The De Nora blog: Water Made Easy

How to Prolong the Life of Your Gas Chlorinator

Jul 23, 2020 8:38:00 AM / by Stanley Shima

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In today’s credit-crunched, bottom-line-oriented economic environment, utilities in the water and wastewater sectors are scrapping plans to replace old equipment with new technologies and are looking for ways to improve existing equipment performance with system upgrades. 

For example, many utilities using chlorine gas disinfection systems are improving their existing equipment rather than replacing these systems. Gas chlorination, an effective treatment method for 100 years, is reliable and very economically priced to meet varying application needs.  

To maximize the utilization of this proven, cost-effective technology, a thorough understanding of gas chlorinator operation is essential. Additionally,once-per-year preventive maintenance utilizing factory original spare parts for chlorinators and other gas feed equipment will promote trouble-free operation.

The following is a review of gas chlorinator operations and valuable trouble-shooting tips to help prolong the life of your systems, reducing downtime and costs.

Direct Cylinder Mounting

Chlorinators meter and deliver a set feed rate of chlorine gas from a chlorine cylinder (Figure 1). A vacuum-operated gas chlorinator reduces the gas pressure from a chlorine cylinder to a vacuum and transports the chlorine gas to the point of application.

The easiest and safest way to make the connection from a gas cylinder to the chlorinator is by direct cylinder mounting. In direct mounting, the chlorinator inlet valve is attached directly to the cylinder valve by a positive metallic yoke connection that is sealed by a single gasket. This gasket should be changed every time the chlorinator is disconnected.

After the chlorinator is placed on the cylinder valve, the gasket connection must be secured. One common error is to over-tighten the gasket connection, which can damage the yoke of the chlorinator, causing a leak. It is necessary to tighten the connection only one half to three quarters of a turn after contact is made with the gasket. After the chlorinator is secured on the cylinder valve, the cylinder valve should be opened only one quarter of a turn. The wrench should be left on the valve, so that it can be quickly closed if needed.

Check for leaks using 26 Deg. Baume’ ammonia vapor, applied under the gasket seal. Chlorine gas in the presence of ammonia vapor produces a white smoke, visually identifying a chlorine leak. If a leak exists, check that the packing on the chlorine cylinder valve is tight. Then, check the gasket.

prolong-gas-chlorinator-fig-1

Interconnection Manifold

Direct mounting of a gas cylinder is not always possible. For example, if several cylinders are used to provide chlorine, an interconnection manifold is necessary.  The following items are needed to make an interconnection manifold:

  • the chlorine gas manifold - made of seamless steel pipe with a chlorine valve for chlorinator mounting;
  • a flexible connector or sometimes referred to as a whip (from the rigid pipe manifold to the chlorine source) - made of zinc-plated copper. This provides a flexible line between the manifold and the chlorine cylinder; and
  • an isolation valve - made of silica-aluminum-bronze and located between the flexible connector and the cylinder valve.

If the gas is maintained in a dry condition from the gas cylinder to the chlorinator, very little, if any, corrosion will result. However, when a chlorine pressure connection is broken, chlorine is exposed to atmospheric moisture. This allows some moisture to combine with the chlorine gas, and if moist air collects in the manifold, acids will form. This corrosion may cause the manifold to fail in time.

The isolation valve is designed to isolate the manifold from atmospheric moisture. Before the chlorine pressure connection is broken, the isolation valve is closed, and the chlorine gas cylinder valve is closed, allowing only minimal amounts of moisture to enter the system. When a new container is connected, and the connection is replaced, the isolation valve and the chlorine gas cylinder valve can be reopened. The use of an isolation valve can easily double the life of a chlorine gas manifold.

The Ejector: Heart of the Chlorinator

The ejector represents the heart of the chlorinator system. It contains a venturi nozzle, and when water pressure is supplied to the nozzle, the differential pressure across the nozzle creates a vacuum via Bernoulli’s Principle.

 It is most important to realize that chlorine is being drawn from the chlorine cylinder to the water via the vacuum created in the ejector. If the ejector is not functioning, the chlorinator will not operate, and gas will not flow. The vast majority of problems that occur in chlorination equipment can be traced to the ejector.

Measuring Water Pressure

A pressure gauge located just before the water inlet of the ejector gives a measurement of the water pressure that is available to the ejector. Measuring water pressure downstream of the ejector is more difficult because the chlorine solution is corrosive.

Downstream pressure is that which exists at a given installation as back pressure, a combination of the static pressure against which the chlorine solution must be injected plus the pressure due to friction losses in the solution line.

Both of these sources of back pressure must be considered because insufficient sizing of the chlorine solution line can amount to a considerable increase in back pressure, which may reduce the differential pressure across the ejector to such a point that the chlorinator will not operate.

Check-Valve Assembly/Check-Valve Failure

Assuming the pressure differential is proper, and the water flow through the ejector is adequate, the check-valve assembly — which is usually part of the ejector — is an area of concern. The check-valve assembly prevents water from entering the vacuum regulator portion of the chlorinator.

 If chlorine solution is injected in the presence of any positive pressure when the water flow through the ejector is stopped, a vacuum will no longer exist in the ejector. A positive pressure, equal to the static head into which the chlorine solution is injected, will exist at the check-valve assembly. If there is no check-valve assembly, the water will be forced into the vacuum regulator portion of the chlorinator. 

