Background Theory
To calculate the pressure-drop in straight pipe, Darcy-Weisbach equation is used:
For the friction factor in turbulent flow regime, the Colebrook-White equation is used
which is as follows:
In addition, the equivalent length method is used to calculate minor losses.
Substituting the f(L/D) term with a head loss coefficient K the following equation
can be used for total head loss coefficient:
Finally, the equivalent length considered for fitting and valves are extracted from the
following table:
Regarding the allowable range for pressure drop in steam piping, following numbers
were referenced:
In addition, the following equation and steam velocity criteria is considered in the
pipe size checking step:
Assumptions












Mass flow rate of each UH is considered as 280 lb/hr.
Mass flow rate for the existing UH is considered to be 2*280 lb/hr.
A condensate load of 4% is considered to be added to the total mass flow rate.
For the existing UH, the inlet and outlet nozzles are 1-1/2".
The elevation provided for UHs is the centerline elevation. The nozzles are
located 1’ ½” away from the center.
One gate vale is considered at the inlet and one gate vale is considered at the
unit heater outlet.
Size of the gate valve for the new UHs assumed to be 2” (the picture for the gate
valve shows that they are 1 ½”).
The main plan is referenced as the unit heater’s location, elevation, and pipe
routes.
Elevation of the steam main is considered 22’ from top of the paving.
Piping material roughness for black pipe is assumed to be the same as cast iron
being equal to 0.01 in.
The effect of added roughness to the piping material due to reuse is not applied.
For the minor loss calculations, elbows are considered to be STD/ SR.
Design Recommendations



The boiler steam line should always come in the top of the main steam header.
The main steam piping should always be insulated.
Air vents/vacuum breakers should be placed on the steam main header to vent
off air and prevent condensate aspiration back into the steam mains and boiler
upon cool down.




The header piping and all subsequent piping should slope a minimum 0.25
inches per 10 ft in the direction of steam flow.
At the base of each slope, a drip leg assembly should be located.
Reducer, if any, used in the steam main line should be eccentric with flat side
down.
The drip legs should be installed at least one per 200 ft of steam main piping or
at any low point condensate dead leg location.

Recommended drain pocket dimensions are shown in below table:

The trap line should be at least 25 to 30 mm from the bottom of the pocket for
steam mains up to 100 mm, and at least 50 mm for larger mains.
It is better to fit a flange or blowdown valve at the bottom of the pocket for
cleaning purposes.
All drip legs should be properly trapped and be drained prior to each cold system
startup.








It is recommended to install check valves after all steam traps to prevent back
flow of condensate into the steam line during shutdown.
Branch line connections shall from the top of the steam main.
It is recommended to position an isolation valve on branch line as close as
possible to off-take location.
If the branch has drop leg, a drain point with steam trap set should be considered
prior to the user isolation valve.
Special attention should be directed toward designing the piping network with
sufficient flexibility to accommodate expansion resulted from hot loads.
Pipe supports should be provided at intervals not greater than shown in the
following table (span required to accommodate dead loads):