Aug-2024

Hidden profits in steam generation

Optimising the steam generation cycle and condensate recovery process for profit.

Ametek

Article Summary

Objective: To identify key areas in the steam generation cycle, condensate recovery system and waste heat recovery process where cost-effective instrumentation solutions offer a tangible return on investment over the short-term. The goal is to reduce heat rate, environmental impact, fuel and water consumption, water treatment and maintenance costs in commercial and heavy industries where steam generation is essential to the production processest.

Overview
- Why Control?
- Steam Generation
     - Boiler/steam drum
        - Deaeration
        - Blowdown
- Condensate Recovery
     - Cost benefits of condensate recovery  
     - Condensate receiver tanks
     - Pump protection
     - Flash tanks and heat exchangers/ condensers
- Makeup Water Treatment
- Energy Management
     - Combustion air, fuel flow & compressed air
- Case Studies

Why Control?
Although the pulp and paper industry is one of the largest producers of steam outside power generation, the primary metals, petroleum refining, chemical process, and food processing industries also allocate significant portions of their total energy consumption, anywhere from 10% to 60%, to the production of steam. Instrumentation plays an important role in key applications throughout the steam generation cycle.

As a consequence, the performance of any level technology relative to instrument induced errors, calibration nuances, and vulnerabilities to process dynamics can have an immediate and adverse impact on fuel consumption while contributing negatively in other aspects of the process, be it make-up water requirements, excessive boiler blowdown, energy transfer, etc. Unfortunately, these other aspects of the process indirectly contribute to the inefficient use of fuel and hinder production throughput and product quality. Adding to this burden is the potential for damage to expensive hardware resulting in forced outages, unscheduled, costly maintenance, and production downtime.

It is not an uncommon practice in this day and age to employ waste heat and/or condensate recovery systems to reduce energy losses and capture valuable condensate. The use of instrumentation technology that cannot adequately or reliably address the control aspects of these processes can inhibit the effectiveness and overall return on investment in these systems and expose hardware to unnecessary damage. Furthermore, processes where electricity consumption and steam generation represent a disproportionate amount of the fuel cost can be plagued with inefficiencies simply due to a technology’s shortcomings on critical applications. Of course, this depends on the fuel type as well as other factors. Nonetheless, when properly addressed, these areas have an immediate and positive impact on costs.

An overview of the processes involved, along with the unique instrumentation requirements for each component, offers insight into the significance of maintaining proper level control and protective measures to realise potential savings in steam generation, waste heat and condensate recovery and water treatment systems common in heavy industry.

This paper highlights the individual areas where the application of specific, proven-in-use level control technologies can lower operation and maintenance costs allowing companies to better compete in today’s global market. As price is usually a key consideration, severe service applications leverage technologies where the cost benefits are realised over the short- and long-term and are linked directly to efficiency. More consideration is typically given to the front-end cost on applications having the least effect on efficiency of the process; but, in reality, reliable measurement is a key factor in normal operation of the process.

Steam Generation
Steam generation and condensate recovery systems can vary in complexity depending on the steam end usage and process requirements, e.g., steam for electricity generation or to support a paper mill operation versus that for a small to mid-size specialty chemical process operation. Figure 1 is a simplified diagram depicting a basic steam generation cycle, scalable to virtually any plant requirement, whether incorporating a fire tube or larger water tube type boiler. It should suffice to highlight critical areas in the cycle where addressing level control concerns can have a profound impact on efficiency, reliability and maintenance.

At the heart of the system is the boiler/steam drum. Regardless of its size, its primary and peripheral functions are as follows:
· Provide ample surface area for the efficient separation of water and steam
· Provide storage capacity to meet immediate boiler feed water requirements
· Facilitate the introduction of chemicals for treatment purposes as well as the removal (blowdown) of impurities

A boiler, fire or water tube, presents an extremely dynamic environment with respect to level control regardless of the control strategy – single-, two-, or three-element. The common denominator in each of these strategies is the level measurement itself.

Applying a technology that improves on this variable in the equation will most certainly aid in controlling the normal water level (NWL) in the boiler/steam drum, allowing it to better serve its primary function of separating water and steam for improved steam quality.

This becomes more important when fluctuations in demand can have dramatic effects on an instrument’s performance during ‘shrink’ and ‘swell’ conditions resulting from pressure changes in the boiler/steam drum. In larger-scale steam production such as that required for commercial power generation (water tube boilers), disruptions in boiler/steam drum level control can have adverse affects on the natural circulation of the process and a plant’s ability to respond to market demand.

Level technologies historically used on boilers rely on inference or buoyancy to determine the level. This in itself leaves them vulnerable to process dynamics (specific gravity, pressure, temperature, etc.) or limits their ability to precisely manage the level for improved fuel economy.

Although corrections can be applied to mitigate the effects, the variables that need to be accounted for increase the level control’s installation, hardware and calibration complexity, which has the unintended consequence of introducing new avenues for error. Eliminating potential sources of error (including human error) as related to an instrument’s fundamental technology is the first step in optimising boiler/steam drum level control.