[FUN] who does segment design?

[Stephen Mitschke]

Most engineering companies have instrument engineers, instrument designersand control system specials. It seems that segment design requires inputfrom all three. The actual location of the device affects wiring lengths: normally thedesigners are the ones most familiar with this. The current draw & functionblocks loaded in each device: seems to me like the instrument engineer whospecified the device would be most familiar with these details. Having allthe function blocks needed for a particular control strategy on the samesegment: either the instrument engineer or the control system guy wouldprobably know this the best. So, on major capital projects, how have other E&C's actually handled this? Who was the person doing the initial grouping of devices on each segment? I'm sure there isn't just one right answer. I'm interested in how othershave actually done it.--------------------------------------------------------John Kealey, Instrumentation Dept.Mustang Engineering

[Stephen Mitschke]

Hi John,I have done numerous jobs on this in the past. I wrote the original rules to do the segment design for the Malampaya project back in '99. I worked exclusively with the control system designer. We never used an instrument engineer. Basically there are a few simple rules to follow and then it becomes fairly straight forward after that.You do not need the instrument engineer to do your segment layout, unless you have already bought the devices and are now trying to make them fit a design.This is the easy way to do it...Before the bids go out:1) Is the project going to be Intrinsically Safe (IS)? This is going to effect device types (must be IS certified). You are going to have to track current draw on the segments and IS will be the biggest constraint on your segment design.2) Start with the P&ID's. Your P&ID's will dictate the Function Blocks (FB's) needed and from there the device types/FB's needed. All you need here is someone who understands the P&ID's and has a good idea of the general location of the devices (does 1 device feed data to another device 2 stories above?) Are you going to do control in the field? I recommend ALWAYS doing simple loops in the field (you should remember this from my talk last year). If you have more complex control that will stretch across multiple segments, then I recommend moving some control up to the host. This is another constraint on your segment design and feeds 3 next.3) From 2 above you can specify the needs in your FF device data sheets. Order the devices that can meet the needs of your P&ID's. Keep in mind that devices that support FB instantiation (you can turn FB's on and off) of multiple FB types are going to be the most flexible in design.Bids are back and you know what devices are going to be used. You can NOT do a decent segment design without taking into consideration 1-3 above. If you do 3 before 2 you are asking for problems...4) Once you know the devices you are using, the FB's in them, the current draw of each device, then and ONLY then can you begin the segment design. This is where you might want to bring in the Instr. folks, but they aren't necessary if you have the detailed FF data sheets for the devices. Use the same folks that were involved with 2 above. If there is no control in the field, segment design is simple, group devices by location and current draw. If there is control in the field, things get a bit more complicated. Keep all devices on a simple loop together. If a device needs to talk to another device (loop, interlock, etc.) keep them on a segment together. I like to work directly on the P&ID's grouping the devices by segment.5) Once 4 is done just transfer the segments in to INtools or other segment/loop drawing program.Keep in mind that you can bring in an expert who can do this for you in a matter hours or bring in someone to train your folks on how to do it. There are quite a few time savers and tricks to doing segment design. Once you have done a project or two, things begin to get much easier. Also, keep in mind that once you have one segment drawing done, the rest are just basically "cut-and-paste".Regards,Kurt ZechKI Engineering

[Stephen Mitschke]

Ian -I hope all is well on your end.I wonder if your FUN readers understand that they can use functions blockslocated in different devices not on a particular segment. There may or maynot be an issue with single loop integrity but if the application calls fora particular function not contained in the available devices on a segment, aFB on a different segment can be used. Also, one can add a fieldbus devicethat is not a measuring device but only contains function blocks that can beused by any device on any segment.Just wanted to make sure your readers think about the full power of thefieldbus technology.Regards,Chuck MicallefCJM Consulting