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SISTEM AIR PENGISI DAN AIR PENAMBAH

Sistem Air Penambah

Pengertian Air Penambah

Sekalipun siklus air uap didalam PLTU merupakan siklus tertutup tetapi didalam sirkulasinya banyak terjadi kehilangan massa air yang antara lain disebabkan oleh adanya kebocoran-kebocoran didalam sistem. Akibatnya diperlukan tambahan air sejumlah tertentu dari luar siklus secara kontinyu. Oleh karena itu Sistem Air Penambah berfungsi untuk memenuhi kebutuhan akan tambahan air tersebut. Mengingat bahwa kualitas air penambah harus sama baiknya dengan kualitas air yang telah berada dalam siklus, maka sistem air penambah dilengkapi dengan unit pengolah air (Demineralizer Plant) yang berfungsi untuk mengolah air sumber (Raw Water) menjadi air penambah (Make up Water).

Sumber Air

Air baku yang digunakan untuk sistem air penambah adalah air laut yang telah dihilangkan kadar garam di Desalination Plant. Hasil dari Desalination Plant disebut Raw Water, Raw Water kemudian diolah di Demineralization Plant untuk dihilangkan ion-ion kontaminan. Hasil penghilangan ion-ion kontaminan pada Raw Water yang diproses di Demineralization Plant kemudian disebut Demin Water. Demin Water yang telah memenuhi syarat sesuai spesifikasi yang telah ditentukan kemudian digunakan dan disebut sebagai air penambah. Air penambah ini kemudian ditampung di dalam make up water tank sebagai cadangan air yang siap menambah kekurangan air pada sistem air pengisi.

Desalination Pant

Desalination plant adalah suatu peralatan yang berfungsi untuk mengolah air laut menjadi air tawar (Destilate). Prinsip kerja Desalination Plant adalah menghilangkan garam-garam air laut sehingga diperoleh air murni. Proses Desalination yang umum adalah dengan cara menguapkan air laut (Evaporation). Bila air laut diuapkan, garam-garamnya akan tertinggal sedang yang menguap hanyalah air saja. Uap dari air laut inilah yang kemudian dikondensasikan sehingga diperoleh air tawar (Destilate).

Demineralization Plant

Demineralization Plant berfungsi untuk menghilangkan ion-ion kontaminan air dan menukarnya dengan ion-ion hidrogen dan ion hidoxil sehingga diperoleh air murni. Air yang diproses pada Demineralization Plant untuk dihilangkan ion-ion kontaminan adalah raw water atau air tawar (Destilate) yang dihasilkan oleh Desalination Plant.

Komponen Sistem Air Penambah

A. Make Up Water Tank (MWT)

Make up Water Tank (Tangki Air Penambah) berfungsi sebagai penampung air penambah yang akan digunakan untuk menambah kekurangan air di dalam sistem. Pengisian air pada MWT diatur oleh Make up Water Supply Valve (MWSV) yang beroperasi secara otomatis sesuai dengan set poin yang telah diatur. MWSV akan membuka jika MWT mengindikasikan level Low dan akan menutup jika level High. Dalam hal ini, level MWT selalu harus dikontrol jangan sampai rendah karena dapat mengakibatkan turunnya tingkat kevakuman Condenser karena yang terhisap dari MWT adalah udara. Jika tingkat kevakuman Condenser turun hingga mencapai pada angka yang tidak sesuai dengan yang telah ditentukan maka dapat mengakibatkan unit trip.

B. Make Up Water Pump (MWP)

Make up Water Pump berfungsi untuk memindahkan air dari MWT ke Hotwell. Terdapat dua unit MWP yang disediakan untuk memompa Make up Water dari MWT ke Hotwell. Dalam keadaan operasi normal, salah satu MWP dioperasikan sedangkan yang lainnya diposisikan Standby. Apabila MWP yang sedang beroperasi trip maka MWP yang diposisikan Standby dapat dioperasikan untuk mengganti MWP yang trip tersebut. Selain itu, MWP yang berada dalam keadaan standby juga dapat beroperasi secara otomatis jika diposisikan auto standby dan parameter condenser mengindikasikan level low sementara Condenser Level Control Valve dalam posisi membuka 80% dan Condenser Level Control Valve Bypass Valve posisi membuka penuh (Full open) maka MWP yang diposisikan auto standby akan beroperasi secara otomatis untuk membantu pengisian air penambah ke condenser hingga parameter level pada condenser mengindikasikan level normal. Gambar berikut memperlihatkan aliran air penambah sederhana yang dipompa oleh MWP.

C. Make Up Water Recirculation Valve (MWRV)

Make up water recirculation valve berfungsi untuk mengalirkan kembali air yang telah dipompa oleh MWP ke MWT dengan tujuan untuk mengamankan MWP dari kelebihan aliran dan tekanan air. Kelebihan aliran dan tekanan ini dapat terjadi jika parameter level pada condenser mengindikasikan level high, maka condenser level control valve akan secara otomatis menutup penuh. Akibatnya air yang dipompa oleh MWP akan terjebak sehingga menyebabkan tekanan dan jumlah air penambah makin bertambah pada aliran menuju ke condenser. Hal ini dapat membahayakan keselamatan MWP karena tekanan yang dihasilkan tersebut. Oleh karena itu untuk mencegah dan mengamankan MWP dari kemungkinan kerusakan, maka MWRV mensirkulasikan air kembali ke MWT. Gambar berikut memperlihatkan sistem aliran MRV sederhana.

