Chamfering ABS and PVC Pipe

Chamfering is an essential part of the construction process for any solvent cemented plastic pipe system (such as PVC and ABS). Chamfering the pipe involves filing, scraping or grinding away the outside 90 degree angle on the end of the PVC pipe and taking it down to  somewhere  in the region of 22.5 to 45 degrees. The angle is not absolutely critical. What is important, is to ensure that there are no sharp edges. Such edges, when pushed into the socket, will have the effect of acting like a blade, pushing any solvent cement ahead of it and the result will be a very poor join and potential for leakage.

There are several different tools that can be used for chamfering pipe. Our pipe engineers favour either a double cut metal file (half round files are preferred by some as they are easier to use on small pipe), a wood scraper of the type used by decorators or a small hand grinder with a zirconium flapper disc. Very often, especially when carrying out a repair in a pipeline, access to the cut pipe is difficult. It is for this reason that it is important to have all three options available in the toolbox.

A PVC or ABS pipe and socket should never be assembled without first chamfering the pipe otherwise there is a very high risk or leakage and possible mecahnical failure of the joint at higher pressures. If the location of the pipe means that it cannot be effectively chamfered, the decision must be made to replace a larger section, so that correct jointing  procedures  can be followed at all times. This can sometimes seem like a drastic solution which will lead to too much downtime, however a failed joint in the repair, will probably lead to much more downtime further on.

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Understanding pipe diameters

There is often confusion amongst new users of industrial plastic pipe as to the actual measurements of the internal bore and external diameter. Over the decades, and indeed centuries, there have been many adapted and modified standards, some of which have been long abandoned, but others which have remnants in today’s specifications.

To understand how plastic pipe is measured, the user must first understand the methods of production and consider the way that industrial plastic pipes are generally assembled:

The machines that manufacture lengths of plastic pipe heat plastic pellets and then squeeze them through a die. This allows the wall thickness and diameter to be controlled. The problem comes in the cooling process, where, like all materials, the hot plastic contracts. If this were left unchecked, the finished product would have a varying external diameter and bore, which would result in a poor fitting product that is next to useless. The pipe manufacturers concentrate on maintaining a fixed external diameter, and let all the shrinkage happen to the internal diameter / bore of the pipe. It is here that we need to appreciate how industrial plastic pipe is assembled. In general, and this is true for both solvent weld pipework such as pvc pipe and electrofusion welded materials such as polypropylene, assembly is by inserting a pipe into a socket. The external diameter of the pipe is therefore critical, as any variation will make a poor joint with the socket. The internal diameter is less critical as it does not have to perfectly match anything else.

The plastic pipe manufacturers therefore control the outside diameter of the pipe. The external diameter of the pipe therefore remains constant, and the internal diameter varies depending on the pressure rating (due to a thicker wall). It is important to note that even with the same manufacturer, wall thicknesses for the same specification of pipe can vary slightly, resulting in a slight bore variation (usually less than 0.5mm). This is certainly the case between different manufacturers and the internal bore should not be relied on for machining purposes.

With metric pipe, and this knowledge, determining the pipe dimensions is simple; te outside diameter of the pipe measures the same as the stated size; so a 50mm pipe has a 50mm outside diameter, a 90mm pipe a 90mm outside diameter etc. Unfortunately the same cannot be said for imperial (inch) pipe. This is an area of great confusion as the size of the pipe refers to the “nominal bore” (or to put it another way “approximate bore”) of the pipe. So, a 2 inch pipe has an approximate bore of 2”. As the outside diameter of the pipe needs to remain fixed, to allow for fitting into sockets etc. the wall thickness of the plastic pipe changes with pressure rating and so, as a result, does the bore…which is why it is referred to as “nominal”. So a 2” pipe does not actually measure 2” anywhere! Many inexpereicnced users will order a 2 inch pipe and expect it to have an outside diameter of 50.8mm but in fact the OD is 60.3mm.

The general rules of thumb are:

Metric pipe – will measure the stated diameter as an external diameter

Imperial pipe – Will not measure the stated diameter anywhere so the user must check first in the table below:

Pipe size (inches)	Outside diameter (mm)
3/8	17.1
½	21.4
¾	26.7
1	33.6
1 1/4	42.2
1 1/2	48.3
2	60.3
2 ½	75.2 (Note that for North American Pipe 2 ½” = 73mm)
3	88.9
4	114.3
5	140.2
6	168.3
8	219.1
10	273.0
12	323.9

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Plastic pipe use in glycol and chiller circuits.

