Steam trapping 5

[KenA]

Is it generally ok to trap directly from high pressure steam header to a lower pressure steam header, without collecting HP condensate and flashing in a vessel? Are there any problems associated with this mode of operation? Sorry for the seemingly simplistic question but someone has just said something to me that has shaken my confidence in my current design!

 

[quark]

Actually that is not a good practice(nor even a practice) to do it that way. You will have plenty of problems with hammering and this may cause accidents. Secondly, if the traps are not designed for the LP back pressure, you will face problems even with HP header.

I strongly suggest not to do it.

Regards,

 

[Fzob]

What pressures are you running at? It can make a big difference in your application.

I have done this with good success many times in trapping 150 psig condensate to 35 psig steam supply. You need to make sure the low pressure steam piping is large enough and designed to handle the extra condensate load.

What I mean by this is the low pressure steam header needs to have a condensate pot close to where you are trapping the high pressure, and it needs to be configured in a way that allows the condensate to travel to the pot without hammering.

Be sure to consider start up loading. In a few application I needed to use a low temp subcooling trap in parallel with my regular trap to remove start up loads to the low pressure condensate system.

 

[rmw]

Is the lower pressure header you are trying to trap into superheated or saturated steam?

I, like fzob would like to know the "from" and "to" pressure before hazarding an answer.

fzob's advice was good.

rmw

 

[TD2K]

I've seen it done. I could imagine you having some potential overpressure issues with a high pressure steam source routed into a low pressure line depending on what scenarios you wanted to consider.

 

[25362]

Why couldn't the condensate flash pot be separately connected to the LP line from its vapor outlet, thus keeping HP condensate and LP flashing steam out of the LP line ? Wouldn't this be a logical and safer arrangement ?

 

[KenA]

Thanks all for the responses. I have 3 levels of steam:
HP 70barg (1000+psig) superheated
MP 35barg (500psig) saturated
LP 4.5barg (65psig) saturated
There is only a LP condensate return system at 1-2barg (15-30psig).
Don't get me wrong, I'm only talking about the steam header traps. Process users (exchangers) of say MP steam have a MP condensate flash drum which (as suggested by 25362) is connected to the LP steam header and LP condensate. This is not a problem. But the long MP steam header is trapped directly to the LP header rather than having an MP condensate header with flash drum. Won't the condensate from the MP header just flash into the LP steam system?
Since the HP steam is superheated, the steam traps should only be discharging during start-up/warm-up. Again won't the condensate from the HP header traps simply flash into the MP steam header?

 

[25362]

Yes flash it would, into about 45% steam and 55% condensate, as long as the traps don't pass steam.

Read carefully the responses above, and consider all hazards and risks stemming from the planned design.

 

[rmw]

Would it be that much flash steam, 25362? I didn't run the numbers, but the previous flash calcs I have done would have indicated to me that there would be more condensate than that. I am not going to run the numbers now, just wondered if you did.

KenA, point being, whoever is right, more than half of what comes through that trap is going to remain as condensate, and that has to be handled in some form in the lower pressure header so that it does not cause hammering, erosion, etc.


26362 advises correctly to proceed with caution. I believe it can be done, but I would want to be VERY careful about how I went about it.

At a minimum, the traps on the LP steam header would have to be sized for their normal condensate load plus the addition of the condensate coming from the MP system.

rmw

 

[25362]

To rmw: you are absolutely right, it should be about 79% condensate and the remainder flash steam. I've misread the enthalpies in Perry's steam tables.

Believe me, I intituively thought as you did, but I didn't have the time or patience to spend in reviewing the figures when referring to KenA "irregular" design.

It is apparent that KenA wishes to save money and efforts by not installing a dedicated flash drum + a condensate collecting line.

KenA: if the HP header is thermally well insulated, the amount of condensate forming could be negligible and probably economically sound and safe to be released to the atmosphere. If the line is long (how long?) one can assume that at the pipe's end users may be converting HP steam into condensate. What are they doing with it ? Is it being recovered ? How ? Kindly comment adding details.