D. Condenser Level Control Valve (CLCV)

Condenser level control valve berfungsi untuk menambah air pada sistem air pengisi tekanan rendah apabila terjadi kekurangan air yang diketahui melalui parameter hotwell yang mengindikasikan level low. Pengoperasian CLCV dan spill over valve terintegerasi dengan level pada hotwell. Jika level air di dalam hotwell turun dibawah level kerja normal akibat kebocoran atau blowdown, maka signal hasil pengukuran level hotwell akan dikirim ke komputer kontrol dan kemudian komputer kontrol akan mengirim signal ke CLCV untuk membuka agar pengisian air ke hotwell mencapai level kerja normal, sementara itu spill over valve harus sudah menutup. Sebaliknya jika level hotwell naik dari ketinggian level kerja normal, maka spill over valve akan menerima signal untuk membuka sehingga kelebihan air dikeluarkan dari sistem air pengisi tekanan rendah ke MWT sementara CLCV harus sudah menutup penuh.

Sistem Air Pengisi


Pengertian Air Pengisi

Air pengisi adalah air yang digunakan untuk memasok kebutuhan air ke boiler. Pada umumnya sistem air pengisi dibagi menjadi dua bagian yaitu, sistem air pengisi tekanan rendah dan sistem air pengisi tekanan tinggi. Aliran sistem air pengisi takanan rendah mencakup rangkain dari hotwell hingga deaerator, sedangkan aliran sistem air pengisi takanan tinggi mencakup rangkaian dari deaerator hingga boiler drum.

Tujuan menaikkan temperatur air pengisi:
1. Mencegah thermal stress
2. Mengurangi kerja boiler
3. Meningkatkan efisiensi boiler

Tujuan meningkatkan kemurnian air pengisi adalah untuk mencegah deposit, kerak dan korosif pada pipa-pipa boiler dan kerusakan pada sudu turbin.

Tujuan menaikkan tekanan air pengisi
1. Mencegah air pengisi yang menuju boiler berubah menjadi uap
2. Air pengisi dapat masuk ke boiler drum.

Jika sejumlah panas yang dibuang ke condenser dapat dikurangi, maka efisiensi turbin dapat ditingkatkan. Hal ini diperoleh dengan mengambil sebagian uap (steam) dari turbin dengan tujuan untuk memanaskan air pengisi di dalam heater (pemanas) sebelum masuk ke boiler drum.

Sirkit Air Pengisi

Sistem Air Pengisi Tekanan Rendah / Condensate System

Sistem air pengisi tekanan rendah juga disebut dengan condensate system (Sistem Kondensat) yang mana mencakup rangkaian dari hotwell higga ke deaerator. Condensate yang ditampung di dalam hotwell dengan tekanan ±0,07Kg/cm2 dan temperatur ±39°C dipompa menggunakan Condensate Pump (CP) hingga tekanan naik mencapai ±17Kg/cm2 dan kemudian dialirkan ke deaerator melewati Low Pressure Heater System (sistem pemanas tekanan rendah).
Komponen-komponen utama pada Low pressure heater system adalah Steam Jet Air Ejector (SJAE) / Gland Steam Condenser (GLC), NO.1 Low Pressure Heater (NO.1 LP Heater) dan NO.2 Low Pressure Heater (NO.2 LP Heater). Di dalam SJAE / GLC, condensate berfungsi sebagai air pendingin yang menyerap heat lost (panas yang terbuang) dari gland steam turbine (uap bekas perapat turbin) dan steam ejector (uap ejektor). Di dalam heater (pemanas), condensate dipanaskan menggunakan extraction steam (uap ekstraksi). Proses pemanasan ini meanaikkan temperatur condensate. Pada aliran condensate menuju ke SJAE/GLC, condensate diekstraksikan untuk menjaga temperatur hotwell melalui condenser spray valve yang bekerja secara otomatis, setelah itu condensate masuk kedalam SJAE/GLC. Condensate kemudian akan melewati condenser minimum flow valve dan spill over valve. Condenser minimum flow valve dan spill over valve terletak sesudah SJAE / GLC dengan tujuan agar SJAE / GLC tidak kekurangan air pendingin. Karena selain dipanaskan, condensate berfungsi sebagai pendingin gland steam turbine (uap bekas perapat turbin) dan steam ejector (uap ejektor) didalam SJAE/GLC. Gland steam turbine dan steam ejector pada SJAE/GLC uap mengalir diluar pipa sedangkan condensate didalam pipa yang terdapat didalam SJAE/GLC. Besar aliran Condensate yang telah dipanaskan didalam SJAE/GLC kemudian diatur oleh deaerator level control valve yang mana memiliki fungsi untuk menjaga level deaerator agar tetap pada level normal. Setalah aliran diatur, condensate dipanaskan didalam NO. 1 LP Heater menggunakan NO. 5 extraction steam sebagai uap pemanas yang diambil dari turbin kemudian condensate dipanaskan lagi di dalam NO. 2 LP Heater menggunakan NO. 4 extraction steam.
Proses perpindahan panas pada LP Heater sama seperti pada SJAE/GLC dimana uap pemanas mengalir di luar pipa yang berada didalam heater sedangkan condensate mengalir didalam pipa. Proses ini menyebabkan temperatur condensate naik sedangkan extraction steam terkondensasi. Hasil kondensasi extraction steam didalam heater kemudian dikontrol oleh heater level control valve sehingga tetap pada level normal. Jika level terlalu tinggi maka emergency drain valve akan bekerja secara otomatis untuk membuang condensate ini ke condenser.

Kedua LP Heater ini dilengkapi juga dengan bypass valve yang dioperasikan jika heater sedang bermasalah. Setelah dipanaskan didalam NO. 2 LP Heater, condensate dialirkan ke deaerator untuk dipanaskan dan dibuang udara dan gas-gas yang terkandung didalam condensate. Deaerator menggunakan NO. 3 extraction steam sebagai uap pemanas, namun pengambilan uap pemanas ini hanya dilakukan pada saat beban sudah mencapai lebih dari 30MW, jika beban belum mencapai 30MW maka uap pemanas diambil dari auxiliary steam yang diatur oleh deaerator auxiliary steam supply pressure control valve. Katup ini juga berfungsi untuk mengontrol tekanan deaerator pada saat kondisi operasi normal. Peralatan utama yang digunakan pada condensate system adalah:

A. Hotwell

Hotwell berfungsi sebagai penampung condenste hasil kondensasi exhaust steam turbine (uap bekas turbin), hotwell terletak di bawah condenser dengan tujuan untuk memberikan tekanan pada sisi hisap condensate pump. Level hotwell dikontrol sehingga tetap pada Normal Water Level (NWL) dengan menambahkan Make Up Water (MUW) yang telah ditampung di Make Up Water Tank (MUWT). Jika level hotwell berada pada level low maka maka condenser level control valve akan membuka dan tetap membuka hingga mencapai NWL. Jika level hotwell telah berada pada level high maka kelebihan air di dalam hotwell dialirkan kembali ke MWT melalui spill over valve yang terletak setelah SJAE / GLC.

