Boiler Piping Modification

[uwm]

We have a boiler we operate at 15 psi. It is rated at 8625 lbs/hr at 150 psi but we operate it as a low pressure boiler. The pipe leaving the boiler going to the 6" steam header was 3". We replaced this 3" pipe with a 6" pipe and are now having problems heating our process tanks to the desired temp. Any ideas on why we lost heat. Also does anyone know how many lbs/hr the boiler would be putting out at 15 psi with the 6" pipe vs. the 3". Thanks

 

[TBP]

What about all of the other components in your system, like temperature control valves, heating coils, traps, etc. If they were sized for the much higher pressure, they won't likely be big enough now. In addtion, you're likely getting a LOT of wet steam out of the boiler, at the greatly reduced pressure. The water carried out of the boiler with the steam will have no latent heat - just sensible.

 

[ApC2Kp]

To umw,
The 150 psig steam has a density that is almost 5 times that of the 15 psig steam. The 6" pipe has to flow 5 times more cubic feet volume of 15 psig steam to get 8625 lbs per hour for heating, without factoring the different heat capacity of the 150 psig steam. The 15 psig steam would have a flow velocity of 164 ft/sec, higher than with the 150 psi steam but still a reasonable velocity. Pressure drop in header will be even more important for steam users at a distance from the boiler. There might be less than 10 psig at the process tanks.
The low 15 psi supply pressure severely limits what you can do on the condensate side of the system. Many low pressure steam users will dump the condensate to sewer or grade instead of attempting to return it feedwater system. Did any steam traps get re-sized for the 15 psig steam? The steam traps could be restricting removal /draining of condensate in process tank heaters, especially with any back pressure from a condensate return system.
Boiler tube heat transfer coefficient could also suffer with vapor film in tubes occurring from the reduced steam pressure. This could result in overheating tubes and earlier tube failures, besides limiting steam production. Did the boiler manufacturer evaluate the 15 psig operation of the boiler?

 

[uwm]

Thanks for the tip on checking out the steam traps, they might be the wrong size now because of the change. The boiler was here when I arrived and it worked ok at 15 psi with the 3" line but when we put in the 6" line we are not able to maintain the proper temp. in the process tank.

 

[ApC2Kp]

uwm,
The steam flow through previous 3" pipe was probably choked, since velocity would have been 4 times that of flow in the current 6" piping, maybe 650 ft/sec theoretical.
The backpressure inside boiler would have been higher, if the header presure was maintained at 15 psig.
The higher pressure probably helped by preventing the vapor film inside boiler tubes and keeping heat transfer coefficient at good higher rate.

 

[unclesyd]

As stated above you need to get with manufacturer about the operating characteristics at 15 psig vs the design at 150 psig. That is a turn down 90% and is extremely high for a steam generator of any capacity. As posted by A2C2Kp you are probably steam blanketing the tubes, a condition that could eventually cause tube damage from over heating.

 

[rmw]

I suspect this is a scotch marine fire tube type boiler or something similar.

Many of these type boilers come std with dual controls, one set for 150 psig, and another for 15. Some boilers are designed and stamped for 150, and sold and operated at 15 psig. I don't see a problem here if the boiler was originally designed (nozzle outlet sizes, etc.) for the lower pressure.

YOu do not say if you had other users, although a 6" header makes me believe you might. Others have run the math on the velocities, and I am not going to check them, but I agree with the choked flow conclusion above, and believe that your header was not operating at 15 psig, while your boiler might have well been.

If I am right about that, at a reduced header pressure, the other users that may have been previously "starved" for sufficient steam flow, are now satisfied, and are pulling more steam than before, "bullying" if you will, for lack of a better technical term, the kettles.

Alternatively, if other users are not the culprit, then your traps are suspect, as now that the kettles can get all the steam they want, they may be flooding the traps that weren't 'pushed' before when the kettles were somewhat starved for adequate steam supply.

rmw