Solve Difficult Leakage Problems

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Contents 1 Solve Difficult Leakage Problems 2 Basic Guidelines for Diagnosis for Internal Leakage problems 3 Video Self Training Exercise 4 Valve Leakage Surveys 5 Steam Trap Surveys 6 Air and Gas leakage Surveys 7 Other Applications for Ultrasonic Noise & Vibration Meters

Solve Difficult Leakage Problems

Have you ever had a hydraulic cylinder that is creeping and asked the question, “Is it the Cylinder Piston or the Directional Control Valve that has the leak”? Have you ever isolated a section of pipework for a weld repair only to find one of the valves you have isolated is leaking fluid, but you don’t know which one? Have you ever had a fluid system performing poorly and you suspect a leak somewhere but don't know where? Have you ever had a steam system that seems to be dropping in efficiency? Have you ever had a compressed air system where the compressors are still working hard even when your plant is shut down? Leakage is a major source of poor operation, waste and energy loss in many industrial plants. One relatively low cost and easy to use tool useful for solving these types of equipment problems is an ‘Ultrasonic Noise and Vibration Meter’. I worked at the Port Kembla Steelworks power station department for 7 years in the 1980’s. This was an exciting period of time with personal computers just starting to be used in maintenance, maintenance planning processes becoming better understood and lots of developments occurring in instrumentation such as for vibration monitoring. An engineer in the main power station had purchased an ultrasonic meter for monitoring steam trap leakage and was not sure what to do with it. As I was heavily involved with condition monitoring, he gave it to me. This was the start of my long and enthusiastic involvement with ultrasonic monitoring.

The great thing about basic ultrasonic meters is they’re so simple. For diagnostics on steam traps the meter was very easy to use. A steam trap is designed to let out the water that condenses in a steam line but not let the steam out. In power stations the discharge from steam traps are piped up to collect the expensive condensate. If a steam trap starts leaking it is not obvious and can waste a huge amount of energy. A steam trap that lets steam through generates lots of very high frequency noise. This is easily detected useing the contact probe of an ultrasonic meter. Internal leakage diagnostics for a wide range of equipment became easy once I understood steam trap monitoring. Below is some basic guideline for diagnosing internal leakage problems.

Basic Guidelines for Diagnosis for Internal Leakage problems

You will need an ‘ultrasonic meter’ to get the best result if you want more than just a conceptual understanding. These devices have been around for 30 years so before you go and buy one ask around to see if you can borrow one or check at the back of your sections instrumentation cupboards in case someone has brought one in the past. For basic leakage monitoring you only need a very simple ultrasonic meter compared to a unit you might require for trending bearing condition or doing large air systems external leakage surveys. The AccuTrak VPE Leak Detector unit I purchased a few years ago from the internet is very simple and quite cheap (US$345 including FedEx postage - View). This unit has a microphone sensor for directional measurement of air born ultrasonic noise and a solid probe that screws over the top of the microphone for direct contact measurement of internal leakage noise. As ultrasonic noise by definition is above normal hearing range, the meter converts or 'heterodynes' ultrasonic sound frequencies down into the normal hearing range. A set of headphones are supplied to help with some types of diagnostics. Headphones are especially useful for slow speed bearing monitoring. The only controls on this meter are the on-off button and the level control slide and the output is just a simple column of led lights.

There are lots of more suphisticated Ultrasoic Noise and Vibration Meters available from a range of suppliers if you have some serious applications that require this.

Three steps for diagnosing internal leakage

1. Where to Measure? -
   
   Understand the types of pressure differences and flows in your system. This includes where the pressure drops and excessive internal leakage might be occurring. Ultrasonic noise will result from turbulent flow where a fluid changes pressure such as at the valve within a stream trap, leakage across an isolation valve or flow across a control valve where a reasonable pressure drop occurs.
2. Select Comparison Points and Compare Levels – If you have similar components (eg steam traps) with similar pressure drops, compare the levels at the same measurement location on each. When you only have a single component for testing, compare its measurement level at the selected location to a point a reasonable distance away along the inlet line to the component. The specific probe contact point for measurement should have a direct solid transmission path to the downstream leakage location. The figure above shows there is lots of flexibility for valves in higher pressure applications as the leakage noise will be high and transmits well through metal components. For components that are lagged you may need to drill a small diameter whole through the cladding (suggest using a cordless drill). The process for using the meter is given below.
   1. Connect the meters contact measurement probe
   2. Turn the unit on (for more complex meters there may be some switches to set eg. setting frequency bands to Fixed and output to linear- Check the manual for meter)
   3. Contact the probe to the first chosen measurement location with about ½ Kg force and adjust the sensitivity control (large knob, slide or up-down arrows) till the output is about at the 50% level. This first point becomes the reference location.
   4. Without making any meter adjustments measure the other comparison point/s and note if the output is substantially higher or lower than the reference location. For most cases you are looking for a large difference in levels between a leakage and a non-leakage situation.
3. Eliminate the Possibility of False Indications from External Noise or Other Issues – There are many reasons why you might measure a higher ultrasonic noise on a component other than internal leakage so you need to eliminate any possible false indications. You don’t want to go changing or repairing a good component due to a false indication. First listen to the suspect component using the meters headphones. A leak will generally be consistent and sound like a leak. A bucket type steam trap may have an occasional normal discharge of condensate that could be mistaken for a leak. The main false indication is from a high ultrasonic background noise on the component from a source other than an internal leak. Typically this is from an external source transmitted to the component through the inlet or outlet pipework or through a component support connection. To eliminate this possibility compare the suspect component measured level to the level away from the component on all of the possible noise transmission paths. If all these levels are lower than on the suspect component then external interference is eliminated. Sometimes a component could have a leak to atmosphere from a flange or a gland that is not obvious. To eliminate this, check that measured level does not significantly increase for these component locations. If your meter is set on high sensitivity to detect lower pressure leakage issues then airborne ultrasonic noise can affect the contact probe. This effect can be eliminated by using your hand or a cover to shield the probe from the airborne interference.

