The power of Velocity Limit (VL)

ControlGlobal.com

Keywords: Velocity Limit, VL, alarm management, process automation, process safety management, automation engineering, process automation technologies and industrial automation systems

This article argues that Velocity Limit (VL), which can be used to detect and take corrective action against faulty signals as part of an overall alarm strategy, is not being used as much as it should be.

By Anil Sharma, Essar Steel

INDUSTRIAL AUTOMATION technology for the process industries is changing at the speed of light, enabling companies to respond to industry trends and market opportunities while continuing to meet quality and compliance requirements.

With these advancements in technology, distributed control systems (DCS) in turn have become more intelligent and can now detect very minute disturbances in the automation process without affecting plant operation.

Recent studies and personal observation, however, show that merely investing in the latest automation controls technology does not guarantee the expected benefits, until and unless its applications are fully utilized. Yet there are so many applications and functions within today’s control systems that many go unnoticed and unapplied.

Process is identified as having one or more variables associated with it that are important enough for them to be controlled. And of all the process parameters in the process industry, temperature is one of the most important. Temperature is the one parameter that doesn’t change drastically even in the trip/startup mode or during adverse conditions. Its rate of rise and fall is always linear.

The Power of VL
In any process application, a DCS is an integral part of the automation process, performing many functions depending on the process requirements. In a DCS, there are many alarm check functions that detect abnormalities in process signal and input signal converters. Some of these functions include Input Open Check, High/Low Alarm Check, Deviation Check, and Velocity Check. Yet one of these functions, Velocity Limit (VL), which can be used to detect and take corrective action against faulty signals, is not being used as much as it should be.

This article will outline the power of VL functionality and explain how it can be used to avoid false alarm tripping, especially when related to faulty indication of temperature and level. The VL function is very much suitable for faulty indications of these two parameters, as these don’t change drastically with the process.

ADVERTISEMENT To avoid plant alarm trips because of faulty indications of temperature and level, utilize the VL function already existing in your DCS.

Application of VL in the DCS for temperature/level loops is very simple. There is always some change in the process parameters if change in the Process Valve (PV) during the scan period of the system exceeds the value assigned in the VL function. When this occurs, an alarm is generated indicating that there is an abrupt or abnormal change in the process. If proper value is assigned to the function, the only possibility remaining is that there is a problem somewhere in a particular control loop.

Today’s control systems are very fast, and scan time is in milliseconds. As the DCS displays the instantaneous value of the process parameters, the VL function samples the process variable, computes the change in it during the scan period, and generates an alarm if the change exceeds the VL setting. To make it simple, if the PV is 75 deg C, and the value assigned to VL is five, then VL will get actuated if PV changes more than 75+/- 5deg. C in one scan period.

During process applications, it is impossible for temperature to rise or fall by 5deg. C in milliseconds. Thus the problem must lie somewhere in that particular loop. On actuation of the VL, that particular instrument goes to calibration through a sequence table, and PV comes to the value assigned in the calculation table. If a controller is in the loop, it will display in manual mode during the calibration, which effectively minimizes plant disturbances.

A Case Study
Temperature loops are very critical for steel industry plant operations, and faulty indication from any of these loops can trip the process and make it unstable. One such loop is the discharge temperature of a process gas compressor, which is the heart of the process. Gas is compressed in two stages. Suction temperature of the gas is brought down from 300 deg. c to 50 deg. C by passing it through different cooling arrangements. Suction pressure is around 400 mBar in normal load conditions, and final discharge pressure is 2.0 Bar. Flow through the compressor is approximately 100KNMdeg. 3/Hr. Gas discharged from these compressors becomes the feed gas for the reformer.

The range of the temperature instrument is 0-100deg. C. Suction temperature is approximately 50deg. C, and discharge temperature is maintained at about 70deg. C with the help of spraying water. Discharge temperature is important from the point of safety of the compressor, and process also demands that temperature should be within limits so as to maintain the chemical composition of the gas. This indication comes to the DCS as a trip value; output from the DCS goes to the PLC through NDO, which actuates the relay and thus trips the compressor. This, in turn, trips the complete process.