Failure of the check valve is usually indicated by some quantity of water in the chlorine gas metering tube. The check-valve may fail for several reasons, the most common of which is an accumulation of deposits on the seat of the check-valve. This means that even though the check-valve closes, it is blocked and not completely sealed, allowing water to pass through to the vacuum regulator.

The second most common failure, depending on how frequently the check-valve is cycled, occurs when the check-valve seat material becomes distorted. This is most common on systems, such as a deep well, where the pressure against the ejector is high, and the check-valve cycles are frequent.

Most chlorinator manufacturers offer several check-valve arrangements, depending on the pressure that the ejector is subjected to when the chlorinator is shut off. Consult the chlorinator manufacturer to determine if the proper check-valve is installed.

Removing Water From the Chlorinator

If water pressure builds in the vacuum regulator, the diaphragm assembly will move in the opposite direction of the chlorine inlet valve, and water will be allowed to vent to the atmosphere in the same way that excess gas pressure builds in the chlorinator. Depending on the capacity of the chlorinator, it may be a simple operation to remove the water. If only a small amount of water enters the chlorinator and the check-valve problem is remedied, the chlorinator can be restarted, and the water will be drawn out with the chlorine gas.

It may be difficult to remove the water from low capacity units (less than 10 PPD/4.5 kg/h) because the quantity of gas drawn through the chlorinator is very small. To safely remove the water from small capacity chlorinators, remove the vacuum regulator from the chlorine cylinder source and allow the chlorinator to pull in air rather than chlorine. This will allow the chlorinator to dry out so it can be reinstalled on the chlorine cylinder source.

Where the metering tube is small, the liquid can cause the metering ball to stick. When this happens, it may be necessary to disassemble the chlorine flowmeter and manually dry it before the chlorinator is returned to operation.

When the check-valve fails, it sometimes becomes so distorted that it blocks the vacuum action, and it appears as though no vacuum is present. If the ejector is operating correctly, suction exists at the vacuum port, and gas will flow.

The Chlorinator’s Vacuum Integrity

The other operational problems associated with chlorinators are related to vacuum integrity. The chlorinator must be vacuum-tight from the ejector to the vacuum regulator inlet valve for a flow of gas to exist. Three significant situations can occur if there is a break in the vacuum.

1. No indication appears on the flowmeter, even though vacuum is being created. This indicates a vacuum leak at the top of the meter tube or somewhere downstream toward the ejector.

If the leak occurs above the float, this indicates the O-rings on the rate valve stem are not sealing. If the gasket on the top of the meter tube is not sealing, it has probably been caused by someone grasping the vacuum regulator by the meter tube when changing cylinders, thus dislodging it from its sealed position. When confronted with this situation, check all vacuum tubing and connections, the rate valve O-rings, and the gasket on top of the flowmeter for vacuum leaks.

O-rings are used in most chlorinators to provide a seal between two parts. In the case of the rate valve, the O-rings slide as the rate valve is moved. Although chlorinators are constructed of materials designed to withstand the effects of chlorine gas, even the most substantial O-rings eventually become brittle. They can remove themselves in the constant adjustment of the gas flow. 

They may disintegrate and be fed out through the vacuum tubing. When this happens, there will be a loss of vacuum through the rate valve. (Most chlorinator manufacturers provide spare parts kits, including O-rings, with their chlorinators).

2. A vacuum leak exists somewhere below the float in the metering tube. This can be caused by a leak at the bottom metering tube gasket. In this case, the flowmeter will indicate a flow, but the flow will be air, not chlorine gas. If any connection below the bottom metering tube gasket leaks, air will register as gas flow in the flowmeter.

3. The ejector is located at the chlorine application point. In the past, it was common practice to mount the ejector and vacuum regulator inside a cabinet. But for safety and economy, the ejector should be located as close as possible to where the chlorine is to be applied. This eliminates pumping the chlorine solution to the point of application, which may be several hundred feet. By placing the ejector at the point of application, a break in the vacuum tubing will cause the chlorinator to shut off immediately, stopping gas flow or damage from chlorine solution feed. 

With a cabinet-mounted ejector, a broken solution line to the application point could allow corrosive chlorine solution to contaminate the area.

There are other advantages to placing the ejector as close as possible to the point of application. For example, this arrangement eliminates the backpressure resulting from friction losses in very long solution lines. When the vacuum regulator portion of the chlorinator is mounted directly at the chlorine source, gas pressure lines and the accompanying problems are eliminated. The gas is conducted under vacuum to the point of application, minimizing the hazards of pressurized chlorine gas and solution piping.

However, the most practical configurations of equipment are not always possible. If solution lines are unavoidable, they should be routed to an area where a break will not cause damage. Also, valves within a solution line should be of the one-quarter turn, PVC variety, because these will cause the fewest maintenance problems and produce the lowest friction loss.

With current global economic concerns causing many industries to reevaluate plans for capital improvements, proper equipment operation and preventive maintenance can be the key to significant cost savings – and the safe operation of critical water and wastewater systems.

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Tags: water treatment, De Nora, chlorination, gas feed, chlorine gas

Stanley Shima

Written by Stanley Shima

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