steam piping arrangement

[spereira]

hi,
fot the revamping of one of our plants we need to increase low pressure steam (1.5 bar) pipe size in order to allow for increased steam production. Actual pipe size is 16" and I've calculated a new pipe size of 20". The question is: I might not have the space on the existing pipe rack to fit this new pipe diameter. On the other hand a new pipe rack will be installed to accomodate other piping that is also needed for this project. To keep the same pressure drop I would need a new pipe, 12", in parallel withe the existing 16" pipe. Is there any disadvantages in using this arrangement. How many connecting points would I need between them assuming that overall pipe lenght would be around 150m (including 4 U shaped tube routings - sorry but I know they are made to account for pipe expansion purposes but I don't know it's name in english) and what size? I was thinking of two, one at each end of the tube and both connections at 12". Is this enough/correct? Other advantage of selecting a parallel arrangement would be cost and erection time (the subtitution of the whole 16" pipe would have to be made on an annual stop (~1 week) which I'm not sure it's enough, even if we pre-manufacture most of the piping)
are there alternatives for these U shaped tube routings? expansions joints maybe, or are these uneconomical for these pipe sizes?
thanks for any comment on this
regards,
Susana

 

[TD2K]

What you call U shaped bends are called expansion loops and yes, they are to accomodate the piping growth over the range of operating temperatures (essentially shut-down up to operating).

Over 150m, 4 expansion loops is more than I would expect especially for this low of steam pressure and associated temperatures but the design and number of those come from your stress analyses people. You might want to check what they are using for a piping design temperature in their stress program. The loops may be smaller than I'm used to seeing, thus they need more of them.

From a process point of view, you are simply looping your 16" steam line to increase its capacity and this is not uncommon.

As you indicated, one connection at each end of the 16" steam header for the new 12" loop would be typical, you don't need to have intermediate points, the total steam flow will distribute itself through the two legs to maintain equal hydraulics. Since you have a 16" header, I would make the connections 12". You can do a check and see if 10" tie-ins and valve and then going up to your 12" header significantly affects the hydraulics. That would be an option if a 12" connection is significantly more difficult to make than a 10". A 10" on 16" tie-in can be done with a weldolet and hot tap, I'm not so sure if a 12" on 16" can be done that way or if a encirclement type fitting might be needed, your mechanical engineer should be able to help you there.

Make sure you provide adequate drip legs and steam traps. If the flow is always going to be one way, you need a drip leg before each elevation right. If the line can flow either way, you'll need another drip leg on the 'far' side of the expansion loop for the case when the steam flow is reversed.

 

[spereira]

thks TD2K

the flow is either way. what do you call a drip leg? I thought I only needed to consider steam traps.
As for the connections I was planning on using reducing 'T' instead of tapping the header pipe.
And, as for the number of expansion loops, they were designed by the engineering company who was responsible for the design of the plant back in 1977. I guess the people back then just built it like it was on the drawings.

 

[MJCronin]

spereia,

In response to your question "What are drip legs ? " Do you have any experience with steam systems at all ?

Drip legs are short vertical "stubs" (18 to 24 inches long)of piping typically installed on steam systems to aid collection of condensate during operation and startup. They are typically terminated with a pipe trap and are usually one pipe size smaller than the horizontal pipe to which they are attached. Steam traps are attached to the side of the "drip leg" and a valved drain is installed on the bottom

During startup, when the condensate load is the largest, a drip leg provide a place to collect condensate and prevent a wave of water from "hammering" the next elbow. They also prevent the quick cycling of steam traps.

Drip legs for saturated steam service should be sized slightly differently than for superheated steam service. ASME B31.1/B31.3 do not provide specific rules about drip legs. Most large companies have developed a one page guide about drip legs

See

http://www.armstrong-intl.com/products/traps/pdf/bult135.pdf

http://www.e-armstrong.com/prod_detail.php?pc=1&ps=104&AS_Session=12761b2b060c7554480ac8970ce508bb

MJC

 

[spereira]

MJCronin

I really don't have a lot of experience with steam systems. So far all we've done was tieing-in the header line to supply low pressure steam to new plants more or less by duplicating existing steam lines with the same flow/pressure drop. But now, the expected steam usage forces us to change the whole header pipe and this means that a new line must be calculated. I had to check the information available on this since I remembered from my university days that there were some "rules" for steam distribution lines calculation, as well as several charts supplied by specialized companies which could give a rough idea of the pipe size. Only after this I would check the number of steam traps needed. I know there are several types of steam traps but I never heard of drip legs before since all we have in our saturated steam lines (low and medium pressure) are steam traps. Do you know how I can calculate the number of steam traps needed for the whole distribution line? are there any guide lines?
Susana

 

[TD2K]

Plants will typically have standards how often and the sizes of your drip legs need to be on distribution headers. You definitely want one each time the steam pipe raises up to avoid condensate collecting in the rsulting pocket. If the header is designed with flat turns or has a long run, you likely want a drip leg every 200 to 300 feet. The load for this trap for normal operations is easy to calculate, it's just the amount of steam that condenses for the heat loss from the system (if your steam is superheated, the heat loss might be made up simply by the loss of superheat but in your case, I'm assuming that doesn't apply).

During startup the load on the steam traps will be many times higher than during normal operations since you are bringing the 'cold' piping up to operating temperatures. Traps are typically not sized for this in my experience as the practice is to have the traps open to atmosphere through their inlet strainer vent valves until the heater is up to operating conditions. However, you can calculate a load for this case if you want by looking at how long it takes to bring your system up to temperature and how much steam condensation that represents. Armstrong has information on this (as do other trap vendors), I'd look around the site linked by MJCronin on Sept 2nd.