A good instrumentation design requires the production of a large number of design drawings, usually in conjunction with some kind of Instrument Database. A list of the more usual ones is detailed below;
DESIGN DOCUMENTS
Instrument Design
Instrument Index - An Instrument Index is a super starting point for any design, it is really a database of all the necessary references which are required in an instrument design, all referenced from the tag number. It would include fields such as Tag Number, Service, Line /Equipment No , Instrument Type, Location, Location Dwg, Cable Routing Diagram, Junction Box Layout Drawing, Manufacturer, Model, Calibration, Measuring, Size & Process Connection, P & ID Number, Process Datasheet Number , Instrumentation Datasheet Number, Hookup Drawings Number, Bill of Materials Number, Loop Diagram Number, Fieldbus Segment Drawing Number and Instrument Standard Number, Relevant Standard Numbers and Purchase Order Number. You will find that some "non instrument" people will say that it is an unnecessary document BUT THIS IS NOT TRUE!
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Instrument Hook Ups - Instrument Hook Up Diagrams detail the accessory and tubing hookup for both process and pneumatic instruments based on the tag number. It should include standard specifications for the welding of hook-up piping, heat tracing & insulation and pressure testing & painting requirements. Generally included are;
Tag number
Numbers of the loop drawing.
Layout & routing drawing and isometric piping drawing containing the particular control loop component.
Elevations of both the primary control loop component and process connection.
Tagging of mechanical (piping or equipment) to instrumentation interface
Tagging of all elements/fittings & valves with item numbers.
Direction of slopes in hook-up lines.
Elevation of Instrument.
Maximum allowable lengths of hook-up lines.
Accessories often detailed include Monoflanges, Double Block and Bleed Valves and Fittings etc.
Material take off with part numbers, number of particular fittings, size, connection, material type, mounting type.
weather shield and tubing specification.
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Instrument Loop Diagrams - Instrument Loop Diagrams detail the instrument connection and wiring from the field instrument to the control system Input/Output (I/O) card. Included are termination box details, connections, wiring including screens etc. A [link Point to another website Only the registered members can access]
Instrument Segment Diagrams - Instrument Fieldbus Segment Diagrams are similar to Instrument Loop Diagrams, except they detail all the fieldbus field instruments along with their respective "spurs", terminators and the connection interface to the fieldbus trunk.
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Instrument Fieldbus Segment Checkers - There are several "free" tools which check the segment design.
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Instrument Block Diagram - The Instrument Block Diagram shows all the instruments in an overview format.
Instrument Datasheet - An Instrument Datasheet is the primary design Instrumentation purchase document, it details all the technical and process data required to select an instrument.
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Instrument Cable Schedule - The Instrument Cable Schedule provides details of all the cables utilised, it typically lists type of cables, source, destination, terminal, size, core, length etc.
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Instrument Specifications - Instrument Specifications are generally produced by the End User company or Engineering design Houses. They provide a detailed description of the design requirements.
Instrument Layout - The Instrument Layout details the location of the instrument and cable routes or pneumatic lines on a "plot plan".
Instrument Air Supply Diagrams - Instrument Air Supply diagrams show the various configurations of instrument air manifolds.
Instrument Tubing/Cable Tray Support Layout / Detail - These drawings detail the tray layout, design of the tray and Material Take Off.
Cable Penetration Layout - Details the penetrations into a panel, showing the cable transit frame and block configuration.
Process Control System DesignIt is worthwhile considering the extensive use of a database system in the production of the design documents which are to be used as input documentation by the system supplier.
Base Graphics
The generation of 'base graphics' MUST be configured by the operating company since a supplier just does not have the experience to produce graphics which accurately reflect the process. It is not simply a job of 'copying' the P&IDs. The most effective way of configuring these base graphics which the supplier enhances is to use the supplier configuration package, thus having the ability to transfer the data to the supplier database easily.
It is also a great idea to use a Database for creation of the Instrument Index , Cable data Sheets, I/O schedules, Message Lists and Motor Schedules as data can then be effectively transferred from one database to another. This does have the added advantage in that errors are minimised once one database has been checked. Mind you if adequate checking does not occur then the problem will be multiplied.
The following input documents should be produced for issue to the Process Control System (PCS) supplier. Some operators consider that it is more effective for the PCS suppliers to create some of these documents but that is just not so in that they just CANNOT have adequate experience to provide a comprehensive enough package.
(1) I/O SCHEDULES - this is the base document around which configuration revolves, information contained within it should include tag number, whether it is an Intrinsically safe or Non I.S. loop, digital or analogue, range, units, critical or non critical loop, report input and alarming priority.
(2) FUNCTIONAL LOGIC DIAGRAMS - these are the base documents which the supplier uses for motor and sequence control. They are usually drawn utilising logic blocks around the logic symbols which are identified in AS 1102.9 - 'Graphical Symbols for Electrotechnology - Part 9 Binary Logic Elements'.
(3) MESSAGE LISTS - these lists are the base document which are used for the generation of reports, alarm and special messages. They are usually configured using a database format which the supplier can easily transfer to his own database.
(4) BASIC GRAPHICS - the rudimentary graphics which are initially passed to the operating company OPERATIONS GROUP for comment and then used by the supplier as the base graphic background which the supplier then enhances.
(5) CABLE DATA SHEETS - these sheets are used rather like termination diagrams where normal termination diagrams do not exist.
(6) MOTOR SCHEDULE - this document details the requirements needed by the energy management system ie priority of tripping.
(7) TERMINATION DRAWINGS - details of all incoming terminations and cables.
(8) FUNCTIONAL DESIGN SPECIFICATION - This document specifies the functional and technical requirements of the system. It should be comprehensive and miss nothing. 'Slimline' specifications DO NOT work and leave the customer wide open for variations.