B. Condensate Pump (CP)

Condensate pump berfungsi untuk memindahkan condensate dari hotwell melalui SJAE / GLC (pendingin Bantu) dan LP Heater ke deaerator. CP dipasang secara vertikal dengan sisi hisap berada di bagian bawah agar condensate dari hotwell dapat mengalir dan masuk ke sisi hisap CP yang berada di bagian bawah tersebut. Pada PLTU Tarahan, dua unit CP disediakan dengan kapasitas 100%. Salah satu CP beroperasi dan satu yang lainnya diposisikan standby. Jika dalam keadaan darurat maka CP yang diposisikan standby akan beroperasi, sebagai contoh bila aliran condensate yang dibutuhkan melampaui kemampuan CP yang sedang beroperasi sehingga terjadi kekurangan aliran condensate, maka CP yang diposisikan standby dapat dioperasikan untuk mengatasi kekurangan aliran condensate tersebut. Jika CP yang sedang beroperasi trip sementara CP yang sedang standby diposisikan mode otomatis maka CP yang standby akan secara otomatis beroperasi untuk menggantikan CP yang trip. Selain itu, CP yang dalam keadaan beroperasi juga bisa trip secara otomatis jika level hotwell terlalu rendah, hal ini bertujuan untuk mencegah CP bekerja dengan NSPH yang lebih rendah dari tekanan minimum yang dianjurkan. Selain termasuk di dalam condensate system, aliran condensate yang dipompa oleh CP juga didistribusikan ke beberapa peralatan, yaitu:

1. Sampling rack (rak sampel)
2. Gland seal
3. Chemical dosing
4. Condenser spray
5. Low pressure turbine casing spray.






Condensate pump







Aliran dari condensate pump

C. Steam Jet Air Ejector / Gland Steam Condenser

1. Steam Jet Air Ejector (SJAE)

Adanya sejumlah gas dan udara yang tidak terkondensasi di dalam condenser akan mengurangi laju perpindahan panas antara uap bekas turbin dan air pendingin. Terhambatnya laju perpindahan panas dikarenakan gas dan udara ini akan menyelimuti permukaan pipa air pendingin, sehingga menghalangi uap bekas turbin melepas panas pada permukaan pipa air pendingin secara maksimal. Pengurangan laju perpindahan panas antara uap bekas dan air pendingin akan menyebabkan penurunan tekanan (vakum) didalam condenser yang berarti mengurangi kemampuan unjuk kerja condenser.
Peralatan penghisap udara/gas condenser berupa SJAE, harus mampu memenuhi dua keadaan, yaitu pembuang udara/gas selama operasi normal dan menaikkan vakum condenser (vacuum up) pada saat start awal. Pada saat vacuum up, peralatan penghisap udara harus mampu engeluarkan secara cepat sejumlah besar udara/gas. Pada kondisi turbin telah beroperasi SJAE tetap dioperasikan untuk membuang udara dan gas-gas yang tidak terkondensasi dari dalam condenser untuk mempertahankan vakum di dalam condenser. Udara/gas dibuang ke atmosfir sedangkan uap untuk ejektor dikondensasi didalam gland steam condenser / kondensor ejektor. Hasil air kondensatnya kemudian dialirkan ke condenser utama.

2. Gland Steam Condenser (GLC)

Didalam sistem perapat turbin, uap sebagai media perapat harus bersirkulasi (mengalir) secara kontinyu. Setelah memberi perapat didalam labyrinth, uap ini menjadi basah dan cenderung terkondensasi karena tekanannya sangat rendah. Oleh karena itu uap perapat ini dialirkan ke kondensor uap gland (gland steam condenser) untuk didinginkan menjadi air kemudian kondensatnya dialirkan ke condenser utama. GLC mendapat pendinginan dari aliran air condensate yang dipompa oleh CP. Oleh karena itu condenser ini terletak pada jalur condensate system. Gambar beriktu memperlihatkan SJAE/GLC beserta sistem aliran pada PLTU Tarahan.

Steam Jet Air Ejector (SJAE) / Gland Steam Condenser (GLC)







Aliran condensate system dari CP melewati beberapa valve antara lain:
 Condensate minimum flow

Condensate minimum flow berfungsi untuk mensirkulasikan condensate melalui saluran resirkulasi kembali ke condenser agar menjaga jumlah aliran condensate pada CP selalu tercukupi (aliran dalam keadaan normal) baik pada saat start awal CP maupun saat kebutuhan air ke deaerator masih minimum (minimum flow). Apabila pada CP terjadi aliran kecil atau tidak ada aliran condensate maka akan dapat mengakibatkan kavitasi dan panas pada CP tersebut. Hal ini dikarenakan aliran condensate yang dipompa oleh CP juga berfungsi sebagai pendingin CP. Letak saluran resirkulasi pada condensate system terdapat sebelum katup kontrol pengisian ke deaerator, dimana saluran ini menghubungkan sisi tekan (Discharge) CP dengan hotwell. Pada saluran resirkulasi dilengkapi dengan katup kontrol dimana pengoperasian katup ini tergantung dari pembukaan katup kontrol pengisian ke tangki deaerator. Gambar berikut memperlihatkan sistem aliran condensate minimum flow secara sederhana.

