Flash Steam Recovery and Flash Vessels
Hot condensate dropping to a lower pressure releases flash steam whether it is wanted or not. A flash vessel catches it at a useful low pressure, separates it from the remaining water, and sends it to a user that needs low-pressure steam. The water goes on to the feed tank.
You'll learn
- Explain how a flash vessel works
- Calculate the flash steam recovered
- Match flash supply to a low-pressure user
- Lay out a flash recovery system correctly
Before this: Flash steam, What condensate is worth
Catching the flash
Condensate from high-pressure equipment is led into a vertical vessel held at a low pressure. Part of it flashes to steam; the steam rises and leaves at the top into a low-pressure steam line, while the remaining water collects at the bottom and drains through a trap to the condensate return.
The vessel is wide enough that the steam rises slowly, letting droplets fall back. Without it, the flash steam goes up the vent of the feed tank or condensate receiver as a white plume — heat and treated water lost to the sky.
How much it recovers
- Condensate arrives saturated at 183 °C.
- Flash fraction, from the flash formula:
- Flash steam: 249 kg/h at 120 °C, occupying 221 m³/h.
- Water to the return: 1,751 kg/h.
- Good users: deaerators and feed tank heating, low-temperature process heating, air heater batteries, space heating, pre-heating stages.
- The user must need at least as much low-pressure steam as the vessel produces, at the times it produces it; surplus has to be vented or made up with live steam through a reducing valve.
Making a flash system work
- Flash vessel pressure sets the yield: lower pressure means more flash, but less useful steam. Choose it from the user’s needs.
- The equipment upstream must still drain: its traps discharge against the vessel pressure, which reduces their differential. Check it, especially on temperature-controlled equipment.
- Fit a float trap on the vessel’s water outlet, a safety valve on the vessel, an air vent, and a pressure gauge.
- Make up a shortfall in flash with live steam through a PRV into the low-pressure line; dump a surplus through a back-pressure relief valve.
- Size the inlet and steam outlet for the flash volume, which is large; use a tangential inlet so the water separates.
Key takeaways
- A flash vessel separates flash steam from hot condensate at a useful low pressure.
- 2,000 kg/h of 10 kg/cm²(g) condensate into a 1 kg/cm²(g) vessel yields about 249 kg/h of steam.
- Match the flash supply to a low-pressure user in time and quantity.
- Check upstream traps can still drain against the vessel pressure.