Pressure safety for the energy and power generation

Paul Boughton

Continuity in power production and energy distribution is critical for the sustained economical growth of any industrial region. Roger Bours reports.

The consideration of protecting critical assets such as power generators, steam circuits, transformers, switchgear, heat exchangers, etc, against unacceptably high internal pressures or explosions is necessary and well introduced into today's risk reduction methodology.

Due to the high-investment costs and long delivery times of the related process equipment, combined with the induced risks related to the presence of concentrated energies, the incidental occurrence of unacceptably high overpressures must be considered.

Such overpressure situation would put the continuity of energy supply at risk and would create unsafe situations for the employees working in the energy industries.

Controlling the energy

Steam-driven energy cycles are inherently linked with pressure risks in the (high-pressure) steam circuit equipment such as steam condensers, heat exchangers and boilers.

Due to the high levels of stored/produced energy damages resulting from such overpressure incidents may be devastating, with injuries or fatality risks being very high and loss of production capability for long repair or replacement periods being eminent.

Biofuels

Where biofuels are handled (coal, biomass, peat) the storage, handling and transportation of such combustible materials must be protected against the effects of dust explosions through the use of industry accepted protection measures.

Suitable protection techniques with proven reliability and performances are available; they include (explosion) pressure relief through the use of rupture discs or explosion venting devices, explosion suppression and isolation measures for coal or biomass milling (when used in co-firing), storage facilities (silos) and bulk handling (transportation and drying).

First line of defence

As a first line of defence pressure control systems are typically used. These monitor the pressure developments in the process equipment and will interact timely with the process control system to limit the pressure to acceptable levels.

As in most cases pressure control systems may not assure the required level of reliability in all service conditions, the use of pressure relief systems is often required.

To attain the required safety against pressure risks the industry has been using pressure relief devices. Such pressure relief devices are categorised in reclosing and non-reclosing types, both offering unique characteristics making the selection process for the design engineer possible.

Relief device options

The industry has been working traditionally with (reclosing) relief valves or (non-reclosing) rupture (or bursting) disc devices to achieve pressure relief action. Both types are accepted for use as independent relief devices, protecting the installations against unallowable pressures.

Reclosing pressure relief devices, commonly referred to as safety relief valves (SRV), pressure relief valves (PRV) or relief valves (RV), are designed to provide opening for pressure relief at the selected set pressure, allow for the overpressure to evacuate and will then reclose when the pressure has dropped below an acceptable level.

Rupture or bursting disc devices are often preferred to achieve instantaneous and unrestricted pressure relief (both overpressure and vacuum pressure).

These disks are designed to provide a leak-tight seal within a pipe or vessel, until the internal pressure rises to a predetermined level.

At that point the bursting disc ruptures, preventing damage to the equipment from overpressure. They consist of a calibrated metallic membrane which will rupture when the set pressure is achieved.

Discharge of pressure

After activation the membrane will remain open resulting in a complete discharge of the pressure from the installation.

Fike offers a complete line of cost-effective bursting discs, holders and customer pressure relief devices which are compliant with global code regulations and designed to meet or exceed industry requirements for performance, reliability, and quality.

On a large number of occasions rupture discs are selected to be used in combination with relief valves. Such combinations can be either in parallel or in series, both of which offer to the user the choice of a combination of features which provides the 'best of both worlds'.

- Case 1: Rupture Disc in Parallel with Relief Valve. When used in parallel, the main objective of the designer/user will be to allow for the relief valve to handle the overpressure situation, bleed the pressure until an acceptable pressure is achieved and allow for the process to continue.

Where the overpressure cannot be effectively reduced by the relief valve, the pressure may continue to rise until the (higher) set pressure of the rupture disc is reached.

Upon activation the rupture disc will provide an additional/back-up relief path for the overpressure, resulting in a safe situation.

- Case 2: Rupture Disc in Series with Relief Valve. Rupture disc devices may be installed upstream or downstream of relief valves, each geometry offering its particular benefits for the user.

A rupture disc upstream of relief valve prevents plugging, corrosion or leakage and allows for in-situ testing.

A rupture disc downstream of relief valve prevents corrosion, fouling or sticking of the relief valve: prevents variable superimposed backpressure affecting relief valve; detect opening or leakage of relief valve.

Conclusions

Pressure relief solutions are common in industry processes, to assure that the investments are safe and a safe working environment is presented to the employees.

The use of rupture discs in combination with relief valves can be done in several combinations. It offers a wide range of benefits to the user and for the environment, cost reduction, emission control, higher safety and reliability levels and improved performance of the plant safety systems are a direct result.

Process system designers need to evaluate the individual effects and make a selection of what geometry applies best for the individual plant requirements.

In most applications the combined solution of rupture discs and relief valves offer customers more value for more benefits - a true definition of 'more for more'.

For more information at www.engineerlive.com/ipe

Roger Bours is Manager Pressure Relief, Fike Europe, Herentals, Belgium. www.fike.com

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