Condensate minimum flow valve
 Spill over valve

Spill over valve berfungsi untuk mengalirkan kelebihan air pada condensate system ke tangki air MWT. Katup ini akan membuka apabila terjadi kelebihan air pada condensate system, dimana saat kondisi level hotwell naik melebihi level normalnya. Gambar berikut memperlihatkan sistem aliran pada spill over valve secara sederhana.

Spill over valve


D. Low Pressure Heater

Setelah melewati SJAE / GLC, condensate melewati rangkaian LP Heater yang mana memiliki fungsi untuk menaikkan temperatur secara bertahap atau sebagai pemanas lanjut yang mendapat panas dari uap ekstraksi turbin. Pemanas air pengisi tekanan rendah pada Unit 3 dan 4 di PLTU Tarahan menggunakan pemanas jenis permukaan (tube), dimana terdapat water box yang terbagi menjadi dua bagian yaitu pada sisi masuk dan sisi keluar. Air mengalir dari sisi masuk water box melalui pipa-pipa berbentuk U ke sisi luar water box dan selanjutnya ke pemanas berikutnya. Tipe pemanas ini adalah shell and tube heat exchanger.

Sistem Air Pengisi Tekanan Tinggi

Pada pokok pembahasan ini akan dibahas tentang air pengisi tekanan tinggi. Condensate yang telah masuk ke dalam deaerator sudah mendapat panas dari LP Heater 1 dan LP Heater 2. Condensate yang telah berada di dalam deaerator kemudian disebut air pengisi. Air pengisi di dalam deaerator dipanaskan menggunakan panas lanjutan dari auxiliary steam dan uap ekstraksi nomor 3. Pada saat start awal dan beban belum mencapai 30 MW, air pengisi di dalam deaerator dipanaskan menggunakan uap dari auxiliary steam dan apabila beban telah mencapai 30 MW maka uap yang di gunakan adalah uap ekstraksi nomor 3. Uap ekstraksi nomor 3 untuk memanaskan air pengisi di dalam deaerator pada saat beban rendah berada pada dalam keadaan vakum, oleh karena itu sumber pemasok uap alternatif untuk memanaskan air pengisi di dalam deaerator pada saat beban rendah diambil dari auxiliary steam. Harus diingat bahwa pasokan uap pengambilan pada beban rendah dari auxiliary steam harus di tutup ketika beban dinaikkan menjadi 30 MW untuk menghindari tekanan berlebih pada deaerator yang dapat menyebabkan safety valve (katup pengaman) bekerja.
Uap ekstraksi adalah uap bocoran yang diambil/diekstrak dari turbin pada beberapa titik dan kemudian disalurkan ke LP Heater, deaerator dan HP Heataer yang digunakan untuk memanaskan condensate dan air pengisi sebelum dimasukkan ke dalam boiler drum. Setelah mendapatkan pemanasan dari deaerator, air pengisi dihisap oleh Boiler Feed Pump (BFP) dan kemudian di alirkan melalui HP Heater 4, HP Heater 5 dan economizer untuk mendapatkan pemanasan lanjut. Pada sisi tekan (discharge) BFP terdapat tiga buah katup atau yang disebut dengan Feed Water control Valve yang bekerja secara otomatis untuk mengisi boiler drum sesuai dengan set parameter yaitu pada level nol atau level middle di boiler drum. Pada discharge BFP terdapat juga BFP Minimum Flow Valve yang bekerja secara otomatis untuk menutup atau membuka untuk disirkulasikan ke deaerator sesuai dengan set aliran yang diatur. Temperatur air pengisi pada HP Heater 4 naik karena air pengisi dipanaskan dengan uap extrasi nomor 2 dari turbin. Pada HP Heater 5 temperatur air naik karena air pengisi dipanaskan lagi dengan uap extrasi nomor 1 dari turbin. Pada HP Heater 4 dan HP Heater 5 terdapat katup bypass yang berfungsi sebagai alat bantu untuk mengalirkan air pengisi pada saat HP Heater 4 dan HP Heater 5 mengalami gangguan. Katup bypass hanya digunakan pada saat emergency karena penggunaan katup bypass secara terus menerus dapat mengurangi efisiensi turbin oleh karena itu katup bypass ditutup pada saat HP Heater 4 dan HP Heater 5 beroperasi. Setelah air pengisi melewati HP Heater 5, air pengisi mengalir ke economizer. Tujuan pemanasan air pemgisi di economizer sama seperti pada HP Heater. Pada economizer tidak terdapat ekstraksi uap dari turbin yang digunakan untuk memanaskan air pengisi melainkan memanfaatkan flue gas (gas sisa hasil pembakaran) pada boiler. Air yang masuk atau melewati economizer diteruskan ke boiler drum lalu turun ke riser melalui downcomer, disini air dipanaskan secara radiasi dengan memanfaatkan pembakaran di dalam furnace (ruang bakar) sehingga air menjadi uap dan masuk ke separator pada boiler drum untuk dipisahkan antara uap dan air. Air masuk pada boiler drum sedangkan uap di alirkan ke superheater untuk di jadikan uap kering sebelum masuk ke turbin.

Macam – macam alat Bantu yang digunakan :

A. Deaerator

Deaerator berfungsi untuk memanaskan dan menghilangkan gas dan udara terlarut dalam air pengisi. Udara dan gas dapat menyebabkan korosi di dalam pipa-pipa air uap ketel uap. Rancangan deaerator yang banyak dipakai terdiri dari bejana penyimpan mendatar yang besar dan diatasnya dipasang satu atau lebih unit-unit pembuang udara (deaerator), ini serupa dengan pemanas kontak langsung. Sebelum air pengisi memasuki bagian pembuang udara, air tersebut lewat melalui vent condensor tipe permukaaan.

