The short answer
Pressure Transmitter Installation
A pressure transmitter reads correctly when the tap is on the side of a liquid line where neither air nor sediment collects, away from turbulence, with an isolation valve at the tap and a bleed at the transmitter, an impulse line that slopes so air returns to the process, the transmitter mounted below the tap on liquid service so the line stays full, the elevation difference between the tap and the sensor corrected in the zero, pulsation from pumps damped, the line and the transmitter protected from freezing, a diaphragm seal where the liquid would plug or attack the line, and the transmitter mounted where vibration and sun do not reach it. Most calibration complaints on pressure transmitters are installation faults, and most are visible from the floor.
Key points
- Tap on the side of a liquid line; the top collects air and the bottom collects solids.
- Transmitter below the tap on liquid service with the line sloping up to the tap, so air goes back to the process.
- Isolation at the tap, bleed at the transmitter: the two valves that make calibration and replacement possible.
- Elevation between the tap and the sensor is a fixed offset; correct it in the zero and write it on the loop sheet.
- Pulsation is damped by a snubber or damping, not by a filter that hides a real pressure swing.
- Freeze, plug, and chemical attack are prevented by seals, heat trace, and fill fluids chosen at design.
The tap
| Service | Tap position | Transmitter position | Reason |
|---|---|---|---|
| Liquid, clean | Side of the pipe, 3 or 9 o clock | Below the tap | Air stays out of the line, solids stay out of the tap, line stays full |
| Liquid with solids | Side of the pipe, slightly above center | Below the tap, or a seal at the tap | Solids settle away from the tap; seals for sludge |
| Gas or air | Top of the pipe | Above the tap | Condensate drains back to the process |
| Steam | Side | Below, with a condensate pot or pigtail | Water leg protects the sensor from heat |
| Tank level | Bottom of the tank, above the sediment zone | At or below the tap | Head measurement; elevation correction |
Position along the pipe matters less than for flow, but a tap directly on a pump discharge nozzle, inside an elbow, or just downstream of a throttling valve reads turbulence and velocity effects rather than static pressure. A few diameters of straight pipe from those features is enough. The tap itself is flush with the inside of the pipe, deburred, and not protruding into the flow.
Valves and lines
- Isolation valve at the tap, a root valve, so the transmitter can be removed with the process running.
- A bleed or vent valve at the transmitter, or a two-valve manifold that provides both, so the line can be vented of air and the transmitter zeroed at atmosphere.
- Impulse line short, of adequate diameter to avoid plugging, and sloped at least an inch per foot: upward toward the tap on liquid service so air rises to the process, downward toward the tap on gas service so liquid drains to it.
- No loops or traps that hold air or liquid where it is not wanted.
- A gauge tee at the transmitter so a test gauge can be fitted for a check without disconnecting anything.
- A calibration port or a manifold that allows a pump to be connected with the process isolated.
Elevation
A transmitter mounted below its tap on liquid service sees the process pressure plus the head of liquid in the impulse line; above the tap, minus that head. The offset is constant and equals the vertical distance times the liquid density, about 0.433 psi per foot for water. It is removed by zeroing the transmitter with the isolation valve closed and the line full, or by entering the offset as a zero correction. Write the elevation on the loop sheet, because a transmitter later moved to a new bracket takes its old zero with it.
Offset = h × SG × 0.433 psi per foot
- h = vertical distance from the tap to the sensor in feet, positive when the sensor is below the tap
- SG = specific gravity of the liquid in the impulse line
Pulsation and vibration
Positive displacement pumps and some centrifugal pumps produce a pressure that oscillates at the stroke or vane frequency. The transmitter follows it and the reading is a blur. A snubber, a small orifice or a porous element in the line, or a pulsation dampener at the tap smooths the pressure at the sensor. Transmitter damping does the same electronically and is fine where the swing is small; it should not be used to hide a swing large enough to matter to the pump. Mechanical vibration from the pump or the pipe is handled by mounting the transmitter on a bracket or a stand rather than directly on the tap, connected by a short flexible line.
