A vacuum system that still pulls vacuum after a service but does not reach the level it used to is a typical situation. The search for the cause usually turns towards the pump, while the real reason is often in the reassembly. Taking a joint apart is not difficult, what is difficult is putting the machine back the way it was. This article covers the three steps that are most easily left out and what each of them costs when it is.
The first step: new sealing parts, fitted the right way round
The sealing faces of an opened flange have to be cleaned, and no traces of the old sealing material, fibres or sludge may be left on them. The sealing parts themselves should be renewed on every reassembly, and especially on pipe runs that have run hot, because rubber parts harden and take a permanent set, so that they look pressed in when they are fitted back while the contact face has actually changed.
Fitting a sealing ring the wrong way round is the mistake that is most easily overlooked. A lip seal turned back puts the lip on the pressure side, the sealing face looks closed and is in fact being pushed open, and the joint may still hold under positive pressure while it leaks gas in on the negative side. This kind of defect is not always visible after the joint has been opened.
The second step: bolts tightened in order and in stages
A flange is sealed by the whole contact face sitting flat, and tightening the bolts in one go or in the wrong order leaves one end tight and the other end open, so the seal fails. The correct method is to work in a diagonal sequence, bring every bolt up to the specified torque and then go round again until the whole face is loaded evenly. The torque is the one given on the drawing, because bolts of different sizes take different torques and the value cannot be judged by feel on site.
The joint has to be checked once more after tightening. Running under vibration can change the bolt torque, and the position of a flange face is not exactly the same hot and cold, so a second check keeps this from reaching the machine the next time it goes into service.
The third step: a staged pressure test after reassembly
A finished reassembly is not a reason to start the machine. Before the system is pressurised, do a staged test: isolate the sections one by one, draw the system down close to its limit vacuum and hold it there, watching whether the reading stays steady. The limit vacuum of the ZYD-100 is ≤5 Pa and the working vacuum is -0.08 ~ -0.0999 MPa, and if the reading falls back clearly during this test, some part of the reassembly did not come out right.
The test can be combined with the decay after the pump is stopped, which shows the state of the seals directly: in a well sealed system the reading falls back gently and stays high for a long time, while a system with a gap falls back noticeably faster. Used together the two methods point at the specific section of pipework that is responsible.
What the three missing steps look like
| The step that was left out | What is seen on site | What to check after the reassembly |
|---|---|---|
| Sealing parts not renewed or fitted back the wrong way | The vacuum falls back slowly, more clearly at higher pressure | Stop the pump and run a decay test, look at the rate of the fall |
| Bolts tightened out of order or to the wrong torque | The vacuum will not come up and the limit vacuum does not reach ≤5 Pa | Open the joint and check the contact face and the tightening order |
| No staged pressure test after the reassembly | It only shows up later in service and the figures fail intermittently | Draw each section down and hold it before the machine goes into service |
Keep the quality of the reassembly in the record
The service record should carry the reassembly as its own entry: how many flanges were opened, which sealing parts were replaced, how the tightening order was executed, and what the readings were after the work. The gas content limit of the ZYD-100 is ≤0.01% (GB/T 423), and a joint that is not sealed properly affects the vacuum level and the gas content at the same time, so afterwards there is nothing to fall back on except the record.
Opening a joint is easy, the hard part is making the system behave after it is closed as it did before it was opened.
