Principles of P&ID Developmente

Engineering Practice

Principles of I. Loop

y *{A diagram Piping model

Isometrics

P&ID Development"The tips provided here wili streamline efforts to

develop piping & instrumentation diagramsMohammad ToghraeiConsultant

le piping and instrumentationdiagram (P&ID) is often consid-ered to be the gold standard forthe proper design, operation and

maintenance of plants in the chemi-cal process industries (CPI), includ-ing chemical, oil-and-gas facilities,mining operations, food-processingplants, and water- and wastewater-treatment plants. The P&ID providesimportant information for manufac-turing and installing equipment andmachinery, piping, instrumention,and safe and appropriate startupand correct operation of the plant.

The P&ID is frequently refer-enced by various engineering dis-ciplines during both the designstages and the operating phase. Itis also referenced in technical meet-ings with equipment vendors andmanufacturers, in hazard and op-erability (HAZOP) studies, in man-agement meetings, and during proj-ect scheduling and planning.

The P&ID is one of the few plantdocuments that is created by multi-ple engineering disciplines workingin concert. These disciplines includeprocess engineering, instrumenta-tion and control (I&C), plot plantand piping (PL&P), mechanical,heat ventilation and air condition-ing (HVAC), and to a lesser extentcivil, structural and architecture(CSA), and the environmental andregulatory group.

Similarly, the information pro-vided by the P&ID allows for thegeneration of various other impor-tant documents, including isomet-ric drawings and models for piping.

instrument lists, cause-and-effectdiagrams, control philosophy, de-scription, alarm-setpoint tables,line-designation tables (LDT), plotplans, loop diagrams, tie-in lists,and many more (Figure 1). Withsuch universal applicability, P&IDsare often affectionately referredto as "primary interdisciplinarydocuments."

Role of the process engineerThe duties of the process or chemi-cal engineer in a CPI project can bebroadly split into two categories equipment sizing and P&ID devel-opment. Therefore, most engineersneed to have skills in both areas.

The former skill calls for knowl-edge related to hydraulic calcula-tions, pump and compressor siz-ing, vessel and tank sizing, processsafety-valve (PSV) sizing, and heat-exchanger sizing. Equipment sizingrequires different skill sets, whichmay vary by level of seniority andby industry segment.

Chemical engineers should havethe knowledge that is needed to sizespecific equipment components re-lated to their industry segment (forinstance, distillation towers for pe-troleum refineries and clarifiers forwater treatment). While equipment-sizing skills are routinely taughtduring the acquisition of an engi-neering degree, the skills neededto develop meaningful P&IDs areoften not formally taught in school,but rather are acquired through "onthe job" training.

The absence of P&ID-develop-

Mechanicaldata sheets

\

Caiculations

IProcess

data sheets

FIGURE 1. P&IDs are technically pip-ing and instrumentation diagrams butthey provide a central repository of es-sential engineering information that isrelevant to numerous other functionsthroughout the planning and operationof most process plants

ment training in academia may re-sult in part from the fact that inher-ently, P&ID development involvesmore art than science. Plus, thecontent and structure of individualP&IDs tends to vary from companyto company, and there is a constantstream of new technologies beingintroduced as older ones are retired.While volumes could be written onthe development of P&IDs, this ar-ticle provides a framework of recom-mendations for P&ID development.

P&ID development activitiesThe block fiow diagram (BFD) is thepreliminary document in the devel-opment of any CPI project. It out-lines the most basic, general infor-mation related to the project. Then,it is the job of the process fiow dia-gram (PFD) to add further detailsto the design before the final docu-ment the P&ID is developed(Figure 2). In general, the BFD cap-tures the theoretical process stepsthat are needed to convert a feedstream to finished products whilethe PFD goes inside of each of theBFD "blocks" and shows the majortypes of equipment that are neededto meet the goal of each block. TheBFD and PFD only show the mainelements of the plant, while the

6 2 CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2014

Block fiow diagram(BFD)

Process flow diagram(PFD)

Piping & instrumentationdiagram (P&ID) Reversible system

FIGURE 2. Before a detailed P&iD can be developed, a BFD and PFD must be devel-oped to identify the major aspects of the process. The BFD identifies primary streamsand unit operations.The PFD expands each BFD block, adding tanks, pumps andsome instrumentation.The P&iD pulls it together with fuller details

Environmentalhealth and

safety (EHS)codes

plant with:Low capital andoperating expensesQuick constructionProper operation

A plant with comerclallyestablished designprocedures

A plant with: Ease of operation Safe operation

FIGURE 3. CPi facilities require cooperationamong three parties. Each has its own responsibili-ties but EIHS requirements are common to aii

P&ID provides more detailed ele-ments, capturing the real plant onpaper while ignoring the scale.

