Steam Digest 2002 An Analysis of Steam Process Heater Condensate Drainage Options

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An Analysis of SteamProcess Heater CondensateDrainage OptionsJames R. Risko, TLV Corporation

ABSTRACT

The production and reliability of performance ofsteam process heaters can be significantly affectedby the condensate drainage design that is em-ployed. The current variety of drainage optionscan be confusing to a system designer who is un-aware of the reasons for each specific design. Anunderstanding of the various types and why theymay be used follows.

BACKGROUND

For simplicity, the terminology “process heater”is intended to mean any steam heat exchanger,coil, or kettle which uses steam as the primaryfluid to transfer heat to a product. While gener-ally intended for production purposes, this ter-minology can also be used to refer to HVAC ap-plications. The reason that all of these heater typescan be grouped together is because even thoughthey incorporate different exchanger designs onthe product side, they all are intended to transfersteam heat in an efficient and cost effective heat-ing manner. In that sense, they all use the steamto provide a certain level of heat transfer and drainthe by-product, condensate, away from the heaterso that new steam can be introduced and con-trolled to heat additional product.

What is intriguing about this simple goal is thatthere are currently a variety of installation designsto accomplish the removal of that condensate, andthese provide various levels of production perfor-mance depending on the environmental condi-tions of the specific heater application. What canbe confounding is that some of these drainageoptions may work very well in one scenario, andyet fail miserably in another depending on theconditions. When successful in a previouslytroublesome application, a particular installationdesign may create a sense of comfort within anengineering department and later become a

standard practice for a facility. Later it can havedecidedly mixed results when used for an appli-cation for which it cannot perform well enoughto meet design expectations. This situation mayhave tremendous energy and production impli-cations and can usually be easily identified inadvance.

IDENTIFYING THE SYMPTOMS

Telltale signs for those installations with unsuit-able condensate drainage include:

Condensate being visibly wasted from the heatexchanger discharge side, either from a hoseconnection at the strainer, or an opened unionor drain valve on the steam trap’s outlet pip-ing. In this case, the condensate is no longeravailable to be returned through the returnline, and its valuable energy is needlessly sac-rificed to grade so that required productionperformance levels can be achieved.The presence of severe hammering in the ex-changer itself or in the return piping down-stream of the heater. There are a variety ofcauses for this type of hammering, but in theworst case its cause can be attributed to sig-nificant amounts of preventable steam loss.Product variance much greater than expecta-tions.Dramatic temperature stratification of theheater’s exterior surface where steam is shell-side.A higher than average maintenance require-ment for head gasket or tube bundle failure.

COURSE OF ACTION

The optimum solution is to specify a condensatedrainage design that removes all condensate fromthe heater rapidly. This is where confusion overthe best design has traditionally occurred. Whilethe target of high performance heat exchanger in-stallation design prevails, a full understanding ofthe options and when their use is indicated is of-ten not clear.

Therefore, the purpose of this presentation is toexamine the common types of heat exchangerdrainage designs, and describe the instances whereeach can perform suitably. Those installationdesigns are slightly different when steam is tube-side versus shell-side, and the piping optionsfor both instances follow:

Steam Digest 2002An Analysis of Steam Process Heater Condensate Drainage Options

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STEAM INLET CONTROL VALVE WITH

OUTLET LEVEL POT (FIGURE B)

This system is basically a modification of the in-stallation shown in Figure A. It has been used as asteam trap substitute in applications of very highpressure and high capacity where these require-ments are beyond the capabilities of a traditionalsteam trap. It has also been used for instanceswhere “stall” occurs and the resulting hydraulicshock has severely damaged the outlet steam trap.

The only difference between the systems shownin Figure A and Figure B is in the design of thetrap. Figure A uses a traditional, self-containedsteam trap, and Figure B substitutes an electronicsteam trap in the form of a level pot receiver, levelsensing from the transmitter, and automatic valveopening and closing through the controller andcontrol valve. Although a more complex system,the condensate drainage function is virtually iden-tical to a simple, mechanical steam trap. Gen-erally, the cost of this electronic steam trap op-tion is greater than a self-contained trap.

POTENTIAL INSTALLATION DESIGNS

Steam Inlet Control Valve with Outlet SteamTrap (Figure A).Steam Inlet Control Valve with Outlet LevelPot (Figure B).Steam Inlet Control Valve with Outlet Con-densate Level Control (Figure C).Condensate Outlet Control Valve and LevelOverride (Figure D).Condensate Outlet Control Valve for Drain-age and Set Point Control (Figure E).Steam Inlet Control Valve with Outlet Con-densate Pump/Trap Drainage (Figure F).

An in-depth knowledge of the various options willhelp in providing the most effective condensatedrainage installation for each given circumstance.With a clear understanding of these options andwhen to use them, the designer will be able tomaximize the energy usage and production per-formance of each application according to thebudgetary constraints of the allocated capital.

