flare header ovepressure protection hips

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Introduction

A properly designed and applied HIPS may be used to reduce loads to existing flare systems or provide additional safeguards where conventional pressure relief devices have proven to be unreliable. When units are expanded, modified, or when a new unit is being integrated into a plant, existing flare capacity may be inadequate. As plant experience increases, new, larger contingencies for flare sizing may be identified. This application brief describes a typical High Integrity Protective Systems (HIPS) to minimize overpressure events in reactors and distillation columns, and thereby reduce the frequency of relieving to the flare.

Industry design practices describe the basic principles that underlie the application of safety considerations for all new and existing plant design. Flare capacity, an essential safety design feature, is typically set by the largest single contingency for a unit. Conventional design of overpressure protection systems require additional flare capacity either by installing another flare system or reducing contingencies of existing flare systems. An alternative is to apply High Integrity Protective

System (HIPS) to reduce some single contingencies to double contingencies, thereby allowing continued operation without compromising safety, or requiring additional expansion or investment in the flare system.

Application Requirements

Overpressure protective devices and facilities are normally sized on the basis of handling the largest release resulting from a single contingency, without exceeding the design pressure or temperature of the equipment. Sizing of pressure relief devices to handle the required relieving rate is based on the various contingency considerations. Flare capacity, which is an essential safety design feature, is typically set by the largest single contingency for a unit.

A simplified typical distillation column and flare design is shown in Figure 1. During normal operation, the distillation column/tower receives feed, which is heated and vaporized in one or more reboilers, then rises through trays, where one or more distillates are drawn off. Thermal energy is introduced by controlling (as a function of product analyzer and/or column temperature and/or pressure) steam or other heat carrying media to the reboiler. Energy is removed in an overhead condenser (not shown). Should column pressure or temperature rise above the normal operating range, the control system will reduce heat addition by closing the steam control valve. Should column pressure continue to rise and approach the pressure rating of the column, a self actuated Pressure Relief Valve (PRV) opens, discharging to the flare system thereby reducing column energy and pressure. The PRV discharge

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capacity exceeds the capacity of the heat source to add energy.

The conventional practice for addressing overpressure is to install one or more PRVs on every vessel. Depending on the substance, the PRV discharges to atmosphere or closed system. The PRV is sized to provide an energy discharge path from the system that is greater than the ability to add energy to the system. Column damage will probably result if the PRV fails to open.

The flare system is designed to conservatively manage the simultaneous discharge of all four columns. If one or more column PRVs fail to open on process demand, then the pressure excursion will likely rise to excessive levels in those columns. A hazard analysis indicates the greatest risk of column overpressure damage is caused by PRV failure and the most frequent PRV demand initiator is loss of cooling to the overhead condenser.

Where temperature excursions may result in overstressing a vessel, pressure relief devices cannot preclude vessel damage. In these cases, a basis for appropriate protection in the form of high or low-temperature alarms/cut-outs, control instrumentation, isolation, depressurizing,

quenching, material selection, and/or other means must be developed.

An alternative method for addressing overpressure is to install a protective system that automatically isolates the energy source when overpressure is detected. Similar systems are currently used where PRVs provide inadequate protection, i.e. reactor high temperature and/or pressure, boiler and furnace fuel isolation systems. Instrumentation may have to be configured as a HIPS consisting of a series of redundant sensors, redundant logic control, and redundant isolation devices. (voting 2 out of 3, etc.).

To address safety and reliability, HIPS criteria include:

No New Hazards. Any changes to the plant or flare system should not increase the hazard rate or risk, and ideally should reduce hazards rate.

Maintains or Improves Plant Availability. System faults should not cause plant upsets. It should continue to operate with the presence of a single fault.

Lower Investment. Introduction of the HIPS should reduce overall plant cost.

