Banging Noise in 2-Pipe Steam Heating System

[lbspersqin]

Building and Equipment:
16 story apartment building, 248 residential units
Two(2)-pipe steam heating system
Two(2) 200 hp low pressure steam and hot water scotch marine boilers; Boiler A and Boiler B
One(1) Vacuum Pump
Two(2) Condensate pumps
Two steam loops serve all 248 apartments. One loop has steam risers labeled A through L. The other loop has riser labeled N through U.

Boiler Operating Parameters:
Anecdotal observations provided by client:
1. A modulating heating and controller panel - "Multi-Mod" sets the max steam temperature and pressure at 116 deg F and 2.5 psi respectively.
2. One boiler usually operates while the other is on standby. Boiler A and Boiler B switch roles as primary secondary once per week.
2. The Vacuum pump maintains a negative pressure of -5 psig.

Observed Phenomenon
1. A banging noise is heard each morning starting at about 5:30 AM when the “Multi-Mod” switches from “Night Mode” to “Day Mode” and lasts until about 8:30 AM. During this period, the banging is steady with a rate of about one knock per second. After this period the noise is intermittent throughout the day.
2. Of the two riser loops, the loop with riser L is the one with the steady banging noise. Steam enters this loop and first serves riser L. Riser L serves 12 apartments above it, the first being apartment 1L. Apartment 1L is where the steady state banging noise is the loudest.
3. If the supply steam temperature is manually set for below 116 deg F at 2.5 lbs of pressure the noise falls off then disappears. When the steam temperature is brought higher than this threshold, the noise reappears shortly thereafter.
4. Although set for 116 deg F, when boiler firing sequence is started, the Multi-Mod panel often displays an overshoot of up to 126 deg F and 5 psi for a period of time and even occasionally reaching 140 deg F on extremely cold days.

Attempted fixes that did not result in the elimination of the noise
1. Several steam traps in the upper apartments served by riser L were replaced. (No improvement)
2. The steam trap at the bottom of riser L was removed. (This lessened the noise)
3. A previous engineer determined the boiler system is over-sized for the building which most likely is causing short-cycling of the boiler operation.

I am seeking possible explanations or this steady state banging noise.

Thank you kindly.

 

[bimr]

Banging is typically due to steam forcing its way through pockets of water. The problem can be traced to:

radiators not sloped to drain (shim the radiator)
steam valve need to be replaced (replace)
steam valve not fully open (valve should be open or closed)

 

[LittleInch]

Sorry but steam temp of 116F? how is this possible at a pressure of 2.5 psig? It doesn't look like a typo as you say it in more than one place...

Does 200hp refer to the heat output of the boiler? - sounds way too low to service 248 appartments

This system looks very specific to your problem, but a lot will go on what tis the layout of the piping and where are the drains and traps and vents?

I just tried "steam heating system knocking" into google and there are lots of answers there.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[TBP]

You have a vaccum system. Did somebody install an inverted bucket steam trap? They don't work well on vacuum systems as they have a water seal in the bottom of the trap body. That seal water will be either sucked-out by the vacuum, or evaporate because of it. that lets live steam enter the condensate system - and let the water hammer begin...

If I understand your system description correctly, the trap on the bottom of the steam riser is supposed to be there. I'd recommend putting it back.

I would post this question on "heatinghelp.com". There are several boards there, one is "Strictly Steam". Someone there will have the answer to your problem. I would also suggest the inexpensive book "The Lost Art Of Steam Heating" by Dan Holohan, who is the guy who runs this site. Your system, it's problems and the solution to those problems will be in that book.

 

[lbspersqin]

LittleInch,

Thank you for your response.

1. You are correct about the steam temp and pressure. I made the classic mistake of taking down information without scrutinizing it. At 2.5 psig and atmospheric pressure saturated steam temp is about 220 deg F. 116 def F is allegedly the temperature from a sensor in the condensate rerun line as measured from a heat timer panel.

2. The gross output power of each boiler is 200 hp. The boilers typically operate as primary secondary but both will fire to meet the demand near, on or below design OA temp.

3. Piping plans and/or riser diagrams are no available.

Thanks again for your response.

 

[lbspersqin]

TBP,

1. I don't know if an inverted bucket steam trap is installed, I will find out.
2. As for the trap at the bottom of the steam riser being removed, that is incorrect; it had been relocated from the bottom of the condensate return riser to a position several feet away on the main condensate return line.

Thank you for the referrals to "heatinghelp.com" and the book I will check these out.

 

[BronYrAur]

What is the difference between "night mode" and "day mode"?

 

[chicopee]

Typical of low pressure steam heating boilers firing to max in the morning when the piping is cold and as mentioned due to improper drainage of water either condensing at the onset or left overnight.