How to Specify a Sample Conditioning System for a Process Analyser

At a glance. Specify a sample conditioning system from the process data and the response time the measurement needs. Take the sample from the centre third of the pipe, reduce gas pressure close to the tap, add a fast loop when the analyser is far away, keep the sample in one phase, filter without adding hold-up volume, and design stream switching, calibration and disposal in from the start. Then factory-test the system with the analyser in place.

Why does the sample system decide analyser accuracy?

An analyser can only measure what reaches it. A sample that arrives late, partly condensed, contaminated by the previous stream or stripped of its reactive components gives a precise but wrong answer. Doug Nordstrom and Tony Waters, quoted in Swagelok's guidance on analyser accuracy, name time delay as the most common cause of misleading results from process analysers.

The recognised practice is API RP 555, Process Analyzers, with IEC 61285 covering the safety of analyser houses. A good specification turns that practice into numbers the vendor can design to.

What process data does the vendor need?

Data item	Why it matters
Full stream composition, including water, H₂S, trace components and solids	Phase behaviour, materials and filtration
Normal, minimum and maximum pressure and temperature at the tap	Pressure reduction, heating or cooling, and pressure rating
Hydrocarbon and water dew points for gas, or bubble point for liquid	Heat tracing and vaporisation design
Analyser type, inlet conditions, sample flow and cycle time	Conditioning targets and response time
Required total response time	Fast loop and line sizing
Distance and elevation from tap to analyser	Lag time and pressure drop
Return or disposal points and their pressures	Fast-loop driving force and safe disposal
Number of streams, and calibration and validation gases	Stream switching and calibration design
Area classification, ambient range and utilities	Enclosure, heat tracing and electrical design
Materials and sour service requirements	NACE MR0175 / ISO 15156 compliance

How fast does the sample need to reach the analyser?

Total response time is the sum of the probe, transport line, conditioning components and the analyser's own cycle. ASTM D7278 is the standard guide for predicting analyser sample system lag times. Swagelok's guidance gives one minute as the industry norm for response, while noting that a gas chromatograph alone takes five to ten minutes per analysis and that the process engineer sets the acceptable limit.

For gas, the transport delay is proportional to absolute line pressure. Take 50 m of 1/4 in tubing with a 0.035 in wall (4.57 mm bore). It holds about 0.82 L. At 10 barg, roughly 11 bar absolute, that is about 9 standard litres of gas, so a flow of 1 standard litre per minute gives a delay of about 9 minutes. Reduce the pressure to 1 barg at a field station next to the tap and the same line holds about 1.6 standard litres: a delay of about 1.6 minutes. As Swagelok puts it, "At half the pressure, you will get half the time delay."

When the analyser is far from the tap, a fast loop is the answer. A high-flow loop carries fresh sample from the tap to a point beside the analyser and back to a lower-pressure return, and the analyser draws a small slipstream through a bypass filter at the junction. Ian Verhappen's overview of analyser sample systems describes how the fast loop also sweeps away material the bypass filter rejects.

Every volume in the path also has to be flushed. Swagelok notes that clearing a filter or knockout pot takes about three times its volume to remove some 95% of the old sample, so keep conditioning components small.

Where should the sample be taken?

GPA 2166, the gas sampling standard, places the probe tip within the approximate centre third of the pipe, away from liquids and debris on the wall. It prefers the top of a straight horizontal run and, near the dew point, at least five diameters from flow-disturbing elements. Take the sample upstream of drums, dead legs and stagnant lines, which add delay, and check long probes for vibration at the flowing velocity. ISO 10715 is the international standard for natural gas sampling.

How do I keep the sample representative?

Keep it in one phase

For gas, GPA 2166 asks for on-line sampling systems to be held at least 30 °F (17 °C) above the expected hydrocarbon dew point. Heat-trace and insulate the lines, use heated regulators, and remember Joule–Thomson cooling across every pressure drop. For liquids, keep the sample at pressure all the way to the analyser. If a liquid must be analysed as a vapour, vaporise it completely and quickly; partial vaporisation fractionates the sample. Hot samples should be cooled before pressure let-down, using sample coolers and pressure reduction units, to prevent flashing.

Filter without adding delay

Use a bypass filter in the fast loop so the flowing stream keeps the element clean. Add a coalescer for fine aerosols and a membrane separator immediately ahead of the analyser to stop liquid slugs. Our analyser sample membrane filters pass gas and reject entrained liquid such as condensate, glycol or amine.

