Piping capacity

[NewMechanicalEngr]

Good morning,

As my handle states, I am a new mechanical engineer and I need assistance from you experienced engineers.

I need to determine if I can branch off an existing natural gas line that currently supplies gas to boilers on the 18 floor of a hotel. I have to tie in a 1-1.5" line for a 30kW backup generator without affecting the performance of the boilers.

There is a 6" main gas line that comes from the street. The line travels up to the 18th floor where it reduces to 4" before going to the boilers. We are installing a 30kW backup generator on the same floor and need to get a natural gas supply. We would like to branch off the boiler supply with a 1 - 1.5" line. However, I need to determine that this will not affect the boilers.

Questions:
1. Is this a very complex determination?

2. Is there any information I need other than what I listed below?
I assume:
Line pressure to boilers
Flow rate to boilers
Pressure and flow rate needed to generator

3. What advice can you give me in regards to the best methods to make this determination?

Any help, guidance, suggestions, etc would be greatly appreciated.

Regards...

 

[PEDARRIN2]

One thing you may need to consider is how to shut off the gas to the existing equipment while maintaining the gas to the emergency generator.

When I have a project with an emergency generator, I am required to place shut off valves outside on the ground floor (where the fire department can get to it) I am also required to put signage indicating what each valve serves.

This allows the fire department to shut off gas to everything but the emergency generator thus minimizing the potential of the gas creating a bigger fire hazard.

You might want to see if you are required to do the same.

 

[TBP]

Look in the fuel utilization code for your jurisdiction. They normally have tables showing line sizes required for various loads, based on the length of the run.

 

[MortenA]

If the 1.5" line is sufficient to supply the generator - then i would assume that there will be no influence. A 6" line has about 16 times the capacity of a 1.5" (when using velocity as the sizing factor).

BTW the difference between the cross sectional area for a 6" and a 4" is still 9 time larger than the area of a 1.5"

Best regards

Morten

 

[TD2K]

Basically, you know the gas flow to the existing unit. You know the gas flow to the new unit (to be supplied through the 1.5" line).

Add the two together (I'm assuming both will be running at the same time at full rate) and see if the combined flow can be delivered through the common section of piping. Essentially, you'll have a greater pressure drop and whether that is acceptable depends on your operating conditions.

Crane's technical paper 410 would be a highly recommended reference to get if you don't have access to a copy. It will work you through these calculations.

As Morten said, I would suspect you don't have a problem but you need to satisfy yourself this is the case. You'll come across this type of an analysis many times.

 

[pmover]

answers to your questions:

1) depends upon knowledge and skill level, but generally not complex for an engineer. one would expect you to be able to perform such a task

2) ASSUME pipe pressure or do you KNOW pipe pressure?
need to know max & normal flow, pressure and temp requirements AT boiler and AT gen set. pipe size shall then be determined to each equipment (if any branch connections) and from source tap exterior of building in order to meet pressure and flow requirements at each equipment.

3) highly recommend adhering to TBP advice in obtaining direction from local jurisdiction fuel gas code. may want to further explore NFPA 37 & 110 requirements and size the piping to limit fuel gas velocities at recommended values. be sure to install proper isolation valves for equipment/piping systems.

otherwise, good luck and have fun!
-pmover