Leak Free Diathermic Oil Systems

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How to Build a Reliable, Practically Leak- Free Therm al Fluid

System

John Cuthbert and Dough Heydrick, Dow Chemical Co.

Posted: October 1, 1999

Proper system design and equipment selection can help keep thermal fluid leaks

to an absolute m inimum .

Thermal fluid systems utilizing synthetic organic- and silicone-based heattransfer fluids are used in industries as diverse as pharmaceutical production,

petrochem icals, m an-m ade fiber production and environm ental test chambers.Operating conditions can vary substant ially, from the relatively constant

tem peratures experienced in large vapor systems ut ilized by fiber plants, t odramatic, several-hundred-degree swings that can occur in t he sam e reactor , a

situation not uncommon in multipurpose production plants.

Regardless of the heat t ransfer fluid t ype, application or use temperature, allsystem operators share a comm on desire: that these systems be as reliable and

as leak-free as possible. Reducing fluid leakage from a thermal fluid system hasseveral benefits, the m ost obvious being economical. I f t he fluid stays in thesystem, new fluid does not have to be purchased to make up losses. Moreover,

environment al and regulatory issues impacting t he operation of heat tr ansfersystems are best addressed by keeping t he fluid in t he system.

A leak-free system also helps minimize plant personnel exposure to the heattr ansfer fluid, which helps m aintain the overall level of industrial hygiene in theplant. In addition, heat transfer fluid that leaks into porous insulation represents

a fire risk. Eliminating leaks, as well as choosing the proper insulation, cangreatly r educe the r isk of t hese types of fires and thus help ensure worker

safety. Finally, som e therm al fluids have odors that plant personnel may findobjectionable. Keeping leaks to a m inimum while maintaining adequate

ventilation will keep such complaints to a minimum.

To a large extent , r eliability and int egrity can, and should, be designed into a

thermal fluid system. The best designs result when users focus adequateattention on selecting t he proper equipment, m inim izing joint s and connections,

and avoiding inappropriate joints and connections. The following guidelines willhelp you select piping, pum ps, valves, flanges, gasketing and expansion j oints -

- all key elements essential t o the design of a reliable, virt ually leak- freesystem. While other approaches may w ork equally well, t hese practices were

developed over many years and have been used successfully in manyoperations.

Star t w ith the Right Connections

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17-04-2012http://www.process-heating.com/copyright/ff5bec343a268010VgnVCM100000f932a...

Basell's HSEDC requires cellular glass (e.g. Foamglas) insulation on oil and peroxide lines / equipment to take care of above hazard.


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I f you are concerned only w ith m inimizing leaks, a properly designed andinstalled all-w elded system will provide mu ch bett er leak protection than a

system w ith f langed and screwed connections. However, t he tr ade-off wit h anall-welded system is a decided lack of flexibility for maintenance operations. For

example, while it is possible to repair or rebuild a valve inline, it is much easierand cheaper t o remove the valve and send it out for repair. I n practice, m ost

therm al fluid systems contain a num ber of f langed and screwed connections --and each is a potential leak point. For this reason (and for the sake of the

system operators who will have to deal with any leaks), a therm al fluid systemshould be designed to include only as many breakable connections as are

absolutely necessary.

Screwed connections should be avoided wherever possible, as bendable tubingwith com pression fit tings has been found to be a satisfactory alternative. Where

screwed connections are desired, however, they should be made from schedule

80 pipe and limited t o pipe diameters less than 1.5" . The tapered threads

should be cut tr ue-t o-gauge wit h a sharp, clean die, washed wit h a good

solvent, then covered with a pipe thread sealant. But, under no circumstancesshould you rely on pipe thread sealant t o make a good joint out of a poor one.

By far, flanged connections are the best comprom ise you can make in t erms ofcost, v ersatilit y and sealing ability. As such, t hey are t he most used breakableconnect ions in therm al fluid systems.

Choose Flang es and Gaske ts Car efu lly

Several types of flanges can be used in thermal fluid systems. Your selectionshould be influenced by m any factors, including system operat ing

tem peratures; operating pressures; purchase and m aintenance costconsiderat ions; perceived perform ance differences in safety and emissions;

desired gasketing; and past operating experience. While ring-j oint, t ongue-in-groove and male- female flanges can be used in therm al fluid system s, the

Regardless of t he num ber

of pum ps, valves and

connectors, thermal fluidsystems can be designed

and built t o keep fluid leaksto a minimum wh ile

providing reliableperformance.

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American Society of Manufacturing Engineers (ASME)- and American NationalStandards I nstitu te (ANSI ) -specified B16.5 raised-face flange is the m ost

prevalent. But even here, there is an im portant choice to consider.

