Having dealt with the merits of high pressure steam distribution and low pressure steam usage, let us now look at how to select the right steam pipeline size for distribution and understand why steam pipe sizing is so important.
The importance of the steam distribution system is to pick steam from the steam generator (Boiler) and deliver it to the user section, as per its requirement both quantitatively and qualitatively.
- Quantitatively: Steam provides the requisite heat load required by the Process for which steam has to be made available in the right quantity. Quantitative delivery envisages maximum steam flow required by the process considering seasonal variations, plant load factors, and process variations. Also, the delivery of steam should be at the pressure required in the process / by the process equipment.
- Qualitatively: Qualitative delivery envisages steam quality viz dryness faction, free of contamination, safety factors to suit process batch cycles expected.
Pipe Sizing for Saturated Steam
For a given application, there is only one technically right pipe size. If the piping design calculations are not done right, the steam pipeline may end up undersized or oversized – both have adverse effects as given below-
What will happen if the steam pipelines are oversized ?
100 NB pipe size selected in place of correct size of 80 NB will have the following implications :
- 100 NB line creates less pressure drop for the same flow rate than 80 NB line.
- So, the steam user will receive steam at the required pressure. There will be no chances of steam starvation and steam availability is reliable.
- The 100 NB line reduces the noise associated with the steam flow.
- However, 100 NB line is more expensive than 80 NB. Also, its installation and insulation costs are greater.So, it is not economically viable to install oversized pipelines.
- 100 NB line has greater surface area, so the heat transfer area increases. Consequently, convection and radiation losses are more.
Thus, the piping losses increase and greater amount of condensate is formed. More steam has to be generated to make up for this loss thus increasing steam costs.
Hence, correct sizing of steam lines as part of the piping design calculations is critical to ensure lowest possible piping losses.
What will happen if pipelines are under-sized?
80 NB pipe size selected in place of the correct size of 100 NB line will have the following implications –
- 80 NB causes a greater pressure drop than a 100 NB line for the same Steam flow rate. Thus, the steam users receive steam at low pressure which may not suit process parameters.
- 80 NB pipe has less volumetric flow carrying capacity as compared to 100 NB. Thus, it will not be able to deliver steam to the steam users at the required flow rate and lead to steam starvation. This again will affect process parameters and hamper production rates/quality of products
- Due to undersized pipe size, the velocity of steam increases and causes water hammer and erosion. Waterhammer has hazardous effects.
Thus, right Pipe sizing for a given application assumes significance. Pipe sizing is done based on two methods:
Velocity method and Pressure drop method of Pipe Sizing
Velocity Method
In fluid velocity method, the steam velocity is assumed and the line size is calculated for the required flow-rates by the following formula:
Q = A x v
Where, Q = Volumetric flow-rate (m3/s)
= (Steam Flow in Kg/hr * Specific Volume M3/kg) / 3600
A = cross-sectional area of a pipe formula (m2) = D24
v = steam velocity (m/s)
Since Q and v are known values in the above equation, A and hence D can be calculated!
The velocities assumed for calculating the line size for the following are:
Flash steam = 15 m/s
Saturated steam = 25 m/s ……for long distance travel
Saturated steam = 30 m/s ……for short distance travel
Superheated steam = 30 to 40 m/s ……depends on piping length and pressure drop.
The steam velocity should not exceed the above values as it can cause heavy water hammer effect and greater pressure drops.
Let us consider following application for sizing of Steam pipes for two plants as per the parameters given below -
Steam Pressure = 7 Barg
Steam Flow at Peak Load = 3025 Kg/hr
Distance between Boiler and user for Plant 1 = 90 Mtrs
Distance between Boiler and user for Plant 2 = 300 Mtrs
Using the Steam Pipe Sizing Table(based on Velocity method) we can select pipe size of 100 NB corresponding to steam pressure of 7 Barg, velocity of 25 M/s
Demerit of Velocity method :
The demerit of velocity method is that it does not consider the length of the distribution line. Thus, it treats both the above plants with varied lengths on the same basis to select the same pipe size of 100 NB for both. It is expected that plant with greater length will have increased pressure drop as compared to smaller length for the same pipe size. We need to estimate pressure drop for the given pipe length and verify whether the same is within acceptable limit.
Using these sizing charts we can correctly select required pipe size for both the plants as below :
Plant 1 – Pipe Length 90 Mtrs Select 100 NB
Plant 2 – Pipe Length 300 Mtrs Select 125 NB
Criteria for allowable Pressure Drop over a given length of Piping
Pressure drop over a given pipe length shall be equal to 10 % of the Steam Inlet Pressure OR 1 Barg whichever is less.
Using this criterion and two tables (Velocity Method and Pressure Drop) we computed Pipe sizes required for the two Plants.