5 replies to this topic

[David Southall]

Hi,

I just wanted to say thanks to Phil (Leckner, if you mind me using his first name) for the help you gave me regarding my post on the old message board.

I checked out what you said and you're absolutely right, I have to set the relief valve at 10% accumulation, with it being permissible to set a second device at a pressure up to 105% of design pressure. I'm going to size it at a pressure higher than that of the valve.

You've just prevented me from handing in a project with incorrectly sized relief devices; so once again, thank-you very much.

Regards,

Dave.

[pleckner]

Dave, you are welcome and you can use my first name anytime (and so can everyone else).

But I think you mis-understand (or I hope you just mis-spoke (or mis-wrote):

1. You SET the relief valve at the design pressure (or MAWP).

2. You SET the rupture disk at 105% of design pressure (or MAWP).

3. In general and normally (BUT NOT FOR YOUR SPECIFIC CASE) you CALCULATE your relieving rate based on 16% accumulation for both devices, not 10%! You only use 10% accumulation when only one relief device is being used in the system and only for a non-fire case.

4. If you are sizing for a fire case, you CALCULATE the relieving rate using 21% accumulation for both devices.

So, don't get confused between the SET PRESSURE and the RELIEVING PRESSURE!!! The set pressure is the point the device is activated. The relieving pressure is the set pressure PLUS the allowable accumulation.

FOR YOUR SPECIFIC CASE where the rupture disk is really sized for runaway and the relief valve is used for everything else, I would size each device as if they were the only device in the system. By this I mean use only a 10% accumulation for each non-fire sizing scenario (control valve failure, blocked-in, loss of cooling water, etc.). For a fire scenario, calculate the relief valve size at 21% accumulation.

In reality, during the runaway, both your relief valve and rupture disk will activate so you really could take credit for both during this scenario and size based on 16% accumulation. As I said above though, I wouldn't and just size the rupture disk as if it was the only device in the system.

One last thing...your relief temperature must be coincident with your relieving pressure.

I hope this helps and I hope this gets to you before it is too late.

[David Southall]

Hi Phil,

that's great, thanks. I thought that was what you meant, but you've clarified it now. I'm assuming you say to treat each device separately because the valve is in effect almost acting 'on its own', because it is at the design pressure and handles most relieving scenarios, but for runaway the disc could be imagined to be handling it 'on its own' again, although it may be worth looking at sizing both based on 16% overpressure in some cases, but not this one (in your opinion). I know I perhaps haven't phrased this very well, but how I imagine things in my own head is a little different to how I'd explain it if I had to give someone an accurate explanation!

I'm just reworking my calculations now (based on 10% overpressure for the valve, 105% of design pressure for the disc, 21% accumulation for the fire case for both), to submit my project this coming Friday (doesn't it take forever to type up calculations in Word? Taking longer than it did to actually do the calculations).

Thanks again for the help and advice, it has been invaluable!

Best Regards,

Dave

[pleckner]

Dave,

OK, I think you fully understand how to handle the relief valve but let's try the disk part one more time. By what you wrote {"I'm just reworking my calculations now (based on 10% overpressure for the valve, 105% of design pressure for the disc,..."} , I'm not confident you fully understand what to do with the rupture disk.

You SET the bursting pressure at 105% of design pressure but you use 10% overpressure in calculating the relieving rate. The 10% accumulation is 10% above DESIGN pressure, not the bursting pressure in this case.

For example, for a 100 psig design pressure, you SET the bursting pressure at 105 psig but you CALCULATE the relieving rate at 110 psig. If the disk and relief line is getting too big, try using 116 psig when calculating the relieving rate (16% overpressure). If it makes a significant difference, then use 16% overpressure.

The Code allows this 16% overpressure if BOTH deivces are expected to be in operation for the same relieving scenario. In your case, both the relief valve and rupture disk will pop/burst for the runaway so the 16% overpressure is allowed.

[David Southall]

Hi Phil,

Yes, that's how I went about sizing the disc. Thanks for all your input on this, I've finally got my project in and I'm very pleased with it (if I do say so myself).

Thanks once again for all the input, and I did make sure to reference you and these pages (I also purchased one of the vertical pressure vessel data sheets to give me some ideas for preparing my own).

Thanks once again for the help, and for having a great website.

Best Regards,

Dave.

[pleckner]

You are welcome and it is our pleasure!