Steam Velocity

RayWohlfarth

I have a head scratcher for me and hoping the people smarter than myself can help. I am doing research for a new book and trying to see if there is a steam velocity when carryover starts. I know Dan H tells us the old boilers used 15 FPS as their norm. I did a study of 50 of the most popular low pressure steam boilers (I Know I don't have a life) and found the average velocity they recommend is 45.77 feet per second. Spirax Sarco recommends 40 feet per second. My question is this, Is there a velocity at which carryover starts? I know there are lots of circumstances that affect it such as water quality and burner firing rate. I have been searching all over the internet and even spoke with a couple steam separator manufacturers without an answer. Thank you in advance for firing your neurons on a Friday.
Ray Wohlfarth

nicholas bonham-carter

In modern boilers, there is no longer the voluminous steam chest which there once was. The risers, and header now serve the purpose of drying the steam, and so the most important part of the carryover equation would be the height of the riser above the waterline, and the design of the header.
As Steamhead has noted, a number of side exit boilers are more prone to produce wet steam, unless piped properly. In these boilers, much lower velocities, from larger diameter risers, are needed to compensate for the short distance from waterline to supply piping.--NBC

Jamie Hall

Just thinking here, having absolutely no concrete or experimental basis...

But. I doubt that there is one single critical velocity. Why? Because the velocity which will allow steam to effectively carry water droplets -- a usable definition of carryover -- will vary with the size of the droplet. Further, it should be different (not saying it is, mind you!) in a horizontal pipe with turbulence, since the turbulence will tend to keep the droplets suspended, than it would be in a vertical riser where, despite the turbulence, the droplets are trying to fall down.

A worthwhile investigation however, and carry on!

RayWohlfarth

Wow Thanks Gentlemen I do believe there is more than just a single velocity number and other factors.

Nicholas, I agree and have seen side piped steam boilers with substantial carryover with a vertical riser of 6 feet or more.

Jamie, I never considered the droplet size to be part of the calculation. Im going to need a bigger calculator.

Hatterasguy, You bring up another great point I have thought of is the disengagement time. The faster the steam velocity, the longer the disengagement time.

It seems that manufacturers are not as much concerned with carryover as they use the near boiler piping for the drying effect.
I appreciate all the help and expertise
Ray

jumper

Don't forget de-acceleration & change of direction to catch liquid.

DanHolohan

Ray, there's a chart on page 43 of Lost Art that shows the maximum velocities allowed when steam has to flow against falling condensate. Go beyond those numbers and the steam wins. Here are the limits:

2" not more than 23 fpx
2-1/2" not more than 26 fps
3" not more than 29 fps
4" not more than 32 fps

I'm looking forward to the new book!

RayWohlfarth

Jumper, Thanks for the good point
Dan, Of course you have it in Lost Art. I should have known. Thanks
Ray

The Steam Whisperer

Stantfin wants a minimum of 3 x the diameter of the header between the last boiler riser and the first take off and 2 times the diameter between take off and also between the last take off and the drip. This all makes sense when you think about what's going on inside the pipes.

The Steam Whisperer

I believe that the velocity of the steam going up the first riser will cause the water to lift up at this point. Leaving some extra length will allow the water to drop back down to the bottom of the header before the next riser, preventing it from carrying over into the system.

RayWohlfarth

Gentlemen Thank you for giving me some things to consider.
Hatterasguy, I am not sure about the disengagement time and still trying to wrap my head around it. Another consideration is the steam pressure of the boiler as the lower the steam pressure, the faster the velocity. For example, I met with a brewer that used a 2 1/2" pipe for his 400,000 Btuh output boiler because it was 39 feet per second at 12 Psig. He was getting carryover in the system. I explained the steam velocity was 66 FPS at 2Psig which would cause carryover
Have a great day
Ray

RayWohlfarth

Thanks Hatterasguy!

Dave0176

I prefer to raise the risers as high as a basement ceiling will permit, always use both risers, and size them accordingly. On paper I prefer to keep my boiler exit velocity around 15 fps or slower, this will give the steam a better chance to to drop water.

Just as an example, one of my most popular steam boiler installs is a Weil McLain EG-40, 125,000 BTU and 325 sq ft steam. I run two 2-1/2" risers a minimum of 30" above the boiler jacket or higher depending on space. The risers turn into a 3" drop header, then I'll put a 3"X12" long nipple before the first take off. Most risers end up around 36" high. I may just use 3" risers from now on because that's what the boiler is tapped for.

ChrisJ

Hatterasguy said:

Dave0176 said:

I prefer to raise the risers as high as a basement ceiling will permit, always use both risers, and size them accordingly. On paper I prefer to keep my boiler exit velocity around 15 fps or slower, this will give the steam a better chance to to drop water.

