Boiler Fan High Vibration and Temperature Analysis

Hello reader! I have some problem at motor vibration and temperature. Here the short story:
Our predictive engineer found level C vibration and occasionally hi temp on our 1400kW Fan Motor and issuing reccomendation to change the bearing.

Our motor has two bearing on DE (NU238ECM & 6238) and one bearing on DE (NU238ECM).
With this arrangement : [NDE NU238 ROTOR 6238 NU238 DE]

When plant periodic outage came, our maintenance team changed the bearing.
After bearing change, we run no load test and resulting in level B vibration and hi temp bearing (DE +-90 degC and NDE +-82 degC) and vibration 2.4mm/s.
Then we replenish grease, anf try another no load test resulting (DE +-80 degC and NDE +-70 degC) and vibration 3.2mm/s.

Then we try to couple with fan, and had 8.4 mm/s axial vibration.

Why it become worse after we replaced the bearing?



Here some condition to consider:
- We did a bearing change in dusty area (ESP has been drained).
- Our fan is axial hydraulic type, which means it has variable axial thrust force.
- Before bearing change, sometimes DE motor bearing temperature going as high as 80 degC, but could be back to normal about 40-50 degC mysteriously (already trending vs fan load and has no correlation at all). NDE bearing always in stable temperature about 40-45 degC.
- With this arrangement : [NDE NU238 ROTOR 6238 NU238 DE] the ball bearing outer race is not fitted on bearing housing, not as NU bearing). The ball bearing outer diameter has about 0.75mm gap with bearing housing.
Is this condition expected by design?


and here the possibilities :::

STD:
1. bent shaft
2. Misalignment -> realigned with thermal growth of motor frame allowance
3. Bearing housing
others idea:
- Shaft damage due to bearing inner race movement
- Shaft deformation due to extreme heat
- Over greasing
- Shaft alignment
- Shaft motor not level
- Static discharge through bearing especially on VFD usage ->bearing insulation & shaft grounding brush
- out of balance coupling
- Rotor has axial force try to center on its magnetic center
- Small burr on shaft shoulder, where bearing fits up to\\
- Shim incorrect installation
- Rotor positioning, by locking DE bearing in proper arrangement by the grease caps and the NDE bearing does the floating to allow for thermal growth of the shaft. If the shaft can be moved axially, then the DE has not been reassembled correctly.



The sudden temperature increase analysis :
- Ball bearing is unlikely except it has high axial force imposed on it
- Suspect on radial/cylindrical bearing beacause they only have to subjected to a small amount of axial thrust. NU bearing in motors, one of them need to be mounted in a way that it can slide axially, otherwise the two of these bearing will thrust against each other beacuse of thermal expansion of shaft.  -> check this sliding arrangement if incorrectly assembled then correct it.

Pengendalian Korosi pada Hollow Stator Bar Generator

Khusus di generator dengan sistem pendingin air pada stator, pada dasawarsa terakhir, perhatian pada kemungkinan korosi pada laluan air pendingin pada stator generator makin meningkat. Hal ini didasarkan pada semangat untuk memastikan bahwa generator akan kontinyu beroperasi tanpa gangguan mayor selama umur harapan suatu pembangkit, misalnya pada PLTU Batubara yang sekitar 25-30 tahun.

Terdapat beberapa cara untuk menjaga laju korosi pada tembaga batang stator, akan tetapi, kita perlu mengetahui apa penyebab dari korosi pada tembaga yang terkena air demineralisasi.

Saat tembaga bertemu dengan oksigen terlarut dalam air, akan terjadi reaksi anodik dan katodik yang akan mengorosi tembaga dan membentuk formasi membran oksidatif.

Reaksi Anodik

2Cu +H2O-2e→Cu2O +2H+

Cu -2e→Cu2+

Cu -e→Cu+

Reaksi Katodik:

O2 +2H2O +4e→4OH-

Cu +2OH-→Cu(OH)2

Cu(OH)2→CuO +H2O

2Cu+ +H2O +2e→Cu2O +H2

Cu+ +H2O +e→CuO +H2

2Cu+ +1/2O2 +2e→Cu2O

2Cu+ +O2 +2e→2CuO

Q : Membran ini akan melindungi tembaga dari korosi lebih lanjut sehingga dapat disebut dengan membran proteksi oksidatif tembaga. Membran ini memiliki struktur dua lapis (double-layer) di permukaan tembaga pada kondisi normal.

Jika terdapat karbondioksida terlarut dalam air pendingin stator, maka akan terjadi reaksi yang akan melarutkan membran proteksi oksidatif tembaga diatas.
CO2 +2H+=Cu2+ +H2O
H2CO3H+ +HCO3-
Kemudian nilai PH akan berkurang pada kondisi adanya karbondioksida terlarut karena
kemurnian tinggi dan performa buffering yang buruk dari air pendingin stator.?

