Friday, April 13, 2012

Making and Importance of Process P & ID (Piping & Instrumentation Diagram)


The successful conversion of a conceptual idea into actual execution of process industry is the basic aim of any process engineer .Normally a project passes through various phases , viz., basic engineering , detailed engineering equipment fabrication /procurement , civil/structural work , erection and commissioning .The conceptual idea , either from the process development cell (for new processes) of from the marketing engineer (as per the customer) requirement , is converted to executable documents during basic engineering stage , and thus governs the successful completion of the project.

Piping & Instrumentation Diagram (P&ID) is one of the important basic engineering documents in the form of drawing , which truly represents the process plant It shows pipelines interconnecting all equipment , instruments , control systems, valves , etc. for the completeness of the process plant .Since detailed engineering work for any process plant depends more on the P & ID , its correctness and completeness is very much essential .Any wrong information provided in the P&ID at the initial stage may to a big disaster or may entail heavy expenses later .Hence , great attention is required for the 
preparation of error free P&ID .Good quality P& ID may deistically cut the engineering man hour requirement for the successful completion of a project .In this article, the details for the preparation of process P & ID are discussed.

Normally process plants have static equipment like storage tanks, columns , heat exchangers, etc. and rotating equipment like pumps, blowers, etc. These equipment are connected to each other by a complicated pipeline network for the completeness of the process. Measuring instruments like thermocouple for temperature measurement , and controlling devices like temperature control valve , are mounted on the equipment or pipelines for operating the plant at desired conditions. Although a single step process like blending of two compounds can be represented in a single Piping & Instrumentation Design (P & ID) , a process having a large number of processing steps like reaction , filtration , drying , etc. would need to have many P & IDs for the complete representation of the process plants .For ease of understanding , the complete process plant is functionally divided into various units .For example a reactor with the other related equipment , pipelines and instruments required for carrying out a reaction process may be covered in one unit while filtration operation my be specified with another unit number accordingly the individual units.

In a big chemical process industry, the processing units are spread over a large area .Although two different units may be far away from each other they are connected by pipelines or the continuous flow of fluid from one unit to another Interconnection between two units is represented by “Interconnecting” P & IDs Similarly, another engineering document, “Utility P & IDs , provides information regarding the distribution of utilities like water ,steam fuel etc. in the required locations. This article deals only with preparation of “Process P & ID” .

Process P & ID provides detailed information of the process plant like type of equipment , the details of pipelines , i.e. , size material , identification number, types of instruments and controlling devices etc. Operation, start-up, maintenance, safety and aesthetic aspects (discussed below ) are mainly considered during the preparation of process P & ID .


Process P & ID is prepared following the operational sequence of the process .For example , say a process needs step like chemical reaction , followed by filtration and drying of the product .Process P & ID shows the corresponding equipment like reactor, filter and dryer in the same order . All equipment is to be shown connected with pipelines, fittings and instruments necessary to carry out the process . Clear understanding of the process is very much essential for the preparation of a good quality process.

Process P & ID provides detailed information of the process plant like type of equipment , the details of pipelines , i.e., size material , identification number , types of instruments and controlling devices , etc.

P & ID Elevation of all equipment are normally are normally fixed in the prior stage .Equipment in the Process P & ID are shown at different levels with marked elevations .For example , head tank for reactant is shown above the reactor in Process P & ID for representing the gravity flow of reactant from head tank to the reactor .Likewise , the other equipment like critical pipelines , instruments , control valves etc., is shown at the appropriate elevations .Normally , heavy rotating equipment like pumps , compressors etc., is located at , lower elevation i.e. ground floor .Therefore , rotating equipment is placed at a lower in the Process P & ID also .


Start-up of any process plant may need some additional steps which are not required for the normal run. Special care should be taken for such steps at the initial stage of P & ID preparation .For example safety seals in the critical gaseous lines need to be filled with liquid before commissioning of the plant .Valve with hose connection is shown in the process P & ID for liquid seal filling .Critical equipment is by passed using temporary pipeline connections during water test run to avoid contamination .Process P & ID is marked with such types of requirements for the plant start-up.


Maintenance is an important aspects to be looked into during the preparation of Process P & ID .For example , electric hoist required for lifting heavy equipment is shown above it. Bypass line with regulating valve for control valve is provided for talking it out for maintenance without affecting the running plant. Some times it is marked in the P & ID to provide sufficient room for cutout in the floor above the vertical tube bundle from the heat exchanger body . If such type of information is not provided initially , it may be missed out in the various engineering stages and may lead to problems at the later
stages . To avoid such possibilities, Process P &ID , should be critically examined from the maintenance point of view during preparation stage .


