Relieving Flow rate for two systems of different design pr connected through Block and bleed valves

[samya]

We have a case where one ethylene sphere of max. operating pressure of 21.6 kg/cm2g (and Design pressure of 24.0 kg/cm2g) are connected through vapor line with an Ethane sphere of operating pressure of 11.0 kg/cm2g (Design pressure of 16.0 kg/cm2g) though double block and bleed valves along with spectacle blind (normally closed).

Also we have a trip interlock based on single PT on PAHH. Do we need to consider operator error (spectacle blind in open position) and isolation valves are passing and resultant overpressure causing a relief scenario for lower pressure Ethane sphere system ?
In case it is required, how to arrive at relieving flow rate ?

Thanks in advance.

SC

 

[TD2K]

Do you have a sketch of the system?

You say you have a single trip on PAHH, what does this trip do?

If the trip is to prevent the ovepressure of the lower pressure sphere, a single transmitter sounds insufficient? Have you done a LOPA analysis and SIL analysis.

< Do we need to consider operator error (spectacle blind in open position) and isolation valves are passing and resultant overpressure causing a relief scenario for lower pressure Ethane sphere system ?>

What I've seen typically depends on the rules set by the customer. Are the valves locked or car sealed closed? Is this just a normal position position shown on the P&IDs and controlled by procedure? How much credence do you take for the operating procedures ensuing those valves and spectacle blind is in the intended position?

What happens if the valves passes or inadvertently opened? From what you've posted, the lower pressure sphere could go up to the operating pressure of the high pressure sphere depending on the PSV capacity on the low pressure sphere, whether you use operating or design in the high pressure sphere and the capacity of the interconnecting piping. That will give you a steady state result, in reality, depending on the volume, you may have to look at a dynamic study.

It looks like worse case you would overpressure the low pressure sphere to 21.6 kg/cm2 which is about 35% above the low pressure pressure sphere design pressure, less if you use maximum operating pressure. The low pressure sphere isn't going to likely fail at that overpressure but you'll have to evaluate it if it's acceptable. I can't say I would want to sign off on this design just based on what you've provided.

 

[LittleInch]

I echo TD2K's points. Connecting two different design pressure rated systems is always a significant issue and requires careful study to understand how they could be connected, what pressure regualting and safety systems ar ein place and what relief systems are required.

For any vessel or pressurised equipment, you need to work out how it could be overpressured, what is stopping it becoming over pressured and what the relief flowrate would be. A forum like this is no way to do that, only establish the basic principles.

Personnaly I always work on the basis that if there is a hard connection like you apparently have, then it is possible for the two to become connected, whether by accident, incorrect procedure or equipment malfunction.

Questions like this often only give us a small snap shot of the whole operating issue and quotes like "a trip interlock based on single PT on PAHH", clearly mean something to you who knows and understands the entire operation, but are meaingless to me without the P &ID, the C&E chart and the operating philosphy / procedure.

So in summary my response ot your questions are:
Yes you need to consider this
Work out the max flow rate that can transfer from the high presusre vessel to the lower one including the feed supply of the higher vessel.

As said above it looks on the info you supply that this is simply not a good idea which will cause you lots of problems and I currently can't understand why you need it or what advantage it gives you compared to the risk of overpresusirising your lower rated sphere.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[don1980]

Samya - Scenario decisions are intentionally left to the user. So, rather than asking what is required, the correct question is what is needed, based on standard industry practices and based on the owners risk tolerance.

API 521 explains standard industry practice for scenario evaluation. A credible scenario is one that involves a single identifiable failure. Base on your post, the potential failure in this case is one of mis-operation (human failure). Multiple independent failure (human or any other type) are normally regarded as not credible (they're called double jeopardy).

In evaluating this case, consider whether an open connection between these spheres is a single or multiple failures. There's no objective right answer to this question. It's a risk management decision. Personally, it looks like a single failure, and I'd want the low pressure sphere PSV to be sized for it.

One of the factors in this risk analysis is the consequence of being wrong. In this case the consequence is very severe. That should cause you to err on the conservative side - further reason for defining this as a credible scenario and sizing the PSV for it.