11 replies to this topic

[djack77494]

process,
Your question does not make sense. I suspect you do not fully understand the concept of settle out pressure. It has nothing whatsoever to do with fire or even relieving conditions. Please do some research on the meaning and come back if needed after that.

[astro]

Agree with djack and would recommend that you study API STD 521, especially section 4 that reviews design philosophy and relief contingencies.

The concept of settle out pressure is relatively straight forward. I found a brief run down of a methodology to determine it here (I haven't gone through the method in detail but from a quick scan it seems to make sense):
Simple Settle Out Pressure Method

In section 4 of API STD 521, I would also recommend you to take particular note of the sections discussing double jeopardy and latent failures. I get the impression your query involves these concepts.

If you're still having problems, then set out your issues and concerns with appropriate detail and associated working that you can attach here. Then we'll be able to offer further guidance if needed.

[fallah]

Just a correction: the pressure of different parts of the system and their relevant volumes are to be used for determination of settle out pressure (not settle out volume).

Going back to your question: in my opinion, maximum operating pressure of each part should be used for calculating settle out pressure.

[astro]

I would point out that it's difficult to respond to a post where the formulation of the question is not entirely clear. It's a 2 way street, but the onus is on the originator to start the dialogue well, otherwise all that results is confusion.

You also appear to be asking for an absolute answer to a somewhat vague question, which invariably will result in a response of "it depends". I indicated this previously and will expand further.

If you take the time to review the standard mentioned, I'm sure that all will become much clearer and the key here is what's the basis of your problem and directly associated with that is, what is the operating, shut down and relief philosophy that you're applying to it? Once you've determined this then you can set out credible scenario(s) and use the pressure profile that matches the associated process conditions in your calculation. If you simply take maximum values without considering what is credible, then you run the risk of over design and this can result in incurring unnecessary cost.

The volume associated with a settle out pressure calculation is taken from the system's limits. Given that you've not provided the forum with any detail of your system's description, it's difficult to provide anything other than general advice. Typically though, these limits are determined by a system's shut down valves or any other mechanism for blocking it in (that fits with the scenario under study). The task is then to estimate the volume of vessels and pipework between those limits.

[fallah]

fallah ,
in both the cases , do we have to take the maximum operating pressure?

In my opinion,yes.

[JoeWong]

how to calculate settle out pressure
case 1 when compressor are working normally and the system falls outside the fire zone
case 2 when the compressor system is on fire

Compression system settle out will occur whenever compressor is shutdown, SDV closed followed by opening of anti-surge control valve REGARDLESS of

i) the compressor is inside or outside of fire zone
ii) the compressor is on fire or not.

Whenever fire occur and detected, the compressor will shutdown (and followed by depressuring if this is provided). Compression settle out will take place when the compressor is shutdown.

to calculate the settle out volume do we have to consider the normal operating pressure of suction scrubber , after cooler and discharge scrubber or do we have to consider the high high pressure

What you need to analyze is the scenario is CREDIBLE for suction scrubber reach high-high pressure (understood as PAHH, pressure just before it trip the compressor), add in the compressor head within the compressor operating curve and reach PAHH on discharge. If credible, you may have to consider.

CREDIBLE here includes the possibility and risk of getting
- PAHH at suction scrubber
- PAHH at compressor discharge
- temperature trip does not trigger
- other trip i.e. speed, oil temperature, vibration, etc do not trigger
- operator sleeping and ignore the alarm (this involve in risk assessment...)
.
.
.

[ethylene]

joe

could you please further explain why the anti-surge control valve is opened following compressor shutdown.

thanks

[JoeWong]

Safe shutdown in my mind...