Steam pressure is governed by certain factors as mentioned below :
- The maximum allowable safe working pressure of a boiler
- The right pressure requirement of a plant process
When steam travels through distribution network, the pressure drops due to :
- Frictional resistance within the pipe which in turn is determined by pipe sizing and velocity.
- Condensation within the pipe
All these factors must be considered while designing the distribution network. Steam should be generated from the boiler at rated pressure for best steam quality. The specific volume of steam at higher pressures is lower and hence steam can be distributed through smaller sized steam pipes. This results in reduced radiation losses and lower capital investment. Below is a table illustrating the impact of pressure on specific volume of steam.
To summarize, generating and distributing steam at higher pressure is advantageous because :
- The quality of steam generated is dry saturated.
- Steam mains required are of smaller size and this results in lower capital costs for piping, flanges, supports, insulation and labor.
While steam must be generated at higher pressures it is advantageous to utilize steam at lower pressures. Reducing pressure at the point of utilization ensures savings. As steam pressure reduces, the latent heat of steam increases. Hence it is always advisable to use steam at lower pressures.
For example- The difference in latent heat between steam at 10 bar absolute and 3 bar absolute is 35.64 Kcal/hg. Now consider that an equipment requires 500 hg/hr of steam. Therefore the heat energy saved will be 500*35.64=17820 Kcal/hr. also the corresponding steam load will be 17820/517.46 = 34.44 hg/hr.
The increase in the latent heat at lower pressure is further illustrated in the table below
Secondly, reducing pressure at the point of usage also ensures that steam delivered has a higher dryness fraction.
Consider steam traveling at a pressure of 9 bar g having dryness fraction of 0.95. The pressure of steam is now reduced to 3 bar g. This pressure reduction leads to an increase in dryness fraction that can be calculated using the formula,
H = Hf + XHfg
Where,
H = Total heat of Steam
Hf = Enthalpy of Fluid
X = Dryness Fraction
Hfg = Latent heat of Steam
At 9 Bar g,
Hf = 182.54 kcal/kg and
Hfg = 481.82 Kcal/kg.
Steam with the dryness fraction of 0.95,
H =640.27 Kcal/kg.
Pressure reducing station is an isenthalpic device (i.e. there is no work done). Hence, ‘H’ at inlet and outlet is same.
With reduced pressure to 3 bar g, Hf = 144.74 kcal/kg and Hfg = 510.29 Kcal/kg. Total heat i.e. H remains constant, however the dryness fraction increases to 0.97. This example illustrates that a reduction in steam pressure enhances the quality of steam.
It is equally important to select correct pressures. For saturated steam, pressure and temperature have a relation over the complete range. With increase in pressure, the corresponding temperature also increases. For heat transfer, it is important to have a temperature difference between primary and secondary fluids.
In this case, steam is the primary fluid and process fluid is secondary fluid. Recommended temperature difference should be between 30 Deg. C to 40 Deg. C. i.e., steam temperature should be higher by 30 to 40 Deg. C. than the final desired secondary fluid temperature.
The pressure of steam selected should be corresponding to the temperature of steam. For example if the final temperature required in a process is 110 degree C., then the ideal temperature value for steam is 145 Deg. C. and corresponding pressure is bar (a) or 3.16 bar(g).
It should be verified that the heat exchanger has the necessary volume to accommodate the required flow rate of steam (at lower pressure).