A: Gas blanketing is bad practice in general, and particularly bad for distillation columns. In addition to wasting nitrogen, which carries away product out the vent valve, it also decreases relative volatility, making distillation more difficult.
If it can’t be avoided, use a proportional controller with split-range valves. Pressure does not have to be held right at set point.
Greg Shinskey
A: Using an integral function, the system will likely behave as described. Why use it? Your controller probably has proportional with gap function which will allow the pressure to wander a little without opening either valve unnecessarily. Even pure proportional will work with offset between the zero signal on each output. One of the first rules of controller tuning is that two integrators will cause oscillatory response unless they are very different in time constant. Your flow into (or out of) a capacity provides one of the integrators. You don’t need the second one in the controller.
Ian H. Gibson, CPChem, RPEQ, FSEng
Process, Control and Safety Engineering Consultant
Victoria, Australia
A: Split range works fine when it is applied properly. If the operation engineer is suggesting the use of two different controllers, that tells me the control philosophy may need to be reviewed and the control strategy re-done in light of whatever the objective is pursued.
It looks to me like the operation engineer does not want N2 to be vented unnecessarily. However, he must understand that implementing two separate controllers or a split- range or a gap controller for the service he’s describing will always result in the limit cycling he illustrated.
Sigifredo “Sigfried” NINO
Invensys Systems Canada Inc.
Q: I would like to contribute to the next edition of the Instrument Engineers Handbook. I would like to know what are the formalities and what are the qualifications required. I am a design engineer working with a Danish oil and gas consultant. Please let me know how to proceed.
Deepesh G.K
Rambøll Oil & Gas
Doha - Qatar
A: I am glad that you are writing from outside the United States, because it is time for us to broaden the character of the Instrument Engineers’ Handbook (IEH) from an American to a global one, so that the readers can benefit from the knowledge of process control engineers around the globe.
In preparation for the 5th edition, I have a folder in which I am collecting the names and addresses of the potential co-authors, their areas of interests and their qualifications (resumés), including past publications if any. When somebody offers to update an existing chapter or prepare a new one, I favor colleagues with operating or design experience, because the IEH is written by users and for users, and therefore I want to keep it totally unbiased, which can be hard (although there are outstanding exceptions) for colleagues working for suppliers.
The IEH is a practical handbook. The dirtier its pages get, the happier I am. If you do not have copies of the index of the three volumes (Vol. 1 on measurement, Vol. 2 on control, Vol. 3 on software), I can send them to you. You can find a complete index to all three volumes at www.controlglobal.com/IEHindex.html. In the 5th edition, I plan to insert new material on energy conservation and on the control of renewable energy processes.
I should also note that I have not yet decided if I should take on the preparation of the 5th edition, and if I do, whether I should retain the three-volume format. One option I am thinking about is to find three competent colleagues, each to edit one of the volumes at my guidance as the series editor. Another option I am considering is to abandon the three-volume format and publish each chapter as a separate book. Therefore, when you send me your offer to participate, you have three options:
- You can offer to update or prepare section(s) in the 4th edition. If this is your preference, name the section(s) you would like to handle.
- If you are experienced as an editor (put information in your resumé), you can offer to take on the editing of one of the three present volumes.
- If you are qualified (please elaborate on this in your resumé), you can offer to take on the conversion of one of the chapters into a separate book by updating some of the sections by yourself, and inviting qualified coauthors to update the ones you are less knowledgeable about.
In view of the above, you can send me your resumé, and tell me what role you would like to play.
Béla Lipták