Background

ABS pipe chilled fluid circuits are often required to operate at temperatures close to, and well below freezing. To achieve this, a closed circuit containing a mix of glycol and water, along with a secondary heat exchanger is used. The ratio of glycol to water is dependent on the temperature required. The lower the temperature that the system will be operating at, the higher the ration of glycol to water is required. If too little glycol is added to the circuit, ice crystals can form, which will block and potentially damage the internal heat exchanger in the chiller unit. Too much glycol in the system will not do any harm, however it will increase the set up and operational cost of the unit (as glycol is more expensive than water!). The freezing points of different ratios are given below;

% Glycol by volume	Freeze protection point (degrees C)
18	-7
29	-12
36	-18
42	-23
46	-29
50	-34
54	-40

Table 1 – Freeze protection ratios for glycol

In order to ensure sufficient margin for error, it is generally recommended that a temperature at least 3 degrees lower than that expected is selected from Table 1. Different manufacturers may have different percentage of glycol in their product; Table 1 assumes 100% glycol.

Plastic pipe choice

The plastic of choice for glycol and non-glycol chiller circuits is ABS. ABS pipe and fittings are rated down to an operational temperature of -40 degrees C (-40 degrees F) and unlike PVC pipe, retain an excellent impact strength at these temperatures. The installation of ABS pipe is much quicker, and easier than steel or other metal pipe systems, as there is a wide range of fittings. The ABS fittings are assembled to the pipe using a solvent cement which is applied with a brush.

System design

The design of ABS pipe systems for glycol circuits primarily follows that of other plastic pipe systems. The number of elbows and tees used should be minimised and where possible, long radius bends like our product 12686, should be used to minimise friction loss and reduce pumping requirements. Although when the correct percentage of glycol in a circuit is used, there should be no formation of ice crystals, it is good practice to install a strainer in the ABS return line, just prior to the chiller. This can also serve to catch any plastic swarf that may be left in the ABS pipe system after installation. A mesh of 0.5mm is recommended. The strainer should be installed with ball valves either side that permit cleaning without draining the complete circuit.

Advice

Should you require further information on ABS pipe system, or any other plastic pipe systems, please feel free to contact one of our sales engineers.

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Technical Bulletin: Plastic pipe threaded fittings

2. Thread sealants for plastic pipe

Thread sealants are an integral part of making a threaded connection with plastic pipe fittings. Without a sealant a thread will always leak, even at almost zero pressure. There are many different types of thread sealant available on the market, and preference is often a matter of habit, rather than performance.

The most common thread sealant used is PTFE tape. Generally white in colour, PTFE tape is supplied in rolls and wound onto the male thread only. The number of turns around the fitting is dependent on the size of the fitting. Prior to applying the tape the thread should be cleaned of any grease or debris by wiping with a cloth soaked in pvc cleaner. The outside of the thread should then be very slightly roughened using a flat file. These two steps help bind and bed the PTFE tape in, to the male thread.

PTFE tape should be wound onto the thread in the same direction as the thread. This will ensure that the tape does not unwind as the plastic pipe fittings are connected together. If the tape is wound on the other way, it will begin to unravel as soon as the fittings are threaded together.

There is a lot of debate over how many turns of PTFE tape should be applied to a thread. The generally accepted amount is between 3 and 5 turns.

Cord products

Cord type thread sealant products such as Loctite 55 are relatively recent additions to the market compared to PTFE tape. One of the key advantages of Loctite 55 over PTFE such cords, as claimed by the manufacturers, is that the thread can be “backed off” by up to ¼ of a turn without any fear of leakage. This enables more precise positioning of fittings, especially in a situation where you are connecting to fixed fittings.

The other advantage of cords such as Loctite 55 is that they can be cleaned off threads very easily if the fitting needs to be removed and reinstalled.

Paste compounds and hemp

Paste type compounds such as Boss white and a myriad of other trade names are generally, for the plastic pipe industry, regarded as having been superseded by PTFE tape and cord. Although they can provide a good seal, care must be taken that the compound, which is usually applied with some strings of hemp (a plant based material used to make natural fibre ropes), are suitable for the application they are being used in. This is especially so where potable water or chemical s are concerned.

The use of paste compounds is also very messy in comparison to PTFE tape and cord products and it is very difficult to make a joint that looks sanitary or with a professional finish to it.

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Heavy Duty Suction / Delivery Hose

A very strong PVC spiral hose that resists shocks and crushing and has a smooth inner bore. Used in heavy duty applications for seawater, water and light chemicals and other liquids in agricultural and industrial applications as well as the construction industry where a damaged, burst hose could lead to delays.

General specification:

• Pressure rating: 9 – 24 bar depending on size.
• Temperature: -5 to +60 deg C
• Assembly: Cam and Groove in combination with hose clamps

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