1. What exactly do metal expansion joints do? Stop getting confused with compensators
Many friends who have just entered the industry ask "are metal expansion joints and compensators the same thing" as soon as they come up. In fact, in our industry, these two words refer to the same kind of thing. You go to the standard, which is called "metal bellows expansion joint" in the national standard, but the on-site workers and purchasing habits are called "compensator". The core task of the metal expansion joint is one: to absorb the displacement of the pipe due to thermal expansion and contraction, mechanical vibration or installation deviation, while ensuring the seal and pressure.
But note – not all compensators are metal. For example, we have rubber compensators, PTFE compensators and non-metallic expansion joints (fabric fibers) on our station. These materials have completely different temperature and pressure resistance ranges. You have to use the rubber one to top the high temperature steam pipe? Then wait for it to paste. Therefore, the first thing in the selection: distinguish the medium and working conditions, and then determine the material.
Two days ago, a purchasing call from a power plant had to use non-metallic expansion joints on high-temperature flue gas pipelines, saying that they were cheap. I asked, "What's the temperature of your smoke?" He said, "850℃". Alas, the upper temperature resistance limit of fabric fiber expansion joints is generally 500-600℃. If this temperature goes up, it will be an accident if it burns through. Later, he recommended a high-temperature axial expansion joint with heat-resistant alloy bellows, which was done. Therefore, don't try to save trouble. When you should use metal, you can't make do with it.
Second, from power station to cement plant: which expansion joint should be used under different working conditions?
The application scenarios of industrial metal expansion joints can basically cover the whole process industry. Pick a few typical ones to chat about.
Power station industry: Main steam pipeline, reheating steam pipeline and water supply pipeline, with high temperature (600℃ +) and high pressure (above 10MPa), must be fitted with corrugated expansion joints for power station industry. Such products usually come with multi-layer bellows, external reinforcing rings, and have special design standards for fatigue resistance life. For large-diameter pipelines like air-cooled islands, the vacuum working conditions have to be considered. This is what the double-hinge expansion joint of air-cooled island vacuum pipeline does. Think about it, once the bellows goes unstable under vacuum, the whole system has to stop.
Cement industry: Air duct, kiln tail and preheater, the conditions are worse than those of power stations-high temperature (common above 1000℃), dust and flue gas corrosion. At this time, ordinary metal expansion joints can't bear it, so we have to use metal corrugated expansion joints in cement industry, lined with wear-resistant guide tubes, and upgrade the material of corrugated pipe to 316L and Inconel. Some places also have to cooperate with flue gas baffle door or desulfurization flue gas baffle door to isolate the system. The deflector is installed backwards? Tsk, the medium flushes the bellows directly and perforates in a few months.
Chemical industry: There are many corrosive media, and PTFE-lined hoses and PTFE compensators have become frequent visitors. However, it should be noted that the upper temperature resistance limit of PTFE lining is generally about 200℃, and if it exceeds, it has to be coated with metal bellows. There are also vacuum working conditions, vacuum special hoses and directly buried expansion joints have their own particularities.
Is there a dedicated model for every industry? right. Hold the universal corrugated expansion joint in your hand, and the probability of rollover is 80% higher.
3. The three parameters that are most prone to rollover during model selection: pressure, temperature and displacement
Choose the metal expansion joint, and most of the rollover is because these three parameters are not thoroughly understood. Say it one by one.
pressure: The design pressure not only depends on the normal working pressure of the system, but also on the hydraulic test pressure and impact pressure. For example, when the steam pipe is heated, the temperature and pressure increase instantly. If the bellows does not leave enough safety factor, it will bulge directly. We have done tests. A certain type of external pressure single axial expansion joint has a design pressure of 1.6MPa. The customer insists on exceeding 2.0MPa, and three of the six bellows are squeezed. Later, he was replaced with a straight pipe pressure balance expansion joint. The pressure balance structure kept the bellows from the end thrust, which was stable.
Temperature: Temperature affects two things-the allowable stress of the bellows material and the calculation of the amount of compensation. For example, the allowable stress of 304 stainless steel at 400℃ is only about 60% of that at normal temperature. Many customers only look at the normal temperature parameters. As a result, when calculated at high temperature, the displacement is all eaten by the creep of the material. For high temperature working conditions, it is recommended to choose high temperature axial expansion joint, and the material can be directly upgraded to 321 or 316Ti.
displacement amount: Here are the most pits. The calculation of thermal expansion of pipes cannot only count linear expansion, but also consider the direction of pipes, the position of fixed brackets and guide brackets. The lateral expansion of the compound hinge saves energy and absorbs the lateral displacement, but the rotation angle cannot exceed the design value; Curved tube pressure balance type expansion energy saving absorbs angular displacement and axial displacement-the wrong compensation type is selected, either the displacement sucks not enough, or the bellows is twisted into a twist. And guess what? A few days ago, at the site of a chemical plant, the pipeline ran with an axial displacement of 30mm, but the drawing said "horizontal". As a result, it was installed in a general-purpose type, and the flanges on both sides were deformed. Therefore, the displacement direction must be clearly confirmed with the technology.
IV. Installation and after-sales: Is the tie rod removed or not? What happens if the deflector is installed backwards?
The installation details look small, but something is a big deal. Let's start with the pull rod. The correct installation method of large tie rod expansion joint Article 1: During transportation and installation, the tie rod and nut are locked tightly for the purpose of protecting the bellows from being stretched or compressed. However, before the system is put into operation, the tie rod nut must be loosened (the adjustment amount must be reserved according to the design requirements), otherwise the expansion joint will not work. Some construction sites are lazy and do not dismantle the screw. As a result, the bellows is deformed under force, and the stress of the whole pipeline is alarmed. Does the screw of the expansion joint need to be removed? Standard answer: According to the design drawing, it is usually necessary to release but retain the limit function.
Let's talk about the deflector. The role of the deflector is to deflect flow and protect the bellows from erosion by high-speed media. The direction of the arrow of the deflector must be consistent with the direction of the media flow during installation. Install backwards? That medium rushes directly toward the bellows root, and the shear stress tears the weld directly. We have a case where the guide tube of the smoke duct of a power plant was installed backwards, and the bellows leaked after three months of operation. Later, the double-seal single-axis circular baffle door was replaced with the expansion joint, and the problem was solved.
After-sales, many customers asked "How many years will it take for expansion and energy saving". In fact, life depends on the number of cycles, working condition fatigue and environmental corrosion. It is recommended to check it regularly, and replace it in time if there are cracks or corrosion pits on the surface of the bellows. Don't wait until there is a leak before shutting down, it will be a lot of damage. In addition, the installation position of supporting equipment such as electric plug-in insulation door and manual plug-in insulation door should also consider the maintenance space of the expansion joint.
In the final analysis, choosing the right metal expansion joint for industrial use is essentially to understand the four things of pressure, temperature, displacement and medium characteristics. Don't be superstitious about "general purpose" and don't save that consulting fee. A project plumbing system several million investment, because an expansion joint selected wrong leads to parking, who can afford it?