DA questions

[bethw]

What effect would changing the pressure within a dearator have on the operation of it? We presently run at approx 20psig and up to 50 at times. Also what effect would the temp. of the makeup have on the steam usage and/or operation of the same dearator? Makeup is only about 1-2% of cond flow(cond. flow is about 400gpm). Makeup water is 35F.

 

[pmover]

first off, i'd be verifying the da gauge pressure as 50 psig seems excessive and relief valve should be open.

operating the da at higher pressures will impact other process systems and likely impact the dissolved gas capabilities of the da tank.

for what justifications is there to increase the da pressure?

-pmover

 

[bethw]

Plant operations for increased eff. call for increasing this pressure at times when we have no other place to put our steam. We also increase to other places also. The relief is set at approx 100psig.

 

[GenB]

It would be a good idea to use the surplus steam to produce electricity instead of damaging your DA system and feed pumps.
ER

 

[rmw]

If the DA vessel design pressure limits are not exceeded, the DA does not care what pressure it operates at. What a DA does is get non condensables out of your BFW, as well as heat the BFW on its way to the boiler, increasing the efficiency of the boiler.

The oxygen soluability of water is already low at 250F, (15 PSIG) and going to 50 PSIG does not get you a whole lot more oxygen removal. However, assuming that the increased temperature does not mess up the circulation in your boiler, the increased BFW temp at 50 PSIG does reduce the amount of fuel you need to burn to bring the water to its boiling point. So, if you have excess steam, and you can sink it there, it is saving you fuel dollars.

To answer your other question, the colder the make up, the higher the DA steam usage. It is a straight thermodynamic book keeping first law calculation.

Check with your boiler OEM as to what the upper limit of BFW is with respect to internal circlation issues in the boiler.

rmw

 

[jproj]

"What effect would changing the pressure within a dearator have on the operation of it?"

Simple answer.... It increases the steam usage and the operating (outlet) temperature. Make-up temp has the opposite effect. Lower the temp, the more steam. Since you only have 1-2% make-up, the make-up temp is not going to make much difference. If the unit is designed for the max. operating pressure, sliding pressure is fine (it's quite common actually).

More complex amswer... What is the design operating pressure range of the deaerator? The only problem with increasing the pressure is that the tray stack (I'm assuming this is a spray-tray unit?) will have increased steam velocities. If the tray stack is not designed to handle that much steam, you are going to have liquid hold-up in the tray stack and liquid blowing out of the vent, which will severely decrease your performance.

 

[rmw]

I would have thought that since raising the operating pressure decreases the specific volume of the steam, that the steam velocities would go down, offsetting the additional demand for heating steam to the tray area to reach saturation temperature for he higher operating pressure.

rmw

 

[jproj]

rmw

Actually, you are correct. Thank you for correcting my mistake. Tray enclosure velocities do go down as you increase pressure (because of the specific volume change, as you stated).

 

[bethw]

I don't believe that this is a tray unit. It has spray scrubber units of some sort that I have just begun to dig into and have only seen pictures of to this point. The unit did not function well on start up years ago (poor O2 removal in the area of 100ppb) so many changes had to be done. Probably given to the lowest bidder at the time like most of the stuff here, and has been modified over the years to make up for that low initial investment. The unit now performs well (down to <5 ppb to 0) with small O2 scav. chemical additions. There are times when this unit seems overloaded (high O2 levels). My main concern was for the effect on O2 removal/wasted steam because of varying operating conditions as mentioned before.

 

[jproj]

bethw:

If you have a spray unit then you really need to find out what the design point is. Scrubber units are designed to operate at a steady state condition. They don't handle fluctuations very well (as you've probably noticed)... especially fluctuations that cause a decrease in steam flow below the design point. This is because of the steam / water contact in the scrubber. Most scrubbers I've seen have fixed orifices that are sized for a certain steam flow. I'm really not sure how increased steam flow effects scrubber units, but if the unit is operated below a certain percentage of it's design capacity, your steam / liquid contact turbulence in the scrubber section will decrease. This will result in a lack of heat transfer and performance issues.

The ultimate deciding factor should be DA and system performance. If you are operating the unit on a sliding pressure while not popping the safety valve and achieving performance (O2 outlet concentrations), then I can't see anything wrong with it.

Hope this helps!

jproj

 

[rmw]

To use a different term to say what Jprog stated, spray DA's have very low turndown ratios, I seem to remember as low as 2:1. On the other hand, tray units, and spray/tray units have higher turndown ratios, but I can't remember the ratio, allowing your BFW flow to fluctuate much more without performance loss.

I seldom encountered spray types, as the types of boilers that I had experience with had widely swinging loads.

Probably back at start up, you were running the boiler at lower loads due to some steam user not being yet started up, and the DA suffered. Once everything was started up, and the BFW flow was at rated flow, it did OK.

In a spray unit, the spray patterns through the nozzles is a function of the flow and pressure drop across the nozzles. At lower flows the spray cones are not so good, and the DA cannot work as designed.

That brings me to a problem I see with the variable pressure operation. At 20 psig, and some pressure at a given flow produced by the BFP operating on its curve, you have a given differential pressure across the nozzles. If you change nothing but the operating pressure of the DA, then you automatically have 30 psig less differential pressure, which will cause the spray cones to vary.

So, unless you are running a turbine driven or VSD electric pump, and can speed it up to match the 30 psi delta p loss, you will have to take that into consideration as well.

rmw

 

[rmw]

A clarification; I referred to the pump supplying the water to the DA as the BFP, and later it occurred to me that the water at this point is still generally referred to as condensate and/or make up, and is not usually referred to as BFW until the BFP takes suction from the DA storage tank, sending it to the boiler.

Therefore, my concern with the loss of delta P on the spray nozzles is with the condensate or make up pump, whichever is appropriate in your system. What ever you call it, I am not accoustomed to seeing pumps at that stage of the water cycle being operated as variable speed, so as to be able to be sped up to produce the desired delta P across the nozzles.

rmw