Most process plants consider steam as an indispensable means of delivering energy. After all, it offers many performance advantages including low toxicity, ease of transportability, high latent heat and low cost of production. Because most of the energy in steam is stored as latent heat, large quantities of heat can be transferred efficiently at constant temperature.
Typical steam systems encompass multiple pressure levels connected to a number of steam producers and steam users or consumers spread across a site. As economies of scale drive operating companies to build ever larger and more integrated facilities, the design of the shared steam utility system becomes extremely critical to their operation. The steam headers often run throughout the complex, tying together myriad units. This creates a highly non-linear control and operability challenge.
It's essential to ensure that steam can be provided to all reaches of the facility without interruption and that the system can be controlled in the event of upsets to maintain stable operation. Improper controls could lead to loss of the entire steam system, trip or damage of critical equipment, off-specification products and, in the worst case, loss of the entire steam system and shutdown of the complete facility. Normally, such design deficiencies become apparent only after an incident — this could be costly or potentially disastrous.
Further, with ever-increasing energy costs, better design, control and operation of the steam system can directly impact the entire facility's overall efficiency, translating into substantial operational savings.
Traditional steam hydraulic analyses assess demand and production issues at different steady-state operating conditions. Such analyses can't predict the steam system response through multiple headers all across the complex during process upsets.
Understanding the response through dynamic transients and ensuring the steam system can handle all expected events without jeopardizing the availability of the facility becomes a critical aspect of the process and controls design of such systems.
This article takes a look at how dynamic simulation can assist steam system design and offers up some tips for staying out of "hot water."
AN IMPORTANT TOOL
Dynamic simulation is a "best available technology" that can be used to evaluate the "as designed" process and control strategy to maximize the likelihood that it can provide stable and uninterrupted operation following steam system or process upsets.
A typical dynamic simulation of the steam system involves building a rigorous first-principles model that includes:
• steam turbine generators and drivers;
• multiple pressure headers;
• pressure letdown stations;
• steam consumers; and
• regulatory and plant master control.
Today commercially available software packages such as DYNSIM from Invensys allow steam system models to be built in a fraction of the time of older programming languages or software platforms (Figure 1).
The model is built and the controls are configured to maintain the steam system at the normal design operating point(s). The model encompasses all regulatory controls, including those specifically designed to manage expected transients resulting from steam-system or process upsets.
The high fidelity model can simulate many steam-system or process upset scenarios in a matter of just days — to predict the system response following such events in a safe and controlled environment on the computer. The model then can be used to determine how best to correct any issues identified during the upset scenarios.