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boiler's control system
Engineering and workplace issues. topic
Posted by M.fahimi on 18 April, 2006 - 8:52 pm
hello,

we are going to establish a boiler with 6 burners and 187 ton/hour capacity, the generated steam of this new boiler will injected to the common outlet header of four existing (old) boilers, control of the old boilers is a pneumatic system that adjusts the firing rate to constant the pressure of the outlet header (there is a pneumatic signal that's caused by a flow transmitter, it's name plant master, by this way these four boilers are pneumatically controlled, all of 4 boilers are in service continiously.)

In the new boiler we propose DCS for control and supervisory (managing operation of boiler), and a PLC for BMS (interlocks and safe operation requirements). and now, there are 3 bugs for us:
1. choice of DCS and PLC manufacturer and model (vendor).
2. coordinations between this new and fast control system by the existing slow control system until the total plant (5 boilers) operates in a effective and unique form.
3. as i told, the existing boilers and all of other plants in this refinery are controlled pneumatic and in the future they want to revamp the control system into DCS/FCS, but now we are looking for a suitable method for making communications between this boiler control system and the revamped control system for future, while we don't know anything (e.g. manufacturer, model, vendor...) about this revamping project, i want to know exactly which servers and related protocols can help us in this way.

but there is another question by me:
in the ESD system, when we need a switch as a sensing element that use in ESD logic, is it better to place a transmitter on the line and in the software explore the switch from the 4-20mA signal, that's coming from trnsmitter (soft switch) or a hard switch on the mentioned line? (which one is more reliable?)

before, i'm really indebted to you forever, thank you,
regards.


Posted by Vin on 19 April, 2006 - 11:25 pm
I would suggest you go for a modular DCS. This will help you in future retrofit for the old boilers as well as you can expand your system as per required stage by stage. You can have all your controls like combustion, drum level, temperatures in the new DCS and ESD on a PLC which can be interfaced with the DCS. Regarding vendors.. DCS: DeltaV from Emerson(Rosemount) is suppose to be a good modular system. PLC Siemens S7 300 is again a good system. Interface: you can go for Profibus on both DCS and PLC.

ESD can be configured very well in the programs.

Hope this helps you...


Posted by Juan Calderon on 20 April, 2006 - 11:32 pm
Regarding the switch Vs transmitter question, I suggest you to use a smart transmitter. For safety systems (like the ESD) you have to consider two types of failures for the devices involved in the safety function (sensor, PLC and final element), for instance safe failures and dangerous failures. Because of their diagnostic features typically transmitters have lower dangerous failure rates than switches, consequently they are safer. On the other hand, also because of the diagnostics features, smart transmitters have the ability to detect a failure and set its output to an under-range or over-range value, then at the PLC you have the ability to detect this failure and you might decide to keep running your boiler (while repairing the fault) or to trip the boiler. This allows you to reach also better process availability (up-time) by minimizing the spurious trip.

On the other hand if you have any Safety Integrity Level (SIL) requirement for your boiler safety function, then you can achieve SIL 2 or even SIL 3 by using one transmitters, while typically to achieve SIL 2 with a switch you need two switches in 1oo2 voting configuration, with the additional problem of higher spurious trip rate leading to less availability.

Juan Calderon.
MCL Control
www.mclcontrol.com


Posted by M.Fahimi on 24 April, 2006 - 11:07 pm
Hi and very thanks, dear Juan,

As you wrote it is needed for two switches in 1oo2 voting configuration to achieve SIL2, and by using only one transmitter SIL2 or even SIL3 is achievable. Please tell more because I don't know how, by using only one transmitter SIL2 or SIL3 is achievable?

regards


Posted by Juan Calderon on 29 April, 2006 - 12:05 am
There are several transmitters (Rosemount, endress+Hauser, ABB, etc) in the market with failure rates and Safe Failure Fractions (SFF) good enough to be used as part of a SIL 2 Safety Instrumented Function (SIF) in single mode.
If you have a SIL 3 requirement, then you need to use typically two SIL 2 rated transmitters in 1oo2 voting, unless you find a SIL 3 certified transmitter in the market, however typically you don't have a SIL 3 requirement for a boiler application.

