At one time I was working for a client who wanted to replace a pressure reducing valve (PRV) in a steam distribution system with a backpressure turbine. As I was working on the heat and mass balance for this project I realized I didn't have definite information about what fuel would be used to provide the additional heat input required to the boiler. I therefore called the client's project engineer and asked the question and a long silence ensued. He then told me that they were not planning on burning any additional fuel, since they were merely replacing an existing PRV with a turbine.
This engineer held the not so uncommon belief that a PRV is an energy waster and replacing it with a backpressure turbine/generator set will recover this "wasted energy". In reality the enthalpy of the steam leaving the PRV is exactly the same as entering (ignoring the negligible heat losses through convection and radiation). On the other hand, when using a turbine to reduce steam pressure, the enthalpy of the steam is also reduced in the process.
There are two potential scenarios in order to maintain status quo with respect to the downstream process. If the PRV services the process directly without desuperheating, a higher steam flow will be required through the turbine in order to compensate for the lower enthalpy. If the PRV is followed by a desuperheater (which is quite common), the desuperheater spray flow will be substantially reduced and the steam flow from the boiler has to be increased accordingly. The bottom line is that in either case, the additional energy required from the boiler (through increased steam flow) is equivalent to the shaft power delivered by the turbine.
Lesson: Don't ignore the first law of thermodynamics when evaluating the pros and cons of changing a steam distribution system.
When inspecting this piece of ductwork internally it turned out it had been sucked together like a paper bag and the actual cross sectional area was maybe 20% of the original. The insulation and lagging was installed like a shell around the ductwork without being attached to the duct walls. From the outside everything therefore looked perfectly intact. There were obviously not enough stiffeners on the duct to withstand the pressure differential.
Lesson: Things are not always what they appear to be.
Checking the pressure gage on the deaerator indicated a positive pressure of 10 psig, which corresponded to the static head difference between the boiler drum and the deaerator storage tank. However, the system had been shut down for a couple of days and the water in the boiler as well as the deaerator storage tank were at ambient temperature. Granted the makeup water was fed from the city water line at maybe 40 psig pressure, but the deaerator was vented to the atmosphere, so how could it hold positive pressure?
The vent valve was verified open, so the only option left at this point was to break out the pipe wrenches and start to disassemble the 1/2" horizontal vent pipe going through the boiler house wall. As soon as I got the vent pipe loose, a whooshing sound was emitted from the vent valve as the the air that had been trapped in the deaerator all of a sudden escaped. With the vent pipe on the floor it was obvious that it somehow had become completely plugged. Rodding it out revealed dirt particles and insect parts that would be associated with dirt dauber nests.
Lesson: Dirt daubers are capable of building a nest in two days that is completely air tight inside a 1/2" pipe. No matter how long you work in a particular business there will sooner or later be a set of circumstances you have never before experienced.