HYDROGEN RECYCLE COMPRESSOR FIELD PERFORMANCE ANALYSIS

Transcription

1 Elliott 15 MB hydrogen recycle comressor at the Valero refinery in Ardmore, Oklahoma, U.S.A. erformance analysis found that cleaning the comressor would bring the unit back to normal oerating conditions and a rerate was not necessary. lans are to install online erformance monitoring software to confirm current erformance and to aid in scheduling comressor maintenance. YDROGEN RECYCLE COMRESSOR FIELD ERFORMANCE ANALYSIS Comressor erformance Analysis rior to Major Overhaul or Intended Rerate Can Reveal Imortant Cost and Time Saving Measures By Ted Gresh Ted Gresh is vice resident of Marketing and Engineering, Flexware, Inc., Graeville, ennsylvania, U.S.A. e has been involved in the design of high efficiency centrifugal comressor staging, field testing of comressors and steam turbines and troubleshooting field erformance roblems for over 30 years. While most of this time was in the Technical Services Det of Elliott Co., he is resently with Flexware Inc. a comany focused on consulting services and software for turbomachinery erformance analysis. Gresh received a BS in Aerosace Engineering from the University of ittsburgh in In addition to numerous aers and magazine articles, he has ublished a book on the subject of comressor erformance, and has several atents related to turbomachinery. For further information or comlete references, contact the author at his address:mtgresh@flexwareinc.com As art of a debottlenecking rocedure, management at Valero an Ardmore, Oklahoma, U.S.A., refinery were interested in analyzing the lant s hydrogen recycle comressor erformance. The comressor string consisted of an Elliott 15 MB barrel comressor driven by an Elliott SBEG-5 condensing steam turbine driver. Of rimary concern for data accuracy were the flow meters and obtaining an accurate gas analysis. Accurate data were of secial concern because confirmation of calculation results was not available. Secial care was taken to assure accurate data. ressure differential data from the comressor gas-flow meter were read directly and used to calculate the flow rate to the comressor. The same was done for the turbine flow meter. Multile gas samles were taken to assure redundancy. The comressor was determined to be oerating at about 71% efficiency, COMRESSORTechTwo

2 about 5% below redicted values. The turbine was oerating at about 44% efficiency, about 10% below its redicted value. While the data showed the equiment needed maintenance to bring it back to design oerating condition, the comressor was oerating at midrange so that a rerate was not required. It was recommended that the comressor be oened for cleaning and insection of the internal labyrinth seals. According to Michael Tibbits, Staff rocess Engineer, the main reason for having the analysis was to determine the extent of rerate of the comressor to increase its throughut. Because the analysis revealed that only minor maintenance was required, the refinery saved considerable time and exense. Valero was most leased with the results. Start-u Following overhaul, all the iing and vessels were filled with air rather than the rocess gas. So, when the comressor first started, it comressed air and eventually nitrogen once all the air was urged. For oeration, the effects are aroximately the same because the molecular weight (MW) for air and nitrogen is 8. owever, this is very different than the rocess gas, which has a MW of 3.6. First consideration was the ower. The ower was limited to the available ower of the driver. And, even if there was unlimited driver ower, the increase in ower required for oeration on nitrogen would end u in a shaft-end failure if the comressor was oerated at the same seed and ressure. To oerate the comressor on nitrogen, it was necessary to reduce both seed and ressure. To estimate required seed and ressure, guidelines and formulas were obtained from a book comosed by M.T. Gresh, entitled, Comressor erformance: Aerodynamics for the User. For simlification, these equations are labeled numerically from 1 through 5. olytroic gas horseower is obtained as follows: G M η 33,000 Mass flow is roughly roortional to MW and ressure as exressed aroximately by the formula: 1MW11 MW If the comressor is to be oerated at 400 sia (7.6 bar) suction ressure while oerating on nitrogen: 35,000 x 3.6 x 1700 x 8 x ,15 ft-lb/lb. (57,165 Nm/kg) In order to achieve this head, the seed had to be reduced. The fan law equation shows that head is roortional to seed. N (3) 35,000 11,89 19,15 N N 8344 rm (1) (4) () The comressor was oerated on nitrogen at 8344 rm, 400 sia (7.6 bar) suction ressure and 100 F (38 C). The discharge ressure is estimated using the formula: + Z1RT1[ n /( n 1) ] sia (49 bar) Nomenclature: G Ṁ olytroic Gas orseower olytroic head, ft-lb/lb Mass Flow Rate, lb/min MW Molecular Weight of Gas Mixture n olytroic exonent N Equiment seed, rm ressure, sia R Gas constant (1545) (molecular weight) T Temerature, degrees Rankine Z1 Inlet comressibility factor n Efficiency, olytroic Subscrits: 1 Case 1 Case olytroic n /( n 1) 1.36 /( ) 19, x55.18x / [ ( ) ] 1 (5) Another variable to consider is the discharge temerature. We had to be sure the discharge temerature did not exceed guidelines for the comressor. In this case the discharge temerature is relatively low and well within limits (Figure ). An accurate oerating curve for start-u conditions on nitrogen is necessary and should be obtained from the comressor OEM. The OEM also can confirm if there are other limitations to consider at this off-design oerating condition. Conclusion A lot can be learned from a field erformance test. Knowing where the comressor is oerating on the curve is valuable information. In this case, it was found the comressor was not the bottleneck and a rerate was not necessary, although an internal insection and cleaning was found to be necessary. Routine monitoring of rotating equiment erformance should be art of a normal reventative maintenance rogram to maintain eak lant roduction rates. 400 COMRESSORTech Two

3 ead & Work, Ft-Lb/Lb Rated ead Rated Work Design t. ead Design t. Work Field ead Field Work Efficiency Rated Efficiency Design t. Efficiency Field Efficiency Flow Rate, ICFM Figure 1. ydrogen recycle comressor oerating data for August 6, 004. Data has been fan law corrected to design seed, 11,070 rm. Summary of Results Comressor Data Gas Mole Formula Fraction Time 7:30 AM 10:30 AM exane C 6 14 Flow, MMSCFD ydrogen 0.94 Orifice D, O roane C 3 8 Inlet T, F i Butane C 4 10 Inlet, sia n Butane C 4 10 Disch T, F Ethane C 6 Disch, sia Nitrogen N Seed, rm 11,89 11,89 Methane C 4 MW Comressor Results Flow, #/min Flow ICFM ead, ft-lb/lb 35,118 35,040 Efficiency ower, Turbine Data Inlet, sig Inlet T, F Exh, g Vac Exh T, F Flow, k#/hr Orifice D, O Turbine Results Flow #/hr 1,568 1,40 Efficiency ower, Table 1. ydrogen recycle oerating data for August 6, 004. Note that the comressor ower is identical to the steam turbine ower. The comressor ower was used as inut data for the steam turbine calculation to determine the exhaust conditions of the condensing steam turbine. COMRESSORTech Two

4 Gas Flex Straight Through Comressor Estimation Figure. Oeration on nitrogen at reduced ressure and seed.

5 Gas Flex Straight Through Comressor Test Results Figure 3. Comressor calculation results.