Figure 1 Conventional Flare Design


Application Solution

Several Triconex clients have installed HIPS to prevent flare header overpressure events. Major implementation steps include:

Conceptual Design. Hazard Analysis. Detailed Design Installation

System Benefits These systems provide the following cost savings and other benefits:

A major chemical company installed a SIL-3

rated TRICON based system on a single new large distillation tower. Hazards analysis indicated the probability of overpressure was reduced to less than 10E-4, the tower relief valve was not connected to the plant flare header. Total erected cost savings exceeded $4 million.

A major chemical company installed a TRICON based HIPS on four additional distillation columns installed during a plant expansion. A hazard analysis revealed no reduction in plant integrity. Project savings exceeded $2 million.

A major refinery installed a TRICON based HIPS overpressure protection system during a recent debottlenecking expansion. Change in feed stock and a larger reboiler would have required a flare header expansion. Project savings exceeded $1 million.

A major chemical company installed a single TRICON based HIPS to provide protection for two exothermic reactors after a management of change review indicated the potential for reactor vessel damage due to increased feed supply pressure.

System Description Figure 2 depicts a distillation column design with an independent High Integrity Protective System (HIPS) installed to minimize demands on the safety valve. The HIPS is designed to equal or exceed Safety Integrity Level (SIL) 3 which is a Probability of Failure on Demand Average (PFDavg) Range of 1E-3 to 1E-4 and conform to ISA S84 Application of Safety Instrumented Systems for the Process Industries; the Draft International Electrotechnical Commission (IEC) 61508 Standard: Functional Safety: safety-related systems, Parts 1 through 7.

During normal operation, the column receives feed, which is vaporized in the reboiler, passes through the trays of the column, and one or more distillates are drawn off. Heat is introduced by controlling steam (as a function of product analyzer and/or column temperature and/or pressure) to the reboiler. Heat is removed by the overhead condenser. Should column pressure or temperature rise above the normal operating range, the control system will reduce heat addition by closing the steam valve. Should column pressure continue to rise, the HIPS will close the control valve and the Emergency Shutdown Valve (ESDV). Should the HIPS fail to initiate action, the operator may close the ESDV by depressing the Manual Trip Push-button. Should column pressure continue to rise, approach the rating of the column, a self actuated safety relief valve opens, reducing energy/pressure and discharges to the atmosphere or flare header. The capacity of the relief valve to discharge energy exceeds the capacity of the heat source to add energy.

The design objective of a HIPS is a system that can continue to provide protection even with a single component failed. It must suffer two or more faults before safety is jeopardized and in most cases before spurious action occurs. The following design, as shown in Figure 2, achieves the criteria, excepting the energy supply isolation valves. A


single fault causing a single valve to fail close will cause a spurious shutdown of the column. The HIPS is a completely independent safety layer with its own UPS power, sensors, logic cabinet and final element. It is designed to measure column pressure using three identical, but separately mounted, pressure transmitters. Each transmitter signal is read by the Safety Instrument System (SIS), scaled to engineering units, tested against setpoint and majority voted. Each transmitter signal is also tested for reasonableness and defaults to a trip vote if out of range. Each transmitter is tested against the median of the three and a diagnostic alarm sounds on 5% deviation.

The Safety Instrument System (SIS) is composed of three input channels, three microprocessor/communication channels, and three output channels, all fully integrated. The SIS continuously checks the health of each channel and compares the results of the three channels. Failure of one channel is alarmed and may be repaired on-line. Multiple channel failures result in fail safe action i.e. outputs de-energized.

When the voting logic receives two or more trip votes, SIS outputs take control of two independent, and diverse valves to isolate the heat source from the reboiler. Valves may be installed in the supply or return from the reboiler. Solenoids SOV1 and SOV2 are deenergized, isolating the air supply and venting the air off the ESDV actuator, allowing the spring to close the valve. The SIS also deenergizes Relays A and B, removing the DCS signal to the current to pneumatic converter that allows the control valve actuator spring to close the valve.

A read only serial communication link is connected to the DCS to present and log significant data such as diagnostic information, discrepancy between pressure transmitters, trip voter status, process pressure, pre-alarm and trip set points, current status, software version, etc. HIPS employs significant security features and self diagnostics to detect and annunciate faults. The system is designed for and will be thoroughly function tested every three months.

