Surge phenomenon in the compressor and how to control it. Turbine, centrifugal, axial or turbine compressors are the heart of many technological processes (in the framework of the article, the author calls them by the common names of commonly used machines in Vietnam). Typically, air compressors are a critical component to plant operations, but it’s still rare for them to have a backup machine installed. The problem of “Surging” pulsation (some documents call it the phenomenon of ups and downs) is a big threat to turbo compressors in particular and centrifugal compression technology in general. Preventing Surge is an important part of technology control. Surging can increase costs due to shutdown and it also causes mechanical damage to the compressors. An effective anti-Surge control system is very important for every turbo compressor (centrifuge).

Watching: What is Surge

What is surging

?Many people think that surging is similar to the cavitation phenomenon in centrifugal pumps, but it is not. Surging is defined as the self-oscillation of outlet pressure and compressed air flow rate, including a reversal of compressed air flow. Each centrifugal or axial compressor has a combination of maximum pressure and minimum flow characteristics. Beyond this point, surging will occur. During surging, a reverse current is often accompanied by a voltage drop (voltage drop). When viewed with the naked eye, the voltage meter at the output fluctuates continuously.Surging is best illustrated by observing the movement of the compressor at the time of operation on the characteristic curve as shown in Fig. 01 below.

Figure 01

To visually invite you to watch the video when the turbo compressor is Surging. You pay attention to observe the outlet pressure gauge, the film wrapped around the air outlet indicates the state of the air being sucked or blown in reverse. In particular, the machine temperature will increase rapidly along with dynamic capacity.

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The Evolution of the Surge Cycle

Consider a turbo compressor system as shown in Figure 02. The outlet pressure is denoted Pd and the outlet pressure to the gas tank or the load outlet pipe is PV.

Figure 02

Now, refer to Figure 03 below, assuming that the system operates stably at point D. If the gas demands decrease, the operating point will be redirected to point A, the Surging point. If the load is reduced enough, the compressor operating point will surpass point A. Beyond point A, the compressor will lose the ability to increase outlet pressure so that Pd will become lower than PV. This is the reverse flow observed during surging. The operating points will then move to point B.

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Figure 03

Point B is not a stable operating point. When reverse flow occurs, the discharge pressure will decrease. This causes the operating point to move from point B to point C. At point C, the flow rate is not enough to create the pressure required to return to point A. Therefore, the operating point will move to point A. point D, where the flow rate exceeds the required load and pressure builds up until point A is reached. This completes one Surg cycle. Subsequent cycles are restarted with another backflow and the process repeats until an external force breaks the Surge cycles.

Consequences of surging

Consequences of Surging may include: 1. Rapid fluctuations in pressure and flow cause process instability2. Increase the temperature inside the compressor 3. Stop the compressor 4. Mechanical Demolition Mechanical Demolition:

The turbine wing was destroyed

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Bearings carry the load in the first stage of surging. One side of the load is placed on the rotor acting perpendicular to the shaft. The thrust bearing carries the load under load and at no load. The seal has friction (seal) Dynamic and stationary parts can touch when the thrust bearing is blocked. Overload. Thermal destruction when surging occurs it will create turbulent flow, the amount of heat generated will be very large, heating the tubor impeller chamber. The obvious sign is that the turbo will be slightly red hot, the entire exterior paint will be burned, rust occurs quickly. This heat also overloads the cooling system, leading to silver lubricating quality. reduced (low viscosity, poor thermal conductivity).

Anti-Surge Control

The only way to prevent surging is to recycle or blow down part of the compressor line to keep it outside the surging limit. Unfortunately, this additional gas flow affects economic efficiency, energy (electricity). Therefore, the system control must be able to correctly identify the compressor operating points as the point to provide the full Recycle current, but not too much. A Surge Limit Line (Surge Limit Line) SLL) is a line connecting different Surge points at different speeds. The anti-surge control point setting shown on the compressor diagram in Figure 04 is a line that runs parallel to the Surge SLL limit line. This line is called the Surge Control Line (SCL). The controller should be able to calculate the offset from the operating points to the SCL.

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Figure 04 The compressor Surge limit is not fixed for any measured variables such as compression ratio or pressure loss through the accelerometer. Instead, it is a complex function in which the gas composition depends, rotational speed, temperature and inlet pressure. A PI controlled closed loop will not be able to stop Surge during times of large or fast turbulence. So such a control wouldn’t be able to stop Surge. Instead, a simple control would be to cycle the Recycle valve opening and closing in response to successive Surge cycles. To control a PI that acts quickly, the “b” value needs to be high. This will result in a reduction in the compressor operating area when the Recycle valve is closed. Therefore, an open control loop is used in conjunction with an anti-Surge controlled closed loop. The overall configuration is shown in Figure 5. A Recycle Trip Line (RTL) cyclic interrupt will be used between the SLLs and SCLs. Slow and small disturbances are managed by a closed control loop that keeps the compressor operating point to the right of the RTL line. For large and fast disturbances, the compressor operating point will approach the RTL line. At this point, the open control loop will be started. This will add a step change that is a function of the compressor operating point which will at this point approach the RTL. In this way, opening the valve quickly will be enough to stop the surging. Amplifier converters are also used in anti-surge control. When the operating points move rapidly towards the SCL line, this amplifier moves the SCL towards the operating point.

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Figure 05

Requirements for Anti-Surge Valve (Anti-Surge Valve)1. The valve must be large enough to prevent surging under all operating conditions. However, a valve that is too large will make it work poorly.2. Stroke speed – a responsive stroke speed is very important. The recommended full stroke time is as follows.