Steam should always be distributed at high pressure as it has the following advantages :
- The specific volume of steam decreases as its pressure increases
Steam required for the process is a certain mass flow, depending on the process heat load. At high pressure steam is compressed and has a lower specific volume. Hence, at high pressure volumetric flow reduces for the given mass flow. For gaseous mediums, as pipelines carry volumetric flow, the pipeline size required to transport steam at higher pressures is less.
Smaller bore pipes, not only reduces the cost of piping, associated pipeline accessories, insulation (lagging) and support structure but also means lower heat losses due to the surface area of pipe being smaller, leading to monetary benefits.
- Reduction in Steam Pressure results in an improvement in its dryness fraction
Generally, in most process industries, saturated steam boilers are used. Steam generated by a saturated steam shell type boiler is typically 98 - 98.5 % dry. When this steam is distributed at high pressure and its pressure is reduced at the point of use, the dryness fraction improves. This happens because high pressure steam at inlet of the pressure reducing valve has a higher heat content and when the pressure reduces it can no longer hold that heat content and the excess heat actually gets used up to dry the steam.
Wet steam has less latent heat to offer, corresponding to the degree of wetness it has, and leads to higher steam consumption for a given process heat load. Improvement in dryness fraction leads to availability of greater heat for the process, thereby reducing steam consumption and thus Opex.
- Generating steam at close to design pressure increases the thermal storage capacity of the boiler
We’ve noticed that many plants generate steam at pressures significantly lower than the boiler design pressure. Their rationale is that the process requires low pressure steam and hence they generate and distribute it at low pressure.
However, generating steam at lower pressure reduces its thermal storage capacity. The thermal storage in the boiler shell acts like buffer. When the process steam demand suddenly increases, boilers with reduced thermal storage struggle to meet the increased quantity of steam thus leading to carryover, pressure drop and steam starvation which is not desired.
Here’s an illustration of how much reduction in thermal storage capacity of a 9 barg rated boiler operated at 6 barg?
If Boiler shell has 5000 kg of water
Its thermal storage at
- 9 Barg = 5000*181 Kcal=9,05,000 Kcal
- at 6 Barg = 5000*166 Kcal=8,30,000 Kcal (8.3 % less)