The system is tested in several ways and frequencies. In many applications, each valve

Figure 2 - Typical Distillation Column Design with High Integrity Protective System


may be fully closed and reopened within a few seconds to verify valve closure through position indicators and process flow indicators. In those applications where process dynamics preclude interrupting steam flow, a bypass valve may be opened and the valve stroked closed.

Summary

HIPS is used in the process industry as a cost-effective alternative to mechanical safety devices such as pressure safety valves with flaring. Several companies have conducted several studies to evaluate and validate the feasibility and reliability of such systems, comparing HIPS performance with traditional overpressure protection devices.

When units are expanded or modified, or when a new unit is being integrated into a standing plant, existing flare capacity may be inadequate. As plant experience increases, new larger contingencies, thereby allowing continued operation without compromising safety, or requiring additional expansion or investment in the flare system.

A properly designed and applied HIPS may be used to reduce loads to existing flare systems or provide additional safeguards where conventional pressure relief devices have proven to be unreliable.


For more information please contact any of our Triconex offices or visit our website www.triconex.com: Triconex Headquarters Irvine, California Tel: +949 885 0700 Fax: +949 753 9101

Triconex Webster, Texas Tel: +281 709 1200 Fax: +281 709 0015 Triconex Baton Rouge, LA Tel: +225 297 5231 Fax: +225 293 0539 Mexico Regional Office Tel: +52 52 63 0123 Fax: +52 53 95 2463

Triconex Singapore Pte., Tel: +65 738 5488 Fax: +65 738 5188 Triconex Solutions, S.A. Cedex, France Tel: +33.1 34 43 26 26 Fax: +33 1 34 43 26 27 Triconex Middle East Dubai, UAE Tel: +971 4 3314 222 Fax: +971 4 3314 666

Triconex Moscow Tel: +70 95 232 05 68 Fax: +70 95 232 05 67 Triconex Saudi Arabia Tel: +966 3 894 0087 Fax: +966 3 895 0050 Triconex Ltd. United Kingdom Tel: +44 1293 574 800 Fax: +44 1293 574 840

About Triconex Triconex is an operating unit of the Invensys Process Systems (IPS) Group and is a global leader in the supply of products, systems, and services for safety, critical control, and turbomachinery applications. Since its inception in 1983, the company has installed thousands of control systems solutions in a wide variety of industries and applications worldwide. Triconex products are based on patented Triple Modular Redundancy (TMR) technology. Today, Triconex TMR products operate in over 4,000 installations throughout the world, making Triconex the largest and most successful TMR supplier in the world. In January 2002, the TRICON Version 9 became the first TMR system to be approved by the U.S. Nuclear Regulatory Commission for use in 1E nuclear power plants. For more information, visit the Triconex home page at http://www.triconex.com

About Invensys Process Systems Invensys Process Systems is part of the US-based Process Management Division of Invensys plc. Invensys Process Systems automation solutions provide measurable performance improvements across a broad spectrum of customer industries. Invensys Process Systems includes focused business units such as Invensys Process Solutions, plus well-known brands such as APV, Avantis, Esscor, Foxboro, Pacific Simulation, Simulation Sciences, Triconex and Walsh Automation.

About Invensys Invensys plc is one of the global leaders in automation and controls. Headquartered in London, England, Invensys operates in all regions of the world through 2 focused divisions Process Management, Energy Management. With just over 75,000 employees, the Group supplies a wide range of products and services, including advanced control systems, remote diagnostics and energy management for process plants, factories, and commercial environments; electronic devices and networks for residential buildings; as well as complete power systems for the industrial, telecommunications, and information technology sectors. For more information, visit the Invensys home page at http://www.invensys.com

2002 Invensys Systems Company. All Rights reserved.

15345 Barranca Parkway Irvine, CA 92618 USA Telephone: 949.885.0700 Fax: 949.753.9136 http://www.triconex.com Document Number: TMCAN12 Issued: 4/02

flare header ovepressure protection hips

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