Selecting Orifice Plate Tapping Points and Impulse Tubing

Before anything else, confirm the tapping arrangement named on the orifice-plate calculation sheet. Tap geometry is part of the differential-pressure calculation; it is not an interchangeable piping detail. Select impulse tubing only after the process service, pressure, temperature, material class, run length, and required response are defined.

Measurement-Basis Confirmation

1. Record the primary-element type, pipe inside diameter, orifice bore, flow direction, fluid phase, operating pressure, operating temperature, and transmitter differential-pressure range.
2. Read the tap designation from the flow calculation, plate datasheet, or approved hook-up drawing. Do not infer it from an existing flange.
3. Confirm that the calculation and mechanical drawing use the same tap convention. A calculation based on flange taps cannot be transferred to vena-contracta, corner, radius, or pipe taps without rechecking the discharge coefficient and predicted differential pressure.
4. Resolve any conflict through the project instrument specification and the approved primary-element calculation before releasing fabrication drawings.

Do not move on until the datasheet, calculation, and hook-up drawing identify one tapping arrangement with the same pipe and plate dimensions.

Tapping-Arrangement Selection

The deciding document is the approved flow calculation. Do not move on until the tap name and physical dimensions agree on every deliverable.

Tap Orientation and Impulse-Line Routing

1. Place both process connections where they sense representative static pressure rather than a deposit, vapor pocket, or condensate pocket.
2. For liquid service, route both legs so they remain liquid-filled and prevent trapped gas. Locate the transmitter and vents according to the approved liquid-service hook-up.
3. For gas service, route the legs to prevent retained condensate and provide drainage where the project detail requires it.
4. For condensing-vapor or steam service, establish equal liquid heads in both legs using the project condensate arrangement. Unequal elevations or fill densities create a false differential pressure.
5. Keep the HP and LP routes together where practical. Similar length, elevation profile, exposure, and insulation reduce temperature-driven density differences.
6. Provide a continuous slope, accessible isolation, vent or drain points, mechanical support, and protection from vibration. Avoid unventable high points and undrainable low points.

Pressure at the transmitter equals tap pressure plus or minus the hydrostatic head of each impulse leg. Any unequal elevation, density, trapped phase, blockage, or leak therefore appears as measurement bias. Do not move on until both routes can be filled, vented or drained, inspected, and isolated without trapping the wrong phase.

Impulse-Tubing Size Selection

6 mm OD is metric tubing, not imperial tubing. It is also not a universal offshore standard. The governing project piping and instrumentation specification must define the accepted tube series, material, wall thickness, fittings, pressure-temperature capability, corrosion allowance, fire requirements, and support practice.

1. Obtain the proposed tube wall thickness and calculate or read its inside diameter. Outside diameter alone does not determine flow resistance or plugging margin.
2. Check the tube and fitting pressure-temperature rating against the design conditions and selected material class.
3. Review fluid cleanliness, viscosity, phase-change risk, ambient exposure, line length, elevation change, and required measurement response.
4. Select 10 mm OD only when the project specification or service analysis calls for the larger bore or mechanical format. Do not use it merely because the run appears long.
5. Document one tube size and fitting series across the complete hook-up; mixed metric and imperial components can assemble incorrectly and leak.

Do not move on until the tubing schedule records outside diameter, wall thickness, material, fitting series, design conditions, and the service reason for the selection.

Manifold Connection and Initial Commissioning

1. Trace the upstream tap to the transmitter HP port and the downstream tap to the LP port. Mark both ends before connection.
2. Inspect the orifice plate flow arrow or bore orientation, gasket position, tap passages, tube supports, and manifold port labels.
3. Pressure-test each completed impulse path using the approved test medium and project test limit. Repair leakage before exposing the transmitter to process pressure.
4. Fill, vent, drain, or purge each leg for the measured phase. Repeat until no trapped gas remains in liquid service and no retained liquid remains in gas service.
5. With the transmitter isolated from differential pressure and both sides equalized through the approved manifold procedure, check zero. Apply any permitted zero adjustment only after the impulse legs have reached their intended condition.
6. Place the manifold into service using the transmitter manufacturer’s and site’s valve sequence. A wrong sequence can impose an unintended one-sided differential pressure.

Do not move on until the assembly holds pressure, the phase condition in both legs is correct, the manifold is in its documented operating lineup, and the transmitter reads zero under equalized pressure.

End-to-End Flow Verification

1. At no-flow or equalized conditions, record transmitter differential pressure, indicated flow, and output. Investigate a persistent offset as unequal head, trapped phase, leakage, plugged passage, or zero error.
2. Introduce stable process flow and confirm that differential pressure rises with flow in the correct direction. A negative value usually points to reversed impulse lines, reversed plate orientation, or an incorrect flow-direction assumption.
3. Compare the observed differential pressure with the approved flow calculation at the actual operating condition. Correct pressure and temperature compensation inputs where the calculation requires them.
4. Check the complete signal path from transmitter through the control system to the operator display. Confirm engineering units, square-root handling, range, alarm direction, and totalizer behavior where those functions are configured.
5. Repeat the zero or equalization check after venting or draining once more. Stable return values confirm that the impulse system is not retaining a movable gas or liquid pocket.

Release the loop only after the physical tap geometry matches the calculation, positive flow produces positive differential pressure, the displayed flow tracks the calculated operating point, and the final equalized reading returns to zero.

FAQ