DP Flow Best Practices with Mulitvariable and Conditioning Orifice Plate

While using best practices for your process control and instrumentation would seem obvious, best practices are not always common practices.  Often times plants are unaware of a better way to control the process.  The common adage is this is the way we have always done it.  Similar to new technology, best practices take current technology and apply them in a way that improves performance and improves reliability.  These are just a few examples to consider with many more being fully developed in future posts.

There are numerous measurements in a process plant that are made through differential pressure (DP).  DP can be used for measuring flow, level or just plain differential across a piece of equipment (like a heat exchanger or filter housing).  Impulse piping is the most common method used to capture the process pressure.  However, it is not the best practice method.  Impulse piping creates a substantial number of potential leak points.  It is also more expensive to install, due to increase material and labor.

DP Flow measurement is the biggest offender of best practices.  Rather than using impulse piping, plants should take advantage of direct mount primary elements.  The primary element, standoff and valve are all integral and the pressure transmitter is attached directly to it.  The only potential leak points are the process connection and valve interface.  This method also creates a faster response time, allowing you to better control your process.  Overall cost of this design is much less once material and labor are considered.

DP Level can be improved through the use of a Tuned System.  It is a common practice to use a balanced system in which the lengths of capillary are equal.  It has been determined that this does not lead to the best overall system performance.  A Tuned System will mount the high-side transmitter directly to the process flange and a single capillary to the low side.  While the head and temperature effects are larger in this design, the overall error is low (the two cancel each other).  Once again, the material and installation costs are less.

Temperature measurements are communicated to the control system through direct wire sensors (RTD or thermocouple) or temperature transmitters.  It is common for high-density measurements to be made through direct connection of the sensor.  Once again, this is not the best practice.  Temperature measurements benefit greatly through the use of transmitters.  Transmitters can be calibrated, the distance between the measurement and control system is higher and problems with the measurement can be communicated.  In the case of high-density measurements a high-density temperature transmitter should be used.  Multiple temperature sensors are wired into the transmitter and a single communication wire is needed for the control system.  In keeping with the theme, the material and installation costs are less.

Engineering firms can take advantage of best practices, too.  While they may increase the cost of the instrumentation, they lower the overall material cost, allowing you to be more competitive.  Best practices commonly allow faster commissioning of instruments, which allows you to finish the project sooner.  Finally, best practices can be tied to process performance which may allow you to charge a premium to your client (plants are willing to invest more with you to achieve higher profit).  In summary lower capital, faster commissioning and improved production can be realized with best practices. 

As you continue with your Reliability Resolutions for 2012, best practices can be a way to attain you goals.  I have only presented a few examples of best practices which should be considered.  Coupled with other suggested strategies of advanced diagnostics and new technology, these are strategies that will create a plant that is reliable, available and profitable.

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