Prinsip Dasar Surge Pada Kompresor [PDF]

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Prinsip Dasar Surge pada Kompresor Surge pada kompressor menunjukkan adanya kondisi operasi yang tidak stabil. Surge itu sendiri hanya terjadi pada kompresor jenis aksial dan sentrifugal. Surging merupakan gejala kompresor akibat dari rendahnya laju aliran yang masuk ke kompresor. Flow tersebut mempunyai batas minimum tertentu. Batasan tersebut tergantung pada kecepatan putar (rpm) dan tekanan discharge serta tekanan suction kompresor. Jika kompresor beroperasi di bawah harga minimumnya, maka kompresor akan mengalami surging sehingga terjadi aliran balik gas. Surging ini ditandai dengan vibrasi yang tinggi dari kompresor tersebut. Akibat dari surging pada kompresor adalah terjadinya kerusakan pada poros, sudu-sudu stator, sudu-sudu rotor, dan bearing kompresor. Secara umum ada dua jenis surge yang terjadi pada kompresor yaitu : 1.



Mild Surge Merupakan surge yang terjadi karena adanya osilasi aliran dengan frekuensi tinggi dalam batas aliran sempit yang terjadi pada tekanan relative konstan. Surge ini tidak terjadi secara simultan pada semua bagian kompresor, namun hanya pada salah satu atau beberapa bagian saja, misalnya pada sudu-sudu akibat adanya aliran balik atau berkurang. Surge ini terlihat berupa :



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Periodic dips (spikes) pada laju aliran, dengan durasi sekitar 0,2 detik.



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Pulsasi pada tekanan discharge, terutama jika diukur di dekat diffuser.



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Naiknya tingkat vibrasi.



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Meningkatnya temperature discharge.



Gambar grafik Mild surge 2.



Violent Surge Surge ini terjadi di seluruh unit, ditandai dengan jatuhnya laju aliran secara mendadak sehingga menimbulkan vibrasi yang sangat tinggi, bunyi bising, dan temperature tinggi yang kemudian memungkinkan terjadinya kondisi trip pada kompresor. Jika kondisi proses yang menyebabkan surge ini tidak segera diatasi, maka surge ini akan berulang dalam selang waktu hanya bebarapa detik saja. Hal ini dikarenakan pada saat tekanan turun, kompresor akan mengembalikan aliran dalam arah positif lagi. Arah positif ini akan menaikkan tekanan sehingga laju aliran akan jatuh kembali ketika melampaui batas surge. Sistem Kontrol Anti-Surge Sistem kontrol anti surge sangat diperlukan untuk menjaga kondisi laju aliran yang masuk melalui kompresor. Hal ini bertujuan untuk menjaga agar kompresor tidak beroperasi di bawah kondisi desain. Secara umum, system kontrol anti surge adalah sebagai berikut:



Gambar sistem kontrol anti surge Pada gambar tersebut dapat terlihat bahwa dalam keadaan normal control valve FCV harus berada dalam keadaan tertutup penuh. Hal ini bertujuan untuk mendapatkan efisiensi yang maksimal. Control valve tersebut akan bekerja bila gejala surging mulai terjadi. Gejala surging tersebut ditandai dengan berkurangnya laju aliran fluida atau flow yang masuk ke suction kompresor dengan disertai penambahan pressure ataupun pressure yang konstan. Jika hal ini terjadi terus-menerus dan tidak ada pengendalian, maka dapat berakibat fatal bagi kompresor tersebut. Pada saat gejala surging mulai terjadi, maka control valve akan mulai membuka. Hal ini bertujuan agar aliran suction yang kurang dapat di back up oleh aliran dari discharge sehingga kompresor tetap dapat dikendalikan pada kondisi desain.



Pengendalian system kontrol anti surge ini berdasarkan pada performansi compressor. Dari performansi kompresor tersebut maka dapat dibuat surge limit line yang menunjukkan batas kondisi minimum yang akan menyebabkan kompresor mengalami surging. Contoh dari grafik surge control line adalah sebagai berikut :



Grafik Diagram surge limit line Pada grafik tersebut dapat dilihat bahwa kondisi surging akan terjadi jika kompresor beroperasi di garis sebelah kiri garis batas surge (SLL). Daerah pengendalian dilakukan pada garis SCL yang berada di sebelah kanan SLL. Pada saat titik operasi mencapai garis tersebut maka FCV akan mulai membuka untuk memberikan backup flow dari discharge kompresor sehingga kondisi surging dapat dihindari. Safety margin merupakan daerah toleransi untuk kompresor bekerja sebelum kompresor tersebut mengalami surging. Safety margin ini terletak diantara SLL dan SCL.



