Assembly procedure of the Compression Fittings (A-LOK series)

 Introduction

It has been chosen to use in the guide the company Parker (A-lock series) because it is one of the leading companies in the world.

As for all other companies, Parker A-LOK tube fittings are designed to provide reliable leak-free connections for instrumentation, process and control, and analyser applications. Manufactured to the highest quality standards, these tube fittings are available in various sizes, corrosion-resistant materials and multi configurations.

As standard, the tube fittings manufacturer’s A-LOK range is manufactured from heat code traceable 316 stainless steel. Other materials include 6Mo, Alloy 825, Alloy 625 and Alloy C-276. Straight fittings are machined from cold-finished bar stock, while shaped bodies are machined from close grain forgings.

The tube fittings are available for imperial tube sizes from 1/16” through to 2” O.D. and for metric sizes from 2 mm through to 25 mm O.D.

Features and benefits of A-LOK instrumentation tube fittings

All Parker A-LOK fittings use an advanced two-ferrule system for reliability and ease of assembly and disassembly. No special tools are required, and the design ensures that all make and remake motion is transmitted axially to the tubing.

No radial movement stresses the tubing, so its mechanical integrity is uncompromised. The fittings back ferrule is hardened by Parker innovative Suparcase surface treatment process, which - unlike many competitive hardening processes – actually increases its corrosion resistance.

Figure 1 below shows exactly what happens between tubing and ferrule when the gland is screwed into the body of the fitting.

How an A-LOK parker fitting works

 Figure 1 - ferrule and tubing installation

As can be seen from Figure 1, the tube comes lightly deformed by the ferrule. Practically the ferrules block the exit of the tube from the fitting when it is under pressure.

Assembly procedure

Before starting, I want to remind you that A-LOK fittings are provided with two ferrules, back ferrule and front ferrule on the tube and body. Remember, the back ferrule is only for A-LOK fittings. See Figure 2 below.

Figure 2 - Front and Back Ferrules

Several companies provide the same technology, but the working principle remains the same; the ferrules bite the tube when the gland of the fitting presses it.

Our recommendation for a secure installation is never to install a fitting with different manufacturers inside, such as the ferrule Parker with a gland Swagelok, or different materials, such as ferrule in stainless steel and tube in Inconel. This is not allowed. The connection will not be secure. A fitting must be installed as it is when opening the package.

Tubing preparation

Figure 3 - Cutting tool

For the tubing cutter, always use the tube cutter as per Figure 3 above. Never use other types of blades like Hacksaws etc. If the saw is used, the tube will be like in Figure 4 below.

Figure 4 - Hacksaw

The tube preparation process should not begin if the tube is damaged or scratched, as in Figure 5 below.

The cleaning tool

Figure 6 - Cleaning tool

Sometimes, this tool is not used, but in reality, it is essential because it can eliminate any metal chips from the tubing and avoid damaging equipment that the newly built line will supply.

The tool includes two sides: one is for cleaning the internal diameter of the tube, and the other is for cleaning the outer part of the tubing. Figure 7 below shows all details (before and after cleaning the tube).

Figure 7 - Cleaning tool

Tube Installation on the Fitting

There are 4 Points that absolutely must be followed:

1. Insert the tube into the fitting until it is fully bottomed.
2. Rotate the nut until it is finger-tight.
3. Mark the nut in the 6 o’clock position.
4. Tighten the nut with a suitable wrench an additional 1-1/4 turns to the 9 o’clock position.

To make it easier, use a marker to highlight (as a reference) some important parts between the tube and the fitting before tightening the gland. Let's see what they are for:

• Mark the entire tube circumference near the gland neck (not shown in figure 9). This reference indicates to the technician if the tube unintentionally comes out (partially) from the fitting during the tightening of the glans, resulting in the connection being unsafe.
• The second mark should be made on the head of the gland (shown in figure 9). This indication shows the technician how many turns the gland completed and how many remain, in our case, 1 and 1/4. This is to avoid extra torque between the tube, ferrule, and gland.
  

Figure 9 - assembly of the tube on the fitting

Usually, after tight the gland on the fitting is required to check if the assembly has been carried out correctly. Use the "Inspection Gauge" supplied by Parker to ensure the connection has been made perfectly. Figures 10 and 11 show the special tool provided by Parker.

The working principle is straightforward. Place the gauge between the nut and the body hex. If the gauge does not fit between them, you have correctly tightened the nut. If you can slip the gauge into the space, the fitting is not properly made up, and you must repeat the assembly procedure.

Another suitable method is to check if the front ferrule (bigger) inside the tube rotates on the tube without it coming out. This shows that the connection has taken place perfectly. If the ferrule doesn't rotate, extra torque has been applied, which is not required. In this case, a new connection is required. Figure 12 below shows the ferrule installed on the pipe.

Recommendations

• Several companies supply these types of fittings with ferrule compress. Never mix any fitting parts between different companies, the tolerance change, and it could be hazardous. For example, suppose you are using a Parker fitting. In that case, you must use all of its internal parts, therebefore, the gland, ferrule and fitting itself, Parker (This is forbidden).
• After the tube and fitting installation should be checked by unscrewing the gland to see if ferrules are correctly fixed on the tube (as mentioned before), a good connection should have the front ferrule (bigger) spin on the tube (without slipping out from the tube) without extra friction. This is called a perfect assembly. For more details, see Figure 12 below.

Figure 12 - Fitting and tube installation

• Several companies provide various diameters of fittings, from inches to metrics. Never mix any fitting between inch and metric. In this case, it will create a hazardous situation. Before installing any fitting or tubing, it must be inspected to confirm the exact size. Remember, several diameters are more or less the same between inches and millimetres. For example, in a 1/2" tube with a 12mm tube, there are few millimetres only of difference; making a mistake is straightforward. For information: the 12mm tube can fit in the inch fitting, while the 1/2 inch tube cannot fit in the millimetre fitting.
• A good thing if you work with both sizes of tubing (inch and mm), like in the example here (1/2" and 12mm), should be available a small sample of tubing in mm and inch, then testing if the tube in inch will fit in mm fitting, or the tube in mm will fit in the inch fitting (in this case between tube in mm and fitting in inch there will be extra space due to due to fitting in inch is a little bigger).
• Another method to identify the correct size between inch and mm is provided by Parker (not all companies are the same). Figures 13 and 14 show the difference. On the fitting in mm (figure 14), there is a lug on the gland, which indicates the mm fitting, while on the inch fitting, this lug is missing (figure 13), which means the fitting is in inches.

• Experience does not mean modifying the connection system or internal parts of the fittings but knowing all the procedures written in the manual is mandatory.
• If you are unsure about 100% of a connection, never leave it like this; it could break in the next few days.
• Never use a fitting with different internal parts of materials, like Inconel with stainless steel, etc. For example, tube in Inconel and ferrule in stainless steel. The tube in Inconel is too strong, and the ferrule in stainless steel is too soft and cannot scratch the tube during the tightening gland on the fitting. The consequence will be an unsafe connection.