Process Heating - Steam Traps

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Process Heating: Steam Traps

Severely malfunctioning steam traps blowing steam in all directions and a steadystream of condensate straight out of the upper trap.

IntroductionSteam traps are automatic valves that discharge condensate from a steam linewithout discharging steam. Steam traps are an essential part of a steam system;without them the steam pipes and heat exchangers would quickly fill withcondensate that would prevent the flow of steam and transfer of heat. Steamtraps should be placed along distribution piping and after all heat exchangers.

The temperature of the liquid condensate discharged from steam traps isdetermined by the pressure in the condensate collection vessel and return piping.Many condensate return systems operate at atmospheric pressure; hence, thetemperature of the condensate is about ! " immediately after beingdischarged from the steam trap. This high#temperature distilled water contains asignificant amount of heat and should be returned to the boiler. "or example, thegas energy savings from returning !,$$$ lb%hr of condensate at $$ " to an &$'efficient boiler, rather than using makeup water at ($ " would be about)

!,$$$ lb%hr x ! *tu%lb#" x +$$ " ($ "- % &$' !&/,($$ *tu%hr

Types of Steam Traps0n general, there are four types of steam traps)

0nverted bucket.

Process Heating: Steam Traps 1


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"loat 1 thermostatic

Thermostatic

Thermodynamic

Source) 2rainger 3atalog, $$$#$$!

0nverted *ucket Traps0n inverted bucket traps, steam is contained within an inverted bucket floating incondensate. 4s the level of condensate rises, it is discharged. 0nverted buckettraps require water within the bucket, called the prime, to operate. This trap ismost appropriate for steady loads such as on distribution systems. Condensateis discharged intermittently.

Source) 56esign of "luid Systems7, Spirax Sarco, $$$.

"loat and Thermostatic Traps0n float and thermostatic traps, condensate is discharged when the rising level of

condensate lifts a float attached to a level. 4 thermostatically operated ventdischarges air from the top of the trap. "loat and thermostatic traps havesuperior air removal characteristics; however, the internal valves and seats mustbe matched to steam pressure or the trap can fail in closed position.Condensate is discharged continuously.



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Thermostatic traps

Thermostatic traps operate on the difference in temperature between steam andcondensate. 4s condensate cools, the volume of an enclosed bellows decreasesand the discharge valve opens. Thermostatic traps always cause somecondensate to remain in the system. Condensate is discharged continuously.


Thermodynamic Traps

Thermodynamic traps have a disk situated on a central orifice. 4s condensatepressure builds, it lifts the disk, passes through the orifice at the center of thedisk and exits through smaller orifices surrounding the disk. "lash steam buildsup pressure on top of the disk and closes the orifice. Condensate is dischargedintermittently. 4s the trap ages, the cycling rate of the disk snapping open andclosed increases.



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Testing Steam TrapsSteam traps are designed to operate about !$ years, but can fail sooner due tocontamination, improper application and other reasons. Steam traps can fail5open7 or 5closed7. 0f a steam trap fails 5open7, it allows steam to pass throughthe trap; hence the energy value of the steam is completely wasted. 0f a trap fails5closed7, condensate will back up into the piping +which reduces steam flow,inhibits valve function and causes pipe erosion- and%or flood the heat exchanger+which reduces or eliminates effective heat transfer-. *ecause of theseproblems, it is recommended that all traps be tested at least once per year.

The most common method of testing steam traps is with an ultrasonic sensor.8ltrasonic sensors amplify high frequency noise from steam and condensate flowinto the audible spectrum. Thus, an analyst can determine whether steam and

condensate is being discharged through the trap by listening to the condensateside of a steam trap. 0f the discharge is continuous, it could indicate that the traphas failed open. 0f no discharge can be sensed, it may indicate that the trap hasfailed closed. Measuring the temperatures on either side of the steam trap canalso provide useful information about whether the trap is working or not.

0n general, there are four types of steam traps) thermodynamic, thermostatic,float 1 thermostatic and inverted bucket. "loat 1 thermostatic and invertedbucket traps can be identified by their distinctive shapes. Thermodynamic andthermostatic traps have similar shapes but can sometimes be identified by thenameplate) 5T67 for thermodynamic, 5TS7 for thermostatic, 5"T7 for float 1

thermostatic and 50*7 for inverted bucket.

Inverted bucket and thermodynamic traps have a cyclic discharge whenfunctioning properly. A steady discharge on the condensate side of the trapindicates that the trap has failed open. Thermostatic and float + thermostatictraps have a continuous discharge and it is difficult to assess whether thesetypes of traps are functioning properly merely by listening to the discharge. The



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best field method available to test thermostatic and float + thermostatic traps is tomeasure the temperature on both sides of the trap.

Temperature readings on both sides of the trap are also instructive. 9roperlyfunctioning traps are generally warm on both sides, but hotter on the steam side

than the condensate side. 4 trap that is equally hot on both sides may havefailed open. 4 trap which is cold on both sides may have failed closed and beflooded with water.

Estimating Savings From Repairing Steam TrapsThe rate of steam loss through a leaking trap depends on the si:e of thecondensate orifice in the trap. rifice si:e is a function of the si:e of the trap andthe differential pressure between the steam and condenstate lines that the trapwas designed for. rifice si:es for Sprirax Sarco inverted#bucket andfloat1thermostatic traps are listed below. rifice si:es for thermostatic and

thermodynamic traps are generally not specified; however the effective orificesi:e is similar to the orifice si:e for inverted bucket and float1thermostatic traps.

