Why should metal expansion joints be installed in circulating water pipe network
It is not just water that runs in the circulating water system, but also three hidden forces: pipe thermal displacement, foundation settlement and equipment vibration. The pipeline expands and contracts with the temperature change, the buried pipe section moves slowly with the foundation, and the vibration of the pump group is transmitted all the way down the pipe wall-these three forces can't be held by rigid connection alone, and the result of the hard top is interface cracking, flange warping and equipment alignment deviation. The expansion joint is the flexible margin left for the pipe network, so that it can move when it should move and stable when it should not move.
In the circulating water system of power station, net circulating water of steel plant and chemical cooling water system, the two most common structures are general corrugated expansion joint and directly buried (fully buried) expansion joint. General-purpose type has strong adaptability, and can be used on pipe gallery and equipment interface; The direct buried type is specially designed for buried pipelines, and it is directly buried in the soil without building a well chamber. Everyone has his own job, don't mix it up. You have to use the general-purpose type to fill the pit of the underground pipe. As soon as the earth pressure and groundwater come up, something will happen to the bellows sooner or later.
Find out these parameters before selecting, otherwise it will be for nothing if you install it
Select the circulating water metal expansion joint, first ask four numbers: diameter, pressure, medium temperature and displacement direction. These four parameters directly determine the wavenumber, wall thickness and material of the bellows. The pressure of circulating water system is generally not high, 0.25MPa and 0.6MPa are frequent visitors, but the diameter is often scary, DN800, DN1200 or even larger. Large-diameter pipes use large-diameter, thick-walled expansion joints, which are much more durable than ordinary specifications-the wall thickness is there, and the fatigue and corrosion resistance are not of the same order of magnitude.
Whether the medium is fresh water or seawater/reclaimed water is directly related to the choice of stainless steel material. Freshwater environment 304 is basically enough, but in water environment with high chloride ion content, 304 is prone to stress corrosion and cracking, and too many people step on this pit. In the reclaimed water and seawater reuse system, 316L or even higher materials are reliable. Don't put the whole system in just to save that little money on materials. Chloride ions corrode the cracked bellows, and you can see it with your eye-the cracks fork like tree branches, are dark in color, and there is a crisp sound at every knock.
Different laying methods determine different structural forms
The stress state of buried pipelines and pipe gallery erection is completely different. Buried pipelines give priority to directly buried (fully buried) expansion joints, which are directly buried in the soil without setting a well chamber. The civil construction cost is saved, and the overall stability of the pipeline is also good. If the transverse displacement of the pipe on the pipe gallery is large, the compound hinge transverse expansion joint is more suitable, which can absorb the large transverse displacement and will not hard transmit the force to the fixed bracket.
The vibration near the pump outlet is large, the pressure fluctuates frequently, the straight pipe pressure balance type expansion energy saving to avoid the blind plate force impact equipment. Anyone who knows the blind plate force knows that it can push the pipe to jump straight when it is started. The ordinary expansion joint absorbs the displacement but can't control the blind plate force. The pressure balance type digests this force by itself with the internal structure, and the equipment end is clean. Selection is not a set of formulas, working conditions are the determining factors. You take a selection form and fill it out from beginning to end. Without looking at what the stress state is at the site, you will have to rework it sooner or later.
The failure of circulating water expansion joint is mostly in these cases
Check the damaged expansion joints at the site, and you will find that most of them are not product quality problems, but mines buried in the selection and installation links. The four most common failure modes: chloride ion stress corrosion cracking, internal erosion wear of bellows, deflector tube falling off, and pre-deformation failure caused by improper adjustment of tie rod nut during installation.
As mentioned before, chloride ion stress corrosion cracking belongs to the wrong material selection. It is often the pot of the guide tube that washes and wears out this matter. The role of the expansion joint guide tube is often neglected, it protects the inner wall of the bellows from being directly washed by the medium, and can also reduce the flow resistance. If the medium flow direction is installed backwards, the guide tube can't play the role of diversion, and the bellows is directly washed by the water flow, so the life can be reduced by half. When you take it apart, the wearing position must be directly facing the direction of the incoming flow.
Deflector shedding is also common. The circulating water system starts and stops frequently, and the water hammer impacts several times, and the weakly welded guide tube falls off. The falling debris got stuck in the corrugated valley, grinding back and forth, and the bellows quickly leaked. Improper adjustment of the tie rod nut during installation leads to pre-deformation failure, which is the most injustice-it is obviously a protective device, but it did not follow the regulations when installed, the tie rod was locked or overadjusted, and the expansion joint started working with internal injuries before working.
Several hard rules for installation and daily inspection
The direction of the arrow of the expansion joint must coincide with the direction of the media flow. The arrow is not a decoration, it marks the direction of the deflector. Install it backwards, and the whole hydraulic condition is in complete chaos. Article 2: Do not rush to dismantle the pull rod and screw before installation, and adjust it after the pipeline alignment is completed. The role of the tie rod is transport protection and initial positioning, you move it ahead of time, and the bellows may have produced pre-deformation. Article 3: During the hydraulic test of the system, the expansion joint should be in a free state, and no additional constraints can be imposed. You tie it to death with a wire, and as soon as the test pressure comes up, the displacement that should be absorbed has nowhere to go, and the tube can't stand it first.
Whether there is water corrosion in the corrugated valley, whether the displacement indication is reset, and whether there is any abnormal noise. Water accumulation in Ripple Valley is a chronic disease. Chloride ions and impurities in the water are enriched at the bottom of the valley, which becomes a den of corrosion over time. The displacement indication does not reset, indicating that the displacement of the pipe has not been completely absorbed, and there may be a jam. Abnormal noise is more direct-a signal of deformation and friction of bellows or loosening of deflector. Circulating water system starts and stops frequently, and thermal shock will cause bellows fatigue to accumulate. This factor is often more critical than pressure in life assessment. When you calculate your life span, you just look at the number of pressure cycles and don't look at the alternating temperature. That's lying to yourself.
This expansion joint thing, which looks inconspicuous, is actually a technical job. Selection, installation, inspection, which link is loosened, it will give you a poke when you least expect it. Don't rush to replace the parts yet. It is much more useful to find out the root cause of the disease than to spend a lot of money on new parts.