Control loops

[Procman]

Can someone please give me a definition of a control loop as shown on a P&ID? What is included in a control loop.

Should interlocks be shown on P&IDs? Interlocks for a paper machine for example are quite complicated and it is the general feel that it should therefore not be shown. Interlocks are however helpful when doing HAZOP studies, but P&IDs are not only prepared for HAZOP studies. How do I convince others in my industry to include interlocks on P&IDs or should they be convincing me not to include it?

 

[Montemayor]

Procman:

Let's get one thing straight: the P&ID is the official and recognized instrument of record on how the process is supposed to work and be controlled safely - up-to-date!. This is the essence and intent of what OSHA dictates in 1910. So, for practical and mandated reasons, you must either:
1) indicate all instruments and interlocks on the P&ID and explain their functions on a separate, complementary document; or,
2) clearly show all instruments and interlocks on the P&ID in a way that explains their functions.

Interlocks are not helpful when doing a HazOp. They are mandatory! How do you intend to prove that the process is inherently safe to startup, operate, and shutdown if you don't thoroughly discuss any installed interlocks?

In my opinion, you should be focusing on the most efficient manner and technique that you can employ in order to fulfill the legal, practical, and engineering requirements while operating your process. This can be either of the two options I noted above. OSHA doesn't care which one you employ - as long as you accurately and clearly depict and explain all the safety items to your operating and maintenance personnel. And don't forget that the information has to be AS-BUILT! In other words, it has to be maintained up-to-date, on a constant basis. So, when you consider what you have to do, bear in mind that day-to-day maintenance and all changes to the process (remember Management Of Change[MOC]) must be kept up on the official documentation. This is the part where the P&ID's can get really crowded and difficult to interpret correctly.

Your dilemma is a practical one and you are going about it in the correct, engineering manner - you're concerned about how to control the correct information and getting it out to the correct individuals in an efficient and accurate manner. This is the first step. Now, you must study all the rest of the parameters that affect your decisions: doing the required maintenance and changes on the P&IDs and the interlock documentation as well as filing and distributing the information efficiently and timely.

Lots of luck. I compliment you on the concern you show.

Art Montemayor
Spring, TX

 

[FrancisL]

The idea that you can show all Control Loops and interlocks on P&ID's is only generally applicable to continuous processes, and even then the P&ID of necessity shows only a grossly simplified version. There generally have to be other documents that further define the details, for example Loop diagrams and Interlock diagrams.
In batch plants it is common for the P&ID's not to show the Loops at all. For example if the plant is configurable (by pipe connections, manifold valves or whatever) then the Valve (say) that a Loop controls could depend on the plant configuration, and the P&ID has no chance of showing the connection between the Control Loop function (as in FIC, LIC, PIC etc) and the Valve. Often such P&ID do not even show the control loops, just the transmitters and the valves.
For interlocks, often only the safety critical ones are shown in the P&ID, and even then only by reference to another document - eg a Diamond with an Interlock number in it.
So don't just assume that the P&ID is the master - and never assume it shows all the control!

 

[Procman]

Thanks for the valuable info! I fully agree with you guys on what should be on a P&ID. However, the argument of the instrument guys in some of our plants are, "We have A FDS (Functional Design Specification) and loop diagrams as well as interlock diagrams. Why do you want to duplicate info?" My argument was that it makes the HAZOP easier as you do not have to look at 3 or 4 documents to get the answer you need. You can immediately see it on the P&ID. Their argument, if the design engineer knows his plant, he should now the answer to the questions and he should know where to find them if he doesn't know the answer.

Montemayor, I like your heated reply on It's Mandatory!
Unfortunately this will only help in convincing our American mills, bbut not our Southern African Mills. Here in Africa it is not mandatory and As Built P&IDs do not exist!!!! They are really "As built drawings" from 40 years ago. Changes made subsequently were not necesseraly updated on the drawings. It will cost the company a fortune to try and rectify this, so we have decided to update drawings during projects. In other words, when we upgrade a section of the plant, we will also ensure that we have "As-Built" drawings. What is happening now though, is that only the upgraded sections are redrawn on separate drawings. The old drawings are never updated.

Also, sections of ourt plants are batch and sections are not. Do we have different standards for those then? Apparantly a paper machine has hundreds of interlocks and to show those on a P&ID is impossible. How do you draw up a standard to show interlocking and detail instrumentation in certain sections but not in other?

 

[smckennz]

Hi Procman
I have yet to see the P&ID that shows the entire process functionality on it.
My expectation of a P&ID is to show all equipment, valves, instruments, piping, controls and perhaps the odd control loop on it.
It is not and (I thought)has never been the place to show all interlock,heavy duty control logic and process conditions, although the odd P&ID will show a few pressures and flows.
For the control/interlock philosophy, I use a functional description, perhaps a flow chart, and sometimes a limit state logic diagram similar to that prescribed by IEC848.
In my opinion the primary purpose of the P&ID is to show physical relationships, secondary purpose; sizing and identification and third purpose if there is room on the paper; some indication of interrelationship (control).
As control systems become increasingly more complex, there is decreasingly less opportunity to describe the control process on something as rudimentary as a P&ID.
If you dont believe me, consider the fuel injection system on an automobile; the physical devices and interconnections give scant indication of the true control process.

Cheers

Steve

 

[moltenmetal]

This is all subjective- there's no hard and fast rule out there. For simple processes it may be possible to reasonably represent both control and interlocking strategies accurately on the P&ID, and where that's possible IMHO you should do it. But for more complex processes, an accompanying control description document is essential. It's just as important that a P&ID convey essential information QUICKLY as it is that it convey all the required information completely. A drawing with too much detail is hard to use for normal purposes. Simplifying the drawing by splitting it into multiple drawings with links between them can actually confuse the issue so much that it defeats the purpose.

IMHO a P&ID should show EVERY valve and connection that the operator can use to access the process fluid. That includes all vents, drains, instrument bleeders, flush ring connections etc. But some P&IDs for some major companies don't even do that- they rely on external instrument installation detail drawings to show these numerous valves and other connections. I'd rather relegate some of the detailed interlocking structure to secondary documents than eliminate valves from the P&ID, that's for sure.