How Fluctuating Pressure Occurs
Expansion vessels are an essential component in plumbing systems that use a booster pump or circulator. They serve a vital role - to allow for thermal expansion and contraction of water in the system without causing dramatic fluctuations in water pressure.
When water is heated, it expands in volume. In a closed plumbing system like those found in most homes and buildings, this expansion will cause the system pressure to rapidly rise if there is no room for the expanded water volume. Expansion vessels give the expanding water a place to go, preventing pressure spikes that could damage pipes, valves, and system components.
Expansion vessels are pre-pressurized tanks, usually cylindrical in shape, that contain a bladder or diaphragm inside. As the system water expands when heated, it flows into the pre-pressurized vessel and compresses the air bladder. This allows the pressure to remain stable in the rest of the system. The compressed air acts like a spring, pushing back against the expanded water. When the system water cools and contracts, the pressure in the vessel pushes the water back out into the system again. This process keeps the overall system pressure in a safe range during normal operating fluctuations.
By regulating these pressure changes, expansion vessels play a critical role in maintaining the health and safety of plumbing systems. Understanding how they work is key for both installing them properly and diagnosing issues that may occur.
How Fluctuating Pressure Occurs
Fluctuating pressure in closed water heating systems is caused by thermal expansion and contraction of the water as it heats up and cools down. When water is heated, it expands in volume. In a closed system, like a residential water heater or boiler, the water has nowhere to expand as the volume increases. This results in a rapid rise in pressure within the system. Conversely, when the water cools back down, such as overnight when the water heater or boiler turns off, the water contracts in volume. The pressure drops dramatically in the system. This heating and cooling cycle happens every day in water heating systems, causing the pressure to fluctuate up and down.
The amount of expansion and pressure fluctuation depends on the volume of water in the system and the temperature change. In large commercial systems, the pressure changes can be extreme, going from 0 psi to over 200 psi. In residential systems, pressure may fluctuate 30 psi or more during a daily cycle. These pressure spikes put stress on plumbing components and can lead to leaks, floods or even explosive failures over time. Proper pressure management is crucial to avoid this damage.
Managing Fluctuating Pressure
Fluctuating pressure in closed heating and plumbing systems can cause major issues if not properly managed. As the temperature of the water in the system changes, the water expands and takes up more volume. This thermal expansion causes pressure spikes that stress pipes, joints, valves and seals. Over time, fluctuating pressure accelerates wear and tear, and can lead to leaks, cracks and even catastrophic component failures.
Without a way to absorb the pressure spikes, the added stress damages system components. Expansion tanks and other devices crack or burst from the repeated pressure cycles. Gaskets and seals deteriorate faster, causing leaks. Pipes can pull apart at joints and fittings when pressure rapidly increases. Valves also experience accelerated wear from fluctuating pressure. In extreme cases, violent pressure spikes can rupture pipes and damage property or cause injuries.
Proper pressure management is crucial for the safe operation and longevity of closed loop heating and plumbing systems. Uncontrolled pressure fluctuations put undue stress on system components, shortening their lifespan and increasing the risk of dangerous and costly failures. Devices like expansion vessels help prevent damage by giving the expanding water volume a place to go while maintaining safe operating pressure.
How Expansion Vessels Work
Expansion vessels are installed in closed water heating and cooling systems to manage fluctuating pressure caused by thermal expansion. They consist of a metal tank with an internal flexible rubber diaphragm that divides the tank into two sections - one contains air and the other connects to the system water.When the system is cold, the rubber diaphragm is expanded toward the air side of the tank. As the water in the system heats up and expands in volume, it pushes water into the expansion tank. This compresses the air pocket, allowing the expanding water to take up space inside the tank. The compressed air acts like a spring, pushing back against the water to maintain pressure in the system. This smooths out the pressure fluctuations and prevents spikes that could damage pipes, valves and other components.
The main components and operation are:
- Metal tank - provides a contained space for expanding system water. Typically made of steel.
