| Gas Got You Down? |
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| By Dave Kerr, Senior Technical Adviser, USABlueBook |
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| Disinfection is a crucial process in virtually all water and wastewater treatment facilities. Liquid Sodium Hypochlorite is commonly used because it is a safe and effective means of disinfecting water and wastewater, but its off-gassing can cause pumps to lose their prime if they are not properly vented. |
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| One of the biggest problems operators face is maintaining chemical feed pumps that lose prime or become “air-bound” by the off-gas the sodium hypochlorite continuously releases. If the off-gassing is extensive enough, the gas formed in the liquid can make its way into the suction tubing and eventually move into the pump head. |
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| When a diaphragm-type chemical feed pump becomes filled with gas, the displacement of the pump head merely compresses and expands the gas, and the pump loses its prime. This condition usually requires manual intervention, and you have to manually re-prime the pump as soon as possible to maintain proper function. |
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| Luckily, there are several ways to minimize or eliminate off-gassing problems: |
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| Proper System Design |
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| Proper system design will help reduce off-gassing problems. You need to minimize suction lift conditions—most manufacturers recommend a 5 to 6 feet maximum for suction lift applications. |
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| Using a flooded-suction setup (placing the pump below the liquid level of the storage tank or day tank) ensures a flooded suction at all times from the tank, and will help alleviate gassing problems from the upstream supply of the pump. Try to keep the suction line length as short as possible. |
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| Degassing Valves |
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| You can also set up the pump head with a gas relief system to prevent it from losing its prime. The older and more common method is using a degassing (bleed) valve that continuously returns a small liquid stream back to the storage tank. |
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| Degassing valves prevent off-gas from forming in the pump head. Off-gas is released and routed into the bleed line along with a small amount of liquid back to the storage tank. |
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| A drawback to using a degassing valve is that the valve will not bleed off gas if the pump is not running. Therefore, the head can gas up and lose prime while the pump is off, and the return line can easily become clogged from the hypochlorite crystallizing in the line. In this case, the system fails to release gas as intended. |
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| Degassing Heads |
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| A more recent innovation for preventing the loss of prime is the use of a degassing head. A degassing head is designed to continuously release off-gas to the discharge piping, whether the pump is running or not. |
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| The degassing head uses a gas port on the pump head, which lets the pump be totally self-priming and eliminates the need for a degassing valve. A degassing head is great for applications where the pump is run intermittently with long periods of time in between cycles. |
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| Peristaltic Pumps |
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| Using peristaltic pumps eliminates the potential for loss of prime because the fluid you are pumping never actually touches the pump, but rather is “squeezed” through a flexible tube. |
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| This squeezing action of the flexible tubing generates a powerful vacuum that moves both gases and liquids simultaneously, without the pump losing its prime. The pumps are completely self-priming up to 25 feet, can operate dry and will not clog from dirt and minor debris. |
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Tech Talk
