Frequently Asked Questions
Common questions about sample probe assemblies, packing glands, and pipeline instrumentation.
What is a sample probe assembly?
A sample probe assembly (SPA) is a precision instrument inserted into a pipeline to extract representative gas or liquid samples from the process flow. It consists of a probe tube, a PG compression seal packing gland, an isolation valve, and optional accessories like retaining chains and stop collars. The probe tip is typically positioned in the center third of the pipe for representative sampling.
What is a PG packing gland?
A PG packing gland is a compression seal fitting that allows a probe tube to be inserted through a pressure boundary while maintaining a leak-tight seal. Conax Technologies manufactures PG models from PG2 (1/4" NPT) through PG9 (2" NPT). The gland uses sealant rings (Teflon, PEEK, or Grafoil) that are compressed by tightening the follower nut, creating a seal around the probe tube.
How do I select the right probe material?
Material selection depends on your process conditions. 304 SS is the standard for clean, non-corrosive service up to 1,600°F. 316/316L SS handles moderate chloride environments. For H2S/sour gas service, use 316 NACE per MR0175. Monel 400 excels in HF acid and seawater. Hastelloy C276 handles wet chlorine and extreme corrosion up to 1,900°F. Inconel 600 is best for high-temperature oxidation up to 2,000°F.
What is wake frequency analysis?
Wake frequency analysis per ASME PTC 19.3 TW-2016 compares the vortex shedding frequency (caused by flow around the probe) to the probe's natural frequency. If the ratio fs/fn exceeds 0.80, the probe is at risk of resonance failure and must be redesigned. The calculation uses the Strouhal number (St = 0.22), flow velocity, probe OD, and cantilever beam natural frequency based on the probe's material properties and immersion length.
What tube OD sizes are available?
Standard probe tube ODs are: 0.250", 0.375", 0.500", 0.625", 0.750", and 1.000". Each OD has specific wall thickness and material combinations available. For example, 0.500" OD is available in 0.035", 0.049", 0.065", and 0.120" wall thicknesses. The 0.500" OD with 0.049" wall in 304 SS is the most common configuration.
Which PG gland model fits my probe?
PG model selection depends on probe tube OD and pipe size. PG5 (3/4" NPT) is the most versatile, accepting tube ODs from 0.250" to 0.750". PG4 (1/2" NPT) handles 0.250" and 0.375" tubes. PG6 (1" NPT) accommodates 0.750" and 1.000" tubes. For pipe-based probes, PG7 through PG9 are used for 1/2" through 1-1/2" pipe sizes.
What sealant should I use?
Teflon (PTFE) is the standard sealant for temperatures up to 500°F. PEEK handles up to 480°F with better chemical resistance. Grafoil (flexible graphite) is required for temperatures up to 850°F and provides superior sealing under thermal cycling. The sealant choice also affects torque specifications — Grafoil requires significantly higher torque than Teflon.
What is the center-third rule for immersion depth?
For representative sampling, the probe tip should reach the center third of the pipeline cross-section. This means the immersion depth (Z dimension) should position the probe opening between 1/3 and 2/3 of the pipe internal diameter from the wall. Our configurator automatically calculates and validates this based on your pipeline size and immersion depth.
When do I need a retaining chain?
A retaining chain is recommended whenever process pressure could eject the probe during insertion or removal. At pressures above 150 PSI, the ejection force on even a 0.500" probe can be significant. The chain attaches between the probe and the gland body, preventing uncontrolled ejection. For pressures above 150 PSI, Conax also recommends using their Easy Tool retracting system.
What coatings are available?
Three coating options are available: SilcoNert 2000 provides an inert, corrosion-resistant coating ideal for trace-level gas analysis (prevents sample adsorption). SilcoNert 1000 offers similar benefits at lower cost. Electropolish per ASTM B912 provides a smooth, passive surface that improves corrosion resistance and cleanability. Coatings are applied to the probe interior and exterior surfaces.