There is no chemical process facility invincible to the dangers of pressure over-pressure, and it is important to not dictate the need for protection against over-pressure. For any situation that requires secure containment of process gas there is a requirement for engineers to also provide flaring and pressure relieving options whenever necessary.
The levels of security are ordered, beginning with the design of an inherently secure method to avoid pressure overloads, and then providing alarms to operators to intervene, and then emergency shutdown provisions via ESD and SIL certified instruments.
Beyond these instrument and design-based security measures, the concept of abatement and containment measures like flares for pressure relieving dikes, physical dikes and Emergency Response Services is employed.
Within the industry of oil and natural gas, the process installations are frequently subject to fluctuating wellhead pressure and flow patterns. These systems of process are now being maintained for protection against overpressure through the installation of high integrity pressure protection system.
A typical high integrity pressure protection system architecture is composed of 3 (3) Pressure transmitters (PT) which continuously record the pressure of the line that is fed to an algorithmic solver. If there is an over-pressure the logic solver triggers the shutdown process of two (2) consecutive Fail-close (FC) valves that are placed in the same pipeline, thereby stopping the flow of fluid.
The pressure alarm (PA) is used to serve the purpose of informing the workers.
The reason for installing the required number of valves and transmitters are as follows
To prevent compromising high integrity pressure protection system capability due to the malfunctioning of a single shut-off valve (SDV) A second valve is installed to offer greater redundancy. Both valves operate on an 1oo2 vote system that determines which fail-close (FC) valve is shut.
To ensure that there isn’t a premature (or false) signal by the pressure transmitter two-oo3 voting is used in place of one of the voting principles. This means that unless two pressure transmitters agree with each other that an overload event, high integrity pressure protection system is not activated.
HIPPS Valve Selection
high integrity pressure protection system Valves are operated by hydraulic or solenoid mechanisms. Two (2) kinds of valves are butterfly or ball type. Ball valves are the best shutdown conditions and range between 2 inches and 56 inches, based of the brand. Butterfly valves are supplied from 2 inches to 100 inches, subject to the manufacturer.
For HPHT applications the piping class may be to 2500#, which is supplied within the ball classification selection with material that ranges from carbon steel to stainless steel duplex, as well as special alloys.
The average stroke time for high integrity pressure protection system valves must be approximately 2 sec. The selection of valves should also take into account the fact that HPHT applications may experience temperatures that can reach 500 deg.C. HPPS valves should also be able to accommodate Partial Stroke Testing capabilities (PST) and the Tight Shutoff (TSO) (e.g. class V, or Class VI in ANSI FCI 70-2), Rapidly acting and tested for Fire Safety to such standards as API 607. Environmental requirements must be adhered to for emissions that are fugitive, such as ISO 15848-1 specifications.
HIPPS Engineering Standards
High integrity protection systems are able to handle a variety of applications, including offshore and onshore well heads flare headers, well heads and chemicals process industries. ASME Section VIII the UG-140 (Over-pressure Safety Systems) offers a variety of uses for which HIPPS are suitable for, including,
Very high Propagation Chemical reactions that lead to the loss of confinement prior to relief device opening , or processes that result in unpractical large venting areas
Runaway Polymerization Reactive, Exothermic or Exothermic reactions that result in huge vapour volumes, making relief devices inadequate to respond to situations of over-pressurization.
To keep the piece short the article focuses of Oil and Gas applications. HIPPS system specifically for use in the oil & gas engineering company are founded on two elements which are performance and prescriptive.
Standards like API, ASME, ANSI to name a few are designed for manufacturing and implementation. Some examples include API 14C (Recommended Methodology for Analysis and Design, Installation as well as Testing of Basic Surface Safety Systems for Offshore Production Platforms), API 6A (Specification for Wellhead and Christmas Tree Equipment) for offshore applications, API 520/521, API 521/521 API 17O (Subsea High Integrity Pressure Protection Systems – high integrity pressure protection system) to mention some.
Another aspect can be found in that of the IEC standards, primarily IEC 61508 and IEC 61511 that are more of performance-based standards that outline how to come to the best solution, rather than providing an answer.
This will leave an opportunity for clarification between suppliers, contractors, and operators and result in a the absence of widely accepted industry-specifications. IEC 61508, for instance. IEC 61508, as an instance, is focused on the operation of the logic solver, not much on the control element that is final.