18 replies to this topic

[chemks2012]

Dear all,

I have one query for you about pressure relief.
 
One of our clients has 10” BD set at 2.75barg  already installed on the reactor [reactor is designed for 6barg to full vacuum] and I have to verify whether it is sufficient for their new process requirement or not.  I have considered one case of two phase flow when steam leaks through the reactor internal coil resulting into two phase flow through this BD. The BD seems OK if steam leaks abruptly by steam coil failure normal flow rate [i.e. without any failure of pressure reducing valve on steam line]. However, if PRV fails, steam flow reaches to the max level and BD seems slightly smaller. As this is the case what would be the best cost effective solution? I am thinking of two options
 
Option#1    Increase the BD size – which is not very cost effective as they have to replace the BD and nozzle [?]
Option#2    Increase the set pressure of BD – It seems even if they set the BD at 3barg, it will be sufficient. I don’t know if I am missing here [?]
 
Also, input on following will be helpful
 
1.       Also, this reactor has also another over pressure protection i.e. relief valve set at 2barg. I am wondering if there is any thumb rule for the set pressure for two devices [here, bursting disc and pressure relief valve] are installed independently. I am asking this because to me set pressure for both devices are too low while vessel is designed for 6barg. OR am I missing anything?
 
Your help will be much appreciated
Thank you!!

[fallah]

chemks2012,

Seems the reactor's overpressure protection system to be designed be able to cope with pressure spike.

Anyway RD could be set at (or below) 6 barg, where the relief load requirement to be met and if still not to be met you should increase the RD size.

PSV as first layer protection could be set a little bit below RD set pressure in order to minimizing its size.

Fallah

[pallavikhatri]

Dear All

I have a rupture disc installed at the upstream of a relief valve.The line size for the inlet to the relief valve is 6" and the orifice size for the relief valve is 4M6. I want to know what will be the size of the rupture disk? Will it be same as the size of the inlet line or will its size be equivalent to the orifice size of the PSV?
I also want to know how the PSV and rupture diska are installed? Are all the PSVs and rupture disk arrangement close coupled?

Thanks in advance

Pallavi

[ankur2061]

Pallavi,

Rupture Disk will be the same size or bigger than the PRV (for low pressure applications) inlet connection and / or line size.

When a rupture disk device is used between the pressure relief valve and the protected vessel, a pressure indicator, bleed valve, free vent, or suitable telltale indicator should be
provided to permit detection of disk rupture or leakage.

Another stipulation for installing a rupture disk below a PRV is that it should be a non-fragmenting type rupture disk.

For installation of a rupture disk in combination with a PRV refer:

"Figure 7 - Typical Rupture Disk Assembly Installed in Combination with a Pressure Relief Valve"

API STD 520 - Part II - Installation

Hoe this helps.

Regards,
Ankur

[fallah]

Pallavi,

The rupture disc upstream of a PSV has to be the same nominal pipe size as the PSV inlet or larger.

"CLOSE-COUPLED" is the most common installation method of a rupture disc in combination with a PSV in which RD holder is mounted directly upstream of the PSV. But care should be taken to insure that there would be sufficient clearance to allow the RD to open without blocking the nozzle of the PSV.

Fallah

[pallavikhatri]

Thanks Ankur and Fallah for the reply.
Can you tell me what is the difference between closed coupled rupture disk and pad flanged rupture disk?

Thanks in advance
Pallavi

[fallah]

pallavi,

Do you mean by pad flanged?

Please note that the discussion relates to installation methods of RD/PSV combination.

As far as i know other than "closed-coupled" installation of RD and PSV combination, there is another case in which RD holder and PSV would be separated by a spacer or pipe spool with 1 or 2 pipe diameters in length.

Fallah

Edited by fallah, 09 April 2012 - 01:13 AM.

[pallavikhatri]

Fallah

Actually at present i am working on a petrochemical unit and in that plant it has been mentioned that the the rupture disk should be pad flanged.
Can you tell me when do we use closed coupled rupture disk with PSV and A spool piece between PSV and rupture disk?

Pallavi

[fallah]

it has been mentioned that the the rupture disk should be pad flanged.

pallavi,

It is mentioned by whom/in where? Ask the source of the mentioning for further clarification and submit the clarification, if any, for our information.

