the impact of increasing the inlet stream. The first line of defense against a surge is provided by the control system or control valves. However, control valves alone cannot totally eliminate a surge, but will only stop a surge from impacting a downstream system. Ultimately, the surge needs to be handled, but by other methods. There are basically two surge-management methods that can be implemented for each piece of equipment or group of equipment in a plant: • Boxing-in a surge in a specific equipment component or series of equipmen • Transferring the surge to an external or auxiliary system Understanding the applicability of each of these techniques requires some knowledge about the inherent dynamic characteristics of the systems from a process control viewpoint. The three dynamic features of each equipment or unit are resistance, capacitance and inertia (dead time) [4]. A brief qualitative explanation of these three features is presented next. If a system is more dominantly a “resistance” type, this system will be able to prevent the surge from transferring to downstream equipment. A piece of pipe is one example of a resistance-type element. A pipe could inherently stop the surge if it is narrow enough. However, because a pipe's main function is to transfer fluid, the designer generally sizes the pipe based on its duty (transferring fluid) and then, if needed, a control valve is placed on the pipe to stop a potential surge. The capability of a system to dampen the surge depends on the “capacitance characteristics” of the system. The higher the capacitance characteristic, the more it is able to dampen a surge. Here, a capacitance-type element refers to whatever element that can be used to temporarily store excess mass (such as liquid volume or gas pressure) or energy (such as thermal or chemical energy). For instance, large-volume equipment generally have a higher capacitance feature. Implementing a surge tank, equalization tank, surge drum (or even pond) is one means of providing a system with sufficient capacity to dampen the surge. Another example of using a high-capacitance system is when transferring a surge to heat-exchange media. Utility heat exchangers use streams such as cooling water, steam, and other media, to transfer the heat to or from process streams. These utility streams are also able to absorb a temperature surge in the system. The capacitance feature of a utility network can be provided in part by pipes in the network (the pipes function mainly as resistance elements but they have some capacitance too), and also their surge tank, as discussed above. A system is called robust against upset when it can tolerate a large surge (as defined for each process parameter) and no upset occurs, thereby allowing the process to proceed smoothly. If an upset cannot be tolerated, one solution is to implement a rate-of-change control loop in the system. The following llist provides some general rules of thumb on the capability of a system to handle surges: 1. Generally speaking, equipment with larger volume CHEMICAL ENGINEERING
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