Despite the simplified drawingshown in Figure 2, P&ID develop-ment goes beyond just expandingthe PFD. There are some small itemsthat are not shown but that need tobe developed by the designer for theP&ID. Still, the development of theBFD and PFD requires exhaustivestudies and rigorous calculationsand simulations. Going throughthese "preliminary" efforts and notbypassing BFD and PFD develop-ment is essential, because everysingle decision for main items onthe PFD could have a big impact onthe project.The main goal of a facilityThe main goal of a process plantis to produce desired quantitiesof various products while meet-ing stated quality goals. A soundplant design will take into consid-eration the owner's wishes for theplant (for instance, low capital andoperating expenses, the ability tobuild it quickly and so forth), thedesigner's requirements (that thedesign procedures can be trustwor-thy and commercially established)and the operator's requirements

(to ensure ease of op-eration and flexibility),while meeting all localenvironmental and safetyregulatory requirements(Figure 3).Essential elementsIdeally, the specific ele-ments captured in anyP&ID should account forfull functionality of theplant in all stages of theplant lifecycle, as outlinedbelow:1. All given elements

including equipmentand piping items must operate well and

reliably during normal opera-tion, within the window of oper-ating conditions that is expectedat the plant. A basic process con-trol system (BPCS) should beimplemented to bring parameterswithin normal conditions. Thefive key parameters of chemicalprocess operations (temperature,pressure, flowrate, level and com-position) may need to be "adjusted"continuously by the actions of theBPCS to ensure that they meetthe requirements at the inlet andoutlet of each component.

2.The element operates well dur-ing non-normal conditions, suchas under reduced-capacity condi-tions, and during process upsets,startup and shutdown. Engineer-ing provisions for working reliablyduring low-capacity operatingconditions, the use of safety-in-strumented systems (SIS) to shutdown the system, and safety-reliefvalves are examples of the types ofitems that can address this stageof plant lifecycle in P&ID develop-ment activities.

3.There are enough provisions toensure ease of inspection andmaintenance; these include in-situ inspection, ex-situ inspection,workshop maintenance and more.

FIGURE 4. The startup of a reversiblesystem often requires a recirculationloop; it should be sized appropriately tominimize costs

All given elements must be de-signed to allow them to be appro-priately isolated, drained, vented,cleaned and flushed (via purging,steaming, or water flushing).

4.Provisions must be made to mini-mize the impact on the rest ofplant when an item, equipment orunit is out of operation.

The following points should be con-sidered when adding different itemsto address any of the above four re-quirements:l.Make sure that no added element

within one stage of the plant'slifecycle will jeopardize anotheritem's function. For example, add-ing bypass capabilities with amanual block valve for a safety-related switching valve (for thepurpose of making the plant op-erational when the switchingvalve is out for maintenance, perItem 4 from the list above) couldjeopardize the operation of theswitching valve in an SIS; that is,the bypass could be left open andtherefore create a safety flaw).

2.Decide if added items can be"merged" with each other or not.This basically involves check-ing if a single shared item canaddress multiple requirementswithin the plant lifecycle or not.Whenever possible, items shouldbe "merged" or "shared" to makethe most of capital and opera-tional costs. In certain cases, thiscan be justified, especially whenan item needs to be added for thepurpose of satisfying Item 3 or 4above. As these specific compo-nents are not in use all the time,a good process engineer will at-tempt to "merge" them with otheritems so they can carry out mul-tiple functions.

However, this last practicecannot be carried out in all situ-ations. From a redundancy pointof view, it is not always good toexpect one item to carry out mul-tiple duties. Technically, one item

CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2014 6 3


could be time-shared when it ismeant to carry out different du-ties at different times (that is,with no overlap in duty duration).When designing for shared duty,keep in mind that this setup mayend up creating confusion amongoperators, may he more prone tocross-contamination, and mayenable a small failure to lead toa big shutdown. Meanwhile, de-signing components to be dedi-cated (not shared) will drive upcosts (if items are expensive),but they will be easier to trouble-shoot, should a failure occur. Onecommon example is the use of amanway pressure-relief valve(PRV) on one shared nozzle ontanks.

Tjrpically, the ability to installshared items is most practicalin batch systems and in systemswith only intermittent operation.In such operations, a given itemcan be used for different dutiesduring different time spans.