Closed-coupled configuration is a good installation method especially in where there is no space for considering spool pipe but care should be taken in the case of using single petal RD due to its potential to block the PSV nozzle.

Fallah

[Lowflo]

Pallavi - as the orifice gets larger, it's inherently more difficult to keep the inlet pressure loss within the allowable limits. That's because orifice sizes increase disproportionately high as compare to the increase in valve body size. That's a real problem for API 526 valves. About the only time you can use a same-size disk is when the PSV orifice is a D or E. With an M orifice, it's almost a certainty that the disk will have to be one size larger than the PSV. Otherwise, you'll have too much inlet loss.

Before deciding where to locate the disk relative to the PSV, consider how you will meet the code requirement for detecting disk leakage/failure. That's normally done witha pressure gauge or pressure transmitter. If you "close-couple" the disk and the PSV you'll have to install the pressure sensor on the small threaded hole in the disk holder. Personally, I think that's a bad design from a mechanical perspective - it's too small and can easily be broken off. Consequently, a short spool piece between the disk and PSV is better. That allows for a mecanically strong nozzle for the PI or PT.

A "pad flanged" nozzle is one without a neck. In other words, there is no nozzle projection. This is used when there's a risk of solids accumulation/formation in the branch connection. Disk manuafcturers can supply a tee with a "pad flanged" branch connection, on which the disk mounts. The face of the disk is essentially flush with the fluid passing through the pipe. So, determine if your service requires such an installation. If not, there's no need for the extra cost.

[pallavikhatri]

Dear Lowflo
Thanks for the information. So in pad flanged rupture disk there is no need to make the rupture disk closed coupled with the safety valv and if there is a spool piece between the rupture disk then what will be the size of that spool piece?
Can you provide me a sketch or some reference of the pad flanged nozzle connection.


Thanks in advance

Pallavi

[Lowflo]

A rupture disk can be located anywhere within the PSV inlet line. The location doesn't matter. All that matters is the inlet losses.

[fallah]

what will be the size of that spool piece?

pallavi,

Normally one or two pipe diameters in length is adequate. Longer pipe length may cause reflective pressure waves reclose (or fragment) the RD petals, while this wouldn't occur in short length.

One more thing, seems with mentioning "pad flanged" you refer to the type of connection for venting the space between RD and PSV by using "Pad Type Flange", otherwise the installation of RD itself upstream of a PSV could be done by: 1) Preferably "Close-coupled" configuration, and if there are limitations 2) Using spool piece inbetween.

Fallah

[pallavikhatri]

Thanks Fallah and lowflo for the valuable information you have provided.
@Fallah: By mentioning pad flanged i doesn't mean the venting space between the rupture disk and PRV infact i meant the connection of rupture disk on the line i.e. as lowflo explained there will not be any nozzle projection.

Pallavi

Edited by pallavikhatri, 11 April 2012 - 09:54 AM.

[fallah]

infact i meant the connection of rupture disk on the line

pallavi,

Seems you are installing a combination of RD/PSV on a process line not on a vessel. Am i correct?

Fallah

[pallavikhatri]

Fallah
Yes you are right we are installing a PSV/RD combination on a line as well as on the vessel

Pallavi

[pallavikhatri]

Dear Lowflo
I have one qusetion related to closed coupled rupture disk. My operating fluid is of polymer service and in my case rupture disk is provided at the upstream of the PSV. My question is, is it necessary to provide the ruture disk closed coupled with the PSV or we can provide a spool piece between the rupture disk and PSV? I understand that if we provide a rupture disk before a PSV and a PT is present at the rupture disk discharge then alos we can provide a spool piece between the rupture disk and PSV even though its a polymer service and there are less chances that the spool piece may get clogged in case the rupture disk leaks.

Regards
Pallavi

[Lowflo]

Pallavi - the disk doesn't have to be "close coupled" with the PSV. If you're concerned about plugging the line, then install the disk on the pad flange. There's no requirement, or need for one, that specifies the space between the disk and the PSV.

[pallavikhatri]

Dear Lowflo
Thank you so much

Regards
Pallavi