Ultrasonic Condition Monitoring
The handling of explosive or toxic chemicals coupled with the escalating economic pressures to reduce operating costs can lead to, potentially, disastrous results. Such disasters can be of a catastrophic nature or a relatively smaller scale involving financial penalties following the release of environmentally hazardous materials to atmosphere. This all leads to increased pressure being placed on Maintenance departments to develop a safe strategy to increase plant up-time and at the same time decrease whole life cycle costing for the plant.
A significant factor in decreasing the whole life cycle costing of the plant and increasing up time to be able to realise the full design efficiency capability of the plant is the ability to control and maintain effectively, the valve population of the asset. Maintaining the valve population effectively, invariably will include some kind of inline testing as part of a fully integrated maintenance strategy, which in turn will involve the development of a cost effective Leak Detection and Repair Program (LDAR).
LDAR Explained
The development and implementation of the system will ultimately determine the effectiveness and success of the LDAR program. The LDAR program is, on the face of it, a simple system, the essence of which is to find a leaking valve and repair it. The part of the process that this paper is involved with will be how the leaking valve is identified. The detection of a problem or leaking valve is essentially a condition based maintenance activity. Monitoring the valves is the first step in the program, which initiates the secondary activity, being the repair of the leak. The LDAR program is therefore easily incorporated into existing maintenance regimes.
Ultrasonic Condition Monitoring of Valves
Geographe Condition Monitoring service offers an integrated solution to valve condition monitoring, with a range of advanced hardware and a suite of software that enables the collection, storage, trending and analysis of valve data in a common database. This tool is particularly useful, when used as part of an integrated maintenance program, in the development of maintenance strategy, particularly in respect to the scheduling of PSV’s. Ultrasonic condition monitoring allows valves to be tested in-line, thus reducing down time, increasing uptime and providing a greater cost reduction in the total outlay associated with the upkeep of plant. The cost savings are generated in a number of ways; one identified saving is as a result of identifying problematic or leaking valves at an early stage. The valves identified at this early stage would be scheduled for maintenance activities at the earliest opportunity. In identifying the valves at an early stage the costs associated with the repair and refurbishment of the valve are reduced, as there is less damage to valve internals due to erosion caused by cavitation and flashing.
Principles of condition monitoring:
As the line media passes through a leak orifice, it generates a turbulent flow with detectable high frequency components. By scanning the test area with the condition-monitoring device, a leak can be detected either through a headset as an increase in acoustic amplitude, noted on a ballistic meter, or the data can be recorded as a digital file and monitored via computer.
Once the digital ultrasonic signal has been transferred to computer further manipulation of raw data, evaluation, and trending can take place. In the event of a small leak the readings taken are open to interpretation and the art of predicting seat leakage or recognising seat leakage or cavitation / flashing is a subjective one. However, one fact is clear, the closer to the leak the greater the increase in amplitude and the higher the meter reading. This is particularly useful in determining if a particular valve is leaking through the seat or if the process is generating the noise. For air borne leaks, should ambient noise be a problem, a rubber-focusing probe may be used to narrow the instrument’s reception field and to shield it from conflicting ultrasounds. In addition, a frequency-tuning device that is fitted to most units can reduce background noise interference to provide a clearer, more defined reading. For internal leak detection, such as valve seat leakage, a contact probe is employed to transmit ultrasonic readings from the valve casting to the measuring device.
For extremely low level leaks (ranging from 1 x 10-3 to 1 x 10-6 STD cc/sec) when minimal turbulence is produced, liquid leak amplification can be used. Liquid leak amplification is an ultrasonic “bubble” test method utilising a liquid with low surface tension, which is faster and more reliable than the classic bubble test. This ‘liquid leak amplification’ is particularly useful when monitoring stem leakage to atmosphere and has two basic advantages over the classic bubble test:
- Bubbles do not have to be seen for leak detection. As bubbles form and collapse they produce strong ultrasonic signals which are easily detectable by the unit.
- Bubbles form and collapse almost instantaneously, so waiting time for bubbles in low level leaks is markedly reduced.
Through our development and marketing of in-line testing systems we have become aware of some dangerously unreliable methods of testing being utilised today. The technology is now here to supersede these unreliable methods and improve upon the overall effectiveness of a fully integrated maintenance strategy. As outlined above ultrasonic acoustic condition monitoring is a tool, which when used effectively as an integral part of any valve related cost effective maintenance routine, provides cost saving benefits. These cost savings manifest themselves as an increase in plant up time, a decrease in the overall life cycle cost of the plant, and increases in overall plant safety.
