ABB 691HT

1. Product Overview

The ABB 691HT is a rugged, high-temperature pressure transmitter designed for demanding process applications where media temperatures exceed the limits of standard electronics. Unlike conventional transmitters that require remote mounting or cooling elements, the 691HT integrates a specially designed diaphragm fill fluid and thermal isolation system, allowing direct flange mounting on steam lines, superheated water systems, and hot oil processes. This model belongs to ABB’s 600T series, known for their long-term stability and resistance to vibration and humidity. The 691HT is the go-to choice when media temperatures reach up to 350°C (662°F) without the need for capillary cooling or heat sinks.

2. Key Features

• Direct process temperatures up to 350°C (662°F) – no cooling tower or remote seal required
• Hermetically sealed electronics housing (IP67 / NEMA 4X)
• HART® protocol 7.0 communication as standard; optional PROFIBUS PA and FOUNDATION Fieldbus
• ±0.15% span accuracy over a 10:1 turndown ratio
• Active temperature compensation every 50ms
• Double-conformal coated electronics board for corrosion resistance
• Local configuration via magnetic switches (no cover removal)
• Adjustable damping (0–60 seconds)

3. Technical Parameters (Bullet / List Format)

Pressure Ranges:

• Minimum span: 0–20 kPa (0–3 psi)
• Maximum span: 0–35 MPa (0–5000 psi)
• Static pressure limit: 41 MPa (6000 psi)

Performance Specifications:

• Reference accuracy: ±0.15% of calibrated span
• Long-term stability: ±0.1% of URL per year
• Thermal effect (per 28°C / 50°F): ±0.25% of span
• Response time: < 100 ms (without damping)

Environmental & Physical:

• Operating media temperature: –40°C to +350°C (–40°F to +662°F)
• Ambient temperature (electronics): –40°C to +85°C (–40°F to +185°F)
• Process connections: 1/2″ NPT female, 1/2″ BSP, or flanged (DN50 / DN80 / 2″ / 3″)
• Wetted materials: 316L SST, Hastelloy C-276 (option), Monel (option)
• Fill fluid: High-temperature silicone or inert fluorocarbon (for oxygen service)
• Enclosure rating: IP67 / NEMA 4X

Electrical:

• Supply voltage: 10.5–45 V DC (HART); 9–32 V DC (FF/PA)
• Output signal: 4–20 mA + HART (2-wire)
• Load capability: 250–1100 Ω @ 24V DC

4. Installation & Maintenance

Installation Guidelines:

• Mount the 691HT with the process connection pointing upward or horizontally; avoid inverted mounting if media contains particulates.
• Use a bypass manifold (3-valve or 5-valve) to allow zero trimming without process shutdown.
• For steam applications, install a siphon tube (pig tail) between the process tap and transmitter to absorb pulsation.
• Do not insulate the electronics housing; maintain at least 50mm of air gap around the housing.
• Ground the housing via the internal ground screw (not through conduit connection).

Maintenance:

• Perform zero trim every 12 months for critical loops (after isolating from process pressure).
• Inspect the diaphragm for pitting or corrosion during plant turnarounds.
• If output drifts non-linearly, replace the sensor module (field-replaceable without recalibrating the electronics).
• Never use high-pressure water or steam to clean the housing; use a damp cloth.

5. Structure & Composition

The ABB 691HT features a modular, two-chamber design that separates the process-wetted sensor from the electronics:

• Process Head (Hot Side): Machined 316L stainless steel, containing a flush or extended diaphragm. A welded capillary tube filled with high-temperature oil transmits pressure to the sensor cell.
• Thermal Isolation Neck: A 120mm long, finned stainless steel neck with conductive heat breaks. This section dissipates heat before it reaches the electronics housing.
• Sensor Module: Piezoresistive silicon sensor with an oil-filled isolation membrane. Temperature sensors embedded in the oil allow real-time thermal compensation.
• Electronics Housing: Epoxy-coated aluminum or 316L SST (optional). Contains the signal converter, HART modem, and terminal block. The cover has a tempered glass window for local display (optional LCD).
• Terminal Side: Dual conduit entries (1/2″ NPT or M20) with stainless steel plugs.

6. Application Fields

• Superheated steam flow measurement (with orifice plate or averaging pitot tube)
• Reactor pressure control in chemical batch processes (exothermic reactions)
• Heavy fuel oil line pressure – eliminates coking issues due to reduced heat loss
• Plastic extrusion melt pressure (with optional flush diaphragm)
• Pharmaceutical autoclave pressure monitoring (SIP/CIP capable)
• Geothermal wellhead pressure – handles scale-forming brines at high temperature
• Diesel engine test stands (exhaust gas backpressure)

7. Safety & Compliance

The ABB 691HT carries multiple certifications for use in hazardous and safety-critical environments:

• ATEX / IECEx: Ex db IIC T4..T6 Gb (flameproof); Ex tb IIIC T85°C..T135°C Db (dust)
• SIL 2/3 capable: Certified per IEC 61508 (hardware fault tolerance = 0, proof test interval up to 15 years)
• FM / CSA: Class I, Division 1 & 2, Groups B, C, D; Class II, Division 1 & 2, Groups E, F, G
• PED 2014/68/EU: Category III for pressure accessories
• NACE MR0175 / ISO 15156: Suitable for sour service (H2S) with optional Hastelloy wetted parts

8. Additional Information

• Ordering note: The 691HT is configurable with various diaphragm extensions (flush, extended 2″, extended 4″) – contact ABB with process details for optimal selection.
• Warranty: 24 months from installation or 30 months from shipment (whichever is earlier)
• MTBF (Mean Time Between Failures): >350 years per Siemens SN 29500
• Replaces legacy models: 691HP (obsolete), 691HS (discontinued)
• Software compatibility: Configure via ABB Asset Vision Basic, Emerson AMS, or PACTware with DTM

The ABB 691HT delivers uncompromising performance where standard pressure transmitters fail due to high process temperatures. Its direct-mount design reduces installation complexity and eliminates common failure points associated with capillary tubes and remote seals.