High integrity pressure protection system (HIPPS) provides a novel approach to protect Process systems from over-pressurization. It is a safety instrumented system that provides protection to Process systems including pressure vessels and piping systems against over-pressure. The over-pressure may be caused due to upset conditions such as control valve failure, blocked outlet or other. Such upset conditions cause unbalanced flow of energy and material leading to accumulation of material or energy in the system giving rise to pressure rise situation. Over-pressure protection can be provided either by mechanical means or by instrumented means. PRV is a mechanical device whereas HIPPS an instrumented device to achieve the same objective i.e. over-pressure protection.
Designed and built according to International Electrotechnical Commission (IEC) standard 61511 (Functional Safety-Safety Instrumented Systems for the Process Industry Sector), an HIPPS consists of a pressure sensor, a logic solver and a shutdown valve as its main components. Pressure sensor and shutdown valves are generally provided with redundancy to achieve desired safety integrity level (SIL). Generally, sensors are provided with 2 out of 3 configuration and shutdown valves 1 out of 2 (graph 1). Whenever pressure of the system reaches a pre-set value which is less than or equal to the design pressure of the system, the logic solver closes the shut-down valve at the inlet i.e. cuts-off the source of over-pressure and thus protects the system from over-pressurization.
Graph 1: Components of HIPPS
Notes:
i) SDV-shutdown valve, PT-pressure transmitter
ii) 1oo2-one-out-of-two voting architecture; 2oo3-two-out-of-three voting architecture
HIPPS can act as a substitute of a pressure relief valve (PRV) or can supplement it depending upon the upset scenarios causing over-pressure. For example, if one of the over-pressure scenarios is fire, the need for the PRV cannot be done away with as HIPPS cannot be provided for fire scenario. In such cases, PRV can be provided for fire scenario and HIPPS for remaining scenarios (such as blocked outlet and control valve failure).However, if HIPPS has toact as a substitute for a PRV, its SIL rating must be at least 2 i.e. its probability of failure on demand (PFD) should not be more than 0.01 as PFD of a PRV is typically 0.01.
Moreover, an HIPPS with SIL-3 can provide higher risk reduction than PRV. Thus, HIPPS can be used for risk reduction of high-risk process systems such as those involving highly toxic material.Further, HIPPS can provide additional risk reduction in combination with PRV where extremely tight risk tolerance is required.Where stricter environmental regulations are in force, HIPPS may be the only option as PRV’s operation may cause harmful impact on environment by releasing greenhouse gases through flare in atmosphere. HIPPS is particularly effective in revamp of an existing plant or unit where installation of additional PRV is constrained by existing flare capacity and providing new flare system or enhancing capacity of existing one is impractical or infeasible. A comparison of HIPPS and PRV can be found in table 1.
Table 1: Comparison of HIPPS and PRV
| Sr. No. | PRV | HIPPS |
|---|---|---|
| 1 | PRV is a mechanical safety device | HIPPS is a safety instrumented system. |
| 2 | PRV provides protection against overpressure by releasing the excess fluid from the system | HIPPS provides protection against overpressure by closing source of overpressure. |
| 3 | Activation of PRV leads to discharge of contents from the system without shutting it down. For the same reason, it causes loss of containment that could be valuable. | Activation of HIPPS may lead to shutdown of the system. There is no loss of containment. |
| 4 | PRV can be used for every commonly known over-pressure scenario. | HIPPS cannot be used for every over-pressure scenario e.g. external fire. |
| 5 | PRV requires a disposal system e.g. a flare system for disposing the discharged fluid from the system. | HIPPS does not require any disposal system. |
| 6 | Testing and maintenance requirement for PRV is relatively less frequent. | Testing and maintenance requirement for HIPPS is frequent. |
| 7 | No similar means are available to lower PFD of a PRV. Thus, PFD of a PRV once designed remains fixed. It is typically 0.01. | SIL of an HIPPS can be increased or PFD decreased by increasing the redundancy level of its components (sensor and shut-down valve) and/or safe failure fraction, or decreasing test interval1,2. |
Notes:
1. According to IEC standard 61511-2, safe failure fraction (SFF) of a component denotes its capability in terms of the extent to which the faults lead to safe condition or can be detected by diagnostics so that a specified action can be taken.
2. In order to ensure that an HIPPS performs its function as per required SIL, its components have to be tested regularly. Such a test is called functional test or proof test and the interval between the two successive functional tests or proof tests is called the test interval.
About Author: Satyendra Kumar Singh, B.Tech. (Chemical Technology) + M.B.A., is proprietor of Satsha Management Services-an award winning design engineering and management consulting company (www.satshamanagement.com). He possesses approximately 30 years’ experience in engineering consultancy in process and energy industries. Satyendra has authored several papers on energy, business and management, which have been published in some renowned journals/magazines such as ‘Chemical Engineering’, ‘Process Worldwide’, ‘Modern Manufacturing India’. He may be reached at satyendra.singh@satshamanagement.com, Ph. +919811293605.
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