The Importance of Sensing Lines to Performance of Boiler Room Instrumentation

-By David Eoff

Sensing lines are everywhere in the modern plant, whether it be a boiler plant, refinery, chemical plant, or water treatment plant. Often the instrumentation problems that stump operators, contractors, and sometimes full-time service technicians are related to how sensing lines are installed and maintained. We’re going to examine some of the most common instruments that employ sensing lines and describe how they are supposed to work, how they should be installed, and what to examine when things aren’t working correctly. These include flow meters, level transmitters, draft transmitters, PRVs, and steam pressure switches and transmitters.

The most important practice for installing instrumentation correctly is reading the instruction manual for the instrument. In this era when the internet is as close as our smartphone, installers usually have access to manufacturers’ installation manuals. Sensing lines are critical to the performance of an instrument, so their correct installation will be shown. Barring that, installers can use some common sense and ask themselves, “What is supposed to be in this sensing line under normal conditions?” The answer to this question will dictate how the sensing lines are supposed to be run.

Steam Pressure Transmitters and Switches

This is the easiest application we’ll cover. The one trick is most steam pressure transmitters and switches cannot withstand typical steam temperatures. So some sort of protective device needs to be installed between the pressure sensor and the steam source. If the transmitter can be located below the steam source, the piping or tubing in between (the sensing line) fills with condensate created by cooling steam and protects the sensor. Note the pressure transmitter in this arrangement will read both steam pressure and the hydrostatic pressure of the water column. This is usually acceptable for pressure switches such as the high steam pressure switches on a boiler, but not for pressure transmitters that are expected to accurately read steam pressure.

In applications where the switch or transmitter needs to be located above the steam source, other means must be employed to protect the sensor. The most common of these is the siphon loop or pigtail. The low-lying portion of the loop fills with condensate that protects the switch or transmitter from steam temperature. An alternative to the siphon loop is the diaphragm seal that passes the pressure in the sensing line onto the pressure sensor but insulates it from high steam temperatures.

The volume between the diaphragm seal is often filled with an in-compressible liquid like glycerin. Steam pressure switches and transmitters on firetube boilers are often located at the top of the boiler and use a siphon loop to protect the switches.

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Correct steam pressure transmitter installation from Preferred Utilities’ Catalog 25

Isolation (or maintenance) shutoff valves are usually included in the sensing lines of pressure transmitters. They are often not allowed with pressure switches. NFPA 85 boiler code does not allow isolation valves between the boiler and the high steam pressure switches. However, some jurisdictions allow an isolation valve to be installed if supplied with a means to physically lock the device in the open position.

Flow Meters

Differential pressure flow meters, whether the flow element is an orifice plate or V-cone, will have sensing lines to a differential pressure transmitter. For natural gas and other cool gases, the differential pressure transmitter is typically mounted above the pipeline. The sensing lines (or impulse lines) run down to the pipeline. If condensate is a concern, the sensing lines are continuously sloped downward so that condensate does not flow down into the transmitter. Just as flow meters require a certain number of upstream and downstream pipe diameters to read accurately, the sensing lines must be installed according to the manufacturer’s instruction to ensure an error isn’t introduce by the sensing line. Bernoulli’s Law dictates the square relationship between differential pressure and flow. Most differential pressure transmitters have a square root function embedded in their electronics. The controller or indicator of the flow transmitter connected may have a square root function as well. It’s important to set up the devices so that the square root of the differential pressure is only taken at one device.

Steam flow meters are a special case because the transmitter needs to be protected from high-pressure steam temperatures. Typically, the differential pressure transmitter is installed below the pipeline so the transmitter is protected by a water leg. But, differences in the height of the water in the water legs can introduce a source of error in the differential pressure and flow reading. So condensate pots are installed as close as practical to the steam, and sensing lines are connected to the bottom of the condensate pots. Condensate pots simply increase the area of the sensing line at the top so that small differences in the amount of water from the high-pressure line to the low-pressure line cause an insignificant difference in hydrostatic pressure. Condensate pots must be piped close to the steam source so that condensing steam will keep the pots full. Condensate pots used on boilers once were spherical. Now most boiler manufactures use an over sized tee fitting as a condensate pot.