Sebagai contoh, PH air murni akan lebih rendah dari 6.7 saat ada 1mg/L karbondioksida terlarut.

Nah, dari penjelasan diatas, dapat kita rumuskan bahwa untuk mencegah korosi pada stator dapat dilakukan dengan :
1. Menurunkan jumlah oksigen dan karbondioksida terlarut pada air pendingin stator
   Dari hasil uji coba pada berbagai referensi, unjuk kerja terbaik didapatkan pada kurang dari 10ug/L oksigen terlarut. Namun best practices didapat di angka 20-30 ug/L.
Sebagai gambaran, konsentrasi oksigen terlarut pada air 25 C yang terpapar udara bebas adalah 1.4-3.2mg/L. Jumlah oksigen terlarut akan berkurang jika temperatur air meningkat, akan tetapi tidak terlalu signifikan.
Metode yang digunakan adalah dengan menggunakan nitrogen kemurnian tinggi sebagai cushion sehingga untuk metode ini hanya dapat diaplikasikan pada sistem air pendingin yang kedap udara (airtight).


2. Mengatur PH
PH dikendalikan pada level 7 hingga 8.9 jika terdapat sejumlah impurity ion seperti besi dan tembaga pada air pendingin stator. Cara terbaik menaikkan PH adalah melalui pengambilan sebagian sirkuit laluan ke mixbed tipe sodium. Saat air pendingin stator melalui mixedbed, Cu2+ dan Fe3+ diubah menjadi Na+ dan anion diubah menjadi OH-.
 Mixed bed tipe sodium dengan kualitas air konduktivity dibawah 0.5us/cm, hardness 0, PH 7-8, konsentrasi tembaga dibawah 10ug/L, maka menurut best practicesnya, rasio resin kation dan anion adalah 2.7 : 1.
Cara ini dapat digunakan pada sistem air pendingin yang tidak kedap udara.

Renewable Energy to Light up The Nation

Indonesia has growing demand for electricity needed at the average of 7.1% from 2009 to 2015[1]. However, there are still many areas in Indonesia that have not got the electricity benefit yet. In contrast, the amount of fossil fuels that are used to produce electricity are going to be depleted by our consuming demand. Exploration and finding rate are not comparable to national consuming rate. Indonesia will only have remain 10 years availability of oil, 31 years of gas and 69 years of coal if we don’t explore new mines and wells[2]. This condition will increase the risk of national energy security because we will be much dependent to other countries.

Indonesia is well known as resourceful country, start from natural resources such as forest products, herbs and spices; to fossil energy resources like coal, oil and gas. Beside those finite energy sources, Indonesia also has abundant renewable energy sources, for example geothermal, hydro, sunlight, wind and sea. Some portion of those renewable energy already extracted and generated to electricity especially for geothermal and hydroelectric.

The advantages of renewable energy utilization are free of fuel cost, long lasting, and environmental friendly. On the other hand, there are also several disadvantages such as inconsistent continuity, lower capacity, higher capital cost and larger space needed which resulted in the narrow growth of national utilization. For instance, Indonesia is estimated to hold approximately 40% of the world’s geothermal reserves[3] but having challenge that many geothermal resources located in distant with load center besides high investment cost[4].

To support United Nations Climate Change Conference COP21 Paris Agreement and government objectives to have renewable energy mixed >23% in 2025[5] we as an energy company have to do an action. Firstly, we have to contribute to increasing renewable energy mix which embodied in Indonesia Power main strategy. We have an objectives stated on the Company’s Long-Term Plan/RJPP to develop new renewable power plant about 883.25 MW in 2017-2024 which means 10.4% of our total planned power plant development will contribute to national energy mix target. We should do more than that. Secondly, we have to start develop off-grid renewable power plant in remote area in addition of Ministry of Energy and Mineral Resources efforts. The development of off-grid system by considering local energy opportunities will increase national electricity ratio, reinforce human capital skills, encourage people productivity and also increase our image and credibility.

Every efforts to light up Indonesia are worth to do because Indonesian constitution was not an ambitions, but a promises we must fulfill to complement our patriots’ sweat. Renewable energy power plants are great solutions. It can applied on-grid to mainly overcome sustainability and climate challenge and off-grid to levelling electricity in whole Indonesia, from Sabang to Merauke equitably.

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[1] Ministry of Energy and Mineral Resources. Handbook of energy economic statistics of Indonesia. 2016.

[2] Kementerian Energi dan Sumber Daya Mineral. Rencana strategis kementerian energi dan sumber daya mineral tahun 2015-2019. 2015.

[3] International Energy Agency. Indonesia 2015-Energy policies beyond IEA countries. 2015.

[4] Pusdatin ESDM. Prakiraan penyediaan dan pemanfaatan energi dengan skenario optimalisasi EBT daerah. 2016.

[5] PP No 79 / 2014 tentang Kebijakan Energi Nasional