Feeding Low Press Boilers with Water Treatment from High Press Plant

[BronYrAur]

I will be adding low pressure (15 psig) boilers to a central plant that currently uses 125 psig boilers. The low pressure boilers will only be used during the summer when my load is greatly reduced. My high pressure boilers will be shut off at that time. I have calculated my summer load and have determined that most of my piping distribution system will be adaquate. I will be paralleling low pressure control valves, traps, etc. I am fairly certain that I have enerything "downstream" covered.

My question today concerns the boiler feed equipment. The high pressure plant has 2 deaerators, a water softening system, makeup water tank, chemical feed system, and a blowdown heat recovery system. Can this all be used on the new low pressure boilers? My deaerator runs at 5 psig, so there will be no problem serving it with the low pressure boilers. I assume that the water softening equipment can remain the same. What about chemical treatment? Will the new boilers require a different water chemistry?? Also, my feed water pumps are currently running at 225 psig. I'm not sure why they are so high, since the boilers opearte at 125, but that's where they are set. Can I still use them with the appropriate PRV, or should a seperate feed water pump be used for the low pressure boilers? Should I even be concerned with blowdown heat recovery? Any other concerns?

 

[BigInch]

Actually your questions raise a lot more questions in my mind. And a whole lot more "concerns". It seems like substantial modifications have been made. And... it doesn't seem like you have much choice other than to run with the existing auxiliary equipment. One would think that, if all that was accomplished in the usual manner using a design engineering company and construction contractor, was not all that you ask already verified? Did they leave out the new operating procedures and all operator training for the new additions to the system.

And I really don't understand how or why you have been operating pumps at 225 psig when boilers are only rated for 125. Maybe you should think about changing the pumps, buying a VFD and, while your at it, add a safety shutdown valve and PSV to that PRV.

What's going on there?

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[BronYrAur]

BigInch,

Nothing has been done yet. I'm in the process of verifying, which is why I asked the questions. As I mentioned, I have a good handle on the "downstream" stuff; I just need a reality check on the boiler feed. The motivation for this is to drop down to 15 psig during a good part of the year in order to eliminate the need for a 24/7 stationary engineer, which is required for high pressure. The plan is to remove the oldest boiler which is a 35,000 #/HR B&W unit, and replace it with 4 low pressure boilers in the range of 4000 #/hr each. This is enough to satisfy the summer reheat and domestic water loads.

I'll try to find out why the current boiler feed pressure is so high. What is the rule of thumb for feed pressure relative to boiler pressure?

What are your other concerns?

 

[BigInch]

That's a "relief"! I thought you were ready to turn the key. Good! You're ahead of them then.

I've only done one boiler feed water transient hydraulic analysis about 8 years ago (but it was for a nuc). As I recall, the feed water pump had the capability to go to the
design operating pressure of the boiler. Makes sense, otherwise one or the other would be wasting the money it takes to build the sys to that pressure rating. If its a modification, the original economics of the system doesn't necessarily apply now, so as long as the pump can at least reach the max pressure of any boiler it will be feeding, plus say 10-15% or so, and still be under the relief setting of the boiler. I'd say that would be a sensible requirement. You might want to wait and see if a boiler operator/designer wants to make a better suggestion. If discharge pressure of the pump can reach over the relief valve setting of the boiler, then I would think a PRegV and a safety shut down valve in the feed line would be applicable, but I DO NOT know what the code has to say about that.

My other concerns were mainly related to preparation of the appropriate operating procedures and getting some training for the operators before they start up the new system. That can obviously wait for a bit now, as long as its coming.

If moving to a lower pressure gas or vapor service, there may be some valves and lines that will need to be resized (upwards), as the actual volume rates will tend to increase with lower pressure, IF there is no corresponding decrease in mass rate. Then the opposite could be true, if there is a relatively big decrease in mass rate, I would expect that some control valves and regulators might need to be reduced in size. You don't really want to have F/PCVs operating at 15-30% of range, if you can help it.

So, since I have no expertise in power gen and their aux systems per say, I should leave it at that and you wait for better advice, other than to say you might want to keep an eye out for general overcapacity issues in all the aux systems.

I usually don't like VFDs, but when you need them, you need them. This may be one of those cases where one could be beneficial.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com