Steam system start-up

[bopeng]

Hello,

I'm curious what the standard is for starting up a facility steam system from the main distribution line. We will be installing a second isolation valve (to provide DPI) and performing some re-routing on a 6" line that feeds the facility 8" header from the main 16" distribution line. With the installation of new piping in an old (60+ yr) system is this also a good opportunity to install a warm-up valve around the newer second isolation valve? Currently there is nothing and previous operation of the system had operators "slowly open" (fully opened within 15-20 minutes) the one 6" isolation valve. Is there also a general rate for warm-up steam headers/piping systems? 50F/hr? Our system is saturated steam at 110psig supplying process equipment and air supply heating coils. Am I being overly conservative in trying to "baby" our facility steam system? I'm fairly new so I'm unfamiliar what is considered normal practice.

Any info would be great. Thanks!

 

[sshep]

Hey bopeng,

You were heating up way faster than 50F/hr if you had this 6" valve open in 15-20min, and could risk water hammer damage (more than thermal shock) if the downstream system is big. We blew a flange off the end of a long pipeline doing that too quickly.

This is where the warm up valve makes it easy.
1) Open all the drains and far end vent.
2) Open the warm-up bypass- it will be something like 1" or 1.5".
3) First condensate will come out the drains as steam condenses to warm the piping, then steam and some dribbles of water.
4) Close the drains and air vents and bring the traps on line to allow the pipeline to finish coming up to pressure.
5) When the pressure is in the branch reaches header pressure, the main valve is lined up

Using the steam HVAP, the mass of downstream piping, and the steam flow through the bypass will give you an estimate of the time to reach step 4 and start closing drains and lining up traps. Lining up traps to a closed return may include draining any cold condensate in the downstream piping to ground (backflowing from the condensate header) to avoid hammers in the condensate system downstream.

best wishes,
sshep

 

[bopeng]

sshep,

Thanks for the pointers. I was quite surprised about the valving that was done to start-up our steam system which is why I stumbled around Google reading and ended up here to confirm all I learned through Swagelok, DOE, etc. Is there any difference in isolating our facility header and warming it up first prior to valving in the individual steam lines (2" and 3") or is it better to use the warm-up valve to bring the whole system up to temperature (process equipment being the exception)? Also, we do not have many drains on the condensate side. Am I wrong to assume trap by-pass lines act in a similar fashion? Are there specific traps that should be by-passed during start-up (drip legs, prior to vertical runs)?

cheers,
bopeng

 

[psafety]

What I have seen in the industry I work is 6" and above require a warm-up bypass. Think .. that steam is warming that line on start-up, in a cold start-up near 100% goes to condensate ... can your traps handle this? NO, once the line is partially filled your moving water at steam velocities.

Several people have been killed by inadequate warm-up methods.

-psafety

 

[sshep]

Hey bopeng,

It is desirable to drain the condensate to ground during start-up because the traps can struggle- some traps don't vent air well (including most inverted bucket traps), the amount of condensate can be greater than the trap capacity (way above normal heat loss), and the trap dP will be below design (stall or reduced capacity). A trap bypass valve will push the air into the condensate return and can lead to hammers when cold condensate hits flash steam in the header, but will help get the condensate out better than a trap alone.

On the otherhand since your previous start-up procedure depended on the traps and was successful, if you don't have the proper drain valves you may not have a choice. I think your warm-up bypass will be a good improvement in any case- i.e. still less chance of accident or damage than the old method.

best wishes,
sshep