Gambar Deaerator

B. Boiler Feed Pump ( BFP )

Boiler feed pump berfungsi untuk menaikkan tekanan air pengisi sehingga air pengisi tersebut dapat mengalir dan masuk ke dalam boiler drum. BFP harus mampu mengisi boiler drum pada penguapan maksimum dengan pembakaran penuh dan ketika katup pengaman superheater dan boiler drum membuka pada saat terjadi akumulasi tekanan. Tekanan discharge BFP harus lebih besar dari tekanan boiler drum, karena disebabkan adanya rugi-rugi tekanan dalam sistem aliran air pengisi sebelum air pengisi tersebut masuk kedalam boiler drum.
Tugas utama pompa BFP adalah memasok air pengisi ke boiler drum namun selain itu digunakan juga untuk menyuplai air pengisi ke beberapa peralatan, yaitu :

Desuperheter spray water
• HP ( High Pressure ) turbin bypass
• Auxiliary steam
• Turbin bypass spray water












Gambar Pompa BFP ( Boiler Feed Pump )





C. High Pressure Heater ( HPH )

High Pressure Heater befungsi untuk menaikkan temperatur secara bertahap atau sebagai pemanas lanjut yang menggunakan uap ekstraksi dari turbin sebagai sumber pemanasan. Pada PLTU Tarahan unit 3 dan 4 pemanas air pengisi tekanan tinggi yang digunakan jenis permukaan atau tube, dimana terdapat water box yang terbagi dua bagian yaitu sisi masuk dan sisi keluar. Air mengalir dari sisi masuk water box melalui pipa-pipa berbentuk U ke sisi luar water box dan selanjutnya ke pemanas berikutnya, tipe pemanas ini adalah shell and tube heat exchanger.

Gambar HPH ( High Pressure Heater )




D. Economizer

Economizer berfungsi sebagai pemanas awal air sebelum dimasukkan ke boiler drum, dimana panas yang didapat dari pemanfaatan flue gas buang sisa pembakaran dari furnace (ruang bakar).

E. Boiler Drum

Boiler drum berfungsi untuk menampung air dan uap juga untuk menjaga boiler pada suatu tekanan tertentu.

MW 50 75 100

O C HP5 203.5 221.5 253.0

O C HP 4 173.5 183.2 201.0

O C Dea 145.5 158.6 169.1

O C LP 2 113.3 123.8 131.7

O C LP1 79.5 82.3 98.8

O C sblm LP 44.0 43.2 42.7










Tabel Beban beserta temperatur : Baca Selengkapnya...

FEEDWATER SYSTEM

Feedwater heaters

Open type—deaerators.

(1) Purpose.

Open type feedwater heaters are used primarily to reduce feedwater oxygen and other noncondensable gases to essentially zero and thus decrease corrosion in the boiler and boiler feed system. Secondarily, they are used to increase thermal efficiency as part of the regenerative feedwater heating cycle.


(2) Types.

1) There are two basic types of open deaerating heaters used in steam power plants—tray type and spray type. The tray or combination spray/tray type unit will be used. In plants where heater tray maintenance could be a problem, or where the feedwater has a high solids content or is corrosive, a spray type deaerator will be considered.

2) All types of deaerators will have internal or external vent condensers, the internal parts of which will be protected from corrosive gases and oxidation by chloride stress resistant stainless steel.

3) In cogeneration plants where large amounts of raw water makeup are required, a deaerating hot process softener will be selected depending on the steam conditions and the type of raw water being treated (Section IX, paragraph 3-38 and 3-39).


(3) Location.

The deaerating heater should be located to maintain a pressure higher than the NPSH required by the boiler feed pumps under all conditions of operation. This means providing a margin of static head to compensate for sudden fall off in deaerator pressure under an upset condition. Access will be provided for heater maintenance and for reading and maintaining heater instrumentation.











(4) Design criteria.

1) Steam pressure to the deaerating heater will not be less than three psig.
2) Feedwater leaving the deaerator will contain no more than 0.005 cc/liter of oxygen and zero residual carbon dioxide. Residual content of the dissolved gases will be consistent with their relative volubility and ionization.
3) Deaerator storage capacity will be not less than ten minutes in terms of maximum design flow through the unit.
4) Deaerator will have an internal or external oil separator if the steam supply may contain oil, such as from a reciprocating steam engine.

5) Deaerating heater will be provided with the following minimum instrumentation: relief valve, thermometer, thermocouple and test well at feedwater inlet and outlet, and steam inlet; pressure gauge at feedwater and steam inlets; and a level control system (paragraph c)







Closed type.

a. Purpose.
along with the deaerating heater, closed feedwater heaters are used in a regenerative feedwater cycle to increase thermal efficiency and thus provide fuel savings. An economic evaluation will be made to determine the number of stages of feedwater heating to be incorporated into the cycle. Condensing type steam turbine units often have both low pressure heaters (suction side of the boiler feed pumps) and high pressure heaters (on the discharge side of the feed pumps). The economic analysis of the heaters should consider a desuperheater section when there is a high degree of superheat in the steam to the heater and an internal or external drain cooler (using entering condensate or boiler feedwater) to reduce drains below steam saturation temperature.

b. Type.
The feedwater heaters will be of the Utube type.

c. Location.
Heaters will be located to allow easy access for reading and maintaining heater instrumentation and for pulling the tube bundle or heater shell. High pressure heaters will be located to provide the best economic balance of high pressure feedwater piping, steam piping and heater drain piping.

