Is it possible to shut down the compressor without going through the compressor PLC in the SIS system?
Thread Content
This post was last edited by *eshj123 on 2024-6-6 at 13:57. The risk assessment results indicate that there is a high risk of overpressure in the outlet scrubber tank of the screw compressor. Therefore, it is required that the SIF circuit involving the PAHH interlock (SIS interlock) on the outlet scrubber tank to shut down the compressor be completely independent from the protective interlock within the compressor skid, which also serves to shut down the compressor when the outlet PAHH reaches a certain level (controlled via PLC). In other words, it is required that the SIF interlock of the SIS system shut down the compressor without going through the compressor’s PLC control, and each of the two interlock circuits must actuate its own relay to achieve shutdown. Gentle experts, do you think this can be achieved? How can this be achieved? Does the relay for PLC interlocked shutdown of the compressor skid cut off the power to the compressor control panel? And can the relay for interlock shutdown of the SIS system cut off the power between the MCCs?
The SIF (Safety Instrument Function) interlocks of the SIS system (Safety Instrumented System) do not shut down the compressor through the PLC control of the compressor; rather, they are typically implemented via the system’s independent safety logic and controls. The following are the specific steps and considerations for implementing this function: Sensor monitoring: The SIS system monitors dangerous parameters in the production environment in real time through various sensors installed in the compressor and its surrounding area, such as pressure sensors and temperature sensors. These sensors convert the data they detect (such as pressure, temperature, etc.) into electrical signals, which are then transmitted to the SIS system. Logical operations: The SIS system contains a series of logical operation and control units that process and analyze the sensor data received. If the SIS system determines that a dangerous situation has occurred or is about to occur (for example, the pressure at the compressor inlet is below the safety threshold), it will decide whether to activate safety measures such as an emergency shutdown (ESD) based on predefined safety logic and algorithms. Emergency shutdown: When the SIS system determines that an emergency shutdown is necessary, it sends control signals directly to the actuators of the compressor (such as emergency shut-off valves, motor control devices, etc.), rather than through the PLC. These control signals immediately shut off or disconnect the power and gas supply to the compressor, causing it to stop operating and thereby ensuring the safety of personnel and equipment. Feedback regulation: After an emergency stop is initiated, the SIS system continues to monitor the operation of the system and makes the necessary adjustments or restoration actions as required. This may include restarting the compressor (after the hazard has been eliminated) or adjusting the operating parameters of the system, etc. Independence and redundancy: SIS systems are typically designed to be independent of systems such as DCS (Distributed Control Systems) and PLCs, so as to ensure that the SIS system can continue to function properly in the event of a failure in the DCS or PLCs. SIS systems also typically employ a redundant design, including redundant sensors, controllers, and actuators, to improve the reliability and availability of the system.
There’s nothing that can’t be done; SIS is essentially a standalone system, independent of the PLC. Add a pressure transmitter or pressure switch to provide interlocks for the SIS.
What I mean is, is it possible to achieve PLC interlocking and SIS interlocking, using different relays in these two circuits to shut down the compressor?
For example, you can use two intermediate relays: connect their normally closed contacts in series to the control terminal of the AC contactor as a control signal. Then, connect the SIS DO terminals to the control of one of the intermediate relays, and connect the PLC’s interlock terminals to the control of the other intermediate relay. In this way, any interlock failure will result in a power loss that shuts down the AC contactor. Of course, that’s the general idea; there will definitely be some deviations in actual use. It’s best to find a systems engineer who can give you advice based on the actual situation.
You can connect the PLC to whoever you need; ultimately, stopping the machine is achieved by de-energizing the control coil. If the signals are the same (passive 220VAC or 24VDC), you can connect them directly; if they’re not the same, adding an intermediate relay will suffice. Just find any electrician or instrumentation technician, and they’ll get it fixed for you
It can be achieved. The SIF interlock design of the SIS system should be independent of the PLC control system in order to enhance the safety of the system. Shutting down the system can be achieved by bypassing the PLC through the emergency stop device of the compressor or other hardware means. .
It can be achieved. The connection from SIS to MCC is normally on; when interlock occurs, it simply gets disconnected. Just connect the DO point relay of the SIS to the electrical circuit.