Question about steam pressure and velocity

[RHorseUSMC]

I am trying to figure out a problem we are having with a steam boiler. First, it is a natural gas fueled boiler rated at 2,000,000 BTU. In the summer time, the boiler supplies heat to the indoor pool through a heat exchanger. There is approximately 10 ft of 1 inch black iron pipe feeding into the top of of the copper heat exchanger. The problem is that over a 2-year period the steam coming into the heat exchanger @ 40 psi has cut a hole into the heat exchanger. My question is does velocity have any part in this? Someone had told me that do to the size of the boiler (2,000,000 BTU) and the size of the pipe feeding the heat exchanger, the velocity of the of the steam is higher and therefore creating a hole in the heat exchanger. Is there a formula to figure this or can anyone just explain it to me. Thank you--Rick

 

[Danger]

I don't really think that I can be of much help here anyhow, but it would be easier if you can possibly post a simple diagram showing relative pipe diameters in the various sections. Also, what form of heat exchanger it is; ie. just coiled tubing in the pool, finned tubing, 2 coils of tubing in a conductive medium, etc.. (The reason for the last is just to rule out other possible causes of the hole such as transfer-medium corrosion from contaminants. As another 'for instance', if your tubing was aluminum hooked to steel, there could be dielectric effects. I don't know if that applies to copper, though.)
PS: You might have better luck with this in the Engineering section.

 

[Astronuc]

One would need to know an additional property (e.g. temperature, specific enthalpy) of the steam in order to determine if it is wet steam (saturated) or dry steam. Is the 40 psi absolute or gage pressure.

It could be a matter of erosion or erosion-corrosion, which is what Danger indicated. In addition to the velocity, one needs to look at galvanic coupling, pH of the steam and condensate, and oxygen levels in steam/condensate.

Is there a U-bend involved?

This should probably be in the engineering forums - Mechanical would be the appropriate forum.

 

[russ_watters]

The BTU rating has very little to do with the pressure/velocity of the steam in the system (ie, a vastly oversized boiler would simply fire less often and produce roughly the same pressure/velocity). What you have there is probably due to corrosion.

Where is the hole, exactly? I'm pretty sure that copper and steel are the metals that don't work-and-play-well-with-others in plumbing. Ie, if you put them near each other without a dielectric fitting to separate them, they will create a galvanic cell and quickly dissolve the steel.

 

[russ_watters]

From the other thread (please don't double-post):

It could very well be an erosion issue. Usually a heat exchanger won't be made entirely out of copper, just the core. Where exactly is the hole in the heat exchanger? Does it happen at a 90° bend? I am imagining a shell and tube heat exchanger. Is this correct?

edit: heh - maybe I locked the wrong one, but this one had more responses...

 

[quark]

A quick estimate shows problems of erosion as suggested by others. Generally boiler capacities are specified either with steam flowrate or the latent heat capacity.

Assumptions
Capacity of the boiler is 2,000,000btu/hr (and not 2,000,000btu), steam pressure is 40psig or 54.7psia, steam is not distributed to any other equipment, feed water is at saturated temperature and steam is saturated.

Calculation
latent heat of steam ~ 1000btu/lb.
qty. of steam generated = 2000lb/hr.
sp.volume of steam at 54.7psia = 7.827cu.ft/lb
Volume rate of steam = 15654cu.ft/hr = 4.348cu.ft/sec.
ID of the pipe ~ 1" (check actual ID by googleing)
So, velocity = 4.384x4x12x12/3.142 = 803.68fps or 245m/s.

If the feed water temperature is 87F then the steam flowrate is 1666lb/hr or 13039cu.ft/hr or 3.622cu.ft/sec. So, velocity will be about 664ft/sec or 202m/s.

For such smaller sizes, I would prefer not more than 45m/s.

 

[russ_watters]

Bump...

