Short, straight access

Brian Rice, Heiner Klinkmann

Around a million differential pressure transmitters are sold worldwide annually. They are interfaced to pipelines in a variety of ways but, most commonly, via an orifice plate. This has proved very reliable and a cost effective method over many years and is used to monitor flow along the processing and transportation pipelines used within upstream and downstream oil and gas facilities, and chemical and petrochemical processing plants. Orifice type devices use a flat plate with a specific-size hole, located within the media pipeline, giving at first sight a relatively simple solution for measuring flow. Media is bled off via two tapping points either side of the orifice plate, and passed to a differential pressure (DP) transmitter where the difference between the pressures of the two flows is measured. However, in handling a pulsating flow, as it is transferred from the orifice to the transmitter system, errors do occur and accuracy is affected.

Typical error sources may include square root error (SRE), inertia effects, coefficient shifts, gauge line error (GLE) as well as data acquisition and processing. Two main problem areas affect flow transfer in DP transmitter systems: in the primary element it is square root error (SRE), and in the secondary section gauge line error (GLE). There is also inertial error that occurs at high frequency or high amplitudes and usually is much smaller than SRE. Coefficient shifts become relevant at extreme amplitudes only.

All orifice type metering instrumentation have an induced error due to pulsation of flow. The square root error is introduced while averaging the differential pressure (DP) across an orifice. Basically, SRE is the difference between the average DP measured at the transmitter and the real DP difference measured across the orifice plate. What should be averaged to determine the true flow rate is the square root of the instantaneous differential pressures. However, because transducers are unable to follow rapid pulsation changes fast enough to measure DP instantaneously, a DP average reading is taken off the transmitter signal. SRE is then the difference between the two DP averages. When the sampling rate goes above one cycle/minute, differential pressure becomes difficult to measure. From this assessment it can be seen that SRE is a data processing error and could be ignored if a true differential pres-sure could be measured at the orifice tapping points in conjunction with a fast response, data acquisition system. Although SRE applies to both closely coupled instruments and to the indirect type of DP transmitter system, GLE is more relevant in the latter case.

Consequently, flow measurement systems with DP transmitters encounter problems mainly when indirectly coupled to a media pipeline. In this type of configuration the two tapping points at a pipeline’s orifice plate are linked to the pressure transmitter by gauge lines. These runs of tubing may include various bends, valves and manifolds, possibly involving some 20 or 30 joints. Consequently, problems associated with flow in this secondary area relate to pulsation, attenuation and shift. Thus, the eventual transmitted signal may be inaccurate by as much as 15 %. Many component parts are used to install the secondary section of an indirectly coupled differential pressure system. It is therefore understandable that the rise and fall of pressure pulsations, plus a shift in the average line pressure, can cause errors in flow measurement. Pressure shift may be due to different diameters and lengths of tubing, the bends that are applied, and also because valves with different internal diameters to the tubing may be inadvertently used.

In order to overcome these problems and achieve more accurate results, a Close-Coupled Instrument Mounting Solution, or CCIMS, has been developed by Parker Instrumentation. A key feature of the design is the direct attachment of the transmitter as part of an integral assembly to the piping flanges, thereby providing short, straight and full-bore access to the DP transmitter that is without constriction. The piping interface and instrument mounting parts of the module can be separated via a quick-release mechanism. Referred to as a fast-fit operation, this mechanism is designed to support separation and replacement of the transmitter from its primary module so that maintenance or calibration can be carried out without shutting the process down.

Although pipeline flanges are machined to an industrial standard to match orifice plate tolerances, in practice there can be considerable misalignment. The tolerances allowed mean that in practice the connection points for the instrument can be misaligned in X, Y, Z and rotational dimensions, as well as the angle of alignment of the flange faces. This problem is solved by Parker’s interface module, with its isolation valves, that incorporate a patented universal tubing joint for overcoming alignment issues when connecting to the two flanges on the pipeline.

A CCIMS unit, which has only five internal joints, can typically be installed in 30 min-utes. On the other hand, indirect systems can take one-to-three man-days to install. There are also benefits for plant maintenance staff by having such a compact, integral unit that, once fitted, can remain unattended, as there are very few parts that will need attention. A lower maintenance requirement also goes hand-in-glove with long term reliability thus reducing the total cost of ownership, factors which are the key to successful operation of the new generations of highly automated, unmanned plants. In the oil industry, for example, a particular problem that raises maintenance cost is the presence of large particles in the oil flow. This can cause blockages in the gauge lines that connect orifice plate sensing points to the transmitter, causing the read-out to freeze instead of returning to zero. The reading then becomes the false pressure level between the sensor and the blockage. Unless some form of monitoring is in place, it may be a while before the fault is apparent. Similar blockage or plugging problems are commonplace in any process involving viscous media and/or cold temperatures, and these can be very costly indeed if they cause a plant to shut down. Consequently, time has to be invested in regular checks to ensure gauge lines are not blocked, or plants must invest in technologies that can automatically detect such situations. It is this close coupling of the CCIMS to the orifice plate and the subsequent elimination of flow lines that gives high and dependable accuracy. Also, because the instrument is allowed to connect as part of the integral unit there is no need for gauge lines. It means that the nature of the media being handled, whether it is a viscous liquid or a gas, will have a much reduced effect on measurement integrity.

In order to suit different applications CCIMS is being offered as a modular family. It can be configured in various ways depending on the interface requirements. For the primary module, initially there will be a number of choices with double block and bleed being the most commonly preferred option. Similarly, for the secondary clip-on module, choice is available including 3- and 5-valve instrumentation manifolds and flow paths for traditional or fiscal measurement situations.

It is fair to say that close-coupling has been a goal of instrumentation component vendors for many years, and a number of manufacturers have made attempts at producing a solution. None of them, however, appear to be capable of being as closely coupled to the media in a pipeline as CCIMS. One particular problem area that other designs have found difficulty with is getting proper alignment between the pipeline flanges and the instrument faces. Among limitations observed in the different models is that there often appears to be little or no adjustment to accommodate flange tolerance; there are also more joints; and some of the instruments introduce bends in the impulse lines and do not, consequently, provide a linear flow path.

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