The following discussion explainsthe activities that are reqruied forP&ID development for each stage ofa plant lifecycle.1. Normal operation. For normaloperation, each item on the P&IDneeds to he able to carry out theduty it has been assigned. Since,in the majority of cases, this is notachievable through equipment de-sign alone, a control system shouldbe implemented on the equipment.The BPCS must ensure that thedesign of the equipment will forcethe equipment to operate within a"window" of expected results, typi-cally at its best operating point.

In a broad sense, a control systemis supposed to bring the five mainprocess parameters flowrate,pressure, temperature, level andcomposition into the requiredrange. "Composition" encompassesmany relevant parameters, rangingfrom viscosity, density and conduc-tivity to octane number and Brixnumber. All utility distribution andcollection networks, and heat-con-servation insulation, must also bedecided at this stage.2. Non-normal operation. Non-normal operations occur under the

TABLE 1. OPTIONS FOR EQUIPMENT MAINTENANCE

In-place

In workshop

In-line

By operators doing rounds

Not applioable

Off-line

By the mechanicai group

Accident

Interlocksystem(SIS)

(specificpoints)

Mechanical protection

Major upsetHigh high

Mild upset

High

Normal

- - - LowMild upset

Low low

Major upsetMechanical protection

"C Accident

Quick hardwareloss -m

Long-termhardware loss

Does not meetprocess goal

Meetsprocess

goal

Does not meetprocess goal

Long-termhardware loss

Quick hardwareloss

FIGURE 5. A diagram depicting upset conditions, such as this, can be defined fortemperature, pressure, ievel, flowrate or composition of each component

following conditions (Each is dis-cussed below):a.During reduced-capacity opera-

tionb. During startupc. During upset conditionsReduced-capacity operation. Oc-casionally, actual plant capacityshould be reduced from the designcapacity for a variety of reasons.Such conditions may result from ashortage of raw materials or an ex-cess of production, or from downtimeof a critical equipment componentor unit. The process engineer usu-ally provides some turndown ratio(TDR) for the plant. TDR is a ratiobetween the normal capacity of theplant and the minimum running ca-pacity that is possible without los-ing the quality of the product. TDRcan be defined for the equipment,for a unit, or for the whole plant.

Some owners or operators ex-pect the engineer to provide a TDRof around two for their plants (thismeans they want to be able to oper-ate the plant at half capacity withoutlosing any product quality). Some

plant items (such as tanks) have aninherently high TDR, while others(such as equipment with internalweirs or vessels with internal feeddistributors) have a lower TDR. Theduty of the process engineer is toprovide the required TDR (definedby the client) for each equipmentcomponent and for the entire plant.

One method of providing TDR isto install multiple smaller compo-nents in parallel, instead of one bigpiece of equipment. Another solu-tion is to implement a recirculationloop around the equipment to com-pensate for the low flowrate.Startup operations. Startup opera-tion could be assumed to be a severecapacity-reduction case. Be causeprocess parameters during startupare not necessarily within theirrange, equipment and instrumenta-tion are not expected to be workingaccording to full operating expecta-tions. However, startup specialistsare often onsite in this stage andcan help to compensate for the tem-porary lack of operability of equip-ment and instruments.


TIPS FOR VENTS AND DRAIN VALVES' Each drain can cover o portion of a system; Vents can covera bigger portion

' Drain or vent size should be manageable (Minimum sizeshould be % in.; limit to 2 in., unless inside of dike)

' Multiple drains and vents should be implemented in a cov-ered area to ensure draining or venting within a reasonabletime

' When the system is small: drain = vent (usually for pipes For volume < 0.5 m^ (such as pump cosings), use VA-IH.drains

> Vents can be one size smaller than the drain' Drains for liquids with viscosity higher than 50 cP could beone size bigger than guidelines stated above

= 2 years

t = 2 years

- time betweeneach maintenance, inspection or cleaning event

ip = time betweeneach plant overhaul or turnaround

2 years

(^ = 3 years t^ = 2 years

FIGURE 6. When planning for isolation valves, the engineeringteam should evaluate data related to the anticipated time for sched-uled maintenance and anticipated turnaround schedules

TABLE 2. OPTIONS FOR ISOLATING A PORTION OFTHE PROOESSFROM THE PLANT

1

2

3

4

TypeBlock valve(with or without lock)Block valve(with lock) and blindDouble bicck valve(with lock) and bleed

Block valve(with lock) and blindand removable spool

Symbol

IXI 1 Process

nIxT 1 Process8

ItXhxHtxT \

ItXI ItXl 1

Process

Process

CredibilityNotaccepteible

c

ISn

afer

is

o

(0

V 7TABLE 3. DIFFERENT METHODS OF REMOVING MATERIALFROM EQUIPMENT FOR INSPEOTION OR MAINTENANCE

1

2

3

Type of"dirt"Solid/semi-solid:removalLiquids:Washing

Gases:Purging

Removal method

Manual Machine-as-

sisted Flushing: By

water Steaming out:

By utility steam Chemical

cleaning: Bychemical solu-tion or solvents

Neutral gaspurging

Ventilation

P&iD

Nothing is needed on P&ID Do we need "clean-out" doors?