d. Design criteria

(b) Each feedwater heater will be provided with the following minimum instrumentation: shell and tube relief valves; thermometer, thermocouple and test well at feedwater inlet and outlet; steam inlet and drain outlet; pressure gauge at feedwater inlet and outlet, and steam inlet; and level control system.
c. Level control systems.
(1) Purpose. Level control systems are required for all open and closed feedwater heaters to assure efficient operation of each heater and provide for protection of other related power plant equipment. The level control system for the feedwater heaters is an integrated part of a plant cycle level control system which includes the condenser hotwell and the boiler level controls, and must be designed with this in mind. This paragraph sets forth design criteria which are essential to a feedwater heater level control system. Modifications may be required to fit the
actual plant cycle.
(2) Closed feedwater heaters.
(a) Closed feedwater heater drains are usually cascaded to the next lowest stage feedwater heater
or to the condenser, A normal and emergency drain line from each heater will be provided. At high loads with high extraction steam pressure, the normal heater drain valve cascades drain to the next lowest stage heater to control its own heater level. At low loads with lower extraction steam pressure and lower pressure differential between successive heaters, sufficient pressure may not be available to allow the drains to flow to the next lowest stage heater. In this case, an emergency drain valve will be provided to cascade to a lower stage heater or to the condenser
to hold the predetermined level.
(b) The following minimum instrumentation will be supplied to provide adequate level control at
each heater: gauge glass; level controller to modulate normal drain line control valve (if emergency drain line control valve is used, controller must have a split range); and high water level alarm switch.
(3) Open feedwater heaters-deaerators. The following minimum instrumentation will be supplied to provide adequate level control at the heater: gauge glass, level controller to control feedwater inlet control waive (if more than one feedwater inlet source, controller must have a split range); low water level alarm switch; “low-low” water level alarm switch to sound alarm and trip boiler feed pumps, or other pumps taking suction from heater; high water level alarm switch; and “high-high” water level controller to remove water from the deaerator to the condenser or flash tank, or to divert feedwater away from the deaerator by opening a diverting valve to dump water from the feedwater line to the condenser or condensate storage tank.

3-30. Boiler feed pumps.
a. General. Boiler feed pumps are used to pressur- 3-41 TM 5-811-6 ize water from the deaerating feedwater heater or deaerating hot process softener and feed it through any high pressure closed feedwater heaters to the boiler inlet. Discharge from the boiler superheated steam in order to maintain proper main steam ternperature to the steam turbine generator.
b. Types. There are two types of centrifugal multi-stage boiler feed pumps commonly used in steam power plants—horizontally split case and barrel type with horizontal or vertical (segmented) split inner case. The horizontal split case type will be used on boilers with rated outlet pressures up to 900 psig. Barrel type pumps will be used on boilers with rated outlet pressure in excess of 900 psig.
c. Number of pumps. In all cases, at least one spare feed pump will be provided.
(1) For power plants where one battery of boiler feed pumps feeds one boiler.
(a) If the boiler is base loaded most of the time at a high load factor, then use two pumps each at 110-125 percent of boiler maximum steaming capacity.
(b) If the boiler is subject to daily wide range load swings, use three pumps at 55-62.5 percent of
boiler maximum steaming capacity. With this ar- rangement, two pumps are operated in parallel between 50 and 100 percent boiler output, but only one pump is operated below 50 percent capacity. This arrangement allows for pump operation in its most efficient range and also permits a greater degree of flexibility.
(2) For power plants where one battery of pump feeds more than one boiler through a header system, the number of pumps and rating will be chosen to provide optimum operating efficiency and capital costs. At least three 55-62.5 percent pumps should be selected based on maximum steaming capacity of all boilers served by the battery to provide the flexibility required for a wide range of total feedwater flows.
d. Location. The boiler feed pumps will be located at the lowest plant level with the deaerating heater or softener elevated sufficiently to maintain pump suction pressure higher than the required NPSH of the pump under all operating conditions. This means a substantial margin over the theoretically calculated requirements to provide for pressures collapses in the dearator under abnormal operating conditions. Deaerator level will never be decreased for structural or aesthetic reasons, and suction pipe connecting deaerator to boiler feed pumps should be sized so that friction loss is negligible.
e. Recirculation control system.
(1) To prevent overheating and pump damage, each boiler feed pump will have its own recirculation control system to maintain minimum pump flow whenever the pump is in operation. The control system will consist off
(a) Flow element to be installed in the pump suction line.
(b) Flow controller.
(c) Flow control valve.
(d) Breakdown orifice.
(2) Whenever the pump flow decreases to minimum required flow, as measured by the flow element in the suction line, the flow controller will be designed to open the flow control valve to maintain minimum pump flow. The recirculation line will be discharge to the deaerator. A breakdown orifice will be installed in the recirculation line just before it enters the deaerator to reduce the pressure from boiler feed pump discharge level to deaerator operating pressure.
f. Design criteria.
(1) Boiler feed pumps will comply with the latest evisions of the following standards:
(a) Hydraulics Institute (HI).
(b) American National Standards Institute (ANSI).
(2) Pump head characteristics will be maximum at zero flow with continuously decreasing head as flow increases to insure stable operation of one pump, or multiple pumps in parallel, at all loads.
(3) Pumps will operate quietly at all loads without internal flashing and operate continuously without overheating or objectionable noises at minimum recirculation flow.
(4) Provision will be made in pump design for expansion of
(a) Casing and rotor relative to one another.
(b) Casing relative to the base.
(c) Pump rotor relative to the shaft of the driver.
(d) Inner and outer casing for double casing pumps.
(5) All rotating parts will be balanced statically and dynamically for all speeds.
(6) Pump design will provide axial as well as radial balance of the rotor at all outputs.
(7) One end of the pump shaft will be accessible for portable tachometer measurements.
(8) Each pump will be provided with a pump warmup system so that when it is used as a standby
it can be hot, ready for quick startup. This is done by connecting a small bleed line and orifice from the common discharge header to the pump discharge inside of the stop and check valve. Hot water can then flow back through the pump and open suction valve to the common suction header, thus keeping the pump at operating temperature.
(9) Pump will be designed so that it will start safely from a cold start to full load in 60 seconds in
TM 5-811-6 an emergency, although it will normally be warmed before starting as described above.
3-31. Feedwater supply
a. General description.
(1) In general terms, the feedwater supply includes the condensate system as well as the boiler
feed system.
(2) The condensate system includes the condensate pumps, condensate piping, low pressure closed heaters, deaerator, and condensate system level and makeup controls. Cycle makeup may be introduced either into the condenser hotwell or the deaerator. For large quantities of makeup as in cogeneration plants, the deaerator maybe preferred as it contains a larger surge volume. The condenser, however, is better for this purpose when makeup is of high purity and corrosive (demineralized and undeaerated). With this arrangement, corrosive demineralized water can be deaerated in the condenser hotwell; the excess not immediately required for cycle makeup is extracted and pumped to an atmospheric storage tank where it will be passive in its deaerated state. As hotwell condensate is at a much lower temperature than deaerator condensate, the heat loss in the atmospheric storage tank is much less with this arrangement.
(3) The feedwater system includes the boiler
feed pumps, high pressure closed heaters, boiler feed suction and discharge piping, feedwater level controls for the boiler, and boiler desuperheater water supply with its piping and controls.
b. Unit vs. common system. Multiple unit cogeneration plants producing export steam as well as electric will always have ties for the high pressure steam, the extraction steam, and the high pressure feedwater system. If there are low pressure closed heaters incorporated into the prime movers, the condensate system usually remains independent for each such prime mover; however, the deaerator and boiler feed pumps are frequently common for all boilers although paralleling of independent high pressure heater trains (if part of the cycle) on the feedwater side maybe incorporated if high pressure bleeds on the primer movers are uncontrolled. Each cogeneration feedwater system must carefully be designed to suit the basic parameters of the cycle. Level control problems can become complex, particularly if the cycle includes multiple deaerators operating in parallel.
c. Feedwater controls. Condensate pumps, boiler feed pumps, deaerator, and closed feedwater heaters are described as equipment items under other headings in this manual. Feedwater system controls will consist of the following
(1) Condenser hotwell level controls which control hotwell level by recirculating condensate from the condensate pump discharge to the hotwell, by extracting excess fluid from the cycle and pumping it to atmospheric condensate storage (surge) tanks, and by introducing makeup (usually from the same condensate storage tanks) into the hotwell to replenish cycle fluid.
(2) Condensate pump minimum flow controls to recirculate sufficient condensate back to the condenser hotwell to prevent condensate pumps from overheating.
(3) Deaerator level controls to regulate amount of condensate transferred from condenser hotwell to deaerator and, in an emergency, to overflow excess water in the deaerator storage tank to the condensate storage tank(s).
(4) Numerous different control systems are possible for all three of the above categories. Regardless of the method selected, the hotwell and the deaerator level controls must be closely coordinated and integrated because the hotwell and deaerator tank are both surge vessels in the same fluid system.
(5) Other details on instruments and controls for the feedwater supply are described under Section 1 of Chapter 5, Instruments and Controls.




