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Analytical Instrumentation
DR-3 Design Report-3, Why Use Analytical Fittings Instead of Industrial Ones* Part II
Yves Gamache, Pres., Analytical Flow Products, Division of Mecanique Analytique Inc. 233, Jalbert St. W., Thetford Mines, QC, Canada G6G 7W1 Tel: 418.338.0004 • Fax: 418.338.2500 • Email:
info@afproducts.ca • Web:
www.afproducts.ca
Torque Related Problem
Tube size of 1/16"OD or 1/32"OD requires less pushing force from the nut to get proper sealing, so less torque from the user. However, a problem arises with this concept when tubing size is 1/8"OD and higher. Remember, most of packed columns are made with 1/8"OD 304 stainless steel tubing, file cut. Indeed, when the nut becomes in contact with the ferrule, it becomes very hard to rotate.
Higher rotating torque is required to move forward the nut in order to get the ferrule gripping on the tube. It's difficult to do so while holding the fitting body with your hand. It requires longer tools and very often a vise is used to hold the fitting body. So imagine yourself replacing such fittings on a column inside a GC oven or on a critical and fragile component of your system.
Often this rotational torque transfer makes the ferrule to rotate or twist. Since the area in contact between the nut and the ferrule is much larger than the area in contact between the tip of the ferrule and the fitting body, this large rotational traction force can't be counterbalance by the ferrule. So until the ferrule becomes really compressed on the tube and forced against the fitting body, it will rotate and/or twist. Such ferrule rotation causes scratches and leaks.
This is a common problem for 1/8"OD tube size and, believe it, even worse for ¼"OD size. This phenomenon is shown in Figures 10.
2. An anti-friction and anti-galling coating has been applied to the front and the threaded portions of nut. This reduces the friction at least by an ordered of 10, eliminating the rotation of the ferrule when the nut is screwed in. So, now the fitting can be easily assembled with small tool and this without the need of a vise.
3. In order to decrease further the required torque to make a proper fitting connection and still getting excellent sealing performance, a relatively thick layer of gold has been deposit on the front portion of the ferrule, on the area where the ferrule seals on the fitting body. Figure 11 shows our new design.
4. We added a fine lip at the bottom of the detail. So, when the nut is screwed in, firstly, the ferrule grips the tubing. Then, as the ferrule is pushed forward by the turning nut, the tubing is compressed against this new fine lip. When proper tubing is used, this creates an effective metal to metal seal. Normally, tube material softer than union material will be preferred. For example tubing made of annealed SS304 and fitting body made of hardened SS316L have resulted in excellent performance.
Having a fine lip, or sealing ring, distributes the mechanical force on a much smaller area increasing the effective seating force; the sealing lip penetrates the tube end. In fact, this concept creates a first metal to metal seal section, with no dead volume. The second section is the one done by the coated ferrule against union body. Optimally, an additional soft layer (ideally gold) would be deposit on the sealing lip, further improving the sealing, and reducing total force required for the desired sealing level. It also improves sealing with tubes that have some imperfection on their end.
5. We have made the front portion of the ferrule longer, in order to occupy more of the empty volume. Doing so reduces the dead volume, as shown in Figure 11.
Figure 10a: Motion of ferrule and nut before the contact of the ferrule with the fitting taper
Figure 10b: Motion of ferrule and nut at the beginning of the contact of the ferrule with the fitting taper
Figure 11: Improved fitting design* Material Considerations Figure 10c: Initial surface finish of fitting taper Need For Improvement
Reducing further the dead volume of analytical fitting,eliminating the ferrule rotation and improving the sealing between the ferrule and the fitting body will be very beneficial for any analytical high sensitivity instrumentation.
Here's how we did it.
1. First, we did change the pitch and shape of the thread on the nut and inside the fitting body. Ultra fine pitch threads are used instead of the ones normally used. Doing so, the force transfer to the ferrule is done in a much smoother way.
*: US Pat.#7,503,203 other patents pending August / September 2011 •
www.petro-online.com Figure 10d: Surface damages caused by ferrule rotation
Many GC column manufacturers ship their packed columns with industrial type fittings, often made of brass. It is common knowledge within industrial fitting manufacturers that the fitting material should always be harder than the tubing material.
In fact, protection of the fitting assembly against loosening coming from pressure pulsation and mechanical vibration comes from the swaging effect of the ferrule on the tubing. See reference [2], page 2-1.
However, in the analytical world, things are different. Pressure and vibration are less critical; sealing is a top priority. A ferrule material softer than the tubing one is important for the good performance of such fitting assembly. It makes sure that the tubing remains undeformed (not swaged) while the ferrule creates a sealing ring on the surface of the tubing.
There comes the choice of gold for the coating of our ferrule. The gold layer improves a lot the sealing; it is softer and easier to mate the stainless steel surface of the fitting body. Gold inertness is also well renowned.
*Patent pending
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