Drip Leg Design

http://cr4.globalspec.com/thread/48613/Drip-Leg-on-Steam-Mains

Drip legs on steam mains, steam traps, etc., are unavoidable features of steam system design.

However, considerable savings can be made in large installations by carrying out the following:

• Effective thermal insulation for all steam and condensate pipework. This also provides skin-burn avoidance as well as utility process economy.
• Collect and return all condensate to the boiler house through a well-designed condensate recovery system. The savings occur in three areas:
• energy consumption - there is a lot of heat to be re-used in the condensate
• water treatment costs can be much reduced by re-using all that hot, low-conductivity material rather than producing it new from the higher-conductivity, cold, town mains supply
• water supply and disposal costs. The volumetric costs of water disposal can be as much as four times that for water supply, though they are often ignored. Rather than tip it down the drain, pay for its disposal and buy new, it can be re-used.

An uninsulated 13 ft. "drip leg" should instead be called a leak!

Your largest energy loss comes from the intermittent operation of your mechanical steam traps which emit live steam as well as condensate as they cycle. In addition, when closed, they do nothing to remove condensate from your supply steam as the drip leg assumes the same pressure as the steam main. What is required is the continuous action of a properly sized Venturi steam trap

Different types of steam traps have been designed because some are better than others in certain applications. Advocating a single specific operating style as a "one-fix" solution does a great disservice .

When looking for system design or efficiency improvement advice from a vendor, one would be well-advised to speak to a reputable supplier who manufactures all types of traps.

Actually that premise has been proven wrong, unless there is more than a 4 to 1 turn-down ratio between min and max load conditions (and it is not economical to install proportional controls) venturi units are the best way to go.

Seems to be a bit of confusion here. When closed, there is no condensate to be removed! In the presence of condensate, a steam trap opens and the full differential pressure forces the liquid to the return system. Once there is no more condensate, the trap closes again. This is quite unlike a "venturi trap" which essentially is simply a perpetually-open hole in the steam system.

Kind of like a constant "leak"

Your largest energy loss comes from the intermittent operation of your mechanical steam traps which emit live steam as well as condensate as they cycle.

Condensates can be recycled back to the boiler...

no, the largest energy/efficiency loses, are impingement of the Steam in the line turning the steam into condensate.

Hello Meejay,

Although you've probably already seen my comments on your other post http://cr4.globalspec.com/thread/48613/Drip-Leg-on-Steam-Mains , readers who haven't may find the following excerpt to be helpful:

..."Usually, a properly designed drip pocket (same size as the steam main) is sufficient. This should also be insulated since any pooled condensate will sub-cool in the pocket. Your photo doesn't suggest any drip pockets. It seems that the smaller-sized drip legs come directly out of the main. This is not very effective at all since there is no effective collection point for the upstream condensate. As a result, you're likely experiencing downstream erosion of the lines and control valves. Additionally, with wet steam, your process will require more energy." ...

If you look at the velocity of the steam along the main you'll realise that water is going to blow straight past the tiny drip leg. You are not going to remove much condensate this way. Ideal is a tee on the end of a run. Steam goes up, water hits the back wall and falls down into a line sized catch pot long enough to give protection of the pool from the gaseous wind above. A drain from that, sized for twice the expected condensate flow (calc needed) will give good results. Steam is expensive, so is water damage to equipment, so is condensate and boiler feed water. Get a professional to overview or design your scheme.

Responding to the latest comments, your drip leg sizes are ok, as long as there is a pressure differential from the drip leg to the return system. To prove it just crack open the ball valve that is on your 'Y'-strainer protecting the drip leg trap. If you have opened it to the right size, you will get a continuous flow of condensate, still in it's vapor condition, being pushed out by the system pressure. If the opening is too large you will exceed the rate of condensate production and begin to lose live steam. Too small and you will back up condensate. A qualified technician can choose the correct size venturi for the application

And would this "qualified" expert also claim that small-bore take-offs serve as effective drip legs??

Hmmm...

Indeed, JoltnJony, your above statement is precisely the reason why steam traps are used rather than simple orifice plates as you advocate!

The opening of real steam traps is proportional to the amount of condensate produced. Condensate formation is never constant, particularly in a modulated process. With a high load, your orifice traps will waterlog the system creating process temperature swings, increased energy usage, corrosion and quite possibly: water hammer. On the other hand, low loads will mean that the orifices blow live steam to atmosphere or pressurize a closed return system thus wreaking havoc with the rest of the processes.

In either case, they're not very nice situations to have to deal with, huh?

I think the confusion may lie in the fact that "orifice traps" i.e.plates and "venturi orifice traps" are two completely separate technologies. The original orifice plates designed by the navy for service aboard ship are in fact only suitable for fixed load systems. The venturi style, based on two phase flow can indeed self regulate and have been proven in numerous applications (including mine) to provide steam usage reduction, virtually zero maintenance and improved safety due to elimination of water hammer....

May I suggest those interested go to "The Steam Trap Handbook" by James F. McCauley, PE ISBN 0-88173-187-0 for an expert's evaluation of these steam traps. In addition, "Timber Processing" magazine issues July/August 04 and Jan/Feb 06 has information of the Lumber Industries experience with venturi steam traps.

The main problem is mounting the traps vertically. The disk in the trap needs gravity to re-seat once the condensate is pushed out. Mounted as you have them, the disk could stay in the "up" position, allowing a great deal of live steam to escape. Venturi type steam traps do not require gravity for their operation

"Need for clarification" ? Indeed.

However one would like to spin it, either "venturi" or "orifice", the basic description of these steam traps (?!) is a fixed hole . Fixed holes do not, by definition, modulate. Sure, at one specific load, the steam to condensate interface near these "holes" might just be acceptable. However, as soon as conditions change, one would either have too much sub-cooling (hence water-logging and its attendant woes ) or, steam blow-through (and wasted energy ). I don't know of too many applications that are steady. Even steam mains have changing condensation rates.

The so-called "technical studies" report that regular steam traps waste energy because steam is allowed to blow through. Come on now, let's get real. What they're really talking about is the generation of "flash steam". This happens as high pressure condensate (upstream of a real steam trap's valve) flashes to steam at a lower pressure (downstream of the trap's valve). Sure, orifice/venturi traps sometimes have less flash but only because they've choked the effective flow of condensate thus greatly sub-cooling it at the discharge (they're even more notorious for water-logging than bi-metallic traps are). On the other hand, at lower loads, venturi/orifice traps blow live steam. There's a big difference between "flash" and "live" steam! In any event, proper design can capture flash steam and re-use it in low pressure applications.

These things came out of naval engineering bowels. On pitching and rolling vessels, yeah, mechanical linkage steam traps have some draw-backs (properly-applied thermostatic traps would be suitable). Since a venturi/orifice trap has no moving parts (remember: it's only a hole), nothing binds when the ship rolls. Steam blow-by? Big deal: If an organization can afford 15,000.00 dollar hammers, a bunch of wasted steam is of no concern. Water-logging and water hammer? No sweat: Unlike shore-based systems, naval equipment is so over-engineered that it can withstand enormous abuse.

Unless your plant isn't permanently mounted to the ground, you'd be well advised to use real steam traps. Not beholden to promote one particular technology, there are quite a few reputable manufacturers out there who can help you choose the best type for your applications.