3-3. Steam power cycle economy
a. Introduction. Maximum overall efficiency and economy of a steam power cycle are the principal design
criteria for plant selection and design. In general, better efficiency, or lower heat rate, is accompanied
by higher costs for initial investment, operation and maintenance. However, more efficient cycles are more complex and may be less reliable per unit of capacity or investment cost than simpler and
less efficient cycles. Efficiency characteristics can be listed as follows:
(1) Higher steam pressures and temperatures contribute to better, or lower, heat rates.
(2) For condensing cycles, lower back pressures increase efficiency except that for each particular turbine unit there is a crossover point where lowering back pressure further will commence to decrease efficiency because the incremental exhaust loss effect is greater than the incremental increase in available energy.
(3) The use of stage or regenerative feedwater cycles improves heat rates, with greater improvement
corresponding to larger numbers of such heaters. In a regenerative cycle, there is also a thermodynamic
crossover point where lowering of an extraction pressure causes less steam to flow through the
extraction piping to the feedwater heaters, reducing the feedwater temperature. There is also a limit to
the number of stages of extraction/feedwater heating which may be economically added to the cycle. This occurs when additional cycle efficiency no longer justifies the increased capital cost
(4) Larger turbine generator units are generally more efficient that smaller units.
(5) Multi-stage and multi-valve turbines are more economical than single stage or single valve machines.
(6) Steam generators of more elaborate design, or with heat saving accessory equipment are more efficient.
b. Heat rate units and definitions. The economy or efficiency of a steam power plant cycle is ex-less efficient cycles. Efficiency characteristics can be listed as follows:
(1) Higher steam pressures and temperatures contribute to better, or lower, heat rates.
(2) For condensing cycles, lower back pressures increase efficiency except that for each particular turbine unit there is a crossover point where lowering back pressure further will commence to decrease efficiency because the incremental exhaust loss effect is greater than the incremental increase in available energy.
(3) The use of stage or regenerative feedwater cycles improves heat rates, with greater improvement corresponding to larger numbers of such heaters. In a regenerative cycle, there is also a thermodynamic
crossover point where lowering of an extraction pressure causes less steam to flow through the extraction piping to the feedwater heaters, reducing the feedwater temperature. There is also a limit to the number of stages of extraction/feedwater heating which may be economically added to the cycle. This occurs when additional cycle efficiency no longer justifies the increased capital cost.
(4) Larger turbine generator units are generally more efficient that smaller units.
(5) Multi-stage and multi-valve turbines are more economical than single stage or single valve machines.
(6) Steam generators of more elaborate design, or with heat saving accessory equipment are more efficient. Baca Selengkapnya...

PLTU ABOUT WASTE WATER TRAETMENT

Process Description


Unit Neutralization Pit

Regular water from power house oil sump pit, condenser pit, turbine room sump pit, boiler area sump pit, check pit 2 & 3 ( When oil is detected in the water ) will collected in the oil removal pit. ( First compartment of Unit Neutralization pit ).

The oil removal pit is provided with a belt type oil skimmer which is the one of the easiest ways of removing hydro carbon from waste water. The oil skimmer needs to be started by operator, from the PLC when oil is detected in the checkpits.