STEAM INLET CONTROL VALVE WITH

OUTLET STEAM TRAP (FIGURE A)

This is the traditional approach to supplying steamand drainage condensate from process heaters. Itoffers a relatively simple installation, with easytroubleshooting and low cost maintenance. It ac-complishes the control value by modulating steamtemperature and rapid drainage of condensatefrom the tube bundle. It is the primary methodof choice where the pressure supplying the steamtrap (P1) is always greater than the back pressure(P2) because it always keeps steam on the tubebundle under these conditions. The figure showscondensate draining by gravity, but this is not anactual requirement. Condensate can elevate withthis design, and it can operate against back pres-sure, provided that the differential from P1:P2 isalways positive.

Is the differential pressure reallypositive?The actual pressure differential P1:P2 may notreally be positive when it appears to be so. For atrue understanding, it is necessary to study thepressure dynamics of the control valve as it modu-lates to achieve the heat balance with the set point.As the heating demand lowers, the control valvewill modulate and lower the pressure in thesteam space. When this occurs, P1 will oftenbecome lower than P2 even though the supply

pressure to the control valve is substantiallygreater than P2. This condition where P1modulates a lower pressure than P2 is known asa “stall”. The effect is that Figure A’s installa-tion design can work very well in all cases untilit is used in applications where stall occurs.Then the system will provide less desirable re-sults.

What happens during “stall”?The system partially floods the tube bundle. Thiscreates a variety of undesirable effects, includinginconsistent product quality and large variationsfrom the set point. Other typical symptomsinclude corrosion, thermal shock, and hammer-ing of the heater with damage to either the heateritself or the outlet steam trap.

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STEAM INLET CONTROL VALVE WITH

OUTLET CONDENSATE LEVEL CONTROL

(FIGURE C)

This solution has been used in some cases to com-bat stall conditions. In this case, a sufficientlyhigh steam pressure is used on the control valveinlet to ensure a positive pressure differentialP1:P2. Because of the possibility of large pressureand temperature changes, some heater surface areais removed by intentional flooding. Limiting theeffective surface area in this manner can lower therange of the control swing.

This insulation will provide positive pressure dif-ferential for P1:P2, but causes increased foulingon the tube bundle which is usually exposed tohigh pressure steam. Troubleshooting this instal-lation will be complex, and corrosion, head gas-ket failure, and thermal shock can be expectedmaintenance issues because the tube bundle isstratified with condensate and steam.

Because of the durability of the components,this system can improve the symptom where aself-contained steam trap has been regularlydamaged from severe water hammer, but it doesnot correct the cause. In this sense, it is only aband-aid, and additional symptoms of corro-sion and production variance from the controlvalue will not be corrected by this option.

CONDENSATE OUTLET CONTROL VALVE

AND LEVEL OVERRIDE (FIGURE D)

A common alternative used by some design engi-neers is to completely eliminate inlet steam con-trol and select outlet condensate control instead.In this design, the outlet control serves as bothcontrol valve and steam trap. The installation doesnot stall because the steam pressure does notmodulate. The result is that the pressure on thetube bundle is always greater than the back pres-sure. The level pot is used to reduce the possibil-ity of live steam loss, and the tube bundle is inten-tionally flooded to remove excess surface area.Then the available tube bundle area becomes thecontrol variable.

Under stall conditions, this system can be a lowercost alternative than the steam inlet control andoutlet level pot design of Figure B. The outletcontrol and level pot are actually sized similarlyto Figure B, but this system completely eliminatesthe cost of the inlet control valve. Additionally,the heater is exposed to higher pressure steam atall times, so its required surface area may be re-duced due to the expected higher temperature ofthe unmodulated steam.

This system can provide acceptable process con-trol in instances of limited demand variation.However, the more the demand changes, there maybe instances of significant deviation from the con-trol value. This is due to at least two factors.Changing water level on a tube bundle is a muchslower process than adjusting steam pressure.Therefore, the process to adapt to demand changesin an outlet control design by moving a water levelis significantly slower than when moving steamthat occurs in steam inlet control installations. Theresult is a greater lag in response to demandchanges. Also, once the water level is moved, thenewly exposed or covered heat transfer area en-counters a drastically changed “U” value. This isbecause of the substantially different heat transferrates between hot condensate and steam. The ef-fects of this difference will be in proportion to theamount of new surface area exposure. The morethe demand change and subsequent tube expo-sure, the more dramatic the change in “U” andthe resultant variation from the control value. Inshort, wider temperature or pressure swings aretypical of this control method.

An additional issue with this design is that theheater’s life begins with intentional flooding, thenexposes increasingly more area as the tube surface

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becomes fouled. Eventually, the entire surfacecan become exposed and still not satisfy the pro-cess demand due to fouling. Then, such sys-tems programmed with a level override prioritycan actually blow live steam through the con-trol valve, thereby pressurizing the return line.This leads to a high energy cost and detractsfrom the production rate of all other heatersdraining into the increased back pressure of thesame return heater.

