Dearator Pressure 2

[flinana]

Hi,

How can the dearator pressure affect the production of a Stem Turbine?

 

[hacksaw]

the dearator serves to remove non-condensibles from condensed steam and feed water make up. Its main function is in conditioning water for the boiler and is not directly connected to any turbines in the system.

If by turbine "production" you mean hp or kw then the usual issues are the degree of superheat at the turbine inlet and the discharge temp/pressures.

 

[flinana]

Since the dearator is being heated by the turbine extractio I thought it would have an effect.
Another thing I find strange is that one of the vents in the deaerator spits water while the other doesnt and it is working at its normal level.

 

[hacksaw]

the dearation steam(in your case the extraction steam), affects dearator performance. It is critical to have enough steam available to maintain dearator pressure in the event of high make up water demand.

The steam vent operation have to do with the internals of the dearator. In the tray type, if you are flooding the trays, you might get the "spitting" referred to. I have not worked with the spray type. Take a look at the operating manual. They usually have a good over-view.

The vent it self is usually a "restriction orifice". If the plant is pretty old, it can be erroded, and require replacement.

 

[hacksaw]

how much inlet superheat to you need to meet the turbine spec?

 

[flinana]

30ºC for starting up and 50ºC above 5 MW

 

[rmw]

While as Hacksaw mentions you need to deaerate your water to remove the oxygen, the steam you use to do it doesn't generate power in your turbine downstream of the extraction point. I assume that your steam has a cost.

That means that you don't want to use any more than you have to, and you want to use it at as low an extraction pressure possible in order to get all the useable work possible out of the steam.

If you have any other way to pre-heat the make up water to the DA, that would help reduce your steam consumption.

The DA is only going to pull the steam it can condense except for the spitting part. That is puzzling.

If you are spitting water, you have some kind of problem internal to the DA. Level control would lbe my first place to look.

rmw

 

[flinana]

Thanks, apparently the problem is that the setam is condensing at the vents tip because it is colder, and thefore it accumulates water!! weird
We run 3 50MW plants and the one giving 1 MW less happens to have about 0.5bar nore pressure in the DA.

 

[rmw]

Are there absolutely no other differences? Are the condenser operating pressures the same on all 3 units? Is the DA water flow rate different on one unit? Is the main steam pressure the same for all 3 units?

You might have 1 MW (2%) difference between 2 units for reasons totally unrelated to the DA; turbine internal seals, for example, or dirty condenser or consenser with more plugged tubes or a large squid on the inlet tubesheet.

rmw

 

[hacksaw]

Dearators use pressure control on the admitted steam, if it is running 0.5 bar higher than the others, that is a setpoint or a valve maintenance issue.



do you have the same steam temperature going to each TG?

 

[hacksaw]

here is a good survey of you system may be of interest

http://www.spiraxsarco.com/resource...s/the-boiler-house/pressurised-deaerators.asp

 

[flinana]

thanks, all temperatures going into de TG are pretty much the same. I believe there is a preheating valve from the hp header leaking to the DA or the continuous blowdown valve. These valves could provide this excess in pressure to the DA.

 

[hacksaw]

if your tg's are off a common header the temps would be close, but should they have separate superheaters then you might have a super heater issue to deal with. 10-15 C superheat can make a difference

(At 102 barg inlet the sat temp is 313.2C for a SH of 68C)

you can identify the possiblity of leakage by taking a look at valve position on the DA pressure control valve, if it is much different that the valves on the other DA's then you likely have fugitive steam getting into the system

 

[flinana]

This is how our system is configured. There are two valves from the main hp header with the fucntion od prewarming the DA. The SP is about 1 bar so these only open during start up after that it is closed. The pressure comes from the TG extraction and from the blowdown.

 

[rmw]

Is it a controlled extraction, and if not, does your DA pressure fluctuate with TG load? DA's that I have seen that operate directly off of an extraction stage are a slave to the pressure in the expansion line at a given load. They are normally designed for full DA pressure at full load and fall off as load falls off.

rmw

 

[hacksaw]

rmw is on to something here and may be compounded by a problem in the condenser

good luck

 

[flinana]

It does fluctuate with load and how is it related to the condenser problema?

 

[hacksaw]

look at your condensate temps and pressures

 

[rmw]

You should have some heat balances for your plant at various loads, say 100%, 50%, 25%, etc. Look at the various pressures throughout the steam expansion through the turbine and the condenser and see if you can establish a relationship to the conditions you are describing above.

rmw