When evaluating the configuration and technology of devices to be used in a SIF to achieve the required SIL, you also need to take into consideration other factors that might introduce common cause of failures preventing you to achieve the SIL requirement, for instance, using the same process taps for two transmitters in 1oo2 voting.

Regards,

Juan Calderon.
MCL Control.
jcalderon@mclcontrol.com


Posted by Anonymous on 20 April, 2006 - 11:03 pm
You can look at MicroMod Automation that offers pre-engineered boiler controller packages, both hard- and software.

http://www.micmod.com


Posted by Abhijit Goswami, Haldia, India on 18 May, 2006 - 12:21 am
I believe you need to appoint experienced consultant / engineering contractor to get your job done.

Before proceeding you need to look into the following,
1. Is it all which you would like to integrate in future or something more too? This depends on your installation facilities.
2. Do you plan to revamp on phasewise manner. I mean, either in one go or in phasewise manner.
3. How is your staffing philosophy.
4. What are the requirements you have from DCS and PLC.
5. How is your budget.

DCS vendors are many where you may choose to have modular & scalable system if the entire revamp is phased. Keep in mind the system architecture & limitation referencing your ultimate goal. See if vendors have boiler specific application algorithms to support you better. You must ensure that what you require is met by DCS/PLC vendors and that can only be done once you have a firm technical specification against which the offers (vendors) are evaluated.

As the 5th boiler will discharge on the same header. Two mechanisms are possible,
1. Firing control in individual boilers are driven by common pressure / flow control. In that case electronic/pneumatic converters are required.
2. 5th boiler is independent where you can still fine tune it to be slower as like other boilers.
But better is to have a dynamic study of the scenerio before proceeding on control schematics.

Burner management or boiler control is safety critical application and hence the PLC must have higher order integrity. Approaches to solve this is many,
1. Do as your neighbour do. (Not a right way of engineering).
2. Implement cost effective solution from local vendor where you know that the supports are always available.
3. Implement safety certified hardware. This is only preferred,
a. If you are guided by land of the law.
b. If the residual risk is much above tolerable risk and you quantify that by implementing the certified certified system you mitigate the risk to tolerable limit.
c. Generally this application falls under SIL-2 catagory with certain boundary conditions.

Switch or transmitter - an universal question..
Switch is simple device. No diagnostics ( if non-smart switches). Lasser components, hence less prone to failure. Cheap. You get less safety but high plant availability.
Transmitters are complex. High diagnostics but still classified as type-B device. Analog data may also help in indicating drifts.Theoritically more prone to failure. Comparatively costly. You get higher safety but lesser plant availability.

If you want both safety and plant availability and have money to afford you may go for 2oo3 voting in sensing to get best trade off in safe failure rates as well as dangerous failure rates.


Posted by Jorge Cano on 21 April, 2006 - 5:36 pm
I recommend A DCS Deltav of Emerson Process and in place of PLC put a SIS of Emerson Process.

So you can integrate both in the same architecture.

Check http://www.easydeltav.com for both systems.

Regards.


Posted by Lubo on 20 April, 2006 - 9:55 pm
I built similar application with Yokogawa CS3000. You can start with small system configuration and add some control stations and operator stations later. CS3000 is extremley modular.

Yokogawa also provides ESDS ProSafe-RS. This one can be conected to DCS system bus. No extra communication line needed.

Lubo
Slovak Rep.


Posted by Gerald Kontriner on 20 April, 2006 - 10:05 pm
The newest trend in automation is to have ESD and DCS integrated within one system. ABB, Emerson, Honeywell and some other automation companies are offering such systems. What are the advantages with such integration: The signals from both systems are easily exchangeable and by using the same configuration tools your engineers just have to learn one system.

Failure rates for field instruments may be obtained from industry databases, manufacturer FMEDA analysis, manufacturer field failure studies, company failure records or other sources. I recommend reading the article "Getting Failure Rate Data" http://www.exida.com/company/articles.asp

Gerald Kontriner
Senior Automation Engineer
Synertec Asia Pte. Ltd
http://www.synertecasia.com

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