Piping: Chapter 5
Leak Checking
In this module, we will learn about how to check leaks in pipe. We will also learn about cleaning the pipes using the purging process. Skip to quiz!
Leak Checking
Before using pipe, we need to ensure that the pipe joint is leak proof. Leakage can occur if pipe is not connected properly. Nitrogen leak test is carried out to check the pipe joints for leaks using the nitrogen gas.
Before starting nitrogen leak detection test, the system must be isolated. For isolating the system turn OFF the system and close all the valves connected to the piping.
Leak checking is carried out using number of components such as:
Nitrogen cylinder
Regulator
Regulator gauges
System gauge set
Hoses
We need to fit the regulator to the nitrogen cylinder. Regulator is a knob that adjusts the pressure of the gas. Gauges are present on the regulator to verify the pressure of nitrogen that will be inserted into the piping.
We also need to fit gauges to the system that we will test. They are called system gauges. For example, these gauges are connected to inlet of compressor and outlet of evaporator. If any leak is present in the system that gauge pressure will drop down to indicate leakage.
Fit the common manifold hose of system gauge set to the nitrogen cylinder regulator. Through this, the nitrogen is inserted to the system.
Close the gauge set handles and nitrogen cylinder handle. Open the regulator by turning regulator knob counter clockwise.
We need to test the system at the minimum allowable system pressure to avoid damage to components. Pressure limit for testing is specified by the manufacturer. The least pressure limit in HVAC system is evaporator pressure and it is 450 psi as per manufacturer recommendation.
Open the handle of nitrogen cylinder and adjust the regulator knob to recommended pressure. Allow the nitrogen to enter into system by opening the low pressure and high pressure gauge set handle.
Maintain the system at the maximum allowable pressure for the 10 minutes duration. Monitor the inlet and outlet side gauges. If two reading dont match there is leakage present.
We can also check leakages using leak detection spray or soapy water to identify the leak point. Leaks present in the material will cause gas bubbles to form at defective spots. If leaks are found, they must not be repaired with the system pressurised.
Slowly vent the remaining nitrogen. Repair any leaks found and then repeat the test procedure.
Purging
Before using pipe, we also need to ensure that there are no impurities present inside the pipe. Purging is a process of removing the already present impurities inside the pipe. This process is carried out by adding another gas.
Nitrogen gas is used for purging because it is dry, non reactive and non-combustible. It will not react with impurities to form another compound. Non-combustible property of nitrogen can prevent potentially ignitable atmosphere.
Pipeline purging with nitrogen gas is a mandatory procedure to start using pipe for gas flow lines. This procedure will eliminate oxygen, water vapor, and other impurities that are retained within a newly laid pipeline network.
When we install an air conditioning system, the refrigerant lines from the indoor unit to the outdoor unit have to be connected. We use brazing process to connect them.
During brazing, oxygen present in the atmosphere reacts with metal. This forms an oxide layer inside the tube. This oxide layer cools and add impurities inside the pipe.
The impurities can be carried by the refrigerant through the system. This contaminant can restrict flow through small tubing such as capillary tube, flow control valve. Restriction of refrigerant flow can damage the system. To avoid this, purging is carried out on newly laid piping.
Connect the regulator to nitrogen cylinder. Insert the regulator hose to one end of the pipe. The regulator is used to control flow of nitrogen through the pipe.
Recall that we need to first close the nitrogen handle and open the regulator knob. Then, Open the nitrogen cylinder valve. Slowly move the nitrogen regulator in clockwise to pressurise the system. We need to purge the system at 1.5 to 2 psi nitrogen pressure.
We need to initiate the flow before heating the pipe for brazing. Allow nitrogen to escape from other end of pipe to remove impurities. Continue to flow nitrogen until the pipe has cooled after brazing.
Avoid an excessive flow rate that builds pressure inside the tube. A high flow rate will tend to cool the tube. This will reduce brazing temperature. A poor braze joint is created at low temperature.
Nitrogen leak test is carried out to detect leak present in the pipe joint. Nitrogen purging is used remove impurities formed inside the tubing during brazing.
Question #1: What is the function of regulator?
Measure leakage amount
Control density
Control temperature
Control pressure
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Answer: Control pressure
The regulator controls pressure of nitrogen. Gauges are present on the regulator to verify the pressure of nitrogen that is going to be inserted into the piping.
Question #2: What is the use of gauges?
Create vacuum
Change temperature
Show pressure reading
Control pressure
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Answer: Show pressure reading
Gauges indicate pressure. They are present on the regulator to verify the pressure of nitrogen that is going to be inserted into the piping.
Question #3: How do you ensure there is leakage present by monitoring gauges?
Gauge show leakage message
Pressure fluctuation indicate leakage
Pressure limit for leakage is noted
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Answer: Pressure fluctuation indicate leakage
Monitor the low pressure and high pressure side gauges. If two reading dont match there is leakage present.
Question #4: How do you recognize the leakage during nitrogen leak test?
Using temperature sensor
Bubbles coming out after applying soapy water
Measuring nitrogen volume
Measuring nitrogen mass
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Answer: Bubbles coming out after applying soapy water
Soapy water is used to identify the leak point.
Leaks present in the material will cause gas bubbles to form at defective spots.
Question #5: Which gas is used for purging?
Oxygen
Nitrogen
Propane
Methane
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Answer: Nitrogen
Recall that Nitrogen gas is used for purging because it is dry, non reactive and non-combustible.
Question #6: What is the need of purging?
To remove liquid nitrogen
To remove burr
To remove oxide layer and impurities
To check leakage
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Answer: To remove oxide layer and impurities
The oxide layer and impurities are formed inside the pipe during brazing.
To avoid this, purging is carried out on newly laid piping.
Question #7: We need to purge the nitrogen inside the piping with higher flow rate. This will improve brazing efficiency.
True
False
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Answer: False
Avoid an excessive flow rate that builds pressure inside the tube during brazing. A high flow rate will tend to cool the tube.This will reduce brazing temperature.