3 replies to this topic

[afd]

In a recent modification at my unit I observed that a pressurized small vertical water drum is designed for a pressure of 28 bar and the rupture disc setting on it is set for 22 barg. This drum is having water upto its tangent line( where a overflow line with valve is provided) and is pressurized with steam at 18barg.

The water used to fill this vessel comes from a pump operating at 28 barg. I remarked that the rupture disc can blow if there is a operator error when filling i.e. if the operator closes the overflowline valve before closing the filling valve. But it was overlooked.

My question is: is it normal to have a safety device set at a pressure lower than that can be experienced by the process?

afd

[pleckner]

The short answer is "yes/no/maybe". The designer can set the relief device at any point up to design pressure or MAWP (if this value is available). There are many reasons why you may want to set the rupture disk to burst at a lower pressure than the vessel design pressure but only the designer can tell you why he/she is doing this for your particular vessel.

For this vessel, is the rupture disk right on the vessel nozzle or is it some distance higher? If the rupture disk is higher, than you must take into account the liquid static head on the vessel; meaning the the rupture disk would have to have a lower burst pressure than the vessel design pressure because by the time the rupture disk were to burst the pressure in the protected vessel will be higher because of the liquid level from the vessel to the rupture disk.

Another reason for introducing a discrepency between design pressure and burst pressure would be if there was a potential run-away reaction scenario; but obvioiusly this isn't an issue in your case.

[afd]

Phil,

I agree with you that it is upto the designers discretion to set the burst pressure somewhere betweent the operating and the MAWP.

The RD is right on top of the vessel nozzle.

My concern is the vessel is being filled from a pump whose operating discharge pressure is greater than the vessel RD and equal to the design pressure of the vessel.

Suppose during filling the operator makes a mistake closes the overflow line valve before closing the make up valve then the RD will burst if the vessel becomes full.

[pleckner]

Your question was:

...is it normal to have a safety device set at a pressure lower than that can be experienced by the process?

and I explained that.

But let's now look at your concern.

Without having any of the necessary details, I have to assume by the way you describe the system, that this is a horrible deisgn!! The design pressure should have been a minimum of 10% or 25 psig (whichever is higher) above the maximum expected operating pressure. Perhaps the designer determined that even though the pump can put out 28 barg of pressure, the maximum expected pressure in the tank would be lower at all times. But this is the wrong thinking. The design pressure should have been determined by using the criteria I give above and compare that to the shut off pressure of the pump. If the pump operates at 28 barg, then the shut off pressure would be expected to be higher and this might have dictated the design pressure of the vessel.

Now, there may be another reason to set the ruputure disk at this lower pressure. If this is a liquid only relief case, and liquid is not very compressible as we know, the rupture disk will probably not burst at exactly 22 barg but at some higher pressure. What this higher pressue really is I don't know and I'm not sure even the vendor can tell you....or perhaps they can with some proprietary computer program. So maybe the designer was told by the vendor to set that burst pressure at 22 barg based on the design pressure of 28 barg and the fact that this is a liquid only relief (if it is).

All-in-all, you have a valid concern for this system as you describe. And rupture disks are not the greatest relief device to use in an all liquid relief case. I would prefer a PSV first or even a rupture pin.

By the way, I'm confused as to how you can pressure this vessel with steam if the overflow line is opened? Obviously I'm missing something here. Do you pressurize AFTER you fill the vessel and it is basically blocked-in?