Steam quality is a measure of the amount of liquid water contained in steam generated by a boiler. It is often expressed as Percent Dryness. Typical low pressure boilers have a Percent Dryness of around 99%, which means that there is 1% moisture in the steam. That 1% is important, because moisture, in the form of condensate, contains less heat energy than dry steam, and as a result a pound of wet steam does less work than a pound of dry steam. Put another way, in order to drive your plant, you will need to spend more money on fuel to accomplish the necessary work, if your boiler is producing wet steam instead of dry steam.
In many boilers, an upgrade of the boiler system can be the solution to improving steam quality. Upgrades might provide improvement if your boiler is more than 10 years old, and the original separators are worn or loose. An upgrade might also improve the performance in a system where the original separators were not well matched to the boilers actual application. The benefits from dry steam are gained in three main areas, which are drier stream, reduced blowdown, and improved purity. To get an idea of the economic impact to each of these areas, let’s take the example of a 125 psig boiler, generating 100,000 pounds per hour of steam at 99% dryness. It uses natural gas at $6.00/MSCF, and operates with 75% condensate return.
At 125 psig, steam contains 1109 Btu/lb, while condensate has only 324 Btu/lb. Since this boiler generates steam at 99% dryness, the 1% that is condensate reduces the work load that can be done by the steam by 0.7%. This doesn’t sound like much, but over the course of a year, it can increase fuel costs by over $150,000 in this example.
The water that is entrained in the steam is not pure. It will contain any contaminants that have made it through the pretreatment system as well as boiler treatment chemicals. These dissolved solids will eventually evaporate out on equipment contacted by the steam, and can form solid masses of deposits. They might start out small, but over time, the deposits could become large enough to restrict steam flow in pipes, through valves or across heat exchangers. in a worst case scenario, these deposits can, and have, caused unexpected plant shutdowns, which can cost a company tens of thousands of dollars per hour.
By reducing carryover of dissolved solids in the steam, the return condensate quality, and as a result, the boiler feedwater quality, will be improved. This will be directly measurable in terms of lower feedwater conductivity.
For this example, our reduced feedwater conductivity allows for an increase in boiler cycles of concentration, resulting in reduced blowdown of heated boiler water. This has a direct impact on fuel usage, which in this case would mean a reduction to the annual gas bill of $31,000.
There are several companies that sell aftermarket steam separators, and there are many designs. A typical installed cost for a water tube boiler like the one in our example would be between $45,000 and $60,000. So, could your plant benefit from an upgraded steam separator? In many cases, the answer is a definite yes.