Soon after the acronym DCS appeared, more than 30 years ago now, discussions about what system is “best” cropped up among control engineers.
The conversations continue today. Which DCS is indeed the best? Or is DCS even needed anymore, when modern PLCs — especially high-end PACs — can allegedly do the job?
Over the years I’ve seen a number of different approaches to the control system selection. Some — usually larger — companies take a very methodical approach, with complex criteria that take into consideration an enormous amount of information. Smaller companies look at certain features that are most critical to them, like the proximity of a reputable engineering outfit capable of handling routine maintenance issues. Precious few seek advice from a consulting company.
The truth is, a good engineering team (or a lone engineer, as may be the case) can make any control system do what the process requires. Sure, you’ll occasionally bump into what I call “quirks” that make you cringe, but that’s almost unavoidable. Even the best selection process may include some features that, while being good for your company overall, aren’t the best for your particular process unit.
So, what do I do when confronted with a quirky system? One thing’s for sure: griping or blaming the system won’t get you anywhere. Take a step back and look at the challenge. The solution inevitably appears. After all, you’re not ripping your control system out entirely.
Let’s get specific. A comprehensive comparison of the multitude of control systems currently on the market wouldn’t fit in this blog post, even if I wanted it to. I can, however, share an incomplete list of features that DeltaV offers to process control engineers. The following is based on my personal work experience — at Emerson, where I had two seven-year stints, and while working for a couple of systems integrators.
I have to say it here: My acceptance of DeltaV, even while working for its manufacturer, was not by any means automatic. I was coming from the RS3 crowd and looked at the then-new system with a critical eye. Well, after all these years I can tell you that, while my heart will forever be with RS3, I wouldn’t give up the features of a modern DCS.
Take the function block (FB)-based configuration. It’s not new or unique to DeltaV, but it is refreshingly straightforward. One can create a custom function block by putting together a number of library FBs and furnishing this module as a “composite,” which for all intents and purposes is a custom function block. No Visual Basic or C++ configuration required. It can indeed be done very quickly by a control engineer as the need arises.
The PID block, which is the core of the process control engineer toolbox, has tons of built-in functionality to address a number of problems that would otherwise require some serious fringe configuration. One example is so-called FRSI options (FRSI stands for Fisher-Rosemount System Inc). This is a collection of unique features, either new or carried over from the older Emerson systems. One such option called dynamic reset limit — in conjunction with “using PV for backcalc” option for the slave PID, along with some other options — allows cascade loops to cope with a wider range of process saturation causes, as opposed to more common and more narrowly defined final element saturation.
This last feature works especially well if you want to establish some process optimization by setting up valve position control — say, to keep valves in parallel VFD-driven pump discharge lines as open as possible. Look at Greg McMillan’s blog for more details on this and other related matters.
If PID-based process optimization is not going to do it for you, a number of compact APC tools are built right into DeltaV controllers. Model-based control, either SISO or multivariable, is right under your fingertips. As a matter of fact, your system comes with a license for unlimited number of single output model-based controllers. Inputs are not licensed at all. You only need a license if you need a controller with two or more outputs. Try a SISO model control today; you can put it on top of the existing PID by utilizing ROUT feature of the PID function block and then switch between them at will.
As far as the hardware side is concerned, I can’t say it often enough — DeltaV analog cards come with HART modem installed on each point. If your valve has an intelligent positioner capable of relaying the actual valve position back to the controller via HART communications, use it. It costs you nothing. Not only can you now easily see how badly a valve is sticking (add actual position feedback to your historian list), but you can use the valve position for “dynamic reset limit” mentioned above and improve the effectiveness of your valve position optimization strategy even more.
If you happen to have wireless transmitters (how else would you get a dryer mid-temperature?) then DeltaV has the ability to deal with unpredictable wireless transmitter scan times. PIDplus (available in DeltaV version 11) is designed to address just such a problem. Check out an excellent piece on the subject by Greg McMillan in InTech magazine.
Integrating wireless transmitters into your system is also straightforward. Rosemount wireless products are based on HART 7 specification that has been adopted as an international standard, IEC 62591Ed. 1.0., so don’t worry about this technology fading away. While Rosemount wireless gateway can be integrated with most other control systems, in DeltaV it is just a little bit less stressful. Emerson’s wireless network uses the “mesh” mode, which means each transmitter can pass through packets from other ones, so there are multiple possible ways for a wireless signal to reach the gateway, increasing the reliability of the system.
Yes, DeltaV HMI application package may not cater to certain engineering tastes. But hey, you get a package deal that comes with lots of useful control features! Don’t let some graphic design idiosyncrasies stand in your way.
While I can’t say that DeltaV is better than the others, I can grudgingly acknowledge that it beats my beloved RS3, hands down!