 

[KenA]

Again - thanks for the input.
The plant (oil&gas separation) is to have 3 pressure levels of steam, as described above. Steam is distributed to various users (reboilers and steam turbines) around the plant via the usual network of steam headers. The main headers are ~500m (~1500ft) total length and are well insulated. LP condensate from LP steam users is collected and returned through an LP condensate return system to the boilers. MP condensate from MP steam users is flashed to the LP steam pressure in drums local to the user(s) and the LP condensate joins condensate return system. The HP condensate, from the HP steam users is similar, except that it is firstly flashed in a drum to the MP steam pressure and then the MP condensate is routed to the MP condensate flash drum as above.

What we don't have is HP and MP condensate headers running around the plant. Therefore the header steam traps from the HP and MP steam headers have no condensate header to discharge into. The choices were (a) trap to the LP condensate or (b) trap to the next lower steam pressure level. We chose (b), but am now a little uneasy.

Is my situation unique? Do all plants that have multiple steam pressure levels all have multiple condensate return headers?

I understand there may be hammering, but surely less than if the higher pressure condensate were discharged directly into the LP condensate header?

 

[katmar]

Ken,

The question which nobody has asked yet, but which to my mind is crucial, is "what are the relative sizes of the headers?"

If the MP header is 4 inch and the LP header is 36 inch I would not hesitate to do what you are proposing. If it is the other way around I would not start to consider it. In between these two extremes you have to do some careful analysis.

Remember that as the pressure decreases the relative density difference between water and steam increases rapidly. This means that LP headers tend to be larger diameter, with proportionally less of the area devoted to carrying condensate. So a bit more condensate should be OK.

However, as long as you have checked the actual numbers and you have carefully selected the points where the MP condensate enters the LP header it will work. By this I mean don't do obviously wrong things like expecting the condensate to flow uphill. Don't laugh - these things happen.

If the mains have not yet been constructed it may be possible to build "mini flash pots" into the LP main. For example, where you have a drip leg in the main go up a size or two for a few feet either side of the tee and introduce the MP condensate there.

regards
Katmar

 

[KenA]

Katmar,

Excellent question. The headers obviously reduce in diameter as the get towards the extremities, but the main lengths have the following diameters:
HP 12" (with a 20" section)
MP 20"
LP 30"
LP cond 20"

 

[quark]

Properties at 65 psg

Steam Enthalpy - 1183.27 btu/lb
Water Enthalpy - 281.864 btu/lb
Latent heat - 901.4 btu/lb

Properties at 500 psig

Water Enthalpy - 452.975 btu/lb

%flashing = (452.975-281.864)/901.4 = ~18.9%

For a 20" pipe in still air at 70deg.F, the condensate formation will be approximately 2.53 pounds/hour/linear feet(with 75% insulation efficiency).Assuming 50 feet trap distance, the total condensate load will be 126.73 lb/hr.

http://www.armstrong.be/pdf/N_101.pdf

So flash steam will be approximately 23.95 lb/hr and condensate will be 102.78 lb/hr.

Density of water at 65 psig is 56.92 lb/cu.ft. So this will occupy a pipe length of [(102.78/56.92)*4]/(3.142*2.5*2.5)=0.37 ft/hr. (for 30" piping)

So condensate hammering may not be a problem.

Regards,

 

[Joesteam]

I wish to impose my two cents, as I have seen this before in plants.
If you decide to do this, put another trap station on the lower pressure line downstream so that the condensate can be collected and removed resulting from the let-down.
Be aware, two big reasons for installing drip traps are for start-up and upsets. In these two conditions you will get way more liquid condensate in the lower pressure line (the above calculations are correct for standard running loads) that may result in massive water hammer. A steam separator on the lower pressure line may be a good idea - at least for piece of mind.

 

[25362]

Depending on the actual HP steam flow rate (linear line velocities), quark's comprehensive estimate on condensation during normal operation, may result in the MP steam having still a quality higher than 95% at the farthest delivery point.

Thus, drops of moisture may not settle out to be removed by the traps along the main header, but would possibly do so at the end of the pipe run, or when encountering obstructions to flow. Multiplying quark's estimates by 30, for the full length of the header, may signify in a condensate load with a potential for water hammer.

Collecting drain bottles should be installed along the MP header to enable condensate to reach the traps for these to function, and to avoid water hammering in the MP steam pipe itself.

Besides, fixtures to protect the LP steam main, such as indicated by Fzob and Joestream, would have to be provided, in particular, when the smaller (reduced) diameters of the LP header end-sections receive flashing condensate from the MP header.