how to size condensate tank?

Stuart Rogers

Hi again,

Thanks to all for your previous help -- we're on our way to a properly operated and maintained system.

Summary: 1925 building, 25 unit co-op in Toronto, 2-pipe steam. Weil-McLain 1280 boiler (4242 sq ft edr; probably too big; 2 years old). I've got some decent pro's in who have skimmed, fixed F&T traps, added outdoor reset, etc., and are now testing/replacing much-neglected radiator traps. Every day things get a little bit better.

But here's the problems I need advice on: The big boiler gets all excited about making steam, the LWCO calls for the boiler feed pump to start up, but before the boiler refills, the burner shuts off on low water. (I think this is the effect of residual oil in the boiler water, because the level in the glass is still bouncing a lot. I'm going to ask the guys to do a TSP-style cleaning along with another skim.) Fifteen seconds later, the burner comes back on, but the 1.5 minute purge means we lose all the pressure. Do we need a bigger pump? The LWCO is a McDonnell Miller 150SHD, apparently non-adjustable. (But when I take the top off, I see a couple of adjustment screws, AND, someone has bent the lever on one of the switches.) Any way to make the pump come on a little sooner?

Second problem: The condensate tank is too small. (Our current tank measures 24" x24" x 20", or about 6.5 cu. ft.) The guys have the float control set to maintain the water about an inch below the overflow; but every time the boiler feed pump comes on, the water level drops about 6 inches, and then we immediately get fresh cold water to bring it back up to level. I figure all my hot condensate is going down the drain, except for the 20-second periods when the pump is running. Should we replace the condensate tank with a bigger one, or add a second tank piped in with the first? How do I calculate what capacity we need? Where should the float control be set so we recapture all our condensate, but don't let the pump run dry?

Happy New Year, and thank you again!
Stuart

Brad White_184

Making Progress

Hi Stuart- sounds like the past couple of months have been productive. Here is the thread from that time as a refresher:

http://forums.invision.net/Thread.cfm?CFApp=2&&Message_ID=359120&_#Message359120

I would definitely take care of the bouncing waterline issue first, by your skimming program. That is wreaking havoc with your ability to control the feed call as you note. I cannot comment on the specifics of the MM 150SHD (I am not a hands-on technician) so will leave that to others.

The make-up or rather the boiler feed rate should be about three times the evaporation rate. Specifically to you, your 4242 SF EDR output would take about 6 to 6.5 GPM. This keeps the water level up ahead of the evaporation with room to spare and allows the pump to rest.

Your float level in the receiver is too high, I would say. Let the water level in the receiver drop to below half or lower before making up water and make sure that the make-up rate is about twice the pump rate (say 10-12 GPM). With your float too high you are not giving the receiver a chance to perform.

Your receiver is probably well-sized at approximately 45 gallon capacity (a common nominal size to be verified of course). This will hold over five minutes of pump run-time. I would check the actual pump flow rate by diverting the flow to a bucket and using a stop watch. See if the flow rate is close to the 6 GPM range. While you are at it, check to see if the boiler feed line and instrumentation piping is clear of mud.

A few thoughts, anyway.

Happy New Year!

Brad

Unknown

Stuart

As Brad has said, it sounds like you are certainly on the right track.
In addition to his great info, you may find more in this pdf to help.
It may sound slightly biased due to the manufacturer,,but good info that applies to all units nonetheless.

Dave

Brad White_184

Thanks, Dave!

If I had that Skidmore document, I would have posted it. Skidmore is my default condensate pump set and was "what I learned on", to use abhorrent English.

Stay warm, dig neat trenches.

Brad

Stuart Rogers

Thanks Dave & Brad

Thanks guys! I really appreciate the info and the link to Skidmore. Brad, you say the boiler-feed rate should be 3 times the evaporation rate; Skidmore recommends that factor for a condensate pump, but uses a factor of 2 for boiler-feed pumps. So that puts my target flow at something between 4.2 and 6.3 GPM (using an evaporation rate of 2.1 GPM). I don't think I can divert the flow to a bucket, but I've calculated that my condensate tank holds about 2.4 gal. per inch of height, so I will measure and time the level change in the sight glass when the pump runs and calculate the flow from that. I'll use the same method to calculate the make-up flow rate, and let you know...

I also found a Hoffman Selection Guide, and it agrees with Skidmore that the tank should be about 42.5 gal. capacity. Allowing for the overflow pipe at the top of my tank, I've probably got 18" of usable height in the tank, or about 43 gal. So you're right again, Brad, the receiver looks to be adequate. ($igh of relief!)

I'll see if the guys can clean and skim more thoroughly next week (supposed to get up to 10 deg. C on Monday, much better than -17 C overnight tonight!), and I'll insist they lower the float level on the receiver. They're also going to install a gauge with a more appropriate pressure range, so I'm sure they'll check the instrumentation siphon loop for mud then.