Just as an example, one of my most popular steam boiler installs is a Weil McLain EG-40, 125,000 BTU and 325 sq ft steam. I run two 2-1/2" risers a minimum of 30" above the boiler jacket or higher depending on space. The risers turn into a 3" drop header, then I'll put a 3"X12" long nipple before the first take off. Most risers end up around 36" high. I may just use 3" risers from now on because that's what the boiler is tapped for.

You really are building some of the finest systems on the Wall. There are those that use 3" headers with an IN-9 and they are congratulated despite a 32 fps velocity in the header.

15 fps is truly exemplary.

The issue I see with this is I have bushed down to two 2" risers on my EG-40 and I run a fraction of an ounce of pressure with the boiler filled 3/4 of the way up the gauge glass and a lot of water treatment. Remember WM recommends half way up the gauge glass.

Also, my TDS in my wet return under those conditions is always between 1 and 2 PPM also suggesting very dry steam.

To me, this suggests two 2" risers is overkill for an EG-40. Two 2 1/2 or 3" is just wasteful if that's the case especially given the price of 2" fittings vs 2 1/2" and 3".

Danny Scully

I have never and will never put a bushing in the outlet of a steam boiler, sorry @ChrisJ

ChrisJ

Danny Scully said:

I have never and will never put a bushing in the outlet of a steam boiler, sorry @ChrisJ

No need to apologize. Like I said, I'm heating my entire house with only 0.14 - 0.43 oz at the boiler. I must have done something right, eh?

It was the first time I ever installed a boiler. If I was doing it over, I would run 3" out 12" up into a 3" to 2" reducer just because I feel it's a better way to go with little extra cost.

However, keep in mind all Burnham residential blocks are only tapped with 2" and according to WM my piping is good for an EG-95 (400,000 BTU/H input) vs my EG-40's pitiful 125,000. I had considered all of this for months before piping the boiler.


The proof is in the pudding and I'm running the boiler intentionally overfilled with a lot of water treatment and have no carry over. I have to assume this means making any improvements to the piping would be futile.

This also performed fantastic when it was an EG-45.
Personally, I see no reason to go larger for the EG-45 and smaller unless you just want to burn money or have bragging rights.






@KC_Jones Didn't you do the 3" into a reducer on your EG-40?

KC_Jones

Yes I did, pretty much exactly as you described. 12" nipple into reducer to 2" then into 3" drop header.

ChrisJ

KC_Jones said:

Yes I did, pretty much exactly as you described. 12" nipple into reducer to 2" then into 3" drop header.

What box?

Henry

Look at any steam trap manufacturers steam handbook. You will find the recommended velocities per pressure. Try Armstrong Steam for one.

gerry gill

You really are building some of the finest systems on the Wall. There are those that use 3" headers with an IN-9 and they are congratulated despite a 32 fps velocity in the header.

15 fps is truly exemplary.

Hatteresguy, can you post some pictures of your installs for us.

EzzyT

Gerry he can't post any pictures of his installs because he hasn't done any installs.

Hap_Hazzard

Jamie Hall said:

Just thinking here, having absolutely no concrete or experimental basis...

But. I doubt that there is one single critical velocity. Why? Because the velocity which will allow steam to effectively carry water droplets -- a usable definition of carryover -- will vary with the size of the droplet. Further, it should be different (not saying it is, mind you!) in a horizontal pipe with turbulence, since the turbulence will tend to keep the droplets suspended, than it would be in a vertical riser where, despite the turbulence, the droplets are trying to fall down.

A worthwhile investigation however, and carry on!

The droplet size is determined primarily by the difference between the density of the liquid and vapor phases and the pressure. The densities of steam and water are the the same for everybody, of course, although water quality, especially TDS, can increase the density of water and, to a lesser extent, steam. Low pressure, believe it or not, is a bad thing in terms of water entrainment, but I don't think it would help significantly to increase the operating pressure from 8oz. to 6lb., and it would be bad for a lot of other reasons.

I wouldn't know how to calculate a Reynolds number for a steam header if my life depended on it, but my impression is that a well-designed header creates a more or less laminar flow that is disrupted by the diversion of the stream into perpendicular risers while allowing any entrained water to continue on a more linear trajectory into the equalizer due to its higher inertial mass.

Steamhead

Hap_Hazzard said:

I wouldn't know how to calculate a Reynolds number for a steam header if my life depended on it, but my impression is that a well-designed header creates a more or less laminar flow that is disrupted by the diversion of the stream into perpendicular risers while allowing any entrained water to continue on a more linear trajectory into the equalizer due to its higher inertial mass.

That's how Dan describes it in Lost Art, on page 23.

Hap_Hazzard

Steamhead said:

That's how Dan describes it in Lost Art, on page 23.

No, he makes it a lot easier to visualize--he's a much better writer--but that's probably where I got that impression. There's nothing I know about steam I didn't hear from either you or Dan or one of the Wallies.