We asked following questions for Foundation Field Bus:
1) No. of Device per segment

2) Scan time for of per segment for close and open loop

3) Whether RTD/TC need to considered as Field Bus Device.

All of our field instrument within 200 m apart and 80% are 100 m apart. Please explain us.

 

1. Theoretically you can have 32 devices per bus "segment" and using
repeaters you can have a bus consisting of many segments, each with 32
devices, so theoretically you can have 240 (logical limit) devices per bus. However, most systems limit to 16 devices per bus. In my personal opinion, engineering is done to maybe 12 devices per bus in most cases, considering that plants tend to limit the number of loops per bus.

2. "Scan time" (in FF called "macrocycle") is often set as low as 250
ms for flow loops and some pressure loops but longer for temperature and level control loops. The scan time is set to meet process requirements. With a 250 ms macrocycle you can have maybe two loops per bus but for slower updates you can have more loops. The balance devices on the bus is open loop monitoring which can tolerate updates up to seconds.

3. You can't connect a TC/RTD directly to the fieldbus. You need a temperature transmitter. The temperature transmitter is counted as a device. However, there are temperature transmitters that support both two sensors and even up to eight sensors, but it is still counted as a single device.

To learn more about fieldbus take a look at the yellow book "Fieldbuses for Process Control: Engineering, Operation, and Maintenance". Buy online:
http://www.isa.org/fieldbuses

Cheers,
Jonas

 

Answer:- 1) Normally it is recommended practically 8-10 devices per segment. But theoretically 32 devices per segment. Note in IS-Area/ danger zone it is recommended 2-4 devices per segment for minimizing I / V limitation from exploitation.

2)For open loop i don't know, but for close loop it is in ms, but always remember scan time for tem and level are always in seconds.

3) i don't know.

 

2-4 devices per bus in hazardous area was true for the old entity concept model of intrinsic safety. It is no longer used for new design (except maybe in Japan). Today, you can put 12-16 devices in the hazardous area using intrinsic safety thanks to FISCO and high-power/high-energy trunk concepts. This makes the solution very economical and easy to design.

If you design correctly, you can easily achieve sub-second scan time using Foundation fieldbus. For instance, 250 ms can be achieved. This is easily possible because Foundation fieldbus is time-synchronized and scheduled, so the entire control loop can be closed in a single macrocycle, from input to output.

To learn more about fieldbus take a look at the yellow book "Fieldbuses for Process Control: Engineering, Operation, and Maintenance". Buy online: http://www.isa.org/fieldbuses

Cheers,
Jonas

 

Dear sanjib

What is Hazardous Area classification for your plant ?

Regards
Jariwala