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Dec 16, 2014
Improving process safety and cutting costs with process ball valve
Head of Industry, Power to X and Gases
How can a properly designed process valve extend plant availability and reliability?
A properly designed valve supports successful plant operation and minimizes maintenance costs. Especially in a demanding hydrocracker application, simply fulfilling standards is not enough. The right valve must be suitable to the process to take advantage of all life cycle and safety targets.
High performance ball valves are widely used in demanding control applications. By combining an advanced process ball valve design with various safety and plant performance expectations, the advantages of using true process valves are easy to recognize.
Operational costs can account for a major portion of the total valve cost. Using properly designed valves with the correct material selection can extend availability and reliability. Valves with good fugitive emission performance increase plant runtime and safety. Also using a noise control solution increases worker safety.
Plus, noise control extends the valve lifetime by reducing the effects of cavitation and vibration. Fire-safe and tested valves according to API 607, which also fulfill functional safety requirements, can be critical components in helping the plant to go into safe state in case of an emergency.
The advantages of properly designed process ball valves can be easily shown in a hydrocracker application.
Typical hydrocracking feedstocks include heavy atmospheric and vacuum gas oils, and catalytically or thermally cracked gas oils. These products are converted to lower molecular weight products, primarily naphtha or distillates. Sulphur, nitrogen and oxygen removal and olefin saturation occur simultaneouslywith the hydrocracking reaction. Typical reactor operating conditions require temperatures of 280 – 475 °C and reactor circuit pressures of 35 – 215 bar, depending on the feedstock and final products desired. The reactions consume hydrogen and are highly exothermic. The key to hydrotreating and hydrocracking reactions is applying bi-functional catalysts that contain an acid function and a metal function.
A hydrocracker sets demanding requirements on valves through plant runtime, emissions, fire-safe and functional safety. These are directly related to valve design and testing.
Cost control with high-performance design
Valve sizing in on-off cases is quite simple, but valve selection requires expertise. The best solution can be achieved when all parts of the valve are selected according to the process conditions. In high performance ball on-off and ESD valves, the critical areas are the material selection and actuator sizing.
Poorly performing valves in the hydrocracker process must be serviced because they will have a direct impact on the efficiency of the process. The cost of unscheduled maintenance can be quite high, up to 70% of the cost of a new valve in some applications. When adding this to the cost of removing the valve from the line and disrupting the process, the total cost will be much higher. Typical hydrocracking plant runtime is 2 to 4 years, which requires reliable equipment and process control, before the catalyst will need regeneration of its deposits.
Many refineries are looking for a safety instrumented function for emergency depressurizing of their hydrocracking process unit upon detecting thermal runaway. Many hydrocrackers are equipped with two different means of depressurizing: a slow system and a fast one. In an emergency scenario, an operator will first attempt to bring the process under control using slow depressurizing. The fast depressurizing system is only used if the first option is not capable of stopping the runaway reaction from continuing. The process can be brought back to a safe state by either manual or automatic depressurizing. To minimize the negative impact on the process equipment, slow depressurizing is always used first.
Valve performance can be one of the weakest links in the safety system, where the failure mode may become stuck. The only way to test for this condition is to stroke the valve regularly. Closing the valve completely is not desirable when the unit is operating. In many refineries, the hydrocracker reactor can be a SIL 3 application.
As part of a safety instrumented system, a high-performance process ball valve with fire-safe and tested construction according to API 607 and online valve diagnostics ensure reliable and safe valve operation so that the process can be brought back to a safe condition.
Proper valve design is key to successful plant operations with minimized maintenance costs. All parts of the valve must be designed according to the process requirements, even if this means exceeding the given standards. This ensures that the process fulfills of life-cycle and safety targets.