Issues related to the modification of the low-pressure pump control circuit

A changeover switch is required to enable switching between local and remote control； Take an normally open contact on the contactor to serve as the operation signal for the DCS; it is advisable to use a relay for isolation in between. On the DCS, create a DO point for starting and stopping, which controls the coil of the contactor through the intermediate relay – it is best to connect the two normally open contacts of the intermediate relay in parallel. For DCS programming, use a 5-core 1.5 mm² cable; there is also a designated point for the central control unit, which shares the same COM terminal as the operating system. For this solution, one changeover switch, one DI and one DO module, two intermediate relays, and one 5X1.5 cable are required.

The plan upstairs is great! Regarding the start/stop signal for powering on/off the DCS, is it better to use one DO signal or two DO signals (one for start and one for stop)? Let’s discuss it together!

Personally, I think there are advantages and disadvantages to each approach. Using a single point for starting and stopping saves money; controlling each point with a regular PLC costs around 100 yuan, while using an imported DCS is much more expensive. But this is a test for the reliability of the intermediate relay. The downside is that when there is a defect in the template and no alternative point is available, trying to replace the template will affect other devices. Using two points to control start and stop is not cost-effective; it requires more templates and intermediate relays, the electrical circuit becomes somewhat complex, which poses difficulties in maintenance and increases the number of potential failure points. An advantage is that, when the equipment is in operation, the hardware of modern control systems generally supports hot swapping. In the event of defects in the template, whether it’s necessary to modify the program or replace the template, this does not affect the operation of either the current device or other devices.

1. Since a “remote/local” selection switch is required, the existing on-site control panel obviously needs to be replaced. Otherwise, add a “remote/local” selector switch in the control room. 2. The pump operation signal is provided by the electrical system in the form of a pair of passive contacts (2 wires) to the DCS as a DI; there seems to be no disagreement on this point. 3. For starting and stopping the pump, the DCS can use one DO channel or two DO channels. But it is important to be clear that when using a single DO channel, the contact must be a long signal: a closed contact indicates start, while an open contact indicates stop. When using 2-channel DO, the contact must be a pulse signal. The operating DO contact is a normally open contact; it remains disconnected under normal conditions, and it closes momentarily upon startup before opening automatically again. The stop DO contact is a normally closed contact; it remains closed under normal conditions, and it opens momentarily when stopping before closing automatically again. 4. If 2 DOs are used for controlling the start and stop of the pump, it is not recommended to use a 3-core cable with a common terminal; instead, it is advisable to use 2-core DOs for both start and stop controls. Since sometimes other contacts may need to be connected in series between these two pairs of contacts in the electrical control circuit, it is best to use a 4-core cable in order not to affect the electrical wiring. Of course, if you have already discussed this with the electrical team in advance, it’s great to be able to save one cable core! 5. It is advisable to use intermediate relays to isolate the DI and DO signals between the electrical system and the DCS, thereby avoiding unnecessary interference.