For all the oases . three options areavailable to show on the P&ID:1. Only washing valves2. Washing valves that are hard piped3. Hard piped washing system with

switching valves for automaticwashing

If it is by inert gas, the same optionsfor "washing" (above options) areavailable here

For venfilation (by natural draft ofair), imake sure there are at least 2nozzles are available

For reversible systems (such asreactors that carry out equihbriumreactions), startup operation can besupported by recirculation. If thesystem is not reversible, the startupoperations can be more complicatedand case-specific. Figure 4 showsthe basics of this procedure.

If recirculation is to be used dur-ing the startup procedure, effortsshould be made to avoid exces-

sively large circulation loops, so asnot to waste money for piping thatis supposed to be used only duringstartup. As much as possible, thedesign should try to use the existingpipe arrangement for the purposeof startup recirculation, especiallywhen high-bore pipe is needed tosupport startup efforts.

The tendency to use the piping ar-rangement that was implemented

for normal operation for the pur-pose of startup recirculation is sostrong that some process engineersforget to think about the startup op-eration during the development ofthe P&ID; they simply assume theywill find a way to accommodatestartup somehow without actuallyplanning for it.Upset conditions. Upset conditionscan be defined as operation of theplant when some of the process pa-rameters are beyond the normalband. In Figure 5, this situationis arbitrarily split into two differ-ent cases mild upset and severeupset for any of the five key pa-rameters (fiowrate, pressure, tem-perature, level, and composition). Inboth cases, during upset conditions,the process goals have already beenlost so the immediate goal is toprotect the equipment (hardwareconservation) and the health andsafety of the personnel and neigh-boring communities.

To address point upset conditions,the facility should be equipped withan alarm system and a SIS. Thealarm setpoints are usually on themaximum (or minimum, in somecases) value of a parameter, and theSIS action will be set to the high-high (or low-low) level. However,some additional alarm setpointsor additional SIS setpoints can beadded, too.

The purpose of this SIS action isto shut down a plant and bring it inthe lowest energy state (in terms oflowest pressure, lowest temperatureand so on) Other than "event-basedSIS" explained above, SIS action(s)can also be activated by the opera-tor. This shutdown is named "opera-tor activated SIS."


The duty of the alarm is to warnthe operator that something hasgone wrong. If for whatever reasonthe operator fails to respond in atimely manner, the SIS system willinitiate the action that the operatorhas failed to, or tried without suc-cess. This allocation of responsibili-ties between alarm system and SISis shown in Figure 5.

If, for whatever reason, the SIScannot mitigate the parameter thathas deviated from the normal pointand it has gone beyond high-high(or low-low) level, then finally a me-chanical item needs to be triggeredto "tame" the system and regaincontrol. Even though a mechanicalsystem (as the last line of defense)can be considered for each of thefive parameters mentioned earlier,pressure safety valves (PSV) are apopular type of mechanical defenseagainst a wild parameter. InstallingPSVs, and routing their release to anappropriate destination, is an essen-tial task during P&ID development.

Winterization is another issuethat is resolved in this stage. Win-terization involves implementingspecific features in a plant design toprevent any impact of cold weatherduring a plant shutdown. For in-stance, winterization efforts typi-cally start with provisions to enable"natural internal drainage" of theequipment and pipes more tolerantitems, such as tanks. Other activi-ties include heat tracing and insu-lation of pipes to prevent freezingor settling of non-drained (trapped)fiuids, and installing uid mov-ers on emergency power sourcesto provide recirculation to preventfreezing/setting in the case of powerloss.3. Inspection and maintenance.Equipment care can be categorizedinto "in-workshop" and "in-place"care, and the latter can be catego-rized further into "in-line" or "off-line" operation. In-place care isusually done by operators makingrounds, while in-workshop care istypically carried out in a workshopby a mechanical group (Table 1).

