Steam Systems
September 24, 2002

Introduction

Steam systems are most commonly associated with sulphur burning sulphuric acid plants, however, acid regeneration plants have steam systems and some metallurgical acid plants.  When the process generates waste heat, the generation of steam is the simplest way to put the energy to use either for process uses or for the generation of electrical power.

Pressure and Temperature Level

The pressure and temperature of the steam to be generated in the steam system is influenced by a number of factors such as:

• Process requirements
• Equipment costs
• Distribution piping costs
• Boiler feed water treatment costs
• Steam turbine driver costs

The material of construction for equipment and piping is affected by the design pressure and temperature of the system.  Below 650 psig, 750°F, carbon steel can be used for piping, tubing, steam turbine casings, etc.   Above this temperature and pressure Cr-Mo alloy must be used with a corresponding increase in equipment cost.

If the steam is being used to generate electrical power, higher pressures and superheat temperatures are desireable since steam turbines are more efficient at the temperatures and pressures.  Higher pressures and temperatures at the turbine inlet also help to prevent wet turbine exhaust conditions which can lead to severe erosion of the turbine blades.  Moisture contents of less than 8 to 9% in the exhaust of condensing turbines is usually specified by the manufacturer.

If the steam is being used only for process heating duties, no superheat is required.  Superheated steam is not desireable since it results in lower heat transfer coefficients.  The heat transfer coefficient of removing sensible heat is much smaller compared to the heat transfer coefficient associated with removing the latent heat from saturated steam (i.e. condensation).

Generally, it is good practice to generate the highest level of steam possible in the plant since it provides flexibility of either satisfying some high level steam process requirements or, by means of a control station letdown or back-pressure turbine, satisfying a process requirement at some lower steam level.

In a typical acid plant steam system there will be typically three different levels of steam distribution pressures; high, medium and low.  High pressure steam will be generally be above 600 psig with a maximum pressure of 900 psig.   Medium pressure steam will be between 150 and 250 psig.  Lower pressure steam will be about 50 psig.

Steam Line Sizing

It is important that the steam generated in the boiler arrives at the end user at the desired pressure.  Steam lines should be sized to ensure that the line losses are not excessive.  Velocities should also not be too high as this can lead to excessive noise being generated.  The following criteria can be used for sizing main steam lines.

• High pressure: 2 to 3 psig/100 ft
• Medium pressure: 1 psig/100 ft
• Low pressure: 0.5 psig/100 ft