What is Leakage?
In regards to compressed air and gases, leakage refers to unintentional venting of the gas to atmosphere. Leaks can arise from corrosion, vibration or material failure and most commonly at joints or component junctions in the pneumatic circuit. Note that leakage is normally considered separate to the venting of compressed air through the action of valves and actuators where the air energy has already been partially used.
Compressed air systems are large users of electricity and can be a significant proportion of the overall plant electricity consumption. It is generally accepted unlikely that perfect sealing can be maintained in pneumatic systems and up to 10% leakage may be deemed acceptable. This sounds like a high percentage but leakage is often much higher than expected with figures up to 40 or 50% not being unusual. This makes maintenance of compression and distribution systems an important part of the plant maintenance scheme.
Why do I need to worry about Leak Detection?
In addition to leaks being the source of wasted air/gas and therefore increased operating costs, they can also be the point of ingress for contamination entering the system. Leaks will propagate and become worse over time if remedial action is not taken with the result that small leaks can grow to significant hidden costs.
Industrial plants tend to have a high degree of background noise and mask any obvious evidence of small leaks, making it hard to understand the scale of leakage. This becomes a completely hidden expense for plant operators and maintenance planners. Identifying all the leak points and rectifying all the faults will reduce operating costs and improve the delivered air/gas quality.
How Do I Detect Leaks?
Leaks may become obvious if the air or gas system contains flow measurement devices as any indication of flow while that part of the system is shut down has to relate directly to leakage.
Listening for leaks is only possible during a shutdown when the whole plant is sufficiently quiet that large leaks can be audible. Smaller leaks can be below the threshold for hearing and therefore the absence of the hiss of leaking gas does not mean that there are no leaks. Even if leaks are heard, they can be very difficult to pinpoint, especially in difficult to access areas.
The traditional direct method that is still widely used is to wash over any suspected joints with a soap solution. In the event of a leak, bubbles will form and judging by the number and size of bubbles, the fault can be pinpointed. This is a very simple method but very time consuming as every joint has to be tested individually. There are also potential issues for contamination, corrosion and plant cleanliness.
For remote identification of leak points, the best method is to use a purpose designed ultrasonic leak detector. These electronic devices employ a microphone designed to be sensitive in the high frequency harmonics typically generated by leaking high pressure gas. The circuitry uses the heterodyne principle to change the frequency of the detected signals so that they become audible through headphones.
Ultrasonic leak detectors enable testing of out of reach areas to quickly determine if access is required to pinpoint leaks. With a range up to 8m it allows operators to quickly survey air/gas distribution systems and models with detachable sensors simplify testing in confined spaces.
The detectors incorporate laser pointers and graduated microphone probes or sensor tips to precisely locate the tiniest of leaks so that they can be marked for subsequent repair.
The degree of leakage is directly related to the area of the orifice and the line pressure. High pressure lines should be checked first as these will be the major source of unnecessary expenditure. Low pressure systems will require more thorough testing as the leaks will be harder to detect.
How Do I Measure Leakage?
Leakage cannot be measured accurately through ultrasonic detection as there are too many variables that would affect the result. The only precise way to measure the leakage and determine the cost of the losses is through flow measurement. Some ultrasonic detectors like the S531 below allow the user to input the sensing distance, line pressure and an average cost for generating a unit compressed air volume from which they can produce approximations of leak cost and flow rate. This is not as accurate as using a flow meter but offers ease of use and speed of evaluation across the site in question.
If the system already has flow meters installed, these can be used to determine the normal plant consumption when operating and again during a shutdown. With no operations, this residual flow is purely down to leakage and by comparing this figure to the operating flow, the percentage of daily or hourly leakage is easily calculated.
Turning these flow figures into costs requires knowledge of the operating costs for the compressor plant and possibly the drying/treatment plant in terms of electrical consumption. The use of a power meter may be standard practice or can be used with clip on sensors for an electrical audit.
Another method to determine the amount of leakage is to measure the flow into the distribution system with the plant not running until the pressure in the air reservoir drops by 1bar. Using this figure in a simple calculation gives a realistic estimate of the degree of leakage that can then be used to extrapolate to annual costs. Since this measurement only takes as long as needed for the pressure drop to take place, there is minimal impact on plant operations.
Inline or insertion flow meters can be used based on the thermal mass (calorimetric) principle or differential pressure from pitot tubes (insertion) or orifice plates (inline).
The readings from the meters can be determined from the plant control system or, if meters are fitted purely for this survey, a portable data recorder.
If required, subsequent tests could employ flow meters on each branch of the distribution network in order to determine where the largest volume leaks are across the plant to aid the identification and rectification on site.
S530 ultrasonic leak detector set with two focus tips, noise cancelling headphones, detachable sensor extension lead, integral laser pointer, charger and transit case
S531 Advanced Leak Detector Set with Camera and Leak Cost Estimation
S418 inline thermal mass flow meter with integral data logger is ideal for measuring flow to determine leakage losses
S401 insertion type and S421 inline thermal mass flow meters for higher flow compressed air and gases
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