Typ ically, an ASME/ ANSI Class 300 raised-f ace flange is chosen over anASME/ ANSI Class 150 raised-face flange. Because therm al fluid system s can

operate at tem peratures as high as 750oF (400oC), a Class 150 flange may nothave sufficient strength for t he anticipated maxim um operating pressures. For

example, at 750oF, a carbon steel Class 150 flange will have a maximumoperating pressure of 95 psig. By contrast, a carbon steel Class 300 flange willhave a maximum operating pressure of 445 to 505 psig, depending on the

mat erial group chosen. Given that the vapor pressures of most heat t ransfer

fluids exceed 95 psig at 750oF, the unsuitability of a Class 150 flange is evident.

I n addition to this, ASME/ ANSI B16.5 standard stat es: "When used above

400oF, Class 150 flanged joints may develop leakage unless care is taken to

avoid imposing severe external loads and/ or severe thermal gradients. Forother classes, similar consideration should be given above 750oF."

At 400oF (204oC), a Class 150 flange will have a m aximum operating pressure

of 200 psig, w hich should be acceptable for most thermal fluid system s.However, even at t his temperature, Class 150 flanges are m ore prone to

leaking t han Class 300 flanges.

Som e therm al fluid systems will need to wit hstand rapid therm al cycling andthe additional stresses induced by expansion and contraction forces. In general,

a Class 300 flanged system is bett er able to maintain the m inimum requiredseating stresses during thermal cycling, reducing the risk of leaks. Class 150

flanges can provide a suitably leak- free system at m ore moderate, relativelyisotherm al operating conditions.

To ensure proper gasket sealing, raised-face flanges must have a proper

surface finish. Manufacturers of the gasket t ypes most comm only used t o joinASME/ANSI B16.5 raised-face flanges typically will specify a phonographic

surface finish of 125 to 250 microinch roughness average.

Gasket choice also is determined by several factors. For starters, a gasket'sconstruction materials must be able to withstand the system's operating

tem peratures and must be chemically compat ible with t he fluid used. Therm al

fluid system s can operate at tem peratures as low as -150 oF (- 100oC) and as

high as 750oF (400oC). At these extremes, elastomers and plastics areunacceptable because of their poor mechanical properties.

Another tem perature- related challenge is resistance to heat generated by an

external fire. In the highly unlikely event that a fire occurs in the area of aflanged joint, the fluid should be contained within the system -- remember,

thermal fluids will burn. A gasket that can withstand extreme temperaturesuntil the fire can be extinguished could prove beneficial in an emergencysituation.

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Most high perform ance, high t emperatu re heat t ransfer f luids are based on

aromatic or silicone compounds. Only a few types of elastomers and plastics arecompatible wit h t hese chemistr ies, so keep th is in m ind when specifying anelastomer or plastic component . Gaskets const ructed wit h m etal and graphite

generally will meet the temperature and chemical compatibility and fireresistance requirement s of a properly designed t herm al fluid system.

In addition to a gasket's construction m aterials, carefully consider it s physical

design. A catastrophic gasket failure -- a blowout caused by unexpected over-pressurization of th e system, for exam ple -- will generate a large fluid leak that

could lead to a potentially serious fire risk.

Spiral-wound and grooved-m etal gasket designs resist blowouts and areconstructed for leak-free operation. Probably the most com m only used in t hese

systems, spiral-wound gaskets have a stainless steel inner ring, carbon steelcentering ring, stainless steel windings and flexible graphite filler. Alternatively,

grooved-metal gaskets have a core ring of grooved 316 stainless steel at least3 mm thick. I n addition, t hey are faced with f lexible graphite and ut ilize a loosecentering ring. So far, their use has been limit ed primarily t o Europe.

Properly assembling the flanged joint also reduces the chance of leaks. Duringinstallation, it is critical to protect the flange surface from nicks, dent s andscratches, which can cause the joint to leak.

Proper flange alignment is equally critical. Mis-alignment can result inoverstressing one side of a flange while leaving the other without sufficient

compression to seal the gasket. Once a flange pair is properly aligned, usestuds and bolts that comply w ith American Society for Test ing and Materials(ASTM) standards. Using ASTM A-193 Grade B7 studs and bolts wit h ASTM A-

194 Grade 2H hex nu ts will provide a fastener strong enough t o supply t herequired sealing force. I ncrementally tighten the well-lubricated fasteners in a

criss-cross pattern to m aintain proper alignment . Applying adequate initialpreload to t he flanged connect ion will m inimize the risk of leaks aft er system

startup.

Reduce Valve Leaka ges

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Valves tend to be a m ajor source of leaks in therm al fluid system s, wit h leakagetypically occurring around t he stuff ing box. Due to t he maintenance

considerations already noted, m ost valves are not welded into the system,

resulting in additional potential leak sources at the flanged piping connections.

