How Vacuum Can Form in a Steam System - Heating Help

I heard from an old friend the other day. This guy works in Brooklyn, NY, and usually on some very interesting stuff. He's not one to say fuggedaboutit when there's a problem on a job. Here's what he wrote.

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"Dan, I just finished installing a new Weil-McLain LGB-4 gas-fired boiler. This boiler is providing steam for a steam kettle in a commercial kitchen in a school. We installed main vents, a bucket trap for the main return line, and a Float & Thermostatic trap for the kettle. We also installed a condensate-receiving tank since we don't have enough height for the condensate to return to the boiler by gravity alone. We have a solenoid valve on the tank for feed water, as well as a tank overflow line to a drain, and a vent line so that the air can leave the kettle and vent from the tank.

"Today was the first time that we fired the boiler, and it went just as we had planned. After we cycled the boiler a few times, though, we shut down the system and watched as something very interesting happened. After the boiler started to cool down, water started to enter into the boiler, even though the feeder on the boiler was satisfied and the water was at the proper level. It just kept rising. And by the way, we have a spring-loaded check valve between the condensate pump and the boiler. I am sending you some pictures of the job."

I was in my car in the parking lot of an open-air shopping mall when I got his . It was pouring rain and I was waiting for The Lovely Marianne to finish shopping, which is sort of like doing time in a Super Max penitentiary.

My buddy had sent a lot of photos attached to his , and my was taking its sweet time downloading them because of me being in the penitentiary and all, so I just called him. Sometimes it's quicker to be old-fashioned, and besides, I didn't need the photos to know what was wrong with this one.

He told me how he and his helper watched the water fly up the boiler's gauge glass. "It was like it was being sucked right in," he said.

"It was," I said.

"What do you mean?"

"The boiler was sucking it in," I said. "Well, actually, the atmosphere was pushing it in, but you get the picture. There was a vacuum inside the boiler. It happens a lot with this sort of installation."

We talked about it and I got him thinking like steam, which always makes this stuff easier to understand. You just have to pretend you're the steam and then think about what you would do inside that system.

For instance, you begin as water in the boiler. The fire comes on and you get hot. When the flame adds enough sensible and latent heat to you, you decide it's time to change state from a liquid to a gas, and at that point, you go off like microwave popcorn. In fact, you go off even better than popcorn. You expand 1,700 times, shoving all the air that's around you toward the steam kettle. The air passes through the kettle, enters the kettle's steam trap, sails right through that trap and leaves the system through the vent on the condensate receiver.

As steam, you're now in the kettle. You can't get out of the kettle because of that steam trap (steam traps trap steam). So you give up your latent heat to the soup in the big kettle, which causes it to boil, and then you start shrinking back into liquid water. And as you do this, it would be wonderful if the air could work its way back into the kettle to fill the empty space you're leaving behind as you condense, but that steam trap is still closed. So what gets left behind in the kettle, and in the boiler (all the way back to the boiler waterline) is a vacuum.

With me so far? Good. Now think of the waterline in the boiler. It has a pressure above it that's lower than the pressure of the atmosphere. The water in the condensate receiver, which vents to the atmosphere, has atmospheric pressure sitting on top of it. In Brooklyn, this happens to be 14.7 pounds per square inch.

Okay, we have two containers of water (the boiler and the condensate receiver), and they're connected with a pipe. One container is under a pressure that's higher than the pressure that sits atop the other container. Separating these two containers of water is that pipe, and in that pipe there's a spring-loaded check valve. This is facing in the direction of the container that contains the low pressure.

Have you ever put air in your car's tires? High pressure goes to low pressure, right? You bet it does. Always. So the water inside the condensate receiver is going to flow into the boiler and flood it. From the outside, it looks like something is wrong with the automatic water feeder, but that device is innocent, so don't curse it and don't replace it; it's not at fault.

"You need to add a vacuum breaker to the boiler," I told my friend. "Put it anywhere above the boiler waterline and you'll be fine."

"Simple as that?" he said.

