Some information about a common method for both saturated and superheated steam to determine velocity/pressure drop/ pipe diameter.
Before taking the next step for calculation, we should choose the calculation method and acceptable velocity range.
1- For short pipe up to 50 m, size the line based on velocity because the pressure drop is negligible.
2- For long runs, check the pressure drop as well as velocity.
The velocity:
1- Should be between 25 to 40 m/s (4920-7872 fpm) for saturated steam. Low velocity as low as 20 m/s (~4000 fpm) is acceptable and, also, higher velocity than 40 m/s would be acceptable providing that the line is short and straight, horizontal and well insulated to make sure the steam is dry.
2- Can exceed above limits for superheated steam, because it is dry and there would be no chance for errosion due to water droplets.
PRESSURE FACTOR METHOD:
What required:
1- Pipe Length (including straight length plus the equivalent lenght because of all fittings, valves, etc.)
Here, there are some shortcuts:
a) If straight length< 50 m —---> Pipe Length= 1.05* straight run
b) Longer than 50 m with a few fittings —---> Pipe length= 1.1 * straight run
c) Longer than 50 m with more fittings —---> Pipe length= 1.2* straight run
2- Inlet and outlet pressure
3- Actual flowrate (process flowrate plus the condensate flowrate). Some of the steam gets condensed due to natural heat transfer. Convection and radiation heat transfer should be calculated to know the condensate load. But, choose 3.5% condesate load per 100 m as a rough estimation. Please note that total pipe length MUST be used to calculate actual flowrate not only the length of straight run.
Example: How much is the actual flowrate when 100 t/hr steam flows through a 250 m straight run pipe with several bends to drive a turbine.
1.2* 250= 300 m
every 100 m produces 3.5% condensate:
(300/100)* 3.5%= 10.5%
Actual flowrate= 1.105* 100= 110.5 t/hr
It means although the turbine needs only 100 ton/hr to operate, boiler should supply to 110.5 t/hr. This is a rough estimation.
Pressure factor method employed one formula and two tables to give us the pipe diameter (internal diameter). The tables should be used to find factors for inlet and outlet pressures. Then, calculate pressure factor:
F= (P1-P2)/L
Where :
P1= factor for inlet pressure
P2= factor for outlet pressure
L= Actual Length
How to validate the result:
Check the velocity to see whether it is in the range?
If yes, enjoy it.
Otherwise, the piping should be modified.
Pressure factor method is good for the time that pressure drop is constant and maximum allowable pressure drop is already determined. So, the pipe length and diameter should be chosen such that velocity is in the allowable range.
This is the last part of velocity/pressure drop/sizing based on Spirax Sarco procedure.
NOMOGRAM METHOD:
There are two nomograms applicable to both saturate and superheated steam.
a) Pressure drop chart
What you have: Steam conditions, steam flowrate and inside pipe diameter
What yoy find: Pressure loss bar/100 m
so you don't need to know the steam velocity. Steam velocity should be validated to make sure it is in the allowable range.
b) Velocity Chart
What you have: Steam conditions, steam flowrate and steam velocity
What you find: Pipe diameter
****STEAM PIPELINE SIZING CHART****
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