Loss dues to Scale in Boiler

Scale forms when minerals in boiler water settles out on the inside of boiler surfaces. Scale accumulates when improperly treated water comes in contact with the hottest parts of the boiler.

The most common components of scale are lime, magnesium and iron oxides (rust). Scale acts as an insulator, preventing the heat from the burner to reach the water.

This increases boiler flue temperature (more heat is lost up the stack) and can lead to damage to the boiler.

Energy Losses From Scale Deposits

Unclean Heat Transfer Surface

Except for natural gas, practically every fuel leaves a certain amount of deposit on the fireside of the tubes. This is called fouling, and it reduces heat transfer dramatically. 

Tests show that a soot layer just 0.8 mm (0.03 in.) thick reduces heat transfer by 9.5 percent and a 4.5 mm (0.18 in.) layer by 69 percent! As a result, the flue gas temperature rises – as does the energy cost.

Boilers that burn solid fuels (such as coal and biomass) have a high fouling tendency, whereas those that burn liquid fuels (particularly refined oils) have a low fouling tendency. Maintaining the boiler at peak efficiency requires keeping the boiler surfaces as clean as possible

Brushes and manual lances may be used.

Large boilers and those burning fuels with a high fouling tendency should have soot blower systems that clean the fireside surfaces while the boiler is operating.

Brushes and manual lances may be used. 

Small boilers, including natural gas-fired boilers and those without soot blower systems, should be opened regularly for checking and cleaning.

Scale Deposition

Remember, 1 millimetre of scale build-up can increase fuel consumption by two percent !!

Rather than shutting down and draining the boilers to visually inspect the cleanliness of boiler waterside surfaces, waterside conditions can be estimated by testing the boiler water while the boiler is running. Certain water treatment chemicals can then be injected depending on the results. Boiler water is tested daily in small, low-pressure boiler plants and every hour in large, high-pressure plants. The water treatment and testing program is critical to ensuring the maximum efficiency and reliable operation of any boiler plant.

Regular Cleaning

An upward trend in flue gas exit temperatures from boiler over weeks or months usually indicates that a deposit has built up on either the fireside or waterside of boiler heat-exchange surfaces. 

The boiler should be inspected promptly & cleaned using suitable means.

Boiler Capacity Selection

Think Gross, Not Net

When selecting the boiler for an application, key factors include the available fuel, floor space and clearances, required operating pressure, and emmission requirements.

Other than finalising process steam requirement losses incurred in steam distribution & heat transfer get ignored. In a large industrial heating system or heating load, this loss can be as great as 30 percent. If the boilers are sized for the net load only, the steam will condense as quickly as it is being utilized and never develop a head of steam.

Always Choose Right Boiler Capacity

Always size the boiler for the gross load, not net load. The difference between gross and net is the heat loss from the process piping distribution system. 

Steam Pipeline Selection

Wrong selection of pipe sizes and pipeline routing can cause many problems in process like low steam pressure, low steam quantity, Uneven steam distribution, Condensate accumulation & hammering in pipelines etc. 

These would adversely effect the plant efficiency and productivity

Consider the velocity of the steam and the outlet of the boiler.

When designing the steam piping, consider the velocity of the steam and the outlet of the boiler.

The velocity should be kept as low as possible to minimize the possibility of bouncing water levels and frequent low water cutouts.

This also will allow a margin of error for upset boiler water chemistry conditions with a high boiler water total dissolved solids level.

Steam Pipeline Statutory Compliance

Comply with IBR Piping Code

The current Indian Boiler Regulations (IBR) dictates & defines the requirements for installation & testing of steam pipelines from boiler to your process. The piping must be hydro-statically tested and documented as indicated by this code section. (In addition, any local jurisdictional requirements must be satisfied.) 

Statutory guidelines of IBR must be understood and applied

Before installing  any new steam pipelines or extending / modifying the existing pipelines, the statutory guidelines of IBR must be understood and applied for as per procedure

One way the hydrostatic testing and documentation can be accomplished is by having the boiler equipment manufacturer install the boiler steam piping, blowdown piping etc. at your factory. The boiler manufacturer would provide a step by step approach and clear instruction for compliance. Experienced  & approved contractors may also be engaged.

Steam Distribution

A wrongly designed steam distribution system may have following effects:

1. Improper steam pressure at various points
2. Insufficient steam availability at machines
3. Increased condensate formation
4. Higher pressure drops
5. Increased initial cost for pipes, valves & fittings

Get Detailed Layout Prepare

Before installation of pipeline systems, please get a detailed layout prepared based on the steam consumption patters in your plant. Care must be given to:

• Steam quantity at different points
• Steam pressure requirement at different points
• A steam distribution header with a PRS may help in proper distribution & utilization of steam

Pressure Ratings & Fittings

Wrong selection of fittings like flanges, bends, valves etc. may not only lead to statutory non-compliance, but may also cause serious safety issues

Improvement Tips

Be aware of flange and fitting pressure ratings. The ratings and certification information are available with the item. A 150-lb flange indicates a pressure temperature rating.

This is not the maximum allowable working pressure (MAWP) of the flange; instead, it is a designation that allows a certain pressure use based upon the installation and temperature to be encountered.

The pressure and temperature ratings can be found in IBR / ASME specification tables.

Condensate Removal

Condensate inside the steam lines causes loss due to reduced steam carrying capacity and condensation of dry steam. Hence it must be removed properly & efficiently from the distribution system

Improvement

A steam trap should be installed at every low point in the system where condensate can collect. The piping should be inclined in the direction of the steam flow to ensure that the condensate will not travel against the flow in order to reach a trap. The critical steam traps in locations such as the steam header should have a bypass with valve around the trap to enable the operator to test and drain the devices on a periodic basis.

Water Chemistry

Since water is the only component going inside the boiler, it can be the greatest contributing factor for its efficient / inefficient & troublesome operation. 

Maintain Water Chemistry

Water chemistry must be maintained per the boiler manufacturer’s recommendations. Water chemistry should be sampled in both the feed water and the boiler water. Water softeners, RO plants, DM plants, carbon filters and chemical feed systems may be incorporated to achieve the proper chemistry. Other considerations include conductivity based on timed surface blowdown, and automatic bottom blowdown to control levels of total dissolved solids.