High Integrity Pressure Protection Systems (HIPPS)
By now if you have been working in the process industries (like chemicals, oil & gas, petrochemicals and so on) for some time, you must have come across the term HIPPS. What is it? It is an acronym for High Integrity Pressure Protection Systems. These protection systems can be considered to a special subset of Safety Instrumented Systems (learn more about them here), that are meant to provide protection to pressurized equipment (tanks, pipelines and so on) against overpressure and consequent rupture.
Traditional Pressure Protection for equipment
Traditionally pressure protection for equipment was implemented using two main devices, rupture disks (also known as burst disks) and safety relief valves. There are three methods of overpressure protection, namely the burst disk, the relief valve and the third method uses a combination of the first two along with a pressure switch or transmitter.
The burst disk, as the name suggests, is a contraption that bursts under overpressure conditions to release the pressure inside the equipment on which it is mounted and protect the equipment itself from bursting. Therefore the burst disk always bursts at a slightly lower pressure than the equipment's design pressure. Conceptually, it is similar to a fuse that blows in an electrical circuit if excess current flows in the circuit. The fuse gets destroyed but the other expensive and/or critical electrical/electronic devices in the circuit are protected.
In case of mechanical safety relief valves, excess pressure in the equipment lifts the valve and the excess pressure is relieved. As soon as the excess pressure is vented, the valve shuts and maintains the pressure inside the equipment. The advantage is that the safety relief valve is not a "single-use" item, it can operate even several times in a hour, in case a process upset results in excess pressure conditions in the equipment. Conceptually it is similar to a circuit breaker which trips in case of overcurrent, but can be used again and again in the same circuit to protect critical and/or expensive electrical/electronic devices.
The third method is a combination of the two devices, a rupture disk and a mechanical safety relief valve. The selection is done such that the rupture disk bursts first (at a certain pressure). The outlet part of this is connected to a pressure switch or a transmitter, that sends an alarm signal to the control room indicating that the rupture disk has burst. The operator is supposed to respond, to tackle the excess pressure in the system. If he does not, or cannot then, when the pressure becomes higher the mechanical relief valve gets actuated.
The traditional methods have worked for a number of years successfully. In fact most plants today use MAINLY the above two methods of pressure protection for equipment. Of course many plants do have pressure transmitters and pressure switches to generate alarms in case of excess pressures, but they are mainly meant for operator intervention and not for automatic pressure relief.
Drawbacks of the present mechanical systems
Despite the fact that these traditional methods form the bulk of overpressure protection, does not mean that that users are very happy with them. There are several drawbacks in these methods; these are given below:
a) Modifications in existing plants-lack of flare capacity
Many times, expansion or debottlenecking projects in plants do not correctly estimate the additional load on flare systems, due to increased throughput. This point only comes up during the HAZOP and other safety and environmental studies. The increased investment in flare headers and the shutdowns that may be necessary to increase the flare capacity, are simply not possible in today's tight economic situations. This is where HIPPS is an alternative. Instead of merely venting the gases to relieve overpressure, a fast acting HIPPS simply shuts down the inlet valves to prevent overpressure from building up in the first place.
b) Environmental Release & Personnel Injury
If a burst disk or relief valve opens, it may discharge environmentally unfriendly substances into the surrounding air, leading to consequential problems of spillage, pollution, environmental damage or personnel injury. In many older plants, many processes are still operated manually which means more humans are working around pressure vessels. Any release of flammable or toxic contents into the surroundings may also endanger the personnel working in the area.
c) Fire and Explosion
If the released material is an explosive material then releasing large quantities of it through relieving devices can increase the risk of fire or explosion. It may also endanger the personnel who may rush to that area to replace the burst disk.
d) Replacement problems
When a burst disk operates, the equipment of part of the pipline gets shutdown. Replacing it and starting the plant again entails a lot of use of resources. Sometimes, it is challenging to find out why a burst disk burst at a particular time and place. Nowadays many times the third method is used (combination of rupture disk, relief valve and pressure switch) . Even then, it may be difficult to find the root cause of the bursting. It may be due to a genuine overpressure condition, or it may be that the particular burst disk operated a slightly lower pressure than it was designed to-it is difficult to pinpoint. Not all rupture disks operate at the exact pressure that they are designed to burst at.
The HIPPS solution
To avoid all these problems, users today are now actively accepting the HIPPS alternative. A typical HIPPS is shown below. In case of overpressure in the line, the quick shutoff valves close the line in a matter of seconds, preventing the overpressure condition from traveling further downstream. Note that the HIPPS shown below can be considered as a special case of a Safety Instrumented System. Note that these systems can have Safety Integrity Level ratings pretty high, like SIL 3 or even SIL 4. This is because the risk reduction that is sought to be achieved by using these are very high. Sometimes these systems are installed in subsea environments, where they MUST operate reliably.
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