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The topic of carrying out processengineering calculations in themost effective manner can beconsidered in these four ways:

1. The purpose of the calculation

2. The content of the calculation3. The method used to carry out the

calculation4. The performer of the calculation

The purpose of the calculationEngineers typically carry out cal-culations for a variety of purposesincluding the following:a) To conduct a technical or eco-

nomical evaluation of a plant orspecific unit (such as a heat andmaterial balance), to carry out

evaluations in the early stages ofthe project, or for use while writ-ing studies or technical memosfor stakeholders

b) To size, rate or specify a piece ofequipment

c) To generate a specific operationalprocedure

Sizing, rating, and specifying arethree distinctly different goals ofcalculation efforts. Process sizingmeans defining all the (process) as-pects of the equipment, so that the

manufacturer can produce it withlittle process knowledge. Sizing ef-forts typically involve calculationsthat aim to identify all the un-knowns associated with one equip-ment component, to enable the fab-ricator to produce that component.

By contrast, when specifying anequipment component or instru-ment system, the engineer must de-fine a specific amount of informationfor the manufacturer, and can thenexpect the manufacturer to satisfythose requirements. For example,when an engineer plans to buy apressure safety valve (PSV), he or

she specifies the required setpointof the PSV, the release rate and afew other items. This effort doesnot require the engineer to design aPSV by sizing the orifice diameter,spring constant and so on.

It is clear that no process engineerdesigns a centrifugal pump just bycalculating the impleller diameter

and vane angles. Today, process en-gineers can expect that the manu-facturer will provide much detailedinformation to support the processof designing and specifying processequipment. As a result, there is verylittle pure designing in todaysspecialized market; rather, thereis always an element of specifyinginvolved when carrying out sizingcalculations.

While specifying and sizing areconcepts related to equipment or

instruments that are planned to beprocured later, rating is an activitythat involves calculations associ-ated with existing equipment com-ponents. During rating, the ques-tion that the engineer is trying toanswer is whether an existing com-ponent can be used in a specific newapplication or not. Rating effortsrequire evaluating current equip-ment for a specific type of serviceand conditions.

The content of the calculationCalculations involve the followingthree steps:

Step 1 requires identifying suit-able input values (assumptions)

Step 2 involves choosing and usingan appropriate methodology one that uses the parameters es-tablished in Step 1 to arrive atan answer that has the requiredaccuracy in the available time slot

Step 3 involves the evaluation

and verification of assumptionsand results

Step 1. Identifying suitableinput values (assumptions). Ju-nior engineers tend to struggle withthis step. Too often in the universitysetting, the instructor provides thegiven values, whereas in real-worldsituations, it is up to the engineer todetermine which input values andassumptions are relevant. This ef-fort requires three sub-steps:1. Find effective parameters. Engi-

neers need to find the parametersthat will impact the results they areseeking. For example, within a spe-cific issue, there could be three pa-rametersM,NandP. In this step,these three parameters need to beevaluated in order to recognize theones that are most relevant to theproblem at hand. At this stage, theengineer should be able to evaluatethe three parameters of M,N,andPto determine that, for example,MandPare the only required param-eters (or the most relevant param-eters) and these will be consideredfurther in Step 2. Or with the level

Feature ReportEngineering Practice

Calculations in Process

EngineeringKnowing how specific calculations

differ can focus your efforts

Mohammad ToghraeiEngrowth Training

2-phaseflow

3-phaseflow

Non-Newtonian

fluidLiquid Gas Different

pairsLiquid/gas

/solid

Line-sizing

Tank/vessel

Hydraulic calc.:

pump/comp CVHX sizing

PSV sizing

Single flow

Design

Increasing difficulty

FIGURE 1. Themost populartypes of calcula-tions are relatedto the sizingof lines, tanks,vessels, pumps,compressors heatexchangers andPSVs. This figure

shows thesegeneral items andtheir variations

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of required accuracy, perhaps onlyMwill be considered in Step 2.2. Select a numerical design value.In this step, a numerical value needs

to be assigned or determined for theselected parameter(s) identified inthe previous step. The magnitudeof the design or rated parameter isthat number (or range) that coversall the different scenarios that couldbe encountered during the operationof the plant, plus a reasonable mar-gin. For example, Mdesign= 6 couldbe a wise decision when parameterMcan accept magnitudes of 2.0, 3.5and 5 during plant operation.

