Recent publication of BG03 – Boiler Blowdown Vessels has led to some concern that existing vessels may not have adequate size vent or drain pipework.
It is essential to bear in mind that the dimensions shown in BG03 are stated to be typical: they are not mandatory and/or minimum. The actual size will depend upon the manufacturer’s design, and in particular the boiler operating pressure, and the size and the equivalent pipe length between the boiler and the vessel inlet.
Water being discharged by blowdown will produce flash steam as it approaches atmospheric pressure, which must form without significant pressurisation of the vessel. Depending upon design this is often limited to a maximum of 0.3 barg. The proportion of flash steam will increase as boiler pressure increases, so it may in some cases be necessary to have a larger flash steam vent than the typical size shown in BG03; conversely, at low operating pressures, a smaller vent will pose no problem.
Whilst the size of the main blowdown valve will initially dictate the flowrate, the equivalent pipe length (taking into account actual length, plus the flow restriction of bends and valves) will determine the flow resistance, and therefore the actual flowrate into the blowdown vessel. Internal design of the vessel may optimise separation of flash steam from water entering the vessel. It is consequently inappropriate for any guidance document to advocate specific connection and pipe sizes. Hence the dimensions shown in BG03 are shown as typical examples, and not intended to be specific.
Owners of tanks installed in accordance with PM60 should only need to consider BG03 section 6 if there have been, or they intend to make, changes to the original installation.