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