Options for oversized steam boiler VLR pump

aircooled81

I have returned to a site I annually maintain a Peerless Steam Boiler, that has always had an issue maintaining the water line. I beleive the Vac/Cond pump is groselly oversized and is the contributor to this issue. I don't see the safety concern as it only floods at start-up, but feel it needs to be remedied eventually.
I'd like to know if there is a way to derate this pump, or use it like a storage tank with auto feed rather than float feed. Or is this pump just too big and needs to be replaced with smaller model? Adding a storage tank and auto feed valve just seems like an expensive bandaid for something that should be corrected, but I am not rulling out any options.
So my questions are, is this a horrible problem, and whats the most cost effective method to deal with this if it is a big problem?

- Boiler is a Peerless 211A-05-W/S-1, net 2,100sqft steam I-B-R 504,000
http://www.peerlessboilers.com/DesktopModules/Bring2mind/DMX/Download.aspx?EntryId=34&PortalId=0&DownloadMethod=attachment
- Pump is a Bell & Gossett 10VLRS1-20, 14GPM, 6CFM, 5.5inhg, simplex
http://documentlibrary.xylemappliedwater.com/wp-content/blogs.dir/22/files/2012/07/s-310d.pdf


What happens at start-up in the beg of the heating season or after PM and flush/skim (cold start) is the water level is fine but the boiler needs to run for about 30 minutes before it reaches pressure. Currently the ex pressuretrol (honeywell Q7800B 1003-1) is as low as I can set it, so it cut's in at 0.9psig and cuts out at 2.4psig. As the boiler steam takes off into the system, the condensate does not return quick enough to fill the large vessel of the VLR, so the make-up water on the Mcdonnel Miller LWCO (51-2) feeds to get the water line back to normal. Shortly after this has taken place the VLR descides it's full and ejects condensate back into the boiler. Then we see the water in the sight glass rise to almost the top. As the boiler continues to fire, typically the remaining condensate in the pump gathers some more and ejects again, this then raises the water line above the sight glass. Now there is no more need for make-up water, so that does not contribute anymore, but now the boiler water line is much higher than it should be, and only lowers to the top of the sight glass before the pump is full again and ready to discharge anyways.
The VLR is set to maintain 3"hg cut-in and 8"hg cut-out. I had adjusted the floats in the upper and lower receivers to what I think is the lowest settings, to keep the water volume and level as low as possible. This has helped a little, as it has forced the pump to discharge a smaller volume of condensate back into the boiler sooner, this is why I can atleast see a little bit of water line in the site glass later in the boiler cycle. But I believe this pump is 4 to 5 times larger than it should have been. I dont know if this was to assist with leak issues but that sounds like the most logical, this system has about 30 radiator stretched out across a 2 story residence built in 1920's, so leaks over the years may have required more vacuum?
Someone replaced it in 2013 with the identical model that may have been in here since the 60's, so explaining to the customer why they should correct this has been difficult, and it's fairly expensive equipment so I have avoided the topic as there have been other more pressing repairs needed in other parts of the system.
BTW, I'm sure this was only replaced because of the leaking steam traps. One return line is almost steam temp and compared to the others it is smoken hot. I have also concluded that until they approve repairs to those failed traps, no way condensate is returning at the rate it should have been, but I dont think this is the smoking gun as to why it takes the pump so long to return condensate back to the boiler.

Jamie Hall

Um... this may be a simple minded thought, but can't you rewire and redo the controls so that the pump runs when the boiler needs water -- that is when the boiler water level is low -- rather than when the tank is too high? As I see it, you would have a float control in the condensate tank which would ensure that there was enough water in the tank, but the pump (which would now be called a boiler feed pump, rather than a condensate return pump) would be controlled by the water level in the boiler, which is what you want, rather than the water level in the tank, which is irrelevant.

You must have at least one LWCO on the boiler. I'd put another one on, but instead of functioning as an LWCO I'd have it control the feed pump (it could act as an LWCO, too, but at a lower level).

The Steam Whisperer

A few things I've done when bumping into this issue are:

Adjust the float level on/off settings of the feed pump very tight so only a small amount of water is fed on each pump cycle. This will often get the pump returning condensate sooner to the boiler so the water feeder won't kick on

Adjust the back pressure on the pump so at full boiler load, the pump is running about 1/3 the time. This is the recommended pump sizing.

Vent the steam mains like made, so condensate will start returning sooner. Lots of main venting will get the steam to the ends of the mains much quicker, so then condensate starts getting to the pump sooner.

Steamhead

And if that doesn't work, then install a boiler-feed tank and pump. This way, as long as a minimum water level is maintained in the tank, no water will feed into the system. The vac pump will pump out when it feels like it, into the boiler-feed tank, and the water won't go to the boiler unless the pump controller on the boiler calls for it. The tank will have to be large enough to hold the amount of condensate that comes back on startup.