"Yep, and then you can fuggedaboutit," I said.

Simple, right? Here's the other place where this can happen, and this is a bit more complicated.

Nowadays, when you replace a steam boiler that serves a big building you'll often find a vacuum pump on the return piping. Engineers specified those pumps because those pumps increased the differential pressure between the steam-supply pipes and the condensate-return pipes. That allowed the engineer to downsize every pipe, valve, and fitting in the building, which cut costs on the installation, but which also means that once it's a vacuum system, it's always a vacuum system.

When you change the boiler, you're probably going to be installing a replacement that doesn't contain nearly as much water as the old boiler held. That's just the nature of modern steam boilers. The new boiler may need a boiler-feed pump to hold the water that used to be in the old boiler, but isn't in the new one. The challenge is that the system still needs the vacuum pump because of the size of the pipes throughout the building.

So here's what you'll do. You'll install the new boiler-feed pump between the new boiler and the old vacuum pump. The vacuum pump will start with the burner and pull the air from the system. It will suck the air from the return lines, the radiators, the supply mains, and all the way back to the boiler's water line. The steam will follow, and when the condensate returns to the vacuum pump, the vacuum pump will discharge it into the new boiler-feed pump.

And that's the problem. You see the new boiler-feed pump has a vent that's open to the atmosphere. It's not part of the vacuum loop that makes up the rest of the system (boiler, to supply mains, to radiators, to return mains). As the vacuum pump sucks on the system, it's also sucking on the air from the surface of the boiler water. At that point, the atmospheric pressure inside the vented boiler-feed pump's receiver will shove the water that's in the receiver into the boiler and flood it.

You can't use a vacuum breaker this time because you have a vacuum maker on the return. Having a vacuum maker and a vacuum breaker in the same system is like having a humidifier and a dehumidifier in the same room. They'll just go to war with each other to see who's the toughest.

Here's how to solve this problem. Instead of using a spring-loaded check valve between the boiler-feed pump and the boiler, use a motorized valve. Control the motorized valve with a pump controller on the boiler. When the boiler needs water, it will open the valve, which, in turn, will start the boiler-feed pump though its end switch. The boiler will get the water it needs, and once the motorized valve closes, the atmosphere won't be able to shove any unwanted water into the boiler.

And then you, too, can fuggedaboutit.

One last thing. On every steam system that has a vacuum pump, you'll also see a small equalizing line that runs from the pump's receiver to the boiler. This line will dip down at some point to form a U-tube that will always have water in it. In the low point of that U-tube there will be a check valve that points toward the boiler.

This line is there to equalize the natural vacuum that can form inside the supply mains and radiators during the fall and the spring when the pipes are cold on start-up. That naturally induced vacuum, caused by that 1,700:1 ratio of contraction when steam condenses, can cause a greater vacuum in the supply lines than the vacuum pump is making in the return lines, and that will keep the condensate from leaving the radiators. Equalizers equalizer.

None of this is that complicated. Just think like steam.

Industrial boilers are essential in many industries, providing the necessary heat and steam for production process. This guide aims to help you choose the right industrial boiler by addressing key considerations and common questions.

1. Analyzing Business Needs: Understanding Steam and Hot Water Demand

Before selecting an industrial boiler, it is essential to understand the business's actual needs, including the supply quantity and parameters of steam and hot water.

• Assessing Total Load Requirement: Understand the steam and hot water needs for production processes or heating. For example, if a factory requires 10 tons of steam daily, the total load range should be set at 10 tons/day.

• Identifying Peak Demand:Determine the duration and frequency of peak demand periods. For instance, peak demand may occur from 8 to 10 a.m., during which steam consumption might reach 15 tons/day.

• Ensuring Economic Efficiency: The boiler should operate at or above 75% of its rated capacity to maintain efficient and cost-effective performance.

• Planning for Redundancy: Consider the need for backup capacity to cover boiler outages. It is generally recommended to have a backup boiler to ensure uninterrupted production or heating.

