Transient analysis of a utility steam system

Dynamic simulation in the analysis of complex interactive systems has proved valuable in determining what effects variations in the dynamic characteristics of a package boiler would have on overall system performance

Brian D Bumgarner, Scott Ray, Surajit Dasgupta, John R Cassata
Kellogg Brown & Root (Now KBR Technology)

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Article Summary

Dynamic simulation and transient analyses are being used more routinely as an integral part of the process engineering design effort. Analyses of transient events directly benefit the resulting process design by addressing important issues related to safety, yield, throughput and other factors, particularly plant stability and controllability, that have a direct bearing on plant operations [Cassata J R, Gandhi S L, Ray S; Dynamic simulation: the technology of today and the future, HTI Quarterly, Winter 1995/96].

Transient analyses enable the design engineer to meet such process objectives as optimising product quality, achieving tighter process guarantees, and shortening startup and shutdown times [Van der Wal G, Haelsig C, Schulte D; Minimising investment with dynamic simulation, Petroleum Technology Quarterly, Winter 1996/97]. It also provides a more accurate basis for establishing capital cost requirements for retrofits [Cassata, Dasgupta S, Gandhi; Modelling of tower relief dynamics, Hydrocarbon Processing, Oct and Nov 1993].

As an analysis tool, dynamic simulation reveals transient behaviour that is often non-intuitive and difficult to envision using steady state methods. It provides the means of evaluating alternatives that can mitigate the undesirable aspects of transient behaviour.

System description. For exothermic processes that reject a significant quantity of heat at high temperature levels, it is often appropriate to recover this waste heat via generation of steam at useful pressure levels. This byproduct steam in turn is often utilised to satisfy some of the energy needs of the process.

Integrating a process with its utility systems to a significant extent leads to serious concerns of overall plant operability and operational flexibility [Depew C et al, Dynamic simulation for IGCC process and control design, Hydrocarbon Processing, Jan 1998]. A good example of the impact that dynamic relationships can have in complex systems is illustrated in a recently designed process that is highly integrated with its utility steam system.

To support the design effort of this world scale petrochemical facility, dynamic simulation and analyses were done of the transient interactions between the steam system and the process. The results of this analysis provided input to the specification of key control system features. Insight gained from the dynamic study resulted in satisfactory startup and operation of the facility at the completion of construction.

The primary reason for doing the dynamic analysis was to determine whether a sudden large increase in steam demand could result in a plant shutdown. Some of the relationships between the steam system and the process on this recent project are listed below:
- Relatively high temperature waste heat is used for boiler feedwater heating and steam generation
- A significant amount of mechanical energy is consumed in the process via steam turbine drivers of large process equipment
- Enough steam is generated from waste heat to operate a turbine-driven electrical generator which, during normal operations, supplies power to some process consumers
- Steam is required as a process feed stream.
Thus, the relationship between the process and its utility systems, when coupled together, is complex.

Figure 1 illustrates relevant portions of the utility steam system. A large quantity of high pressure superheated steam is generated in process Waste Heat Boiler B-101 and supplied to the High Pressure Steam Header (HP Header). Process heat, at somewhat lower temperatures, is recovered in boiler feedwater upstream of B-101 via process exchanger E-101.

The high pressure steam generated by B-101 is utilised primarily in a large steam turbine, KT-101, which drives a process gas compressor. This compressor provides process gas to an exothermic reactor after which the heat of reaction is rejected in E-101. A minor amount of high pressure steam is consumed by other users. KT-101 is an extraction-condensing turbine that supplies a large fraction of the needs of the Medium Pressure Steam Header (MP Header).

It is possible to let down high pressure steam to the MP Header; however, no let-down is intended during normal operations, only during upset conditions or during startup. Although a large fraction of the medium pressure steam that is produced originates as extraction steam from KT-101, Package Boiler B-102 is provided to satisfy the remaining medium pressure steam requirements.

The process has a variety of medium pressure steam consumers, with the key ones being Steam Turbine-Driver Kt-102 and the process itself which requires medium pressure steam as a feedstock. KT-102 is a condensing turbine that drives an Electrical Generator.

The Electrical Generator and Package Boiler B-102 are sized to provide the entire electrical needs of the process if necessary. During normal operations, however, a substantial fraction of the electrical load is imported from the local electric utility.

Control philosophy
The control systems implemented for the components shown in Figure 1 are relatively complex. The two primary objectives of the control scheme are control of steam flow to the process and control of the MP Header pressure. Other control objectives include, among others, HP Header pressure control, control of the KT-101 steam turbine, control of steam flow to KT-102 and maintaining process conditions.



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