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HOW TO SELECT A CPU FOR PROCESS PLANT

Created by: Uppi at: 7/28/2010 8:02 AM (15 Replies)

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7/28/2010 8:02 AM	Rate (0)
Uppi Posts: 17 Rating: (0)	Can u please guide to select a CPU for a process plant
Uppi Posts: 17 Rating: (0)
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7/28/2010 10:59 AM	Rate (0)
Gunners Posts: 130 Rating: (18)	Dear Veerababu, Actually i think your question is very open. That is, there are many criteria to consider a particular CPU for a process plant. You can elaborate as to what kind of processing plant you are talking about. However the following detailscanbe considered while selecting a CPU for a process plant: 1) No.of inputs/outputs (normally process plants have lot of I/O's ranging from 500 - 100000) 2) No.of Closed loops (if more closed loops then you need faster scan time) 3) No.of PID loops (More processing power goes into each closed loop) 4) Type of plant (Type of process) . If it is a critical process in a plant then you need to select CPU with high availability. 5) Type of Instrumentation system. (Whetherit is HART/4-20mA/ Profibus/FoundationFieldbus/Modbusetc) 6) Type of architecture ( Distributed or Centralized Control System) 7)Also particular care should be taken, however, to include a realistic estimate of the performance required by the human-machine interface (HMI), particularly involving video andgraphics. 8) Whether the CPU is for basic process control or for safety related applications (SIL requirements etc) 9) Amount of Logic (no.of calculation blocks). These are some of the most important considerations which we have to take while selecting a CPU. Ofcourse, depending on the type of plant there can be many more aspects to take care while selecting a CPU. Hope this helps. regards,
Gunners Posts: 130 Rating: (18)
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7/30/2010 2:52 PM	Rate (0)
dmertens Bronze Member Joined: 10/3/2007 Last visit: 6/25/2024 Posts: 188 Rating: (34)	The first consideration should be if redundancy or fail-safe is required asthis would already limit your choise to S7-4xxH CPU's. The next consideration is program size (the number of IO's can be a guide in this, but that's not always the case as the process may be extremely complex involving a lot of calculations) and the required processing speed. Also the need for segmentation of your plant (fully independent operation) may require you to multiply the number of PLC's based on the number of independent segments. So you see we will really eed a lot more information in order to provide you with even the roughest and most preliminary estimate.

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8/6/2010 3:20 PM	Rate (0)
mrich Advanced Member Joined: 5/20/2009 Last visit: 3/14/2022 Posts: 32 Rating: (5)	I agree with 'Gunners'. There are a lot ofapects to consider when selecting a CPU. But to get a general idea ofthe capabilities of the various CPU's you could refer the table provided in the 'Technical Brochure for the Simatic PCS7 Process Control System'. In the 2009 version you'll find the table on page 41 (sorry, to big for anattachment). Or on page 29 of: https://support.automation.siemens.com/NL/adsearch/resultset.aspx?region=NL&lang=en&netmode=internet&ui=MTk4NjE1MTgA&term=brochure+pcs7&ID=25211022&ehbid=25211022&docid=25211216

