(44a) Automation Strategies: Avoiding Accidents During the Process of Draining Water Into Spheres LPG

Introduction

This paper presents, as its main goal, alternative proposals to the methodology of the water drainage practiced in parks of LPG spheres. The study was encouraged by analysis of an accident which happened in the Duque de Caxias Refinery, Rio de Janeiro, Brazil, in 1972. At that time, the water drainage system consisted of a manual opening of the water valve by one employee and measurement of the approximate period of time consumed for the entire drainage of the liquid, until the closing of the valve. During the drainage period, the assigned employee should stay in the location.  In the case in reference, the employee went out, and did not return to close the valve at the necessary time. A large gas leak ensued, with consequent freezing of the control valve. The leak could not be stopped, thus generating several explosions. After the accident, the system received many improvements, such as inclusion of secondary valves, an inclined floor, automatic action valve, etc. The drainage procedure, nevertheless, continued to be manually operated.

In order to propose more efficient and safe solutions to the draining, a program of dynamic simulation process was carried out for the withdrawing of the water and gas drainage.

Base Case

Initially, a system was simulated in the absence of the controller, maintaining the drainage even after the removal of all the liquid. The temperature of the running water current suffers a considerable drop when a passage of gas occurs, this temperature being defined as a “controlled variable”. The “manipulated variable” was defined as the drainage of the tank, controlled by the drainage valve.  

PID Controller

It is believed that the temperature dynamics can safely and exactly anticipate the moment in which the water drainage ends. This way, in order to suggest the automation of the drainage process, it was initially investigated the use of a traditional PID controller. Several simulations were made, varying the parameters of the controller and the temperature established for the set-point.

At the beginning, the valve stays in a complete open position, thus allowing the water to drain. As time goes by and the amount of water diminishes, the gas starts to go out and the temperature of the way-out current also drops. The controller works on the valve, with the intention of reducing out-flow. The temperature continues to drop, while there is a passage for the gas. The controller finally works in order to completely close the valve, not allowing the draining process to go any further.  

Comparing the behavior of the analyzed variables related with the values of the parameter “P” of the controller, one may notice that the larger the “P”, the quicker the valve will close. Elevated values of “P” minimize the amount of the gas released thus reducing the risk of the valves freezing. We shall highlight that elevated gains of the controller induce the valve to act practically as an on-off valve.

Monitoring Processes

“Monitoring processes” (Seborg et al., 2003) work in a way similar to the PID’s controller. The monitoring can be used as a supervising and control tool, warranting that the variables be maintained within specified limits, rapidly detecting abnormal operation conditions, implementing corrective actions, etc.  

In this paper, the measured variable is the temperature of the way-out flow. This temperature is passible of natural floatation, such as alterations on the environmental temperature. This normal variability can reduce the efficiency of the traditional control approach, which is based on a fixed set-point.

Through this simulation a measure of the way-out temperature was registered during the first ten minutes of the drainage, obtaining the average value of 29.17°C. Through the previous simulations it was noticed that the drop of the temperature provoked by the beginning of the gas out-flow was of 0.5°C. This was the maximum value of deviation adopted in order to determine the closing action of the on-off valve. This approach allowed the closure of the valve just two minutes after the start of the gas release. The rapid interruption of the process reduces the risk of freezing, being an interesting option to improve the safety of the procedure. 

SEBORG, D.E.; EDGAR, T. F.; MELLICHAMP, D.A. Process Dynamics and Control, 2 ed., Ed. New York: Wiley, 2003.