Selecting a Molten Sulphur Flow Meter for a 6-Inch Line

On the panel, the fault usually appears as an unstable flow value, a low-flow dropout, or a reading that no longer agrees with the process balance. For molten sulphur at 150 °C in a 6-inch steam-jacketed line, start with a jacketed or heavily heat-traced vortex meter when cost and control measurement drive the decision; select a jacketed Coriolis meter when direct mass flow and higher accuracy justify the larger installation cost. Thermal continuity comes first: if sulphur cools or freezes in the meter, changing transmitters or tuning filters will not fix the measurement.

Read the symptom before selecting the meter

Start here. Compare the displayed behavior with line temperature, flow demand, valve position, and upstream/downstream pressure. Separate a hydraulic or thermal problem from an instrument problem before replacing hardware.

A stable temperature indication at one point does not prove that the complete meter body is hot. Flanges, impulse connections, insertion fittings, and unjacketed transitions can form local cold spots.

Understand what creates the bad reading

A vortex meter infers volumetric flow from the frequency of vortices shed by a bluff body. Its usable low end depends on flow velocity, fluid properties, meter geometry, and Reynolds number. When the flow falls outside the linear operating region, the displayed value can lose accuracy or drop out even though material still moves through the pipe.

A multivariable vortex design can calculate Reynolds number and compensate for low-flow nonlinearity. It may also use density information to calculate mass flow. This can preserve a larger line size where reducing the pipe solely to raise velocity would add pressure loss or create another jacketed transition.

A Coriolis meter measures mass flow directly and can also provide density. That makes it attractive when the required result is mass rather than volume, especially for material balance or custody-transfer evaluation. Coriolis meters have been used successfully on molten sulphur in smaller lines with a bolt-on steam jacket and heat-transfer mastic between the jacket and meter body.

Meter technology does not remove the thermal requirement. Cooling changes the fluid condition and can build a restriction inside or beside the sensor. A partially cooled meter may show unstable zero, excessive pressure loss, or an apparent calibration shift before it becomes fully blocked.

Select the measurement principle

Define the measurement duty before requesting a quotation. Do not select from nominal pipe size alone.

• Use vortex for flow control: It offers a lower-cost path than Coriolis and has field experience in liquid sulphur service. One cited capability is accuracy up to 0.5%, but verify the proposed meter's stated accuracy across the actual Reynolds-number and flow range.
• Use multivariable vortex for a wide turndown: Consider it when low-flow nonlinearity is the main risk or when reducing the 6-inch line is undesirable.
• Use Coriolis for direct mass flow: Favor it when mass accuracy, density measurement, or custody-transfer evaluation controls the project. Confirm that the selected size can meet pressure-loss, heating, and mechanical installation limits.
• Evaluate insertion multivariable vortex for the large line: It may cost less than a full-bore Coriolis meter at this size. Review insertion hardware, sealing, removal method, velocity profile, and tracing around the fitting.
• Treat differential-pressure options as a separate design: An averaging-pitot arrangement has worked in other hot process measurement, but that experience does not establish suitability for molten sulphur. Check tapping heat, plugging risk, density compensation, and required range before using it.

Give the supplier minimum, normal, and maximum flow; operating pressure; the 150 °C process temperature; expected density and viscosity range; required units; allowable pressure loss; accuracy target; and whether the result must be mass or volume. Ask for the calculation at minimum flow, not just at the design maximum.

Install the selected meter without cold spots

1. Confirm that every wetted component, seal, lining, and sensor arrangement is compatible with molten sulphur at the stated operating conditions. Obtain the limits from the proposed meter documentation.
2. Review the manufacturer's orientation and straight-run requirements. Keep elbows, reducers, valves, and other profile disturbances outside the required distances.
3. Specify a steam jacket or heavy heat tracing for the meter body. For a bolt-on jacket, use the approved heat-transfer compound or mastic between the jacket and body where the design calls for it.
4. Extend heating across mating flanges, reducers, insertion fittings, and adjacent pipe. Insulate the complete assembly after checking the steam and tracing paths.
5. Place isolation and control valves so they do not create an unacceptable distorted velocity profile at the meter. Keep a throttling valve away from the meter by the distance required for that design.
6. Wire the transmitter, configure engineering units, and enter only the fluid and pipe data required by that meter. For calculated mass flow, check the density source and its units.
7. Heat the assembly before admitting sulphur. Confirm steam flow or tracing operation at the meter body and each transition, then introduce flow using the plant operating procedure.

Do not rely on ambient insulation as the heat source. Insulation reduces heat loss; it does not restore heat removed during a shutdown or maintain a cold flange without active heating.

Verify the range and the installed result

1. At zero process flow and stable operating temperature, inspect the raw signal and transmitter status. Record any vibration-related zero activity rather than masking it immediately with damping or a cutoff.
2. Run at minimum expected flow. For vortex, confirm that the operating point remains inside the supplier's usable Reynolds-number and velocity range.
3. Increase flow through normal and maximum operating points. Record indicated flow, valve position, pressures, meter temperature, and any diagnostic flags at each point.
4. Compare the totalized result with an independent material balance over a period long enough to reduce the effect of level resolution and process transients.
5. Interrupt neither steam nor tracing during the test. A thermal disturbance tests the heating system, not the meter's normal accuracy.
6. Verify that scaling matches the control system: mass versus volume, time base, totalizer multiplier, decimal placement, and density compensation must agree.

Acceptance must cover the whole operating envelope. A good reading at normal flow does not prove low-flow performance, and a stable display does not prove accuracy if a large damping value hides dropout.

Avoid fixes that waste time

• Do not resize the transmitter range before proving that the primary meter produces a valid signal at low flow.
• Do not add damping to hide unstable shedding, pipe vibration, two-phase flow, or partial solidification.
• Do not reduce the pipe automatically. A reducer may improve vortex velocity, but it also adds pressure loss, thermal transitions, and more locations that require heating.
• Do not select Coriolis solely because it is called more accurate. Check the proposed size, pressure loss, heating arrangement, installation geometry, and required mass-flow uncertainty.
• Do not use a nominal 0.5% vortex figure as the installed-system accuracy. Range position, Reynolds number, piping effects, fluid condition, transmitter configuration, and verification method all contribute.
• Do not leave insertion hardware or flange necks outside the heated envelope. Small unheated metal sections can become the first restriction point.

FAQ

Stop commissioning if the meter cannot remain heated, pressure loss rises, diagnostics persist, or the indication fails the minimum-to-maximum flow check. Escalate to the meter manufacturer's official application or technical-support channel with the sizing data, piping layout, heating arrangement, configuration record, diagnostics, and test results.