9 replies to this topic

[BillyRainbow]

Good day.

We are going through a risk based inspection exercise to determine if all the equipment on the plant is properly protected against overpressure, and are struggling to determine why we don't have to install a relief valve (RV) on the steam side of a distillation column reboiler. I have never seen a RV on the steam side of any reboiler.

The configuration is as follows:

Process fluid on tube side and steam on shell side. The steam flow rate is controlled by a fail-close control valve. The column has a RV sized for fire case, taking account of the inventory in both the column and the reboiler.

My question is:

In the event of a fire there is a high likelihood that the instrument air to the control valve will fail and the control valve will fail close. This seems to constitute a causal relationship, ie. not a case of double jeapardy, but I have no idea how long it will approximately take for the instrument air to fail.

The steam will then be blocked in between the control valve and the steam trap and the pressure will increase above the design pressure due to the temperature increase from the fire.

Does the ASME/API standards require a RV in this case?

[Bobby Strain]

In all my years in the business, I never heard this one. API is only recommended practice. ASME doesn't recommend anything specific; the equipment should be adequately protected. Seems not to be a problem to me.
Bobby

[fallah]

In all my years in the business, I never heard this one. API is only recommended practice. ASME doesn't recommend anything specific; the equipment should be adequately protected. Seems not to be a problem to me.
Bobby

Bobby,

Some API's (at least 521, 520,...) are now standards not just recommended practices. On the other hand, if mentioned reboiler's shell is considered as a pressure vessel, it shall be protected by a pressure relief device as per ASME Sec. VIII. In the case you haven't seen RV on the shell side of a reboiler, it might be protected by a common pressure relief device would protect the relevant column at the same time.

Fallah

[paulhorth]

Billy,
I think you are right, there is a fire case overpressure scenario possible on the steam side, due to expanding steam and boiling condensate..
I have not had to deal with steam heated reboilers very much, and those that I have encountered have level control valves on the condensate outlet, not steam traps. These LCVs would usually be fail-open, so in this situation there would not be a blocked-in volume on the steam side. Changing the trap for a LCV would eliminate your problem, at less cost than fitting a relief valve.
A HAZOP of the system should identify overpressure cases like this one, and thereby force the designers to consider how to deal with it.

Paul

[paulhorth]

I would add that the configuration of the reboiler makes a difference. The area exposed to fire is obviously much greater for the shell side than for the tube side, where it would be only the channel and end head. So, with steam on the tube side, as with a kettle or a horizontal thermosyphon, the heat input to the steam from a fire would be quite small. These configurations are what I have usually seen. Nevertheless, if the steam can be blocked in, a check should be made on the effect of heat input from fire.

Paul

[Lowflo]

Billy,

If the reboiler condensate is removed by a steam trap, then I regard that as a vapor-filled vessel, and a PSV doesn't provide any meaningful protection for such vessels in a fire scenario. The shell is a pressure vessel, so it still needs relief protection per the applicable code, but in these case I don't size the device for fire.

If the reboiler condensate is collected in a drum/pot then I generally size the steam-side relief device for fire exposure. That's because you have an inventory of water that will boil during the fire.

All codes require pressure vessels to be protected from overpressure (typically done by installing a relief device) but there are no pressure vessel codes that dictate the sizing basis (scenario) for relief devices. The user is responsible for determining the sizing basis.

[Nomanwer]

Guys,

Just want to know is there any restriction from API on installation of relief valve on reboiler Flange cover? if yes kindly mention reference.

Thanks

Noman

[fallah]

Noman,

Do you mean body flange? Anyway API 520 part II would give you some guidelines might help you out in this regard.

[Nomanwer]

Ends of reboiler are flanged and blind flange is installed on that. Actually relief valve is installed on the vapor return line to column. On upstream of relief valve rupture disc is installed. My client is having problem of bursting of rupture disc. After their investigation they concluded that this problem is due to the stresses. I have done stress analysis and vapor return line is fail due to over stresses. After this i have communicated them to change the place of RV to top of relief valve but one of the guy in their process department said API does not recommend to install RV on Flange cover.

I just need to know this is true or not coz i didnt find any thing like this.

[fallah]

Noman,

In fact, because there wouldn't be any valve between vapor return line and the column, normally both column and reboiler shell side would be protected by the same PSV on the column. Please uplaod a simple sketch of the system for further clarification.
Anyway there is no special limitation in API 520 part II to install the PSV on such blind flange on the top of the reboiler. The limitation might arise from governing ASME code ( may be secVIII).