The Problem

One of the control-system’s primary functions is to respond to changing process conditions dynamically. When process parameters start to fluctuate, the control system uses adaptive PID techniques to minimize fluctuations, and to stabilize the processes once again.

When process upsets do occur, there is a scramble among operator personnel to contain fluctuations in the hope that the plant does not trip.

The Consequence

Wild fluctuations due to process upsets accelerate plant wear and tear. In addition, the quality of the plant outputs may be compromised, leading to financial loss, and loss of reputation.

The inability to deal with upsets also compromises plant safety - when everyone is scrambling to prevent a plant trip, it’s easy to overlook safety-critical alarms and miss early warning signs of an impending disaster.

The Cause

Process upsets are inevitable, and in some cases are inherent in the nature of the product being produced. Take for example a subsea oil slug. These occur naturally and cannot be avoided. What happens is that a large mass of oil and gas surges up through the subsea piping. Since the transient is fast, it cannot be trapped by the subsea control system. The slug then enters the processing plant and ripples through the entire production line.

Since these events occur infrequently, it makes no sense to tune the control system only for the dynamics of the process during the upset, as this approach would render the system unsuited for times of stable operations. One of the most common situations we come across is where some of the control loops have been placed in manual mode of operation. During process upsets the control system has no control over these loops, and hence cannot adequately address the problem.

The Solution

There are multiple approaches to addressing this problem:

✓ End-to-end process simulation should include scenarios where large process upsets occur. The simulation will pinpoint the physical limitations of the plant to deal with upsets – it’s then a management decision as to the extent that additional investment in plant infrastructure is required to offset potential issues.

✓ Simulation also gives operators a chance to find out firsthand what the dynamics of an upset feel like. They can then prepare themselves by ensuring they understand procedures during upsets, and by trying out various techniques to bring processes back under control.

✓ Finally, the control system has to be tuned appropriately so that controllers are not placed in manual mode. There may be reluctance to place loops in auto when they have historically been the cause of trips, but demonstrating technical proficiency of problematic loops is imperative to winning over the confidence of engineering and operating staff.