Trial-and-error efforts, which

typically require an educated guessat the beginning and some adjust-ments made along the way basedon final results, are often required.Certain rules-of-thumb can helpengineers to make a better pre-liminary guess, and this can helpto decrease the calculation time.The reason for this iterative routeis that each methodology has somelimitations, and at the end of thefirst run of calculations, any limi-tations should be checked to make

sure they have been met.Step 2. Choosing the right meth-odology. This step is very impor-tant. The selected methodology mustbe accurate and at the same time,the required engineering time forthat method should be justifiable. Askilled process engineer should beable to identify calculation method-ologies for each situation that meetthe specific accuracy and time re-quirements or constraints.

For example, during pipe sizing,

an engineer should be aware of thestandard pipe sizes that are alreadyavailable on the market. Designinga given pipe that is sized to, for in-stance, two decimal points, does notmake sense, as the market alreadyprovides defined pipe sizes.

Meanwhile, in some cases, one sce-nario can be converted to anothercase, to enable the use of a simplerversion of the methodology andshorten the calculation time. Sucha conversion effort is only accept-able if this new scenario is the moreconservative approach. However, insome cases, the more simplistic and

conservative approach may be tooconservative in a way that makesit unacceptable. In such cases, theoriginal albeit more complicated route must be taken. This mayoccur in situations in which theworst-case scenario has led to ex-traordinarily oversized equipmentor equipment that is overly difficult

or costly to construct.Discussed below are some of the

popular simplifying conversionsthat are widely used today:1. The thermodynamic approachversus the kinetic approach. Mostoften, the thermodynamic approachis selected due to its well-estab-lished methods and its popularity interms of published works and data.For example, finding the evapora-tion rate from a surface of a liquid ischallenging. It depends on different

parameters and requires the con-sideration of various phenomenaincluding heat transfer and masstransfer. However, by changing theapproach from kinetic (in whichthe evaporation rate is calculated)to thermodynamic (in which onlythe saturated pressure of the vaporabove the liquid surface is calcu-lated), the effort will be simplified.2. Assuming steady-state vs. un-steady-state conditions. Most often,the steady state is selected, thanks

to its well-established methods andsimplicity of its calculation usinginexpensive software or even aspreadsheet progra. For example,analyzing PSV opening phenomenais not an easy task as it is inher-ently an unsteady-state operation-However, during PSV sizing, weoften pick the largest flowrate thatis considered a steady-state case fora mere few milliseconds during theopening of the PSV.Step 3. Verifying assumptionsand results. This is not merely theend of the task, but is a very crucialstep, the results of which may call

for a second run-through or recon-sideration of Steps 1 and 2.

As mentioned, the design processmight need some iteration to reachthe most appropriate results. Forexample, if the calculation is sizingor specifying, it will result in one ofthe following three conditions:1. A case involving an extremely

inexpensive or small piece of equip-ment. In this case, the equipmentoften cannot be custom-made butmust be selected off-the-shelf. As aresult, the final piece of equipmentmay not be exactly what is requiredvia the calculations, and adjust-ments may be required to mate thepiece into the overall configuration.For this reason, it may be efficient tohave the specifications of standard-ized (off-the-shelf) equipment onhand throughout the course of the

calculations, so that adjustmentscan be made as the calculations arebeing constructed.2. A case involving moderately sizedor moderately complex equipment.In this case, the equipment will typ-ically be custom made in the shop.It is important to keep in mindthe dimension limits of the shopin question. If the item is too large,it will have to be fabricated in thefield, and the price will be consider-ably higher. In an illustrative case, a

