Steam in the industry today:
The steam system is a part of all major industrial processes today. All major industrial energy users devote significant proportions of their fossil fuel consumption to steam production i.e. food processing (57 %), paper & pulp (81%), chemicals (42%), and petroleum refining (23%).
Since steam is used for one and all process industries i.e. Food & Beverages, chemicals, pharmaceuticals, paper, textile, power, oil & gas, sugar, cement, rubber etc. So, it becomes very important to have very efficient and economical steam systems as this will directly determine the cost of living for people across the globe. Quality of steam plays a very important role in making the steam system efficient and effective as an improved quality of steam directly impacts all the above industry economics.
Quality Of Steam?
The quality of steam refers to the percentage of vapour present in the steam mixture. Steam quality is a measure of how pure or dry the steam is, and it is typically expressed as a percentage.
Pure, dry steam has a quality of 100%, meaning it contains 100% vapour and no liquid water. As steam leaves a boiler, it often contains some amount of liquid water droplets mixed in with the vapour. The percentage of vapour compared to the total mass of the steam-water mixture is the steam quality.
For instance, if steam consists of 95% vapour and 5% liquid water, its quality would be considered 95%. A lower steam quality means a higher percentage of liquid water, which can negatively impact the efficiency and performance of steam-based systems.
Measurement Of Steam Quality:
Throttling Calorimeter
Figure 6 illustrates a common design of a throttling steam calorimeter. Steam is drawn from a vertical main steam line via a sampling nipple. It flows around the first thermometer cup for measurement, then passes through a ⅛” orifice in a disk positioned between two flanges. Afterwards, it moves around the second thermometer cup for further measurement and is finally vented into the atmosphere.
The instrument, along with all connecting pipes and fittings, should be well-insulated to reduce energy loss that could impact the accuracy of the measurement.
The small orifice may encounter problems with corrosion particles passing through the device. As a result, effective steam filtration should be included as part of the measurement system.
The discharge steam piping needs to be short to prevent any back pressure below the disk area, which could cause measurement errors.
Separating Calorimeter:
The separating calorimeter works by mechanically removing entrained water from the steam, and collecting it in a reservoir. The amount of water is either displayed using a gauge glass or drained for weighing. Figure 8 depicts this type of calorimeter.
Steam exits the calorimeter through an orifice of a known size, allowing its total quantity to be either calculated or weighed. This type of calorimeter typically includes a gauge that indicates the pressure inside its chamber and the steam flow over a set time, with the scale calibrated through testing.
Like a throttling calorimeter, it should be properly insulated to prevent heat loss from radiation.
Factors Affecting Quality Of Steam:
a) Dryness of Steam:
- Boiler water carryover depends upon the design of the boiler
- The water level in the drum is not being maintained properly
- Fluctuating steam loads in the plant
- Improper insulation in the steam lines
- Improper steam line layouts and trap installation
- Malfunction of steam traps.
b) The correct quantity of steam in the process:
- Steam must be supplied in the required quantity at the process.
- There must be no starvation even during startup when steam consumption is at its peak.
- Steam lines must be correctly sized and we cannot have either undersized lines or oversized steam lines. One size higher steam line tends to have 40 % more Initial investment and 20 % more radiation losses. Smaller line sizes lead to steam starvation leading to product quality issues.
c) Steam supplied has to be at the optimized pressure:
- Steam to the process must be supplied at the least possible steam pressure to have the maximum latent heat transfer.
- Increased steam pressure than required can lead to a loss in the thermal efficiency of the process.
- The below table clearly shows that latent heat increases as steam pressure reduces, so we use the lowest possible steam pressure for our processes.
d) Air and other non-condensable gases:
- Air enters the steam lines during start-up. Whenever a process is completed and as system is depressurized air is drawn into the system.
- Boiler feed water has dissolved gasses ie Co2 and Oxygen mainly and some part of Nitrogen which is present in the air. As boiler feed water is heated it releases these non-condensable gases and these mix up with the steam and travels along with the steam into the process.
e) Clean dirt-free steam:
- Steam can have contamination from fabrication left over, and welding slugs during the initial commissioning of the steam lines.
- Steam can also have contamination from rusting and scaling of the steam lines over some time.
- Impurities can enter steam lines by poor boiler operations i.e. drum level not maintained properly, higher blowdown TDS and even maintaining lower bed temperatures.
- Improper water treatment also leads to impurities in the steam.
Problems Associated With Poor Quality Of Steam:
- Reduces heat transfer efficiency because of wet steam: A major problem associated with poor quality of steam is the reduced thermal efficiency of the process. Condensate entrapped in the steam has sensible heat close to 16 % (varies slightly with pressure) which is much lower than the latent heat which steam has (close to 94%). This implies that less energy is released to the process when we have wet steam.
- Wet steam also forms a thin layer of condensate along the walls of heat exchangers which reduces the ability of steam latent heat to be transferred to the process.
- Boiler water carryover and condensation taking place in the steam lines lead to the accumulation of condensate in the steam lines. This condensate travels along with the steam at a very high velocity and gains high kinetic energy. As this form of condensate touches any equipment, bends etc. it creates a lot of noise and vibration which is called water hammering.
- Equipment failures: condensate passing through steam lines at high velocities when hits any valves or traps generally erodes the internals over a period. This reduces the life span of the products installed on the streamlines.
- Reduced thermal efficiency due to the presence of air and non-condensable gases in the steam system: air being a very poor conductor of heat often reduces the thermal efficiency of the process when it is present in the steam system. Air forms a thin film of layer on all heat exchange areas thereby reducing thermal efficiency for any process.
Ways To Improve Steam Quality:
Improve dryness of steam:
- Operate the boiler near to design pressure as this will ensure better quality of steam going into your process.
- Steam boilers need to be adequately sized.
- Do not let boiler TDS become high.
- The addition of feed water needs to be modulated instead of on-off.
Installing a 3-element control will help in reducing boiler carry-over.
Steam Line Sizing And Velocities:
The steam distribution network needs to be properly designed so that we receive the designed temperature and steam pressure at the usage points. Line pressure drop needs to be minimized. Steam has to reach the process at the desired pressure i.e. we have to design the system so that the pressure drop should be maintained at the minimum. We also need to ensure that the correct quantity of steam reaches the process and there should be no steam starvation.
Condensate Removal From Main Steam Lines And Other Good Engineering Practices:
Condensate removal from main steam lines needs to be properly done through properly designed steam traps and proper installation.
Use Of Moisture Separators And Strainers:
Properly designed Moisture separators and strainers need to be installed as per proper engineering guidelines.
Air venting and its advantages:
Automatic air vents need to be installed across your steam distribution network.
If we follow and address the above points you can have very good quality steam at your plant and in turn highest level of thermal efficiency in your thermal processes
Author:
Mr. Narpendra Singh
Chief Executive Officer
Incosteam International Pvt Ltd