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Correct flowmeter installation from Preferred Utilities’ Catalog 25

Drip legs are often installed on steam flow transmitters to blow scale out of the sensing lines. The flow transmitter will not read accurately until the sensing lines fill up with condensate again to the midpoint of the condensate pot. Most differential pressure transmitters used for flow measurement will come equipped with a three-valve manifold. Often the three-valve manifold flanges directly to the differential pressure transmitter. The three-valve manifold allows the instrument technician to isolate the transmitter from the steam source and to check the zero on the transmitter. Often taps are included on the three-valve manifold to facilitate calibration. Care must be taken to only close the equalizing valve after the two isolation valves are closed. Otherwise, the transmitter will see process pressure on one side of the diaphragm that could damage the transmitter.

Drum Level Transmitters

Drum level transmitters are piped similarly to steam flow transmitters and function very similarly to steam flow transmitters. A steam flow transmitter senses steam pressure on the low-pressure (downstream) tap and steam pressure plus pressure drop across the flow element on the high-pressure (upstream) tap. A drum level transmitter senses boiler steam pressure plus a full water leg on the high-pressure tap of the transmitter, and steam pressure plus a variable height water leg on the low-pressure tap. A condensate pot is installed at the top of the high-pressure leg to maintain the water level at a constant height on high-pressure tap of the transmitter. Again, a three-valve manifold needs to be installed for calibration and zeroing of the transmitter.

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Correct steam flowmeter installation from Preferred Utilities’ Catalog 25

When starting up a boiler from a cold start, the technician must open the fill cap and manually fill the condensate pot for the transmitter to read correctly. This must be done when there is no pressure on the boiler and the vent valve is open. When the boiler starts producing steam, condensate will continuously form in the cooler condensate pot and run into the steam header to maintain a constant-height water column.

Drip legs are usually installed in the impulse piping of drum level transmitters for cleaning as well. Like a steam flow meter, a drum level transmitter won’t read accurately for a while after blowing out the sensing lines. Sensing lines, also referred to as impulse lines, for steam flow transmitters and drum level transmitter must be continuously sloped downward so that air pockets cannot develop. Any up-and-overs-in the sensing lines can cause a potential air gap that will cause an error in the measurement. Flow meters or drum level transmitters that quit reading correctly, or respond sluggishly, could have clogged sensing lines. Steam and water lines accumulate a black sludgy mixture of rust and scale that will eventually clog sensing lines. Usually applying high pressure steam to just one side will clear the blockage. Properly constructed sensing lines will include plugged tees and crosses that serve as clean-out ports. Occasionally these clean-out plugs will have to be removed and the pipe attacked with pipe cleaning brushes.

Draft Transmitters

When we apply the question, “What is supposed to be in this sensing line under normal conditions?” to a draft pressure transmitter, the answer is nothing, or flue gas. The problem with flue gas is it can be nearly saturated with water vapor that will condense in cool sensing lines. Transmitters that do not need recurring maintenance or setup like the Preferred JC-22-XMTR should be mounted above the tap in the stack. The sensing line should be continuously sloped down to the tap so that condensate runs back into the stack and can’t block the sensing line. Draft transmitters typically read less than an inch of water column, so even a small water droplet in the sensing line can cause the transmitter to read inaccurately.

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Correct drum level installation from Preferred Utilities’ Catalog 25

Direct sensing draft controllers like the Cleveland Draft Regulator or the Preferred JC-22-PL2-1006 are mounted at, or below the stack damper they are linked to. Both draft controllers need to be accessible to technicians, so the sensing line is often run down to the draft controller. To avoid the draft transmitter in these controllers filling up with condensate, a long condensate leg is installed with a drain valve at the bottom. The sensing line tees off this vertical line and is run up to the draft controller so that any condensation in the line to the transmitter runs down into the dip leg. The drain valve needs to be opened periodically to drain the condensate out of the entire line. This can be done while the burner is off, or the draft controller control loop can be put into manual temporarily while the sensing line is drained.

When installing instruments with sensing lines, the manufacturer’s installation instructions should incorporate the practices recommended above. You can also consult the engineering section of Preferred’s Catalog 25. Pages 342 to 348 include installation instructions for generic pressure transmitters, flow meters, and special instructions for steam devices including steam flow meters and drum level transmitters.

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Correct draft transmitter install from Preferred Utilities’ Catalog 25

If you are trouble-shooting a piece of instrumentation that is giving suspect readings, or is occasionally giving inaccurate readings, inspect the sensing lines to make sure they were installed correctly, are clear of obstructions, and are filled with the fluids they are supposed to contain. Ask yourself, “What is supposed to be in this sensing line under normal conditions?” The answer will help you troubleshoot a part of common instrumentation that is often overlooked.

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