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Process Heating: Steam Traps Severely malfunctioning steam traps blowing steam in all directions and a steady stream of condensate straight out of the upper trap. Introduction Steam traps are automatic valves that discharge condensate from a steam line without discharging steam. Steam traps are an essential part of a steam system; without them the steam pipes and heat exchangers would quickly fill with condensate that would prevent the flow of steam and transfer of heat. Steam traps should be placed along distribution piping and after all heat exchangers. The temperature of the liquid condensate discharged from steam traps is determined by the pressure in the condensate collection vessel and return piping. Many condensate return systems operate at atmospheric pressure; hence, the temperature of the condensate is about 212 F immediately after being discharged from the steam trap. This high- temperature distilled water contains a significant amount of heat and should be returned to the boiler. For example, the gas energy savings from returning 1,000 lb/hr of condensate at 200 F to an 80% efficient boiler, rather than using makeup water at 50 F would be about: 1,000 lb/hr x 1 Btu/lb-F x (200 F – 50 F) / 80% = 187,500 Btu/hr Types of Steam Traps In general, there are four types of steam traps: Inverted bucket. Process Heating: Steam Traps 1 Float + thermostatic Thermostatic Thermodynamic Source: Grainger Catalog, 2000-2001 Inverted Bucket Traps In inverted bucket traps, steam is contained within an inverted bucket floating in condensate. As the level of condensate rises, it is discharged. Inverted bucket traps require water within the bucket, called the prime, to operate. This trap is most appropriate for steady loads such as on distribution systems. Condensate is discharged intermittently. Source: “Design of Fluid Systems”, Spirax Sarco, 2000. Float and Thermostatic Traps In float and thermostatic traps, condensate is discharged when the rising level of condensate lifts a float attached to a level. A thermostatically operated vent discharges air from the top of the trap. Float and thermostatic traps have superior air removal characteristics; however, the internal valves and seats must be matched to steam pressure or the trap can fail in closed position. Condensate is discharged continuously. Process Heating: Steam Traps 2 Source: “Design of Fluid Systems”, Spirax Sarco, 2000. Thermostatic traps Thermostatic traps operate on the difference in temperature between steam and condensate. As condensate cools, the volume of an enclosed bellows decreases and the discharge valve opens. Thermostatic traps always cause some condensate to remain in the system. Condensate is discharged continuously. Source: “Design of Fluid Systems”, Spirax Sarco, 2000. Thermodynamic Traps Thermodynamic traps have a disk situated on a central orifice. As condensate pressure builds, it lifts the disk, passes through the orifice at the center of the disk and exits through smaller orifices surrounding the disk. Flash steam builds up pressure on top of the disk and closes the orifice. Condensate is discharged intermittently. As the trap ages, the cycling rate of the disk snapping open and closed increases. Process Heating: Steam Traps 3 Source: “Design of Fluid Systems”, Spirax Sarco, 2000. Testing Steam Traps Steam traps are designed to operate about 10 years, but can fail sooner due to contamination, improper application and other reasons. Steam traps can fail “open” or “closed”. If a steam trap fails “open”, it allows steam to pass through the trap; hence the energy value of the steam is completely wasted. If a trap fails “closed”, 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). Because of these problems, 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. Ultrasonic sensors amplify high frequency noise from steam and condensate flow into the audible spectrum. Thus, an analyst can determine whether steam and condensate is being discharged through the trap by listening to the condensate side of a steam trap. If the discharge is continuous, it could indicate that the trap has failed open. If no discharge can be sensed, it may indicate that the trap has failed closed. Measuring the temperatures on either side of the steam trap can also provide useful information about whether the trap is working or not. In general, there are four types of steam traps: thermodynamic, thermostatic, float + thermostatic and inverted bucket. Float + thermostatic and inverted bucket traps can be identified by their distinctive shapes. Thermodynamic and thermostatic traps have similar shapes but can sometimes be identified by the nameplate: “TD” for thermodynamic, “TS” for thermostatic, “FT” for float + thermostatic and “IB” for inverted bucket. Inverted bucket and thermodynamic traps have a cyclic discharge when functioning properly. A steady discharge on the condensate side of the trap indicates that the trap has failed open. Thermostatic and float + thermostatic traps have a continuous discharge and it is difficult to assess whether these types of traps are functioning properly merely by listening to the discharge. The best field method available to test Process Heating: Steam Traps 4 thermostatic and float + thermostatic traps is to measure the temperature on both sides of the trap. Temperature readings on both sides of the trap are also instructive. Properly functioning traps are generally warm on both sides, but hotter on the steam side than the condensate side. A trap that is equally hot on both sides may have failed open. A trap which is cold on both sides may have failed closed and be flooded with water. Estimating Savings From Repairing Steam Traps The rate of steam loss through a leaking trap depends on the size of the condensate orifice in the trap. Orifice size is a function of the size of the trap and the differential pressure between the steam and condenstate lines that the trap was designed for. Orifice sizes for Sprirax Sarco inverted-bucket and float+thermostatic traps are listed below. Orifice sizes for thermostatic and thermodynamic traps are generally not specified; however the effective orifice size is similar to the orifice size for inverted bucket and float+thermostatic traps. Cast Iron Float and Thermostatic Steam Traps (FT, FTI and FTB) Spriax Sarco Product Manual, 2001, pg 386 dP(psi) / NPT (in) .5, .75, 1 1.25 1.5 2 15 0.2180 0.3120 0.5000 0.6250 30 0.2180 0.2280 0.3900 0.5000 75 0.1660 0.3120 0.3120 0.4210 125 0.1250 0.2460 0.2460 0.3220 200 0.1000 Cast Iron Inverted Bucket Steam Traps (B Series) Spriax Sarco Product Manual, 2001, pg 436 dP(psi) / NPT (in) .5, .75 0.75 1 1.25 2 15 0.2500 0.3750 0.5000 0.6250 1.0625 30 0.1875 0.3125 0.3750 0.5000 0.7500 75 0.1563 0.2500 0.2813 0.3750 0.5625 125 0.1250 0.2031 0.2500 0.3438 0.5000 180 0.0938 0.1563 0.2188 0.2813 0.4375 250 0.0700 0.1406 0.1875 0.2500 0.3750 The rate of steam loss through an orifice is given by: Steam flow (lb/hr) = 24.24 lb/(hr-psia-in2) x P psia x [D inch]2 x C where P is the pressure of the steam, D is the diameter of the orifice and C is the fraction of the orifice that is open (Design of Fluid Systems: Hook-ups, Spirax-Sarco, 2000, pg. 57). Process Heating: Steam Traps 5 Steam Trap Implementation Costs Malfunctioning steam traps can frequently be repaired for less than the cost of a replacement trap. However, the costs of new steam traps from the Grainger Catalog are shown below. Source: Grainger Catalog, 2003-2004, pg 3152. Process Heating: Steam Traps 6 Example Savings Calculation Consider the following example to quantify savings from replacing a failed 0.5-inch inverted bucket trap rated at 180 psi if actual steam pressure is 120 psig. From the table above, the orifice size for this trap is 1/32-inch. Assuming that the orifice is 50% open, the steam loss through the leaking trap is about: 24.24 lb/(hr-psia-in2) x 135 psia x [3/32 inch]2 x 50% = 14.5 lb/hr The latent heat of steam at 120 psig is about 872 Btu/lb and the saturation temperature is about 350 F. Assuming that 100% of the condensate is returned at 200 F, and that the boiler is 80% efficient, the natural gas savings from fixing the steam trap would be about: 14.5 lb/hr x [872 Btu/lb + 1 Btu/lb-F x (350 – 200) F] x 6,000 hr/yr / 80% = 111 MBtu/yr 111 MBtu/yr x $5.93 / MBtu = $658 /yr According to Grainger Catalog 2001-2002, inverted-bucket steam traps for ½-inch pipe connections with a max operating pressure of 125 psig cost about $92 each (pg. 3,438). In addition, we estimate that installation of the new traps would cost about $30 per trap. If so, the cost of replacing the trap would be about $122. The simple payback would be about: $122 / $658 /yr x 12 months/yr = 2 months Condensate Return Systems “Open” condensate return systems use gravity to transport condensate and flash steam from individual traps into vented receivers. The rising level of condensate lifts the float and energizes an electric pump that returns condensate to the feed tank near the boiler. In open condensate return systems, some of the condensate “flashes” into steam as it is discharged from steam pressure inside the steam trap to atmospheric pressure in the open condensate return line. In open condenstate return systems, the heating value of the flash steam, which is typically about 25% of the heating value of the pressurized condensate, is lost