【2026 Control Systems】Which units’ control can be handled by DCS, and which must use CCS (ITCC)?

I. Lines to be drawn in accordance with the standards §1 stipulates that these standards apply to the design of control systems for large centrifugal and axial flow compressors driven by steam, gas, flue gas, or process gas turbines. The provision further specifies the models required to be in accordance with SH/T 3144 and API Std 617, with \"large\" referring to compressors whose shaft power is greater than 1500kW. The provision notes also list the items that are not applicable: the control systems for reciprocating compressors, ventilators, blowers, integral gear compressors, and expanders, as well as the motor components of motor-driven centrifugal and axial flow compressors. These units are not within the scope of mandatory requirements of these specifications; the control schemes are determined by the project itself. In practice, the DCS is often used directly for tasks such as adjusting the load in reciprocating machines, controlling the inlet vanes of blowers, and managing the backflow in small and medium-sized electrically driven centrifuges – the capabilities of the DCS are sufficient for these tasks.

There is one category at the boundary that requires attention: motor-driven centrifugal/axial flow compressors. The motor portion is not covered by these specifications, but the design requirements for the control system of the compressor portion can refer to these specifications. In other words, for a high-power electric drive centrifuge, the methods for preventing surge should still be followed as specified in this standard, with the motor side being considered separately.

II. The line drawing is here: Speed. Table 1 provides a comparison of the three options. DCS+electronic speed regulator: sampling rate of around 250 ms, with a response time on the order of seconds, which does not meet the requirements for high-speed control of compressor units ； It is sometimes used for simple cascade performance control, but there are no professional control strategies or approaches.

There is also a hybrid solution in between – DCS + electronic governor + single-loop regulator. Each device is capable of achieving its own objectives, but it is not possible to implement coupling and decoupling control between multiple circuits. Once a fault occurs in the system itself, it is difficult to resolve such issues, let alone do so online ； Although the speed is sufficient, there is often a tendency for each party to act independently, resulting in communication delays ； Since they are products from different manufacturers, it is often difficult to coordinate the services.

CCS (ITCC) features a millisecond-level sampling rate and a fixed execution speed; the control loop time for the unit is generally less than 50 ms, with a maximum of 20 ms, and it also includes accident recall functionality. §6.6.1.3 Implementing this requirement at the hardware level: the anti-surge, extraction steam, performance, and speed control loops should all be based on the same controller; all critical inputs should be scanned at least once every 10 ms, and all control algorithms should be executed at least once every 50 ms. The same controller is used to eliminate, at the root level, the inter-loop communication latency associated with ad-hoc solutions.

Surge and overspeed in large steam turbine-driven units develop on a second-by-second basis; a sampling interval of 250 ms combined with response times on the order of seconds means that by the time the controller detects an anomaly, the condition has already progressed far. This is why turbine-driven units with a capacity of over 1500 kW do not use a DCS – it’s not that the DCS is unable to handle the logic, but rather that it cannot keep up with the speed.

III. Common tasks of CCS: Decoupling (§4.6): Real-time decoupling among the anti-surge control of the compressor, performance control, and extraction steam control as well as speed control of the turbine； Dynamic decoupling among the various anti-surge control circuits of multi-stage compressors. Load distribution (§4.7): For multiple compressors operating in parallel or in series, the load is distributed reasonably based on the overall process load requirements and the power capacity of each unit, while taking into account safety margins to avoid unnecessary recirculation or shutdowns when reducing the load. Independent overspeed protection (§4.9): The drive turbine shall be equipped with an independent, SIL3-certified dedicated electronic overspeed protector of the 2oo3 type, and a maximum turbine speed limit shall be set in the CCS speed control circuit – the overspeed protection is not dependent on the CCS itself. Uninterrupted operation in case of faults (§4.10): The CCS shall be able to detect faults in the anti-stall control and speed control systems and generate corresponding protection outputs; uninterrupted control shall still be possible even in the event of a single hardware failure.

IV. With CCS in place, what is the role of DCS? CCS does not replace DCS; the two have distinct roles. §6.8 Defining the functional aspects of CCS: anti-surge control on the compressor side, performance (load) control, trip protection (which can also be implemented by the SIS), speed control on the turbine side, extraction control, and overspeed protection. The provisions in 6.8.3 specify that compressor process control – including liquid level control in the tanks between different sections, as well as monitoring and control of inlet and outlet pressure and temperature – requires decoupling operations within the CCS due to its interaction with other control functions; only after this decoupling can signals be sent to the control valves, rather than simply being handled by the DCS.