Those inevitable technical parameters in the bidding documents: how to look at caliber, pressure, temperature and compensation amount
When bidding for metal expansion joints, many people were dumbfounded when they first saw the technical parameter table-how much DN, how much PN, design temperature, compensation amount, a bunch of numbers were swung there, and not many people could really understand them all. Let me say dry goods directly: these four parameters are the bottom line, and you can't sign a contract if you have one less.
Caliber(DN) The simplest is the nominal diameter of the pipe, but don't just look at the numbers, but confirm whether it is a flange connection or a butt welding connection. Like what we have in our stationUniversal corrugated expansion jointThe hole spacing of DN100 and DN150 flange bolts is different. If you make a mistake and can't install it on the site, the construction period will be smashed.
pressure(PN) should be divided into design pressure and test pressure. Many bidding documents only write "1.0MPa", but the actual system may have water hammer impact, and the instantaneous pressure can reach 1.6MPa. At this time, if you choose according to PN1.0, the wall thickness of the bellows of the expansion joint is not enough, and the life will be cut in half directly. It is recommended that it be explicitly requested at the time ofMetal hose pressure standardThe corresponding test pressure value is usually 1.5 times the design pressure.
TemperatureIt's a killing move. Carbon steel bellows is fine below 350℃, but if it exceeds 400℃, stainless steel 321 or 316L must be used. If the temperature of the flue gas pipe is up to 600℃, it must be consideredHigh temperature axial expansion jointAnd even add an internal insulation layer. Two years ago, there was a project bid that said "design temperature 450℃". As a result, 304 stainless steel used by the supplier cracked in half a year-because 304 had serious intergranular corrosion at 450℃. The temperature parameter must be clearly written whether it is continuous operating temperature or instantaneous peak temperature. The two concepts are far from each other.
Compensation amountMore serious. Axial compensation, lateral compensation and angular compensation, the three kinds of displacements should be listed separately, and only one total displacement should not be given. Such asCompound hinge transverse expansion jointMainly absorbs lateral displacement, you insist on it pulling axial, and the bellows immediately wrinkles. It is best to ask the bidder to provide a calculation sheet of compensation amount in the bidding documents, not just a number.
Type selection is not building blocks: the requirements of working condition media and installation position on the material and structure of metal expansion joint
The parameters are matched, but the material is not selected correctly, which is the same as not selected. Is the medium steam, hot water, flue gas or corrosive gas? Three operating conditions correspond to three different numbers of routes.
For steam pipes, stainless steel 304 or 316L is preferred, and the wall thickness of bellows is not less than 1.5mm. Especially pay attention to if there is intermittent condensation of steam, there will be stress corrosion inside, at this timeDirect buried (fully buried) type expansion jointThe outer anti-corrosion layer has to be specially strengthened. The medium contains chloride ions (such as desulfurization flue gas), so be sure to use 316L or even 254SMO super stainless steel, otherwise pitting will make you doubt your life. In our stationDesulfurization flue gas baffle doorThe matching expansion joints are all made of stainless steel, just because this hole is too poisonous.
What about the installation location? Horizontal pipes and risers have different requirements for expansion joints. Horizontal installation should consider self-weight sag, and large tie rod or hinge structure can offset the influence of gravity; Vertical installation must pay attention to the arrangement of guide brackets to prevent the bellows from becoming unstable. Two days ago, I met a client.External pressure single axial expansion jointIt was installed on a vertical pipe, but it was not guided. As soon as the system ran, the bellows bent. Therefore, it is necessary to clarify the installation orientation and bracket setting requirements when bidding.
You can't be lazy about structural selection. Absorption axial displacement findingUniversal corrugated expansion joint； For absorbing lateral displacementCompound hinge transverse expansion joint； Large amount of compensation and space-limited, upperStraight pipe pressure balanced expansion jointOrCurved tube pressure balance expansion joint, these two can balance the pressure thrust by themselves, saving money on the main fixing bracket. AndRotary compensatorIt is suitable for low pressure and large compensation conditions, but it is not used in corrosive media.
Those "low-price winning bids" that pit people: how to identify false parameters and cut corners
The words "winning the bid at a low price" are basically equivalent to "waiting for rework" in the expansion joint industry. I'll say a few real cases, and you can compare them.
Reduce wall thickness. The tender requires the wall thickness of corrugated pipe to be 2.0mm, and some manufacturers achieve 1.2mm, which is euphemistically called "optimized design". In fact, for every 0.3mm reduction in wall thickness, the fatigue life decreases by more than 30%. How to check? At the time of acceptance, take an ultrasonic thickness gauge for sampling inspection, or look at the pressing record-under the normal test pressure of 1.5 times, the bellows with insufficient wall thickness will bulge locally.
