P&ID: A roadmap for the rest of the trip
An ISA standard on the horizon for "faster, better, cheaper" P&IDs
By Thomas McAvinew
The P&ID is the first design drawing where equipment is identified with a tag, a combination of unique letters followed by a number. For instrumentation, this tag number would be per ANSI/ISA 5.1-2009, Instrumentation Symbols and Identification.
The old adage, “A picture is worth a thousand words,” is familiar worldwide, and in the process industries, the design document commonly know as the Piping and Instrumentation Diagram (P&ID) is such a picture.
This graphical representation depicts piping and mechanical equipment, including associated instrumentation hardware and control functions for a given process system or sub-system. This document, also known as an Engineering Flow Diagram, Mechanical Flow Diagram, or more recently, Process and Control Diagram, is intended to be a multi-discipline tool for subsequent detailed design. Long gone are the days of being able to literally build a facility from the P&ID, or its historic predecessor the Flow Plan or Diagram, with its relatively simplistic pneumatic control system. From the P&ID, for example, pipe, equipment, motor, and instrument lists are generated that result in diagrams with ever-increasing levels of detail that finally comprise the construction package that goes to the general contractor and the subcontractors.
But how does this keystone document, this roadmap for the design team, get developed? Well, it has been said there are two things you do not want to watch being made: laws and sausage; and in some ways, P&IDs could be added to the list.
There are as many styles of P&IDs as there are engineering and operating firms. Levels of detail and how much to show on a typical 22" x 34" sheet are ongoing issues. Instrumentation, in particular, is variously represented from simplified depictions to detailed ones. And as if those issues were not enough, there is the ever-present pressure to develop the perfect P&ID.
Design freeze? You must be joking. I recall a project where a project manager came back from lunch to find a process engineer marking up a P&ID that had already been signed off for issue to the client. The project manager cupped his hands and bellowed, “Drop the red pencil! Back away slowly!”
There are certainly subjective and objective issues here. The subjective ones are within the realm of responsibility of the engineering and project managers, but one would think that somewhere there was a standard for developing and checking P&IDs. Many practitioners in the field of instrumentation, controls, and automation have often wondered why ISA did not have a standard for this.
Why a standard?
While in a typical operating company or engineering firm P&IDs are the primary responsibility of the process engineering department with the task of drawing or CAD drawing the work of the piping/mechanical folks, this work is too often done in a bit of a vacuum. Sure, I&C staff get to participate in the review and revision process, but it is usually late in the P&ID development process, requiring the use of many red pencils.
Of course, it would not be a bad idea to get us involved sooner, but would it not also be better if some of this was done right the first time? In other words, P&ID development should be a joint venture between the process, piping/mechanical, and I&C staff.
Individual firms may have an in-house standard for P&IDs, but the development of this key diagram form is most likely an acquired skill. That is, the rules are largely unwritten, and there is typically no perceived need to codify this skill set, which is simply passed on from generation to generation. This may work fine within an individual company as long as staff longevity exists—something which is on the decline—but, particularly among engineering firms, consistency is a desirable goal. Enter Process Industries Practices (PIP) of the Construction Industries Institute, operating out of the University of Texas at Austin. In November 1998, this industry association first issued PIP PIC001, Piping and Instrumentation Diagram Documentation Criteria. Most recently revised in April 2008, this criteria’s purpose was to provide the means of minimizing the costs to process industry facilities of providing consistent and comprehensive documentation including the legal requirements of OSHA regulations.
About that time, the ISA Standards and Practices Board approved the formation of a committee, ISA5.7, to develop a P&ID standard, and after several attempts to focus on content, the PIP document was found to exist. So rather than reinvent the wheel, the committee contacted PIP, and after considerable discussion over a couple of years to resolve legal/copyright issues, an agreement was reached for ISA to use the PIP as the basis for the ISA standard.
This was a win-win agreement. PIP is not an ANSI-approved standards-writing organization, and this was a way for them to expand the influence of their base document beyond their membership, while for ISA it had the obvious benefit of providing a tremendous jump start.
A true P&ID standard
The groundwork has now been laid for a true P&ID standard from ISA, as compared with the newly revised ANSI/ISA-5.1-2009, Instrumentation Symbols and Identification. This standard has long been referred to as a P&ID standard by many practitioners simply because its most common use has been on P&IDs. This is probably due to the fact that when first published in 1949 as a 14-page Recommended Practice (RP5.1), it was titled, Instrumentation Flow Plan Symbols.