2. Determining Steam and Hot Water Parameters

After understanding the demand for steam and hot water, specific parameters need to be determined to select the appropriate boiler.

• Saturated Steam Parameters: Industries such as textile dyeing, food processing, rubber manufacturing, and petrochemicals need to determine the saturated steam parameters according to production process requirements to maintain consistent temperatures. For example, the textile dyeing industry requires constant high-temperature steam for dyeing and setting, necessitating a boiler that can stably provide saturated steam at specific pressures and temperatures.

• Pressure and Temperature: Generally, the boiler pressure should be higher than the required saturated steam pressure to compensate for pressure losses in pipelines or networks. For instance, if production requires a steam pressure of 1.5MPa but there might be a pressure drop of 0.2MPa during transmission, the boiler should provide at least 1.8MPa. Adding a 25-30% pressure margin, the boiler should be designed to provide approximately 2.25MPa to ensure stable and safe system operation.

3. Steam Boilers vs. Hot Water Boilers: Which is Right for Your Needs?

Hot water heating is more energy-efficient and comfortable than steam, but many factories still need steam for heating in production processes. Choose based on the following scenarios:

• If you need a large amount of steam but a small amount of hot water, use a steam boiler with a heat exchanger to produce hot water.

• If you need a large amount of hot water and a small amount of high-pressure steam, use a hot water boiler and install a small steam boiler for steam needs.

• If both steam and hot water demands are small, choose a boiler that can produce both.

• If both steam and hot water demands are large, consider the entire system to choose suitable equipment.

4. Classification and Application of Industrial Boilers

Industrial boilers are classified based on structure, pressure, water circulation type, fuel, and capacity.

By Boiler Type:

Global coal consumption reached an all-time high in and the world is heading towards a new record in . However, due to the complex characteristics of coal, specific designs are required for certain types. Common types of industrial boilers include:

1. Chain Grate Boiler: This boiler is suitable for burning various types of coal, particularly higher quality coal. With the support of the furnace arch, it can handle coal with a higher calorific value. However, chain grates have high metal consumption, so lower calorific value coal requires a larger grate area, increasing metal consumption.

• Suitable Fuel: Medium to high-quality coal (calorific value above kcal/kg).

2. Moving Grate Boiler: This boiler stirs coal during the combustion process, making it suitable for various coal types. Due to its inferior cooling compared to the chain grate, it is ideal for burning coal with high ash content, as the ash can protect the grate bars from overheating and burning out.

• Suitable Fuel: Coal with high ash content (calorific value between and kcal/kg).

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3. Pneumatic Throwing Grate Boiler: This boiler has wide fuel adaptability but is unsuitable for burning anthracite. Anthracite requires a long rear arch, while the pneumatic throwing grate needs an open furnace and secondary air. This boiler features semi-suspended combustion and sensitive load adjustment but produces flue gas with high dust levels. Measures such as controlling coal particle size and optimizing secondary air use are necessary to reduce dust content.

• Suitable Fuel: Various types of coal, but not suitable for anthracite.

4. Circulating Sand Bed Boiler: This boiler is suitable for burning various coal types with a calorific value above kcal/kg, whether high or low quality. It has low metal consumption, is cost-effective, and offers significant development potential, particularly in environmental protection and burning low-quality coal.

• Suitable Fuel: Any coal with a calorific value above kcal/kg.

5. Fine Coal Ppwder Boiler: This boiler is suitable for large-capacity industrial applications due to its complex auxiliary equipment. However, it is not economically efficient for smaller boilers. In regions such as central and southern areas, pulverized coal boilers and dual-purpose boilers with bagasse are commonly used for capacities above 10 tons/hour. Using pulverized coal boilers for small industrial applications is not recommended.

• Suitable Fuel: Not suitable for small boilers with a capacity below 20 tons/hour.

By Boiler Design:

• Fire Tube Boilers: Suitable for small-capacity boilers (up to 4 tons/hour). Simple structure, suitable for small applications.

• Water Tube Boilers: Suitable for large-capacity and high-pressure applications (above 4 tons/hour). High safety, improved combustion efficiency.