Choose the wetted materials

SS 316L suits most hydrocarbon service. Chlorides, wet acid gases and seawater exposure call for alloys such as Monel, Hastelloy or Inconel 625, and sour service must comply with NACE MR0175 / ISO 15156. Reactive components such as H₂S and moisture adsorb on steel surfaces and slow the response, so keep lines short and consider coated or passivated tubing.

Switch streams without cross-contamination

Use double block-and-bleed valves so a leaking valve cannot carry one stream into another, and keep every stream flowing while it waits to be analysed. Modular surface-mount systems built to ANSI/ISA-76.00.02 reduce internal volume and footprint.

Design calibration and disposal in

Introduce calibration and validation gases through as much of the conditioning path as practical. Return the fast loop to process or route it to flare or a closed vent; toxic and flammable samples should not be vented to atmosphere.

Step-by-step specification checklist

1. Collect the process data in the table above for every stream.

2. Set the required total response time and calculate lag to ASTM D7278.

3. Locate the tap and size the probe.

4. Decide on field pressure reduction and whether a fast loop is needed, and identify the return point.

5. Define the phase strategy: dew point margin, heat tracing, cooling and vaporisation.

6. Select filtration and liquid removal with minimum hold-up volume.

7. Specify materials, including sour service and chloride limits.

8. Specify stream switching, calibration and validation gases, and their automation.

9. Define sample return or disposal.

10. Choose the enclosure, from panel to walk-in shelter, with utilities, gas detection and area classification.

11. Require a factory acceptance test with the analyser fitted: leak test, flows, response time and alarms.

Frequently asked questions

What is a sample conditioning system?

It is the panel, cabinet or skid between the process tap and an online analyser. It filters the sample, reduces its pressure, heats or cools it and controls its flow, so the analyser receives a clean, representative sample at the conditions it needs. It also handles stream switching, calibration gas and sample disposal.

What is a fast loop and when do I need one?

A fast loop is a high-flow circuit that brings fresh sample from the tap to the analyser area and returns it to a lower-pressure point. The analyser takes only a small slipstream. You need one when the transport distance would otherwise make the lag time longer than the measurement allows.

How do I calculate sample lag time?

Divide the internal volume of each component by the actual flow through it at its operating pressure and temperature, then add the results and the analyser cycle time. For gas, the delay scales with absolute pressure, so reducing pressure near the tap shortens it. ASTM D7278 sets out the method.

How far above the dew point should a gas sample be kept?

GPA 2166 asks for on-line sampling systems to be held at least 30 °F (17 °C) above the expected hydrocarbon dew point. The margin allows for uncertainty in the dew point and for local cooling, such as Joule–Thomson cooling across regulators.

Why use double block-and-bleed valves for stream switching?

A single valve that leaks lets one stream contaminate another, and the analyser cannot tell. Double block-and-bleed arrangements vent the space between two closed valves, so leakage goes to vent rather than into the analysed stream.

Should the sample be returned to process or vented?

Return it to process wherever a lower-pressure return point exists, because that avoids product loss and emissions. Otherwise route it to flare or a closed vent system. Toxic and flammable samples should not be vented to atmosphere.

How Tech Inc. builds sample conditioning systems

Our sample conditioning systems cover probe and primary conditioning at the tap, heated or unheated gas conditioning, liquid conditioning, fast loops with return, and multi-stream selection with calibration connections, in SS 316L as standard with Monel, Hastelloy or Inconel 625 on request. We build wall panels, cabinets such as our heated sampling cabinet for gas chromatographs, and walk-in analyser shelters with HVAC and gas detection, and supply gas panels and stream-switching systems for calibration and selection. Our analyser shelters and sample conditioning systems follow API RP 555 practice and IEC 61285, and the wider analyser systems range covers the rest of the measurement chain.

Each system is engineered to the client's process data, P&ID and standards, then designed, built and factory-tested in our own facilities with the analyser in place before it ships. Tech Inc. is part of an engineering group founded in 1990, with offices in Canada, India, Estonia and Saudi Arabia, and our equipment is supplied to national oil companies and refineries. For why representative samples matter to analytics, see AI in oil and gas.

Send your stream data, analyser details and response time target to mail@techincresearch.com or call +1 (289) 622-1032 for a P&ID, GA drawing and proposal.