The term Flange Thermal Displacement is too common in pipelines in power plants, cement plants, and chemical plants. To put it bluntly, as soon as the pipe is heated, the flange connection jumps. You think the flange isn't tightened? Not. You thought it was an aging gasket? Not all of them. The real driving force behind the scenes was that the thermal expansion displaced the pipe, and the flange faces were crooked.
The flange surfaces are staggered, and the bolts are tight and loose; The spacer is squeezed out in a small section that looks like a tongue sticking out; Not long after the pipeline started, the flange seeped, tightened the bolts for several days, and then leaked again after a while. What's more interesting is that some flanges are good in the cold state, but they leak when they heat up, and they leak badly when the temperature difference is big. For this kind of problem, you only change the gasket and add torque, which is a cure for the symptoms but not the root cause.
What happens if you don't compensate? Leaky flanges and cracked pipes are no small matter
The consequence of carrying hard is to run and leak if it is light, and to say it is an accident if it is heavy. The flange thermal displacement is not absorbed, and the stress in the pipeline system will always accumulate. The flange bolts are subjected to additional bending stress, the gaskets are repeatedly squeezed, and the sealing surface fails sooner or later. If the medium is high-temperature steam or corrosive gas, leakage is a potential safety hazard.
The plumbing itself isn't much better. The stress is concentrated in the elbow, tee and fixed bracket, and it is possible that the weld cracks and the pipe wall cracks after a long time. Moreover, the thermal displacement is repetitive, and the pipeline is repeatedly subjected to fatigue load every time it starts and stops and the load fluctuates. A friend from a cement factory said when chatting that their kiln head pipeline cracked three times a year, welded and cracked again, and finally solved it with a compensator. As a result, the life of the equipment is reduced and the maintenance cost rises.
To compensate for flange thermal displacement, should I use corrugated expansion joint or metal hose?
This is a type selection problem, depending on the displacement type and working condition. Corrugated expansion energy saving absorbs axial displacement, transverse displacement, and even angular displacement, depending on the structural form. The universal corrugated expansion joint is suitable for absorbing axial displacement; Compound hinge transverse expansion joint specializes in transverse displacement; If the pipe diameter is large and the pressure is low, nonmetallic expansion joints (fabric fiber expansion joints) can also be used, and the temperature resistance and compensation amount are not bad.
What about metal hoses? Its advantages are good flexibility, can absorb multi-dimensional displacement, and also take into account vibration reduction. If flange thermal displacement is accompanied by significant mechanical vibration, or the line has installation deviations to absorb, metal hoses are more appropriate. However, the pressure resistance and axial stiffness of metal hoses are different from those of corrugated expansion joints. In the case of large diameter and high pressure, they must be carefully calculated.
So, who exactly to choose? Depends on which displacement dominates your pipes. Mainly axial, with corrugated expansion joints; Multidimensional direction, with vibration, with metal hose. If this measurement is wrong, the compensator will be installed for nothing.
Three parameters that are most easily overlooked in model selection: displacement, pressure and temperature
In many projects, the drawings mark how many millimeters the thermal displacement of the flange is, but many people start to check the model when they get the parameter table, and three key parameters are taken over.
- The amount of displacement.Many customers only give a total thermal elongation of the pipe, without distinguishing between axial or transverse. The design of the expandable joint is based on the displacement direction. If you report the axial displacement as the lateral displacement, the selected product will not have that ability at all. It is necessary to give the axial displacement, the transverse displacement, and preferably the angular displacement data respectively.
- Stress.The pressure here is not as simple as the design pressure of the pipeline. It is necessary to clarify whether it is the test pressure, the operating pressure or the maximum transient pressure. The bellows of the corrugated expansion joint is a thin-walled piece, and the higher the pressure, the lower the fatigue life. When we make corrugated expansion joints for power station industry, the pressure parameters must be accurate to two decimal places after MPa, and the stress calculation of corrugated pipes is completely different.
- Temperature.Temperature directly affects the allowable stress of the corrugated pipe material. For the same expansion joint, the allowable displacement can be more than 50% different under the working conditions of 200℃ and 400℃. In addition, in high-temperature working conditions, we have to consider whether internal insulation layer or guide tube is needed, otherwise the bellows will directly face the high-temperature medium, and the life will be greatly reduced. The specific function of the expansion joint guide tube is to protect the inner wall of the bellows and reduce the direct action of high-speed fluid erosion and temperature.
These three parameters are not filled in by patting the head. Without which, manufacturers can't accurately calculate the fatigue life.
Installation and Predisplacement: The Details That Make the Compensator Really Work
Select the right type, install and pull the crotch, still leak.
Pre-displacement is the most easily overlooked link in the installation of corrugated expansion joint. The so-called pre-displacement means that during cold installation, the expansion joint is pre-stretched or compressed for a certain distance in the opposite direction of hot displacement, so that it just returns to the neutral position in hot state. The purpose of this is to make full use of the displacement capacity of the expansion joint and avoid the displacement exceeding the limit in the hot state.
How to adjust the tie rod nut on the expansion joint? In the transportation state, the expansion joint is generally locked with a tie rod to prevent damage to the bellows during transportation. Once installed in place, loosen the tie rod nut to allow the expansion joint to move freely. Some construction teams don't understand that the nut is directly welded or not loosened, and the compensator is equal to a rigid short joint, which can't absorb any displacement. The role of the expansion joint tie rod is to transport support and protection, not to keep you permanently locked.
In addition, the fixing brackets and guide brackets of the pipeline system must be set according to the design requirements. The expansion joint can only compensate its own section. If the fixed bracket is not firmly placed, the displacement distribution of the whole pipe system will be messed up, and the compensator may be pulled out by the displacement exceeding the design value.
A Case of Practical Treatment of Pipes in a Cement Plant
Two years ago, there was a cement plant project. The inlet and outlet pipeline of kiln head waste heat boiler, DN800 pipe, operated at 350℃, and the flange connection leaked repeatedly. From the initial quarterly tightening of bolts to the repair welding once a month, the maintenance personnel really couldn't bear it.
The axial displacement is about 18mm and the transverse displacement is about 6mm. The pressure is not high, 0.15MPa. After we bring the data back to the manufacturer for calculation and analysis, the suggestion is: install a metal corrugated expansion joint in cement industry near the heat source side of the pipeline to absorb axial displacement; At the same time, a section of metal hose is connected in series on the pipe section on the other side to absorb lateral offset and equipment vibration.
Pre-displacement was made during installation, and cold state pre-stretched 8mm. After it was put into operation, I stared at it for more than three months, and the flange surface was clean and never leaked again. Later, the customer reported that the labor and material cost spent on leak treatment in the previous quarter was almost the price of half a compensator.
The problem of flange thermal displacement is solved at one time by choosing the right compensation scheme, so there is no need to fight against leakage every year. Carry it hard, that's saving up bills for the next downtime. Don't be vague when it is time to put on the compensator, but the premise is that the scheme is accurate and installed correctly.