Efficient use of Steam

Sir Oliver Lyle“The Efficient Use of Steam”

• Steam Possesses Many Outstanding Qualities:– Very high heat content– Gives up heat at constant temperature– Produced from water (cheap and plentiful)– Clean, odorless, tasteless– Its heat can be used over and over again– Can generate power, then be used for heating– Can be readily distributed and easily controlled

1947

Latent heat vs pressure

Dryness fraction

Steam distribution system

Generating and distributing steam at higher pressure : Advantages

• The thermal storage capacity of the boiler is increased, helping it to cope more efficiently with fluctuating loads, minimising the risk of producing wet and dirty steam.

• Smaller bore steam mains are required, resulting in lower capital cost, for materials such as pipes, flanges, supports, insulation and labour.

• Smaller bore steam mains cost less to insulate.

Steam Piping : Features

• Steam pipes should be laid by the shortest possible distance.

• Provision for proper draining of condensate.

• For example, a 100mm well lagged pipe of 30-meter length carrying steam at 7 Kg/cm2 pressure can condense nearly 10 Kg. of water in the pipe in one hour unless it is removed from the pipe through traps.

• The pipes should run with a fall (slope)of not less than 12.5 mm in 3 meter in the direction of flow.

What is the Function of Steam Traps?

• Steam traps are automatic valves used in every steam system to remove condensate, air, and other non-condensable gases while preventing or minimizing the passing of steam.

• If condensate is allowed to collect, it reduces the flow capacity of steam lines and the thermal capacity of heat transfer equipment.

• In addition, excess condensate can lead to "water hammer," with potentially destructive and dangerous results.

• Air that remains after system startup reduces steam pressure and temperature and may also reduce the thermal capacity of heat transfer equipment.

• Non-condensable gases, such as oxygen and carbon dioxide, cause corrosion.

• Finally, steam that passes through the trap provides no heating service. This effectively reduces the heating capacity of the steam system or increases the amount of steam that must be generated to meet the heating demand.

What steam trap does ?

                                                                  

                                                                                   

                                                                                               

Figure 2. 

Trap Installation

Where the heat goes

Efficient Steam Utilisation

• Avoid steam leakages• Provide dry steam for process• Utilising steam at the lowest possible pressure• Insulation of steam pipelines and hot process

equipment• Minimising barriers to heat transfer• Condensate recovery• Flash steam recovery• Proper selection and maintenance of steam traps• Proper sizing of steam and condensate piping• Reducing the work to be done by steam

Steam leaks

Avoiding Steam Leakages

Leaking Steam Pipe / Valve

Weak whistlingAlmost invisible steam jet

800 litre oil per year 800 litre oil per year

Audible Leak

2,000 to 4,000 litre oil per year2,000 to 4,000 litre oil per year

Visible Leak

Weak hissingVisible steam jet

Live steam vs Flash steam

Provide dry steam for the process

• Disadvantages of wet steam– Less heat content, Extended process time,

Irregular heating, Barrier to heat transfer, Overloading of steam traps

• Disadvantages of superheated steam– Poor heat transfer coefficient, takes time to give

up superheat by conduction

• Benefits of dry steam– Heat transfer is rapid and regular

Providing Dry Steam for Process

Use Dry Saturated steam for processes

Steam Separators to be fitted at point of steam use

Provide a little superheat to ensure dry saturated steam at the process end

Steam separators

Utilising steam at the lowest possible pressure

2151.3 KJ/kg

579.4 KJ/kg

2054 KJ/kg

716.8 KJ/kg

2730.7 KJ/kg 2770.8 KJ/kg Total Heat

Latent Heat

Sensible Heat

2.4 bar, 121.5oC 6.8 bar, 164.3oC

Steam should always be generated and distributed at the highest possible pressure but utilised at the lowest practicable pressure

Operating a boiler at lower pressure ?

• It is the connected load, and not the boiler output, which determines the rate at which energy is used.

• The same amount of energy is used by the load whether the boiler raises steam at 4 bar g, 10 bar g or 100 bar g.

• Standing losses, flue losses, and running losses are increased by operating at higher pressures, but these losses are reduced by insulation and proper condensate return systems.

• These losses are marginal when compared to the benefits of distributing steam at high pressure.

Pressure reduction

Optimal Insulation

50 mm insulation compared with an uninsulated pipe: 320 - 29 = 291 W per m

263 litre oil per year

50 mm insulation compared with 100 mm insulation: 29 - 19 = 10 W per m

9 litre oil per year

Heat loss, 89 mm black steel pipe, 90 oC

Uninsulated320 W/m

100 mm insulation19 W/m

50 mm insulation29 W/m

...But don’t Over-Insulate: There is always an optimum insulation level (1-3 years payback)

Insulation of Steam Pipelines and Equipment

An uncovered flange is equivalent to leaving 0.6 metre of pipe line unlagged.

if a 0.15 m steam pipe diameter has 5 uncovered flanges, there would be a loss of heat equivalent to wasting 5 tons of coal or 3000 litres of oil a year

Heat loss through uninsulated flanges

How to calculate heat losses from flanges and valves

• A 100 mm dia pipe has 8 pairs of flanges and two valves, and carries saturated steam at 7 bar g. Ambient temperature is 10°C. Find out the loss through the falnges and valves

Equivalent length of fittings:(8 pairs of flanges @ 0.5 m) + (2 valves @ 1.0 m) = 6.0 m of pipe

Saturated steam at 7 bar g:Steam temperature = 170°CTemperature difference (pipe to ambient temperature) = 170°C - 10°C = 160°C Heat loss per metre of 100 mm pipe (from Table next slide) = 999 W/m

Heat emission from pipes

Heat emission from bare pipes

Direct Utilization of Steam

Direct Steam use involves both Latent Heat and Sensible Heat

Use temperature controller in Direct Use to avoid steam wastage

Minimising barriers to heat transfer

Resistance to heat transfer of water is 60 – 70 times more than steel and 500 – 600 times than copper

Resistance to heat transfer of Air is 1500 times more than steel and 19,000 times than copper

Effect of air and water filmSteam at 1 kg/cm2

Steam at 0.75 kg/.cm2:Air and water film reduced by 50 % ; Quicker process time

Air Venting

0.25 mm thick air film offers same resistance to heat transfer as 330 mm thick copper wall

Install Air vents where air is likely to be stagnant

Boiler Fuel Saving by Condensate ReturnSaving in percent if condensate is returned to the boiler instead of draining

For every 6OC rise in boiler feed water temperature, there is a 1 % raise in boiler efficiency

Ogdon Pump

Flash Steam

Flash steam available in % - S1 - S2

L2

S1 - Sensible heat of high pressure

condensate

S2 - Sensible heat of steam at lower

pressure (at which it is flashed)

L2 - Latent heat of flash steam at lower

pressure

Flash Vessel

Flash steam utilisation: Example

Flash steam

Thermocompressors

Steam Distribution System

• Ensures that adequate quantity of steam that is dry and free of air, reaches the plant at correct pressure

• Diameter of piping should be optimum to minimise pressure drops, investment and operating costs

Reducing the work to be done by steam

• Have shortest route of piping• Remove moisture mechanically to the fullest

before steam drying / avoid bone drying• Optimise humidity of drier exhaust• Explore process integration• Use thermostatic controls• Remove / blank redundant lines• Productive use of machinery (Maximise equipment

loading)

• Look for cheaper alternatives of doing the job (waste heat boilers, thermic fluid heater etc)

Reducing the work done by steam

Limit excessive process temperatures

Intermittent peak demand