Environment
- Freezing: an impulse line full of water freezes and either breaks or holds a false pressure. Heat trace and insulate, or use a diaphragm seal with a fill fluid rated for the temperature, or mount the transmitter directly at the tap inside a heated enclosure.
- Sun: a transmitter in direct sun sees temperature swings that shift the zero and shorten the life of the electronics and the display. A sunshade is cheap.
- Flooding: in a vault, above the flood line, with the conduit sealed and a drain for the conduit.
- Corrosion: wetted parts compatible with the liquid; seals for hypochlorite, ferric, and other chemicals; the manufacturer material table decides.
- Access: readable display, reachable terminals, a place to stand. A transmitter that needs a ladder is a transmitter that is not checked.
Diaphragm seals
A seal puts a diaphragm at the tap and fills the space between it and the sensor with a fluid, so the process never enters the line. It is the answer for sludge that plugs, chemicals that attack, liquids that freeze or solidify, and taps far from where the transmitter can be mounted. The cost is temperature sensitivity of the fill fluid and a slower response; both are managed by short capillaries, matched lengths on differential applications, and shading. A seal that has lost its fill fluid reads slow and low; the seal, not the transmitter, is what gets replaced.
Frequently asked questions
- Can the transmitter mount directly on the tap without an impulse line?
- Yes, and it is the simplest installation on clean water where the tap position is accessible, with a root valve and a bleed. Add a bracket if the pipe vibrates and a sunshade if it is outdoors. Direct mounting removes the elevation offset and most of the line problems.
- Why does the reading drop for a few seconds when a pump starts?
- It may be real: a suction pressure or a discharge pressure transient at start. If the swing is larger and longer than the process explains, air in the impulse line compresses and expands with the pressure and exaggerates the transient; bleed the line.
- How long can an impulse line be?
- As short as the installation allows. Every foot adds a chance of plugging, freezing, and trapped air, and slows the response. A few feet is normal; tens of feet is a reason to consider a seal with a capillary or remote mounting of a transmitter at the tap with the readout elsewhere.
- The transmitter is rated for the ambient temperature. Does it still need shade?
- The rating is for survival and stated accuracy under conditions that a sunlit surface routinely exceeds; a dark enclosure in the sun can be 40 degrees above ambient. A sunshade keeps the transmitter within its accurate range and extends the life of the display and the electronics.
Related topics
- Pressure TransmittersGauge, absolute, and differential pressure transmitters: how they sense, how range, turndown, and accuracy specifications work, the process connection and its accessories, and the installation errors that show up as calibration problems.
- Impulse LinesThe tubing between the process tap and the pressure or DP transmitter, and everything that goes wrong in it: slope and routing for liquid and gas, elevation head and how to correct for it, air and condensate, blocking and manifold valves, freezing, plugging on dirty service, and the diaphragm seal alternative.
- Pressure CalibrationWhat calibrating a pressure transmitter means: as-found and as-left, the five-point test and hysteresis, tolerance and the reference standard, sensor trim versus output trim versus reranging on a smart transmitter, gauge versus absolute, and intervals.
- Differential PressureThe differential pressure transmitter as an instrument: what it measures, static and overrange ratings, the three-valve manifold and the order of operations that protects the cell, and the applications from filter head loss to membrane TMP, flow, and level.
- How to Calibrate a Pressure TransmitterA five-point calibration with a pressure source and a reference: isolate and vent, record as-found, decide whether to adjust, trim the sensor and the output separately, verify as-left, and record it so the next calibration means something.
- Surge Protection for Signal CircuitsProtecting 4-20 mA loops, discrete inputs, network cables, and radio feeds from lightning and switching surges: where surges enter, the device types and how they clamp, the grounding that makes them work, the resistance and capacitance they add, and where to put them at a remote site.
Direct contact
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