Stuff that Comes at You Too Fast

Last year we had disturbing thoughts on how fast upsets were particularly disruptive and anything you can do to slow them down makes the job of a loop much easier. In real processes, step disturbances are quite rare. However, there are some noticeable cases (e.g. on-off level control, interlocked and sequenced valves, and compressor surge control) where stuff comes at the loop too fast.

If level switches are replaced with a Hart or Fieldbus properly applied level transmitter, correctly tuned level controller, and throttling control valve with a digital positioner, you will make everything smoother downstream. The cost of the better automation system will pay for itself in terms of better reliability and visibility and reduced variability.

On-off valves must in many cases be sequenced and interlocked. The effect of these valves may be underestimated. Even for large valves with slow stroking times, most of the time is spent on the flat portion of the installed characteristic. For example a reactor air feed isolation valve had been deliberately slowed down by a restrictor on the actuator to take 145 seconds to stroke to allow the air compressor surge control system time to open its vent valve. An analysis of the installed characteristic revealed that there was actually only 1.7 seconds between when the flow dropped below the anti-surge controller set point and the flow hit the compressor surge line. The total time on the steep portion of interest in the installed characteristic was less than 3 seconds. The speed of the upset could have been regulated much better by a programmed partial reduction in air feed flow set point followed by a fast closing of the on-off valve to prevent reverse flow. I am must make it very clear at this point that a control valve should not be considered as a replacement for an on-off valve, or vice-versa. They serve distinctly different purposes. A control valve needs to have minimal seating friction for throttling and an isolation valve tight shutoff for isolation, which may be conflicting objectives.

Once compressor surge starts, not much can be done by a flow controller because it is like going over a cliff. The precipitous drop in flow occurs in less than 0.03 seconds. This was mistakenly interpreted as requiring a special microprocessor with a scan time of less than 0.05 seconds when really the control valve on big compressors often didn’t do much of anything for a second or more because you physically couldn’t move enough air out of the big actuator for the fail open vent valve. Also, the feedback controller needed to do something before it hit the surge curve. Once a compressor is in surge, an open loop back up is used to get out of surge because a flow reversal occurs every second or so totally confusing the controller.

For more details on compressor surge control see the books Centrifugal and Axial Compressor Control and A Funny Thing Happened on the Way to the Control Room (reprints available through UMI). Next week we will talk about the use of a simple open loop backup configured in a DCS to assist a PI loop for those applications were you need fast recovery for property and environmental protection.

Compressor anti-surge control is an extreme case but there are many applications particularly for parallel trains of equipment and batch to continuous transitions where it is advantageous to slow down disturbances by a coordinated startup and shutdown of flow set points.

Pressure waves travel at the speed of sound in the fluid (e.g. 1100 fps) whereas composition changes travel at the pipeline fluid velocity (e.g. 5 fps). The pressure waves can also reflect back and forth (e.g. water hammer), which like surge can be totally disruptive. Whether you are talking about pressure or composition changes it is wise here as well to slow them down by ramping the flow controller set point in the DCS rather than restricting the air flow to the actuators of on-off valves in the field. It is also beneficial to use pressure transmitters instead of pressure gages. Operator typically cannot outrun a pressure wave to get to the right one. In some cases we don’t know even whether a pressure upset is originating upstream or downstream let alone where it specifically starts. If you think about it, some field pressure regulators are also best replaced by pressure loops in a DCS to provide more visibility and control over the propagation of pressure waves and the allocation of pressure drops to prevent interaction and cavitation.

Momentum balances, normally not a part of dynamic process models, are required to simulate pressure waves and surge.

Top Ten Stuff That Comes at You Too Fast

(10) On-off level controlled flows

(9) Sequenced and interlocked flows

(8) Strong acids and bases

(7) Pressure waves

(6) Compressor surge

(5) Dunk shots

(4) Ice pucks

(3) Late night car commercials

(2) Corporate Restructuring

(1) Retirement