- Rubber diaphragm or bladder - flexes to separate air and water. Allows air compression.
- Air charge valve - used to charge the air side to proper pre-charge pressure before installation.
- System connection point - inlet/outlet for water to flow in from heating system as it expands.
As the system heats and expands, water flows into the tank, compressing the air pocket. When the system cools and contracts, the compressed air pushes water back out into the system, maintaining pressure. This charges and discharges process counters the pressure fluctuations.
Diaphragm Design
The diaphragm is a flexible membrane inside the expansion vessel that separates the air cushion from the system water. Its flexibility allows the diaphragm to move in response to fluctuating system pressure. Most diaphragms are made of butyl rubber, which provides good flexibility and durability. Some key factors in diaphragm design include:
- Diaphragm thickness - A thicker diaphragm is less flexible but more durable. Typical thicknesses range from 0.5 mm up to 1.5 mm.
- Reinforcing layers - Multiple layers of nylon or polyester fabric help reinforce the rubber. More layers increase durability but reduce flexibility.
- Shape - Conical or dished shapes allow more diaphragm movement and flexibility compared to flat diaphragms.
- Size - The diaphragm needs to be large enough to handle the maximum volume changes in the system. Oversizing allows more flexibility.
The flexibility of the diaphragm directly impacts the expansion vessel's ability to absorb pressure spikes. A diaphragm that is too stiff will not expand enough, leading to excessive pressure build up. Finding the right balance of flexibility and durability is key to proper diaphragm design. Proper installation and maintenance is also critical to prevent diaphragm fatigue over time.
Pre-Charge Pressure
The pre-charge pressure is the amount of air pressure contained in the expansion vessel's diaphragm at the time of installation. This is set by the manufacturer and should be indicated on the vessel's label.
Proper pre-charge pressure is critical for allowing the expansion vessel to absorb fluctuations in system pressure. The optimal pre-charge pressure depends on the installation, but is typically around 80% of the minimum working pressure of the system. This allows the diaphragm to counteract rises in pressure above the system's static pressure.
If the pre-charge is set too low, the diaphragm will fully compress before reaching the system's maximum pressure. This provides no buffer for additional pressure spikes. Conversely, if the pre-charge is set too high, the diaphragm will not fully compress when pressure rises. This reduces its overall capacity to absorb pressure fluctuations.
When installing the expansion vessel, use a pressure gauge to check its pre-charge pressure. Compare this to normal system pressure to ensure optimal pre-charge pressure between 60-80% of static pressure. Adjustments can be made by adding or releasing air with a bicycle pump. Proper pre-charging is essential for the expansion vessel to operate as designed.
Installing Expansion Vessels
Proper installation of an expansion vessel is critical for it to operate effectively. The key considerations are sizing, placement, and piping configuration.Sizing
The expansion vessel must be sized appropriately for the volume of water in the system. Generally, the size is calculated based on the total volume of water, the maximum operating temperature, and the pre-charge pressure. The manufacturer will provide sizing guidelines specific to their vessels. Undersizing the expansion vessel can lead to excessive pressure, while oversizing wastes money and space.
Placement
The expansion vessel should be installed on the suction side of the circulator pump. This placement protects the vessel from high pressures, ensures proper filling, and allows for thermal expansion of the entire system volume. The vessel should be placed close to the boiler on a horizontal plane. Long pipes between the vessel and boiler can cause water hammer and other issues.
Piping
The expansion vessel must be installed with proper connections to the system piping. Standard practice is to install an automatic air vent on top of the vessel, and a drain valve at the bottom. The piping to the vessel should minimize restrictions and friction losses. All piping must follow the manufacturer's instructions. Incorrect piping can reduce effectiveness and lead to safety hazards.Proper installation combined with regular maintenance ensures the expansion vessel works effectively to allow for pressure fluctuations and protect the system. Following the manufacturer guidelines for sizing, placement, and piping is critical.
