The Magazine for the World Sulphur and Sulphuric Acid Industries

January · February 2001Number 272

� Ensuring uniform heat distribution in sulphur vapour lines

SULPH.F/COVER 25/1/2001 4:55 pm Page 1

SULPHUR PROCESSING

Sulphur No 272 January . February 2001

Certain hot process piping ser-vices require thermal mainte-nance devices to keep stream

temperatures within design limits. Insuch cases conventional pipe jacket-ing has been regarded as sufficientbut often too expensive. An alterna-tive to pipe jacketing, tube tracing,does not effectively prevent prob-lems caused by temperature varia-tions along the process pipe wall.Frequently, tube tracing is used as aneconomical way to try and compen-sate for heat loss. In low temperatureapplications with broad temperatureenvelopes, steam tracing can be effec-tive. However, in those applications,the position of the tracers, and thetemperature distribution in the pipematerial are not considered impor-tant. A new system for process tem-perature maintenance or uniformpipe wall temperatures, comparablein price to tube tracing, consists ofcontoured, bolt-on trace elements.These elements are strategically posi-tioned after modelling heat dynamicsin the operating piping system. Indealing with the actual problemscaused by unwanted heat variation,frequently the key principle is assur-ing uniform pipe wall temperature.To assure uniformity, it is necessary

to manage the heat distribution inthe pipe wall. Proper management ofheat distribution depends on thenumber and placement of heatingelements. This placement in turndepends on accurate modelling ofprocess thermodynamics. Therefore,

the heating element vendor shouldapply accurate process modellingsoftware to assure uniform pipe walltemperature. An example of theresult of this type of modelling isshown in Figure 1. Heat from steamat 159ºC is applied from a contoured

Ensuring uniform heatdistribution in sulphur

vapour linesUniform temperature distribution in sulphur pipeline walls can be

achieved by applying heating elements which are strategically positioned using accurate thermodynamic process modelling software. A well-designed system protects

the process from interruptions caused by corrosion or blockage of the pipeline. David R. Hornbaker of Controls Southeast, Inc. reports on a typical case in point.

Source: CSI

Fig. 1: Calculated pipe wall temperatures for an Alberta-based SRU

132ºC

133ºC

136ºC

141ºC

147ºC

steam=159ºC

30 in. process pipe4 control trace heating elementsgas=130ºC

midpoint between control trace elements

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heating element to a pipe wall atdefined intervals. Directly under theelement, the wall temperature is147ºC. Note the decrease in walltemperature as the distance from theelement increases, until a minimumof 132ºC is reached.

At this point heat conductedthrough the wall from another heat-ing element causes the wall tempera-ture to rise again.The wall tempera-ture varies within a defined range andthe warmest and coolest points canalso be readily identified.

Application of contoured pipetracing after thermal analysis preventsproblems of condensation and associ-ated pipe corrosion caused by mixedvapours in sulphur processing.

Case history: sulphur recoverySulphur recovery units in refineries orgas plants usually include tail gas ordegas vapour lines. These lines arecommonly heated to prevent the con-densation of either sulphur or water.In most cases, the gas in the line is atan elevated temperature and can beassumed to be at or above dew pointtemperatures. There are two dewpoints, one for water and one for sul-phur. It has been common practice toemploy conventional tube tracing (orin some cases electric tracing) to heatthe line. Despite this practice, con-densation and resulting corrosion areoften still a problem. The cause isuneven or inadequate heating of thepipe.

A gas plant operator can achievemore uniform heating of the pipe. It isnecessary to conduct a detailed ther-mal analysis of the pipe, gas, heatingelements and insulation. Then theoperator installs an innovative bolt-ontemperature management system.Thermal analysis reveals that the flow-ing process gas may actually be cool-ing the pipe wall. Conventional steamor electric tracings are usually notheating the pipe uniformly, resulting incold spots where condensation occurs.

Condensation can be preventedby using high-performance bolt-onsteam tracing material, properly dis-tributed around the pipe. This casedescribes an installation at a naturalgas plant sulphur degassing unit incentral Alberta, Canada.

Steam tracing allowscorrosionThe degas vent piping at the Albertaplant had been in place for about sixyears and had suffered serious corro-sion, particularly in and around thelow spots.The original steam tracinghad been done with conventional5/8 in. tube tracing employing 500kPag saturated steam. Vent line sizeswere 20in., 24in., and 30in. diameter,running about 55 meters from thedegas pit to the incinerator.They hadbeen installed with 10 tracing tubes onthe 20in. line and as many as 13 tubeson the 30in. line.The tracing was gen-erally located around the bottom ofthe pipe in horizontal runs and on aconvenient side on vertical runs.Theanalysis model showed that 13 tubeswas the very minimum that could beused if they were spaced at equal inter-vals around the 30in. pipe.

