Rosemount 1151 to 3051: Obsolete Transmitter Migration

The Rosemount 1151 is one of the most widely installed pressure transmitters ever built, and it is obsolete. Plants across power generation, refining, water treatment and petrochemicals still run thousands of them, and every failure raises the same question: replace like-for-like from remaining stock, or migrate to the 3051. This guide covers what the migration actually involves mechanically, electrically and in terms of documentation, and how to specify the replacement so it drops into the existing impulse piping and control loop without a re-engineering exercise.

What you have: reading a 1151 model string

A 1151 model number reads as a concatenated option string - something in the shape of 1151DP4E22B1. Each block is a decision the original engineer made, and each has to be carried across or consciously changed.

• DP - differential pressure. Also GP (gauge), AP (absolute), HP (high line pressure DP), LT (level transmitter with flanged connection), DR/GR (draft range, very low DP).
• The digit after the measurement type is the range code. This sets the calibrated span limits and is the number that most often needs checking against the actual process range rather than copied blindly.
• The next character is the output code - E for 4-20 mA analogue being overwhelmingly the most common installed variant.
• Following blocks encode housing material, process connection, diaphragm and fill-fluid material, and options such as hazardous-area certification and integral indicator.

The single most important field to confirm before you order anything is the wetted material - the isolating diaphragm and the fill fluid. 316L stainless with silicone fill is the general-purpose default. Hastelloy, Monel, tantalum and gold-plated diaphragms exist because a previous engineer found the default failed in that service. Inert (halocarbon) fill exists because the service is oxygen or another application where silicone is a hazard. Carrying the wrong material across is how a transmitter fails in weeks.

Why the 3051 is the migration target

The 3051 is the current-generation successor and shares the fundamental capacitance sensing approach, the same 4-20 mA two-wire loop architecture, and - critically - a compatible process connection geometry through the Coplanar and traditional flange options. That combination is what makes migration a swap rather than a redesign. Practical differences worth knowing:

The rangeability difference is the one with the biggest practical consequence. A 1151 range code chosen for a 6:1 turndown often forced the engineer to select a range that was a compromise. A 3051 of the correct range code can usually cover the same duty with margin, which means one spare covers more tag numbers - a genuine inventory reduction.

The mechanical question: does it fit the existing manifold

This is where migrations succeed or generate rework. The 1151 uses a traditional flange arrangement with a specific bolt pattern and process connection centre distance. The 3051 is offered in both a Coplanar flange and a traditional-flange configuration, and the traditional-flange option is what preserves compatibility with existing 1151 manifolds and impulse piping.

1. 01Identify the existing manifold - typically a three-valve or five-valve manifold bolted directly to the transmitter body.
2. 02If you intend to retain that manifold, specify the 3051 with the traditional flange option and confirm the process connection thread (1/4-18 NPT or 1/2-14 NPT) and the flange bolt spacing against your existing installation.
3. 03If you are willing to replace the manifold, the Coplanar arrangement is more compact and gives a cleaner installation, but you are now ordering a manifold as well and the impulse tubing may need re-forming.
4. 04Confirm bolt material. Standard carbon-steel bolting is not acceptable in many offshore, sour-service or coastal installations where the original specified 316 stainless or B7M.
5. 05Check the vent/drain position - top or bottom - matches the service. Getting this wrong on a gas service leaves a liquid trap in the impulse leg and the reading drifts.

The electrical and control-system question

A like-for-like 4-20 mA replacement is electrically transparent: same two-wire loop, same 24 V DC loop supply, same load resistance limits within normal design margins. The migration issues are elsewhere.

• HART revision. If your handheld or asset-management system is configured for an older HART revision, a newer transmitter may need its HART revision setting adjusted to communicate. This is a configuration change, not a hardware incompatibility, but it will look like a dead device until someone realises.
• Device description files. Asset-management systems (AMS, PRM, PDM) need the correct DD/EDDL or DTM installed for the new device type. Order this into the migration plan; do not discover it at commissioning.
• Loop calibration records. The new transmitter has different reference accuracy and different range limits. The loop calibration certificate, the instrument index and the P&ID datasheet all need updating. In a regulated or SIL-assessed plant this is mandatory, not housekeeping.
• Failure-mode direction. Confirm the alarm direction (fail-high or fail-low) on the new unit matches the control system's expectation and the safety philosophy. A transmitter that fails low into a system expecting fail-high will not raise the alarm it was relied on to raise.

Hazardous-area certification: carry it across exactly

If the existing transmitter carries an explosion-protection certification, the replacement must carry an equivalent one, and "equivalent" is defined by the site's hazardous area classification drawings, not by what looks similar. Intrinsically safe (Ex ia) installations depend on a certified barrier and a documented entity-parameter match between barrier and field device - swapping in a flameproof (Ex d) device does not preserve the protection concept. Confirm gas group, temperature class and the certification scheme (ATEX, IECEx, or the relevant national scheme in the destination country) before ordering, and keep the certificate with the loop file.