Material replacement. 201 stainless steel was used instead of 304 and Q235B was used instead of 16Mn. The corrosion resistance of 201 stainless steel is much worse than that of 304, and it will rust in the second half of the year in a humid environment. Third-party material composition reports can be requested during bidding, or suppliers can be directly requestedCorrugated expansion joint for power station industryA warranty like that.
Deflector stealing. What is the role of a deflector? The FAQ in our site has a detailed explanation: protect the inner wall of the bellows from high-speed media erosion. However, low-priced goods often do not make guide tubes, or the wall thickness of guide tubes is only 0.5mm. If the medium contains particulate matter, the bellows will wear out in one month. Don't just look at the appearance, take a flashlight to illuminate the interior. Does the length of the deflector cover the full length of the bellows? Is the thickness not less than 1.5mm?
Reduction of tie rod and nut. The role of the tie rod is to transmit pressure thrust and prevent the bellows from over-stretching. Some manufacturers shrink the diameter of the tie rod from M24 to M20, and the nut uses ordinary carbon steel instead of stainless steel. As a result, after the system heated up, the nut was rusted, and the tie rod could not be adjusted or even broken. Be sure to ask suppliers to provide when soliciting bidsHow to adjust the tie rod nut of expansion jointDescription, and proof of the material of the tie rod.
Don't just look at the appearance during acceptance: the details of stiffness, guide tube and tie rod nut are the key
When the goods arrive, don't rush to sign them. Check firstStiffness and Calculation Formula of Bellows— — Each expansion joint has a design stiffness value, which will affect the stress distribution of the pipeline if it is too large or too small. Use a simple method: put the expansion joint vertically on the ground, press the standard weight, measure the compression amount, and reverse push the stiffness. If the deviation exceeds ±15%, the product will be returned directly.
The positioning of the guide tube is also very particular. The direction of the arrow of the guide tube must be consistent with the flow direction of the medium. If it is installed backwards, it will cause vortex erosion. How to judge? Look at the arrow mark on the outer wall of the expansion joint, or look at the position of the flange bolt hole-usually the bolt hole is centered or centered.
The preload force of the tie rod nut should also be checked. Many installation units screw the nut too hard, and the expansion joint is equal to failure. Correct practice: before installation, adjust the tie rod nut to the "cold tight" position (the manufacturer will mark it), and then loosen it to the working position after the pipeline runs and heats up. If you are not clear, you can refer to our websiteCorrect installation method of expansion joint of large tie rodThat article.
Supplier inspection: Hard indicators such as welding process, pressure test and warranty period cannot be saved
Don't just look at the quotation, production strength is the life saver. How to inspect? Keep an eye on the three links.
Is the longitudinal weld of corrugated pipe automatic argon arc welding or manual welding? The stability of automatic welding is much higher than that of manual welding, and the weld forming is regular and the heat affected zone is small. Go to the workshop to see if the welder has a pressure vessel welding qualification certificate, and bellows welding should hold a certificate to work.
Each expansion joint must be hydraulically tested before leaving the factory, and the pressure value, holding time and whether there is pre-deformation are recorded. Some manufacturers only do sampling inspections, or even do not do tests, and directly issue "certificates of conformity".
Common controversial points: What to do if the size of the expansion joint model does not match the design
The goods arrived, and the installation size did not match the design drawings. The length is 20mm shorter, the flange bolt hole is misaligned, and the type test tailstock is not slotted... I have seen this kind of thing no less than ten times. The processing process is in three steps:
First step, checkModel and size of expansion joint。 Take out the bidding documents and product nameplate to see if the model code is consistent. Many manufacturers have their own internal model codes, which do not correspond to the national standard. At this time, let the manufacturers provide a model comparison table.
The second step is to measure the critical dimensions. Use a tape measure to measure the total length (L), flange thickness, bolt hole center circle diameter, bolt hole diameter. The deviation within ±3mm is generally acceptable, but it must be replaced if it exceeds the range. In particular, the deviation of the flange bolt hole exceeds 1mm, and the bolts can't be threaded at all on the spot.
Step 3, make a written record and make a claim. Take photos, measure data, sign and confirm by both parties, and then hold accountable according to the terms of the contract. Don't think about "making do with it". The wrong size of the expansion joint will cause the pipeline stress to exceed the standard, ranging from leakage to pipeline pull-off.
Spend more time in bidding, and less crap in acceptance. The metal expansion joint is the "joint" of the pipeline system if you choose it correctly, but the time bomb if you choose it incorrectly. I hope this article will help you step a few less pits in your next tender.