Pressure Sensors, Transmitters and Impulse Connections

Pressure Measurement

Pressure is defined as normal force exerted per unit area ($P = F / A$). In power engineering, pressure is monitored to prevent structural rupture of pressure vessels, supervise fluid movement, and indicate phase changes.

1. Elastic Sensing Elements

Mechanical pressure indicators convert fluid pressure into physical displacement using an elastic metallic sensing element:

• Bourdon Tubes:
  • C-Type Bourdon Tube: Consists of an oval or elliptical metal tube bent into an arc of approximately $250^\circ$. One end is anchored rigidly to the instrument casing where process fluid enters; the opposite "tip" is sealed and free to move. When internal fluid pressure rises, the oval cross-section attempts to become circular, causing the tube to straighten. This uncurling displaces the sealed tip. A mechanical linkage, geared quadrant (sector gear), pinion gear, and hairspring amplify this tip motion and rotate a pointer across a calibrated dial. The hairspring eliminates gear backlash and mechanical play.
  • Spiral Bourdon Tube: Formed by winding an oval tube into a flat spiral of several turns. Internal pressure produces a cumulative tip deflection proportional to the number of turns, generating greater mechanical movement than a C-tube without requiring high-ratio multiplying linkages.
  • Helical Bourdon Tube: Formed by winding the tube into a three-dimensional helix. It handles high pressures (up to $700\text{ MPa}$ / $100,000\text{ psi}$) and produces large rotational tip deflection capable of driving heavy potentiometers, pen recorders, or direct digital transmitters.
• Bellows Elements: A bellows is a one-piece, thin-walled, cylindrical metallic vessel with deep circumferential folds (corrugations). When pressurized internally, the bellows expands axially against an internal or external calibration spring. The opposing spring provides precise linear displacement proportional to pressure. Bellows are widely applied in low- to medium-pressure services ($0\text{ to }350\text{ kPa}$ / $0\text{ to }50\text{ psi}$) and differential pressure switches.
• Diaphragm Elements: Consist of flat or corrugated circular metallic discs (or flexible elastomers) clamped tightly between two flanges. Fluid pressure deflects the center of the disc. Diaphragms provide high surface area, making them sensitive to very low pressures, including furnace draft ($Pa$ or $\text{mm } H_2O$) and ventilation static pressures. Diaphragm seals are also used as chemical barriers to isolate corrosive, viscous, or slurry fluids from delicate transmitter electronics.

2. Electronic Pressure Transmitters

Modern plants transmit electronic signals ($4-20\text{ mA}$) to control systems using solid-state transducers:

• Piezoresistive Transmitters: Utilize a silicon diaphragm with integrated micro-machined semiconductor strain gauges arranged in a Wheatstone bridge. Fluid deflection induces mechanical stress in the silicon lattice, changing its electrical resistivity. This produces an electrical output directly proportional to applied pressure.
• Capacitive Pressure Transmitters: Feature a flexible metallic sensing diaphragm positioned midway between two rigid, stationary capacitor plates immersed in silicone dielectric oil. Process pressure applied to isolating diaphragms deflects the center sensing diaphragm, decreasing the distance to one capacitor plate while increasing the distance to the other. The resulting differential capacitance ($\Delta C$) is electronically converted into a highly stable $4-20\text{ mA}$ DC signal.

3. Piping Protection: Siphons & Manifolds

• Siphon Loops (Pigtails): Under ASME Section I rules, pressure gauges and electronic transmitters exposed to live steam must be protected by a water seal siphon (a circular pigtail or U-tube loop). Prior to opening the steam root valve, the siphon loop must be filled with cold water. When steam enters the loop, it condenses against the trapped water column. The water seal prevents live, superheated steam from directly entering the Bourdon tube or transmitter capsule, preventing loss of temper, thermal zero-shift, and catastrophic failure of the elastic element. In cold outdoor environments, siphons must be heat-traced to prevent freezing.
• Manifold Valve Blocks (3-Valve & 5-Valve Manifolds): Differential pressure (DP) transmitters are installed with multi-valve manifolds to permit zero calibration, flushing, and maintenance without plant shutdown:
  • 3-Valve Manifold: Contains two block (isolation) valves (High Pressure - HP, and Low Pressure - LP) and one central equalizing valve.
  • Manifold operation: Use the manufacturer’s sequence and the actual impulse-line configuration. Do not open an equalizer with both process blocks open merely because a generic checklist says so; it can connect process taps or disturb reference legs. Equalization does not itself establish depressurized isolation.
  • 5-Valve Manifold: Adds two vent/bleed valves between the block valves and the equalizing valve. This allows operators to bleed trapped process fluid to a safe drain and verify absolute zero shutoff before performing calibration.

Range, span and a pressure example

The lower range value is the process value assigned to the low output; the upper range value is assigned to the high output. Span is their difference. For a transmitter calibrated from 0 to 1,000 kPa with linear 4–20 mA output, 250 kPa corresponds to 25% span and 4 + 0.25 × 16 = 8 mA. A 12 mA signal corresponds to 50% span, or 500 kPa. A broken wire producing zero current is different from the valid zero-pressure signal of 4 mA.

Check whether the instrument measures gauge, absolute or differential pressure. A differential transmitter can have high static pressure on both sides while indicating a small difference. A zero differential reading therefore does not prove the device is safe to disconnect. Root valves, manifolds, vents and trapped cavities must be evaluated under the approved isolation procedure. Line plugging, a leaking impulse connection, frozen condensate or an incorrect reference-leg condition can produce a misleading measurement even when transmitter electronics are working.

Source notes

Official scope and technical references checked 2026-10-17: Reference 1; Reference 2. Archived DOE handbooks are used for physical principles, not current Canadian legal requirements.