Pressurized Condensate Tank-Air or Steam Pad?

[KA4MIE]

We have a condensate collection tank that receives clean condensate from a number of processes throughout the plant, prior to being pumped through polishing filters and on to the deaerator. The condensate returns are pumped from each department at various temperatures depending on process requirements.

The bulk temperature in the collection tank is 250F, but the tank is pressurized to 30 psig by compressed air in order to avoid flashing in the return lines, some of which are rather long (300 to 800 linear ft)

This is a long standing practice, but it has been suggested that the use of compressed air to pad the tank could contribute to dissolved oxygen in the condensate, and the use of steam to pressurize the tank should be considered. There is a large 60 psig steam line close to the tank that we could tie-into to provide 30 psig padding in the tank vapor space. The question I'm wrestling with is how do you calculate the steam usage for padding the tank, specifically how do you calculate the condensation rate at the liquid surface for vapor in contact with subcooled liquid? If anybody has tried this, I'd appreciate some guidance. Heat loss through the tank wall is fairly straightforward.

I've tried several approaches but I wind up having to make some assumptions as to degree of mixing or surface turbulance. Condensate is introduced well below the operating level, but I would still expect a fair degree of surface movement.

A fall back position might be to try nitrogen padding, if the cost of steam padding was prohibitive and it could be proven that the current padding system is contributing signifigantly to dissolved oxygen to the boilers.

 

[TBP]

If the DA is functioning correctly, there should be no problem with pitting in the boilers with your current set-up. I have seen boilers seriously pitted from improperly selected or piped feedpumps. Air can get drawn in around the pump seals or packing resulting in corrosion in the feedwater lines and boilers. The DAs in these plants were working just fine mechanically, and there was the required level of oxygen scavenager present in the storage sections. The actual cause of the corrosion problem was at the feedpump & associated suction piping.

 

[sailoday28]

The air padding was to prevent flashing,etc.
Won't there still be a flashing problem if you now pad with steam?

Regards

 

[KA4MIE]

Thanks for the replies. As for the flasing problem, this occurred in some of the higher temperature return lines to the tank whenever the tank pressure was lowered. When pressure is maintained in the tank vapor space at 30 psig or above, the flashing does not occur. The return temperatures run from 205F to 270F.

The thinking was that a steam pad at 30 psig would be as effective as air and lower the possibility of air entrainment at the surface.

My concern is that using steam to pad the tank may prove to be more expensive than compressed air, which is why I was looking for a way to estimate the condensation rate of the steam padding flow.

 

[25362]

Have all remote possibilities of hammering been considered ?

 

[KA4MIE]

Thanks for the reply,

I don't know that all the posssible causes of water hammer have been investigated, but this has been a long standing practice(before my time). I do know that at one time we gradually lowered the tank pad pressure to see how low we could go, and the higher temperature returns starting hammering, which was attributed to flashing.

The condensate control valves in the return lines are all located near the process equipment, so the long return runs back to the tank are all on downstream of the valves.

 

[25362]

Is there a remote possibility of "losing" the water level with steam entering the condensate line prior to reaching the deaerator ?