9 replies to this topic

[Alistairm]

I'm drawing a P&ID for a mol sieve dehydrator system (details found in recent thread: http://www.cheresour...n-for-tsa-unit/)

Can anybody suggest what type of valves should be used in such a system:

Which valve should be used for switching valves? From my research, ball, globe, or orbit valves seem to be the main choice.

Which valve should be used for isolation valves? It seems butterfly valves are the commonly used valve. However, from talking to people it also sounds like a butterfly valve cannot be used for tight shut-off for such a large gas flow (pipe approx 750mm @ ~65bar)

Additionally, as the gas flow is split into two adsorbing units at once, can the butterfly valves be used to help control a 50/50 split into the two adsorbers? I have attached a sample portion of a P&ID that illustrates the situation - The pressure differential across both adsorbers are measured and sent to the DCS. The DCS then controls the butterfly valves to adjust the flow for a 50/50 split. The butterfly valves are also used for isolating the adsorbers.

Symbols for the P&ID:
TSO - tight shut off
PDI - pressure differential indicator
KS - Digital Control System (DCS)

As for additional instrumentation - I intend to put only a composition analyser near the end of the bed to detect water. All other indicators (i.e. pressure & temp) will be outside the vessel.

And just a general question - When representing flanges on a P&ID, when is it appropriate to show also show spade/blind in the flange?

Regards

tonyflow1

Attached Files

•  forum valves.jpg   27.68KB   88 downloads

[Zauberberg]

Hi Tony,

1. For cycle switching valves, opt for Metso/Neles, or Orbit valves. I personally prefer Metso/Neles as my experience with Orbit valves is not so good (leaking); ball-type valves are normally used for Mol Sieve adsorber service.

2. Isolation valves: ball or gate valves. Do NOT use butterfly valves as they certainly cannot provide isolation in gas service, especially at high pressures (we know, we tried, and we have removed them in the plant in Africa);

3. Normally there are no flow-distribution valves in these kind of units. Make sure that inlet piping is as symmetrical as possible, to prevent maldistribution. I believe there is no need for differential pressure control as the flow variations should be marginal - unless you have seriously damaged one of the beds. In that case, you can use isolation valves for temporary flow adjustment but solve the cause of maldistribution and do not treat the consequences - that's the golden rule of design and troubleshooting;

4. Analyzers: moisture content, and CO2/H2S analyzer (optional) as some grades of Mol Sieve remove these contaminants as well. Compositional analyzer for Hydrocarbons is not required (it should be somewhere upstream of Amine Unit to detect C5+ as they will freeze-up in the Crygenic section); apart from these, you will need some pressure, temperature indicators, as you say;

5. Yes, definitely indicate the position of spacers/blinds in the P&ID. Where else can you show them? See attached sample of P&ID for more information.

Cheers,

[Zauberberg]

Sorry, I forgot the sample P&ID. Here it is.

Attached Files

•  P&ID.JPG   130.66KB   201 downloads

[Alistairm]

Thank you for the P&ID.

Would you have a symbol list and the notes to go with the diagram?

I see the system with the 3 switching valves in parallel, but where is the isolation valve system?

It appears that the TDI across the vessel is controlling the switching valves, yes?

Regards,

tonyflow1

[Zauberberg]

Switching valves operation (opening/closure) is determined by the Mol Sieve sequence logic, most commonly on a timer basis i.e. certain time in adsorption cycle then switching to regeneration and cooling, and vice versa for beds being in heating/cooling cycle. Cycle timings is determined by the adsorbent vendor.

Some advanced process control schemes employ calculation algorithms to predict useful life of the bed based on inlet temperature, pressure, and outlet dew point (... = water adsorbed per unit of time), and subsequently extending or shortening active cycles, but this is too unreliable and unpredictable to bi considered as a serious alternative to the common, timer-based sequencing.

Isolation valves are there in the drawing - gate valves. "VT" is the valve vendor code, nothing of special interest.

Try Google-ing around, there is a plenty of reference documents showing common P&ID symbology. A few files attached below.

http://www.pids-a-dr..._PIDSymbols.htm

Attached Files

•  Valves.jpg   81.02KB   42 downloads
•  Process Diagrams.pdf   1.58MB   151 downloads

[Zauberberg]

PIP (Process Industry Practices) have published a Sample P&ID available at: http://www.pip.org/p...celist.asp?p=5#

Also from the IPS:

http://217.174.18.60...pr/e-pr-170.PDF
http://217.174.18.60...pr/e-pr-230.pdf

Edited by Zauberberg, 18 September 2010 - 09:36 PM.

[Alistairm]

Thanks,

All those sources are great for learning P&ID. Students learning P&ID would be very well advised going through these. It took some time but I found that the most extensive symbol list is the one I have attached. I found it gave the most extensive symbol list, which answered my questions. I hope others can benefit from this.

Regards

tonyflow1

Attached Files

•  ISA FUNCTIONAL ID LETTERS.PNG   95.4KB   63 downloads

[Alistairm]

Just a couple of questions:

How come there is only a single isolation valve? Why not a positive isolation system such as a double block and bleed?

I'm trying to write up a cause & effect diagram and a logic diagram for shutdown. Are there any examples out there that I should be aware of?

Regards

tonyflow1

[Zauberberg]

Double Block & Bleed applies for systems rated 900# and above, and for H2S concentrations above 500ppm. The Mol Sieve unit - in your case - does not belong to such category.

As for the C&E charts I doubt you can find something on the public network but in case you have some particular questions, post them here and we'll try to find the answer.

[Zauberberg]

Here are some interesting documents available online and related to real plants/projects:

http://www.charstech...gn_Criteria.pdf

http://boonchoo.org/... Phylosophy.pdf

Regarding DB&B, I believe you will find a plenty of variations as far as pressure/H2S concentration threshold values are concerned. Some standards consider these limits as 600# rating and 100ppm H2S.