Safety/Overpressure Protection Overpressure Protection System Compared: PSV or HIPPS?
Protecting process systems from overpressure scenarios — Protecting valuable equipment and piping from overpressure is usually achieved by pressure safety valves. Yet, sometimes, these measures face limitations due to facilities constraints and other factors. High Integrity Pressure Protection Systems offer an alternative approach.
The effective design of a pressure vessel and piping system entails that all components operate safely and according to their design objectives under all the conditions, including upset conditions. The upset conditions such as fire, blocked outlet, control valve failure and others can lead to excessively high pressure in the system.
When the pressure due to such an incident exceeds the design pressure of the system, it can cause the rupture of the vessel or piping which in turn can cause severe hazard for human life, plant assets or the environment. Therefore, it is essential to protect the system from the effects of over-pressure.
There are two ways of protecting the system from overpressure. One, by using mechanical devices such as a pressure safety valve (PSV). Other, by using a safety instrumented system such as high integrity pressure protection system (HIPPS). An HIPPS protects the pressure vessel and piping systems by removing the source of overpressure when pressure in the system reaches a pre-set value. This value must be less than or equal to the design pressure of the system.
For example, if a pressure vessel is pressurized by an incoming stream of a gas, it can be exposed to a pressure greater than its design pressure under certain upset conditions as discussed above if the source-pressure of the incoming stream is higher than the said design pressure.
Providing an HIPPS will close the incoming stream as soon as the pressure of the vessel reaches a predetermined value which is less than or equal to the vessel’s design pressure.
An HIPPS is designed and built according to the International Electrotechnical Commission (IEC) standard 61511 (Functional Safety — Safety Instrumented Systems for the Process Industry Sector), part 1, part 2 and part 3. It consists of pressure sensors, logic solver(s) and shut-down valves as main components. The shut-down valves remain open as long as pressure sensors sense a value less than the predetermined value (that corresponds to less than or equal to design pressure of the system). As soon as pressure sensors sense a value equaling the predetermined value, the logic solver based on the signal from the pressure sensors gets activated and closes the shutdown valves. Pressure sensors and shut-down valves are generally provided with redundancy to meet the required level of performance of the HIPPS (Fig. 1).