A practical migration sequence

1. 01Pull the existing model string and serial from the transmitter tag plate, and photograph both the plate and the installation including the manifold and impulse legs.
2. 02Retrieve the loop datasheet - process range, design pressure and temperature, fluid, wetted material requirement, hazardous area classification, and the control system tag.
3. 03Confirm the actual operating range against the calibrated range. Migrations are the moment to correct a range that was wrong for twenty years.
4. 04Specify the 3051 with traditional flange if retaining the manifold, matching wetted material, matching process connection, matching certification, and the appropriate range code with margin.
5. 05Order the DD/DTM files and confirm the asset-management system version before delivery.
6. 06Bench-calibrate against a traceable reference before installation and issue a calibration certificate.
7. 07Install, leak-test the impulse legs, perform a loop check to the control system, and update the instrument index, loop folder and P&ID datasheet.

When to keep the 1151 instead

There are legitimate reasons to source a remaining 1151 rather than migrate: a single failure in a plant with a stock of matched spares and a validated calibration procedure; a SIL-assessed loop where changing the device requires re-doing the safety verification and there is no window to do it; or a unit due for decommissioning within a short horizon where the migration effort will never be recovered. Where that is the case, the constraint is availability of genuine units, and the risk is that the "new old stock" offered on the open market is refurbished, re-labelled or counterfeit. We supply new and genuine OEM only and will decline rather than fill from that channel.

What to send with a transmitter enquiry

1. 01The full existing model string, photographed from the tag plate.
2. 02Measurement type and the actual process range with units, not just the range code.
3. 03Wetted material and fill fluid requirement, and the process fluid.
4. 04Process connection and whether an existing manifold is being retained.
5. 05Output protocol required and the control system it reports to.
6. 06Hazardous area classification, gas group and temperature class, with the certification scheme required in the destination country.
7. 07Quantity, tag numbers, and whether calibration certificates are required per unit.

Impulse lines: where measurement errors actually originate

Most differential-pressure measurement problems are not transmitter problems. They are impulse line problems, and replacing the transmitter without addressing them simply moves a good instrument onto a bad installation.

• On gas service, impulse lines must slope upward from the tapping to the transmitter so condensate drains back into the process. A liquid slug in one leg produces a fixed offset that looks like a calibration error.
• On liquid service, lines slope downward so gas bubbles rise back into the process. A gas pocket produces an unstable, noisy reading.
• On steam service, condensate pots must be at the same elevation on both legs, and both legs must be filled and equalised before the transmitter is put into service. Unequal fill is the classic cause of a level or flow reading that is wrong by a constant amount.
• Freezing is a real failure mode in cold climates and requires heat tracing or a remote-seal solution.
• Plugging on slurry and crystallising service is addressed by purge, flushing rings or remote seals - not by a better transmitter.

A migration is the right moment to inspect and, where necessary, re-route impulse lines. The instrument is off the line, the fitter is present, and the cost of doing it then is a fraction of doing it as a separate job later.

Remote seals and capillary systems

Where the process is hot, corrosive, viscous, crystallising or must not be allowed into impulse lines, a diaphragm seal with capillary connection isolates the transmitter. This solves a set of problems and introduces another set that must be specified deliberately.

1. 01Fill fluid must suit the process temperature range. Silicone fills have defined limits; high-temperature and inert fills exist for specific duties.
2. 02Capillary length affects response time and temperature-induced error. Keep capillaries as short as the installation allows and equal on both legs of a differential system.
3. 03Ambient temperature swings act on the fill fluid and produce a measurable error. On long capillaries in an outdoor installation this can be significant, and it is calculable in advance.
4. 04The seal diaphragm material must suit the process fluid, exactly as the direct-mount diaphragm would.
5. 05Head effect from the vertical separation between seals must be compensated in the calibration.

Calibration, verification and the records that must follow

A replacement transmitter should arrive with a manufacturer calibration certificate, and it should be verified before installation against a traceable reference. Verification is not recalibration - it confirms the instrument reads correctly at defined points across the span, and it produces a record that closes the loop between what was ordered and what was installed.

• Check zero and span at minimum, and preferably at 0, 25, 50, 75 and 100% of span, rising and falling to reveal hysteresis.
• Record the as-found and as-left values. On a replacement the as-found is the factory condition.
• Confirm the configured range matches the intended range and that the units are correct - a transmitter configured in kPa reporting to a system expecting mbar produces a plausible-looking wrong answer.
• Confirm the damping setting. A damping value carried across from a noisy old installation can mask a genuine process response.
• Update the instrument index, the loop folder and the calibration schedule.

In a plant operating under a management system or with SIL-assessed loops, these records are the evidence that the change was controlled. Their absence is a finding regardless of whether the instrument works.