7 replies to this topic

[premjagan]

Dear sirs,

 

                       In industries, we do have normally trip systems, to safeguard vessel or machine or certain section. For example in any Run away reaction possibilty in certain reactor, we have high temperature trip to protect the vessel from damage.In this case i initially experienced from my first company, the kind of trips like VESSEL HIGH TEMPERTURE,COMPRESSOR SUCTION POT LEVEL HIGH  dont have bypass provision at all. I thought that such kind of trips which related to machine damage , vessel damage should not have bypass provision in all industries.But in my next company, i came to know it is wrong thought. Every trips have a bypass provision in my new company irrelavant for the reasons.

 

I want to know, Is there any standards for bypass provision for trip switches in design?And Is there any common rule for all industry for certain trips which never put bypass provision?

 

  Regards

  Prem Jagan                 

[Bobby Strain]

There are no standards. Your company establishes the requirements considering insurance provisions, among other things.

 

Bobby

[Aranga]

Prem,

 

Every interlocks can be bypassed by instrumentation people (In the Electrical & instrumentation control room). But the Approval to be obtained from top most authority of operation(manager), & Process engineer, and supervisor mandatorily.

 

   Now it is available in DCS also.. as MOS. (maintenance override).

Edited by Aranga, 17 April 2013 - 01:07 AM.

[riven]

Firstly, all alarms and loops can be bypassed physically in the field by changing terminals in junction boxes or removing electrical/pneumatic supplies etc. So I assume you mean being able to bypass the alarm from the control room or station.

 

For basic process control systems, alarms should only be acknowledged by an operator. Bypass/disabling/deleting of the alarm and its trip function is possible but should only be available to a shift manager/engineer log in (traceable change) and would need to be covered by an upfront management of change signed off by a process safety expert and plant manager. It is obviously a bad idea to be easily able to bypass trips.

 

What are regulated are the SIL systems or specific 'safety related control' systems. These can be bypassed in part but should never have the option to fully bypass/disable under any circumstance. For example if you have two separate instruments controlling temperature and the voting system to activate the trip is 1 out of 3, it is feasible to allow one instrument to be disabled at anyy one time (say for maintenance). There is a stringent calculation around this in terms of reliability. This would be covered by comprehensive 'safety related control specification' documentation. High integrity pressure protection systems or HIPPS are an example of a SIL 2 or SIL 3 rated system. Burner management systems for e.g. stoves typically come as being able to meet the SIL 2 standard from many European and US manufacturers.

 

References

http://www.61508.org..._sheet_rev3.pdf

SIL Standards IEC 61508 and IEC 61511

[premjagan]

Yes sir i m asking about the provision of POS ( process override switch) only.. we can bypass any trip with the help of instruments by providing jumper etc. for the purpose of maintenance or malfunction.But the POS may be the chance of misuse...

Edited by premjagan, 16 April 2013 - 08:05 AM.

[Dacs]

This highly depends on the process itself, in consideration with how you'll run your plant during normal, upset, start-up and shutdown.

 

So I don't think anyone can give a blanket guide on if and when to provide bypass for trips.

 

While in fact it can be misused, POS sometimes is a necessary evil. For instance, you're starting up a furnace that has a trip for loss of flame (BAL) which trips the whole system, you can't start this up without a bypass for BAL since obviously you don't have ignition when initially starting up a plant.

 

I have to add though, I'd expect that the needed bypass mentioned above is hard-coded on the logic and I'd be very uncomfortable placing a push button for this one.

Edited by Dacs, 16 April 2013 - 09:47 PM.

[Aranga]

 First time I come to know about the terminology POS. Thanks.

 

But, we cannot bybass an interlock by ourself..,  There must be a written approval from supervisor. Atleast a intimation and record. So it cannot be mis used.

[Jome128]

If no automatic trip, the dependency on the alarms or operator are high. So, usually to be Required for overrides/inhibits/bypasses swtich, these may be included in functional logic or in limit switch