One way to reduce leaks from valves is to eliminate the stuffing box. This can

be done by ut ilizing valves with a metal bellows (as the prim ary seal) incomb ination wit h high t emperature graphite packing (as a secondary seal).

Unfortunately, the cost and space requirements of bellows-seal valves havelimited their use in thermal fluid systems.

When choosing packed valves, pay part icular at tent ion to the valve stems,

stuffing boxes and packing specifications. I n general, nonrotat ing r ising-stemvalves are preferred to quarter-turn valves. For rising-stem valves, thesuggested stem-sealing surface roughness should be no more than Ra 0.8

micron (32 RMS). Also, the stem straightness should be specified within atolerance of 0.039" ( 1 mm ) per 39.37" ( 1,000 mm ), length total indicated

reading, and t he inner wall of the stuffing box should have a maxim um

roughness of Ra 3.2 micron (125 RMS).

As wit h gasket m aterials, flexible graphite is the valve-stem packing m aterial

best suited for th e entir e operating r ange of most t herm al fluid systems. Onepossible improvement to graphite-packed stem seals is to apply a live load to

the packing follower. This will ensure t hat as the packing wears, it will rem ainproperly compressed against the valve stem.

Globe, gat e and rising-stem ball valves are the preferred choices for thermal

fluid systems. General specifications for 2" or larger valve types are shown in

table 1. Specifications for smaller valves will vary.

Table 1. Globe, gate or rising-steam ball valves are designed

to t hese specifications (for valve sizes of 2" or larger) are t hethe preferred choice for thermal fluid systems.

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I nsulate to Reduce Fire Risk

One strong driving factor behind th e push t o design leaks out of a th ermal fluidsystem is reducing the risk of fire. Regardless of the connector or valve sealused, the potential for leaks still exists. To reduce the risk of fire when a leak

does occur, care should be taken in valve orientation and pipeline insulation.

I t is well understood t hat a combustible fluid can ignite at t emperatures wellbelow its published autoignition t emperature if spread in a th in film. The high

surface area present in m any t ypes of insulation can promote this phenomenonwhen soaked with a thermal fluid. To minimize this risk, use a closed-cell

insulation in t he imm ediate vicinity of likely leak points such as valves and

connectors. Fibrous insulation can be used for pipe runs between connectorsand valves, but install a metal drip r ing to separate t he fibrous material fromthe closed-cell insulation. A properly installed drip r ing ensures that any fluidthat gets under the insulation on one side of the ring cannot migrate down the

pipe and soak insulation on the other side.

When installing valves, consider taking additional precautions. Because valvestems are potential leak sources, t hey should be installed wit h t he stem in a

horizontal position if possible (provided the valve manufacturer does not adviseagainst t his orientation). I n th is configurat ion, should a leak occur, the fluid w ill

drip away from the valve rather than down into the insulation around the valve.

Pick the Most Effective Pum p and Sealing System

Figure 1. Putting a nitrogen purge on thestuffing-box vent eliminates oxygen, which in

turn reduces coking and prolongs seal life.

Table 2. Although single-

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Historically, heavy-du ty API centerline-m ounted pum ps have been preferred for

therm al fluid systems operating above 350oF (175oC). These pumps typically

use either standard or cartridge metal-bellows seals.

There are several limitations to this choice of sealing system. While properlyinstalled single mechanical seals can provide reasonably leak-free operation, by

their nature, mechanical seals cannot be considered a zero-emission sealingsystem. Furthermore, mechanical seals are precision-engineered pieces of

equipment -- sensitive to vibrations, misalignment, excessive temperatures anddirt. All of t hese factors must be considered dur ing pum p design, installationand operation.

I n addition, accurate alignment is essential to ensure proper operation of singlemechanical seals. Therefore, a pipe stress analysis must be conducted on thepump to ensure t hat t he allowable forces on the pum p flanges are not

exceeded.

One positive is that many styles of single mechanical seals are available. Some

can operate at tem peratures as high as 800oF (427oC) without f iltered, cooled

seal flushes. Other seals require that the seal-face temperatu re be kept below acert ain point t o avoid heat distortion. Such seals typically require filt ration and

cooling of the seal flush to less than 400oF (204oC).

With seals, again, t here is a trade-off t o consider. Seals that do not requireextra cooling and filtering cost m ore; seals that do r equire cooled and filtered

seal flush are less expensive but reduce thermal fluid system efficiency.

Another choice that m ust be m ade is the seal-face const ruction m aterial. Thereare two alternatives: hard face-to-hard face (typically silicon or tungsten

carbide to silicon or tungsten carbide) or hard face-t o-soft face (silicon ortungsten carbide to carbon). The best choice for a given application will depend

on the specific system conditions. For systems exceeding 400oF (204oC),secondary seals typically are graphite.

