Installations of instrumentation in cold climates (where temperature can fall below -40C) have historically been expensive, unreliable and suffer from slow response. Newer instruments and technologies have become available that allow users to dramatically reduce cost, improve reliability and safety, and speed response time. Case Studies with quantified benefits will be included from operating sites in Fort McMurray. Note that this was previously published in “Chemical Engineering”, April 2012.
Mark opened noting that we can use the intelligent in today’s smart measurement devices to identify hidden problems with ‘traditional practice’ and new best practices. For example, for a differential pressure flowmeter, both of the temperatures of the process and the sensor can be monitored to predict the installed sensor temperature.
One best practice is to eliminate heated enclosures. These enclosures have historically been used to prevent transmitters from freezing which can cause damage to the transmitter, the display stops updating, and impact the transmitters’ accuracy/drift. The downside of these heated enclosures are the cost and risk of operation. These risks include failure of heater off = transmitter freezing, failure of heater on = transmitter cooking, on-off heater causes output shift when turned on, another component to fail and need replacement, fugitive emissions contained in enclosure.
Modern transmitters are much less affected by cold temperatures. Rosemount pressure transmitters include options to cold-start and operate to -50C, option for LCD to continue updating to -40C, CSA approved to -50C, and the sensors characterized to -50C. Mark noted that oil and gas producers in Northern Canada are installing transmitters outside of heated enclosures in Fort McMurray. He summed up these thoughts that new installations of Rosemount transmitters should almost never require heated enclosures.
The caveat is that some approvals are not valid below -40C and this case is usually limited by government regulators and test agencies. Examples may include those certified for safety instrument system functions, custody transfer applications, shipboard, and more. Also wireless batteries must be de-rated below -40C.
A second new best practice is the elimination of heated sensing lines & capillaries. These have historically been used many process fluids harden and plug when they cool. Heating capillaries avoids oil freezing but have a very high installed & operating cost and complicate installation & maintenance. When the heat turns on: oil expands in sealed system causing an output bump and pressure cycling, level applications: independent thermostats on upper and lower capillaries causes level cycling.
Unfortunately, wet / dry legs are even worse than seal/capillary systems in cold climates. The wet/dry leg requires the same heating to avoid condensation or evaporation which causes a large drift and requires frequent maintenance (drain/refill). They also are just as expensive and require higher maintenance than heated capillaries.
Thermal optimizer (T.O.) are the best solution for hot process and cold ambient. They work by using the process heat to warm the capillary tube and fill fluid. It’s designed to ensure ‘just enough’ heat is transferred and not overheat transmitter. It provides oil hot enough for quick response and eliminates need for capillary to dissipate heat and eliminates need for heat tracing. More than 4000 T.O.s have been installed in Alberta since 2010.
Mark explained that new thermal range expander is available for hot process, cold ambient and long capillaries. It uses two types of oil: High MW = very hot process and low MW = cold ambient (even for long capillary runs).