Maintenance
To keep expansion vessels working properly, periodic maintenance is required. The maintenance tasks for expansion vessels include:
- Checking the pre-charge pressure. Over time, the pre-charge pressure in the vessel can decrease. Using a pressure gauge, check the pre-charge pressure and restore it to the proper level if needed. Refer to the vessel's documentation for the specified pre-charge pressure.
- Inspecting the diaphragm or bladder. Depending on the vessel design, inspect the rubber diaphragm or bladder for signs of deterioration or cracking. A damaged diaphragm will prevent the vessel from functioning properly. Replace the diaphragm if worn.
- Flushing the vessel. Sediment and debris can accumulate inside the vessel over time. Periodically drain and flush the vessel to keep it clean.
- Testing safety valves. If the expansion vessel has a safety valve, test it periodically to ensure it is operating properly at the indicated pressure setting. A faulty safety valve could lead to dangerous over-pressurization.
- Checking connections. Inspect the piping connections to the vessel to ensure they are tight and not leaking. Loose connections can allow air to escape from the system.
- Reviewing documentation. Consult any manufacturer documentation to see if there are additional maintenance recommendations for the specific model of expansion vessel in use.
Following the manufacturer's maintenance guidelines for each vessel is highly recommended to maximize service life. Well-maintained expansion vessels help provide years of reliable operation.
Signs of Failure
An expansion vessel that is failing or has failed will exhibit certain symptoms that indicate it needs maintenance or replacement. Here are some key signs to watch out for:
- Frequent rise and fall of system pressure - If the pressure is fluctuating a lot more than normal, the expansion vessel may not be handling the changes in pressure properly. This could point to a loss of pre-charge pressure or a diaphragm leak.
- Water logging from internal leaks - If the internal diaphragm has been compromised, water can leak into the pre-charge chamber. This reduces the amount of available expansion capacity. Signs of this include water dripping or spraying from the air valve on the expansion vessel.
- External corrosion - Corrosion on the outside of the vessel may indicate internal corrosion as well. Expanding corrosion can lead to leaks or sudden ruptures.
- Refilling system pressure frequently - If you find yourself needing to top up the system pressure more often, the expansion vessel may not be maintaining pressure properly between fills.
- Noisy plumbing - Knocking or hammering noises from pipes when faucets are opened or appliances turn on/off points to extreme pressure fluctuations that the expansion vessel cannot handle smoothly.
- Higher than normal water bill - A rise in water usage with no other explanation could mean small leaks are occurring due to pressure fluctuations exceeding the vessel's capacity.
Regularly monitoring your expansion vessel for these signs of failure can help prevent major issues and water damage. Call a plumbing professional as soon as possible if you suspect the expansion vessel needs replacement.
Alternatives to Expansion Vessels
While expansion vessels are the most common way to manage fluctuating pressure in closed-loop hydronic heating systems, there are some alternatives:
- Air scoop - An air scoop is installed on the supply side of the loop to remove air bubbles from the water. This allows air to escape the system rather than compress in the piping and components. The downside is that air scoops don't accommodate water expansion.
- Pressure relief valve - A pressure relief valve opens to discharge water when pressure exceeds the set point. This protects the system from excessive pressure but wastes water and energy.
- Pressure reducing valve - A pressure reducing valve maintains a set system pressure by allowing water to flow to the return line. But this also wastes energy by circulating excess water.
- Fill valve - An automatic fill valve can add water to the system as needed. However, adding fresh water introduces oxygen which can lead to corrosion.
- Design for expansion - Systems can be designed with enough space capacity in piping and components to accommodate expansion. But this isn't always practical and increases the system cost.
While other methods have some benefits, expansion vessels remain the most widely used solution. When properly sized and installed, they provide a simple and reliable way to absorb pressure fluctuations. Expansion vessels don't waste water or energy, require minimal maintenance, and help prevent system damage.