to similar selection logic.
Number of layers of bellows. GIS metal expansion joints are usually made of single-layer or double-layer stainless steel bellows, 304 or 316L material. The double-layer has an extra layer of protection, which is suitable for occasions with stricter requirements for leakage risk.
Comparison of GIS metal expansion joints with different structural types
There are three main structures of GIS metal expansion joints on the market, which will really cause problems if you choose the wrong one.
- Axial type: Most commonly, can only absorb axial displacement. For example, the straight pipe pressure balance expansion joint is compensated by the expansion and contraction of the bellows itself. The advantages are simple structure and low cost; The disadvantage is that it cannot withstand radial forces, and the pipe system must have guide brackets to limit lateral displacement.
- hinge typeFor example, the transverse expansion joint of the compound hinge limits the axial displacement by the hinge and only compensates the angular displacement. This is suitable for pipeline turns, or scenarios where space constraints do not allow axial expansion and contraction. The plane of the hinge must be consistent with the displacement direction during installation, which many people ignore.
- Pressure balanced typeFor example, curved tube pressure balance expansion joint and double straight tube bypass pressure balance expansion joint. It has a balanced bellows inside, which can offset the blind plate force generated by the medium pressure. The advantage is that the thrust force is small and the requirements for the fixed bracket are low. If there is a strict limit on the thrust of the equipment port in the GIS pipe system, this kind of thing must be used.
In addition, there are external pressure single axial expansion joints, with bellows on the outside and medium on the inside, which have strong external pressure resistance ability and are suitable for vacuum or negative pressure conditions. The double hinge expansion joint of the air-cooled island vacuum pipe in our station is similar to this logic.
The easiest details to overlook before and after installation
I met a customer two days ago, saying that the expansion joint leaked after less than half a year. I looked at the site and saw that the transport screw was not removed during installation. That thing is used when leaving the factory to prevent the bellows from being transported and deformed, and it must be removed after installation! If it is not disassembled, the expansion joint can't expand and contract normally at all, and the thermal expansion stress of the pipe is all suppressed on the bellows, which will soon crack by fatigue. Regarding this, we specifically talked about "Does the screw of the expansion joint need to be disassembled?" In the Q&A of our station, the conclusion is very clear: it must be disassembled.
Installation direction of the guide tube. GIS metal expansion joints usually have a guide tube, which is used to prevent high-speed airflow from washing the inner wall of the bellows. There is an arrow on the guide tube,The direction of the arrow must point to the direction of the media flow。 Once installed backwards, the airflow directly impacts the root of the bellows, and it takes less than two months to wear out. For the specific principle, please refer to the question and answer of this site "Specific function of expansion joint guide tube".
Protection while welding. The wall thickness of the bellows of GIS metal expansion joint is very thin, only 0.3~0.5mm, and a little more welding heat will burn the bellows through. The correct way is to weld the flange or pipe on the pipe first, and then connect the expansion joint bolts to avoid direct welding. If welding is really needed, the bellows must be wrapped with a damp cloth, or cooling water must be passed.
Routine maintenance and troubleshooting
The maintenance of GIS metal expansion joints is actually not complicated, but it has to be done regularly. It is recommended to check the appearance every six months: see whether there are corrosion spots, scratches and bulges on the surface of the bellows. If the pressure of SF6 gas drops and leakage is suspected, first check the flange sealing surface and welded joint of the expansion joint with a leak detector. Many times the leakage point is not in the bellows itself, but the aging of the flange gasket-this gang of people changed the gasket and solved it.
In addition, check whether the tie rod nut of the expansion joint is loose. The role of the tie rod is to limit excessive displacement and prevent the bellows from being pulled or crushed. If you find that the tie rod nut is loose, you must relock it according to the method of "How to adjust the tie rod nut of the expansion joint", but remember: the tie rod nut is not used to adjust the actual displacement, but only a limit protection device.
"How long is the life of the expansion joint?" According to the question and answer in our station, the design life under normal working conditions is generally more than 10 years. But it depends on the actual conditions of use. Frequent temperature and pressure fluctuations, or impurity particles entering the pipeline, will shorten the life. If there are dense fine cracks on the surface of the bellows, it is the precursor of fatigue failure, so you have to change them quickly.
Although GIS metal expansion joint is a small part, it plays the role of "flexible guardian" in the whole GIS system. It is cheap to select a different drawing, and it is troublesome to install a different drawing. A daily look can save a lot of maintenance costs. If you are not sure, look through the articles on our station about corrugated expansion joints, high-temperature axial expansion joints and general-purpose corrugated expansion joints for power station industry. Many principles are similar.