Overpressure Protection System Compared: PSV or HIPPS?

Related Vendors

DÜPERTHAL Sicherheitstechnik GmbH & Co. KG

Italvacuum, Srl

Donadon SDD Srl

Level of performance of an HIPPS is expressed in terms of Safety Integrity Level (SIL) which is described in the following paragraphs. Higher SIL means higher level of performance. The following are the main applications of HIPPS:

• To eliminate a particular overpressure scenario from the design basis of a safety system. For example, an HIPPS can be applied to eliminate overpressure scenarios of closed outlet, control valve failure, etc.

• To replace a PSV or reduce its capacity. Whereas an HIPPS can replace a PSV in certain overpressure scenarios, it cannot in certain other scenarios such as fire. However, in latter cases, it can be used to reduce PSV capacity.

• To reduce the capacity of disposal system (e.g. flare system) by eliminating need for or reducing the capacity of a PSV.

• To provide better risk reduction than a PSV, if required, by reducing its PFD to less than that of the PSV. Such a higher risk reduction may be required in high-risk process units e.g. those involving highly toxic materials.

• To provide additional risk reduction by using HIPPS in combination with a PSV.

• To provide protection against overpressure where a PSV is ineffective or impractical.

Advantages of HIPPS

Following are the advantages of HIPPS over PSV:

• Whereas a PSV protects against over-pressurization by releasing the excess fluid from the system, an HIPPS protects the system by cutting-off the supply of fluid to the system. Thus, a PSV may necessitate installation of facilities for disposal of the fluid contained in the system e.g. a flare system, whereas no such facility is required in case of an HIPPS. Thus, the use of HIPPS can reduce the cost of flare system substantially.

• A revamp of an existing plant may require capacity expansion of existing flare system if PSV is installed. This may involve substantial cost addition. The use of HIPPS instead of PSV may lead to substantial cost saving by eliminating the need for increase in capacity of existing flare system as it does not involve any fluid release. Moreover, capacity expansion of existing flare system may be impractical due to plot size limitation. In such a situation, HIPPS may be the only option available as a means of over-pressure protection.

• While PSV’s operation may cause harmful impact on environment by releasing greenhouse gases in atmosphere, HIPPS protects environment by not releasing any emission in the atmosphere. Thus, application of HIPPS may get preference over PSV where stricter environmental laws are in force.

• HIPPS can provide better risk reduction than PSV by increasing its performance level. Performance level or SIL of an HIPPS can be increased by increasing the redundancy level of sensor and final control element and/or SFF of its components, or reducing test interval. For example, performance level of an HIPPS can be increased from SIL-1 to SIL-2 by increasing number of sensors as well as shut-down valves from one to two with 1 out of 2 (1oo2) voting with SFF 60 % to 90 %. Similarly, performance level can be increased from SIL-2 to SIL-3 by increasing number of sensors as well as shut-down valves from two to three with two out of three (2oo3) voting with SFF 60 % to 90 %, or by increasing SFF to >90 % and keeping the number of sensors as well as shut-down valves as two only (1oo2 voting). An HIPPS with SIL-3 can provide higher risk reduction than a PSV whose probability of failure on demand is typically 0.01. Thus, HIPPS can be used for risk reduction of high-risk process units such as those involving highly toxic materials.

• When installed in combination with PSV, HIPPS can reduce the risk to extremely low levels.

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