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Building an Effective Operator Interface for Complex APC
Applications

Erin S. Percell
Applications Engineer
ExxonMobil Chemical Company

Louis M. Michaud
Applications Engineer
Imperial Oil

Abstract

To consistently use an advanced process control application, an operator must
have a user interface that succinctly shows key information about the state of the plant
and of the application. The interface must provide means to drill down and explore
specific application behaviors. The more complex the application, the more difficult it is
to provide both effective summary information and simple methods for further
exploration. Effective presentation of key plant data will improve application service
factor and performance.

This paper discusses operator interfaces for DMCplus and DMCplus Composite
systems at ethylene plants in ExxonMobil Chemical. For plants attempting to run as full
as possible, operators are particularly interested in understanding what are the current
throughput-limiting constraints. The paper emphasizes display of appropriate constraint
information.

Most ethylene plant DMCplus systems consist of multiple controllers and
subcontrollers -- too many to watch simultaneously on individual displays. The main
"entry portal" for the interface is an overview display. This overview is at a higher level
than the traditional DMCplus subcontroller display because it presents information about
the plant over the scope of the entire DMC/Composite system. This is the main display
an operator uses to monitor the plant.

The overview indicates what section of the plant is currently limiting production.
This paper presents a method of identifying the limiting area of the plant by monitoring
which DMC subcontroller has more active constraints than unconstrained manipulated
variables. The overview also shows when constraints have been given up, which often
indicates a need for the operator to take corrective action. From the overview, the
operator can navigate to the correct subcontroller display to find more information about
the constraint violation. The operator can also navigate from the overview to a
constraint history, or to online documentation.
Introduction

In order for an operator to consistently use an advanced process control
application, he must have a user interface that succinctly shows key information about
the state of the plant and of the application. The interface must also provide a means
for the operator to drill down and explore specific application behaviors. The more
complex the application, the more difficult it is to provide both effective summary
information and simple methods for further exploration. Effective presentation of key
plant data will improve application service factor and performance.

Many modern ethylene plants have implemented model-based multivariable
controls as an integral part of their operating strategy. These control applications are
useful for honoring a number of plant constraints simultaneously and often for
maximizing throughput for a unit. However, the interfaces for these applications present
information to the plant operator in a different fashion from what he is often accustomed
to seeing. As the number of multivariable control applications increases, some method
must be provided to show the operator the few key pieces of information he needs to
make correct decisions.

Consider the analogy of an automobile dashboard. An automobile is a complex
machine, but under most normal operating scenarios, there are only a few pieces of
critical information that the operator/driver needs before him at all times. The
automobile speed is usually displayed prominently on the dashboard because it should
be monitored often; fuel level is displayed because it is occasionally checked; some
other gauges and indicators are present only to warn (visibly and/or audibly) when
something is wrong. The dashboard also houses controls within easy reach for a
number of actions that the driver may frequently take: steering, adjusting climate
controls, or setting the cruise control, for example. Similarly, in an ethylene plant, an
effective interface is one that lets operators see key operating data and take common
actions from one location. Identifying the key pieces of information and presenting them
in a single place will help the operator to monitor the applications more effectively.
Ultimately, effective monitoring is a key component of high service factor, since
operators are more likely to allow the application to run when they can easily see what it
is doing and why.

The behavior of a multivariable control application depends heavily on the set of
constraints against which it is operating. For plants running close to full capacity,
operators have a particular interest in understanding the current throughput-limiting
constraints. Therefore, the appropriate "dashboard information" for such a control
system should include some detail about plant constraints.

This paper is based on operator interfaces for multivariable control systems at
two ExxonMobil Chemical ethylene plants: the Baton Rouge Chemical Plant (BRCP) in
Baton Rouge, Louisiana, USA, and the Imperial Oil Sarnia Chemical Plant (SCP) in
Sarnia, Ontario, Canada. Within ExxonMobil, Aspen DMCplus� ("DMCplus" or "DMC")
software is used for multivariable constraint control. Several ExxonMobil plants (including BRCP and SCP in this paper) have implemented DMCplus Composite
("Composite") to coordinate the solutions of the individual DMC controllers in the
ethylene plant and to maximize throughput. However, the information presented here
should transfer readily to other commercial solutions for multivariable control and
throughput maximization.


The Overview Display

Most ethylene plant multivariable control systems consist of a number of
applications ("controllers" and "subcontrollers" in DMCplus), too many to watch
simultaneously with a detailed individual display for each. Therefore, we have found it
useful to construct an overview display to serve as a sort of "dashboard" for the entire
multivariable control system. This overview is at a higher level than the traditional
DMCplus subcontroller display because it presents information about the plant over the
scope of the entire DMC/Composite system. This is the main display that an operator
will use to monitor the plant; it generally eliminates the need for operators to page
frequently through multiple displays for individual applications.

Many operators are accustomed to thinking about the plant as a sort of front-to-
back flowsheet, so it may be helpful to arrange data for individual unit operations in an
order that approximately mimics the flow plan for the plant.

Because every plant's operating circumstances and control system design are
different, the perfect content for an overview display will vary from site to site, and the
plant's operations personnel should be thoroughly consulted about their interface needs.
However, there are some features that have been found helpful at both BRCP and SCP
that should generally translate to other sites. These features are listed in Table 1 and
are individually described below.

Table 1. Information Content for the Overview

Status of Individual Applications
Current Value for Major Unit Constraints
Target Rate for Each Furnace/Feed
Entry Mechanism for Major Operator-Entered Limits
Current Rate for Each Furnace/Feed
Limit Violation Indicators
Constraint Indicator(s)
Countdown Timer
Production Rate Data



Status of Individual Applications

There are a number of different controllers and subcontrollers in the complete
DMCplus Composite suite. The ON/OFF statuses for each of these applications should
be shown, along with each application's status in the Composite suite (i.e., participating
or not). The operators might be allowed to change these statuses from this display; see
more discussion on this subject in the section "Frequent Actions."
Target & Current Rates for Each Furnace/Feed

Because feed maximization is a common operator priority, and because feed
comes from several cracking furnaces that may vary in their capacity and/or feed type, it
is useful to display both the current feed rate and the target rate coming from the
Composite feed maximizer. The current rate provides a snapshot of the current feed
situation, and the target indicates the direction in which the plant will move in the
immediate future. If more than one feed type is present, it is also useful to display totals
for each feed type.

Constraint Indicator(s)

Especially in an environment where the operating objective is to maximize
production, operators need to be able to quickly identify the throughput-limiting
constraint for their feed-maximizing application. There are at least two reasons for
watching the identity of this constraint: in the long-term, the plant may wish to
understand its debottleneck opportunities; in the short term, operators need to consider
whether every manual action has been taken that could help alleviate that constraint
and thereby increase the production rate. See additional information in the section
"Constraint Determination."