Inline care can be considered theinspection of operation equipment,while off-line care is equipment re-

TABLE 4. EXAMPLES OF REMOVABLE SPOOLS (RS)

Engineering PracticeItem

Centrifugalpump

Progressive-cavity pump

Sheli-and-tubeheatexchangerVessels andtanks

Potentiallocation of RSSuction anddischarge side

Discharge side

Tube side

Lines out offlanged heador blindednozzles

P&ID example

RS

RS

TABLE 5. OPTIONS TO DEAL WITH LOST ITEMS IN A PLANTOptionThe exactreplica Inparallel

A similar item inparallel

Bypassing the absentitem

Redirecting the in-flowto a "reservoir" for lateruse

Upstream tank storesthis inlet flow and thedownstream tank pro-vides outlet flow for ashort period of time

The inlet flow is sentpermanently for ulti-mate disposal and thestream will be wasted

The absence of anitem doesn't generateany upset in the rest ofplant or whole plantshould be shut down

Schematic

Bypass

Reservoir

P&ID example

Pond

AFurnace

To flare

Steamgenerator

pair or maintenance. Each requiresdifferent types of provisions for theequipment on the P&ID.

Operators making rounds couldbe equipped with portable sensors;if not, then he or she must rely onthe use of the senses: Sight: To observe, for instance,

leakage, vibration, overfiow oftanks, fiuid levels, flame color andshape

Sound: To sense vibration, cavita-tion, hammering, PSV release, ex-plosion and more by listening

Touch: To detect vibration Smell: To detect fire, leakage, PSV

release to atmosphere, and moreTo support the work of the operatormaking rounds, specific items canbe put on the P&ID. These may in-clude sight glasses to check liquidlevels, catalyst levels or filtering-


TABLE 6. AN EXAMPLE OF P&ID DEVELOPMENT FOR A PUMP (FOUR PHASES OF OPERATION)Case P&IDNormal operafionPuffing fhe pump cail-ouf wifh fhe required informa-fion on fop of P&ID sheef

Cail-ouf should be puf on P&iD

Placing a reducer/expander to mafch sucfion anddischarge side of fhe pump, if needed (may needa fop-flaf eccenfric reducer af connecfion)

Hi CJ-

Adding a permanenf sfrainer fo prevenf damage fofhe pump

iVlai


made to either use test points andfixed gages that transmit informa-tion to the control room, or to imple-ment a control loop that depends onsome parameters based on the criti-cality of the parameter.

Meanwhile, in-place, off-line caremay include chemical or solventcleaning, steaming-out, pigging op-erations and so on. Depending onthe operation-specific requirements,different items should be imple-mented (such as chemical cleaningof valves).

For all off-line care a specific ar-rangement must be made to ensurepositive isolation of the system fromthe rest of the plant. This arrange-ment t3rpically comprises isolationvalves, drains, vent valves and so on.The isolation system is discussed ingreater detail below.

For in-workshop care, the provi-sions defined by in the P&ID areitems that will allow the equipmentto be removed from their founda-tion easily and safely. However,the characteristics to satisfy thisrequirement are not always shownon P&IDs (mainly to avoid clutter-ing of the P&ID). For example, ifequipment needs to be hoisted forremoval, this engineering detail isoften not shown in the P&ID. Itemsthat must be shown on the P&ID in-clude the following: Isolation valves that allow the

equipment components to he de-tached from the rest of the plant

Drains and vents Removable spools (RS) that would

be used around the equipment toallow it to be "untangled" from thesystem by removing the piping sys-tem interference; this allows foreasy equipment transfer tothe workshop

When it comes to preparing foroff-line care, with regard to de-signing isolation systems, the fol-lowing three questions should heanswered:l.To which equipment should the

isolation elements be added?2.Where do they need to be placed

"around" the equipment?3.Which t}^es of isolation systems

or elements should be used?To answer the first question, pro-

viding isolation valves is not nec-essary for all the equipment in aplant. Isolation valves are requiredto isolate the equipment from therest of the plant if the equipmentis expected to need "off-line care"at frequent intervals, in time du-rations that are shorter than thescheduled plant turnaround times).For instance, if (based on histori-cal data), the unit expects to needoff-line care every three years butthe entire facility for which youare developing the P&ID will needplanned turnaround work everytwo years, there is no need (atleast theoretically) to put isolationvalves upstream and downstreamof the unit. This concept is shownin Figure 6.

In some cases, companies don'tprovide isolation systems for es-sential equipment, such as heatexchangers. The logic is that theyessentially cannot afford to put theheat exchanger out of service, soadding isolation valves would be ir-relevant.

The answer to the second questionis that the isolation system shouldbe added on all downstream or up-stream connecting pipes, as close aspossible to the equipment. However,some companies challenge this andquestion if there is real needed toput isolation valves on, for example,a vent pipe to atmosphere or not.

