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:
- Some styles are subject to fatigue in service.
- They require careful handling and installation to avoid mechanical
- Most types require special holders.
- Care is required to install the disc correctly (i.e., right side
- Burst pressure is sensitive to temperature.
- Special types are required for low operating pressure margin
- Performance sensitivity to deposition on the rupture disk surface
is an issue in particular services (e.g., from polymeric or other
- Potential failure at a pressure lower than its nominal bursting
pressure, particularly in liquid service or where pulsating flow can
occur, must be considered.
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.
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:
- Determine the potential for an exothermic runaway reaction.
- Assess the initiating events that could lead to an exotherm.
- Determine whether prolonged venting of the system would result in
potentially hazardous concentrations of high boiling point material in
the system or other hazardous conditions.
- Assess whether a conventional pressure-activated relief valve
system is capable of providing protection against overpressure. This
assessment may require developing a dynamic model.
- If a conventional pressure relief system cannot provide adequate
protection, consider whether a non-reclosing device (e.g., a rupture
disc) is acceptable in terms of safety and the potential for rupture
during “normal” operations. If it is acceptable, then design and
install such a system.
- If a non-reclosing device is unacceptable, then assess the
initiating event frequencies for the various scenarios leading to an
exotherm and determine what independent protection layers exist in the
- Assess whether the existing risk level is tolerable or not.
- Determine whether there are any “inherently safe” concepts that
can be applied to the process to minimize or eliminate the risk
associated with an exothermic runaway.
- If the current risk level is unacceptable, then assess the
magnitude of the risk reduction required and examine the requirements
of the ASME Code Case 2211 where applicable.
- Determine whether there is the potential for two-phase flow in any
section of the system following the opening of the relief device. If
there is, then utilize DIERS methods to size components. If redundant
relief valves are utilized, then ensure that the relief header system
and flare can handle the total flow if all relief valves open.
- Design and install a control system to provide protection against
overpressure. This will involve determining the required overall
system availability and the equipment redundancies and the test
frequencies required to achieve this overall availability. Ensure that
all components of the system are examined for their contribution to
the overall availability and that potential “common cause” issues are
reviewed. The design process should include an independent safety
- Define the criteria to be used for opening and closing the relief
valves (e.g., temperature, dT/dt, etc.).
- Define whether any other action is to be taken by the control
system (e.g., closing valves, adding quench, etc.).
- Ensure that test frequencies for equipment and systems are defined
- Ensure that all components of the relief valve and actuator are
designed to withstand the mechanical stresses imposed when the relief
valve is opened for testing when there is only atmospheric pressure
under the relief valve.
- Document all issues associated with the study to ensure knowledge
is retained within the organization and that the basis for the system
design, testing, etc. is available.
- Implement a tracking system to monitor the testing and other
requirements necessary to ensure that the required system availability
- “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.