THE 'TAIL END CHARLIE SYNDROME'
It has always been the case that the Controls/ Instrumentation design could not be finalised until the piping design has been completed because the instrument locations are unable to be adequately determined. This of course is true if total accuracy is required, however, if you have a schedule problem there is a method of achieving 90% accuracy at a very early stage, picking up the remaining 10 % at a later time.
This method utilises the base vessel layout and allocates instrument positions on a 'best guess' basis (usually they are very close to the final position) and 'driving' package vendor terminations at edge of skid. The suppliers we have found are not adverse to this approach as it does do a fair bit of design for them.
THE 'LOST' TAGS
It is always a problem as to just where you pick up internal system tags and tags which do not appear on the P&IDs. A convenient method of picking up these tags is to use a document called a Instrument Line Diagram. The ILD is essentially a point list and can be in diagrammatic or data format.
This document however must be treated with great care in that it can become a monster if you are not careful. Keep it as simple as possible, utilise it by all means as a tool for creating 'temporary P&IDs' when package P&IDs are not available but ensure that the tagging system used does not cause problems later.
CHECKING
It is absolutely essential that all documents produced are cross checked, to not check is false economy as eventually the supplier will pick up errors and it takes significantly more effort at that time to rectify them. Considerable cost overruns can result from poor cross checking.
SAFETY INSTRUMENTED SYSTEM DESIGN DOCUMENTS
The utilises several documents to develop the necessary logic these being:-
SHUTDOWN PHILOSOPHY - this is the most important document associated with the Combined safety System in that it lays down the philosophy applicable to it. In this document are listed the hierarchical shutdowns. One must not lose sight of the fact that although the system has the ability to implement very critical shutdown features it also implements less critical unit and process Shutdowns.
For instance on offshore platforms the usual stages of shutdown are as follows:-
UNIT SHUTDOWN - this, the lowest level of shutdown, causes the individual units to stop.
PROCESS TRAIN SHUTDOWN - an individual Process Train will shutdown on occurrence of any applicable trip.
PROCESS SHUTDOWN - on this occurrence the complete process stops but utilities remain running, in effect it is a process 'stop' with NO BLOWDOWN in order to facilitate a easier startup on rectification of the problem.
EMERGENCY SHUTDOWN - This action results generally from fire or Gas being sensed on the platform, obviously a fire in the Galley or in a room in the accommodation does not cause a ESD but more serious events in the Process, Wellhead or other critical areas will result in an ESD. An ESD is actually a Process Shutdown with Blowdown and isolation of the platform trunkline. The blowdown results in flaring of the gas component of the platform inventory whilst the liquid component is maintained within the various process vessels. When co-incident fire detection in the process or wellhead areas occurs one of the two strategically placed firepumps start and deluge occurs automatically.
On some platforms main power is shutdown and the emergency generator starts when an ESD occurs whilst on others main power is maintained by the generators switching to Diesel except when there is fire in a critical area such as the wellheads. This approach is advocated in that maintaining lighting ensures that at night the firefighting crew can see what they are doing.
TOTAL PLATFORM SHUTDOWN - This shutdown hopefully will never require operation during the life of the platform since it usually is the result of abandonment. There are generally only two or three TPSD pushbuttons which are under the control of the Platform Operations Manager. The result of this action is total blackout of the platform including isolation of batteries except for some navaids which continue to run. The intent of this shutdown is to maintain some battery power for when the 'black start team' reboard the platform.
Other documents used in the development of the CSS configuration are as follows:-
I/O SCHEDULES - these detail the fundamental configuration such as tag number, IS or NIS, alarm limits, analogue ranges etc.
PSD/ESD CAUSE AND EFFECTS - these documents which are based on the Process Cause and Effects are used by the CSS supplier as the basis for the logic. The usual appearance of them is to have the cause on the left hand side with the effect at the top with a 'X' matrix.
FIRE AND GAS CAUSE AND EFFECTS - These documents are similar to the PSD/ESD C&E described above except that they do not have logic symbols incorporated (matrix only).
The logic is developed by the vendor based on the above documentation on the CSS CONFIGURATION PACKAGE. This package is deliberately separate from the executive software of the system since it is very important that software previously developed is not corrupted in any way. After completion the software is tested extensively before being included in the overall software package. Great emphasis is placed on ensuring that the executive software cannot be accessed by unauthorised personnel and once the system is operational the configuration package is usually located onshore.
MESSAGE LISTS/CABLE DATA SHEETS/ TERMINATION DRAWINGS - As previously described for PCS.
When designing and specifying a CSS it is important to remember that it does have a fundamental common mode failure point this being of course the software. It is all very well to have duplicated and triplicated hardware but if there is a common software bug just what can be done to overcome the problem. Well the answer is that the requirements of API RP14C should be followed in that there should be a primary and secondary safety system. Usually the primary being the electronic system and the secondary, safety relief valves.
Where there is no possible alternative to having a single electronic system then it is absolutely imperative that DUAL sets of software are used which have been written by DIFFERENT personnel. Having to use this route has great disadvantages in that it is very complex, extremely costly and difficult to maintain. The RULE is therefore - devise some form of secondary system.
How to read P&IDs - Dave Harrold - Instrumentation detail varies with the degree of design complexity. For example, simplified or conceptual designs, often called process flow diagrams, provide less detail than fully developed piping and instrumentation diagrams (P&IDs). Being able to understand instrumentation symbols appearing on diagrams means understanding ANSI/ISA's S5.1-1984 (R 1992) Instrumentation symbols and identification standard. S5.1 that defines how each symbol is constructed using graphical elements, alpha and numeric identification codes, abbreviations, function blocks, and connecting lines.Thanks to Control Engineering
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