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8/25/2010 12:09 PM	Rate (0)
Uppi Posts: 17 Rating: (0)	Thank you pratap and sorry for the late response...i was on site and i came today and can u help me to select S7 400 cpu for th following Inf PID-58 MOTORS-81 ON-OFF VALVES-68 AI-161 DI-364 AO-73 DO-208 The above info is regarding my next project.....
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8/26/2010 7:55 PM	Rate (5)
NickM Posts: 25 Rating: (9)	VEERABABU, Since this is a PCS7 system (I assume that it is), it may be better to think in terms of total Process Objects (POs), since that is how PCS7 is licensed. Basically a PO is a motor or valve, a monitored AI or DI, a PID, a sequence (SFC), etc. Looking at your list,Ihave estimated the following POs: PID-58 MOTORS-81 ON-OFF VALVES-68 AI MONITOR POINTS=TOTAL AI - PID=161-58=103 DI MONITOR POINTS=TOTAL DI - MOTORS - 2xVALVES=364-81-136=147 UNKNOWN AOs=TOTAL AO - PID=73-58=15 UNKNOWN DOs=TOTAL DO - MOTORS - VALVES=208-81-68=59 58+81+68+103+147+15+59=531 531+20% SPARES=638 This is a rough estimate, but it helps when you are selecting the CPU. Assuming a singleCPU (or single H CPU), the sizes in POsfor each CPU are: 412-3H - ~250 POs 414-3 - ~250 POs 414-4H - ~250 POs 416-2 - ~800 POs 416-3 - ~1200 POs 417-4 - ~3000 POs 417-4H - ~2500 POs The size of the memory cards in the CPUs will alsolimit how many POs they can handle: 1MB - ~50 POs 2MB - ~180 POs 4MB - ~300 POs 8MB - ~800 POs 16MB - ~3000 POs For example, a 414-3 CPU can handle ~250 POs, but a 2MB memory card would limit it to ~180 POs. So with 638 POs, a 416-2 CPU with 8MB memory card should be about the right size for your application. This is a crude estimate, and many factors affect the size of the memory and processing speed of the system. Pratap's post has some very good considerations in it, and I would highly recommend taking note of his suggestions. Also the PCS7 Engineering System manual is very helpful in estimating systems. Here is a link: http://support.automation.siemens.com/WW/view/en/36050990 I couldn't access the link for the PCS7 Technical Brochure thatmrich provided, so here is a link that I have: https://eb.automation.siemens.com/goos/catalog/Pages/Templates/collateral.axd?id=3865&versionid=539 This has the information on the CPUs and the amount of POs that each CPU can handle, based on the size of the memory card. Regards, Nick
NickM Posts: 25 Rating: (9)	Last edited by: NickM at: 8/26/2010 11:15 PM Added link to PCS7 Technical Brochure Last edited by: NickM at: 8/26/2010 7:56 PM
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8/27/2010 8:30 AM	Rate (0)
Uppi Posts: 17 Rating: (0)	Thank you Mr. Nick It is really helps to my next project and i gone throughUrattachmentand i saw theselectionofCPUbased on PO.But i calculated the remaining PO's as perUrcalculation but it is not matching.Can u please help in this issue..
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8/27/2010 2:43 PM	Rate (0)
AndreyCh Posts: 17 Rating: (0)	I also take into account maximum number of communication jobs if I use ALARM_8P in CPU400. The number of communication jobs is directly concerned with number of alarms in this case. CPU417 - 10000 communication jobs (almost unlimited)) CPU414 - 1200 CPU412 - 600 You can check this in the Properties Window of CPU in HWConfig.
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8/27/2010 3:49 PM	Rate (0)
NickM Posts: 25 Rating: (9)	VEERABABU, I will explain where my numbers came from in more detail, and I hope it will make more sense. PID-58 MOTORS-81 ON-OFF VALVES-68 These numbers are from your list, I think they don't need any explination. AI MONITOR POINTS=TOTAL AI - PID=161-58=103 Each PID has an AI for the process value, and usually there are AI Monitor Points in a system. Without more details from your system, I estimated the number of AI Monitor Points by taking the Total AI and subtracting the number of PIDs. DI MONITOR POINTS=TOTAL DI - MOTORS - 2xVALVES=364-81-136=147 Each single speed Motor has a single DI,and each two state Valve has two DIs, and usually there are DI Monitor Points in a system. I estimated the number of DI Monitor Points by taking the Total DI, subtracting the number of Motors, and subtracting 2 times the number of Valves. UNKNOWN AOs=TOTAL AO - PID=73-58=15 Each PID has an AO for the control value. I estimated that there would be a PO for each of the extra AOs, so I estimated the number of Unknown AOs by taking the Total AO and subtracting the number of PIDs. UNKNOWN DOs=TOTAL DO - MOTORS - VALVES=208-81-68=59 Each single speed Motor has a single DO,and each two state Valve has a single DO.I estimated that there would be a PO for each of the extra DOs, so I estimatedthe number of Unknown DOsby taking the Total DO, subtracting the number of Motors, and subtractingthe number of Valves. AndreyCh, That is also a good consideration, I had trouble with a PCS7 in a box system last year because we exceeded the limits of the communication jobs. Regards, Nick
NickM Posts: 25 Rating: (9)
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9/1/2010 5:10 AM	Rate (0)
Gunners Posts: 130 Rating: (18)	Hi Veerababu, Sorry for my late reply.I was out on tour..... Anyways....i think nick has explained you very well ho w to select the CPU for your I/O counts which you have given. If you still have anything to discuss, you can...... regards,
Gunners Posts: 130 Rating: (18)
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