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Maritime Forum

TOPIC: Boiler Feed Water

Boiler Feed Water 28 Mar 2017 04:43 #4766

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(a) Sketch a boiler feed water line diagram with all components showing.

(b) What is the significance of PH value in boiler feed water.

(c) What is electro chemical corrosion.
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Boiler Feed Water 06 Apr 2017 07:23 #4774

(a)


(b) When we measure pH, we’re measuring the power of Hydrogen. The number refers to the relative concentration of hydrogen (acid) and hydroxide (base) ions in solution.

It comes into play with steam boilers because a pH between 7 and 9 (slightly alkaline) is just right.

If the pH is lower than that, the water will start eating the pipes. If it’s higher than that, the water won’t corrode metal, but it may begin to foam, and that’s not good.

Say, you’re on a job and you have a steam boiler that’s taking on a lot of feed water because the system has leaks.

Here’s the real problem for that: Fresh feed water brings with it carbonates and bicarbonates, which are both natural and normal. The challenge starts when the water boils because carbonates and bicarbonates break down and leave the boiler as carbon dioxide. This gas flows through the system with the steam, and if you’re not venting the system well, the condensate will absorb the carbon dioxide as the steam gives up its latent heat to the pipes and the radiators. We call that Henry’s Law.

The system winds up with carbonic acid in the returns and that’s bad news because carbonic acid removes the thin film of rust that naturally forms on the insides of steel pipes. That mild rusting is a good thing to have because it helps protect the underlying metal from further corrosion. By stripping away the surface rust, the acid makes more fresh metal available for munching. Before long, you have leaks.

This is why it’s good to insulate the return lines in any steam system. The cooler the water gets, the more it will absorb gases such as carbon dioxide (Henry’s Law). Keep the condensate as hot as possible; it will be less acidic and the return lines will last longer. The boiler system will also use less fuel.

And even if you fix the leaks in the system, you still have to keep an eye on the pH of the water because many people add chemicals to their boilers to avoid corrosion. These chemicals shove the pH toward the alkaline side of the scale. When the pH reaches 10, corrosion becomes impossible. That’s what makes chemical drain cleaners safe for pipes. That nasty stuff has a very high pH (think lye).

The trouble really starts when someone gets overenthusiastic with the chemicals and the pH reaches 11. At that point, the water will foam, and that leads to wet steam, which wastes fuel because it robs the steam of its latent heat. The steam boiler has to run his burner longer to heat the building, and with lousy results.
Use litmus paper to check the pH whenever you blow down a low-water cutoff. It’s a simple test and easy to do, and by getting the water’s pH just right, you can solve a lot of problems that are costing money.

And please keep in mind that all steam systems are open to the atmosphere (through the Hotwell), so there’s always going to be some evaporation and a need for feed water. But steam boilers that do only space heating reuse nearly all of the water because the condensate returns to the boiler from the piping and radiators. You shouldn’t have to add too much water to these systems unless they’re leaking.

Feed water is cold and contains lots of oxygen. People who maintain boilers deal with that oxygen by passing the feed water through a deaerator, but you’ll rarely see a deaerator on a space-heating boiler.

Henry’s Law tells us that gases dissolve in liquids in direct proportion to pressure and temperature. So the colder the water, the more oxygen it will contain. That oxygen comes out of solution as the water boils, and it can eat holes in the boiler, right at the boiler’s waterline.

Feed water also contains suspended solids. The more feed water you allow in, the more solids you’re going to get. The solids collect on the surface of the water as it boils. They surround the steam bubbles as they form, making them tougher. Tough bubbles resist breaking, and that also leads to foaming and wet steam. The finer the suspended particles are, the more they will collect in the bubbles, and the worse the foaming will be. You probably can’t see these solids but they’re there, and this is why you must keep the feed water to a minimum. Fix those leaks.

(c)
Electrochemical corrosion is practically the simplest and the most common corrosion process that is observed. Generally seen in metals.

You can define it as a process of individual metal forming its salt byproducts.

For this few things are needed. An agent (which is most commonly water) and a depolariser. This dipolariser can be anything from oxigens to a free anion available in acids to a another positive ( in comparison ) metal. Nitrogen is almost and inert gas.

The basic equation is simple.

M (metal) - M+ + e.

This free electron is released from the outermost atom and goes to join the electron deficient depolariser. Forming salts/ oxides etc.
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