Consider Dynamic Simulation for Steam System Design

Models can provide crucial insights for dealing with upsets and transient conditions.

By Ian Willetts, Abhilash Nair and Charles Rewoldt, Invensys Operations Management

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A dynamic simulation analysis of the steam system helps precisely understand transients in the system. Simulating upsets enables monitoring flows and pressure drops across pipe segments as a function of time, to identify violations of design criteria during the transients. The greatest benefit from this type of analysis occurs when it's performed closely with the engineering design of the system. At that point, incorporating necessary design corrections incurs the lowest possible cost and impact on schedule.

Confirm steam system controls. Dynamic simulation also can be used to evaluate the proposed control strategy around the integrated steam system. It can help get the control system right the first time, thereby saving valuable time during commissioning, helping ensure stable operation during day-to-day operations and keeping the system up and healthy during some of the worst-case scenarios the facility could experience.

An upset, like loss of a boiler, has the potential to bring down the entire steam system, causing shutdown of critical process units. Because a dynamic simulation model incorporates all the controls, analysis can determine if the as-built controls can maintain stable operation after an upset. The model allows easy configuration and testing of control alternatives that might improve steam-system response. Feedforward signals to boiler controls, low/high selector clamps on letdown stations, priority settings on steam headers, set-point staggering across the facility on various control loops are some of the important handles that can be quickly changed and fine-tuned using a dynamic simulation analysis. These parameters can prevent nuisance trips and shutdowns and can accelerate startup.

Identify steam load-shedding strategies. A critical outage of major steam producers for scheduled maintenance or due to an unforeseen trip requires adjusting steam demand to balance supply and demand across the complex. If backup boilers can't make up the difference or are slow to respond to the upset, it will become necessary, for example, to switch from steam-driven turbines to electric drivers (if available) or to identify which less-critical units should be taken offline and for how long to protect the more-critical equipment and units.

When transient demand exceeds transient production, as in the case of multiple boiler trips, a steam shedding strategy must be initiated quickly to counter the upset before steam networks reach unacceptable pressures. Developing a steam shedding plan that could be implemented during a major upset is critical to maintaining the availability and un-interrupted operation of the steam system.

Dynamic simulation can be a great help with this evaluation as it can be used to test and evaluate critical shed lists and to develop a strategy — prior to startup and operation — that least impacts the economic profitability of the overall complex. It allows analysis of either reducing steam consumption or dropping steam users outright based on priority and criticality. Both feedback pressure-driven and feedforward event-based shedding strategies can be easily configured and tested.

KEEP OUT OF HOT WATER
Dynamic simulation quickly is becoming an accepted technology for performing in-depth steam system analyses that can't otherwise be done except by trial and error in the plant. Engineering companies can benefit greatly from performing such analyses by following the tips described here and other best practices as early in the process lifecycle as possible.

Moreover, the simulation software platform models become assets within the company and can be re-used beyond the design environment to support plant commissioning and for the development of operator training systems.

Avoid Common Design Errors
Dynamic simulation can aid in a number of areas of steam system design, including:

Properly sizing lines. One of the common errors encountered in steam system design is incorrectly sized distribution piping. Undersized lines have higher velocities and pressure drops, leading to insufficient flow and pressure of steam to users. Undersized lines also increase the risk of erosion, noise and hammering. On the other hand, oversized lines are expensive and cause higher heat losses, impacting the quality of steam. In addition, flows through steam pipes can undergo drastic changes. Understanding this phenomenon through simulation is crucial for accurately estimating and verifying line sizes.

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