What is the thrust when the blind plate is blocked? Calculation method of thrust of blind plate of expansion joint
When pipeline pressure test, purging or system isolation, once the blind plate is sealed, the medium pressure directly acts on the cross section of the blind plate. This force is transmitted along the pipe to the expansion joint, the bracket, and the fixing point. You go to the site and look at the pipe sections that have problems-the expansion joint tie rod is bent, the bracket is displaced, and the fixing point weld is cracked-nine times out of ten, the root cause is the blind plate thrust is not clearly calculated.
The essence of blind plate thrust is that the pressure acts on the area to produce an axial force. The formula is spread out in one sentence:F = P × A。 F is the blind plate thrust in N; P is the medium pressure in MPa; A is the effective pressure area of the blind plate in mm². However, in practice, how to take a and which value to use P are all particular.
The formula itself is not complicated, but the complexity lies in the value of the parameter
F = P × A, Anyone can write this formula. The difficulty is: Is the diameter of the blind plate calculated according to the inner diameter of the pipe or the middle diameter of the sealing surface?
In most cases, the blind plate is installed between two flanges, and the medium pressure acts on the gasket sealing position, which is the middle diameter of the flange sealing surface, not the inner diameter of the pipe. Take DN400 pipe as an example. The inner diameter is 400mm, and the medium diameter of the sealing surface may reach 410mm. The area difference between the two is about 5%. On high-pressure pipelines, this gap is converted into thrust of several tons.
If the expansion joint is close to the blind plate, the bellowsEffective areaIt should also be included in the calculation. The effective area of the bellows is the equivalent area of the axial thrust generated by internal pressure on the bellows. This value is directly given in the product sample, and it is taken according to the manufacturer's data, so there is no need to push it yourself. Universal corrugated expansion joint, straight pipe pressure balance expansion joint, compound hinge transverse expansion joint, all types of samples have this parameter. Can't find it? Call the manufacturer to ask for it, don't estimate it yourself.
Different types of expansion joints have completely different force paths
The same blind plate thrust, mounted on the general corrugated expansion joint and mounted on the straight pipe pressure balance expansion joint, the calculation result may be the same, but the force distribution is very different.
The universal corrugated expansion joint itself has no self-restraining ability, and the bellows will produce a great axial thrust under the action of pressure, which is all borne by the main fixed bracket. In other words, if the bracket can't hold it, the expansion joint will be installed in vain. Two days ago, I met a customer. The steam pipeline of DN500 in the workshop was pressure tested, and the blind plate was sealed. The general corrugated expansion joint tie rod next to it was directly bent into a bow, because the bracket was not designed according to the thrust.
Straight pressure balanced expansion joint is different. It relies on its own working bellows and balanced bellows to offset the internal pressure thrust. The blind plate thrust is mainly carried by the equipment itself, and the requirements for fixed brackets are much smaller. The double hinge transverse expansion joint bears the internal pressure thrust by the tie rod, but it can only absorb the transverse displacement, and the blind plate thrust will still be transmitted to the bracket. The structure of external pressure single axial type expansion joint determines that it has strong external pressure capacity, but it can't reduce the thrust of blind plate.
This expansion joint you used, can it withstand blind plate thrust, or can it only absorb displacement? This question is asked wrong, and there is all trouble behind it.
Several Key Corrections in Practice
The test pressure tends to be higher than the operating pressure. For pipelines with a design pressure of 1.0MPa, the hydraulic test may be 1.25 times or even 1.5 times. At this time, the blind plate thrust must be calculated according to the pressure test pressure, not according to the operating pressure. It is quite safe to calculate according to the operating pressure. When you press it, the bracket will be wasted directly.
The temperature correction cannot be ignored either. When high-temperature pipeline is running, the allowable stress of materials will decrease. If the allowable stress at normal temperature is used in the calculation of blind plate thrust, it must be reduced accordingly under high-temperature working conditions. How to take the safety factor? It depends on the type of bracket-the fixed bracket is generally 1.5, and the guide bracket and sliding bracket are 1.1 to 1.2. Don't blindly enlarge it, and don't stiffen a coefficient. How much should be taken according to the actual working conditions.
Example: A DN400 steam pipeline
Assume a DN400 steam pipeline with a design pressure of 1.6MPa and a hydraulic test pressure of 2.0MPa. The inner diameter of the tube is about 400mm, the middle diameter of the sealing surface of the blind plate is 410mm, and the area is about 132000mm². Calculated at 2.0MPa:
F =2.0×132000=264000 N, Approximately 26.9 tonne force.
That's not a small number, right? If a general-purpose corrugated expansion joint is installed on the pipeline, the main fixed bracket must solidly hold the thrust of 26.9 tons; If replaced with straight pipe pressure balance type expansion joint, the force of the support can be reduced greatly. This comparison tells you directly: the calculation of blind plate thrust is not just a mathematical problem, but directly determines the bracket design and expansion joint selection.
Three of the easiest things to overlook on the scene
First, is the temporary bracket near the blind plate strong enough during pressure test? Many temporary brackets are welded with several steel pipes, which look quite stable, but when calculated as thrust, it is completely insufficient. Don't save this.
Second, is the limit device that comes with the expansion joint adjusted in place before the pressure test? The tie rod of the double hinge transverse expansion joint is loose when it leaves the factory, and the nut must be locked tightly before pressure test. If you don't lock it tightly, the pull rod will push the bend directly during the pressure test, and it will be too late to cry.
Third, the blind plate should be positioned a little farther away from the expansion joint, so that the thrust can first pass through the natural corner of the pipe to digest a part. If it can't be avoided, it is necessary to add a temporary fixing bracket between the expansion joint and the blind plate.
Calculation method of blind plate thrust of expansion joint? After all, F = P × A, the formula itself is worthless. What is valuable is that you know when to use which P, which A, and who exactly carries the calculated force. Understand these, and then talk about the design of the bracket and the selection of the expansion joint, so that it is not easy to go wrong.