careless designer may design a tankwith diameter half a meter abovethe limits of the shop, and thereforeunnecessarily cause a price jumpbecause the tank suddenly changesfrom a shop-fabricated item to afield-fabricated one. Shop limita-tions are imposed not just by themachinery-manufacturing limitsin the shop, but by the road restric-tions or overpass weight limits thatwill affect the transportation of theequipment to its final destination.3. A case involving a large piece ofequipment. In this case, the equip-ment will typically be custom

CHOOSING THE BEST MEDIA

Some companies accept calculations that are done by hand, while others will utilize or evenrequire the use of specific software. With hand-done calculations (as opposed to those car-ried out using a spreadsheet program), a clear disadvantage is that any adjustments will

require all of the subsequent steps to be re-done (That is, there is no way to link equations sothat they will be automatically updated if a given parameter is changed).

By comparison, spreadsheet programs, such as Microsoft Excel or an equivalent data pro-cessor, are designed so that any changes will result in the computer adjusting all subsequentsteps automatically. The disadvantage to this is that the calculation process is not always vis-

ible, as some formulas may be buried within a cell. To maximize the advantages and minimizethe disadvantages of a spreadsheet program, the following steps should be taken:

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made in the field at a higher cost.This high cost is due to the factthat skilled workers and variousrequired utilities will need to beshipped and present at the field inorder to construct the larger compo-nents. As a result, this case is oftenused as a last resort.

The performer of thecalculationGenerally speaking, each group ofprocess engineers is expected to beable to carry out specific types ofcalculations, based on this roughclassification:Junior/intermediate design pro-

cess engineers:Sizing of tanks andvessels; hydraulic calculations forpumps, compressors and controlvalves; sizing of heat exchangers;andPSV sizing after receiving releaserates from an intermediate or se-nior engineerIntermediate/senior design pro-cess engineers: All of the above

items, in multi-phase cases (thatis, two-phase flow for pipe sizing);separators (two- and three-phaseseparators, gas-knockout drums,sedimentation vessels, clarifiers);other industry-specific items (suchas distillation towers for petroleumrefining, and ion-exchange systems

for water treatment); PSV sizing(performing the first steps of PSVsizing including defining pressur-izing scenarios, finding the credible

and governing scenario(s) and re-lease rate calculation)Senior design process engineers: Re-actors, especially complicated ones,and industry-specific equipment. n

Edited by Suzanne Shelley

AuthorMohammad Toghraei,P.Eng., is currently a con-sultant and instructor withEngrowth Training (www.engedu.ca; Phone: 403-808-8264; Email: moe.toghrae@engedu.ca). Toghraei has over

20 years experience in the fieldof industrial water treatment.His main expertise is in thetreatment of wastewater fromoil and petrochemical com-

plexes. For the past seven years he has taken ondifferent technical and leadership roles in watertreatment areas of SAGD projects. Toghraei hasreceived a B.Sc. in chemical engineering from Is-fahan University of Technology and an M.Sc. inenvironmental engineering from the Universityof Tehran, and is a member of APEGA.

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1. Ensure that all of the calculations are interconnected in such a way that altering the inputs willalter the results accordingly. Take care in this effort, as any disconnections may be disastrous,because adjustments to the input will not result in appropriate alteration to the output.

2. Ensure that all manual input is visible and clearly marked to give the designer the flex-ibility to alter it. Any hidden manual input may not be recognized by a new designer as apotentially adjustable value.

3. Ensure that new users do not disable any pre-existing macros in the program.4. Ensure that iteration calculations are enabled where required.5. Ensure that the calculations are legible both on the computer and in print.6. Ensure that when the flexibility of adding a new row or column is provided in a template,

all of the information is transferable. Such flexibility exists in some modular pump-sizingspreadsheets.