Pressure Levels of Steam Generation and Distribution

Not sure but I would say efficiency, by that I mean selecting the correct pressure and temperature of steam for your application, with the minimum of heat loss at the exhaust. Avoiding putting excessive pressure and temperature to waste after use? Unless higher ranges are fed into lower systems? But then that may have to be bled off if not used, leading to a miss match of demand at the boiler.

Regards JD.

Hi, nvmani!

I would agree with JDRetired and add that the different pressure levels might well also depend upon the application. Above all else, refineries rely upon efficiency and economy of scale. All those varying conditions of the steam may be sourced from one VHP boiler location.

If they are used to treat viscosity or the steam is injected into the product at varying stages during the refinement process, then their application might be different at each pressure/temperature rating depending upon the nature of the product at those stages of refining.

They might also be used to power machinery that moves the crude through the various processes to the various finished products; and in that case several of them (or all of them) might be used in tandem to produce the power to move the process.

It's commonplace for steam generation to work on several different pressures and temperatures in every application where it is employed to effect efficient use of that type of energy transmission.

Mark

What I've commonly seen in refineries, is that steam is generated at a high pressure (say 1200 psig), then stepped down to several other levels (600 psig, 250 psig, 75 psig, 50 psig) and de-superheated at each stage to produce mains with saturated steam at each pressure. What you then have in the main pipe racks are large pipes filled with steam at known temperatures, which each process unit taps into according to their need. For instance, steam eductors to drive sulfur pit vapors to a tail gas treating unit may use 75# steam, many applications use 50# steam for heat tracing. A particular process stream may require 250# steam to maintain a minimum temperature of 400 deg F, and so on.

Various stages of the manufacturing process have different temperature and energy requirements. Some stages use superheated steam for extra energy, some require saturated steam for a more constant temperature gradient. Once the process parameters are known, the designer then considers trade-offs, comparing the efficiency and quality of the manufacturing process with the cost of the piping and vessels required for the temperature/pressure/flow combinations. Sometimes the process is modified to simplify the steam requirements; other times, doing so would degrade the quality of the finished product, so the additional steam conditions must remain in the design. The final system design is always a compromise.

Once the main process is finalized, the designer looks for ways to use energy from the process exhaust. Heating & de-aerating incoming feed water, heating the building (or cooling with absorption plants), potable water heaters, etc. are all used to get back as much as possible of the energy originally transferred into the steam.

The bottom line is that energy is usually the largest single cost in manufacturing, even more than the raw material. The various temperature/pressure combinations are the most efficient & economical for that stage of the process. The reducers/desuperheaters are one-time expenses included in the capital cost of the project. The payback in lower per-unit production cost makes the various steam levels cost-effective.

To further elaborate on previous threads:

- Distributing steam at high pressures is done to allow more steam (lb/hr or kg/hr) to be distributed in smaller pipe. The lower the steam pressure the higher the specific volume, the higher the steam pressure the lower the specific volume.

- High pressure steam is also determined based on the required steam pressure needed at the furthest end of the steam distribution system.

- Superheated steam (or very high pressure saturated steam) is idea to do work - such as a turbine to create electricity

- When separating crude oil into its various constituents different temperatures are needed, and since there is a direct relationship with the steam pressure and temperature (when the steam is saturated) - it is easy to provide tighter control of the process.

- An additional steam use is steam tracing of the pipe (or steam jacketed pipe). This is needed to help the various oils and sulfur flowing through the pipe & not crystallizing (in the case of sulfur).

- Usually the lowest steam pressure used is for storage tanks and at the DA Tank.

You can go to the following link to further understand the steam distribution system:

http://www.spiraxsarco.com/resources/steam-engineering-tutorials/steam-distribution/introduction-to-steam-distribution.asp