Application of Temperature Activated Relief Devices Part 2

By Martin Gollin

A previous article described the situation where conventional pressure-activated relief valves may not be able to provide sufficient venting to protect a system from exceeding its maximum allowable working pressure (MAWP). In such cases, overpressure protection may be provided by system design. This article highlights some ways that this may be done.

Rupture Disc or Other Non-Reclosing Device

Although conventional pressure-activated relief systems may not provide adequate protection against overpressure resulting from some types of exothermic runaway reactions, an appropriately sized and installed non-reclosing device may be suitable (e.g., rupture discs, buckling pin devices, breaking pin/shear pin devices, etc.). These devices may be effective, as they remain open, by design, until the system has reached atmospheric pressure and VLE effects do not arise. Therefore, the venting will provide adequate cooling to lower the temperature, and hence the reaction rate, and prevent overpressure. However, the use of rupture discs has some practical drawbacks1. For example:

Installing redundant rupture disc assemblies in parallel, and isolating them individually to inspect and replace the discs on a regular basis, could mitigate some of these issues. It may also be effective to install a nitrogen purge under the rupture disc to minimize the potential for deposition. However, many companies choose not to use rupture discs due to the problems identified above and, particularly for large continuous processes, the safety, operational, and economic consequences that would arise if a rupture disc activated unnecessarily.

Temperature-Activated Relief Valve System

A temperature-activated relief valve system can be set to open a relief valve at a given temperature and close it at a given temperature. This opening and closing is achieved by using an actuator (attached to the valve) and a control system. The use of a temperature-activated relief valve system allows control of the process variable (i.e., temperature) that directly affects the rate of reaction. The relief valve can be opened either at a specified temperature above the operating temperature, or when the rate of temperature rise meets a specified value. This enables the onset of an exothermic reaction to be detected and action taken at an earlier stage than would be possible using conventional pressure-activated relief valves. By keeping the relief valves open until the temperature of the system has reached a level where the rate of reaction is essentially zero, the system can be brought to a safe condition, while venting the least amount of material. The lower the opening set point temperature and the higher the closing set point temperature, the smaller the amount of material that is vented from the system. An additional advantage to using temperature as the basis for opening the relief valve is that the lower the temperature, the lower the reaction rate and the lower the potential for two-phase flow effects in the inlet and outlet piping. Holding the valve open may also minimize issues concerning two-phase flow through the relief valve itself. While the use of a temperature activated system can be highly effective, care must be taken in its design and installation so that the required availability is achieved. This usually requires the use of redundant components (sensors, processors and valving), plus the ability to functionally test the system on-line.

Design Process

To effectively analyze whether a control system is required to protect a system from the effects of an
exothermic runaway reaction, the following steps may be required:

Reference

  1. “Guidelines for Pressure Relief and Effluent Handling Systems,” American Institute of Chemical Engineers, Center for Chemical Process Safety, New York, 1998, ISBN 0-8169-0476-6.