What exactly does the non-metallic compensator technical specification book say? Don't be fooled by a bunch of numbers
When you get a copy of the technical specification of non-metallic compensator, the first thing you see is often a series of parameters-temperature, pressure, displacement, material code... The layman has a big head, and the expert has to check it repeatedly. To put it bluntly, the specification book is the "contract drawing" between the manufacturer and the user, which tells you under what conditions this compensator can work and how much tossing it can carry.
Many purchasers or new engineers are easily fooled by those seemingly accurate numbers. For example, a certain project has to withstand 120℃, but as a result, the equipment leaked six months after being installed-why? Because the specification book says "instantaneous temperature resistance", while the actual working condition is long-term high temperature. Therefore, the core of reading the specification book is not to look at the size of the numbers, but to look at the "working boundary conditions" behind the numbers.
Core Parameter One: Temperature, Pressure and Medium. Three data determine whether you can buy the right one
These three parameters are an "iron triangle", and none of them will work. Let's talk about the temperature first: the skin materials of non-metallic compensators (such as fluororubber, silicone rubber and polytetrafluoroethylene) have a clear temperature resistance range. Our commonNon-metallic expansion joint (fabric fiber expansion joint)The skin is usually coated with glass fiber cloth + silicone rubber, and the long-term working temperature is within 200 DEG C; If it exceeds 250℃, it must be coated with ceramic fiber or PTFE. The specification is marked "design temperature", and you have to confirm whether it is continuous temperature or peak temperature-don't take peak value as the norm.
What about the stress? Non-metallic compensators are generally used in low-voltage systems and usually do not exceed 0.1 MPa. However, the pressure fluctuates greatly in some flue gas pipelines and dust removal pipelines, and the "nominal pressure" written in the specification book should consider the safety factor. The most typical pit I have seen: the one marked 0.05MParubber compensatorIt was installed at the outlet of the pump with a pressure of 0.08MPa, and it exploded.Remember: The pressure bearing capacity of non-metallic compensator is shared by the skin and frame, and the strength of the skin will decrease after aging, so the pressure margin should be at least 20% when selecting.
The media thing is more critical. Is it smoke? Sulfur-containing? Oily steam? Some media can corrode the skin or insulation. For example, in the desulfurization and denitrification project, the medium contains acidic gas, and the ordinary silicone rubber skin will be embrittled in two or three months, so it is necessary to use PTFE compensator or add anti-corrosion coating.
Core Parameter 2: Displacement-axial, transverse and angular. If it is not correct, it will be scrapped directly
This is probably the most overlooked link. Many users only pay attention to the temperature and pressure, and the displacement amount is randomly estimated. As a result, the compensator is directly "broken" or "squeezed" when installed.
Axial compression/tension, lateral misalignment, angular deflection. The allowable values for each direction are usually given in the specification book. But be warned: these three displacements are "mutually constrained". For example, if you require axial compression of 20mm and lateral displacement of 10mm at the same time, then the allowable value in a single direction has to be reduced-usually manufacturers will provide combined displacement curves, but many specifications only write individual values. Therefore, after getting the specification book, be sure to ask clearly: If multi-directional displacement exists at the same time, how to calculate it?
The thermal displacement calculated by the pipeline layout engineer should be typed into a written document and sent to the manufacturer for confirmation. Don't "piece together" in the specification book yourself.Especially rectangular pipes, such asRectangular non-metallic expansion joint, angular displacement calculations are prone to errors-pipe expansion is not a standard straight line and may generate torsional torque.
Structural details: skin, insulation layer, frame, guide tube, each layer is special
The description of the structure in the specification book is often a lot of English abbreviations and layers. Let's take it apart and say:
- skin: Outermost layer, in direct contact with the medium. The common structure is "fluororubber + glass fiber cloth + polytetrafluoroethylene film", multi-layer composite. The specification book will write "four layers of skin" or "five layers of skin". In fact, the more layers, the better. The key depends on whether each layer of material matches the working conditions.
