Vent sizing on condensate recovery tank 1

[ione]

I’m dealing with the sizing of an atmospheric condensate recovery tank made of stainless steel AISI 316, with a capacity of 3000 l. The vessel should collect condensate from a process that consumes 3000 kg/h of steam at 12 barg. Flash steam mass flowrate will be about 530 kg/h (approx 17.6%), considering the tank is atmospheric. I have sized the vent which connects the tank to the atmosphere on the basis of a steam velocity of 7.5 m/s at 0 barg, and got an 8” diameter pipe. But a doubt has arisen in my head. Have I to consider the scenario with the upstream steam trap failing open allowing 3000 kg/h of steam flowing to the tank and size the vent pipe according to this mass flow rate, instead of the 530 kg/h I’ve taken into account? This will lead to a 20” pipe diameter.

Any insight would be much appreciated.

 

[willhammett]

You can always go with the smaller pipe size and insert a psv if applicable. If you want to ensure no steam reaches the condensate tank you could also have a pressure control loop on the condensate pipe that can open a valve venting to atmosphere, and simultaneously close a vlave down stream of the psv. we use a setup similar to this on our steam header going to flare and associated steam condensate drain to control downstream pressure (our modulated though and is not used in an open/closed setup)

 

[Chance17]

It is very unlikely that a trap sized for condensate flow would also pass an equal amount of vapor steam.
I would look at the steam trap's actual orifice diameter and calculate limiting steam (orifice)flow.

 

[quark]

Chance17 is right on the money. Specific volume of steam at 13 bara is 0.15cu.mtr/kg. So, 3000kg/hr is about 450cu.mtr/hr, where as you are designing the return line for 0.53cu.mtr/hr (approximately). IF you want to be extra safe, I would second the idea of calculating limiting flow (and also countercheck what is the corresponding pressure drop. It should be less than or equal to 12 bar). Nevertheless, standard calculations of condensate return and flash vessels don't consider accidental live steam.

Further, it is possible, if at all, when there is only one source that feeds the flash tank. Likelihood of multiple traps failing together is remote.

 

[ione]

Thanks guys for replies so far.

If with “limiting flow” you do mean choked mass flow rate, this should be well above 3000 kg/h. If I’m not mistaken, with an orifice of 50 mm diameter, that is the actual steam trap bore, and an upstream pressure of 12 barg, the choked mass flow rate should be approx 10300 kg/h. FYI there’s just one steam trap, so it seems to me the 3000 kg/h can realistically be considered as the worst scenario to take into account.

 

[quark]

Choked flow rate is meaningless unless you have connected the flash vessel just at the outlet of the steam trap (and choked flowrate is unreal in your condition with 3000kg/hr at source). If you have a pipeline downstream the trap leading to flash vessel, calculate the pressure drop at various flow conditions. Flow corresponding to a maximum pressure drop of 12 bar is the maximum flowrate in the system.

My rough calculations say that at 2000 kg/hr steam flowrate, you have about 1 bar drop/10meters for a 2" sch40 pipe. For 3000 kg/hr, it is 2.19 bar.

A description about your process may help us think about cutting down the redundant design. What is the application?

 

[ione]

quark,

Thanks again for your contribution and for keeping alive this discussion.

I am not that sure about the value you’ve reported for pressure drop/10meters for a 2" sch40 pipe. What I got with 3000 kg/h of steam at 12 barg upstream pressure is approx 0.4 bar pressure drop.

Further to the application we do have a heat exchanger consuming 3000 kg/h of steam at 12 barg pressure. From the steam trap to the vessel there are approx 20 m of 3” sch. 40 ASTM A106 grade b pipe.

 

[quark]

ione,

You are correct. I forgot to change steam density in my spreadsheet. That is why, they say GIGO, with software. I got slightly higher drop of 0.63 bar/10 meters (pipe ID - 52.5mm)at 3000 kg/hr. I am using Babcock Empirical formula given in Ludwig. The weblink seems to be using the same formula [link

http://www.engineeringtoolbox.com/steam-pressure-drop-calculator-d_1093.html

]Steam Flowrate[/url]

In any case, we have plenty of pressure to drop. So, if at all you want to be on safe side (not conventional), 3000 kg/hr can be easily passed through the condensate line.

Let us see the conditions which ensure live steam flowrate at 3000kg/hr.

1. There is no automatic steam control valve in the inlet, which is being controlled by the cold fluid leaving temperature,

2. There is no flow on the cold side or one or both of the cold fluid valves are closed and

3. Steam trap totally failed.

 

[ione]

quark,

Now I’m not following you.

If the steam trap failed open, the performance of the HE would drastically drop as the latent heat wouldn’t be exploited in the heat transfer process. Consequently the leaving temperature of the cold side would hardly reach is target thus allowing steam to flow through the HE. In conclusion the automatic steam control valve would remain open despite the fact it is managed by the cold fluid leaving temperature, as the process would keep going on demanding steam.

 

[quark]

ione,

When the trap fails to close, i.e 100% open all the time, it will let out live steam, but definitely not corresponding to full load condition.

Suppose you have an On/Off valve on steam supply, with a 100% leaking trap, there is 3000 kg/hr steam flow into the HX. 3000kg/hr steam condenses into 3000 kg/hr water at full load conditions and there is no condensation at zero load condition (except for overheating of cold fluid).

There will always be some load corresponding to cold fluid flow inside the HX, so steam condenses (but not at a rate of 3000kg/hr). Even after the set temperature is reached, there will not be 3000kg/hr live steam leaking as your cold fluid may get overheated.

If you have a proportional control valve, then partial valve closure happens with respect to cold fluid temperature and thus reducing steam flowrate.

Steam traps, except thermostatic traps, can only take out condensate formed because of heat transfer and they are not devices to condense steam. So, there is always heat transfer first, before traps come into picture. That is the reason I said that there will be condensate formation whenever there is load in the HX. Hope this clarifies things.

 

[ione]

Thanks quark,

Now I see your point and I’m with you when you say that condensation will anyway happen in the HE, despite the fact the steam trap fails open, and so the full load of 3000 kg/hr live steam is an unrealistic scenario. But part of the condensate will flash into steam because of pressure reduction. I’ve reached the conclusion it is hard, if not impossible, to define the amount of steam that will flow to the flash vessel with the steam trap out of service. Said that, how would you size the vent? I do mean would you just consider the flash steam flow rate I reported in my OP (530 kg/hr of flash steam)?

 

[quark]

Yes, I would go with just flash steam flowrate for venting. Steam System Design (Hook-Ups) by Spirax Sarco has a chart for vent sizing purely based on flash steam flowrates.

Ludwig Volume 1 doesn't mention any calculation procedures for vent sizing, yet the vent vessel design is done only for flash steam.

Though accidental, you would never want live steam going through trap. More than pressurised flash vessel, you have trouble with delayed process at the HX. So, size the vent as per flash steam flowrate, put a pressure switch on the flash vessel, generate an audio/visual alarm at the location of HX. Process guys come to know that there is trouble with the trap.