Steam Trap on Condensate Line

[oil198]

Hi,

I am looking to recover condensate from a heat exchanger back to the boiler. Condensate will gravity drain from the exchanger to a condensate tank and then it will be pumped back to the boiler.

I am confused as to whether or not I need a steam trap on the condensate line from the HX to the condensate tank. Is it recommended to recover the "flash steam" and send that back to the HX? Otherwise the steam is going to flash and vent to atmosphere when the condensate enters the tank. Can anyone advise. Thanks.

 

[rmw]

You need to do a heat balance around the Hx. You haven't given any information from which to give an informed tip, but it is quickly appearant that flash steam can't climb back uphill to the operating pressure to get back into the Hx unless of course, all this happens at atmospheric pressure???? or you intend to thermocompress it back up to the Hx operating pressure.

Unless you have a user that can take the steam at the operating pressure of the flash/condensate tank, you are going to lose the flash steam.

Or unless unless your Hx is putting out such subcooled condensate that there will be no flash steam from the condensate tank.

Well, I'm through guessing now.

rmw

 

[Compositepro]

You do not need a steam trap if you have a pressurized condensate receiver with a pump to return the condensate to the boiler. There are prepackaged systems available to do what you ask.

 

[oil198]

Yes, I was thinking the same thing but was a bit confused when looking at vendor systems which all incorporate a steam trap on the condensate line to the condensate tank/receiver.

Typically if the pressure differential is 75 psi you can return the flashed steam to a low pressure user, but this is not the case in my situation. I guess I just want to know why you need a steam trap on the condensate line from the HX to the recovery tank?

 

[oil198]

Compositepro: This will be an open condensate sytem with no back pressure. The condensate tank will be open to atmposphere and the piping will be sloped downwards from the HX to the tank then pumped to the boiler.

 

[Compositepro]

Well, then you must have a steam trap to keep the high pressure steam from free-flowing out the vent of the condensate tank. Why would you wonder about that?

 

[oil198]

What is the issue with having flashed steam venting to atmposhere from the condensate tank?

 

[oil198]

Just to further clarify, steam to the HX is low presure steam at about 40psig.

 

[Compositepro]

Flash steam comes from condensate that has passed through a steam trap and flashes at the lower pressure. Without the trap you will have free flow of live, high-pressure steam into the condensate tank and out the vent.

 

[oil198]

with the steam trap, does the flashed steam remain "live" within the system or is there additional piping to route it somewhere?

 

[rmw]

The flashed steam flows in the condensate line as two phase flow until it gets to a place where it can separate.

rmw

 

[TBP]

Remember that all a steam trap really is, is an automatic valve. It opens when it sees air or water, and closes when it sense steam. That's it.

Assuming that you'll have a temperature control valve on the steam line ahead of the HX, then you must have a trap. Remember that the 40 PSIG will not always be available, such as at partial load - particularly at low load - you can have VERY low pressures in the HX and at the steam trap inlet as the control valve throttles-in. When the valve closes, you'll pull a vacuum in the HX. A vacuum breaker and having the condensate drain by gravity to a vented receiver is the correct arrangement.

The other way to pipe this, is to use a control valve on the condensate outlet, instead of a trap. In this arrangement, the steam is supplied at line-pressure, through an isolation valve. You'll have 40 PSIG steam pressure in the HX, and available at the condensate control valve inlet at all times. You can size the HX to permit subcooling of the steam, if you wish. But that's an economic decision.

 

[abdullah]

In fact you have to use trap if the Re-boiler steam not totally condensed. Other wise use flush drum with level control and the vapor will be sent back to the re-boiler top.
But remember that steam should enter the re-boiler near the saturation point by the means of de-supper heater otherwise you will have problem to recover the condensate .

Regards


ASA-KUWAIT

 

[EricWales]

You need a steam trap in the condensate line out of the heat exchanger to maintain the chest pressure and hence the higher condenstaion pressure of the steam. The Steam trap should then discharge to a flash drum - this flashes low pressure steam off to be used elsewhere and has a second trap off the base to an atmospheric condensate tank which should have a vent condenser to capture vented steam and return condensate to the condensate tank to be pumped back to the boiler.

Alternatively, if there is no use for the LP steam, then the heat exchanger trap should discharge directly to the atmospheric condensate tank.

 

[JohnGP]

As indicated above, you need a condensate trap to prevent uncontrolled release of steam through to the condensate tank. With a suitably sized trap in place, and given the pressure that you have, the amount of flash steam (in lbs of steam) will be less than 10% of the quantity of condensate. Being at atmospheric pressure, there's limited opportunity available to reuse though.