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Steam System

Steam distribution system transports steam from the boiler (generator) to the various end uses depending upon the type of uses such as process heating application and power generation. Arrangement of pipelines and other several accessories to supply right quantity and quality of steam at the required point of utilization is steam distribution system. Steam is a common heat transport medium used in process applications and power generation because of its outstanding qualities such as ability to release heat at constant temperature, high heat content, ease of control and distribution and cheap and inert. It is used for generating power as well as for process heating applications in industries such as sugar, paper, fertilizer, refineries, petrochemicals, chemicals, food, synthetic fibre and textiles. Although distribution systems may appear to be passive, in reality, these systems regulate the delivery of steam and respond to changing temperature and pressure requirements. Consequently, proper performance of the distribution system requires careful design practices and effective maintenance. Steam distribution systems can be broken down into three different categories: under-ground, above-ground, and building sections. Selection of distribution components such as piping, insulation, valves, steam separators, steam accumulators, steam traps, steam meters etc can vary depending on the category.

Figure 1: Steam Distribution system
Source: US Department of Energy (2004), Improving steam system performance: A source book for industry.

Factors affecting the quality of steam

Most importantly, steam should be free from moisture. Saturated steam tends to give up its latent heat as it travels in the pipeline and becomes wet. Wet steam contains water droplets that have not evaporated. As these droplets do not contain any latent heat, they do not contribute much to the heat transfer. Hence, water should be removed before it enters the steam using equipment. A moisture separator at the entrance of the equipment serves the purpose where the water droplets are separated out and drained out through a trap.

The second important factor that affects the quality of steam is the presence of air. Air gets into steam space through various joints, etc., whenever the steam is condensed. This is due to partial vacuum formed because of the condensation. It is difficult to make joints 100% leak proof. Dissolved gases in the feed water like, CO2 & O2, also causes air carry over into the steam. It is important to remove this entrapped air from the steam that is supplied to the equipment. The removal of air is necessary for three reasons: air reduces steam temperature; air reduces rated heat transfer and air interferes with heat distribution.

 

Steam Distribution System

General layout and location of steam consuming equipment is important for efficient distribution. Steam pipes laid by the shortest possible distance would provide optimum efficiency together with a proper sizing of pipelines, proper provision to drain the condensate and proper distribution of moisture separator with traps.

Piping

If the pipe is too small, the pressure drop would be high. On the other hand, if it were too big, the surface heat losses would be more. Normally pipe sizes are optimized based on carried velocity or pressure drop.

SteamVelocity in m/sec
Exhaust steam 20-30
Saturated steam 30-40
Super heated steam 50-70

If the specific volume is known, the flow W, in kg/h, can be calculated as:

W = 0.00287 d2V/U

Where
d = Diameter of the pipe in mm
V = Velocity in m/sec
U = Specific volume in cum/kg

Minimizing pipe size will nevertheless reduce capital costs and surface heat losses

Insulation

Thermal insulation provides important safety, energy savings, and performance benefits. Surface heat loss forms a large portion of the heat loss that occurs in the steam distribution system. It is essential to have an efficient insulation on all hot surfaces including surfaces of distribution pipelines and steam consuming equipment. The recommended thickness of insulation will mainly depend on the desired surface temperature after insulation. The energy and cost savings will depend on the size of the pipe diameter and length run, the temperature of the hot surface and the surroundings, heat temperature coefficient and the number of hours of operation of plant/process. Simplified formula for heat loss calculation is given below:

S = [10+ (Ts-Ta)/20]*(Ts-Ta)

Where,

S = Surface heat loss in kCal/hr/m2
Ts = Hot surface temperature in oC
Ta = Ambient temperature in oC
Total heat loss/hr = S*A
Where 'A' is the surface area in m2

Steam traps

Steam traps are essential for proper distribution system performance. The main purpose of a steam trap is to discharge condensate without releasing steam. There are four main types of steam traps as given below that accomplish this in different ways.

Mechanical Traps Thermostatic Traps Thermodynamic Traps General Traps
Operates on the difference in density between condensate and steam
1) Float traps
a) Plain Float
b) Trip Float
2) Bucket Traps
a) Open top Bucket
b) Inverted Bucket
Operates by sensing a difference in temp between condensate and steam
1) Balanced Pressure
2) Liquid Expansion
3) Bimetal Traps
Operates on the forces generated by flashing condensate and steam flowing through orifice
1) TD trap
1) Impulse
2) Pilot operated
3) Labyrinth
4) Orifice plates

Selection of steam trap for particular application is critical for effective condensate removal. In the same line Trap testing at regular intervals and identification of malfunctioning traps in a systematic manner is necessary. Several methods of testing may be employed such as checking for high temperature at inlet, installing sight glasses or ultrasonic detectors at outlets, etc. Traps incorporated with sensing devices are available, which can easily be checked manually or with a computer based monitoring system.

Condensate recovery

Steam condenses after giving off its latent heat in the heating coil or the jacket of the process equipment. A sizable portion (about 25%) of the total heat contained in the steam leaves the process equipment as condensate (hot water). If this hot condensate is returned to the boiler house, it will reduce the fuel requirements in the boiler. For every 6°C rise in the feed water temperature there could be a 1% saving in boiler fuel consumption.

Since the condensate is a pure form of water, it can be used as boiler feed water without further treatment. This not only reduces the fuel consumption in the boiler, but also results in saving raw water and the chemicals required to treat it. The economics of condensate recovery depend on the quantum of condensate recovered, the temperature of condensate, type of boiler, plant layout and the distance of transportation of the condensate.

Payback of energy saving options

Experience from the past has shown that implementing energy saving options in steam distribution system is highly profitable with payback of investment of less than 3 years.

Table 1: Payback of investment of energy saving options for steam distribution system

Options Estimated payback period
Arrest all steam leakages Immediate
Ensure proper insulation of pipes, valves, flanges and fittings About 1 year
Recover flash steam from condensate < 1 year
Recover all condensate from traps 1 year
Replace dysfunctional traps < 1 year
Optimize the pipe size, length, orientation for reducing losses 2-3 years
Ensure proper air venting for better heat transfer < 1 year

Source: NEEP 2012-2016, IGEA

 

Energy Efficiency in Steam Distribution System

  • Repair steam leaks to minimize avoidable loss of steam
  • Minimize vented steam to minimize avoidable loss of steam
  • Ensure that steam system piping, valves, fittings, and vessels are well insulated to reduce energy loss from piping and equipment surfaces
  • Implement an effective steam-trap maintenance program to reduce passage of live steam into condensate system and promotes efficient operation of end-use heat transfer equipment
  • Isolate steam from unused lines to minimize avoidable loss of steam and reduces energy loss from piping and equipment surfaces
  • Utilize backpressure turbines instead of PRVs to provide a more efficient method of reducing steam pressure for low pressure services
  • Optimize condensate recovery to recover the thermal energy in the condensate and to reduce the amount of makeup water added to the system, saving energy and chemicals treatment
  • Use high-pressure condensate to make low-pressure steam that exploits the available energy in the returning condensate

References

US Department of Energy (2004), Improving steam system performance: A source book for industry

Bureau of Energy Efficiency, 2010 Guidebook for National Certification Examination for Energy Managers and Energy Auditors: Book 2

Asian Productivity Organization, 2010 Training Manual on Energy Efficiency for Small and Medium Enterprises

Nepal Energy Efficiency Programme, 2012-2016, Investment Grade Energy Audits