1. Atmospheric Discharge
In many installations, the relief device effluent is routed directly to the atmosphere. The appropriateness of this design depends on the nature of the effluent, the proximity of personnel and equipment to the discharge location, the operating company’s policies, the facility’s environmental permits, and construction costs. When direct atmospheric discharge is chosen, the “effluent handling system” consists of as little as the relief device’s outlet fitting, or as much as several pipe segments, fittings and a block valve.
2. “Closed” Systems
In many other situations, the discharge side of a relief device is routed to a “closed” effluent handling system. In this context, the word “closed” designates any type of discharge arrangement other than the one in which the relief device discharges directly to the atmosphere. Such systems take a variety of forms, but they nearly always contain one or more of the following components.
Closed discharge systems always involve some degree of discharge piping. In most cases, the discharge of many relief devices is tied together into a common relief header. This “header,” whether coming from one or many relief devices, then leads to the next general component of the effluent handling system. In some cases, this is simply a different point in the process – the effluent is merely recycled. In other cases, the discharge is routed to some type of phase separation equipment.
Nearly all closed relief systems involve the installation of a knockout drum – a liquid-vapor separation vessel – to minimize the presence of liquids in the fluid reaching the disposal system, which is usually not equipped to handle liquids. Occasionally, the relief effluent contains one or more substances that are particularly hazardous or valuable. In these cases, extra separation equipment may be installed to capture these components in the relief stream. This equipment may include sparge vessels, in which the relief stream is bubbled through a liquid, which serves to capture the relevant component. Catch tanks of other design are also used as appropriate. The less hazardous components in the relief stream are then allowed to pass to some type of disposal system.
Disposal systems usually involve some sort of combustion apparatus, whether it be an elevated flare tip, a ground flare, or a pit flare. These systems are typically equipped with a variety of sub-components designed to ensure clean and reliable combustion, and to prevent entry of air into the effluent handling system.