How to make pipe sizing in vacuum system

simonverville

I would like to know how to determine pipe size in a vacuum system.
On most of pipe charts, we see the affect of steam pressure on maximum flow rate but we never see the imapct of vacuum (of the condensate side) on the flow rate.
Is there old pipe chart that exists that would consider vacuum heating system ?

Jamie Hall

In a closed system -- such as a steam heating system -- it is the pressure differentials which are of interest, not the absolute pressures, in determining flow.

Therefore... it makes no difference whether you have 2 psig steam in the boiler and 0 psig (atmospheric) in the returns or 0 psig in the boiler and -2 psig (vacuum) in the returns.

Size the pipes as usual.

simonverville

Ok, It makes no difference on a delta P point of view... However, with a lower pressure comes a lower density, thus for the same flow rate, steam will travel at a higher velocity. So it could exceed maximum allowable velocity, no?

Jamie Hall

The differences due to different absolute pressures would be minor... unless you are really up at the fringes. There reaully is no maximum velocity for steam once it's in the mains, but you don't want to go supersonic...