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1 April 2005

Ultrasonic spots leaky valves

There are significant savings involved in identification and monitoring of critical valves.

By Hans Wagner

It is vital to recognize challenges in the oil and gas industry regarding leak identification, quantification, and monitoring of critical areas such as valves, flanges, bends, or joints.

Today, with all the refineries, process plants, and production facilities, it is essential to take action against unwanted leaks and environmental complications.

In some countries, the contents oil and gas companies are subject to strict regulations from their governments. Governmental regulations do not only require the oil and gas companies to report the specific leaks, but also to report quantities of the leak.

Even though the regulations are there, enforcement does not necessarily happen. By using ultrasonic systems, the operators are able to not only comply with governmental regulations, but also keep the level of security very high.

Research shows significant savings are involved in identification and monitoring of critical valves. We'll look at the possibilities with ultrasonic systems for both topside and subsea monitoring of leaks. Further testing is required for subsea monitoring, and projects are in the pipeline.

Losses through passing valves are a general problem for operating sites, and significant cost savings are possible by being able to identify passing valves non-intrusively and thus enable operators to take early action to reduce the loss by repairing the faulty valve.

There are several areas where benefits arise from valve leak detection: reduced losses to flare, reduced losses to atmosphere, reduced losses from process, and maintenance planning.

The monitoring system has these components:

• Ultrasonic intelligent sensor
• Intrinsically safe power supply unit (with power barrier, signal barrier, RS485/RS232 converter)
• Laptop computer
• Leak monitoring software (graphical display of leaks, generates reports)
• Two-pair cable from sensor to PSU—one pair for power, one pair for signal
• Metal clamping bands
• Silicon compound (for best possible contact between sensor tip and pipe/valve)

If all the leaks are noisy

The physical principal used to detect passing valves is well known and is acoustic emission.

All valves passing will emit a frequency range with varying amplitude depending on the volume of the leak. An acoustic transducer can detect this noise or vibration. The physical source of leak noise is the varying pressure field associated with turbulence in the fluid. Turbulence results from flow instability where inertial effects easily dominate viscous drag. For flow in a cylindrical path, the Reynolds number (R) reflects turbulence.

R = rnD / m
r = Fluid density
n = Velocity of the leaking fluid
D = Leak diameter
µ = Fluid viscosity

Turbulence starts when the Reynolds number is between 1000 and 10,000, giving a lower limit for leak detection under ideal conditions.

In practice, the conditions are never ideal, so there are a number of other considerations that come into play. The most important ones are:

• The leak path is not cylindrical but is usual complex for small leaks, resulting in much higher turbulence, hence more noise from real leaks than predicted.
• The product loss may not come from one single leak but from multiple leaks distributed around a valve seat. If these are below the critical point for turbulent flow, then no noise emits.
• If all the leaks are noisy, then the signals will add up to something that is not the same result as one single leak path.
• The noise level at the source is not what the sensor has to detect; the signal has to travel through the valve body to where the sensor mounts on the outside of the tank, and this distance varies for different valve sizes, as does the signal attenuation.
• The final factor is background noise from nearby turbulent flow or from normal plant noise and vibration; this is why measurement position is important.

There are a number of mechanisms that contribute to the 'noise' or vibration signals associated with leakage across valves including turbulent mixing, shock associated noise, cavitations—liquids only, mechanical source—parts of valves that resonate.

A number of reasons can cause the different mechanisms; the most common are seat peening damage, deposit build up between the nozzle and the seat, or simply scoring of the valve plug.

Installation and real time

The best position is as close as possible to the source of signal generation where there is good contact for signal transmission between valve and transducer. One must maximize the contact area between the measured component measured and the transducer.

Where possible, the measurement position should be a smooth, flat, or surface with a large radius. Preparation of the surface at the measured position is not normally necessary, but any loose paint or rust deposit must go so the transducer couples directly to base metal or firmly adhered surface. There can be no insulation or air gaps between the measure point and the signal source.

Other potential signal sources/background noise, which rarely present a problem, but need some consideration when measuring valves thought to be passing include:

• Noisy steam flow, where there are steam tracing lines to/around the valve body
• Nearby atmospheric steam leaks, impinging directly on the transducer or valve body
• Compressed air leaks or bleeds
• Nearby regulating control valves
• Elbows, tees, or the like causing noisy turbulent process flow nearby

To be able to use the gathered information from the sensor as well as possible, the information needs to be of recent quality; the monitoring must happen in real-time. The sensor will monitor every second, minute, day, week, etc. It is of critical importance that the appropriate personnel receive the alarm from a leak.

The signal from the sensor should also be a digital signal, due to the advantages of digital signals over analogue signals. The sensors can give an output of RS485, RS232, Modbus, Profibus, CANopen, Relay option, and 4-20mA. By using the digital option, the operator can have a two-way communication with the sensor, i.e. download new settings or new developments in the sensor software.

Predicative leak equation

When considering the range of areas that can benefit from leak identification and quantification, there are many cost savings involved.

Identification and quantification of leaking valves on process plant can lead to significant savings, reduced losses to the environment, and enhance maintenance and operations. Specific areas and instances where benefits can arise from valve leak identification and quantification include:

• Reduced losses to flare through leaking relief valves (and their bypasses), control, and blowdown valves. Even where a flare gas recovery system is in operation significant savings realize as high value gas, like propane for instance, does not have to downgrade to something less valuable like fuel gas.
• Reduced losses to atmosphere where valves discharge through vent pipes to atmosphere.
• For maintenance planning, the identification of leaking valves prior to shutdown.
• Troubleshooting—tracing sources of loss and cross contamination, identifying losses through pump and compressor recirculation valves, and monitoring deteriorations in plant performance.

The non-intrusive acoustic leak monitor offers a means to quickly and reliably identify and monitor leakage through valves and to quantify the leakage. In order to identify and quantify if a valve is leaking, the valve must be in closed position, and there must be a differential pressure of at least one bar across the valve. The leakage through the valve produces

an acoustic emission signal, which is detectable using a non-intrusive leak monitor. The leak rate is then determined after the specific parameters are input into the predicative equation.

Saudi Aramco, an international petroleum company, has achieved savings of approximately $1.5 million by identifying leaking valves before shutdown and through better maintenance planning by using non-intrusive leak monitors. This sends a clear message that savings are obtainable using this type of equipment actively in maintenance planning and NDT planning.

Another example is PGS, which made a decision on if they should continue producing with one particular valve, or replace it. They used a non-intrusive leak monitor not only to identify the leak, but also to quantify it. The leak through the valve was not substantial, and they decided to keep on using the valve. They found it more economical and within the safety parameters to leave the valve operating.  CE

Behind the byline

Hans Wagner is president of ClampOn Inc. The article comes from his ISA EXPO 2004 paper and presentation Innovative techniques to deal with leaking valves.