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The lowdown for engineers

With reducing refrigerent leakage now a legal obligation, service and maintenance engineers have a vital role to play.

Service and maintenance engineers can have a significant impact on reducing refrigerant leakage. Good service and maintenance reduces current leaks and future leakage potential. Poor service and maintenance increases leakage potential and the overall environmental impact of the rac system. Leakage is reduced by effective, thorough leak detection and repair, coupled with good service practice. FACT: A catastrophic leak of 300 kg from a retail pack has the same global warming effect as 43 minutes of electricity consumption by the whole of Wales.

Reducing leaks matters because most of the refrigerants we use can damage the environment if allowed to escape. For example:

  • HCFCs (such as R22 and R408A) deplete ozone
  • HCFC and HFCs (such as R134a, R404A, R407C, R410A) are powerful greenhouse gases. Remember, a system with insufficient refrigerant charge uses
    more power, costs more to run, and leads to additional service costs and reliability problems. There is a legal obligation to reduce leaks and you are a key part of this. Because of the end user’s new obligations under the European Fluorinated Gas (F-Gas) Regulation, the service engineer now has to:
  • Leak test systems once a year if the charge is between 3 and 30 kg and twice a year for systems with more than 30 kg
  • Log refrigerant charging, recovery and leak tests/repairs

Be qualified to handle refrigerant. The new F-Gas qualification is now available (City and Guilds 2079). You will need to have this qualification by July 2011. FACT: It is illegal to add refrigerant to a leaking system without first finding and repairing the leak(s).

Leak testing in practice

Leak testing is a key part of reducing refrigerant loss, but often insufficient care and time are taken to do the job well. To maximise the effectiveness of the leak test procedure you need to:

  • Review the refrigerant log (now required under the F-Gas Regulation) to check where leaks have been found previously
  • Choose the most appropriate method, for example - a quick sweep with an electronic leak detector followed by a leak detection spray (bubble solution) to pinpoint the leaks
  • Be methodical and take your time
  • Check the whole system, including fusible plugs and pressure relief valves and their vent lines
    - pressure couplers (eg for switches and gauges)
    - Inside pressure switches (service valve stem glands and then
    cap them)
  • Tighten Schrader valve if necessary and then cap them. Ensure the cap O-ring is in good condition and that the cap is
    tight
  • If the suction pressure is low (eg below 1 bar g, 15 psig) you will need to increase the pressure to find leaks:
  • Simple condensing unit systems - switch them off (do not pump down)

Central plant - switch off as a last resort. FACT: The first leak you find is usually not the only leak - make sure you check the whole syste

The choice of leak test method

For direct leak testing, either leak detection spray (eg soapy water) or an electronic leak detector with a detection threshold of 5g/yr can be used. You could use a combination of the two - do a general sweep with the electronic detector and then pinpoint leaks with soapy water. In some cases you will need to recover any remaining refrigerant and pressurise the system with dry nitrogen (OFN). Usually a pressure of 10 bar g (150 psig) is sufficient. Nitrogen can be supplied with a trace of either of hydrogen or helium specifically for leak testing. This enables leaks to be found at a lower pressure, but a special leak detector sensitive to the helium or hydrogen is required.

When using nitrogen for leak testing ensure you connect it to the system safely. The photo (opposite page, top) shows a recommended assembly which avoids the use of a standard manifold with a sight glass and uses a braided steel hose.

Getting the most out of your electronic leak detector

Many electronic leak detectors are not accurate because they are not properly maintained and checked. Use a reference leak to check your detector is working correctly - just opening a cylinder or a connection on the system to check your detector is not accurate enough. The photos below shows a simple calibrated reference leak that fits onto the cylinder valve. When the valve is opened, the flow through the device is approximately 5g/year. If your leak detector does not pick this up, it needs servicing.

Heated diode detectors

  • the diode needs changing, usually after 100 hours use.

Infra red (IR) detectors

  • the IR sensor needs changing less frequently. Avoid contaminating the detector with oil, and replace the filter (where fitted) regularly.

