Not long after the overhaul of the device, the pipe at the outlet of the pump was forcibly cracked, the pump body was deformed, and the seal leaked completely. When removed, the "expansion joint" installed between the pipe and the pump port is an ordinary stainless steel hose, which can't bear the thrust generated by thermal expansion at all. Alas, the typical install is install, which is equal to not install.
What exactly is the pump coupling expansion joint carrying? Not just a bellows
The pump joint is installed at the inlet and outlet of the pump. Its function is not only to absorb the heat displacement, but also to protect the pump port. The pump casing is made of cast iron or cast steel, which belongs to brittle materials and cannot withstand excessive external forces and bending moments. If the load caused by temperature change, water hammer effect and foundation settlement of pipeline system is directly transmitted to the pump port, it can cause vibration and noise at the slightest, and directly crack the pump shell at the worst.
Therefore, the metal pump joint expansion joint is essentially "unloading force"-reducing the reaction force of the pipeline to the pump port to the range allowed by the pump manufacturer. This "allowable range" is usually very small. For example, the API 610 standard requires that the force on the pump port should not be greater than a certain value, and different pump types and calibers are different.
Calculate the displacement and thrust before selecting the model, otherwise it will be for nothing
What caliber, pressure and length? Then there was nothing more. This is the biggest pit. When choosing metal pump coupled expansion joint, the displacement amount and blind plate force must be calculated first.
Calculating displacement is simple, but no one does. The linear expansion coefficient of carbon steel pipe is about 12×10⁻⁶/℃. If the temperature of a 10-meter pipe rises from 20℃ to 120℃, the expansion amount is 12 x 10⁻⁶ x 10,000 mm x 100 °C =12 mm. This 12mm is the axial displacement that the expansion joint needs to absorb. Don't underestimate this 12mm. If the pipe is rigid and fixed, this force can push the pump askew.
Then count the blind board force. The blind plate force is equal to the medium pressure multiplied by the effective area of the bellows. The effective area of the bellows of DN100 is about 0.012㎡. If the system pressure is 1.6MPa, the blind plate force is 0.012×1,600,000 ≈ 19,200N, that is, the thrust is close to 2 tons. If this 2-ton force is directly attached to the pump port, the consequences can be imagined.
Thermal displacement, medium pressure, allowable force at pump port. The manufacturer designs the wave number, number of layers, effective area and overall stiffness of the bellows according to these parameters. The universal corrugated expansion joint on the market can indeed replace the metal pump joint expansion joint to a certain extent, but it should be paid attention to whether the stiffness and end force parameters meet the requirements of the pump port. If the selection and design do not match, the final trouble is yourself.
Different working conditions, great differences in structure and material-how to choose between high temperature, corrosion and high pressure?
The pump joint expansion joint looks similar in length, but the lining is very different. Under normal temperature and clean water conditions, 304 stainless steel is enough. When the medium temperature exceeds 300℃, the creep strength of ordinary austenitic stainless steel drops sharply. At this time, high-temperature alloys such as Inconel 625 or Incoloy 825 have to be used, and the waveform should be made into a multi-layer thin-walled structure to reduce the stiffness. When dealing with corrosive media, 316L is sometimes not useful, so it has to be replaced with Hastelloy C-276 or lined with PTFE. The products of this station have PTFE-lined hoses and PTFE compensators, which are specially designed to deal with strong corrosion conditions.
There's a way to stress, too. Under high pressure conditions, the wave root of bellows is a weak link, and too high pressure will unstable. The solution is to increase the number of layers or add strengthening rings. Under the working conditions above 15MPa, single-layer corrugated pipe basically has no safety, so multi-layer structure must be used. In addition, when the flow rate of the medium in the tube is high, a guide tube must be added. The material of the guide tube must match the medium, otherwise the high-speed fluid washes the bellows, and it can be worn out in a few months-the guide tube itself is to prevent the fluid from directly washing the inner wall of the bellows and protect the bellows from being worn out by the medium, which has been explained repeatedly in the FAQ.
Install and debug those pits: whether the tie rod is removed or not, and which way does the guide tube face?
Install this piece, there are always people stepping on pits in every project, and they are all stepping on the same pit.
The tie rod is the long screw that fixes the bellows when it leaves the factory. Don't dismantle it as soon as you arrive at the scene. Those tie rods are used for factory positioning and transportation protection to prevent the bellows from being over-elongated or biased during hoisting. However, before the hydraulic test of the pipeline system, it is necessary to confirm whether the tie rod has been loosened or removed-if you test the pressure with the tie rod, the expansion joint will become a rigid short joint, which will not compensate at all, and in serious cases, the bellows will be cracked. Specifically, according to the installation drawing provided by the manufacturer, different products have different structural treatment methods. In addition, temporary support must be added to fix it during pressure test, and the blind plate force cannot be smashed all on the expansion joint.
The direction of the deflector, this is a cliche, but people are wrong every year. The guide tube flows in the direction of the medium, and the direction is reversed. The inside of the bellows is directly subjected to the medium scour. Install it in the direction of pump port or pipe? Look at the flow direction of the medium, the direction indicated by the arrow is the flow direction, and the opening of the guide tube faces downstream. When installed backwards, the guide tube not only does not protect the bellows, but also forms vortex and aggravates vibration.
Plus, don't forget weld protection. When the scene is hot, the welding slag splashes on the surface of the bellows, which will destroy the passivation film of stainless steel and cause pitting corrosion. It is best to cover the bellows section with a fire blanket before welding.
What is the difference between metal pump joint and metal hose and general corrugated joint?
Many people confuse these three things, thinking that they are all bellows and can be used universally. Actually, three different products. In the product information of this site, metal hoses and universal corrugated expansion joints are classified side by side, which is not the same thing.
The core function of metal hose is flexible connection and vibration isolation. Its main characteristics are low bending stiffness and can absorb lateral deflection and installation deviation. It is usually used for flexible joints at pump inlet and outlet, and basically depends on it in large vibration occasions. However, the axial stiffness of metal hose is low, so it is generally not suitable as a thermal displacement compensation element. Its displacement compensation ability is mainly aimed at installation deviation rather than thermal expansion during operation. If you use it to absorb large thermal displacement, its life will be greatly reduced.
The main mission of the general corrugated expansion joint is to absorb the thermal expansion displacement of the pipeline. It is selected according to the compensation amount, pressure and temperature, and the two ends are usually welded or flanged. The stiffness, displacement and fatigue life are calculated according to the stress analysis of the pipeline. The general-purpose corrugated expansion joint does not check the force of the pump port, and it is necessary to check whether the thrust exceeds the standard when it is used at the inlet and outlet of the pump.
Metal pump coupling expansion joint is an expansion joint specially designed for pump port application. It also integrates the functions of limit rod, external pressure balance, etc., and limits the pressure and thermal displacement reaction force generated by the bellows within the range that the pump can bear. Simply put: the metal hose is "flexible connection", the universal corrugated expansion joint is "thermal compensation", and the pump joint expansion joint is a combination of "flexible connection + thermal compensation + low reaction force". The three are similar in shape, but the design starting point and parameter requirements are completely different.
Finally, the pump joint expansion joint is not something that you can just choose a model and install it. From displacement calculation and thrust checking to structural selection and installation and maintenance, each step affects the life of pumps and pipelines. Don't wait for the pump to crack the pipe to remember it-by then, the cost of downtime for maintenance is far greater than the small purchase difference you saved.