Steam is used to provide process heat and mechanical power. Steam loss in the process, is a major problem faced by industries. A well designed steam distribution system can reduce the losses and improve the efficiency of the steam system thus reducing energy costs.
This paper examines the sources of inefficiency in the steam distribution system and discusses practical options to promote greater efficiency in the distribution of the steam.
The function of the steam distribution system is to get the steam to where it is needed and return the condensate to the boiler, doing both as efficiently possible. Distribution heat losses account for 3 to 10% of the total energy generated in a boiler system. Energy management can reduce the heat loss by improving the insulation, detecting and repairing steam and condensate leaks, maintaining the steam traps and condensate pumps, and providing water treatment. A well designed steam distribution network can improve the efficiency of the steam systems. For optimum performance of the distribution and steam enduse equipment, a supply of right quantity and quality of steam is of vital importance.
The losses in the steam distribution system can be in the form of:
• Pressure losses in the distribution pipe lines.
• Steam leaks in joints, valves, gauges, etc.
Principal factors determine pipe sizing in a steam system:
1. The initial pressure at the boiler and the allowable pressure drop of the total system. The total pressure drop in the system should not exceed 20% of the total maximum pressure at the boiler. This includes all drops—line loss, elbows, valves, etc. Remember, pressure drops are a loss of energy.
2. Steam velocity. Erosion and noise increase with velocity. Reasonable velocities for process steam are 6,000 to 12,000 fpm, but lower pressure heating systems normally have lower velocities. Another consideration is future expansion. Size your lines for the foreseeable future. If ever in doubt, you will have less trouble with oversized lines than with ones that are marginal.
3. Properly select, size, and maintain the distribution system steam traps.
4. Insulate all distribution system pipes, flanges, and valves.
5. Ensure that steam mains are properly laid out, sized, adequately drained, and adequately air vented.
6. Ensure that distribution system piping is correctly sized to maintain appropriate system pressure drops.
7. Ensure that distribution system piping is adequately supported, guided, and anchored; and that appropriate allowances are made for pipe expansion at operating temperatures.
A practical steam distribution system should necessarily compromise between the above ideal conditions and several other factors. Lack of attention to these will significantly increase operating costs, either because of reduction in overall efficiency or increase in maintenance costs or both.
4.5.1 Energy Conservation
• Steam piping layout
Steam piping transports steam from the boiler to the end-use services. Important characteristics of a well-designed stem system piping are that it is adequately sized, configured, and supported. Installations of larger pipe diameters could be more expensive, but can reduce the pressure drop for a given flow rate and also help to reduce the noise associated with steam flow. Hence, one consideration should be given to the type of environment in which the steam piping will be located when selecting the pipe diameter.
Important configuration issues are flexibility and drainage. Piping, especially at equipment connections, needs to accommodate thermal reactions during systems start-ups and shutdowns. Piping should be equipped with a sufficient number of appropriately sized drip legs to promote effective condensate drainage and should be pitched properly to promote the drainage of condensate to these drip lines. Typically, these drainage points experience two very different operating conditions, viz., normal operation and start-up. Both load conditions should be considered in the initial design.
Mechanical type moisture separators with traps should be provided in piping at interval, to separate the fine moisture particles in the steam. Automatic air vents should be fixed at the dead end of steam mains to allow removal of air / non-condensable which tends to accumulate in steam space.
• Steam pipe sizing and redundancy
Proper sizing of the steam pipelines involves selecting a pipe diameter which gives acceptable pressure drop between the boiler and the user. Pipe sizing can be done either based on the velocity or on the desired pressure-drop.
Pipe sizing can be done from the general recommendations on line velocities of different fluid based on the specific volume of steam for the chosen distribution pressure and quality of steam, whether wet or superheated. The velocities for various types of steams are:
• Superheated 50-70 m/sec
• Saturated 30-40 m/sec
• Wet or Exhaust 20-30 m/sec
Unused steam piping experiences the same losses as the rest of the system. It is therefore imperative to isolate the unused steam lines immediately. Pipe routing is made for transmission of steam in the shortest possible way, as so as to reduce the pressure drop in the system.
• Steam pressure
The steam distribution pressure should be adjusted in accordance with the pressure generated and the pressure required at the consumer side. If steam piping already exists then the pressure should be adjusted for lower operating cost.
However, at the designing stage, it is desirable to consider steam distribution at the same pressure as the source, or at a moderately high intermediate pressure; if the generation is at very high pressure. Distribution the steam at the same pressure that of source has the following advantages:
• The steam velocity along within the pipes will be lowered and this reduces both noise and erosion
• It provides stable pressure at the user end due to lower pressure drop and higher operating margins.
• The capital cost is reduced as the pipe line is so smaller in size.
Nonetheless, for long distribution systems, it is economical to super-heat the steam to minimize the steam losses. The piping needs to be properly sized and well insulated.
Estimating pressure requirements for small distribution systems is relatively simple; viz it should just meet the minimum user requirement, unless future expansion of the system or new equipment requiring higher pressures is envisaged.
For systems where only a small quantity of high pressure steam is actually required, but where large quantities of low pressure steam are used, the possibility of separating the two should be considered.
4.5.2 Steam line sizing calculation
• Operating Pressure ( P ) :- 4 kg/square cm
• Operating Temperature ( T ) :- 150
• Mass flow Rate ( ? ) :- 0.0625 TPH
• Design Pressure ( Pd ) :- 4*1.2= 4.8 kg/square cm
• Design Temperature (Td ) = Operating Temperature + 10
= 150 + 10
• Specific Volume At Design Temperature and Design Pressure From Steam Table
? = 0.4709 m³/kg
• Allowable Stress (? ) = 1202kgf/cm² ( From ASME B31.1 Power Piping Table a’1)
• Assume Velocity ( V ) = 22 m/s
• Required Inner Diameter Of Pipe, ( d )
• Select Outer Dia. Of Pipe From ANSI B36.10 Carbon Steel Seamless Pipe Data Book
D0 = 33.4mm
• Minimum Thickness ( Tm ) = + A
= + 0.75
= 8 mm
• Theoretical Velocity =
= 14.68 m/s
Theoretical velocity is less than the assumed velocity so that the calculated diameter is suitable for steam carrying pipe for these conditions.
As above all the pipe sizing can be calculated and is given in the sheet format.
4.6 Insulation of Steam Distribution System
4.6.1 Existing Condition Of Insulation in Plant:
The steam distribution system of plan is very badly insulated. Its insulation was done 2 years ago and at present the condition of steam pipe insulation is very poor.
The figure 4.9 shows a photograph part of steam distribution system. It is clearly visible that insulation is badly needs repairing and also at some places no insulation is provided, causing great heat loss. The insulation provided on the steam pipe lines is glass wool and thickness of insulation is also not sufficient. So we have collected the data about different insulation material. Also we have given list of local insulation carrying consultancy. Here are some insulation material properties