CHAPTER 3: STEAM DISTRIBUTION & UTILIZATION
INTRODUCTION
STEAM DISTRIBUTION SYSTEM
What is steam?
- The gas formed when water transforms from a liquid to a gas.
- When energy is added to the water, its temperature rises until it reaches saturation point ( boiling point).
Types of steam
2) Unsaturated (Wet) steam.
3) Superheated steam.
1) Saturated (Dry) steam.
Why do we use steam?
To be the transport and provision of energy
Benefit using steam:
1) Efficient & economic to generate.
2) Easy to distribute.
3) Easy to control.
4) Easily to transferred to the process.
5) Steam plant is easy to manage.
6) Flexible.
Steam quality
- Clean
- Dry
- Free from air & incondensable gases
- Correct temperature & pressure
- Correct quality
The essential link between steam generator & point of use.
Operation of steam distribution system.
- Steam moves from the boiler along the main pipe (steam mains) t0 the point where its heat energy is required.
- Pipework is initially cooler than the steam, so steam on contact with the cooler pipes will begin to condensate immediately.
- On start-up of the system, the condensing rate will be at its maximum due to the maximum temperature difference between the steam & pipework.
- -This condensing rate is commonly called the "starting load"
- Once the pipework has warmed up, the temperature difference is minimal, but some condensation will occur as the pipework still continue to transfer heat to the surrounding air since the surrounding air is cooler than steam. This condensing rate is called the "running load"
- Condensate falls to the bottom of the pipe & will then drained from various strategic points in the steam main.
- Steam enter the process plant & transfer its energy in warming up the equipment product (starting load) & then continue to transfer heat to the process (running load).
- The condensate formed in both the steam distribution pipework & in the process equipment was then return to the boiler feedtank.
Most important components:
1) Pipes.
2) Drain points.
3) Branch lines.
4) Strainers.
5) Filters.
6) Separators.
7) Steam traps.
8) Air vents.
9) Condensate recovery.
10) Insulation
Material:
Carbon steel / copper
Pipeline sizing:
Correct pipeline sizing is important.
Can be calculated based on pressure drop/velocity
Oversized pipework
Undersized pipework
- Higher material and installation costs.
- Increased condensate formation.
- Lower pressure at point of use.
- Risk of steam starvation.
- Risk of erosion.
Piping layout
Steam mains should be installed with a fall of not less than 1:100
Ensures that condensate can reach steam trap.
Consideration must be given to:
- Distance between drain points.
- Design of drain points.
- Condensate in steam main at shutdown.
- Diameter of drain pipe.
- Location of drain points.
Take steam away from main steam
Shorter than main steam
Branch line connections taken from the top of the main line. If connection are taken from the side & bottom, they can accept the condensate & debris from the main.
Stop scale, dirt & other solids in flowing liquids / gases.
Protect equipment from their harmful effects, thus reducing maintenance.
Types of strainers:
1) Y-type
- Almost universally used.
- Can handle high pressure.
- Have lower dirt holding capacity: required more frequent cleaning.
2) Basket type Strainer
- Less pressure drop.
- Preferred type of liquid application.
- Larger dirt holding capacity.
- Only for horizontal pipelines.
- When used on steam system, drain plug is needed to remove condensate.
- Commonly found in duplex arrangement (parallel position)
Types of Screens:
1) Perforated Screen
2) Mesh Screen
Holes punched in flat screen
Large holes ( 0.8mm - 3.2mm
Remove large debris
Fine wire into mesh arrangement
Small holes (~0.07mm)
Remove small debris
To remove smaller particles.
Used in application:
Dirty steam can cause rejection of the product
Minimal particle emission required from steam humidifiers
Direct injection of steam into a process
Remove suspended water droplets from steam
Water in steam can cause a number of problems:
Reduce plant productivity & product quality
Erosion & corrosion of valve & fittings
Scaling of pipework & heating surfaces from impurities
Erratic operation & failure of valve & flow meters
Types of separators:
1) Baffle Type
2) Cyclonic Type
3) Coalescence Type
Function: "Purges" condensate out of the steam system & allows steam to reach destination as dry as possible.
Type of steam traps:
1) Thermostatic Steam Traps
2) Mechanical Steam Traps.
3) Thermodynamics Steam Traps
Function: To remove air from the steam
Types of air vents:
1) Automatic air vent on jacketed pan (vessel)
2) Automatic air vent on end of main.
The presence of air in the steam reduces the temperature of the steam & offers a resistance to heat transfer
Air vent location:
- Within low lying steam trap opposite high level steam inlet.
- Opposite low level steam inlet
- Opposite end of steam inlet
What is condensate?
Distilled water with heat content
Discharged from steam plant and equipment through steam traps
Condensate recovery for?
Reuse in boiler feed tank. deaerator/ as hot process water
Heat recovery through heat exchanger
Reason for condensate recovery
Water changes
Maximizing boiler output
Effluent restrictions
Boiler feedwater quality
Insulator: Low thermal conductor that keeps heat confined within/ outside a system by preventing heat transfer to/from the external environment
Benefits:
Reduction of fuel consumption
Better process control
Corrosion prevention
Fire protection of equipment
Absorbing of vibration
Suitable materials for steam & condensate line:
2) Glass wool
3) Rock wool
1) Cork
4) Asbestos
Financial reason
ASSESSMENT OF STEAM DISTRIBUTION SYSTEM
Traps that fail in the "open" position cause energy losses. Condensate not returned into the steam causes boiler to heat up new water to make steam.
Types of steam trap assessment:
2) Sound testing
3) Temperature testing
1) Visual testing
Open discharge
Sight check
Test tees
Sight glasses
3 way test valves
Ultrasonic leak detector
Mechanics stethoscopes
Screwdriver
Metal rod with a human ear against
Infrared guns
Surface pyrometers
Temperature tapes
Temperature crayons
1) Assessment of steam trap
2) Assessment of heat losses from un-insulated surfaces
3) Assessment of savings from condensate recovery