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Jul-2010

Developments in impulse line blocking diagnosis

Impulse line blocking diagnosis is emerging as a versatile approach to the prediction of process disruption

Neil Hankey
Yokogawa Electric Corporation
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Article Summary
Modern fieldbus instrumentation enables field devices to send a wealth of auxiliary information such as diagnostic results and alarm notifications to the host system. In flow measurements using differential pressure transmitters, the pressure fluctuations found within most processes, and normally averaged within the device, can be used for monitoring the health of the impulse lines and, in particular, detecting a blockage condition. Differential pressure transmitters using digital sensing and built-in intelligence can greatly enhance the process of impulse line blocking detection.

Differential pressure (DP) flow-meters — composed typically of an orifice plate, a DP transmitter, plus other components — are widely used in many applications because they are reliable, robust flowmeters that require no wet calibration. The simple design of orifice plates and modern DP transmitters makes them extremely reliable. However, the impulse lines that transmit process pressure to the DP transmitter are vulnerable to blockages caused by fluid condensation, freezing or corrosion. Impulse line blocking is a problem that must be addressed to ensure operation and efficient maintenance.

Normally, the pressure fluctuation encountered within the process can be used as an effective way of detecting the blocking status of the impulse lines. However, if the pressure fluctuation level is low, it may be difficult to detect blocking because the detection threshold (that is, the change in the pressure fluctuation) may be very small and could lead to spurious alarms. In particular, special care must be taken with most gas flow applications, as the pressure fluctuation level is much lower than that found within liquid flow applications.  

Furthermore, there are a number of sources of pressure fluctuation that may interfere with the detection of impulse line blocking, including piping vibrations and gas entrainment in the liquid-filled impulse lines. This article discusses the factors that may interfere with impulse line blocking diagnosis and proposes practical techniques to overcome these challenges, as follows:
• Provide selection criteria for fluid type, density, viscosity and static/differential pressure fluctuations
• Check piping vibrations, low flow rate and gas entrainment in the liquid-filled impulse lines
• Adjust detection threshold levels in order to avoid false alarms and missed detection.

Technical overview
The technique of impulse line blocking diagnosis was initially developed for applications involving fairly clean liquids found in refineries and chemical plants. However, users would like to apply the technology to other types of fluids within different industries.

This diagnostic technique is normally only applied to hydrocarbon liquids of relatively low viscosity (less than ~10 cSt). The more viscous the fluid, the more the pressure fluctuation is damped by the process, which, of course, limits the role of impulse line blocking diagnosis. When gases are of high pressure and high density, there are opportunities for using the technique. It is, however, normally more applicable to wet gases because of their relative high density.

For steam applications, condensate pots are usually installed between the pressure taps and the DP transmitters. These pots work like an accumulator, largely attenuating any pressure fluctuations and making it impossible to use impulse line blocking diagnosis.

DP generating devices
Impulse line blocking diagnosis is generally applied to orifice-type DP generating devices, since these are the type most commonly used for measurements within the process industry. Here, the pressure fluctuations are considered as being generated by the turbulent flow passing through the orifice plate. The orifice seems to be advantageous for detecting impulse line blocking, because — especially downstream of the orifice — flows occur with large vortices and eddies that generate the necessary pressure fluctuations. However, recent investigations have shown that pressure fluctuations along piping both upstream and downstream of DP devices are essential. This means that venturis, pitot tubes and DP generating devices other than orifices are potential candidates for impulse line blocking diagnosis if pressure fluctuations above a certain threshold can be detected.

Interference effects
Factors that can interfere with impulse line blocking diagnosis include piping vibration and noise, flow disturbances, flow rate variations and gas entrainment.

Piping vibration and noise
Impulse line blocking diagnosis relies on detecting changes in pressure fluctuations within the differential pressure or static pressure measurement. Usually, reference data for the pressure fluctuations that occur normally within a known, clean system without blocking are memorised and then compared with actual data in the detection phase. When one-side blocking of either impulse line occurs, the fluctuation of differential pressure increases. When both sides exhibit blocking, the fluctuation tends to decrease. In real processes, however, piping vibration may change with varying flow rate or valve operation, and these changes can affect the pressure fluctuation being detected. Water hammer noise can also generate abrupt changes in pressure in a different way.

Flow disturbances
Flow disturbances like cavitation or pulsation can affect pressure fluctuations. Cavitating flow may affect the frequency pattern of pressure fluctuation, while pulsating flow may increase the pressure fluctuation’s amplitude. It is therefore desirable for any diagnostic technique to avoid the influences of such effects. This is normally done by limiting the detecting region of the impulse line blocking.

Changes in flow rates
The flow rate at the detection phase might differ from that at the learning phase when the reference data are taken. In normal liquid flows, pressure fluctuation increases with greater flow velocity, so it is possible to compensate for the amplitude of fluctuation by taking into account the change of flow velocity.
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