Oct-2015

Fired capital equipment: key plant components

Fired capital equipment is an often neglected asset associated with plant operations. The global petrochemical and refining industries remain volatile as natural gas and oil prices fluctuate.

CARL RENTSCHLER, HARALD RANKE and GOUTAM SHAHANI
Linde Engineering North America

Article Summary

In recent years, the shale gas discovery created an industry boom as prices returned to pre-2000 levels. This led to a positive impact in the refinery and petrochemical industries as owners moved to take advantage of this attractive fuel and chemical feedstock. The industry hit an unanticipated snag as oil prices plummeted by over 50% in the second half of 2014 and into early 2015. Companies pulled back on planned projects as they assessed the likely trend for the future. The message learned from this industry volatility is that trends can change suddenly, and the outcome is not always predictable.

The challenge for the petrochemical and refinery industries is to effectively manage their assets and planned expansions through the ups and downs. This means that equipment must be economically integrated into the plant, keeping in mind that the lowest total cost is not always the lowest upfront cost. Even the smallest equipment can have a huge impact on plant economics, and this makes it necessary to carefully integrate all components. This is best done by companies that offer expertise in all components and equipment of a plant.

An often neglected asset is the fired capital equipment associated with a plant. Fired capital equipment can represent relatively minor capital costs of refineries and chemical plants, and is often overlooked as a major contributor to plant life operating costs. In fired capital equipment we are referring to hydrogen reformers, ethylene cracking furnaces, fired heaters and oxidisers. These components are often ‘life organs’ of a facility, yet they do not receive the attention deserved as major contributors to the profitability of a facility. Often treated as buy-out items, these components must be carefully integrated into the plant design to facilitate optimum plant performance. The lowest cost fired heater is usually not the best long term solution. Instead, the total life cycle cost including initial capital, energy efficiency and maintenance cost must be evaluated.

While fired capital equipment generally does not have many moving parts subject to wear, it is subject to extreme temperatures which can create significant degradation. The components common to most fired capital equipment include burners, refractory, casing, tubes, piping, stack, crossovers, tube/piping supports, and perhaps air preheaters, fans and pressure vessels. In simple terms, using these components the design goal is always to create economical and safe designs that can withstand the elements over time. There is constantly a battle over how far to take designs in an attempt to fend off possible issues. Some of these issues will be discussed later as they relate to fired capital equipment.  The point is that fired capital equipment typically represents 5% to 15% of the plant cost and may not always receive the same focus as larger components, although their failure can have the same consequence on plant operation as a major component.

Fired capital equipment can consume a significant portion of the energy used in a refinery. It has been estimated that fired heaters alone can consume 40-70% of total energy consumed in a refinery. In addition, the design efficiency is typically in the 75-90% range15, with the operating efficiency even less. Given this scenario, there is a large opportunity to optimise plant operation by focusing on fired capital equipment. A few of the papers listed in the bibliography have addressed this topic.

In addition to the energy consumption side of fired capital equipment, its critical value is demonstrated by the extreme conditions under which this equipment operates. Table 1 gives typical operating conditions for various types of fired capital equipment. As can be seen, these conditions are severe and require special consideration during design. Thereafter, the equipment must be monitored over the life cycle to assess potential problems. Typical trouble spots will be highlighted later in this article. Keeping the fired capital equipment operating as efficiently as possible is a challenge for refineries and petrochemical plants. 

Linde has designed and built fired capital equipment for a variety of applications covering a wide range of operating conditions in terms of temperature, pressure and heat duty. Examples of these different applications are presented in Table 1.

The expertise gained from designing the most sophisticated steam methane reformer under extremely demanding conditions is available within the Linde organisation for optimising even the simplest of refinery heaters. The design tools and methodology are based on a combination of theoretical and empirical techniques, which have been compiled over decades.

Design considerations
The design objectives of fired capital equipment are to maximise heat delivery of the process-side feed while minimising fuel consumption, account for varying fuel quality, minimise equipment wear and maintenance intervals, minimise emissions (for example, stack and noise) and maximise safety. This requires the careful balancing of different parameters during design. Typical heater specifications address: (1) heat duty, (2) process flow rate, (3) temperatures in/out, (4) pressure drop allowable and (5) fuel consumption. Special requirements often include: (1) fouling factor, (2) heat flux, (3) inside surface temperature limit, (4) tube velocity requirements and (5) residence time requirements.

Fired heaters have been designed and built for over seven decades. While this is a mature technology, improvements continue to be made. This is due to changing requirements, including more demanding service, more stringent environmental regulations and the advent of new materials of construction that can withstand extreme conditions. One of the major drivers has been to improve energy efficiency. While energy prices have recently declined, especially in North America, this is not expected to be a permanent situation. Additionally, higher energy efficiency is associated with lower emissions. Therefore, it is always good engineering practice to maximise energy efficiency, taking capital cost into account. For example, it is possible to add an air preheater to recover sensible heat from the flue gases. However, this adds capital. The appropriate level of air preheat has to be selected based on the current and future price of energy.

Heat flux is another important design variable. The higher the heat flux, the lower will be the heat transfer surface area and therefore capital cost. New higher Ni and Cr alloys enable higher heat flux. These have to be selected based on cost and expected life.

Finally, it is important to consider modularisation. Typically, modularisation reduces field construction time and improves quality. However, shipping costs will likely increase. It is important to consider and then balance various design considerations to develop an optimal design. In this regard it is important to work with a company that has extensive experience across a wide range of applications – both small and large – that has been developed over decades.

An example of a large ethylene furnace recently completed on an EPC basis in the US Gulf Coast is shown in Figure 1. A key challenge in executing this project was to obtain the necessary construction resources in a very active market.