Once the single mechanical seal is installed, you can take several operational

steps to extend it s life. Seal faces are sensitive to vibrat ion, so prom ptly

eliminating pump cavitation is critical. Seal faces also are sensitive to abrasion,so the therm al fluid system should be operated and m aintained to generateminim al solids.

mechanical-seal pumpstypically offer lower long-t erm

cost of own ership, seallesspumps t ypically offer m uch

longer MTBF, r educing th echance of associated process

downtime and environmentalchallenges over the life of your

therm al fluid system.

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Startup can present solids problems if caution is not t aken. During start up of a

new system t hat w as not cleaned, it is not unusual for m ill scale or othercorrosion products to break free. These corrosion products typically are very

fine and cannot be removed wit h start up str ainers. I f t hey reach the seal faces,

they could cause damage. Eliminat e this problem by cleaning t he system beforefilling it with the heat transfer fluid, or by circulating the fluid through a bypass

filter that will remove the particles during startup.

Solids also can damage a seal if they form on the outside of the seal itself. Allmechanical seals leak a small amount of fluid dur ing norm al operation. When

this fluid escapes to the atm ospheric side of t he seal, it com es into contact wit hair. Oxygen in t he air reacts w ith t he fluid, causing a carbon buildup on the seal

faces that could, if severe enough, cause the seal to fail. Putting a nitrogenpurge on the stuff ing-box vent elim inates oxygen, wh ich in t urn reduces coking,

thus prolonging seal life (figure 1).

Dual mechanical seals also have been used on pumps in thermal fluid systems.Many of the considerations for single mechanical seals also must be confronted

with dual seals, with one significant addition: A dual-seal system requires abarrier fluid. Because this barrier fluid can leak into the system , t he fluid choiceis important. I t m ust not react wit h the heat t ransfer fluid yet m ust be as

therm ally stable. For these reasons, the heat tr ansfer f luid generally also is

used as the barrier fluid.

A Zero Altern ative

In contrast to pumps equipped with mechanical seals, sealless pumps can be

considered zero-em ission pumps. I n t he past, however, size and operatingtemperature limitations usually rendered these pumps inadequate for thermalfluid system s, so they were not used often in these applications. But now,

design and m aterial improvements are m aking these pumps a more viablechoice for t herm al fluid system s.

Sealless pumps eliminate many of the leakage issues associated with

mechanical seals. The chief dr awbacks of sealless pumps, either canned ormagnetic-dr ive, are t heir inabilit y t o handle solids, int olerance of any cavitation,

less efficient motors and higher initial cost.

A recent in- house survey pointed t o four tr ends:

The average repair cost of a single-mechanical-seal pump is significantly

less than the average repair cost of a sealless pump.

Sealless pumps have a much longer mean time between failure (MTBF)than pumps using single mechanical seals.

Considering only these two influences, the single mechanical-seal pump

offers the lowest long- term cost of ownership, although t he gap continues

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to get smaller.

When process downtime and environmental challenges associated with any

failure are considered, the longer MTBF and generally leak-free failuremode of sealless pumps can outweigh the lower repair cost for single-

m echanical-seal pumps (t able 2).

Pipe Loops vs. Expansion Joints

When piping is heated or cooled, it will expand and contract, and the resulting

stresses in the piping system must be relieved. Steel pipe "U" bends or loopsare the m ost comm on stress relievers.

If you decide to use bellows expansion joints, take care in considering the

construction materials. Occasionally, austenitic stainless steel bellows jointshave failed because the heat t ransfer fluid was contaminated by inorganic

chloride ions, resulting in stress corrosion cracking. The choice of Inconel 600or Monel 400 can reduce the risk of stress corrosion cracking. Failures in

bellows expansion joints stemm ing from improper installation and poormaintenance also have been known to occur.

Conclusions

Reliable, largely leak-free thermal fluid systems can be built and operated.Minimizing the number of connections is a key ingredient, as is proper design

and assembly of flanged joints. Using sealless pumps will reduce emissions. Ifsealless pum ps are impractical for your system , properly installed and

maintained pum ps wit h m echanical seals will give sat isfactory perform ance.

Where possible, avoid using screwed fittings and where practical, choose pipe"U" bends over bellows expansion joints.

One additional point: While careful system design can go a long way t oward

ensuring r elat ively leak- free perform ance, proper m aintenance over the life ofthe system must not be ignored. For example, while your system may last 20

years, t he valve packing will not. At some point, the valves will have to beserviced.

In general, no matter how much time and effort you expend on designing

reliability and integrity into your thermal fluid system, over time some leaks willoccur. However, proper system design and equipm ent select ion, coupled w ith

regular rout ine system m aintenance, will help keep their num ber, size andfrequency to an absolute minimum.

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