Help me understand steam boilers and pressure safety valves

by **saepl** on Sun Aug 14, 2011 12:59 pm

Safety valves for espresso steam boilers are set to about 3 bar (45 psi). I always thought that the steam boilers were filled right to the top with cold water and then heated. But I am not thinking this is the case because typical household water pressure is about 60 psi so that would cause the valve to open. Thus must mean that the boilers are only about half full if water before they are brought to steam temperature. Does that make sense?

by **kize** on Sun Aug 14, 2011 1:21 pm

Safety valves are put into place to protect vessels (in this case boilers) from exploding due to some failure in a working model. I'm no scientist but believe you need water, air, a heat source and a sealed vessel to create pressurized steam. To reach steam in a vessel there has to be an air space within for the water to boil.

by **Peppersass** on Sun Aug 14, 2011 1:21 pm

Yes, that's right. The boiler "buffers" enough steam to compensate for the fact that the heater can't produce steam fast enough to keep up with your frothing in real time. At least, that's a computer geek's description

You're also correct that if the boiler is filled with water to capacity under typical line pressure, the OPV would open. I've seen it happen on my machine when the autofill probe detection failed (long story.)

The fill level varies. My GS/3 fills a little over halfway with water. The level can be adjusted by changing the angle of the level probe.

by HB on Sun Aug 14, 2011 1:34 pm

saepl wrote:Thus must mean that the boilers are only about half full if water before they are brought to steam temperature.

Or half empty, depending on your point of view.

kize wrote:I'm no scientist but believe you need water, air, a heat source and a sealed vessel to create pressurized steam. To reach steam in a vessel there has to be an air space within for the water to boil.

Not to put too fine a point on it, but steam boilers have another valve ("vacuum breaker") to allow air to escape when it first pressurizes so the boiler is filled only with water and water vapor. Otherwise, a steam boiler's temperature controlled by a pressurestat will be too low due to false pressure. Those who are interested can search on "saturated steam" for more than one would ever want to know about the subject.

by **Psyd** on Sun Aug 14, 2011 4:31 pm

HB wrote:Not to put too fine a point on it, but steam boilers have another valve ("vacuum breaker") to allow air to escape when it first pressurizes so the boiler is filled only with water and water vapor.

All the vacuum breaker valve is is a glorified flap that is designed to allow pressure one way, neh? pressure from the inside is not allowed out (once sealed by inside pressure reaching a certain point) and outside pressure, if greater than inside, is allowed in. Meaning that if you cool down your boiler, it will allow outside pressure to fill the gap created by the cooling steam instead of creating a vacuum.
Is the vacuum breakers' design to allow the air out when the boiler is heating, or is that a by-product?
I always knew how it worked, but wan't completely sure of the intent of it being there: To keep the boiler from going into vacuum when it cools down (and perhaps sucking gunk up the steam tube?), or if the intent was to keep air from the steam, and the anti-vacuum properties are the happy by-product.

by **Randy G.** on Sun Aug 14, 2011 4:46 pm

I have an explanation here.
A point sometimes missed is that when a steam boiler reaches its temperature, regulated by the pressurestat, the water and the steam are at the same temperature. The water and steam are held under pressure greater then atmospheric pressure, and at a higher temperature than 212F. In that situation, the water is, in effect, a steam storage system. As some steam is released through the steam valve, the pressure that is keeping the water from boiling is released, some of the water boils, and more steam is being "created" and released.

by HB on Sun Aug 14, 2011 6:11 pm

Psyd wrote:Is the vacuum breakers' design to allow the air out when the boiler is heating, or is that a by-product?

You're right that it serves two purposes. The boiler can withstand the vacuum that forms when it cools, but as you noted, it may suck icky stuff from the steam wand into the boiler; that's why an anti-syphon device is required on NSF certified espresso equipment.

by **Peppersass** on Sun Aug 14, 2011 6:28 pm

HB wrote:You're right that it serves two purposes. The boiler can withstand the vacuum that forms when it cools, but as you noted, it may suck icky stuff from the steam wand into the boiler; that's why an anti-syphon device is required on NSF certified espresso equipment.

How can the steam boiler suck icky stuff from the steam wand when the steam valve is closed?

Although not completely necessary, the vacuum breaker on a PID-controlled steam boiler, like the GS/3's, prevents a false reading on the front-panel steam pressure gauge after initial warmup, and an unexpected pop when you let the air out through the steam wand.

by HB on Sun Aug 14, 2011 8:03 pm

Peppersass wrote:How can the steam boiler suck icky stuff from the steam wand when the steam valve is closed?

Imagine a small cafe with a one-group espresso machine. You know, the kind with a staff that rarely wipes the steam wand. At the end of the day, the wand is caked with a thick, brown-ish layer of hardened milk. As part of their closing procedure, they fill a pitcher with hot water and soak the wand in it; the idea is to soften the milk enough that the holes of the steam tip can be unclogged the next morning. If they neglect to close the steam valve tightly, the valve seals are dry, or they intentionally leave it open to facilitate (?) cleaning of the inner barrel of the steam wand, the result is a slurry of hardened milk and water being pulled into the steam boiler when it depressurizes. Disgusting, eh? :shock:

Of course this would never happen in a respectable cafe, but NSF standards are designed to reduce the risk of contamination, even if it would be better avoided by training the staff proper food handling procedures.


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