Steam System | Energy Conservation in Plant


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Document Category Engineering
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Steam System – Boiler plants are used extensively by industry. Every boiler requires a distribution system to transport the steam. Steam is distributed to various processes via a pipeline network.

Steam System | Energy Conservation in Plant

A distribution system has three main elements: pipes, insulation, and traps. Efficient energy use depends on their correct operation.

The steps taken in quick assessments to evaluate a steam system are limited to a visual inspection. The inspection will enable the operator/energy surveyor to identify pipe size, distribution pressures, missing insulation, and leaks. Some of these steps are often repeated during the normal operation time. The energy performance of a steam system is evaluated by testing the system for:

– Steam pressure and temperature
– Improperly functioning steam traps
– Leaks and missing or damaged insulation

Steam pressure is measured by means of pressure gauges. The pressure is measured at existing pressure sensor points. A calibrated gauge head is used in place of existing heads. The existing head reading is noted, and then it is unscrewed from its position and replaced with the calibrated gauge head. The new reading is noted. A wide variation between the readings indicates that the existing gauge is defective. A steam pressure significantly lower than that expected for the steam line is an indication of a steam leak.

Steam temperature is measured using either a surface probe thermocouple or an infrared pyrometer. The surface probe thermocouple is placed in direct contact with a bare portion of the steam line, and the pipe temperature is indicated on the thermocouple readout. This temperature will be slightly lower than the actual steam temperature, but the temperatures difference will be very small.

The energy surveyor can use the temperature in conjunction with steam tables to verify steam pressure when it is not possible to measure pressure directly. The infrared pyrometer is used to measure steam pipe temperature without making physical contact. The emissivity of the steam pipe is determined by reference to an emissivity table.

Alternatively, paint can be applied to the surface to be measured. The paint is manufactured to yield a known emissivity; the emissivity valve is set on the pyrometer.

Any steam main will condense some steam owing to radiation heat losses from the surface of the pipe. For example, a 100-mm (4-inch) insulated pipe 30 meters long in 10°C air will condense 16 kg of steam per hour. Although this amount probably represents less than 1% of the pipe capacity, at the end of an hour the pipe would contain not only steam but also 16 liters of water. Provision must be made to remove this water from the steam main, or the main would eventually become flooded. This is achieved by using drain pockets and steam traps. Similarly, it makes good sense to run the steam main with a fall in the direction of the steam flow.

Steam typically travels at velocities between 65 and 80 km per hour. If condensate were draining in the opposite direction of steam flow, it would be difficult for the water to collect and hence, be removed from the pipe. Also, it would make the steam wet and could cause water hammer. Water hammer occurs when a slug of water is forced along a pipe by steam flow.

The steam pushes the slug of water until the pipe changes direction. The slug hammers against the pipe and can cause erosion and eventual pipe failure. By allowing the flow to be in the same direction, drain pockets can be situated at regular intervals (30-50 meters), and draining can take place. Drain pockets must be of adequate size to collect the water. Small drain pockets will not cope with the problem, and the main will become waterlogged. A 100-nun drain pocket will serve mains up to 150 mm; a 150-mm drain pocket will serve a 200-mm main, and so on.

It is important for the plant operators to;
Prevent steam loss and maximize the use of the energy contained in the steam (latent heat)
Remove condensate to maximize heat transfer and prevent steam hammers.
Remove air and other non-condensable gases to improve heat transfer coefficient and reduces corrosion.

Briefly, types of steam traps include, Thermostatic traps such as liquid expansion trap; balance pressure trap and bimetallic traps, Mechanical traps such as; ball float trap and inverted bucket trap and Thermodynamic traps.

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