For the safe operation of any process plant , safety measures cannot be overlooked .Hence , the preparation of Process P & ID needs the attention of safety aspects also .HAZOP study is carried out keeping in view each and every safety and operation step .For example , the exhaust of obnoxious gases near the plant operators is harmful to their health .Such gases are diffused in the atmosphere at a higher elevation .These safe elevations are normally marked in the process P & ID Safety interlocks are provided to ensure safe operation of the process plant .For example, failure of a running pump for a critical operation may disturb the complete process plant . To avoid this situation , the stand -by pump should be started without any time lag .- Necessary interlock is shown in the process P & ID .Some metallic pipelines and equipment handling hot fluids are insulated for body protection .Here heat saving may not be an important criteria for the selection of insulating , but it is essential for the operator’s safety .
Such safety requirements are clearly marked in the P & ID .Requirement of safety valves in pressurized system, minimum flow lines for centrifugal pumps , high and low alarms etc. , are to be critically seen during the preparation of Process P & ID .


Though it is not concerned with the plant operation, aesthetic look of process P & ID is important for better presentation of the document and ease of understanding for the user .Complex presentation of the pipelines, i.e., lots of cross-crossing , may even lead to wrong pipeline routing and hence due attention should be paid to the aesthetic aspect during the preparation of process P & ID.

Process P & ID Preparation

The following documents are essentially required for the preparation.image

Making of any Process P & ID

1. Process Flow Diagram (PFD) 
2. Equipment List 
3. Fluid List 
4. Equipment layout 
5. Interlock Description 
6. Vendor information (for related equipment).

Details of these documents are given in the Annexure. Checklists for vessel,pump, pipeline and instrument are prepared and presented in figs1 to 4.These checklist are self-explanatory and include all essential information to avoid any pitfalls during the preparation of process P & ID.

Although experience plays a major role systematic approach is equally essential for producing error-free process P & ID. Table 1 highlights the major steps for the preparation of any process P & ID.

Typical Example

A hypothetical case is considered here for better understanding of the process P & ID preparation .Before proceeding further , let us first understand about the overall process .

Process Description

Process flow sheet for the selected case is shown in fig.5 In this process the main products is produced by reacting the reactants ‘A’ and ‘B’ in a batch reactor (jacketed) 03 R001 A/B/C/D (four numbers) fitted with agitator. Initially fixed amount of reactant ‘B’ is pumped using transfer pump 02 P001 A/B (one operating + one standby) to the reactor from the storage tank 01T001 .In parallel, reactant ‘A’ is pumped using transfer pump 01P001 A/B (one operating + one standby) from storage tank 01T001 to the over head tank 03T001 . Prefixed amount of reactant ‘A’ is fed to the reactor at a constant flow rate during the course of the reaction . Exothermic heat of reaction is removed by cooling water circulating in the jacket of the reactor to maintain the reaction temperature .Homogeneity of the reaction mass is maintained by using an agitator .


Generated gases are continuously removed from the reactor and fed to the waste gas scrubber (packed bed column) 05C001 .Solvent is fed counter currently to the scrubber for absorbing the generated obnoxious gases .Once the batch reaction time is over , product slurry after cooling is transferred to storage tank 04T001 . Later product slurry is pumped by slurry is pumped by slurry pump 04P001A/B (one operating + one standby) to the filter unit 06F001 where the liquid is filtered out and filtered out and filter cake is washed with water Reactant ‘A’ is recovered in the Recovery unit 07U001 from the filtered liquid and recycled back to the process .Finally product cake is dried in dryer 08H001, pulverized 09A001 and packed for shipment .

Process P & ID Preparation

The enclosed area marked in the process flow sheet (fig 5) has been considered for the preparation of the Process P & ID .Process P & ID (fig.6) is developed based on the steps discussed in table 1 and checklist presented in fig 1 to 4 Line sizing is done for the minimum pressure drop at low cost piping, i.e. economic size of pipeline (this is beyond the scope of this article) .All equipment, instruments and lines are numbered for their identification .For example, 03R001 A represents .Reactor –A : 50-FA-03-05-XA1 represents line for reactant ‘A’ Intel to the reactor (where line size is 50 NB , fluid code is FA, unit number is 03, line number is 05 and pipe class code is X A1). All instrument Society of America standards .

Related Article: Piping & Instrumentation Diagram


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