7 HIPPS Logic

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Traditional systems

In traditional systems over-pressure is dealt with through

relief systems. A relief system will open an alternative outlet

for the fluids in the system once a set pressure is exceeded,

to avoid further build-up of pressure in the protected system.

This alternative outlet generally leads to a flare or venting

system to safely dispose the excess fluids. A relief system

aims at removing any excess inflow of fluids for safedisposal, where a HIPPS aims at stopping the inflow of

excess fluids and containing them in the system.


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Conventional relief systems have disadvantages such asrelease of (flammable and toxic) process fluids or their

combustion products in the environment and often a large

footprint of the installation.

With increasing environmental awareness, relief systems are

not always an acceptable solution. However, because of their

simplicity, relatively low cost and wide availability, conventional

relief systems are still often applied.


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Advantages of HIPPS

HIPPS provides a solution to protect equipment in cases

where:

high-pressures and / or flow rates are processed.

The environment is to be protected.

The economic viability of a development needs improvement

The risk profile of the plant must be reduced.


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HIPPS is an instrumented safety system that is designedand built in accordance with the IEC 61508 and IEC 61511

standards.

The international standards IEC 61508 and 61511 refer tosafety functions and Safety Instrumented Systems (SIS)

when discussing a device to protect equipment, personnel

and environment.

Older standards use terms like safety shut-down systems,

emergency shut-down systems or last layers of defence.


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Components of HIPPS

A system that closes the source of over-pressure within 2

seconds with at least the same reliability as a safety relief

valve is usually called a HIPPS. Such a HIPPS is a

complete functional loop consisting of:

Sensors, (or initiators) that detect the high pressure,A logic solver, which processes the input from the sensors

to an output to the final element,

Final elements, that actually perform the corrective action

in the field by bringing the process to a safe state. In caseof a HIPPS this means shutting-off the source of

overpressure. The final element consists of a valve,

actuator and solenoids.


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Example of a HIPPS system

The scheme above presents three pressure transmitters (PT)connected to a logic solver. The solver will decide based on

2-out-of-3 (2oo3) voting whether or not to activate the final

element.

The final elements consist here of two block valves that stop

flow to the downstream facilities (right) to prevent them from

exceeding a maximum pressure.

The operator of the plant is warned through a pressure alarm

(PA) that the HIPPS was activated.


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This system has a high degree of redundancy:

Failure of one of the three pressure transmitters will not compromise

the HIPPS functionality,

as two readings of high pressure are needed for activation.

Failure of one of the two block valves will not compromise the HIPPS

Functionality, as the other valve will close on activation of the HIPPS.


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One must not confine self to the above design as

the only means of materializing the HIPPS

definition. One must always think of the HIPPS

generically, as a means of isolating a source of a

high pressure when down stream flow have been

blocked, isolating the upstream equipment (source

of the high pressure) in a highly reliable manner.


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Be this source of the high pressure a pump (in case of

liquid) or a gas compressor (in case of gas), the aim of

the HIPPS in these cases is to reliably shut down the

pump or the gas compressor creating the high

pressure condition in a reliable and safe manner.


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Standards and design practices

The ever-increasing flow rates in combination

with the environmental constraints initiated the

widespread and rapid acceptance in the last

decades of HIPPS as the ultimate protection

system.


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The International Electrotechnical Commission (IEC) has

introduced the IEC 61508 and the IEC 61511 standards in 1998

and 2003. These are performance based, non-prescriptive,

standards which provide a detailed framework and a life-cycle

approach for the design, implementation and management of

safety systems applicable to a variety of sectors with different

levels of risk definition. These standards also apply to HIPPS.


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The IEC 61508 mainly focuses on electrical/electronic/

programmable safety-related systems. However it also

provides a framework for safety-related systems based on

other technologies including mechanical systems. The IEC

61511 is added by the IEC specifically for designers,

integrators and users of safety instrumented systems and

covers the other parts of the safety loop (sensors and final

elements) in more detail.


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The basis for the design of your safety instrumented system is

the required Safety Integrity Level (SIL). The SIL is obtainedduring the risk analysis of a plant or process and represents the

required risk reduction. The SIS shall meet the requirements of

the applicable SIL which ranges from 1 to 4. The IEC standards

define the requirements for each SIL for the lifecycle of theequipment, including design and maintenance. The SIL also

defines a required probability of failure on demand (PFD) for

the complete loop and architectural constraints for the loop and

its different elements.


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The requirements of the HIPPS should not be

simplified to a PFD level only, the qualitative

requirements and architectural constraints form an

integral part of the requirements to an instrumented

protection system such as HIPPS.


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The European standard EN12186 (formerly the DIN G491)

and more specific the EN14382 (formerly DIN 3381) has

been used for the past decades in (mechanically)instrumented overpressure protection systems. These

standards prescribe the requirements for the over-pressure

protection systems, and their components, in gas plants. Not

only the response time and accuracy of the loop but also

safety factors for over-sizing of the actuator of the final

element are dictated by these standards. Independent design

verification and testing to prove compliance to the EN14382standard is mandatory. Therefore the users often refer to this

standard for HIPPS design.

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7 HIPPS Logic