Instrumentation Designing

[ranagr]

I need detailed response of my below mentioned queries related to design engineering of instrument works.

1-The difference between documents "instrument index" and "instrument I/O list".

2-The difference between "segment wiring diagrams" and "instrument termination diagrams".

3-Is data sheets preparation regarding "PCV" and "PSV" in instruments scope of work?? How, PSV and PCV are sized?

4-What is difference between "Fail close" and "Fail open" position of control valves.

5-The difference between "RTD" and "thermocouples".Which is better for temperature measurement.

6-What is difference between "FFB (Foundation field bus) " and "conventional" protocol.Define the conditions where these are applicable.

7-Why "digital signals" are used for on/off operations?? and analogue signals for control/measure operations?.Please highlight the basic difference of both methodologies.

[sameenkhan]

See my reply in BLUE....

I need detailed response of my below mentioned queries related to design engineering of instrument works.

1-The difference between documents "instrument index" and "instrument I/O list".

Instrument index consist of types of instrument installed in the plant whereas instrument IO list shows instruments connected to BPCS/SIS Systems...

2-The difference between "segment wiring diagrams" and "instrument termination diagrams".
Both can be part of instrument loop diagram... depending on complexity & no of terminations involved... segment wiring diagrams & instruments termination diagrams are referred in Instrument loop diagrams.... segment wiring diagram shows only one segment of the entire loop whereas instrument termination diagrams shows how instrument is connected to BPCS... e.g. a Gas Chromatograph (GC).. to BPCS it is instrument, but it depends on how GC is sending data to BPCS or how BPCS is reading data from GC... it could be via two. three , four , 5 , 10 or 25 wire connection or via some industrial communication protocol.. now Instrument termination diagrams shows how both instrument & BPCS are connected...

3-Is data sheets preparation regarding "PCV" and "PSV" in instruments scope of work?? How, PSV and PCV are sized?

If you are involved in commissioning of new plant, then data sheet will be provided to you as part of As Built documents by EPC...If you are in maintenance then in case there is new installation of PSV or PRV or CV then it is responsibility of instrument engineer to collect data from Process Engineering/Project Engineering and prepare a data sheet...Sizing of PSV or PRV is not easy and I would suggest that you should start with simplest Control Valve rather than jumping directly to PSV or PRV... Each vendor provides sizing tools for its CV/PSV/PRVs... and basics of CV sizing remains same most of the time.. but it may differ, all is subject to how vendors has designed the Valve...

4-What is difference between "Fail close" and "Fail open" position of control valves.

Both terms are used when Safe State of Valve is considered..(Please refer to Plant HAZOP documents for definition of Safe State for each valve)... Fail Close or Fail Open means in case of failure of air supply, 4-20mA or 24Vdc or CV diaphragm rupture, the valve will go to pre-determined safe position i.e. Close or Open respectively...

5-The difference between "RTD" and "thermocouples".Which is better for temperature measurement.

Principle of operation for both is different...
RTD is relatively more accurate and exhibit linear characteristics from low to medium range temperatures ... Whereas TC are relatively less accurate but exhibit linear characteristics from low to very high ranges temperatures...

6-What is difference between "FFB (Foundation field bus) " and "conventional" protocol.Define the conditions where these are applicable.
There is not such things as Conventional Protocol... Please re-phrase your question...

7-Why "digital signals" are used for on/off operations?? and analogue signals for control/measure operations?.Please highlight the basic difference of both methodologies.
Because you can't use it other way around.... Please re-phrase your question with some problems??

A word of advice... This forum is for discussion/problem solution... Some questions you have asked requires a big explanation, I would suggest you to buy & read few instrumentation & control system books.... It would be beneficial for you and also for rest of the members if you ask question too the point and if necessary give an explanation with some examples...


Regards,
Sameen

[ranagr]

Thanks Sameen.

[ali.abbas]

Have a few more comments on a few of your questions.

3- Sizing of PRV and PSV are not necessarily the responsibility of the Instrument Engineer. Sameen is correct as far as new installation is concerned. But for maintenance, it depends from plant to plant since responsibility may be distributed separately in different organizations. As an example, the plant I work at, designing & sizing of all kinds of valves falls under the domain of Process Engineering. They will develop data sheets which they will then hand over to the instrument engineer for procurement of the valve. Once valve is procured, the project engineer (mechanical engineer) will have it installed in the field and the instrument section will be responsible for electrical and pneumatic connections. As for the PSV, that is completely out of Instrument Engineer's domain. It is designed by the Process Engineer and installation and maintenance falls under domain of stationary equipment maintenance section.

7- I agree with Sameen that it is not possible to have it the other way round. Digital signals have just 02 states (on & off). For control purposes, generally the requirement is to have infinite intermediate values between say 0 - 100%, something that is quite unachievable through use of digital signals.