 

[Artman]

You're scouring your pipe if indeed that boiler is dropping 40 psig through a 1" pipe over 10' length. That indicates a velocity of 37000 feet per minute. You should probably run that through a 2.5" or 2" pipe minimum (approximately 10,000 fpm, which is okay for process work, which has a suggested range of 8,000 to 12,000 fpm).

 

[Astronuc]

I am having a little problem with 2,000,000 Btu or Btu/hr.

If it is 2,000,000 Btu/hr, that is 586 kW! I prefer metric or SI.

Now this is heating an 'indoor' pool. I know there is a significant mass of water, but then the heat loss can't be too significant.

So what is with the 2,000,000? It just seems like overkill.

 

[Danger]

So what is with the 2,000,000? It just seems like overkill.

I was unaware of the BTU:kWh relationship, so this didn't set off any flares for me. (My little propane heater that can barely warm up the inside of a car is 12,000 BTU.) Maybe there's a lobster restaurant affiliated with the pool facility.

 

[russ_watters]

So what is with the 2,000,000? It just seems like overkill.

It is bizarre, but it really shouldn't matter - the burner should just cycle a lot. The unit would not really be putting out 2000mbh.

But 40 psi seems like overkill too...Hmm, wait, rereading the OP, I get the impression that maybe this is a large system in a decent sized building and its just that the indoor pool is the only load in the summer. That isn't terribly unusual, then. I'm doing a study right now on a building that has steam heat (sorta) and only uses the steam to make domestic hot water in the summer.

 

[Astronuc]

586 kW = 586 * 1000 W blow dryers (driers?), so perhaps that is correct. So the Btu (or Btu/hr) is peak rating or max capacity, I assume. Perhaps that is necessary in winter? I wonder - Olympic size pool? I wonder what heat loss there is.

It's not clear what the heater is used to heat, other than the pool.

40 psia or 40 psig - it makes a difference. I don't believe that it is the pressure drop over a short section of pipe (i.e. it's not psid).

I prefer to work with absolute pressure.

 

[Louis Calandros]

Opinion on steam cut

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I am trying to figure out a problem we are having with a steam boiler. First, it is a natural gas fueled boiler rated at 2,000,000 BTU. In the summer time, the boiler supplies heat to the indoor pool through a heat exchanger. There is approximately 10 ft of 1 inch black iron pipe feeding into the top of of the copper heat exchanger. The problem is that over a 2-year period the steam coming into the heat exchanger @ 40 psi has cut a hole into the heat exchanger. My question is does velocity have any part in this? Someone had told me that do to the size of the boiler (2,000,000 BTU) and the size of the pipe feeding the heat exchanger, the velocity of the of the steam is higher and therefore creating a hole in the heat exchanger. Is there a formula to figure this or can anyone just explain it to me. Thank you--Rick

Rick, There is some information not brought out in your question about why you may have a problem with steam cutting a hole in your heat exchanger such as are your pipe joints welded or treaded? What type of metal is your exchanger made of? Are the joints welded, threaded or silver soldered? I have experience in welding 1200psi steam lines and the biggest contributor to failure lies in the thickness of the piping (schedule 40,60 etc.), joint design or metalurgical characteristics of components in the system.
You state that you use black iron pipe to the exchanger but do not state what material is used for the exchanger itself. You do not state if the internal tubing of the exchanger is failing or is the joint at the exchanger failing? Is this system of your design and are you following prescribed plumbing codes? Was this system commercially installed or are you the installer? This system has been inplace for two years. Is it used daily or seasonally? Have you had other problems associated with other components of the system that may be contributing to its demise? What maintenance schedule do you follow? Is the water treated that is used in the boiler? Are we talking true steam or heated water? Who told about the deficient pipe size?
I think you need to answer these questions first before you need to worry about the velocity question. Steam is molecularly abrasive in its own right. At 40 psi you are talking about having to have a high pressure boiler license to even operate that thing in a commercial environment in the state of Ohio. I think I have added more to your dillema than I have helped. I think you have already approched a certified plumbber in the time I have written you. Lou