Parallel Deaerator Control 1

[typhonic]

We plan to install two Deaerators in a steam plant each sized for 60% of full load. Plant normal load is 25 - 50% of full load. What is the best way to control steam and water into the Deaerators?

Currently, we plan to
1) pump water from condensate return through one valve controlled by Condensate Tank Level (with overide if Deaerator Storage Tank level is high). The flow will be split in the piping to both deaerators.
2) supply makeup water from softeners through one valve controlled by Deaerator Storage Tank Level. The flow will be split in the piping to both deaerators.
3) supply steam through one valve controlled by Deaerator Pressure. The flow will be split in the piping to both deaerators with orifice plates in each leg to help balance flow.
4) header the deaerator storage tanks together at the bottom to balance the level and pressure.
5) provide separate level transmitters on each Deaerator Storage Tank with averaging to the level control.
6) provide separate pressure transmitters on each Deaerator with averaging to the pressure control.
7) provide separate safety devices.

Normal operation will be both Deaerators online and operating at 5 PSI.

Question: Would it be better to use two Condensate valves, two Makeup Water valves and two Steam valves?

Thanks

 

[TBP]

I'm assuming that you're using two DAs in order to avoid plant shutdowns for internal inspections and other maintenance functions. If this is the case, I'd pipe each as a separate unit, and install valves at the suction header for the feedpumps such that either DA, or both could be in service.

 

[pmover]

in addition to tbp comments,
i consider piping make-up water to the coldest point in the system (i.e. condensate tank) to minimize thermal shock and to take advantage of any condensate heating before the da tank. it has been my experience that this has been done at several locations.
-pmover

 

[TBP]

pmover - I've run operations and maintenance at several plants where the arrangement of mixing condensate and make-up water in a tank before being sent to the DA has caused me far more problems than it's ever solved. I'm sure there are plants where it helps by trimming peak steam loads somewhat, but my experience is that that's a pretty marginal benefit for the extra money spent on a tank and pump set that has to be bought, installed, operated and maintained. A properly designed and installed DA won't suffer thermal shock.

 

[tkordick]

I used to work at a plant with a similiar arrangement. The plant complex as a whole was never shut down, only operating units inside the complex were, so we were required to supply steam all the time. We had two DA's both of which had their own control valves.

We took the condensate return and went to a tank where it was then pumped through the polishers and into another tank. From the second tank it went to the deaerator heaters then drained to the deaerator tank and to the boiler feed pump suction.

The piping was split prior to the control valves with isolation block valves upstream and downstream of each control valve along with a globe bypass valve around the control valve. The outlet of the deaerator tanks were tied together downstream of a block valve.

This arrangement allowed both tanks to be in service or either one taken out of service for maintenance. Indication for both tank level and pressure was sent to the control room for each DA. The operator would select one DA to control the feedwater flow and the steam flow and the other DA's valves duplicated the input. Each tank had a level control overflow valve that went back to the "polished water tank" in the event too much water went into the heater. In addition, each heater had it's own safety relief valves to prevent overpressurization and it's own vacuum breaker which kept the unit from going into a vacuum state when taken it out of service.

By having this arrangement, you could work on the DA and the control valves associated with it for periodic maintenance. If you only have one set of control valves, any maintenance required on the control valves will require manual control for both the feedwater and steam via a bypass around the control valves.

Hope this helps.

Tom

 

[typhonic]

Thanks for the help folks.

 

[jproj]

typhonic:

Condensate should be allowed to flow virtually unrestricted into the deaerators (a condensate return tank should not be needed). Make-up water flow should be controlled by the level in the deaerator storage tank. Condensate return and make-up water should be mixed in the line prior to entry into the deaerator.

Normally, steam is controlled by one valve and the two deaerators are linked by a balance line (to equalize any pressure difference). A balance line on the storage tanks is also advisable to (as you stated) equalize storage tank levels. Usually, the deaerated water outlet is just connected to a common header that feeds a common pump(s). Isolation valves can be incorporated into the above arrangement if one unit should need to be taken out of service for any reason.

The one valve arrangement is sufficient for control, considering the two tanks will be linked anyway. There is really no need for two sets of controls unless the units are operating independantly.

Hope this helps!

jproj