3 Pos Selector Switch

I need some help guys, I need to modified a control Logix 5000 plc ladder.

I have 2 BFW pumps running with and auto/manual selector in auto if there is low pressure the other one starts.

I need to add a 3rd pump, but we want to include a 3 selector switch to disable one for maintenance and the rest two will continue work as before we added the 3rd.

Any help?

Regards

There is not one solution to your problem, but many. The right one depends on what you have in your program now. ZIP it and post (or print it to a PDF file), and someone will help you figure it out.

Lancie1

Got it, here there is actual ladder of BFW pumps, as I mentioned I need add a 3rd pump with the a 3 position selector to disable one of them eventually.

Soriancg,

Thanks for posting the program. It would take awhile to figure out how to add the 3rd pump.

The main problem is going to be how to tie in the 3-Position Maintenance switch (to remove one pump) with your existing logic. For example, if you "remove Pump A" for maintenance, then the logic should now be looking at Pumps B and C instead of Pumps A and B. In other words, the logic must substitute "Pump B" everehere the the old A occurs, and "Pump C" for the old "Pump B". That can be done with indirect addressing, but the current program is not set up for that method. Your current program only has posibility of using two pumps, so the transition to 3 pumps will require an extensive re-write.

The next question is: how much of the current program are you willing to give up to get the new functions? How extensive a re-write are you prepared to acccept? If this is a running boiler, then people often get upset with revisions that cause problems, so this is not something to tackle lightly.

I do most sincerely hope you are planning on having a lock out disconnect for each pump. I suggest you attach an auxiliary switch to the lockout disconnect and wire that to the PLC. If the pump is being maintained the disconnect will be off and your PLC can then respond to that.

Alaric,

You are the MAN for this job! However, I doubt if you would want to rewrite a program like this without a little compensation and a lot more information.

Alaric,

Your mention of motor disconnects got me to thinking about another option - just add three 3-pole double-throw disconnect switches (commonly available up to 600 Amps) in front of each pump motor, as shown below. This provides local safety disconnect switches AND allows selection of any two-of-three motors running from the two existing pump power circuits. Interlock switches can be mounted on each disconnect to prevent PLC from feeding more than two at a time, and to signal when a disconnect switch is in the OFF position.

A similar circuit COULD be tacked onto the existing PLC program, using internal PLC relays to route either of the existing two Pump Run Outputs to one of three pumps, depending on PLC selector switch settings.

|  |                               DISC SW. A

|  | Existing Contactor “1”         +-------+

|  | (Old Pump “A” Work Bit)      1 |       |

|  +----------| |------------+--------O     | PUMP A

|                            |  OFF | ---O-----( A )

|                            |    2 |       |

| Existing Contactor “2” +------------O     |

|(Old Pump “B” Work Bit) |   |      |       |

+----------| |-----------+   |      +-------+

                         |   |

                         |   |     DISC. SW. B

                         |   |      +--------+

                         |   |    1 |        |

                         |   +--------O      | PUMP B

                         |   |  OFF | ---O------( B )

                         |   |    2 |        |

                         +-------------O     |

                         |   |      |        |

                         |   |      +--------+

                         |   |

                         |   |     DISC. SW. C

                         |   |      +--------+

                         |   |    1 |        |

                         |   +--------O      | PUMP C

                         |      OFF | ---O------( C )

                         |        2 |        |

                         +-------------O     |

                                    |        |

                                    +--------+

Lancie1

Regarding your question, you are right this program is in operation. I can do any modifications, but these mods have to work at the first time, I cannot start and stop as easy as I could because I do not have too much time to do that.

I need to mention that we have installed electrical switches beside the pumps to operate them manually or remote via HMI's running Wonderware screens. This switches are wired direct to pump motor breakers. The switch I need to install will be operate by a Wonderware screen buttons.

This switches are wired direct to pump motor breakers. The switch I need to install will be operate by a Wonderware screen buttons.

Click to expand...

Are you saying that Wondware operates these switches directly, and the PLC program is not involved? I don't know how to help with that situation.

Lancie1

Wonderware interact with PLC ladder for example auto and manual command are toggle via screen button. But PLC does all logic involve

I hesitate because I know that Boiler Feedwater Pump control is not something to tinker with lightly. Fixing a feedwater pump circuit so that it will not run because "it is not it's turn yet", even if the boiler pressure is going through the danger point due to lack of water- that is a design no-no.

I have seen feedwater pumps that had a big manual bypass disconnect switch that bypassed ALL the control circuits except the power supply fuse. It depends on how extensive you want to be in keeping the boiler from blowing up.

I have 2 BFW pumps running with and auto/manual selector in auto if there is low pressure the other one starts.

Click to expand...

Are you sure the second pump starts due to low pressure? (The only pressure switch I see is the Deaerator Pressure, and that is monitored to make sure there is enough water on the Suction side of the BFW, not the outlet side. This switch SHOULD NOT prevent a BFW from starting IF NEEDED.)

Most boiler feedwater controllers here are code-required and start the pumps based on boiler water level (NOT DEAERATOR PRESSURE). If the level drops to a danger point, the second pump starts automatically (hard-wired, not through a PLC program). The pressure switches are not considered reliable enough to control this critical element. The water level controllers are usually special devices (McDonnell-Miller water level controllers here in the US) that have been approved by all government agencies to safely control the boiler water level.

Lancie1

Lets see rung # 2, here we have a Deaer.BFWPresSwLo this is a Low Pressure switch wich monitors discharge pump pressure if its activated second pump starts.

Also Deaer.DearLvlSwLo monitors deareator low level and wont let any pump to run, and will stop them, as you comment this is to protect BFW Pumps.

Sorian,

Yes, I see those pressure switches, but they are labeled "DEAERATOR BFW PRESSURE SWITCH LOW". My question for you is WHY is the DEAERATOR there at all? Are these pumps supplying water to a deaerator tank? If so, they are really Deaerator Supply Pumps, not Boiler Feedwater Pumps.

Deaerator Supply Pumps have a different function and are controlled differently - they are not nearly as critical as real Boiler Feedwater Pumps (which must operate at the same output pressure as the boiler) and must supply water under all conditions to keep the boiler water level above the danger point.

This is the kind of program where helping you could lead to a dangerous situation. It is not something that should be atttempted remotely via a web link, without having complete details, specifications, and drawings of the boiler control system.

Lancie1

In our plant design there are several PLC located in diferent areas, Deareator PLC is the one who receive status of LOW BFW discharge pressure and deareator low level, these are located near of deareator. So PLC deareator produce LOW BFW PRESSURE SWITCH AND LOW LEVEL SWITCH tags witch are consumed by DAS PLC this controls BFW START/STOP pumps.

Who thought that it logical to use the name a relatively minor function (Deaerator) to represent the Boiler PLC? This seems like asking for misunderstanding. It sure makes it confusing to see "Deaerator" in front of "Boiler Feedwater".

Sorry, but I don't think I want to help use pressure switches to control boiler feedwater pumps. I pray that this is a low-pressure boiler. That might save you.