- Thermal insulation: Usually ceramic fiber blanket with a thickness of 20-50mm. If the temperature of the medium exceeds the temperature resistance of the skin, the insulation layer is the "talisman". However, the thermal conductivity of the insulation layer will change after compaction. If this is not written in the specification book, you have to ask the manufacturer.
- frame: Carbon steel or 304 stainless steel is generally used. Carbon steel is cheap, but it rust out in less than three years when used in corrosive environments. The specification book will say "Frame Material Q235", and you have to confirm whether you need an anti-corrosion coating inside and outside.
- guide tube: This stuff is critical. If the guide tube is not written in the specification book, you should be careful-without the guide tube, the medium directly washes the skin and wears out extremely quickly.Specific Function of Expansion Joint Guide TubeIs to direct the media flow to the center and avoid direct impact. Some operating conditions (such as dusty flue gas) even require double-layer guide tubes.
Two days ago, I met a customer in the cement industry and usedMetal Corrugated Expansion Joints in Cement Industry(Actually, he should have used non-metallic ones), and as a result the dust got stuck in the ripples and couldn't be cleaned up. Therefore, the structure selection depends on the actual application scenario, and the idea of metal compensator can't be copied.
Guide to selection and avoidance of pits: Don't use non-metal as metal or rubber as fabric
First, the non-metallic compensator cannot withstand too much axial thrust. Metal expansion joints can be limited by tie rods, but non-metallic frames have low strength. If the pipe pressure is high and the thrust is high, it is best to use a pressure balanced compensator (such asStraight pipe pressure balanced expansion joint) or add a fixing bracket. Install a non-metallic compensator on the main steam pipe? That's seeking death.
Second,rubber compensatorAndNon-metallic expansion joint (fabric fiber expansion joint)It's not the same thing. The rubber compensator has good elasticity and low pressure resistance, which is suitable for water or weak acid and alkali media; Fabric fiber expansion joint has high temperature resistance and corrosion resistance, which is suitable for flue gas and hot air ducts. Whether it says "rubber compensator" or "non-metallic compensator" in the specification book, the temperature span can be several hundred degrees different.
In addition, some manufacturers will classify "PTFE compensator" as non-metal, but PTFE has limited temperature resistance (usually below 200℃). If you encounter high-temperature acid-containing medium, you have to use fluororubber + PTFE composite skin, which can't be carried by single-layer PTFE.
Installation and Maintenance Note: Pre-tension, limit screw, bolt torque, these details determine life
The last page of the specification book is usually installation instructions, but many people don't even look at it. In fact, the information in this can help you save a lot of rework money.
Pre-stretching: In order to compensate for the cold shrinkage and heat expansion of the pipe, the non-metallic compensator needs to be pre-stretched or pre-compressed when installed. The specification book will say "pre-stretch amount Xmm", but the actual field temperature is not necessarily the standard temperature, you have to convert it. For example: the working temperature of the pipeline is 350℃ and the installation temperature is 20℃, so the pre-compression amount should be 30% ~50% of the thermal expansion amount (because the non-metallic material itself is also elastic). Don't know how to calculate? Call the manufacturer's technical phone directly, don't guess.
limit screw: During transportation and installation, there will be a positioning screw (also called limit screw) on the non-metallic compensator to prevent the skin from being stretched and deformed during transportation. The specification book will say "Remove limit screw after installation". However, many people forget to dismantle it. As a result, the compensator can't deform, and the pipeline stress is fully pressed on the flange, resulting in leakage. So the first thing you do when you come to the site is to check if the screw is loose.
Bolt torque: Flange bolt tightening torque usually has a reference value in the specification book, such as M16 bolt torque 80~100N·m. However, the flange surface of non-metallic compensators is usually soft (especially rubber compensators), and when tightened too tightly, the sealing surface will be crushed. It is recommended to use a torque wrench and tighten it diagonally in two times according to the requirements of the specification. Also, it is best to choose weather-resistant steel or galvanized bolt material, otherwise the rust will not be removed next time