To answer the third question,it should be stated that there aredifferent type of isolation systems.Tahle 3 summarizes these methods.

Decision needs to be made aboutthe type of isolation method. Theisolation method depends on fac-tors, such as the equipment envi-ronment (for instance, for confinedspaces or non-confined spaces),the fluid type (aggressive or toxicor not), and the pressure and tem-perature of the system. Usually thefirst type of isolation (Table 2) doesnot provide enough "positiveness."Possibly the only application of thisisolation method is for instruments.In such an application, the isolationvalve is called a root valve.

The next step for making equip-ment ready for periodic removal isto bring it to "non-harmful condi-

tions." This means having provi-sions that will allow all five keyprocess parameters to be broughtinto a safe range: Ensuring safe temperatures:

Options include allowing timelapses, or options for cooling down(or warming up, in the case ofcryogenic services) streams. Forsome systems (for instance somebatch operations) that require amore rapid cooling (or warming)by cooling streams

Making pressure safe: Venting iswidely used

Ensuring appropriate flowrates:As long as equipment is isolatedfrom the rest of the plant, there isno flow going into it, and it is nota point of concern

Making levels safe: Drainage op-tions are needed for tanks, vessels,pump casing and more. Some gen-eral rules for sizing and installingdrain and vents are in the Box (p.65)

To ensure safe compositions, thebody of the equipment (externaland/or internal) must be safe interms of exposure. These provi-sions involve proper cleaning ofthe equipment.

Table 5 shows options for mak-ing the composition safe for dif-ferent types of materials insideof the equipment. Washing andpurging (through ventilation) areespecially important for walk-inequipment.

The last step as mentioned aboveis to provide removable spools (RS).Sometimes required RS are alreadypresent due to previous activitieson the P&ID. Tahle 4 provides someexamples.

Allocating a utility station in dif-ferent locations of the plant, and de-ciding about the required utilitiesfor each utility station, is anotheractivity to address this stage of theplant lifecycle.4. Operability of the plant in theabsence of one item. The designerneeds to decide the impact of equip-ment loss on the rest of plant opera-tions and take engineering steps tominimize its impact. The wide rangeof answers and decisions should in-clude the following:


TABLE 6. AN EXAMPLE OF P&ID DEVELOPMENT FOR A PUMP (FOUR PHASES OF OPERATION) (continued)Case P&IDMaintenance / InspectionAdding a pressure gage on discharge and/orsuction side

I . .^, SIS COMMANDr p n q RUN STATUSL115J COMMON THOUBLE ALARM^> UR STATUS STOP

; SHUT DOWN COMMAND

i

Adding block valves in the suction and dischargeline (such as a gate valve) in order to isolate thepump during maintenance

-txl

s/s COMMAND' M M RUN STATUS

" COMMON TROUBLE ALARML/R STATUS STOP

SHUT DOWN COMMANDS/S

?-\h'-d-NITM-

Consider vent and drain valves in the pumpsuction and discharge sides and in the pump cas-ing

, - , _ , s /s COMMANDK P M N RUN STATUSk i l 5 j COMMON TROUBLE ALARM^ t ' ^ L/R STATUS STOP

I SHUT DOWN COMMAND

Consider the use of a piping spool piece to facili-tate dismantling

It is already created and exists

Installing pump insulation for personal protection Service temperature is 40C and there is no need for personnelprotection insulation

Production interruptionDefine the pump sparing philosophy Based on RAM analysis, a second pump with the same arrange-

ment is added (to provide 2 x 100% capacity)

l.A parallel, exactly similar sparesystem can take care of fiow thatwould result fi^om the loss of a givencomponent. Examples include sparepumps or spare heat exchangers(in highly fouling services). Theinstallation of spare equipmentis popular for fiuid-moving equip-ment, since interruption of servicein pumps and compressors caimotbe handled through other belowoptions. One important example ishaving two fire pumps installed inparallel, with two different t5rpesof drives (for instance one with anelectromotor and the other using adiesel drive pump).

2.A parallel component can be usedand the fiow can be redirected

to the alternate component in-stead. Examples include having amanual throttling valve (such asa globe valve) in the bypass lineof a control valve, or placing a by-pass line for a PSV together witha pressure gage (or pressure tap-ping) and a globe valve.

3.The feed to the equipment can besimply bypassed temporarily withmarginal impact on the operationof the system.

4.The feed to the equipment canbe redirected temporarily to an"emergency reservoir" (such as atank or pond), and processed laterby returning it back to the system.Usually this option is available forliquid streams.

