Safety gasket in steel mill rolling stands issue

[Mech2325]

Hi everyone

I am writing this post with regards to a problem being encountered with one of the safety gaskets in automatic hot rolling mill stands.

Are there any rolling mill engineers here so that I may explain the issue in detail??

 

[Compositepro]

Explain away. We will try to help as best we can.

 

[MikeHalloran]

This one, I gotta hear.


Mike Halloran
Pembroke Pines, FL, USA

 

[Mech2325]

The one highlighted in red is a quick release coupling mechanism (QRC)through which the roll chocks (highlighted in yellow) are adjusted. This is a hydraulically operated mechanism utilizing hydraulic oil to adjust the roll gap between two rolls of a rolling stand.

The safety gasket comes with the QRC to prevent the flow of oil during normal operation. This gasket is very thin, about 1 mm. If, during normal operation, the billet temperature is not as hot as it should be (i.e, about 1100 deg centigrade), the gasket goes off releasing the oil and allowing the roll choke to move to release stress on the chocks. When a billet, that is not that hot, passed between two rolls during the rolling process, it would exert stresses on the rolls that are held in place by the hydraulically operated roll chocks. So, the stress would transfer onto the roll chocks. If the gasket does not go off, it would induce stresses on the chocks.

Now, this is absolutely fine.

However, the problem is that, during roll gap adjustment, the roll chocks are to be adjusted. What happens, during this phase is that, as soon as the chocks move to assist in rolls movement, this gasket does not take the pressure and goes off causing the oil to leak.

Is there any way, we can avoid this??

I hope I have explained the problem. If there is anything not clear, do tell me.

Waiting for your replies

 

[berkshire]

This sounds more like a rupture disc than a gasket.
B.E.

You are judged not by what you know, but by what you can do.

 

[Mech2325]

Well, is there any solution you know off?

 

[MikeHalloran]

Why is there a QRC in the circuit?
E.g. are you supposed to disconnect it while adjusting the rolls, so as to protect the rupture disc?



Mike Halloran
Pembroke Pines, FL, USA

 

[SnTMan]

Or maybe a relief valve?

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[zeusfaber]

It seems odd that making offload adjustments generates higher pressures than shoving a hot billet through the rolls does.

Are you using too big a pump to make the adjustments?

Is there a valve somewhere on the other side of the circuit that ought to be open (and isn't) before you attempt to adjust the chokes (working on the principle that the fastest way to overpressurise a system is to pump against a deadhead)?

You could probably come up with a shuttle valve arrangement to blank off the line to the bursting disk whenever oil is flowing into the system from the QRC, but I think doing so would be to invite trouble: If the shuttle gets stuck, the first you'll know about it is when you're trying to understand why a cold billet has destroyed your machine.

A.

 

[Compositepro]

Yes it does sound like your gasket is a rupture disc. My guess is that your hydraulic lines/valves are too small to to relieve oil pressure fast enough when a cold slab hits your rolls and opens the gap or your chocks move to open the gap, so you blow the rupture disc. There may even be a closed valve in your hydraulic control lines when this happens, which prevents pressure relief.

Are the "chocks" wedge blocks, where a steel wedge is pushed between the bearing blocks to adjust the gap? If you move a wedge with a hydraulic actuator while there is high oil pressure holding the rolls together, it is very likely that there will be stick/slip occurring. When wedge slips it moves very suddenly and more than desired. This causes the piston in the cylinders holding the rolls closed to move suddenly, requiring that oil be relived quickly. This oil is already at high pressure, close the your rupture disc rating.

On calendar rolls that i have worked with, the wedges were moved by a manual screw mechanism, which required quite a bit of practice to get a feel for how to do a fine gap adjustment. As the screw was very slowly turned, you could feel the torque build until the wedge slipped and there was a distinct decrease in torque. A hydraulic actuator would conceal this effect.

The solution is to place a spring loaded relief valve(s) very close to the hydraulic cylinders and have it relieve back to the oil reservoir through adequately sized hose or pipe.