In subsequent revisions, however, the title changed, and the scope and purpose statements indicated the standard was applicable to all forms of documentation—something that has been specifically emphasized in the new 126-page revision. That emphasis, plus the 25-year-old aim of the late ISA5.1 Chair Ray Mulley, the current Chair Jim Carew, and the committee to make ANSI/ISA-5.1 the central repository of symbols, should make the main focus of this standard obvious.
There may well be some questioning as to why ISA would want to get involved in areas of other disciplines, but the interrelationship of I&C, piping, mechanical, and process content on P&IDs should be considered, and as has been alluded to previously, there are efficiencies to be gained by reducing rework early in the development process. If better cooperation and the realization of achieving the goal of developing P&IDs “faster, better, cheaper” can be accomplished by this P&ID standard, then there should be no question.
Currently with Don Frey as chair, the ISA5.7 Committee is moving forward with the immediate task to harmonize the PIP document with the newly approved ANSI/ISA-5.1. It is anticipated that the standard will be issued in 2010.
ABOUT THE AUTHOR
Thomas C. McAvinew, PE (email@example.com) is a senior control systems engineer with Jacobs Engineering in Golden, Colo. He is a Life Senior member of ISA and is the Past VP of the Standards and Practices Department. He also co-authored the ISA published book, Control Systems Documentation, 2nd Edition, with the late Raymond Mulley.
What information goes on a P&ID?
An agreement must be reached regarding what instrumentation information is to be included on a Piping and Instrumentation Diagram (P&ID). Since the document is a shopping list of equipment, piping, and instrumentation, it could be argued that everything must be included. However, work schedules and drawing clutter have some influence on the decision. Not all control schemes can be developed before the document is needed for other purposes; not all instrumentation details need to be shown on a P&ID. There are other drawing types for details. There is such a thing as information overload; the line must be drawn somewhere.
Obviously, space and locations must be reserved for piping and equipment, so all primary and final elements must be shown. The same applies to valves, sample points, PSVs, level glasses, TIs, PIs, flow elements, and so on. If the instrumentation is directly connected to the process, it should be shown.
Such things as valve sizes, set points, and failure positions need not be shown. They appear elsewhere, and it is unwise to place them where they possibly will not be revised at a later date. This type of information appears more logically on instrument data sheets.
Away from contact with the process is a controversial area. There are those who argue that only the basic loop should be shown—nothing more. On the other side are the larger companies that tend to perform their instrument design on the engineering flow sheet. Where is the line drawn?
The purpose of the P&ID is to depict the process in detail and to give some idea as to its control. Not all instrument details are shown—there are just too many. The basis already established is all the instrumentation connected to the process and that information necessary to the operator are shown in detail, but not the information that appears more readily elsewhere. Information necessary to the operator includes recording, indicating, controlling, alarming, and the availability of push buttons; it does not necessarily include square root extraction, for example.
Where does the information appear if it is not included on the P&ID? The answer is that it is on other documents: loop diagrams, logic diagrams, electrical control diagrams, ladder diagrams—whichever is most appropriate. Each company has to establish its own clearly delineated standards for documentation.
Categories of instrumentation
The authors’ preference for P&IDs can be explained as follows. Basically, three major categories of instrumentation appear on the P&ID: analog-type controls, discrete controls, and the operator interface. Analog-type controls are those that deal with continuously controlled variables; discrete controls are those that are involved with logic. The operator interface involves everything the operator sees or touches in the supervision of the process. The P&ID should show everything the operator will see or touch—every indicator, recorder, controller, alarm, and push button that is connected with the process. The P&ID is the major source of the information about the process, so the operator must be able to see what will be available.
Discrete controls are probably the most complicated. Being complicated, they are difficult to depict, so they tend to be neglected. Many companies have attempted to invent a simplified logic to use on P&IDs, generally to no avail. The best approach to discrete logic is to show all the inputs and outputs to and from a rectangle that is identified as a logic block and then reference the drawing where the logic will be developed. This approach has the benefit of allowing development to proceed at a later time, yet giving the essential information for development of the engineering flow sheet—the inputs and outputs.
Analog-type controls are the most difficult to deal with because they seem to be the easiest to understand. The controversy concerning how much instrumentation should be included on a P&ID really revolves around how much analog-type instrumentation is to be shown, if the reader accepts the authors’ treatment of discrete logic. The roots of the controversy lie not so much in differences of opinion as in different company operating procedures.
Source: Control System Documentation: Applying Symbols and Identification, 2nd Edition, by Thomas McAvinew and Raymond Mulley