I wanted to bring additional visibility to a great thread in the LinkedIn Process Control group on advanced regulatory control versus advanced process control. The post, So, ARC Can “Replace” APC? is based on a blog post, Advanced regulatory versus model-predictive control. The LinkedIn post’s author draws two points from the blog post:
- ARC is implemented at the Process Control System level while APC (or MPC [model predictive control]) more precisely needs usually a server and communications to the PCS
- ARC has a limited number of variables compared to MPC.
Any other differences?
Presently, there are 14 comments. I wanted to highlight two of them, from Emerson’s Lou Heavner and prolific author, Greg McMillan.
If the choice is between PID with override and feed forward vs MPC, there are simply some things that MPC can do that PID cannot and some things that based on training and skill are arguably easier with MPC. Deadtime dominant processes are a challenge for feedback control and the deadtime limits the ability of feedback control. A Smith Predictor may help, but may not be as robust as MPC. MPC can more easily and robustly handle deadtime dominant processes and I think there is little dispute on that point.
On the issue of constraint limits, with override control, the override controller takes over when a constraint is about to be violated in both MPC and ARC. But in a multivariable controller (typical MPC) the process can ride on one constraint and still drive toward another constraint until the degrees of freedom are saturated. This just doesn’t really happen in ARC. Finally, in interactive processes, feed forward and/or tuning can be used to decouple the interactions. Feed forward can be difficult for many to tune. It can be on the level of difficulty as MPC with modern process dynamic identification tools or worse. MPC will not only decouple the interaction, but can actually coordinate the controller moves in a way that optimizes the response and which feed-forward is unlikely to do.
Greg wrote: Continue Reading ▶
Controlling the pressure levels is one of the challenges many instrumentation and automation engineers face. Pressure regulators are often used for this task, since they are self-regulating and don’t require a control loop connected to the plant distributed control system (DCS).
I saw this post, Pressure Regulator Animations
by Emerson’s Michael Calaway
in the Emerson Exchange 365 community.
He shares links to some pressure regulator animations, a couple for the MR95 Pressure Reducing Regulator and one for a gas delivery over- and under-pressure configuration.
Let’s look at the second one with an EZH pressure regulator with a monitor in series. It is an interactive demonstration helping to show various scenarios of how pressure is maintained even in failure situations. You’ll need to view it on a device that supports the Flash file format.
Continue Reading ▶
As technologies advance through the adoption phases and into mainstream use, a common set of questions usually emerges. For example, as the IEC 62591 WirelessHART communications standard has moved from its introduction to the market in 2007 to present day, its use has increased widely across industries, geographies and applications. We’ve chronicled many of these stories in the wireless category of this blog.
I share this background to highlight a great EE Publishers article, Q&A – Wireless technology in process applications
. The question and answer session is with Emerson’s Dale Perry
who addresses common questions about WirelessHART, a:
…self-organising protocol utilising a time-synchronised, self-organising, and self-healing mesh architecture.
From the article, I’ll highlight a few of the most common questions I see in my travels through the social networks, and leave the rest for you to read if you’re interested.
How far the wireless communications can travel is a common question. Dale responds:
The transmission distance between two wireless devices is a function of the antenna type and the distance the antenna is above the ground or the structure of the installation. The specified distance is 225 m for a standard antenna mounted 1,8 m above the ground/structure. Optional high gain antennas with transmission distances up to 1 km are available and should be mounted a minimum of 3,4 m above the ground/structure. Since WirelessHART is a self-organising mesh technology with each radio able to communicate with all other radios, networks can cover much longer distances through “hopping” whereby transmitters closer to the gateway relay data for those further away.
Battery life for wireless instrumentation is another common question. He explains: Continue Reading ▶
Here in the US, we read many stories of oil tanks being filled to the brim with increased production from shale sources and federal law barring export. Whether in the US or anywhere across the globe, managing and reporting on the levels in these tanks require accurate measurement. Also, given the importance of these measurements in safety and environmental protection, accuracy and reliability is required, especially for older facilities with manual or aging tank gauging systems.
In a Tank Storage magazine article, Gauge Emulation: a cost-effective solution for tank gauging upgrade projects
, Emerson’s Hans Westerlind
shares how technology advancements make it possible to incrementally replace outdated mechanical level measurement devices with accurate electronic devices.
Older tank gauging systems often had proprietary communications between the measurement devices and the control system:
This electrical interface and protocol software was more or less specific to one manufacturer. Level gauges or temperature devices from another manufacturer could therefore not be connected to this bus. If the user wanted an extension of the system, a partial upgrade or an exchange of a number of tank gauging units, the only practical alternative was to purchase them from the same supplier as before.
Also these older measurement technologies:
…consist of mechanical level gauges as float or servo gauges. It is not uncommon that users have to accept very high maintenance costs, poor performance and unreliable measurements…
At a refinery or large tank farm, it can be a difficult undertaking to modernize the tank gauging systems: Continue Reading ▶
A refinery produces many products, one of them petroleum coke. AFPM defines it:
Petroleum coke (petcoke) is one of many valued consumer products produced during the oil refining process. Crude oil is processed into gasoline, diesel fuel, jet fuel, lubricating oils and waxes, leaving some residual crude that usually undergoes additional processing. The crude residue may be further refined by a process known as coking to produce transportation fuels as well as petcoke, which has a variety of uses as an alternative, cost-effective fuel.
The history of petcoke goes back to the 19th century:
Petroleum coke was first made in the 1860s in the early oil refineries in Pennsylvania which boiled oil in small, iron distillation stills to recover kerosene, a much needed lamp oil. The stills were heated by wood or coal fires built underneath them, which over-heated and coked the oil near the bottom. After the distillation was completed, the still was allowed to cool and workmen could then dig out the coke and tar.
I share this because I heard a great story from Emerson’s James Beall
about the application of model predictive control on a delayed coker unit
. Wikipedia describes this unit:
A delayed coker is a type of coker whose process consists of heating a residual oil feed to its thermal cracking temperature in a furnace with multiple parallel passes. This cracks the heavy, long chain hydrocarbon molecules of the residual oil into coker gas oil and petroleum coke.
From the same Wikipedia entry, you can see some of the complexity in this unit: Continue Reading ▶