SCADA Vs BMS (BACNET)

[HeshamM]

In the application of district cooling plants, which system is better to control and monitor the process of producing chilled water and the distribution on the buildings Heat Exchanger Rooms. SCADA or BacNet BMS system?

 

[Ashereng]

I am a bit off topic here, but for controlling chilled water, I would nnot used either a SCADA or BMS (Burner Management System).

I use either a tradiction PLC or DCS type of control systems.

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[HeshamM]

As a mechanical engineer, when it comes to the BMS/Ethernet/100bps/Modbus/VPN... I feel helpless. However, What I meant with BMS is the Building Management System with Dirict Digital Conrollers connected on the automation level through Ethernet/IP connection.
I know that Scada system with PLC controllers are much more accurate and reliable and it is used widely in the industrial processes. My question was, in the application of the chilled water system (where buildings will be connected to District Cooling Plants through a fiber optics Wide Area Network) which system can be more efficient and feasable?

 

[ScottyUK]

If you can install fibre infrastructure to tie all the locations together then this is DCS territory, although a well designed network of PLCs could also be made to work. Any DCS vendor would be capable of putting this system together; not all PLC programming outfits would be capable of taking on a major distributed project. Check out your vendors!

I'm slightly bemused by

...Scada system with PLC controllers are much more accurate and reliable...

Most if not all DCS manufacturers would take issue with that concept. Both DCS and PLC technologies are very reliable and capable of excellent performance when the application software is well designed.

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Sometimes I only open my mouth to swap feet...

 

[simsd]

Neither system is any better for this application.

First what level of accuracy are you looking for? If you are looking for extremely accurate temperatures and flows then you will need to think about your sensors and field devices first.

Who will be running the plant once it's built? SCADA is very difficult to mainatain and program and extremely expensive. We just installed a panel for LADWP that makes a fan kick into high speed and open a damper on three AHU's when it's too hot. It's all Allen-Bradley with a color tough screen. This is no lie - it's a $ 42,000.00 thermostat.

So here's the deal. First you need to figure out what type of front end you want (The black box that provides the interface to the PC). This will be an ethernet connection. Don't worry too much about speed as you are not running a printing press.

Second figure how you want to control everything and what end devices you want (valves, sensors etc.).

Now decide how "open" you want the system. If you want all of the controllers to communicate via Lonworks or BACNET then you would probably have a traditional HVAC controls contractor doing the installation.

If you decide you want Modbus as the communications medium or something else, then you could expect an electrical contractor with a System Integrator to do the work. My concern is that district cooling doesn't need to be too complicated. I would think the traditional controls HVAC controls route would work for you. Industrial System Integrators are great at designing rocket ships.

Consider a front end system like Tridium. It can handle Lonworks, BACNET and Modbus along with a few other protocols in one box.

 

[jraef]

As time has progressed there is very little difference between DCS systems and PLC systems. In fact, the hardware platform for the new Siemens PCS7 DCS is exactly the same as the S7400 PLCs. The difference is in the user interface and control loop configuration, so you should really look at it from your own ease-of-use viewpoint and not get hung up splitting hairs on hardaware performance. Either systems is more capable than you need to be as accurate as you want for that simple of an application.

 

[techzone12]

I would vote for a DCC system, it's cheaper and it does the job. You won't be compromising on the quality if you choose the right BMS integrator.
HVAC sytems don't normally require high processing speed, which is usually provided by PLC's. In addition to that, DCC BMS systems are easier to program, operate and maintain.

 

[techzone12]

Sorry, I meant to say DDC (Direct Digital Control)....

 

[Dayundone]

I formerly worked sales for a reputable BMS contractor. The BMS is far cheaper and chilled water was no problem to handle (can you tell I was in sales). As far as the architecture, the open protocols offered more flexibility but proprietary protocols were still used in various routines requiring both contractors to work together whenever mods had to be made. The system can be built to make proprietary protocol a minimum but that comes at a considerable expense.

As far as durability, I've seen both flake out. For that application and cost, I'd recommend a BMS solution, but not from my former company.

 

[dcasto]

PLC Direct and their software. Thermal couples and fixed range pressure transmitters. You can use the the control software as a HMI. jreaf is right on, if anyone can tell me the exact (no opinions) reason for a "DCS" system over a PLC SCADA I like to know it.

 

[ScottyUK]

I don't think there is an 'exact' reason, and pretty much everything here is someone's opinion. I wouldn't want our plant automated using the cheapest hardware available simply because I don't believe that the compromise in safety and reliability is worth the saving. I have spent too much time getting rid of cheap equipment which turned out to have a very high demand on maintenance and where failure of a cheap device had huge consequential costs on lost generation for me to think otherwise. My opinion, from my industry. Others will have their own views.

Most of the DCS manufacturers offer a 'one-stop-shop' to automate a process, providing hardware, software and application expertise. The PLC integrators have done a good job catching up, but a PLC solution will still typically pick and choose from third party vendors. This introduces risk in the eyes of those financing the build of traditional high capital value like power plants, risk which is perceived to be lower if a turnkey contract can be awarded. Note the use of 'perceived'; I am convinced that many DCS vendors owe their survival in part to this risk-averse attitude.


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Sometimes I only open my mouth to swap feet...

 

[mls1]

ScottyUK makes a good point but the application here is a building chiller, not a refinery cracker or other such critical load. I like the DirectLogic PLC stuff for low cost, easy installations that are not critical. They are cheap but they work and work well. I've had one power supply fail and it wasn't one I put in so my luck has been good.

Mixing third party products is risky but the risks can be reduced by carefully evaluating specifications and completely bench testing the system.