Belt oil skimmer work because of the difference in specific gravity between oil and water. Water has a specific gravity of less than one because of these differences, oil flats to the top of the water where it can be removed. A belt of oil skimmer uses oil oilophilic material ( usually made of ss or plastic ), in the form of belt to break the surface tension of the water to attract and collect the floating oil. The belt passes through a set of wiper blades via a motorized head pulley where the oil is wiped off both sides of the belt and oil is collected into oil skimmer pit. Water moves to intermediate compartment in the unit neutralization pit from the bottom of oil removal pit and then it is transferred to the third compartment by overflowing from second compartment. The third ( main ) compartment of the unit neutralization pit is provided with the air line and headers from the unit neutralization pit blower. The operation of unit neutralization pit blower will depend on the water level of water in the pit. The low level of water in pit will trip the blower.

The pH meter installed in the neutralization pit will keep a watch on the pH level of water. The acceptable range of pH in water discharged from the pit is 6.5 to 8.5. Additional caustic dosing line is provided in the UN pit. If requirement arise then the diluted caustic manual valve to the inlet line can be opened to raise the pH of the unit neutralization pit so that it is maintained within the acceptable range. This shall be a need based operation only.


Pump Operation

Two Unit Neutralization pit pumps are provided to transfer the water from the Unit Neutralization pit to waste water storage ponds 3 & 4. At high level f the water in the pt, selected pump will start and remain ON till the low level in the pit is reached.

The filling of water in WWS 3 & 4 depends on the level of water stored in them. In the beginning, the valve in the inlet line of WWS ponds 3 is open and the water is filled in the pond. This valve is be shut off on the high level of the pond and the valve in the inlet line of WWS pond 4 will open and remain open till its high level. In case of high level in both WWS ponds 3 & 4, the running UN pit pump will trip.


At high level of WWS pond 3 & 4, selected pump of respective pond will start and remain ON till the low level of that pond. In case of high level of clear water pit during system start up, the selected WWS pond pump should not start and when during operation, if the water level becomes high, then the pump should trip. In case of pH in pH control and oxidant pit going beyond acceptable range, WWS pond pump has to be tripped.


Waste Water Storage Ponds 3 & 4

The regular waste water from the Unit Neutralization pit, laboratory sump pit, administration building general waste sump pit coal analysis pit, common header from water treatment building sump pit, chlorination building sump pit and filtrate from the filter press is transfer to waste water storage ponds where it is aerated so that the concentration of solids attain a uniform value. Two no of WW storage ponds are provided in one filling and one empty condition, so as to take the benefit of continuous aeration of the water in the pond. The blowers will be OFF only in the condition of low level respective ponds. One common header is provided from the outlet of both blowers which will ensure the continuous air supply to pH control and oxidation pit.


pH Control and Oxidation Pit

Water from the WWS ponds is transferred to the first compartment and oxidation pit via the WWS pond pump. Her pH is sensed by pH analyzer. Any deviation from the acceptable range will open the valves provided in the line dosing lines to the pit.

In case of pH going below 6.5 ( set value in PLC ), the valve in the inlet of caustic dosing line will open and remain till pH attains a value of 7.5 ( set value in the PLC ). In case of pH going above 8.5 ( set value in the PLC ), the valve in the inlet of acid dosing line will open and remain till pH attains a value 7.5 ( set value in the PLC ). For the acid and caustic dosing the pH values should be fed in the set parameters at the PLC as per requirement. These values are settable and can be changed as per requirement so that the final outlet pH from the neutralization pit is between 6.5 - 8.5.

Coagulant ( Ferric Chloride ) and Coagulant aid ( Poly Electrolyte ) is dosed in the mixing compartment of the pH control and oxidation pit. This dosing only carried out when the level of pH control and oxidation pit level is healthy. The dosing rate of coagulant and coagulant aid is manually adjusted based on JAR TEST ANALYSIS and according to the water quality. The dosing of coagulant and coagulant aid should be done continuously when the plant is running. Normally 1 - 2% concentration of ferric chloride ( coagulant ) is dosed and coagulant aid of 0.05% is dosed. The dosing rate of the chemicals can be varied by adjusting the storke of respective dosing pumps. An agitator is proved in the mixing pit. The operation of agitator corresponds with the level of water in the pit. Air line from common header outlet line from WWS ponds blowers is prvided which is ensures the continuous air supply to pH contrl and oxidation pit for oxidation and help in chemical mixing.


Coagulant and Sedimentation Tank ( Clarifier )

The water from pH control and oxidation pit is transferred to the coagulation and sedimentation tank. Here the clarifier mechanism provided helps the TSS to settle down in the tank and overflowing the clear water through side wall launders to clear water pit. The sludge settled in the bottom of the clarifier is transferred to the suction line of sludge slurry transfer pumps.


Sludge Enrichment Tank ( Sludge Thickener )

The sludge from the of coagulant and sedimentation tank is transfered to sludge enrichment tank by sludge slurry pump where the Coagulant ( Ferric Chloride ) is dosed. The dosing is carried out manually by opening the valve, and only when the sludge slurry transfers pumps on. Normally the coagulant dosing valve at slude thickener is closed. Dosing of coagulant to be done only when the sludge is being transfered to the sludge thickener from the coagulation and sedimentation tank. Sludge thickener mechanism provided helps the thick sludge to settle down in the tank and overflowing through side wall launders and transfer it to WWS ponds. The sludge settled down in the bottom of the tank iis transferred to the suction of sludge enrichment transfer pumps.

This sludge is then transferred to the dehydrator ( Filter Pressure ) via two sludge enrichment tank pumps ( Screw Pumps ). The operation of sludge enrichment tank pumps is interlocked with the level switch. The pump will only start when there is sufficient level in the sludge enrichment tank and trip when the low level is actived. The pump starts after settable time for ( 2 Min ). A valve is provided for flushing the suction line of sludge enrichment tank pump. The operation of the valve is settable time based. For this section start stop facility is provided to drain the sludge and to operate the pumps as per required. To drain the sludge manually the sludge section has to be kept in manual mode and open the flushing valve and start the pump from PLC. Baca Selengkapnya...