PT Voting system vs. SIL 2 1

[rafcu]

Dear colleagues,

My customer ask me replace a pressure transmitter voting system 2 out of 3 for a single transmitter SIL 2, I do not agree because the ESD system will be more sensitive to the pipe problems, Do you have more reasons?, or do you agree with my customer?…

Regards,

Rafcu

 

[Ashereng]

rafcu

A TUV certifided SIL2 PT can be used in a SIL 2 application. There are many models to chose from.

Does your customer's safety standards require 2oo3 for SIL2? If so, then you still need two PTs.

I am always very leery of a customer's verbal request. The onus is on you rafcu, the engineer, to do what is right, allowed, legal, and to use your judgement. If you have a concern, especially a safety system related one, I would do a lot of thinking and engineering.

Without more information, I can not give an answer, or opinion.

 

[sdl]

Rafcu:

Ashereng is correct. Your client will loose availability (i.e. increased nuisance trip rate). You will have to do calculations to determine if the PFD is adequate as well as if the MTTFS will meet their standards/requirements.

I don't see this as a huge dilemma. Do the calculations and show them to your client. The decision is then in their court. Your function is to give them all of the information required to make an educated choice.

sdl

 

[jsummerfield]

Please clarify your phrase "the ESD system will be more sensitive to the pipe problems". Does the customer want to replace the transmitter, transmitters or the voting system?

What pipe problems exist or do you refer?

A triple voting system can use one transmitter with triple electronics for the I/O and processor or solver. A triple voting system can also have three transmitters. Either is better for reliability than a simplex system with one transmitter and one set of electronics. It may be difficult to meet SIL-2 without triple transmitters. Triple transmitters are better than a single for reliability - worst for cost.

John

 

[sdl]

jsummerfield:

There are many manufacturers that make a transmitter to meet a SIL 2 application (most state: in a simplex configuration) and a SIL 3 application in a redundant configuration.

sdl

 

[Ashereng]

rafcu,

In most of the SIL 2 applications that I have seen, the required solution is typically 1 (usually SIL 2 rated/cert) or 2 transmitters (SIL 2 rated/cert or not). I have not seen a SIL 2 application where the required solution is 3 transmitters. I guess yours is the first for me. As such, I don't have any experience with replacing 3 transmitters with a single SIL2 certified transmitters. Again, "Does your customer's safety standards require 2oo3 for SIL2? If so, then you still need two PTs."


With respect to John's post, I would like to add that, usually, each transmitter's signal (any sensing signal for that matter), goes to a single input channel in the safety system. It us unusual for a single sensor to go to multiple channel in a safety system. There are several reason.

One reason is that it is best to keep the safety system simple and clear - and one-to-one is simple and clear.

Also, another reason is that usually, the weak link is the sensor (including element, transmitter, wiring, all), and not the safety system (I/O module, back plane, inter-connections, processor, power, all). In fact, the safety system is usually the strongest link in the safety chain.

The strength in the safety chain, in decending order of strength, is usually:
1) safety system
2) sensing element
3) final element (valves)

Please note "usually", as there are exceptions.

 

[Ashereng]

Oops,

"Does your customer's safety standards require 2oo3 for SIL2? If so, then you still need two PTs."

should read

"...then you still need three PTs".

Sorry, bad typing.