EBEBRATT-Ed

You can try any of the things mentioned above but:
1. you need a boiler feed tank large enough that you do not add any make-up water to the system while the boiler is starting a cycle. The feed tank needs to store enough condensate to feed the boiler without adding make-up water until the condensate returns. It also must store the condensate "hanging up" in the system when it does return without overflowing.

Most undersize the BFT then you are constantly adding and dumping water which will lead to destruction of the boiler feed piping and eventually the boiler

a couple of things to check for:
Make sure the boiler is not overfired. If you can reduce the input the you will need less feedwater until the water returns

Make sure all traps and air vents are working properly. This will help the condensate return faster. Check for clogged returns

AMservices

How do air vents work on a vacuum system?
I haven't worked on big vacuum systems yet, only retrofitted vacuum pumps on smaller system, so I'm confused by how a vacuum can form with open holes.
I agree with @Steamhead
"The laws of physics will always override the laws of economics"
That what the customer needs to understand.

The Steam Whisperer

For main vents in a vacuum system you can use high capacity steam traps as crossover traps at the ends of the mains

aircooled81

Wow great insight all.

- I have 2 end of main steam traps, both need to be rebuilt. Not sure if the bucket is working on both, but the steam side leaks on both.

- I have considered boiler feed tank, but never installed one before. I am curious about using the condensate tank as a feed tank, sure looks like it has enough volume but I better check.

- yes, steam traps (prob 8 of 32) are leaking, last year I was able to rebuild 22 of them, Only 2 checked out still working, and I could not convince ownership to let me check the remaing 8. Their condensate return line (from these 8 unchecked traps) is smoking hot so I know one to all are leaking. Who knows how many have survived a season of downstream steam and water hammer.

Yes, this place has start-up water hammer.
Side note, there are a handful of poorly or completely incorrectly sloped condensate returns that are large contributor to water hammer at start-up, and delayed condensate return. These repairs have been suggested and are awaiting approval.

- I did set the floats as low as I thought I could go, OEM mentioned 1/4" water in sg on upper resevoir as min setpoint for float, I snuck a bit below that. Only problem is if you turn the pump off mid cycle (for service or to shut down the system on timeclock), sometimes you can loose the prime in the resevoir. I have had to adjust it to accomodate that, guess 1/4" at SG is there for a reason.

- I like the idea of adjusting the back-pressure on the pump, I think the idea there was to slow the volume of condensate that would return to the boiler. Since I run this boiler up to 2.4psig, I could probably get away with a pressure as low as 3 or 4 psig. But pressure does not exactly equate to volume, so this is something to toy with. I'll just need to cut a guage on the discharge before the check valve and gate valve. Good call.

- the boiler is overfired in my opinion, but I have not performed a stack gas analysis. It's just currently making the suggested 3.5" manifold pressure. Peerless offers something called the mod-u-pak. I have considered pricing out this option, as it will allow for up to 3 firing ranges. Low and behold, I tested the low fire gas valve long ago, turns out it's bad, solenoid doesn't even open that valve. Nor does this boiler currently have the additional pressuretrol to operate a low fire valve anyways. The infrastructure needs to be repaird and added too, to allow for 2 to 3 stages of firing, it was not on the top of the priority list...... but if low fire will help the issue of an oversized cond/vac pump, I'm all ears.

Looks like I have a good sized list to tackle here before I opt for a BFT or retrofit the ex vac/cond pump.
Thanks again for the insight all!

The Steam Whisperer

Hi,


With the pump adjustment the idea is not to go as low as you can go, but keep the defferential between on and off the the minimum, so the pump shoot out short bursts of condensate more often instead of waiting a long time and then dumping a large load of condensate.

Definitely get the traps and piping working....that could solve your problem all by itself.

Pumpguy

Have you checked that equalizer line(s) are installed and operating properly?

Missing, failed closed, or clogged equalizer lines or valves will prevent induced vacuum due to condensing steam from equalizing with return line pressure. When this happens, steam side pressure becomes lower than the return side pressure, and condensate gets held up in the system due to the reversed pressure differential.

At the next cycle, the system fills with steam, which is now at a higher than the return line pressure, and the held back condensate now flows downstream like it should, causing a flooded condition you describe.

The attached files show how equalizer lines should look and be piped. The dropped check valve arrangement is more common, but either the check valve or thermostatic trap type will work.

That said, my first recommendation would be, as others have recommended above, the installation of a separate boiler feed tank and pump set, or at least, decreasing the float switch level settings at the vacuum pump to decrease the cycle volume.