Regardless of whether one uses a graphite/Vespel® ¼” canister ferrule or a metal ferrule, the forces upon a fitting increase dramatically as torque rises above 5 pounds. Unfortunately, this degree of torque may become necessary to achieve a leak tight seal when connecting a fitting to a canister stem with a thin WALL THICKNESS of [0.028 – 0.035].
A little Known Answer to a Leaking Canister Issue
These forces can easily deform a fitting – which may flare out where the metal is thinner (Figure 2). When a fitting’s threads become deformed, a nut may no longer fit properly and the effects of galling can become increasingly obvious. Thread geometry is designed to easily slide, but when threads become badly deformed, a part may completely cease to perform its connective “fitting” function. To completely avoid this issue, Entech Instruments, Inc. manufactures canisters and field samplers with high-quality tubing and stems with a wall thickness of at least [0.049].
If tubing is coiled, the tubing – by definition – will have a thinner wall as well as a somewhat irregular seam that may require greater force to achieve a leak-tight seal. These higher forces can leave the ferrule with “no place to go” and thus deform the weakest areas of a fitting.
Unfortunately, some other canister and sampling equipment manufacturers utilize cheaper, thin-wall tubing (Figure 3). Some might argue the larger tube opening of a thin wall stem to be an improvement for the sake of faster flow rates during canister cleaning, but testing has revealed virtually no appreciable increase in cleaning times when using Entech’s [0.152 I.D.] (thick walled) tubing. The top illustration in Figure 3 shows that an increased wall thickness is vital to avoid tube deformation and subsequent fitting damage.
Once a thin wall stem becomes buckled and damaged, the canister may be a complete loss since stem damage is irreparable. So, what can be done if you already own multiple canisters that have thin walled stems? One easy fix for this issue is to utilize a press fit tube inserted into the thin walled stem itself. This reduces the potential for fitting deformation by over-tightening by hand.
Canister Bellows Valves
Diffusion and Residence Time Contamination Risks
A commonly used bellows valve has been sliced in half to expose the inside (Figure 1). Note the bellows area under the knob of this all welded system. The bellows area includes over 1cc of dead space, into which volatiles may easily diffuse as they find their way past the sealing bullet and up into that space.
Long term chemical storage within a canister is dependent upon inert surfaces such as SiloniteTM – which promote chemicals to remain in the gas phase prior to analysis – as well as valves and fittings that are compact and inert. Since diffusion is based largely upon TIME (among other things), accurate sample storage within a canister may become increasingly less reliable as volatiles utilize time to equilibrate with the large dead space volume found inside the bellows area of this type of valve. Compounding this problem, time is often not used in effective portions to properly clean canister valves, and valve contamination may persist even after a standard cleaning process. Background contamination in a canister may steadily increase over time while it sits waiting to be rotated into the field for sampling. Within the space of three weeks, a can that originally tested as clean may contain contamination that is well above allowable background levels. Using a bellows valve is not recommended since it can also be over-tightened in the field, is un-repairable, and ultimately much more expensive than the TOVTM over a canister lifespan.
Toxic Organics Valve - TOVTM
The Best Solution for Canister Valve Over-Tightening and Contamination Issues
The Toxic Organics Valve (TOV-1) as shown in (Figure 4). Has been specifically designed for the collection of volatiles in ambient air into SiloniteTM coated canisters. This valve features a compact flow path without ANY bellows area (bellows dead space can exceed 1cc in some valves and allow volatiles to diffuse and create sample losses or contamination). The TOVTM also features no internal welds (weld scars seen on other internal valve surfaces can increase active surfaces). Best of all, the Toxic Organics Valve is designed so that it cannot be over-tightened by hand and a valve repair kit is readily available that includes replacement diaphragms and the orange bullet seen in (Figure 5) to return the valve to a state “as good as new” even if the valve seat should become deformed by hard particles.