This is only case study of simple loop control that was normally found in oil & gas process platform. The discussion will simply talk about either it is direct control action or reverse and also either it is fail close or fail open. The source of this article is knowledge sharing from my discussion on mailing list firstname.lastname@example.org. A very very senior instrument engineer had explained us how to understand and how to design control action and control valve fail selection in simple loop.
Fail action means what control valve position should be in case of the failure of instrument air and or power off. The fail action shall not make danger to the system. The fail action shall not make the system will reach overpressure that will tend to explossion. The fail action shall make the system to lower energy state. If we have production separator vessel and want to protect the vessel from overpressure by regulating the pressure and release the pressure to flare system, so we must select fail open control valve. If we select fail close, in case of fail we will get danger to the system. Although we still have SIS sytem thru BDV, and mechanical system thru PSV to protect the vessel, however we must think about the safety from the first failure possibility.
Above is case#1 we have a simple control loop with level transmitter, level controller, and control valve. The control valve shall be fail close because we do not want the liquid will flood the downstream system. The control action of LIC shall be direct because increasing of liquid level in the tank will need the opening increase of the control valve. Remember above is fail close control valve which has meaning that lowest signal (let say conventional 4 mA) given to valve from the LIC will make fully close of the valve, increasing signal will have increase opening, and maximum signal (let say conventional 20 mA) will have fully opening of the valve. Increasing process value will get increasing analog input and the control action is increasing analog output, such kind above loop is called single close loop direct control action.
Above is case#2 we have a simple control loop with level transmitter, level controller, and control valve. But it is consider more safe flooding the downstream during fail of control valve. The process safety engineer shall conduct hazard and operability analysis prior to decide how safe the system in case of failure. In this case the decision to let the control valve in open position during failure. Fail open position of the control valve means the lowest signal (let say conventional 4 mA) sent to control valve will make it fully open, increase signal will make it closing, and highest signal (let say conventional 20 mA) will make it fully closed (noted). Once the process liquid level is increasing it is required to flow more flowrate to the downstream. Once the process liquid level is increasing, the signal (analog input) from the transmitter is increase also. Once the process liquid level is increasing and need more flowrate to the downstream, so the control valve need more opening, it means the control valve needs decreasing signal from LIC. The increasing signal from the input will cause the the decreasing signal of the output is the meaning of reverse control action. The people is normally fault by refering the reverse control action from behaviour of output signal LIC to valve and the valve opening (see noted). It is not like that. To evaluate control action we should evaluate the analog input from transmitter and the action of the controller on analog output to control valve. Once again the control action is NOT relation between valve signal and valve opening. Increasing process will get increasing analog input and the control action is decreasing of analog output to valve, such kind above loop is called single close loop reverse control action.
Sarjana Teknik Fisika ITB, 2004, sekarang bekerja di J Ray McDermott, Batam-Indonesia.
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