Increasing the capacity or yield of existing chemical process sites through expansion or upgrades is often a cost-effective means of increasing capacity without incurring the higher costs associated with embarking on a new greenfield construction. In order to complete upgrades to existing facilities successfully, the works required must be given careful design consideration with particular attention given to process engineering and plant safety. Large-scale projects may require considerable alterations to plant layouts and complex engineering alterations that can pose quite challenging considerations from a safety viewpoint. Indeed, the safety implications can often be as challenging as those posed by a greenfield site development.

Small-scale upgrades by their very description may seem minor however, they can also pose considerable safety design challenges and implications. 

In an effort to mitigate risk and reduce the incidence of serious hazards, it is crucial to examine and fully understand some common pitfalls that can contribute to major accidents at chemical process plants.  

Important Systems

Relief-valve systems: Relief valve devices are essential to maintaining plant safety and avoiding potentially devastating accidents. Unfortunately, however, proper sizing and implementation of safety relief valves is sometimes misunderstood or overlooked. When existing process plants expand, the additional throughput capacity that must be handled by the relief valves in the facility can often push the limits of the original design. To compensate for this, it may be necessary to replace existing relief valves for new devices. In some cases, valve replacement is not enough. If an existing relief valve is replaced with a new device and the existing inlet or outlet piping size and configuration is retained, it can sometimes create a bottleneck around the new device. Furthermore, the existing relief collection header piping systems may be inadequate for the new higher-pressure levels which are likely to be encountered by the upgraded system. A thorough review of the entire plant is therefore necessary to determine exactly what needs to be replaced and how it should be completed.

When carrying out upgrade works on an existing process plant, consideration should also be given to relief valve systems which are not in the immediate location of the works. Relief valve systems which are located hundreds of meters away can also be impacted by the works and therefore, should not be overlooked as part of any design. If any relief valve is inadequate or fails to meet the proper specifications, the effects could be disastrous.   

As part of any design or upgrade works, there can sometimes be a tendency to overestimate the capabilities of a particular piece of equipment. As an example, equipment such as a boiler might be capable of handling an increase in pressure as the result of a new development. However, the existing connected piping or valves attached to the boiler may not be able to handle the pressure increase. 

It is recommended that a full and complete audit of existing and any proposed new relief-valve systems should be a part of any plant modification, expansion or upgrade. It is vital that plant operators collaborate with contractors to ensure that no part of the audit is overlooked, misunderstood or rushed. Problems with safety relief valves can potentially be one of the most devastating and dangerous problems encountered at a chemical process plant.

Pipe racks: Existing piping should be thoroughly assessed and reviewed during any type of plant upgrade or expansion. In a mature plant, pipe racks can often exhibit little or no room for expansion. If the existing facility has in the past undergone modifications, it can be commonplace to discover that some of the existing pipe racks are overloaded over and above their original rated design capacity. This can pose a serious safety risk. For this reason, it is always prudent to add additional pipes to increase flow. 

Any limitations with existing pipe racks should if possible be identified early in the design process so that there are no unexpected costs for rack expansions. 

Piping: The presence ofdead-leg pipes can pose a potential hazard. These are pipes which have been disconnected from old redundant process systems and have been left behind in-situ. Dead-leg pipes can pose a safety risk and furthermore, take up unnecessary space on pipe racks. Any stagnant liquids left in the disconnected dead-leg pipes can lead to pipe corrosion or rupture in the event of the liquid freezing. 

Process plants often have dead-leg pipes that have not been part of an active process for years. As dead-legs increase in number around the site, the risk to the plant multiplies with them. When dealing with fluids such as hazardous chemicals, even small amounts of pooling fluid in dead-legs can lead to significant ruptures or leaks. Process plants should have regular management programs in place to inspect piping. While there are scenarios where it may make sense for a dead-leg to remain, any dead-legs deemed unnecessary should be carefully drained and removed from site. It is good practice to include these works as part of any plant expansion or modification projects.

Piping and pipe racks can be found everywhere in chemical process plants. It only takes a problem in one of those pipes for a safety incident to occur. For this reason, it is 

recommended that a thorough inspection of existing pipes and pipe racks is necessary as part of any planned expansion or modification. 

Access and confined-space issues: The addition of new equipment onto an existing plant can have a significant impact on plant layout. Large vessels installed to increase output can also create access/confined space hazards. In general, placement of new equipment can create unsafe confined spaces or limit access to existing critical plant infrastructure. While a certain number of confined spaces in a chemical process plant may be unavoidable, designs should try to minimize them as much as possible. 

During the construction works, there are often temporary installation requirements which can restrict access to certain areas of the existing plant. Any additional equipment that is necessary to complete a project, such as scaffolding, should be placed carefully to ensure that it does not obstruct  or restrict vital access areas. 

Even under the best of circumstances, confined spaces can be extremely dangerous. Potential safety hazards associated with confined spaces include exposure to hazardous chemicals, the ignition of flammable liquids or gases and asphyxiation.

Labelling/Identification (especially for chemicals and piping): Having proper equipment labelling and documentation is the simplest and the least costly way to improve safety. Unfortunately, it can also be one of the most overlooked ways of improving safety. Further information on labelling can be found in the ANSI/ASME A13.1 pipe-marking standard. 

Conclusion

This blog attempts to outline some items for consideration when expanding or upgrading an existing chemical process plant. It by no means covers all aspects of any design of such a facility but more attempts to document a few items worth considering as part of this type of development. 

A previous blog on relief valve sizing may also be of interest as it also addresses some interesting points in pressure relief devices. 

References:

1. www.chemengonline.com