In the Manufacturing and Process industry compressors play an important role in production of goods and services as application ranges from provision of compressed air to instruments and various utilities across a facility via air compressors, to the use of gas compressors for compression of gases and discharged to pipelines over long distances for export and other applications in the oil and gas industry. Our focus here though, will be on gas compressors and its application as applied in the process industry. The most dangerous and devastating issues affecting compressors is the problem of antisurge as surging of a compressor leads to potential damage of property and loss lives if not properly checked. Usually compressors are equipped with specialized antisurge control systems to prevent against such disasters as there are several control measures in place for these systems, but first take a look at compressors and how they operate before going deep into the world of antisurge control systems.
WHAT IS A COMPRESSOR AND HOW DOES IT WORK?
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume, usually by taking in gas at a lower pressure which comes in through a suction end and compressing it to a higher pressure to be transmitted via a discharge end. Compression of a gas naturally increases its temperature as the working principle of compressors are similar to pumps, both increase the pressure on a fluid and both transport the fluid through a pipe. As gases are compressible, the compressor tends to reduce the volume of gas, but as Liquids are relatively incompressible, the main action of a pump is just to transport liquids. The proper operation of a compressor system comprises a combination of various instrumentation and control devices which allow the compressor to be started or stopped, provide information on the compressors operating conditions in other to keep the compressor in stable operation, maintain the value of process variables such as suction and discharge flow, temperature and pressure, and prevent overloading of the compressors driver. Usually a turbine is used to drive a compressor with the most commonly used type being the gas turbine. Gas turbine engines derive their power from burning fuel and air in a combustion chamber under constant pressure conditions and using the fast-flowing combustion gases to drive a turbine, similar to how steam is used to drive steam turbines. The resulting hot gas which is now the working gas is then converted to rotating energy for the blades and this is used to ultimately drive an external Gas compressor. Some OEM’s(Original Equipment Manufacturers) are capable of providing a complete package with the turbine and compressor coming from the same source and delivered as one unit but most times, more than one OEM system is used to achieve this turbine-compressor package as usually, you have a case of the turbine being from a particular OEM and the compressor itself being from a separate OEM with both coming in as turn key solutions.
A typical Gas turbine in its housing
A centrifugal-type gas compressor
There are different types of gas compressors which can be grouped as;
Centrifugal compressors are one of the most popular types of gas compressors, they use a vaned rotating disk or impeller in a shaped housing to force the gas to the rim of the impeller, increasing the velocity of the gas. A diffuser (divergent duct) section converts the velocity energy to pressure energy. They are primarily used for continuous, stationary service in industries such as oil refineries, chemical and petrochemical plants and natural gas processing plants. Their application can be from 100 hp (75 kW) to thousands of horsepower. With multiple staging, they can achieve extremely high output pressures greater than 10,000 psi (69 MPa).
Antisurge Control System
In a simple and isolated illustration lets look at a compressor with a simple control system that maintains a constant discharge pressure, the main components we will consider here will be the pressure controller and compressor inlet vanes. The pressure controller consists of a sensor that detects the pressure at the discharge of the compressor. If the discharge pressure deviates from its setpoint the controller tries to control the movement of the vanes to correct this deviation, as long as gas flow through the compressor matches the discharge pressure, the discharge pressure will always remain constant. When the demand for gas increases, discharge pressure begins to drop, and the controller tries to compensate for this by letting the inlet guide vanes to open more to allow more gas into the compressor to compressed in other to increase the discharge pressure. On the other hand, if demand decreases, discharge pressure increases and the controller tries to compensate for this by closing the inlet guide vanes to reduce the amount of gas which is compressed, invariably decreasing discharge pressure. For a given discharge pressure, a compressor has a minimum flow rate below which the compressor becomes unstable and leads to a series of momentary reversals of flow through the compressor causing a phenomenon referred to as surge or surging. A surge in a compressor causes violent fluctuations in discharge pressure, low gas flow, excessive vibration, and a muffled banging sound inside the compressor as the signs of a surging compressor cannot be mistaken. To tackle this issue, the flow rate of the gas going through the compressor must be above the minimum flow rate, or surge point. A simplified approach to achieve this is to recirculate a portion of the discharge gas back into the compressor, in order to rapidly reduce the pressure build-up by opening a blow-off or recycle valve in the discharge system to maintain the required flow. Blow off causes waste of expensive process fluid, therefore, recycle is preferred and mostly carried out by 4-20 mA electronic operated control valve which is termed as “Anti-surge Control Valve”
A Typical Antisurge Control loop
In practice, there is no difference between anti surge control valve and ‘general’ control valves. The main difference is that the anti surge control valve has a certain required fast opening time to avoid the surge. This time constraint will make the anti surge control valve need some additional accessories such as volume booster and three-way pilot operated valve. The valve accessories that will be used may differ from each anti surge control valve manufacturer; this is not a problem as long as the required opening time is achieved.
The Antisurge controller uses analog inputs to determine the location of the compressor operating point and provides an analog output to a recycle valve, modulating its opening as a function of proximity to surge. The control variable for the antisurge control loop is a deviation of the compressor operating point from the surge limit line. In advanced controller Loops, the control action can be split into three actions;
- PI Action
- Recycle Trip action
- Safety on Response
Antisurge Control Lines
PI action is dealt with small and steady-state disturbances while recycling trip action will cope with sudden and quick process disturbances as shown in the below figure. Two lines are introduced right to SLL for this control algorithm. Surge control line (SCL) and surge trip line as shown in the above figure. PI action will be initiated when the operating point touches the SCL line and RTL action will be initiated when the output line touches the RTL line. RTL generates a step opening response until output returns to the safe area.
Total Response = PI Control response and Recycle Trip response.
Note that the derivative action is separate from the derivative action of the normal PID controller as this D action is only concerned with the opening of the control valve, whereas in PID controller derivative action takes part in both opening and closing the valve.
In special controllers there is a safety response which moves the surge control line to the right if the operating point moves to the left of the safety on line (usually located some distance after the SLL). The SOL defines an operating limit beyond which the compressor is assumed to be surging. When the operating point of the compressor moves to the left of the SOL, the safety on response will increment the surge count and add the additional safety margin to move the SCL and RTL to the right, thus reducing the likelihood of another surge event.
It’s a common practice that these loop controllers are not physical standalone controllers, but instead they are controller functions implemented in specialized/dedicated controllers like that of Compressor Control Cooperation’s(CCC) who specialize in Antisurge control systems through their fast and robust controllers platform which include the series 3++, series 5M, series 5 Vanguard and the high performance and most recent Prodigy controller, which have been proven to withstand the test of time with a confirmation for process optimization used to drive some of the most critical equipment in process – turbomachinery trains. From power supplies through I/O processing, to system communications, modular redundant architecture of CCC hardware ensures your turbomachinery and process keep running.
The Prodigy controller in a simplex configuration
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