when it is vented to the atmosphere. Source: “Design of Fluid Systems”, Spirax Sarco, 2000. Process Heating: Steam Traps 7 Newer closed condensate return systems use steam pressure in “pressure-powered pumps” to return condensate to the boiler. Closed condensate systems do not lose heat due to flashing and eliminate maintenance problems associated with the pumps and seals in open return systems. Source: “Design of Fluid Systems”, Spirax Sarco, 2000. Process Heating: Steam Traps 8 AR X: Repair or Replace Failed Steam Traps Annual Savings Project Cost Simple ARC: 2.2113.2 Resource CO2 (lb) Dollars Capital Other Total Payback Natural Gas 1,409 mmBtu 159,200 $11,892 $180 $60 $240 1 month Analysis The plant uses steam to deliver heat to its process. Properly functioning steam traps assure that all latent heat in steam is delivered to the process by preventing high-pressure steam from passing into the low-pressure condensate return lines. Steam traps are located on the downstream side of the process heat exchangers. When a steam trap fails open, steam passes through the heat exchanger without condensing, and most latent heat becomes wasted. We inspected each steam trap inside the plant to find potentially failed traps. Our inspection consisted of measuring the outer pipe temperatures immediately upstream and downstream of the steam traps. If the temperatures on both sides of the trap were nearly the same, the trap was most likely failed. Of the twenty traps we inspected, the two that appeared to be failed were the one downstream of tank #3 and the one downstream of the heat exchanger near tank #9. The former showed an outer pipe temperature of 260 F both upstream and downstream, and the latter showed an outer pipe temperature of 285 F both upstream and downstream. Recommendation We recommend repairing or replacing the steam trap downstream of tank #3 and the trap downstream of the heat exchanger near tank #9. Because temperature readings upstream and downstream of the steam trap were almost identical, the traps have most likely failed wide open. Estimated Savings The boilers produce 100-psig steam. The rate of steam lost through a failed trap depends on the type of trap, its steam pressure rating, and the inlet and outlet pipe diameter. The plant’s traps are ¾” pipe diameter inverted bucket traps. Although we could not find a pressure rating on the traps, we assume they are rated at 125 psig, which is slightly higher than plant steam pressure. According to Spirax-Sarco Product Catalog 2001, the orifice size for an inverted bucket steam trap with the above inlet/outlet and pressure specifications is 0.125 inches. The rate of steam loss through an orifice is given by: Steam flow (lb/hr) = 24.24 lb/(hour-psia-in2) x P psia x [D inch]2 where P is the pressure of the steam and D is the diameter of the orifice (Design of Fluid Systems: Hook-ups, Spirax-Sarco, 2000, pg. 57). Thus, the steam loss through each leaking trap is about: 24.24 lb/(hour-psia-in2) x (14.7 + 100) psia x [0.125 inch]2 = 43 lb/hour Process Heating: Steam Traps 9 The latent heat of 100 psig saturated steam is 1,190 Btu/lb, and its temperature is 338 F. We assume boiler makeup water is delivered at the average outdoor temperature of 50 F. From the Steam System Analysis section of the report, the boilers are about 79% efficient, on average. The hourly natural gas savings from repairing or replacing the two failed traps would be about: 43 lb/hr-trap x 2 traps x [1,190 Btu/lb +{(338 – 50) F x 1 Btu/lb-F}] / 79% = 160,896 Btu/hour The annual natural gas savings would be about: 160,896 Btu/hour x 8,760 hours/year x (mmBtu /106 Btu) = 1,409 mmBtu/year 1,409 mmBtu/year x $8.44 /mmBtu = $11,892 /year The CO2 emission savings would be about: 113 lbs CO2/mmBtu x 1,409 mmBtu/year ≈ 159,200 lbs CO2 /year Estimated Implementation Cost According to Grainger Catalog 2005-2006, an inverted bucket steam trap rated at 125 psig with ¾” connections costs about $90. The cost of two traps would be about: $90 /trap x 2 traps = $180 We estimate that it would take one hour to replace each failed steam trap. At a labor rate of $30 per hour, the project labor cost would be about: $30 /hour x 1 hour/trap x 2 traps = $60 The total project cost would be about: $180 + $60 = $240 Estimated Simple Payback ($240 / $11,892 /year) x 12 months/year = 1 month Process Heating: Steam Traps 10 AR 717: Replace Failed Steam Traps Annual Savings Project Cost Simple ARC: 2.2113.2 Resource CO2 (lb) Dollars Capital Other Total Payback Natural Gas 1,096 mmBtu 123,800 $9,502 $4,500 $900 $5,400 7 months Analysis The plant’s two boilers produce steam to space heat the plant during winter. Steam traps prevent live steam from passing into