Plant Power Failure

Good morning all...

I am using SLC 5/03 (1747-L553)

The objective:

In the case of a plant power failure, equipment controlled by the control system shall be programmed to automatically reset upon failure. All equipment and motors shall be automatically restarted after power failure by the control system in an orderly fashion.

This is directly from a control description that has been provided to me. I was very skeptical about the automatic restart, as I have never seen that specification anywhere in industry. This is a municipal project and I have been assured that this type of design has been implemented before.

My question is: what would be the best way to do this? I have many scenarios in which the process has to be in a given mode, one valve has to be open, one valve has to be closed, the process has to be in auto, etc. for a given pump to start. That said, if the power goes down and someone takes the process out of the needed mode, I want to make sure that on restart, the pump doesn't unexpectedly fire up. Has anyone programmed a routine similar to this before?


Thanks,
Justin

Raises red flags to me. There are may be obvious steps to automatically take upon restart, but with such a questionable state of everything a whole system reset seems like the kind of thing that you would want an operator to direct.

Perhaps restarted doesn't have to mean put back in a running state - would a hot standby work?

Perhaps restarted doesn't have to mean put back in a running state

Click to expand...

I would feel much more comfortable if this was the case...

What exactly do you mean by hot standby? Would that just mean that the system is ready to run upon some sort of intialization from the operator?

If the spec doesn't already define "automatically restarted after power failure by the control system in an orderly fashion", then I would suggest you write a spec defining it. Submit your spec for approval before writing any logic.

I was referring to basically BSing the satisfaction of the requirement with a more technically applicable solution. Yes, means getting all the devices to a "ready" state for an operator to prompt.

I like Steve's suggestion of formalizing the spec for the customers approval. If they want it to start up on a more "auto-magic" basis then you can deal with concerns on a case by case basis.

justin lutz said:

I would feel much more comfortable if this was the case...

What exactly do you mean by hot standby? Would that just mean that the system is ready to run upon some sort of intialization from the operator?

Click to expand...

Auto start is a fact of life and has some very good applications:
emergency generator on power failure
On reenergize
Sewer pumps
fire pumps
elevators
ventilation equipment especially in "hazardous production areas" ie plating lines, etc.
boiler feed pumps
lighting (non emergency)


Dan Bentler
ALL production equipment should shut down and have an operator start them. There are cases where they should be fed from generator I know and allowed to auto restart when power is restored ONLY to let them cool down.

You cannot do auto restart on everything in a plant - some equipment does not have the need and it may be unsafe ie a drill press
AND your distribuition equipment may not be able to handle the total simultaneous restart current.

Dan Bentler

Steve,

Point taken, however it does not look as though there will be time to submit a more definitive spec for the client's approval.

Dan,

In response to your examples of good applications for automatic start, my particular application is for a set of wastewater sludge pumps. They are remotely controlled and there will not be an operator locally present to control them at the pump site. I am not concerned with an overcurrent situation at startup, as there will be a maximum of 2 pumps starting at once, which is the case for normal operation.

Thus, I restate my question to the forum, how would I go about setting up the logic for an auto start of my pump(s)?


Thank you,
Justin

Easiest way would be to check for alarm conditions for each of your pumps.
If none exist then when control power is present and all permissives are there, just run as you would under normal circumstnaces (as if a RUN command was present)

I always use a RESET PB for my safety modules (the operator has to press the reset in order to energize the safety module which in turn energizes an MCR and so on).
Though, if you do use one, it can be set for an Auto Reset if thast seems to be a problem.
Once the module is ok and the rest of the permissives are there, that would be all you need to start running in auto mode I'd think.

Thus, I restate my question to the forum, how would I go about setting up the logic for an auto start of my pump(s)?

Click to expand...

Presumably these pumps don't run continuously. There will be some logic that starts and stops the pumps under normal operating conditions. You will need to parallel the contact that starts the pump(s) under normal conditions with a bit that becomes true when power is restored.

An example:

 

Start      Stop       Preconditions       Pump

-] [----+--] [----+----] [----------------( )-

        |

Power   |

Restored|

-] [----+

        |

        |

Pump    |

-] [----+

It would be good to define certain amount of time Tu first. If power is restored before Tu has elapsed than, you should allow auto restert, otherwise you shouldn't. Auto restart is good especially in situations when equipment experience power dips ( very short power fail). This way you'll not stop the process. On the other it is pretty much dangerous to implement unconditionally auto restart functionallity.

justin lutz said:

Steve,

Point taken, however it does not look as though there will be time to submit a more definitive spec for the client's approval.

Dan,

In response to your examples of good applications for automatic start, my particular application is for a set of wastewater sludge pumps. They are remotely controlled and there will not be an operator locally present to control them at the pump site. I am not concerned with an overcurrent situation at startup, as there will be a maximum of 2 pumps starting at once, which is the case for normal operation.

Thus, I restate my question to the forum, how would I go about setting up the logic for an auto start of my pump(s)?
Thank you, Justin

Click to expand...

Justin
Seems to me you should take some time and ensure both you and your customer have a firm agreed to understanding of what their needs are and what their wants are regarding these pumps.

This is a sewage plant so I assume these are some pretty good size pumps (at least 50 HP) so the equipment costs are appreciable.

Thinking on it a bit
The pumps are controlled from a remote control room.
Each pump has "manual run - auto run - off" control in control room.
There may be a "manual run - auto run - off" switch installed at the pump. If mine there would be.

Manual switches would energize the relay to each pump starting it.

Auto would let PLC control the pump with control parameters dealt with in PLC logic (level, pressure, time or whatever paramter is used to control the pumps)

Loss of power to pumps could be one of several ways.
1 Normal utility power is lost and the pumps are not fed off an emergency generator.
2. Someone opened the disconnect to the controller.
3. Pumps overloaded overloads tripped the starter.

I will assume #1.
The easiest is to let control power do the work for you. Loss of power results in loss of control power thus dropping out the relay. On power return control power is back relay reshuts and pump starts. PLC would not have any control over this.
All it would do is to put the pump in run or stop mode.

Next is for PLC to sense that there is or is not power available to the pumps. If no power it turns the pumps off and maybe sets off an alarm. When power returns PLC senses it and if there is a demand for pump PLC restarts it.

Next is for PLC to determine that pump should not be running or shoud be running that there is power available and then start the pump. It would also confirm the pump is running
AND pumping
either with a discharge pressure switch or flow switch and or current sensing.

All depends on how much control you want on these.

Dan Bentler