Leveraging the inherent properties of steam

Are you losing out by not leveraging the inherent properties of steam to your advantage?

Studies across thousands of plants covering diverse sectors show that plants often fail to effectively leverage the properties of steam to their advantage. This article highlights two very important properties concerning steam distribution and its utilisation.

1. The specific volume of steam decreases with increasing pressure. Thus, just by opting to distribute steam at high pressure you can make it possible for steam to be carried through smaller bore pipelines. The use of smaller bore pipelines brings significant advantages in day to day running costs (OPEX) due to lower surface area (lower radiation losses) and CAPEX (lower material costs on pipes, insulation and racks and supports).

The illustration below indicates the required pipeline size (commercially available) to cater to a steam flow requirement of 2.5Tons/hour when distributed at about 9bar(g), which is close to the boiler rated pressure for most process boilers i.e. 10.54bar(g), vis-à-vis distributing it at say 3bar(g) by installing a PRS within the boiler house itself

The radiation loss above is computed considering a steam pipeline length of 100 meters with commonly used rockwool and aluminium cladding with surface temperature as +15 o C for both cases.

Besides significant radiation losses (fixed losses that the plant will incur throughout the year) by selecting to distribute at a lower pressure the plant will also incur a higher cost on account of piping, pipeline accessories, support and structure and insulation by about 20%.

However, since more steam can flow at higher pressure due to its lower specific volume, using higher pressure than design in ejectors for example would lead to higher steam consumption than required. For example, a plant supplying 8.9barg steam to a four stage ejector instead of 7 bar(g) ( recommended OEM design supply pressure) was leading to a 22% higher steam consumption.

2. Latent heat of steam increases with decreasing pressure. Most of the energy content of steam is its latent heat which is utilised in indirect heating applications. Thus to put it simply, using steam at the lowest possible pressure, means more useable heat is actually available for the process, thereby resulting in lower steam consumption (see table below).

As a good engineering practice it is best to select the supply steam pressure as recommended in the steam table; i.e. selecting the pressure that corresponds to a value equal to maximum process temperature + 30deg C.

Typically, we observe that using steam at pressure higher than required easily leads to 2-4% higher steam consumption in the process on an average. The examples below are from a chemical plant we recently audited.

At one of many heating vessels in the plant we found that the process required a temperature of 99deg C. While this process temperature could easily be achieved by using steam at say 2.5barg the plant was actually using steam at 4.5barg. The latent heat of steam at 2.5barg is 513Kcal/kg which is higher than the latent heat of steam at 4.5barg i.e. 501kcal/kg. Thus, to meet the batch heating load requirement of 402,254Kcal, 803kgs of steam would be required at 4.5barg whereas only 784kgs of steam would be required at 2.5barg; a difference of almost 2.4%.

This practice of using higher steam pressure than required is prevalent across industry segments – such as Food and Beverage, Chemicals, Pharma, Textiles, to name a few – and losses incurred as a result can easily be curbed by observing the above mentioned practices.