Sizing a bypass loop for permanent DOCA sensor installation

A bypass loop gives the DOCA optical sensor a controlled sample of compressed air without forcing the full production flow through the instrument. This arrangement is useful when the main line operates at high pressure, contains valuable product gas, or cannot tolerate a restriction that could affect manufacturing.

Correct sizing begins with the sensor’s approved operating range, not the diameter of the plant header. The loop must deliver a representative sample in liquid, aerosol and vapour forms while maintaining adequate velocity, stable pressure and a predictable response time.

In Australian facilities, permanent monitoring is often added during a planned shutdown in Sydney, Melbourne, Brisbane or Adelaide. Metric pipework, kPa pressure readings, long service distances between regional sites and strict site permit systems all influence the practical design. Pharmaceutical and hospital installations may also need documentation that supports GMP, clean-room and infection-control requirements.

Start with the sampling objective

Identify where oil contamination could enter the compressed-air system and what the DOCA installation must detect. A point downstream of the compressor, receiver, dryer, coalescing filters or distribution header may show different contamination behaviour. If the objective is product protection, place the sampling point near the final use point rather than relying only on a compressor-room measurement.

The bypass should take a continuous, representative sample without creating a dead leg. Use a short take-off from the main line, preferably from the upper or side section where condensate will not collect. Avoid placing the inlet immediately after an elbow, valve or filter outlet if turbulence could make the sample unrepresentative.

Check whether the installation must monitor liquid carryover, aerosol droplets, vapour-phase hydrocarbons or all three. The answer affects tubing length, drainage, temperature control and the need for upstream separation. The DOCA project’s technical documentation and current sensor datasheet should define the accepted sample conditions.

Calculate flow, pressure and response time

Begin with the sensor’s required sample flow, inlet pressure and allowable pressure range. The bypass flow is the sensor flow plus any intentional bleed or purge flow. The take-off valve, regulator, filter, needle valve and exhaust path must pass this amount at the lowest expected header pressure, not only under normal operating conditions.

A practical sizing check is to calculate pressure loss through every component and compare the total with the available pressure margin. Small-bore tubing can create substantial friction loss, especially over long runs. If the sensor needs stable pressure, install a regulator with suitable capacity and verify its performance at the minimum and maximum plant pressures.

Response time depends on the internal volume of the loop and the actual sample flow. Approximate transport delay by dividing the volume of tubing and fittings by the sample flow rate. Keep the run short and use the smallest internal diameter that still meets the sensor flow requirement and contamination-control objectives. Excessively large tubing can delay alarms; excessively small tubing can restrict flow and increase blockage risk.

Build the loop around safe pressure control

A typical permanent arrangement includes an isolation valve at the header, a high-pressure-rated take-off, a particulate or coalescing element where required, a pressure regulator, a flow-control device, the DOCA sensor and a safe vent or return connection. The order depends on the sensor design and site risk assessment, so the manufacturer’s installation instructions take precedence.

Provide overpressure protection if a regulator failure could expose the sensor to the full header pressure. Components should be rated for the maximum design pressure, temperature and contamination environment. In Australia, the design should be reviewed against applicable state or territory Work Health and Safety requirements and relevant pressure-equipment standards, including AS 1210 or AS 4041 where applicable to the equipment and pipework.

Use isolation valves on both sides of serviceable components, and include a controlled depressurisation point. A bypass that cannot be safely isolated will complicate calibration, filter replacement and fault investigation during production.

Choose tubing and site the equipment

For high-purity air, select materials with low adsorption, low particle shedding and suitable resistance to oil and cleaning agents. Stainless steel tubing is often preferred for demanding pharmaceutical, electronics and clean-room environments, while approved polymer tubing may suit shorter, lower-risk runs. Confirm compatibility rather than treating all stainless grades, seals or elastomers as equivalent.

Keep the sensor away from vibration, direct sunlight, washdown spray and extreme temperature changes. In hot Australian plant rooms, summer heat can change regulator behaviour and sample conditions; in coastal locations, corrosion protection and enclosure selection deserve particular attention. A shaded, accessible mounting position improves inspection and reduces unnecessary tubing length.

Installation checks that protect sample quality

• Keep the sample line continuously sloped where condensate could form.
• Avoid unnecessary tees, pockets, flexible hoses and unheated dead legs.
• Label the sample direction, isolation valves and depressurisation points.
• Provide enough clearance to remove the sensor and service filters.
• Use fittings rated for the pressure and cleanliness class of the installation.

Verify performance before handover

Commissioning should confirm that the bypass reaches the required operating flow and pressure during low, normal and peak header conditions. Record readings at the take-off and at the DOCA inlet to identify excessive pressure drop. Check for leaks with an approved method and confirm that the vent location cannot expose workers or contaminate a clean area.

The alarm response should be tested using an approved reference or controlled challenge procedure. Avoid introducing oil into a production air system merely to prove the sensor. The test record should show sample-point identification, calibration status, pressure and flow readings, materials, component certificates and any deviations from the approved design.

Records worth retaining with the installation

• Header pressure range and calculated bypass-flow requirement.
• Tubing length, internal diameter and component pressure ratings.
• Regulator, relief-device and filter specifications.
• Sensor calibration, alarm settings and commissioning results.
• Maintenance intervals, isolation steps and responsible personnel.

A well-sized bypass loop delivers a stable, representative sample while preserving production-air performance. Use the DOCA sensor limits, the site risk assessment and applicable Australian requirements as the design basis, then have the completed installation reviewed by the responsible engineer and commissioning team before permanent operation.