3ast 0ron "loat and Thermostatic Steam Traps +"T, "T0 and "T*-

Spriax Sarco 9roduct Manual, $$!, pg 9T +in- .(, ./(, ! !.( !.(

!( $.!&$ $.


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Malfunctioning steam traps can frequently be repaired for less than the cost of areplacement trap. Cowever, the costs of new steam traps from the 2rainger3atalog are shown below.

Source) 2rainger 3atalog, $$Example Savings Calculation3onsider the following example to quantify savings from replacing a failed $.(#inch inverted bucket trap rated at !&$ psi if actual steam pressure is !$ psig.



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>ewer closed condensate return systems use steam pressure in 5pressure#powered pumps7 to return condensate to the boiler. 3losed condensate systemsdo not lose heat due to flashing and eliminate maintenance problems associated

with the pumps and seals in open return systems.




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AR : Repair or Replace Failed Steam Traps

4F3) .!!atural 2as!,@$?

mm*tu!(?,$$ D!!,&? D!&$ D=$ D@$ ! month

Analysis

The plant uses steam to deliver heat to its process. 9roperly functioning steamtraps assure that all latent heat in steam is delivered to the process by preventinghigh#pressure steam from passing into the low#pressure condensate return lines.Steam traps are located on the downstream side of the process heatexchangers. Hhen a steam trap fails open, steam passes through the heatexchanger without condensing, and most latent heat becomes wasted.

He inspected each steam trap inside the plant to find potentially failed traps. ur

inspection consisted of measuring the outer pipe temperatures immediatelyupstream and downstream of the steam traps. 0f the temperatures on both sidesof the trap were nearly the same, the trap was most likely failed. f the twentytraps we inspected, the two that appeared to be failed were the one downstreamof tank I< and the one downstream of the heat exchanger near tank I?. Theformer showed an outer pipe temperature of =$ " both upstream anddownstream, and the latter showed an outer pipe temperature of &( " bothupstream and downstream.

Recommendation

He recommend repairing or replacing the steam trap downstream of tank I< and

the trap downstream of the heat exchanger near tank I?. *ecause temperaturereadings upstream and downstream of the steam trap were almost identical, thetraps have most likely failed wide open.

Estimated SavingsThe boilers produce !$$#psig steam. The rate of steam lost through a failed trapdepends on the type of trap, its steam pressure rating, and the inlet and outletpipe diameter. The plantJs traps are K7 pipe diameter inverted bucket traps.

4lthough we could not find a pressure rating on the traps, we assume they arerated at !( psig, which is slightly higher than plant steam pressure. 4ccordingto Spirax#Sarco 9roduct 3atalog $$!, the orifice si:e for an inverted bucket

steam trap with the above inlet%outlet and pressure specifications is $.!( inches.The rate of steam loss through an orifice is given by)

Steam flow +lb%hr- @.@ lb%+hour#psia#in- x 9 psia x A6 inchB

where 9 is the pressure of the steam and 6 is the diameter of the orifice +6esignof "luid Systems) Cook#ups, Spirax#Sarco, $$$, pg. (/-. Thus, the steam lossthrough each leaking trap is about)



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@.@ lb%+hour#psia#in- x +!@./ 1 !$$- psia x A$.!( inchB @< lb%hour

The latent heat of !$$ psig saturated steam is !,!?$ *tu%lb, and its temperatureis


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AR #$#: Replace Failed Steam Traps

4F3) .!!atural 2as!,$?=

mm*tu!


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Recommendation

He commend maintenanceJs steam trap maintenance program and recommendthat the inspection continue until all of the plantJs failed traps are replaced.

Estimated Savings

6uring our visit, the boiler produced psig steam. 4ccording to maintenance,the boiler is turned down to about psig after first shift and on weekends. Therate of steam loss through a leaking trap depends on the si:e of the condensateorifice in the trap. The si:e of the orifice depends on the type of the trap, itssteam pressure rating, and the inlet and outlet pipe diameter. 4ccording tomaintenance, the inlet%outlet pipe diameter to all steam traps are either E7 or K7,and all are rated at


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psig)/.! lb%hr#trp x ($' A?=@ *tu%lb 1L!$$' x + $$- " x ! *tu%lb#"B % &$' @,


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D@,($$ 1 D?$$ D(,@$$

Estimated Simple Pay!ac"+D(,@$$ % D?,($ %year- x ! months%year / months



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%&'()* + AR : Fix ,ea"y Steam Traps

4F3) R4nnual Savings 9roGect 3ost Simple

9aybackFesource 3 +lb- 6ollars 3apital ther Total

>atural 2as!/@

mm*tu!?,/$$ D!,? D


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steam pressure. 0n this case, the 5float P thermostatic7 traps had K7 >9T pipeconnections and steam pressure averaged about !( psig. "loat P thermostaticsteam traps for these specifications have an orifice diameter of $.!& inches+Spirax#Sarco 9roduct 3atalog, $$!, pg.


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The 3emission savings would be about)

!!< lbs 3%mm*tu x !/@ mm*tu%year N !?,/$$ lbs 3%year

Estimated Implementation Cost4ccording to 2rainger 3atalog $$!#$$, float P thermostatic traps for

Process Heating - Steam Traps

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