Video Self Training Exercise

It’s always nice to do some simple quick training exercises when first using an instrument before going onto site. This eliminates the initial confusion and fumbling that usually happens to me when trying to use something new.

View UTube Video (This is my first attempt at doing a video so don't expect a professional production)

Valve Leakage Surveys

Valve internal leakage is often a relatively hidden problem that can significantly affect equipment and plant performance. Even on simple pumping systems a leak in a check valve, bypass valve or changeover valve can cause loss of pumping volume. Sometimes the problem is just an incorrectly closed valve or an object in the line not letting a valve close fully, which can often be fixed very easily. Pumping systems should be tested for valve leakage during commissioning and again if there are any changes in performance over time.

Many organisations recognise the opportunity from doing valve leakage surveys on boilers and associated steam systems well before a major planned outage. This is so there is plenty of time to fully plan any valve repairs and replacements and to eliminate unnecessary repairs on valves that are still working fine. Valves generally are low maintenance items but there are dangers ignoring them altogether. Many valves are operated only occasionally and when they do there is a greater chance that there may be build-up on sealing faces causing poor sealing and leakage. In higher pressure applications this leakage can often cause damage to the sealing faces. The solution is usually just to exercise the valve until the valve seats properly. As long as a pressure differential across the valve can be achieved a simple ultrasonic meter is ideal to confirm effective valve sealing.

Steam Trap Surveys

I found two main problems with steam traps when I was working in the power station. The most common initially was where traps were isolated, blocked or failed closed, which can be determined by the steam trap being at near ambient temperature. Steam traps are installed for a purpose and at worst moist steam entering a turbine could cause Millions of dollars in blade damage. The other problem I found was steam traps leaking through excessively and the diagnostic process given above is what I used for leakage fault finding. As with maintenance generally you need to understand how your equipment works. There are 4 different types of steam trap working mechanisms. Some team traps such as the thermostatic type may have a consistent small level of leakage which should not be mistaken for a failure. Check out these two articles on steam trap monitoring steam trap monitoring and steam traps

To start off do an initial survey and depending on the number of problems you find, the equipment criticality and your sensitivity to energy loss repeat on a 2 to 6 monthly basis. The first survey is always a challenge as finding all steam traps in a system is often a significant project. Starting with the easy 90% that can be found on a walk around and discovering the rest over time using piping diagrams is an option to make getting started more practical. For problems found, ensure you disassemble failed traps to understand the failure cause and immediately retest repaired traps on commissioning. A likely problem is trap blockages caused by rust or other solid objects plugging a trap. If this is the case, consider installing a strainer with a blowdown valve at the inlet to the trap. If you are having consistent leakage problems make sure your steam trap is designed to suit your application.

Air and Gas leakage Surveys

Probably the most common use of Ultrasonic Noise & Vibration Meters is not for detecting internal leakage but for detecting external leakage on air or gas systems. Almost all industrial plants have air compressors either for specific process applications or for general service air. It has been estimated that on many larger service air systems 50% or more of the air requirements is to service leaks. Costing of air leaks. If you use more expensive gases like Nitrogen or Carbon Dioxide then there is even a stronger need to be able to find and fix leaks. Some of the causes for these leaks are given below.

• Pipe connections and valves
• Out-of-service equipment with air left on
• Hoses and quick connect plugs
• Air Traps and drains (liquid)
• Manual blow-down valves
• Filters, gaskets and seals

It is often pointed out that you don’t need an ultrasonic meter for leak detection as just human hearing and soapy water can do the job. This is true but anyone who has used an ultrasonic meter for external leak detection knows that the increased speed and effectiveness that it gives easily justifies the purchase of even the most expensive meter. Some of the reasons for this are given below.

• The strong directional nature of the ultrasonic meter microphone makes it easy to pinpoint a leak from a distance.
• The strong directional nature of the ultrasonic meter microphone makes it easy to pinpoint the exact location of the leak even with closely packed components like pneumatic solenoids
• The ultrasonic meter allows for a rough quantification of the size and severity of each leak found.
• The meter can find leaks in vacuum systems that are often almost impossible to find without this technology.
• Using an ultrasonic noise generator the meter can find leaks in tanks, vessels and compartments without pressurisation.

Other Applications for Ultrasonic Noise & Vibration Meters

There are many other applications for Ultrasonic meters other than leakage detection. Some of the standard ones are given below.

• Early detection of bearing defects in higher speed rolling element bearings
• Detection of low speed rolling element bearing defects using listening
• Monitoring lubrication and the application of grease to bearings
• Monitoring medium & high voltage electrical equipment for arcing, corona and tracking
• Monitoring or locating the mechanical friction within components

As with many technologies the number of applications you can find for Ultrasonic Noise & Vibration Meters is only limited by your imagination.