the condensate return lines. Steam traps are generally located on the downstream side of the heat exchangers that deliver useful heat to the building. Properly functioning steam traps assure that steam’s latent heat is delivered to the heat exchanger before returning to the boiler. Latent heat not delivered to the heat exchanger before passing through a steam trap usually becomes wasted. Maintenance recently inspected a portion of the plant’s steam traps and found that 18 of the 20 examined traps had failed. Maintenance replaced the failed traps. Maintaining steam traps is a laudable practice that saves natural gas energy. The photographs below show two types of steam traps in the plant. Inverted Bucket Steam Trap Float & Thermostatic Steam Trap Recommendation Process Heating: Steam Traps 11 We commend maintenance’s steam trap maintenance program and recommend that the inspection continue until all of the plant’s failed traps are replaced. Estimated Savings During our visit, the boiler produced 8-psig steam. According to maintenance, the boiler is turned down to about 2 psig after first shift and on weekends. The rate of steam loss through a leaking trap depends on the size of the condensate orifice in the trap. The size of the orifice depends on the type of the trap, its steam pressure rating, and the inlet and outlet pipe diameter. According to maintenance, the inlet/outlet pipe diameter to all steam traps are either ½” or ¾”, and all are rated at 30 psig. The plant had inverted bucket traps as well as float & thermostatic traps. According to Spirax-Sarco Product Catalog 2001, the orifice size for an inverted bucket steam trap and a float + thermostatic steam trap with the above inlet/outlet and pressure specifications are 0.1875 inches and 0.2180 inches, respectively. To be conservative, we assume all of the plant’s traps have a 0.1875-inch orifice. The rate of steam loss through an orifice is given by: Steam flow (lb/hr) = 24.24 lb/(hour-psia-in2) x P psia x [D inch]2 x C where P is the pressure of the steam, D is the diameter of the orifice and C is the fraction of the orifice that is open (Design of Fluid Systems: Hook-ups, Spirax-Sarco, 2000, pg. 57). Assuming that the orifice failed 50% open, the steam loss through each leaking trap at 8 psig and 2 psig steam pressure are about: 8 psig: 24.24 lb/(hour-psia-in2) x (14.7 + 8) psia x [0.1875 inch]2 x 50% = 9.7 lb/hour- trap 2 psig: 24.24 lb/(hour-psia-in2) x (14.7 + 2) psia x [0.1875 inch]2 x 50% = 7.1 lb/hour- trap The latent heat of steam at 8 psig is 956 Btu/lb and the saturation temperature is 235 F. The latent heat of steam at 2 psig is 964 Btu/lb and the saturation temperature is 222 F. Management estimates that 100% of the condensate in the steam system is returned. We assume the condensate is returned to the boiler at 200 F. We measured the boiler to be 80% efficient. We mentioned earlier that latent heat in steam that has passed through a steam trap usually becomes wasted. However, in the case of space heating, some latent heat is dissipated through the condensate lines, thus becoming useful. We estimate that only 50% of this heat gets wasted. Using these values, the hourly natural gas savings from fixing each steam trap at 8 psig and 2 psig steam pressure would be about: 8 psig: 9.7 lb/hr-trp x 50% [956 Btu/lb +{100% x (235 – 200) F x 1 Btu/lb-F}] / 80% = 6,008 Btu/hr-trp 2 psig: 7.1 lb/hr-trp x 50% [964 Btu/lb +{100% x (222 – 200) F x 1 Btu/lb-F}] / 80% = 4,375 Btu/hr-trp Process Heating: Steam Traps 12 According to maintenance, the 20 traps examined constitute about 25% of the plant’s traps. If so, about 80 traps exist in the plant, and 60 remain unexamined. Eighteen out of the 20 examined traps were failed, indicating a 90% failure rate. Conservatively estimating that 75% of the remaining traps are malfunctioning, the number of failed traps is about: 60 traps x 75% = 45 traps Since first shift is about eight hours long for five days per week, the percentage of time during the winter that the boiler produces 8 psig steam about: (8 hours/day x 5 days/week) / 168 hours/week = 24% Thus, the boiler produces 2 psig steam for the remaining 76% of the time. According to maintenance, the boiler operates between September and April, which is about 7 months per year. Thus, the annual natural gas savings from replacing the failed traps would be about: [(6,008 Btu/hour-trap x 24%) + (4,375 Btu/hour-trap x 76%)] x 45 traps x 24 hours/day x x 30.4 dys/mo (avg) x 7 mo/yr x (mmBtu /106 Btu) = 1,096 mmBtu/year 1,096 mmBtu/year x $8.67 /mmBtu = $9,502 /year The CO2 emission savings would be about: 113 lbs CO2/mmBtu x 1,096 mmBtu/year ≈ 123,800 lbs CO2 /year Estimated Implementation Cost According