The Professor: Advanced Leak Detection

November 1, 2010
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Figure 1. In using a micro-foam solution for leak testing, allow up to 15 minutes reaction time.


My Oct. 4 column discussed leak detection and provided details on basic detection methods. This column will look at more advanced methods.

You may recall that the Oct. 4 column outlined various classes of leaks with standing leaks as the most common. How to deal with such leaks was covered in that column as the first classification of leaks.



PRESSURE-DEPENDENT LEAKS

The second classification was the pressure-dependent leaks which can only be detected as the system pressure increases.

So we will begin with a discussion of how to test for pressure-dependent leaks.

First, you need to pressurize the low side to 150 psig and the high side to 450 psig using dry nitrogen. The equipment rating plate usually states the maximum pressure permissible. Also, always make sure that valving and other components can take these pressures whether they are original equipment or not. If the high side and low side cannot be split by ways of isolation valves, pressurize the entire system to about 350 psig if permissible.

Warning: Never use pure oxygen or air to raise the pressure in a refrigeration system. Pure air contains about 20 percent oxygen. The pure oxygen and/or the oxygen in the air can combine with refrigerant oil and cause an explosive mixture. Even some refrigerants when mixed with air or oxygen can become explosive under pressure. Pure oxygen and the oxygen in the air will oxidize the system’s oil rapidly. In a closed system, pressure from the oxidizing oil can build up rapidly and may generate pressures to a point of exploding.

The second step in testing for pressure-dependant leaks is to always conduct proper bubble testing by thoroughly saturating all surfaces with a microfoam solution. Allow up to 15 minutes reaction time for the microfoam to expand into a visible white “cocoon” structure (Figures 1 and 2). Use an inspection mirror to view any undersides and a light source for dark areas.

Third, starting at the compressor, coat all suspected surfaces. Continue to coat all suction line connections back to the evaporator section.

Fourth, spray coat all fittings starting at the discharge line at the compressor to the condenser coil. Spray coat all soldered condenser coil U-joints.

Fifth, from the condenser, continue to spray coat all liquid line connections including the receiver, valves, seams, pressure taps, and any mounting hardware. Continue the liquid line search back to the evaporator section.

Sixth, any control line taps to the sealed system must be spray coated the entire length of their run all the way back to the bellow device.

Seventh, expose the evaporator section and coat all connections, valves, and U-joints.

Notice that the first sequence of searching started with the compressor and suction line due to their large surface areas.

The next sequence began with the discharge line, went across the condenser to the liquid line connection, and then to the evaporator section. The evaporator section is the last and least desirable component to pressure test in the field.

Figure 2. Proper bubble testing for leaks includes thoroughly saturating all surfaces.

TEMPERATURE-DEPENDENT LEAKS

The third classification of leaks is temperature dependent. All mechanical connections expand when heated. The connections on refrigeration and air conditioning systems are usually of soft metals such as copper, brass, or aluminum. These metals actually warp when heated, then contract and seal when heat is removed.

The procedures to deal with that are:

1. Place the unit in operation and raise the operating temperature by partially blocking the condenser’s air intake.

Warm water may also be used for system pressurization. Water chillers are usually pressurized using controlled warm water. When dealing with chillers, valve off the condenser and evaporator water circuits. Controlled warm water is now introduced on the evaporator tube bundle. This causes the rate of vaporization of the refrigeration to increase, causing higher pressures in the evaporator.

One must slowly control the amount of warm water introduced to avoid temperature shock to the evaporator. The rupture disc on the evaporator may open if the pressures are raised too high. There are special fittings available from the chiller manufacturer to equalize pressure inside and outside of the rupture disc to prevent rupture. Please consult with the chiller manufacturer before attempting to service or leak check any chiller.

An electronic leak detector may be used while the system is running. However, running a sys- tem usually causes a lot of fast air currents from fans and motors that may interfere with electronic detection. It helps to cover the unit with a blanket or sheet to try to collect escaping refrigerant gases. The leaking refrigerant will be easier to pick up with an electronic detector if it can collect somewhere, instead of being dissipated by air currents.

2. Spray coat all metal connections with a microfoam solution (Figure 3) one at a time and observe for leakage. Rewet any extremely hot surfaces with water to keep the fluid from evaporating too quickly.

3. When testing evaporator components, you may induce heat by placing the unit into defrost.

Figure 3. An example of a micro-foam solution used for leak testing.

VIBRATION-DEPENDENT LEAKS

The fourth classification of leaks is vibration dependent. Leaks that only occur while the unit is in operation are the rarest of all leaks. They are cracks that open and close from physical shaking. However, studies have shown that certain components and piping on refrigeration units will develop vibration leaks.

An electronic leak detector or a microfoam solution can be used while the unit is running. Again, drafts have to be minimized when the unit is running for use of an electronic detector. If an electronic detector is used first, a blanket or sheet should be used to help collect escaping gases and minimize air currents.

If a microfoam solution is used, place the unit in operation and spray coat the following areas with the solution. Look for large bubbles or foam cocoons formations. Large bubbles will form on larger leaks (Figure 4) and foam cocoons will form on small leaks.

Below are areas to spray coat:

• All compressor bolts and gasket edges;

• Suction line connection at compressor;

• Suction line connection at evaporator;

• Discharge line connection at compressor;

• Discharge line connection at condenser;

• Vibration eliminators;

• Any joint or fitting on unsupported pipe runs;

• Expansion and solenoid valves;

• Capillary tube connections; and

• Sight glass.

Figure 4. Large bubbles form on larger leaks.

COMBINATION-DEPENDENT LEAKS

Dealing with combination-dependent leaks - the fifth classification of leaks - involves overlapping the procedures al- ready mentioned. At least two, and usually three, procedures should be merged into one procedure. This type of testing requires a high order of skills and observation techniques. Each suspected component must be isolated and tested in the following manner:

1. A valve or fitting is subjected to high pressure.

2. Spray coat the valve or fitting.

3. Tap the component repeatedly with a rubber mallet to induce vibration. If there’s no leakage, then go to step 4.

4. Gently add heat to the component. If no leakage, continue on to another component.

CUMULATIVE MICROLEAKS

The sixth and final classification of leaks discussed over these past two columns was a cumulative microleak that is measured using a helium mass spectrometer. Such superfine leak testing is beyond the normal operations of the service technician. Microleaks are considered an acceptable amount of leakage in our industry at this point in time.

Note: The technical information and most photographs contained in this and the Oct. 4 column were used with the permission of Refrigeration Technologies.

Publication date: 11/01/2010

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