5.The storage tanks upstream anddownstream of the componentshould have enough residencetime to continue operations. Thisway, if the component goes outof service, the upstream stringof equipment can still feed theupstream tank and downstreamcomponents can still be fed bythe downstream tank. This ar-rangement will prevent a surgethat could impact connected plantcomponents.

6.The feed to the equipment is re-directed temporarily to a waste-receiving system or fiare.

7.Whole plant or unit should shutdown: This option should beavoided, if possible. However,



sometimes this is inevitable whenthe equipment of interest is a keyasset in the facility.

Table 5 summarizes these optionsfor a P&ID.

Spare pump optionsWhile the discussion below focuseson pumps, the guidelines apply toany other types of spare equipmentas well. A spare pump, depend-ing on the criticality of the service,could be "an installed spare" or "aworkshop spare." A workshop sparepump is not installed but can bemoved from the workshop and de-ployed within a short period of time(say, 24 hours).

Decisions related to any of theabove two options (in terms of in-stalled spares or workshop spares)can be based on different param-eters, including the following: Mean time between failure

(MTBF) of the equipment Mean time to repair (MTTR) of

the equipment Cost of maintenance Value of the "lost production"For installed spares, if the ambienttemperature of the space aroundthe a pump is far from the operat-ing temperature of the pump (forinstance, differs by 100 to 150C),the pump should be "a hot standbypump" (or "a cold standby pump" forcryogenic service) to make sure itwill not experience thermal shockduring the startup. Otherwise thepump could be installed with nospecific "stand-by provisions."

If a spare pump is supposed to"sit" beside more than one operat-ing pump, another feature thatshould be decided is whether thespare pump is a common spare(to be available for several operat-ing pumps), or is intended for usewith just one dedicated pum; thusa spare should be installed for eachoperation pump.

For spare pumps, the user mightexpect that all pumps should beable to act as both an operatingpump and a spare pump for anyof other pumps that may be outof service (where no specific sparepump has been designated). Whenproviding common (shared) spare

pumps (that may need toperiodically function asspare pumps for all otherpumps), the specific pip-ing arrangements aroundthe pumps will need to beelaborated on the P&ID.

Table 6 shows an exam-ple of P&ID developmentfor a pump in one case. Thistable only provides the re-quired thought process forthe development of a pump P&ID asan example.

Additional important itemsIn addition to the last four stagesof the lifecycle of a plant (discussedabove), a few other items must beconsidered:Future plans. If there is any planfor expansion, or any prediction forimplementing new, under-reviewinnovations in future, this needs tobe addressed in the P&ID to facili-tate the implementation of the fu-ture changes with minimum impacton the operating plant.Insulation to safeguard personnel.Equipment and pipes with skintemperatures that are greater than60-75C (especially for metallicitems), and those that are locatedin crowded areas within reach ofworkers, must be insulated. This in-sulation is called personnel protec-tion (PP) insulation on the P&ID.Useful rules-of-thumbWhether the design engineer (inthe role of P&ID developer) iscapturing general items (such ascontainers, fluid movers, heat ex-changers and so on) or more spe-cialized items (such as liquid-ex-traction towers, filter press and soon), these general rules-of-thumbcan help:l.Much of the equipment that we

buy for any given plant is not"custom built equipment," so wecannot expect the components tooperate exactly according to thedesired operating points. Even forthe case of custom made items, weusually expect that the equipmentwill operate in a pre-determined"window" of operation. The resultis that almost all equipment in

Too few tools

Badly operatingplant

Fiexibiiityin operation

Too many tools

Confusion

FIGURE 7. A balance must be sought between"too much" and "too little" when developing theengineering details for a P&ID

CPI facilities should be broughtcloser to the desired operatingpoint through the use of a controlsystem. Key parameters that mustbe controlled include flowrate andhead for pumps, and heat duty forheat exchangers.

2.Check the required temperatureand, pressure for each item (inletand outlet) and make sure theseare matched with process needs.If they are not matched, take ac-tion to address them.

3.Check the required flowrate foreach item. What is the minimumflowrate that can be handledwithout negative impact on theprocess, and what is the mini-mum flow that can be accommo-dated before there is potentialharm to the equipment? If thereis a chance of harm from low flow,then plan accordingly to protectthe equipment from it.

4.Check the required composition ofthe streams going into the equip-ment and note the special carethat should be taken. For exam-ple, a positive-displacement pumpis prone to plugging if the liquidcontains large suspended solids.In this case, a strainer should beinstalled.

5.Be sure to account for the requiredutilities and their temperaturesand pressures.

