Help w/ UPS Philosophy 1

[controlnovice]

I just got wind that a 'sister' plant was reviewing their UPS systems to determine if they are needed or not.

I just started with the company and was on a conference call discussing UPS systems and SIS systems.

Their philosophy is that the process is designed fail safe: loss of power and all instruments fail to the safe position. There is no emergency (2nd line or emerg gen) power there either. In addition, they state that since the process does not have a SIL (Safety Instrument Level: another layer of protection, usually in the form of additional instrumentation and/or PLC seperate from the main control system) rating, then no UPS is required.

I'm concerned that on loss of power, the operators will have no indication of what is happening to the process because, without a UPS, the control system and HMI will be out of power. In addition, loss of historical data from the control system, possible loss of the controller database, the controller itself, and any indication for the operators seems too much of a chance for more errors.

What are other philosophies for the installation of a UPS? What are they suppose to feed? What are they not going to do for us?

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This is normally the space where people post something insightful.

 

[eyec]

this one topic could consume volumes of needed input. you ask some very direct and responsible questions that can probably best be answered by a team of those most familiar with the inner-workings of your plant and their interaction with computerized automation of all systems.

personnaly, i think installing a UPS system without having backup energy systems for plant operations is a false sense of security; safety and operations-wise.

a UPS is only for temporary power in the case of total loss of power.

it will be interesting to read what others post.

Steven C
Senior Member
ThirdPartyInspections.com

 

[rcw retired EE]

A properly maintained UPS with a 30 or 60 minute battery may keep the plant controls operating long enough to put the plant in a controlled shutdown with all monitoring systems active. This may be better than a crash when power fails. But if the plant still needs compressed air to operate valves or power to run motors or actuators, the UPS will not do much except let the operators watch it crash and print out pages of alarms.

I also look at UPS power to control systems as a power quality improvement. I had too many processes locked up by momentary power dips creating false signals in 120VAC powered PLC inputs. The PLC interpreted a voltage dip as a limit switch or E-Stop open/close action. UPS power for those controls isolated the system from voltage issues.

But there is something to be said for a simple system that fails safe on loss of power and can resume functioning on power return without too much operator intervention.

A UPS costs money and needs to be maintained and tested. Just having one can give a false sense of security. A UPS failure can bring down the whole plant.

 

[Skogsgurra]

In the best of all worlds, there is a DC distribution supplying all controls and computers. No AC supplying any device - not even printers or screens. This DC system has an identical back up system and the transfer switch is no switch, but a couple of heavy duty diodes.

The DC supply is backed up by directly connected batteries, one for each system.

Such a system will be more than complicated, next to impossible, to introduce in an existing factory but simple to install in a newly built factory. It will will keep computers and security systems alive and it will shut down the plant orderly (except for high-powered sections).

Some operations are very well suited to this kind of solution while steel works and paper mills may be less auited. There are a few installations like this running around the world and I think that one can learn a lot about UPS "philosophy" by reading about them.

BTW: I think that SIL (in the OP) should be Safety Integrity Level. Or is there a new acronym for an intermediate or extra level of instrumentation that is there to increase safety? If that is the case, it is an unfortunate choice of acronym. Confounding the two seems very likely.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[controlnovice]

Skogs - you are correct. SIL is Safety Integrity Level. I was probably thinking SIS as I typed....

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This is normally the space where people post something insightful.

 

[exodus02]

Hi!

THis might be too late but I just want to share my point of view which might be of help.

In any control system, it is highly recommended that there should be a UPS. In my experience as an instrumentation and control system engineer, I have not yet encountered a control system without UPS. Control system is meant to monitor and control your system at any condition at any given time. That is why, UPS always comes together with the control system. You are correct when you say that what can you see in the control station (workstation/console) when there is a failure and you do not have UPS. Your control system serves as your eyes and ears, therefore, it defeats the purpose if it is not functioning.

With regards to back-up power, e.g. generator, it would now depend on circumstances. You have to conduct an analysis of benefit/cost ratio. This includes the risks you are to take without having a back-up power. The opportunity cost if you do not have a power for a period of time versus the cost if you put up a back-up power.

For info, in our design of control system, the UPS is considered vital and very important.

 

[raptor6458]

Hi,

This one seems a bit late too.

Most of the time UPS feeds vital instrument loads (for safe shutdown of the process) and telecommunications.

And yes, if "all" instrument are fed via hydraulic or pneumatic controls then UPS will not be considered. HMI or PC/LAN servers will be using "small" UPS system with a back up time of say for about 30mins or 60mins.