7 replies to this topic

[Keith84]

This is going to be a pretty general request, and I may sound a like a student asking it, but here goes.

 

I'm looking for recommendations for resources (books, websites, papers, etc.) that really focus on the design of control loops in a system.  The actual control schemes that you can put right on your P&ID for illustrating the methods for controlling a chemical process system.

 

I own several books on control and took the a controls class in college, but everything seems to be designed more for EEs than Chem Es and MEs.  The books usually jump right into details like dynamic response curves and performing Laplace transforms and PID tuning and all of that.  Now, I don't deny that this is important for the actual tuning of the control system, but there's rarely any discussion of what you're measuring, what you're controlling, how you're controlling it, and why you're controlling it a certain way.  And yet, as a chemical engineer, these are the things you must know to properly draw the P&ID for your process.

 

So I'm looking for something that's more geared towards chemical and mechanical engineers that have to come up with the control philosophy of the system.  There are innumerable situations and questions that need answers, and I'm not expecting them here.  Some examples off the top of my head might be:

 

 

Do all two phase vessels require level control and in what cases does it need to be continuous adjustment and in what cases does can it just be handled by emptying and filling the vessel based on high and low level switches?

 

How do you control the composition of the products of your distillation tower?  You could add more or less heat to the reboiler or change the condenser conditions or modify the split on the reflux control valve or any combination of these or even some other scheme.

 

Where is it preferable to use a three way control valve instead of two separate control valves?

 

Where should pressure and temperature be monitored on various compressor setups, and how should these measurements affect the control (via internal or external bypass or possibly via VFD) of said compressor?

 

Here's one from a current customer: where is the best location to put a temperature control valve for maintaining a specific temperature out of a cross exchanger?  They have their thoughts, and mine are a little different.

 

Must a higher pressure line going into a lower pressure line always have a pressure control valve or regulator to drop the higher pressure to the lower pressure?

 

And then of course there's the whole topic of alarm points and safety/shutdown responses, which is a probably as big a topic as the normal operating control of a system.  Startup control is yet another unique area that can't be ignored.

 

 

I could probably spend days coming up with reams of questions like this, and I know that most, if not all, of these are quite general or maybe some are even misguided.  But that's exactly my point: it's hard to find solid answers to a lot of these and similar questions.  I can read and write P&IDs and come up with control loops that I believe make sense.  That's based on various amounts of experience I've picked up and reading I've done along the way in my short career.  However, there are always questions in the back of my mind as to whether we've got an optimal arrangement or if we've got valves in a suboptimal place or if we've missed an alarm point or interlock.  There are always other eyes (my boss, other engineers, the customers, HAZOP guys) looking at the P&ID, but I don't have total faith in any of them, so the questions remain in my head.

 

Given the long history of chemical processing equipment and plants, there's got to be something out there that directly discusses the control philosophies for various common scenarios and pieces of equipment.  There's also got to also be plenty of practical advice for process control system design.  Does some compendium of knowledge like this exist?  Kind of like a Perry's, but a comprehensive collection of just control scenarios.  Or perhaps there's a dozen different places I should look to fill out the picture.  We all know what the common parts of any plant are: compressors, pumps, towers, separators, reactors, heat exchangers, the piping between all of these things, etc.  All plants have their unique aspects, but most of these control scenarios should be fairly well-established, I have to think.

 

I'd appreciate any recommendations that you all can give me.  Thank you.

[Bobby Strain]

Try this.  http://www.pc-education.mcmaster.ca/

 

Bobby

[breizh]

Keith84,

A few documents /links to support your query.

 

http://www.s88control.blogspot.fr/

 

http://www.controlgu...008/082408.html

 

http://www.driedger.ca/

 

 

Good luck

Breizh

Edited by breizh, 29 July 2014 - 09:35 PM.

[kabtik]

Do all two phase vessels require level control and in what cases does it need to be continuous adjustment and in what cases does can it just be handled by emptying and filling the vessel based on high and low level switches?
1. Not in all cases. In cases where the quantity of the denser phase is relatively small then switches can be used. We have a knockout drum in the plant upstream of a wet gas compressor and all we have are level switches. When the level gets to 50% the pump comes up to drain the drum. For instance if you have 100,000 bbl/day of crude coming to a separator and it carries just about 10bbl/ day of water, you don't want to put a sort of too elaborate a control scheme to handle that. Simple switches and a small pump that runs perhaps just a couple of times a day will do.
2. One way I think you can learn faster is to look at existing designs and imagine what could happen if it was done other wise. For example you may ask why is that the level control valve of a surge drum whose basic functionality is to guarantee stable flow to a downstream unit placed upstream of the drum. Imagine placing the control valve downstream of the drum. What happens

Edited by kabtik, 29 July 2014 - 09:58 AM.

[Art Montemayor]

I fully agree with your statements that most instrumentation books seem to dwell and concentrate on stuff like dynamic response curves, performing Laplace transforms, and PID tuning.  To get directly and practically into the “meat” as to how a process is actually controlled with hardware and logic, I have found that two books on Process Control that deal with these specifics and which I have always referred to and continue to recommend are:

• Process Control Systems; McGraw-Hill; 3^rd Ed.; ISBN 0-07-056903-7
• Distillation Control; McGraw-Hill; 2^nd Ed.; ISBN 0-07-56894-4

Both are by F. Greg Shinskey, who I consider the best author of Process Control Articles and texts.  I’ve also read a lot of papers by Bella Liptak, who is also very experienced, but I feel Bela lacks the simple communications that Shinskey champions.  Greg led the Foxboro Engineering staff for many years and is acclaimed as an expert but, more importantly, as a very practical engineer and excellent communicator.  I suggest you try these texts out.

[Keith84]

I sincerely apologize for not getting back to this thread sooner, but I greatly appreciate every recommendation.  I think my dad actually has the first edition Distillation Control book by Shinskey, but I'll see what I can do about finding the most recent versions Art mentioned.  Right now I'm furiously reviewing the P&ID and layout of our most complicated plant ever...with the customer here.  I can tell as I go through that there are better ways to implement many of the control schemes, and once I can get some time to breathe, I intend to check all of the recommended resources here to figure out the best way of modifying things for our plant.  Again, thank you very much, everyone. 

[sheiko]

Hello,

I can recommend you a book from Norman Lieberman "Troubleshooting process plant control".

[Amin Va]

I recommend the book: "Process Instrumentation Applications Manual" by Bob Connel, written in a very plain and understandable language without going into detailed math.

Edited by Amin Va, 29 November 2014 - 10:43 PM.