6. What are the weak points of theitem and what requirementsshould be taken when designing aproper SIS system for the item?

7.Which parameters must be moni-tored by the operator makingrounds? Think about the five keyparameters: Temperature, pres-sure, level, flowrate and composi-tion.

8.Be sure to acknowledge which as-7 0 CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2014

pects of each component need in-spection or monitoring.

9.Review any history of item fail-ures (in terms of frequency andtime for maintenance) and act ac-cordingly to address them.

10. Consider the impact of an itemgoing out of service. What stepscan be taken to minimize theimpact of this on the rest of theplant? Is it possible to have a sim-ilar system as a spare?

Common challengesDuring the development of aP&ID, the need to choose betweencompeting options is a commonchallenge. Here are several commonscenarios:"Should I show a given detail on themain body of P&ID by a schematic,or can I capture it in the note area?"The P&ID is a pictorial diagram. Asmuch as possible, the P&ID shouldcapture relevant schematic shapes."Should I add the item or not?" Itemsshould be added to give requiredflexibly to the operator. A plant withinsufficient "facility resources" is dif-ficult to operate.However, from theother side, this is also the case for aplant with more than enough pipecircuits, control valves, alarms andSIS actions. For example, a plantwith too many alarms can "overload"the operator and result in a loss ofurgency from the operator when analarm does activate (Figure 7)."Adding more doesn't hurt." This isa popular statement when P&IDdevelopers try to "bj^ass" conduct-ing a rigorous evaluation for thenecessity of an item on the system,and thus place it with no real ne-cessity. However, designers shouldremember that in some cases, add-ing an item might not necessar-ily increase the capital cost of theproject if the item is small andrelatively inexpensive but maystill increase the operating cost be-cause of required inspection, main-tenance, related utility and chemi-cal usage and more. In additionto that, any new item added to thesystem provides a new opportunityfor mistakes, cross -contamination,leaks and other problems."Should I add it here on the P&ID

or will it be captured in other docu-ments?" The P&ID is supposed tobe a common document that can beused by quite a few different disci-plines. Incompleteness is an inher-ent feature of it. Furthermore, theP&ID is supposed to be kept in theplant, for easy use by operators. Ifit is too cluttered, its usefulness isdiminished.

Generally speaking, all processequipment should be shown inP&IDs. Sometimes, non-processrelated P&IDs (such as gearboxesand lubrication systems) shouldalso be shown on the main P&ID oron auxiliary P&IDs. Meanwhile, ifthey are not shown on P&IDs, theirdetails can be found in vendor doc-uments.

All pipes and pipe appurtenancesexcept bends and elbows are shownon P&IDs. Flanges should be de-picted, if there is a specific reasonfor them. Specific piping items thatare not shown on the P&ID can befound on piping models.

When it comes to instrumenta-tion and control system, thingsbecome more debatable. The threemain items of integrated controland safety system (ICSS) elementsare: Regulatory control system(BPCS), the alarm system and theSIS. Almost everyone agrees aboutbasic process control items shouldbe shown on the P&IDs. They aremainly the elements of the controlloops. For alarming systems, thesame clarity exists. The main de-bate is usually on SIS systems, interms of the question of "down towhich level of detail the safety in-terlock loops should be shown onthe P&IDs?"

Different companies follow differ-ent directions."Based on my past experience..." Theinherent creativity required in cre-ating P&IDs may become hindered,if for every single case one refersto "past experience." As unlikely asit may seem, the "this is what hasbeen done before" mentality is notthe most efficient way of developingthis document. That being said, thetechnological innovations, availabil-ity of materials, quality of raw ma-terials, required quality of products.

capacity of the system, and ambi-ent temperatures and pressureswill most likely differ for each newproject. When developing P&IDs, apreviously effective method may beentirely ineffective in the currentproject, while a method that hasproven useless in the past may workperfectly well this time around.

Edited by Suzanne Shelley

AuthorMohammad Toghraei iscurrently an independentconsultant and is the instruc-tor of several P&ID-relatedcourses offered throughProgress Seminars Inc. (Web-site www.engedu.ca; Email:[email protected]).Toghraei has more than 20years of experience in pro-cess engineering. For thepast seven years, he has

held different technical and leadership rolesrelated to oil removal and water treatment forsteam-assisted gravity drainage (SAGD) proj-ects. Toghraei holds a B.Sc. in chemical engi-neering from Isfahan University of Technology,and an M.Sc. in environmental engineering fromthe University of Tehran, and is a member ofAPEGA. He is a certified professional engineerin Alberta, Canada.

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