Doug Greaves
U.S. Product Manager, Temperature & Pressure, ABB Measurement & Analytics
[email protected]
abb.com/pressure
Q: Since heat transfer depends on proximity to the heat source, isn’t mounting the transmitter further away and using longer impulse lines a logical and easy first step?
A: Most standard transmitters like the ABB 266DSH DP (differential pressure) and 266HSH Gauge are connected to the process via impulse tubing.
Thoughtfully planning the path of the tubing can be a cost-effective way to extend transmitter service life without special seals, gaskets or material considerations.
Follow these recommendations when planning for a high-temperature application:
- While some distance is recommended, keep in mind that too much distance can dampen dynamic response.
- Note that impulse lines act as cooling fins, effectively reducing temperatures 150 °F per foot in common ambient conditions.
- Each impulse line should lead to a dead end, meaning no leaky connections and tightly closed instrument manifold valves. Sustained flow of hot medium to the transmitter effectively negates any cooling advantage gained by distance.
- In most cases, if the pipe is insulated, the transmitter should not be.
- Generally recommended impulse line sizing for lengths up to 50-ft is 1/4 in. to 3/8 in. for water/steam/dry gas and 1/2 in. to 1 in. for wet gases/oil/viscous and dirty liquids. Beyond 50 feet, increase impulse line size to 1/2 in. and 1 in. to 2 in., respectively.
- When multiple impulse lines exist for the same measurement point (e.g., DP, redundant P), run lines together to maintain equivalent temperatures and try to keep each line the same overall length.
- In cryogenic applications, impulse lines work in the opposite direction, effectively warming process fluids before reaching the transmitter.
Q: When using impulse lines for differential pressure-based tank level measurement, what are the considerations between dry leg and wet leg installations?
A: It really depends on the process medium. Does cooling the headspace gas never, sometimes or always result in condensate? If normal is “no” or “some,” a dry leg impulse line can likely do the job. If condensate is normally expected, use a wet leg impulse line instead, where the headspace impulse line is kept full of liquid to a fixed height. Both options, however, come with maintenance considerations. A dry leg will require a collection pot that's periodically drained, while a wet leg will require liquid level to be maintained at a known height and protected from freezing by heat tracing or compatible antifreeze additions.
Steam is a specific case where wet legs are commonly used, with condensate expected between the transmitter and process. Sometimes condensate pots are used to make sure impulse line condensate isn’t boiled off during high demand loads, particularly with superheated steam.
Another typical application in power and steam generation is boiler drum level measurement. ABB 266CSH Multivariable with active level calculation can compensate for DP drum level due to both wet leg condensate density changes caused by temperature fluctuations as well as real-time water and steam densities in the boiler based on the measured static pressure and on-board steam tables.
Q: What about blockages in impulse lines?
A: In an ideal case, mount the transmitter to allow the impulse line a 10% slope back to the process. This will allow liquid to drain or gas to vent back to the main process line, depending on the defined normal condition.
Continuous purging is a time-tested method to keep lines clear, but maintenance and operating costs across a plant can be prohibitive. Alternatively, ABB 266 Series transmitters come standard with a Plugged Impulse Line Detection (PILD) feature that can be trained to the high-frequency signature of the normal background pressure, and signal a diagnostic alarm to the operator if it detects a change in the signature indicating a blockage.
Q: The cost of installing impulse lines—and keeping them clear—is certain to add up, and failure to do could be unsafe or cause a shutdown. Are there other options?
A: Remote seals, specifically all-welded seals, remove the wet/dry leg decision as well as the need to install and maintain impulse lines in high temperature applications. Consider an ABB 266 Series transmitter complete with S26 remote seals like a doctor with a stethoscope. The measurement is taken directly at the process and transmitted back to the sensor. In the case of the remote seal, the pressure is conveyed through fill fluid. In high temperature processes, the key consideration is specifying a compatible fill fluid based on the highest temperature at the lowest pressure that will be encountered, such as during a cleaning cycle. Compare these conditions to the vapor pressure curve of the fill fluid to make sure it won’t boil behind the sensing diaphragm.
ABB transmitters with remote seals can also be direct or remote mounted to the process. When remote capillaries are used, it's best to keep them as short as possible and of equal length to minimize response time. And because ABB seals are designed in-house, our experts can also engineer custom solutions like cooling extenders, or use special materials for constructing the wetted elements.
In some colder ambient conditions, heat tracing on the capillaries and a heated enclosure may be necessary to maintain fast response times. Another option for improving the response times of hot processes in cold environments are “electronic” remote seals. In this configuration, which we call a Digital Diaphragm Seal, an 266HSH and 266HRH are electronically coupled to produce a DP output. Often used for level applications, this approach removes the need for lengthy capillary legs; response effects are eliminated, and larger tap-to-tap dimensions are possible.