Protecting A Centrifugal Compressor From Surge Ali Ghanbariannaeeni and Ghazalehsadat Ghazanfarihashemi, Tehran, Iran



Surge is defined as the operating point at which centrifugal compressor peak head capability and minimum flow limits are reached. Actually, the working principle of a centrifugal compressor is increasing the kinetic energy of the fluid with a rotating impeller. The fluid is then slowed down in a volume called the plenum, where the kinetic energy is converted into potential energy in form of a pressure rise. When the plenum pressure behind the compressor is higher than the compressor outlet pressure, the fluid tends to reverse or even flow back in the compressor. As a consequence, the plenum pressure will decrease, inlet pressure will increase and the flow reverses again. This phenomenon, called surge, repeats and occurs in cycles with frequencies varying from 1 to 2 Hz. So, the compressor loses the ability to maintain the peak head when surge occurs and the entire system becomes unstable. A collection of surge points during varying compressor speed or varying inlet gas angle is fitted as surge line. In normal conditions, the compressor operates in the right side of the surge line. However, during startup/emergency shutdown, the operating point will move towards the surge line because flow is reduced. If conditions are such that the operating point approaches the surge line, flow recirculation occurs in the impeller and diffuser (Figure 1). The flow separation will eventually cause a decrease in the discharge pressure, and flow from suction to discharge will resume. Surging can cause the compressor to overheat to the point at which the maximum allowable temperature of the unit is exceeded. Also, surging can cause damage to the thrust bearing due to the rotor



shifting back and forth from the active to the inactive side. This is defined as the surge cycle of the compressor.



Anti-Surge Control Systems These systems detect when a process compression stage is approaching to surge and subsequently take action to reverse the movement of the operating point towards the surge line (SL). This decreases the plenum pressure and increases the flow through the compressor, resulting in stable working conditions. It is normally achieved by opening a control valve in a recycle line (Anti-Surge Control Valve or ASCV), returning the discharge gas to the inlet of the compressor via a suction cooler. The resulting increase in compressor inlet volume flow moves the operating point away from surge. Due to inaccuracies in measurements and response times of transmitters and valves, Anti-surge control achieves a surge control line (SCL) parallel to the surge limit line. The control line is offset to the right of the surge line by a margin; typically equal to 3- 10% of inlet volume flow at surge (Figure 2). However, a lower margin is also desirable because higher efficiency could be obtained by closing the recycle valve.



Figure 2: Surge Protection Definitions. In real operation, compressor performance curves in the coordinate system are unique for constant given suction conditions. This means that the variation of inlet conditions leads to changing compressor performance curves. On the other hand, since inlet temperature decreases, inlet molecular weight increases or inlet pressure increases, the differential pressure across measuring device would go up for the same inlet capacity. So for the purposes of control, new coordinate system is used which must be invariant (or independent) to changes in inlet conditions. Therefore, the effects of inlet temperature, gas molecular weight and compressibility factor are not required to be considered in the controlling system. In this regard, several invariant coordinates are obtained by compressor manufacturer. They use dimensional analysis for the generation of these invariant systems (or compressor map). The two most important systems are: 1) Compressor polytropic head (or differential pressure) versus square of flow rate in suction and



2) Pressure ratio versus square of flow rate in suction. Flow through the compressor suction is equivalent to the pressure drop in orifice or venturi installed at the inlet or outlet of compressor. Thus, pressure loss in orifice or venturi can be calibrated as a function of compressor flow rate. A compressor map is illustrated by superimposing both performance and system resistance curves independent of rotational speed (Figure 3). For compressors with inlet guide vanes, compressor map is represented by a new family of curves that do not depend on suction conditions either. This additional coordinate could be a function of either guide vane position or equivalent rotational speed.



Figure 3: Compressor map. Anti-Surge Controller System And Algorithms Proportional–integral (PI) and proportional–integral–derivative (PID) are two major control algorithms which are used to control imperfectly known compression systems. The basic procedure of these algorithms is that the controller output should be a function of the difference (Error, e) between two values which should be controlled (process variable, PV) and its set point (SP) (Figure 4). When operating in the stable region at the right-hand side of the SP-line, where the error, e, is positive, the controller output is forced to be zero and integrators should be reset to avoid wind-up. As the flow decreases due to a disturbance, PV decreases as well and — at a certain point — where PV< SP, the error, e, becomes negative. Here, the controller comes into action opening the anti-surge valve. This action pushes the PV back to the stable region at the right-hand side of the SP-line. Moreover, small disturbances should not lead to big reactions. But a fast and resolute opening of the valve is required when the control line is exceeded in the direction of the surge limit. Therefore, the controller has a nonlinear gain behavior when the controller deviation



(PV-SP) is negative. Figure 4: Compressor controller schematic. Earlier matter about nonlinear gain controller leads to considering derivation term in logic control of system. Actually, effect of the derivative (D-action) term is that it often allows the control response to be accelerated without increasing the risk of instability, because it is a measure how fast the system is responding and action will tend to counter the oscillatory action. However, it will also make the system more sensitive to signal noise. Thus, the simpler PI algorithm is sometimes more useful than full PID control. But it (PI) is limited in speed of its response and is unable to take the machine out of surge in the event that the operating point crosses the surge line. In other words, the D-action should be incorporated by changing the set point due to a change in flow (dF/dt). When the flow decreases rapidly (dF/dt