Sight glass

[Lopez Mike]

I'm interested in ideas about where to position a sight glass.

The boiler is a Beckman VFT-30. 18" diameter drum and has a water leg.

The question is how far above the lower tube sheet might be O.K. for the bottom of the glass and how far below the steam exit might be O.K. for the top of the glass. I'm guessing in the realm of 3" for both numbers but am looking for input.

Any thoughts on where the best 'normal' level for the water during operation might be? Nearer to the top for more steel to water contact or further down for some possible drying of the steam. I hesitate to call it superheat.

Right now it has a big long glass (13" fitting to fitting) with the extremes within a couple inches of each limit. I'm considering getting a much shorter glass. Maybe half as long. This boat will sometimes be operated (under supervision!) by less experienced people and I'd rather let them think that the level needs to be kept in a narrower range. But should I position such a short glass higher, lower, or in the middle?

Thanks.

[fredrosse]

From a previous post:

The ASME Boiler & Pressure Vessel Code "The lowest visible water level in a gage glass shall be at least 2 in. (50 mm) above the lowest permissible water level". I often question ASME rules, but on thought it soon becomes obvious that the ASME Code is on the proper side of the issue.

This rule is to assure that someone who can see no water level will immediately know to shutoff the fire. Hopefully any operator of a boiler knows to kill the fire immediately if there is no water showing in the boiler. If the gauge glass shows levels into the danger zone, then there is a chance that the operator might think all is well, because he (or she) can see a water level. If the glass is somehow marked to show the minimum safe level, that would be OK, ..... BUT...... the glass will be replaced some day, and this marking might very well not be replaced on the new glass.

A thumbrule, keep at least 10 diameters of tube length covered with water above the tubesheet, where diameter here refers to the inside diameter of the tube. If you ues turbulators, then this value can be reduced somewhat.

On the Margaret S. Boiler, (16 in dia shell, 48 tubes, 1.25 x 0.095 x 18) the Try Cocks are located at 4 in, 5.75 in and 7.5 in below the steam outlet. I have found no problems when running in this water level range. The sight glass extends 1-1/2 in above the top try cock, and 1-1/2 below the bottom try cock.

If you force your boiler to high output, stay away from maximum level to avoid possible priming. Sending a slug of water to the engine is not good. On the Margaret S. there is a centrifugal separator tank with a steam trap at the bottom, thus if the boiler primes it will not get to the engine.

[Lopez Mike]

Thanks Fred.

Wow! That column looks like it came off of boiler ten times as big. I was planning something a bit more modest.

One of the reasons that I'm tearing my setup apart if that the person who cobbled this mess together connected the glass to the boiler with a couple of street els at each end. That way he could adjust the distance between the ends as the spacing of the boiler fittings was a given. The down side is that the street els cannot be tightened down all the way thus the only thing keeping the two ends of the glass assembly from pulling apart is some friction in the pipe threads and perhaps a bit of grip on each end of the glass at the packing glands. Sad.

So I'm building a water column along with replacing the junk valves and schedule 40 nipples everywhere. There are lots of extra entry points to the boiler. A good thing. He plugged the extras with nipples and caps rather than plugs. Weird.

[fredrosse]

I made that column from a piece of 2-1/2 inch Schedule 40 Pipe, and wanted to be parallel to the Top and Bottom shell side connections on the boiler, that is why it is so long at 20 inches, but I have room for it. I also used street elbows to attach the water column, and I got good alignment by having the elbows and unions in place while I tack welded the column connections onto the column. All attachments are SA-105 3000 pound Thread-O-Lets. Using Schedule 80 Half Couplings would have probably been OK, but the ASME Code does not allow that. The overall cost difference is minor, so I would recommend you use the Thread-O-Lets.

All piping attached to the boiler is Schedule 80, and all Screwed Fittings are 300 pound Malliable Iron (Rated for 300 PSI Steam).

The second photo shows the separator on top of the boiler before assembly. A piece of 3 inch Schedule 40 Pipe, 12 inches long, with butt welded ends