selection criteria for PLC

Hello,
How are you,I hope things are going well for you.

i want to know that if you are going to install any plant or industry then how you decide about control system (DCS,PLC), when P&I done regarding that plant/industry and you know about total input,output ,parameters, PIDs, PI etc.
and you stuck on deciding CPU for control system then which parameter you consider for CPU selection.

we are going to install WHR (waste Heat Recovery) of 12MW. i have assignment that i have present that how to decide for Control system for any plant or industry.
i have to explain a GENERAL RULE FOR PLC SELECTION but it must be logical when i have to talk about specs.

i also want to know about communication protocols that which plc use what protocol.


Please guide me in this way i am still student and working as a Trainee in Cement Industry.

Thank you


Regards,

This is like asking what type of Jam do you put on your toast.
That is your choice, Yours.... What are you comfortable with? In your locale what is the Standard that is used most. As far as Protocol I lean heavily towards Ethernet, and on PLC I am comfortable with Allen Bradley, Siemens as well as Mitsubishi. But that's me.
Total inputs, PID, all depends on the design of the system, do you know EXACTLY what you are controlling, then it's easy things that require an on off go digital I/O, variable use Analog. then the rest is easy, I have yet to find a Processor that couldn't handle the task.

Hello,
if you want to select for your own project then how you will choice for CPU and Brand.

Thanks,

First of, you should find it reassuring to know that almost any of the major brands will have a model that will do the job.

If, as it seems, you are new to the PLC world, then you probably want to choose a brand that has a local distributor with good technical support staff. Any extra cost for hardware will be more than offset by the value of the assistance you can get.

Of course, if there are already PLCs on the site then that brand should be on the list of those you will investigate, assuming of course they have worked well and the local support has been adequate. You may not be restricted to that brand, but they should be considered.

The first step is to identify your inputs and outputs. If you only have a few then a micro-PLC will do the job. If you have hundreds then you need to have a powerful PLC.

Next identify your communications requirements. Will you connect to a local HMI touchscreen at the PLC? Will you be connecting to a PC based SCADA system? Will you be connecting to a network of PLCs?

Next look at the programming package for each brand under consideration. Is it compatible with your PC's OS? Does it seem complex or easy to learn? Is the same package used for multiple models, or will you end up with many different packages?

Look at the touchscreens from the same supplier. Are they easy to configure? Is the communications compatible? Do they have the graphics and trending features?

Finally, and NOT the most important, look at the cost. Remember this is a system, with a lot of the total cost going to programming and commissioning. The least expensive hardware may not equal the least expensive final cost. And it doesn't matter if they give you the PLC if it won't do the job reliably.

Thank you sir for reply.
what about size of memory .
is there any equations on the half of input output for memory.??


sir we have to provide interface to operators only through servers.

memory needs generally do increase with I/O count, but only because the logic tends to be more complex. I suggest that you have a description of the logic needed available for discussion with your local suppliers when you contact them.