Condensate Receiver Tank Regulation.

[murphymok]

Hi all, I might have talked about this problem already.

I am designing a steam condensate receiver tank that services both HP (200 psig) and LP (15 psig) condensate from the building. Due to the building's budget, I am trying to design a single tank to service both pressure condensate. Is there any regulation and standards regarding condensate receiver tank design? My concern is the mixing of HP and LP condensate that may cause flash steam or water hammer.

thanks.

 

[Montemayor]

Murph:

Your concern about flash steam being caused by mixing HP and LP steam condensate is unfounded. Flash steam is formed when your steam traps release a quantity of condensate from a high pressure level to a lower one (presumably your proposed condensate receiver is at the lower pressure). You are going to form flash steam any time you adiabatically release the higher pressure condensate (at both the 200 psig and the 15 psig levels – albeit much less at the latter).

What pressure are you planning to operate your condensate receiver at? It’s going to have to be less than the 15 psig condensate you plan to release into it – I assume you’re going to operate it at atmospheric and, therefore, it’s going to be vented with an open[/] vent. That means you’re going to generate a healthy steam plume through that atmospheric vent. This is a trade off you inherit by combining the two pressure expansions into a tank that must be operated at essentially atmospheric pressure. You have to size and vent the tank safely – even though it is an atmospheric vessel. In my opinion, you should size the vent (if you are using one) for the scenario where the steam trap remains open and you have choke flow on the escaping steam vapors. If you don’t, that scenario could build up the pressure inside the vessel to the point of rupture. I would make the necessary calculations to make sure you have enough vent relief or protection.

Water hammering is another totally different subject and it isn’t caused merely because of the mixing. There other factors that enter into this effect.

I hope this helps.


Art Montemayor
Spring, TX

 

[murphymok]

Art,

You are right, the condensate tank is at atmospheric pressure the vent line goes outside the building.

How do I calculate the optimal venting pipe size?

Thanks, great help.

Murphy

 

[sailoday28]

For normal operation, I would calculate the flashed flow rate via the vent. Flow from the tank to the atmosphere will be by natural circulation (natural draft) to the relase point.
Add a slight margin of an extra pipe size, if possible.

Art has suggested looking at failed traps. Therefore,
Secondly, calculate, the steam flow via failed open traps. Perform energy and mass flow balance to justify that vent is still adequately sized. If you find that there is a slight pressure increase, you may again want to increase vent size or increase design pressure of tank.
You will also find a new design temperature for this second case. Also note and take into account that some of the new inflow may leave as steam via the liquid blow down line. Note the possibility of new design temperatures for the vent and liquid blowdown lines.

Thirdly, check with state laws with regard to blowdown of flashed liquid, if it is going off site.

 

[Montemayor]

Murph:

There are some factors that you haven't mentioned that play a role in this application. Are you in the USA? I have to ask because we have people writing in from all parts of the globe - some of which don't identify with the strict safety regulations found in the USA. I'm going to assume that you are and that you are familiar with the engineering method(s) of identifying over-pressurization hazards as well as fluid mechanics.

One of the bad things that can go wrong with the proposed operation is that one of the two steam traps feeding your condensate vessel will fail in the open position. This is the scenario where the steam trap remains open and you have choked flow on the escaping steam vapors. The maximum capacity possible going through the open trap and through your system all the way through and out the open vent must not be restricted to cause a pressure accumulation along this route. If it does, you will build up back pressure – perhaps in the condensate vessel itself. Again, you haven’t stated the design pressure you plan to have for the condensate receiver, so I have to take you on your comment of having a tight budget. This would mean to me that you obviously plan to use a simple, atmospheric vessel that is not pressure-designed according to ASME section VIII. If so, then you’ve got a serious problem in confronting the above scenario. You can do it with an atmospheric vessel, but you must prove that you don’t accumulate pressure during the worse scenario just stated. In other words, you must identify the highest possible steam rate through the open trap and ensure that the vent on your condensate receiver does not restrain its escape from the system and cause a back pressure on the vessel beyond its design point. This, I believe, is a credible and possible scenario of something that could happen to your installation. You can design your condensate receive to withstand a high pressure (200 psig?) but ultimately you have to identify the worse case scenario in order to size the receiver’s outlet nozzle and vent pipe.

I believe that it is safe to state that the second steam trap (the 15 psig one) would not fail at the same time because this would be the classical “double-jeopardy” case – this is not credible because the two traps are operating totally independent of each other. As I stated before, this is a classical trade-off engineering decision. You are not getting off lightly on the budget monies that have to be spent to recover the condensate. If you used two, independent and pressurized condensate receivers you might have to pay more, but you wouldn’t have the steam plume and the higher over-pressurization concern.

Others on this forum may differ with my opinion, but I stand by what I have understood and recommend. I believe this is a serious problem and I also think that you are dealing with it as such. I hope this helps you out.



Art Montemayor
Spring, TX

 

[sailoday28]

How do I calculate the optimal venting pipe size?

Since vent flow is by natural draft, flow losses should be kept to a minimum.

The larger the tank diameter is the less the entrance losses to the steam vent (maybe more costly a tank).

 

[murphymok]

Hi Guys,

I am in Toronto Canada. I have been trying to find out the bylaws in regards to the requirement or regulation of sewage disposal. Not much luck there.

The building I am analyzing takes 200 psig steam and reduces down to 15 psig. There are various 15 psig steam traps directing the condensate back to the condensate receiver tank. There are two 200 psig TD traps servicing the 200 psig main line before the pressure reducing station, they both go to the same condensate receiver tank right now.

The existing condensate pump unit has failed and the return condensate is dumped directly to sewage without cooling. Due to the budget factor, I am designing a receiver tank that will service all return condensate, both 15 and 200 psig. The tank will be at atmospheric state where a vent pipe goes outside the building.

So, I want to ensure my design is correct and sufficient for the building's requirement. The tank will have a temperature sensor and cooling water valve to keep the condensate below 150 F (bylaw here). The concern I am having is of course the mising of the HP and LP condensate.

Thanks.

 

[TD2K]

For Sailor's question, there isn't really an 'optimum'. The pressure in the tank will be set by the amount of flash steam you have to handle along with the size of the vent pipe and its length. If the vessel is sized for 15 psig, I would suggest trying to operate it with no more than a few psi in it and if a 4" pipe (for example) means you run at 3" WC, great. Consider also during cold weather how you will handle any condensate if it comes out of the end, will it create a freezing problem?

Try some different sizes and see what happens. I echo what Art is saying to make sure you are checking the vent capacity at the worse case steam load assuming equipment failure. You do not want to overpressure this vessel.

 

[sailoday28]

TD2K (Chemical) I was quoting the originators question.
As --optimum might be refering to cost.
I believe the vessel was to be designed for approximately 0psig. --- With natural draft driving the vent steam.
Dependent upon the length and number of fittins for the vent line, tank diameter (affecting inlet velocity loses to the vent) will come into the cost equation.

With regard to your comment relating to freezing, I believe the flash tank should drain via a loop seal with properly pitched piping.
With regard to overpressure, see my previous comments.
Regards
Sailoday28