DCS and MCC control of pumps

The DCS generates control signals for both the main pump and the backup pump; the terms \"main\" and \"backup\" are relative, and the DCS does not make any distinction between them. Since it is interlocked, once the process parameters exceed their limits, if you start a backup pump, it will be too late to shut it down manually.

The root cause of the problem lies in the pump’s operating procedures; the electrical and instrumentation systems must meet the process requirements. CASE 1: The A/B pumps operate one at a time, with each serving as a backup for the other. This operation will alternate between the \"normally open pumps,\" allowing the two pumps to operate in turn and thus ensure even wear. If interlock refers to process parameters such as pressure PSL or flow rate FSL being low, the DCS must send DO start signals to both pumps in the MCC. Since one pump is already in operation and the PSL/FSL is directly associated with that pump, it is not necessary to determine which pump is currently running in order to trigger the start signal based on process parameters. The on-site electrical control panel is equipped with a ‘manual/auto’ selector switch; the main pump should be set to ‘manual mode’, while the standby pump should be set to ‘auto mode’. Since the DO start signal can only activate the motor that is in \"automatic\" mode, it is possible to put the standby pump into operation. In short, which pump the start signal is effective for is determined by the electrical secondary circuit + the “manual/auto” selector switch. The stop signal for DO is valid for both “manual” and “automatic” modes. CASE 2: Pumps A/B are normally not turned on; they serve as backups for each other. For example, in the case of electric fire pump A/B, the interlock is triggered when the pressure drops to PSL. ”Both the manual/automatic \"selection switches should be set to\" automatic mode.\" First, a two-position selector switch (a DCS switch can also be used) should be set to select the primary electric fire pump (A or B), thereby achieving the purpose of having them as backups for each other. I suggest bringing in the operation signals; the interlock can be designed as follows: Priority 1 selects Pump A, with PSL---1A; Priority 1 selects Pump B, with PSL----1B. If Pump B is running, then PSL (with a delay)----2A; if Pump A is running, then PSL (with a delay)----2B. Manual remote operation from the control room to start Pump A – 3A; manual remote operation from the control room to start Pump B – 3B. As long as either 1A/2A/3A is active, Pump A will start ； If 1B/2B/3B is valid, the B pump will be activated. Note: For 2A/2B, the pump operation signal is not necessary; I have also taken into account the possibility of false signals from the instrument PT, which is why the pump operation signal is used in an AND operation. However, this AND configuration also carries risks: if the primary pump does not start, the secondary pump will also fail to start, in which case the control room must manually start the pump based on the PSL alarm and the operating status of the pumps. You need to discuss with the process team whether to introduce operating signals for interlocking; there are advantages and disadvantages to both options. CASE 3: Diesel fire pump. As far as I remember, there is a thin pipe connecting the fire water supply to the control panel of the diesel generator; a low-voltage switch is used inside to start the generator. This is an independent interlock mechanism within the control panel – you might want to consult a hydraulic engineer regarding this. The operating status of the electric pump does not need to be sent to the diesel generator. "My understanding is that the start/stop operations related to process parameters are handled by the instrumentation, while those that are not related to process parameters, such as the start/stop of standby pumps, are handled by the electrical system. Is that correct? ? " It can be like this. But the current trend is to achieve this through DCS. The simplest approach is to control the start and stop of the standby pump independently of the process parameters; since the process also requires that the control room be able to operate it remotely, the \"mutual standby function\" is achieved through the DCS, as the operation signals are already sent to the DCS anyway.

After so many days, someone finally replied. Thank you so much; the response was very detailed. Thanks!

Actually, Haiyou on the second floor has already given the answer. It’s just a bit troublesome to switch to automatic mode with two pumps; if “motor restart” is added as well, it requires careful consideration. The fire pump should be equipped with remote manual start. Remember to follow the standards of the United States: the push buttons are sent directly to the MCC in a panel format, and even though you also have a PSL start signal, it still goes to the MCC.

The issue you mentioned is definitely related to the DCS controlling the MCC, which is the motor control center. Generally, there is no manual operation involved; it’s unlikely that human intervention would be used due to time constraints

After reading the comments from all the experts, I learned something

The issue of using pumps A and B as backups for each other depends mainly on the control strategy employed; a DCS can also facilitate the switching between pumps A and B as backups, in which case the DCS needs to be aware of the conditions for switching and carry out automatic switching based on those conditions.