By Outlet Pressure:

• Atmospheric boilers, low-pressure boilers (below 2.5MPa), medium-pressure boilers (2.5MPa to 6.0MPa), high-pressure boilers (above 6.0MPa), ultra-high pressure boilers, subcritical boilers, supercritical boilers, ultra-supercritical boilers.

By Water Circulation Type:

• Natural Circulation Boilers: Rely on the density difference between water and steam for circulation. Advantages include simple structure, stable operation, and low maintenance cost. Suitable for medium and low-pressure applications.

• Forced Circulation Boilers: Use mechanical pumps for water circulation, suitable for high-pressure and large-capacity applications. Higher efficiency and flexibility, but more complex and costly to maintain.

By Fuel Type:

• Solid fuel boilers: such as coal-fired boilers.

• Liquid fuel boilers: such as oil-fired boilers.

• Gas fuel boilers: such as gas-fired boilers.

• Waste heat boilers: utilize waste heat from industrial processes.

• Waste material boilers: use industrial waste as fuel.

By Capacity

• Small boilers: less than 20 tons/hour.

• Medium boilers: between 20 and 75 tons/hour.

• Large boilers: more than 75 tons/hour.

Other Factors:

• Size: Different manufacturers may have varying dimensions for the same model, so users need to know the specific size.

• Load Fluctuations: Boilers must handle load variations, ensuring stable and efficient operation under different conditions.

• Environmental Pollution: Choose low-pollution boilers that meet emission standards.

• Automation: Highly mechanized and automated boilers improve efficiency and reduce labor costs and operational risks.

• Water Quality: Boiler performance and lifespan depend on water quality, which must meet standards.

5. Determining Boiler Capacity and Quantity

Choosing the right capacity and number of boilers is crucial:

• Total Load Range: Based on production or heating needs, determine the total load range and peak load. For example, a food processing plant with a daily steam demand of 5 tons but peak demand of 8 tons in the morning and afternoon. Set the total load range at 8 tons/day.

• Equipment Capacity: The boiler's economic load should be above 75% of the rated capacity. For example, a boiler rated at 10 tons should operate above 7.5 tons for efficient operation.

• Backup Capacity: Consider backup capacity for boiler outages. A textile plant with a daily demand of 12 tons can have two 12-ton boilers, with one as a backup.

6. Water Treatment Techniques for Industrial Boilers

Water treatment is critical for efficient boiler operation and longevity. Key considerations include:

Water Quality Requirements and Treatment Methods:

• Water Softening: Remove calcium and magnesium salts to prevent scaling using ion exchange technology.

• Deaeration: Prevent corrosion using thermal, vacuum, or chemical deaeration methods, with thermal deaeration being the most common.

Water Treatment Characteristics:

• In-Boiler Treatment: Add softening agents to the boiler to convert scale-forming substances into sludge, which is then discharged. Advantages: simple and cost-effective, but requires regular cleaning.

• External Treatment: Softens water before it enters the boiler, thoroughly removing salts and impurities. Advantages: more effective, reduces scaling and corrosion, but higher cost and requires professional maintenance.

Chemical Dosing Applicability:

• No water-cooled wall tubes.

• Reliable sludge removal during operation.

• Mud formation from dosing does not affect boiler safety.

• Low steam quality requirements.

7. Maintenance and Safety Measures for Industrial Boilers

Regular maintenance and safety measures are vital for optimal boiler performance and lifespan:

• Regular Maintenance: Includes inspections, cleaning of smoke tubes and water tubes, replacing worn parts, and monitoring control systems.

• Safety Measures: Operators should receive professional training and follow all safety regulations and procedures.

Conclusion

Choosing the right industrial boiler requires thorough research and careful evaluation of factory needs. Low-pressure boilers are suitable for light industry applications such as food and beverage, textiles, chemicals, and pharmaceuticals. In contrast, medium to high-pressure large boilers, with high automation and remote control systems, are better suited for power plants, ships, and heavy industry.

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