Because they considered the six-

year lifetime of the original pipe to beeconomically unsound, plant manage-ment sought to find a better way tomaintain pipe temperature. Theyreviewed the experience of similarplants equipped with ControTracesystems. In some instances, these sys-tems had been effective in comparableservice for over ten years.

The gas plant operator analysedthe various parameters involved inheating and cooling the pipe. Theyused a proprietary finite-differencecomputer model developed byControls Southeast, Inc. (CSI), themanufacturer of the ControTrace sys-tem.They created a model for gas flowthrough the pipe and generated tem-perature profiles of the piping systemunder various operating conditions.

Conventional trace sizing pro-grams simply deal with the pipe sizes,temperature, insulation and ambientconditions.These programs calculatetotal heat loss from the pipe and deter-mine the number of heating tracersneeded to replace that amount of heat.No consideration is given to how theheat gets distributed to all parts of thepipe. Adding more heat than is beinglost does not assure that all points onthe pipe will operate above the con-densation temperature.

Better analytical toolThe more comprehensive analyticaltool provided by CSI models showshow much heat is going into the pipe.It measures how that heat is distrib-uted by conduction along the pipewall, through the insulation, and byconvection, to the process itself. Theanalysis uses the following data sets:pipe size; wall thickness and materialproperties; insulation thickness andproperties; worst-case ambient tem-perature; wind conditions; process gasproperties; and flow rates.From all of these properties and flowconditions, CSI was able to calculate aheat transfer coefficient specific to theAlberta plant.

By applying the model, plant man-agement and CSI determined detailedtemperature profiles for various condi-tions. Variables included heating ele-ments and their spacing, steam tem-perature, insulation thickness, andflow conditions (from maximumexpected to zero flow). They deter-

SULPHUR PROCESSING

Sulphur No 272 January . February 2001

Fig 2: ControTrace installation.

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mined the optimum number of ele-ments and their placement so that pipewall temperatures would preventprocess condensation.

Plant management then conducteda material cost and installationexpense analysis specific to the Albertaplant. It revealed that even though thematerial cost for ControTrace washigher than for tube tracing, theinstalled cost was expected to be sig-nificantly less. Because the manage-ment of the Alberta plant expectedlonger pipe lifetime with theControTrace system, both the perfor-mance and economic criteria were the-oretically met. Management awardeda contract to CSI for the necessarythermal modelling/design and fabrica-tion of the new bolt-on tracing system.Installation was done by the plant andplant contract personnel.

Effective heat transferControTrace heating elements aredrawn from SA178 carbon steel into arectangular shape. One side is curvedto fit the outside contour of the pipe.The ControTrace elements used onthe sulphur pipeline are nominally25mm by 50mm.They are registeredin Canada with a pressure rating of2380kPag at 340°C. Heat exchangebetween the steam and the pipe isgreatly enhanced by the use of a thinfilm of heat transfer mastic. This

results in improved and more pre-dictable performance. Not only doesthe ControTrace system use fewer ele-ments (tubes) than the traditionalround tubing, it is significantly moremechanically stable.

The sulphur pipeline continued tooperate during the installation of theControTrace system. In spite of this,the actual installation cost was lessthan originally predicted. Most of theinstallation was done from scaffolding,although some of the bolt-on tracingwas shop mounted prior to pipe hang-ing.The ControTrace is supplied pre-

fabricated into rings and headeredpanels. (See figure 2: photo for typicalstraight run.)

During installation, the non-hard-ening heat transfer mastic was appliedto the trace surface.Then the individ-ual panels and ring halves were boltedtogether. It was necessary that thepanels and rings be pulled snuglyagainst the pipe and flanges. Meetingthis requirement was not difficultbecause the ControTrace is veryrugged and can withstand large bolt-ing pressures. The installation crewsaid that the ControTrace productwas easier to install than tube tracing.(See figures 3 and 4: two photos showinstallations around an elbow and ata pipe support.)

Following installation the systemwas insulated. Insulating over theControTrace was no more difficultthan insulating pipe of similar sizewith tube tracing. Trapping of thesteam system was based on plantstandards and steam loads takenfrom the thermal modeling program.Interconnection of steam betweenindividual components was donewith hard piping in accordance withplant standards.

At startup, pipe wall temperatureswere checked with a surface pyrome-ter. Temperatures quickly came tovalues in the expected range. Sincethen, periodic pipe wall temperaturemeasurements have been made withall values falling in the expectedrange. S

SULPHUR PROCESSING

Sulphur No 272January . February 2001

Fig 3: ControTrace on an elbow.

Fig 4: ControTrace at a pipe support.

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