Preventing the development of new leaks
You have the opportunity to reduce the risk of further leakage when you are servicing or maintaining systems:

  • Check that pipes are not vibrating excessively and are not chafing. Correct if necessary
  • Check pipe clips are adequate and in good condition - replace if necessary
  • When tightening flanges, tighten the bolts evenly and to the correct torque
  • When tightening flare nuts use a torque wrench to achieve
    the correct torque:
    - For quarter-inch pipe, tighten to 14 to 18 Nm
    - For three-eighths inch pipe, tighten to 34 to 43 Nm
    - For half-inch pipe, tighten to 49 to 61 Nm
    - For five-eighths inch pipe, tighten to 68 to 82 Nm
  • When replacing flared components, use flare solder adaptors rather than manually-made flares

Do not leave line tap valves on systems (one cabinet manufacturer has found that 40 per cent of line tap valves left on
systems leak). It is important to ensure all valves are capped. Uncapped valves are a very common source of leakage.


Where to start
Leakage commonly occurs in the following places:
Shut-off valve spindles
Causes:

  • Wear of the packing gland between the valve body and spindle shaft as it becomes compacted with age and use.
  • Overheating during installation.
  • Caps not fitted.
    Solutions:
  • Ensure that the gland is tightened.

Wrap the valve with a damp rag while brazing.


Schrader valves
Causes:

  • Valve cores damaged during brazing
  • The cores not tightened correctly during replacement.
  • Deterioration of internal seals over time
  • Caps not fitted or have no O-ring seal
    Solutions:
  • Remove the core when brazing the fitting in; ensure the valve body has cooled before replacing the core
  • Use the correct tool to replace / tighten the core

Ensure the cap is fitted and has a seal (in good condition) in
place.


Flares
Causes:

  • Loosening of the flare nut due to thermal expansion / contraction due to a wide temperature variation, especially where those at the outlet of expansion valves
  • Poor joint preparation (causing leakage from initial installation
  • Over tightening, leading to damage at the copper flare face and the flare nut
  • Under tightening of the flare

Solutions:

  • Where possible avoid using flare connections, if they cannot be avoided
  • Use flare solder adaptors (factory produce flares). Ensure the
    copper seal is located correctly.
  • If you have to make a flare, cut the pipe work with a pipe cutter and de-burr using the correct tool. Use an eccentric flaring tool and ensure the correct amount of pipe is protruding through your flaring block
  • Check the flare size and that it does not foul the flare nut on the pipe
  • Lubricate the flare and nut face with a small amount of refrigeration grade oil
  • Don’t over or under tighten the flare nut - use a torque wrench to the setting provided by the equipment manufacturer. Mechanical joints and flanges
    Causes:
    There are a variety joints and flanges on a system - eg drier core lids etc.
  • Incorrectly prepared joint
  • Not replacing gaskets.
  • Uneven tightening of flanges

Incorrectly torque used for tightening bolts
Solutions:

Avoid using PTFE on HFC refrigerants - use an appropriate thread sealant

  • Replace gaskets on flanges and remove all the old gasket material before applying the new one.

Tighten flanges down evenly applying the ‘opposites’ rule until the flange is seated correctly. Use a torque wrench to carry out the final tensioning of flange bolts.

O-rings
Causes:

  • O-rings are widely used in components such as sight glasses, solenoid valves and shaft seals.
  • Wear, hardening or flattening, especially when subjected to extremes of temperature.
  • Leakage after retrofitting due to a different reaction to the new oil.

Solutions:

  • Check (for roundness and flexibility) and change seals rather than re using the existing ones, especially during a refrigerant retrofit.
  • Oil seals before fitting them.

Ensure the replacement seal is suitable for the system oil and refrigerant.

Line tap valves
Causes:

  • Poor fitting of the line tap onto the pipe, or being fitted tobadly formed or flattened pipe work.
  • Use of the wrong size line tap.

Loosening of the line tap valve due to movement and vibration.


Solutions:

  • Ensure the correct size of tap valve is being used and read the instructions for its installation.
  • Fit a line tap to access a system, and then braze a Schrader connector to replace it (do not leave the line tap valve on the system).
  • Leak test any line taps found fitted and replace them if possible.