Additionally, without any special provisions, theheater will remain flooded at shutdown and tubecorrosion will be exceptionally high in thesecases.

CONDENSATE OUTLET CONTROL VALVE

FOR DRAINAGE AND SET POINT CONTROL

(FIGURE E)

This installation is virtually identical to Figure D,except that the cost and leak protection of the levelpot is eliminated. The installed cost is lower bythis elimination, but the energy consumption canbe significantly higher due to live steam lossthroughout the heater life. All other character-istics of Figure D remain.

STEAM INLET CONTROL VALVE WITH

OUTLET CONDENSATE PUMP/TRAP

DRAINAGE (FIGURE F)

The installation in Figure A is a perfect design inwhich stall and hydraulic shock conditions do notoccur. However, for those severe conditions, Fig-ure A cannot be used, and the other designs ofFigures B through E were most likely developedto deal with them. Unfortunately and for the rea-sons explained above, those other designs are notalways optimal when dealing with stall. Figure Fprovides a suitable maximum benefit design forstall conditions.

The system utilizes a piece of equipment knownas pump/trap combination. This can be either asingle combined pump/trap unit, or employ twoseparate products in an engineered package. Usu-ally suitable for pressures up to 150 psig steam,pump/traps allow the system to adapt quickly todemand changes and drain condensate under allpressure conditions.

The design requires that inlet steam control is used.This is to provide for the most rapid adjustmentto demand changes possible. In an inlet steamvalve installation, the control valve adjusts thesteam pressure and temperature rapidly, as soonas the sensor detects a variance against the controlvalue. Condensate is always drained from the sys-tem by the pump/trap, so the exposed surface areais constant allowing the modulated steam pres-sure to equalize to the required load. The pump/trap has a multi-functional capability.

As a Trap. When the pressure differential of P1:P2is positive, the steam space pressure drives con-densate through the unit, and steam is containedby the included trap. Operation under these con-ditions is similar to Figure A.

As a Pump. When the pressure differential ofP1:P2 is negative, then the condensate fills thepump cavity and is pumped downstream beforethe process heater can be flooded. The result isthat the heater tubes are always exposed only tosteam and not to flooded condensate. The mainpoint is that only the small pump body receiveshigh pressure steam, not the entire process heateras occurs in Figures C, D, and E.

Where suitable to be employed, Figure F systemsof steam inlet control and pump/trap drainageminimize energy waste, high control variance,

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corrosion, thermal shock, and stratification dur-ing production. When properly designed, theyalso drain the equipment during shutdown toavoid the high corrosion that occurs from stag-nant condensate. They can minimize fouling asthe steam temperature used is always the lowestpossible to achieve equilibrium with the de-mand. The motive steam used to pump thecondensate in the system is returned to the pro-cess heater to utilize its heat in the process. Thisguarantees an extremely low cost pumping so-lution while maximizing the production ratesof steam process heaters.

CONCLUSIONS

The author’s preference is for the drainage meth-ods of Figure A and Figure F as primary solutions,and then Figure B when neither of the first twoalternatives will meet the application requirements.In cases where neither Figures A, F, or B will meetthe application demands, then Figure D may beconsidered provided that the user accepts the limi-tations of this design.

ACKNOWLEDGMENTS

The PowerPoint graphics included with this pa-per were drawn by Mr. Tracy Snow and Mr. SteveKortheuer, both Sr. Engineers in TLV’s Consult-ing and Engineering Services Department.

REFERENCES

1. Mathur, Jimmy, 1973. “Performance of Steam Heat-Exchangers”, Chemical Engineering, September 3, pp.101-106.

2. McCarthy, Alfred J. & Smith, Brian, R., 1995. “ReboilerSystem Design: The Tricks of the Trade”, ChemicalEngineering Progress, May, pp. 33-47.

3. Mehra, Davinder K., 1985. “Troubleshooting Prob-lems With Heat Exchangers”, Chemical Processing, Au-gust, pp.36-38.

4. Pathak, Vijay K. & Rattan, Inder S., 1988. “TurndownLimit Sets Heater Control”, Chemical Engineering, July18, pp. 103-105.

5. Risko, James R., 1997. “Maximize Productivity FromSteam Process Heaters Through Proper CondensateDrainage”, TLV Design Guide.

6. Risko, James R., 1996. “Managing Steam Condensateis Key to Heater Efficiency”, Chemical Engineering, Julypp. 114-119.

7. Risko, James R., 1996. “Draining Condensate FromSteam Process Heaters”, Plant Engineering, March, pp.74-76.

8. Risko, James R., 1994. “Complete Condensate Drain-age From Equipment”, TLV Design Guide.

9. Risko, James R., 1988. “Handling Steam Conden-sate”, Plant Engineering, March 10, pp. 146-149.

10. Schwartz, Marvin P., 1982. “Four Types Of Heat Ex-changer Failures”, Plant Engineering, December 23, pp.45-50.