Feedwater Heater Drain to Condenser

[C6HDJOCK]

All:
I am providing piping for a feedwater heater drain (level control) to the condenser. Heater pressure is at 250psig and condenser at 2.5psia. Control valve will need to be sized for flashing flow, but what about the downstream piping? How do I size the piping for flashing flow? Maximum flow is 50gpm through a 3" line to the CV. Do I need to increase the size of the line downstream of the CV?

Thanks, in advance, for any helpful information in the design of this type of system.

 

[rmw]

Try this link and see if it will do what you want it to;

http://www.processassociates.com/process/fluid/dp_2.htm

For sure you have to up-size that piping. Guaranteed you are going to get the flashing and have the two phase flow.

You should take that into consideration in your piping orientation with respect to bends (Long radius) and potential dips or low spots in the line.

You need to read up on two phase flow, because there are regions that you want to avoid. I would have to do some reading to refresh my memory, but I you can do that as easily as I can. Lots of stuff on the net. I had a problem with it 3-4 years ago and found lots of help on the net, and I think on this site. Do a search on Eng-tips as well.

Secondly, you are going to have to be careful where you put the nozzle into the condenser. This is a high energy stream and it can cause a lot of damage if it impinges on condenser tubing or critical support pieces. Use a sparger pipe or STRONG impingement plate. Then bring it in in a location such that if those fail, you still won't hit precious metal.

Make double sure that the maximum flow that you calculate is accurate, and then over size your piping for safety sakes. Valve seats have a way of wearing and passing more than what was the original intention.

I would assume that by level control you mean 'excess' level control and that this is not the primary level control in the heater, but the high level dump. Normally the heater level control valve would cascade the drains to the next lowest heater, and at 250 psig, there ought to be several heaters in the string between this one and the DA or the condenser as the case might be.

If it is the high level dump, take into account that whatever upset that caused the high level might also cause the operating pressure to be higher than 250 psig and size your valve and piping accordingly. If it is due to a burst tube or tubes, burst or even sheared tube or tubes, lots of HP BFW can run the pressure in this heater up real fast.

I have seen lots of condenser damage caused by high energy drains that ended up passing more flow than the designers wrote down on the drawings. I have seen heater drains from heaters much in the same range as your pressure peel 3/4"-1" thick full pen welded impingement baffles back like tuna can lids.

Be careful and post back if you still need help.

rmw

 

[C6HDJOCK]

Thank you RMW for the information. I suspected it would be more difficult than hoped. Your reference will be helpful. This plant is small (20MW) and has only one FW Heater. There is no cascade to lower pressure heaters. I am assuming that it will be as difficult to send the condensate to the condenser as to route it to a flash tank and pump the condensate back to the condenser. Still flashing and still the same problems.

I will upsize the piping downstream of the CV, insure the CV seat is sized and has the material for this service, and ensure the Condenser Manufacturer has contended with the high energy flow. What more could a mother ask?

I appreciate your prompt response. If you don't mind, I will keep you on my list of "good guys" and pick your brain at a later date.

Thanks again,
c6hdjock

 

[rmw]

Does this unit have a deaerator? FWH's above the DA in the larger plants normally cascade down to the DA. FWH's below the DA cascade to the condenser.

Is this the normal drain, or a high level drain as I guessed.

One thing you can do is to put the control valve as close to the final destination as possible. If it ends up being the condenser, then locate the valve right by the condenser close coupled if possible, and you only have to handle the two phase flow for a very short distance.

Everything upstream of the CV is liquid.

rmw

 

[C6HDJOCK]

RMW
This unit does have a DA that is "above" the FWH and this connection in question is the normal drain. I utilized the two-phase flow link and find a modest amount of pressure loss, and what appears to be a large errosion potential.

I think the suggestion to place the CV next to the condenser makes the most sense since all else is not easily or accurately determined. As long as I dump to the hotwell in a straight shot, any errosion will be avoided.

Thanks again for your assistance...

jal

 

[rmw]

C6H....,

Let's quickly define "above".

By saying "above or below" I was referring to the pressure ratings of the units, not physical height. That said, most FWH's that I know of that cascade down to the DA have the DA located at a higher level than the FWH's.

The 250 Psig operating pressure will surely push condensate to quite a high level.

I would suspect that your DA is in the 5-15-50 Psig range, which would make it "below" the FWH, referring to cycle pressures even if you have to look up to see it.

If makes sense to take this condensate to the DA and flash it there, utilizing the flash steam for heating the condensate from the condenser rather than using raw steam to do it, or having the condenser have to handle the flash, which, of course be more in the sub-atmospheric condenser than an elevated pressure DA.

Run the numbers on the fuel cost savings by not having to have to use raw steam to heat all the condensate, vs heating only a portion, and doing it with otherwise wasted heat.

I hope this makes sense, and am sorry that I did not define 'above, below, up or down' well enough.

rmw

 

[C6HDJOCK]

RMW:
No problem, I understand the difference between above/below on the FW train. As you can probably tell, the client I am dealing with has scant documentation on the existing plant, little design basis information on the equipment he is purchasing, and no direction on how he wants the plant configuration. Yet, I am trying to come up with an appropriate design.

I think you may have hit it with regard to the depository for the heater drain condensate. Rather than take it to the condenser and heat the hotwell, I probably should take it back to the DA tank. The DA tank is at a higher pressure and temperature than the Hotwell, but less than the FWH. There would be plenty of driving pressure in the heater to get the flow back to the DA tank. A control valve on the heater drain at the existing steam supply to the DA would allow the heater condensate to flash into the tank.

Pressure in FWH = 248psia, Presure in DA = 28.7psia
Temp of heater drain = 270F, Temp of DA = 248F
Extraction steam to DA = 326F/59psi

I will explore further - Thanks again for your thoughts.

jal

 

[rmw]

I would recommend that you bring this condensate in to the storage tank of the DA.

The heater drains are already way above the oxygen saturation temperature of the water in the storage tank, so you are not going to do much more deaeration of that water. So it does not make sense to take that water through the DA, and the flash steam can rise from the storage tank up through the DA counter flow doing the scrubbing and heating that is needed.

Some DA's have a connection for returning condensate so that the flash can be utilized in the heating section of the DA.

You still may need some heating steam to be supplied by the current Steam supply valve. Make sure it has enough turn down to operate at a lower flow rate with some of the heating steam being now provided by the heater drain condensate.

rmw