Thanks again -- another progress report will come :-)

Stuart Rogers

flow rate too high

I timed the boiler feed pump last night, and it is moving more than 9 GPM by my best estimate (4" height in 54 seconds). The make-up water flow is about the same, maybe a little slower.

How do I bring the boiler feed rate down to the 4 to 6 GPM rate I'm supposed to have? There is a ball valve downstream from the pump; can I just turn that partway off?

Thanks!
Stuart

Unknown

Yes, you can

A ball valve isn't the traditional way to do it, but use what you have.

Noel

Stuart Rogers

thanks, Noel

I'll give it a try tonight.

Stuart

Stuart Rogers

It Worked! I'm euphoric!!!

Guys,

I can't tell you how much better I feel about our system, as of tonight. We still have a long way to go to reach steam Utopia, but I have to say -- Wow! are we doing better!

I've trimmed the modulating pressuretrol so we actually STABILIZE at 2 psi!!! (Last January, we were running 7-9 psi!)

I've cut back on the boiler feed pump flow, and the inflow of cooler water no longer shuts down the boil in half the 12 sections of boiler -- the pump runs much more often, for much less time, and the boiler chugs away without even noticing! I have no more burner shutdowns on false low water! No more sudden suction from the back half of the boiler when cool water condenses all the steam bubbles!

The steam guys changed the float level on the condensate tank, so my hot condensate no longer heats the floor via the drains -- we actually collect hot condensate in a condensate receiver now!!!!

The first skimming helped; we need another, but the bounce in the sight glass no longer makes me dizzy.

You guys have given me the confidence to shepherd this project, and tell my local steam guys what I want -- and I'm going to be a Hero to 25 families who have suffered horribly for 15 winters with banging pipes, cold rads, a sauna in the basement and halls, and a heating bill that could fund a presidential campaign.

OK, off the high horse, I'm gonna go sit by my quiet, comfortable living room rad for a bit, then hit the hay. Thank you, Dan, thank you Steam Pros of the Heating Help World!!

Stuart

Brad White_184

What a fantastic

way to start the day, Stuart! Thanks for keeping us updated!

tim smith

Regarding condensate storage

One problem I find with some old vintage buildings is during the shoulder season, the boiler runs shorter cycles to heat the building, if it is a fairly spread out system, the water does not make it back in time and therefore feeds new water. Finally the condensate comes back and overflows. I find I have to oversize by quite a bit for some old systems. Most of these systems had large volume firetube or large cast boilers that could take all the water. Most modern day boilers don't have near the volume. Just .03 worth, Tim
Noel or Brad, what do you think of this idea??

Unknown

I agree.

Brad White_184

Surge tank

That is something common on larger institutional steam boilers. These would be sized for about 20 minutes of storage as a starting point and more if a spread-out system. Because these are coupled with deaerators and boiler feed units, the actual storage volume can be quite large and it also provides an opportunity for treatment of the feed water.

But your core point is correct- the steam output rate is a practical constant so the system will fill with steam in a relatively short time. Then you wait...

Stuart Rogers

shoulder seasons

Yes, that may turn out to be a concern. Our boiler is pretty big (probably too big) and puts out steam at a high rate. I'm prepared to see problems like this in the spring.

But that's ok -- it will be the first time in many years that we've had a properly functioning system, and only after we finish the remedial work will we have a baseline to judge from.

On the good news side (again!) our trap failure rate is (so far) running at only 60%, whereas I used 90% in my estimate spreadsheet. And there's only 5 F&T traps, not the 15 I used as a worst-case scenario. So we're way ahead on rehab costs, and the gas bill will be a fraction of last year's.

Tony Conner_2

Dropping...

... the steam pressure would have helped that little feedpump beyond words. With the 2 PSIG operating pressure it'll actually be able to shove water into the boiler. At 7 - 9 PSIG in the boiler, I'll bet the flow out of the pump was just a trickle, and not nearly enough to keep up with the boiler water that was leaving as steam.

I've seen this scenario a couple of times on much bigger, high pressure boilers. A boiler runs at 125 PSIG, and somebody picks a feedpump with a discharge pressure of - wait for it - 125 PSIG. The boiler will usually get enough water to run on low loads, but as soon as the steam load picks-up, the water in the glass drops like a stone and the burner trips. Flow in pipe is all about pressure drop. No pressure drop, no flow. With the boiler at the same pressure as the feedpump discharge, the feed piping is essentially just a long, skinny storage tank.

VA_Bear

The Recommended Flow Control

B&G and the other condensate return pump OEMs show a cutoff ball valve, a check valve and a plug valve in series. The plug valve is used to throttle the centrifugal pump's output and reduce the operational noise of the pump.