to maintenance, replacement steam traps cost about $100 each and take about 1 hour to install. The materials cost of 45 replacement traps would be about: 45 traps x $100 /trap = $4,500 According to management, operators make about $16 per hour. Assuming maintenance staff make $20 per hour, the project labor cost would be about: 45 traps x 1 hour/trap x $20 /hour = $900 The total project cost would be about: $4,500 + $900 = $5,400 Estimated Simple Payback ($5,400 / $9,502 /year) x 12 months/year = 7 months Process Heating: Steam Traps 13 UD0693 - AR X: Fix Leaky Steam Traps Annual Savings Project Cost Simple ARC: ? Resource CO2 (lb) Dollars Capital Other Total Payback Natural Gas 174 mmBtu 19,700 $1,229 $236 $60 $296 3 months Analysis During our visit, we observed two severely malfunctioning steam traps blowing steam in all directions and a blowing steady stream of condensate straight out of the upper trap (see Figure below). The traps discharged steam in this manner for about 10 seconds every minute. Recommendation We recommend fixing these traps as soon as possible. Estimated Savings At a minimum, it is apparent that the traps are discharging live steam in addition to condensate for about 10 seconds each minute. In addition, it is likely that the traps are also discharging live steam directly into the condensate line even when no steam is visibly leaking from the exterior of the traps. We will use these two cases to bracket the possible savings from replacing these traps. The rate of steam loss through a leaking trap depends on the size of the condensate orifice in the trap. The size of the orifice depends on the size of the trap, which is rated according to the size of the NPT pipe connections, and the steam pressure. In this case, the “float & thermostatic” traps had ¾” NPT pipe connections and steam pressure Process Heating: Steam Traps 14 averaged about 15 psig. Float & thermostatic steam traps for these specifications have an orifice diameter of 0.218 inches (Spirax-Sarco Product Catalog, 2001, pg. 386). The rate of steam loss through an orifice is given by: Steam flow (lb/hr) = 24.24 lb/(hr-psia-in2) x P psia x [D inch]2 x C where P is the pressure of the steam, D is the diameter of the orifice and C is the fraction of the orifice that is open (Design of Fluid Systems: Hook-ups, Spirax-Sarco, 2000, pg. 57). Assuming that the orifice failed 50% open, the steam loss through each leaking trap is about: 24.24 lb/(hr-psia-in2) x 30 psia x [0.218 inch]2 x 50% = 17 lb/hr The latent heat of steam at 15 psig is about 945 Btu/lb and the saturation temperature is about 250 F. Based on our boiler analysis, we estimated that about 75% of the condensate is returned to the boiler at 150 F. The remaining 25% of steam is lost to leaks and evaporated and replaced with city water at an annual temperature of about 51 F. Based on our boiler analysis, the boiler is about 80% efficient. Using these values, the natural gas savings from fixing each steam trap would be about: 17 lb/hr x [945 Btu/lb + (75% x (250 – 150) F + 25% x (250 – 51) F) x 1 Btu/lb-F] / 80% = 22,700 Btu/hr According to management, the boilers are turned on at noon on Sundays and run continuously until 11:00 pm on Fridays, for a total of about 131 hours per week. If the traps leak steam continuously, the savings from fixing the traps would be about: 2 traps x 22,700 Btu/hr-trap x 131 hours/week x 50 weeks/year = 297 mmBtu/yr 297 mmBtu/yr x $7.08/mmBtu = $2,103 /year If the traps leak steam continuously for 10 seconds every minute, the savings from fixing the traps would be about: 297 mmBtu/yr x 10/60 = 50 mmBtu/yr 50 mmBtu/yr x $7.08/mmBtu = $354 /year Assuming that the true cost is the mean of these extremes, the savings would be about: (297 mmBtu/yr + 50 mmBtu/yr) / 2 = 174 mmBtu/yr ($2,103 /yr + 354 /yr) / 2 = $1,229 The CO2 emission savings would be about: 113 lbs CO2/mmBtu x 174 mmBtu/year ≈ 19,700 lbs CO2 /year Process Heating: Steam Traps 15 Estimated Implementation Cost According to Grainger Catalog 2001-2002, float & thermostatic traps for 3/4-inch pipe connections with a max operating pressure of 15 psig cost about $118 each (pg. 3,373). In addition, we estimate that installation of the new traps would cost about $30 per trap. If so, the material cost of replacing the traps would be about: 2 traps x $118 /trap = $236 The labor cost would be about: 2 traps x $30 /trap = $60 The total cost would be about: $236 + $60 = $296 Estimated Simple Payback The simple payback would be about: $296 / $1,229 /yr x 12 months/yr = 3 months Process Heating: Steam Traps 16

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posted: | 2/19/2011 |

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