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.

Read More

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

Read More

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.

Read More

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.

Read More

Tips and techniques for gluing plastic pipe

Some thermo plastic pipes such as pvc pipe, cpvc pipe, and abs pipe are connected together using a solvent weld technique, where a glue is applied to both the pipe and fitting. When pushed together in a socket / spigot arrangement, the glue melts the outer layers of plastic from the pipe and socket so they then melt together and dry, forming a complete seal.

The process of joining pvc and abs pipe together seems, at first hand, a simple one. It is however full of small technicalities that once mastered, can save a lot of time and money.

Common errors are as follows:

Putting the pvc or abs solvent cement on the pipe (or spigot) first. In almost all cases, the plastic pipe glue should be applied to the socket first, as this can then be put down, the pipe glued, and then the fitting picked back up and pushed onto the pipe. If this is done the other way round, the pipe / spigot, when covered in glue, cannot be laid down anywhere.

Gluing valves – When a valve is glued you must be very careful not to let glue run down into the valve body or the mechanism may seize. Apply the glue to one side of the valve, sit the valve vertically, with the glued part of the pvc or abs valve at the bottom and then apply the glue to the pipe. Push the valve, downwards onto the pipe and leave to set in this position. If there is any doubt that some glue may have entered the body of the fitting, turn the handle a few times to ensure the vale stays free. With abs pipe systems this has to be done very quickly as abs sets much faster than pvc.

Twisting the pipe – Many inexperienced users of Plastic Pipe Fittings believe that twisting the pipe, as it enters the socket, will result in the glue being more evenly smeared around the joint in the fitting. This is incorrect; rotational force applied to the plastic pipe and fitting as they are being joined results in the glue becoming denser in some places and void in others. It is acceptable to apply a small amount of rotational movement to the pipe, one it is fully home in the socket, to align it correctly. This movement should be less than 10 degrees and should take place immediately that the pipe has been pushed home. Once again, with abs pipe and fittings, there is very little time for any movement as the solvent welding process is so quick.

Sharp edges on the pipe – Trying to glue a freshly cut piece of plastic pipeinto a fitting is a recipe for disaster. The sharp, cut edge of the pipe, will scrape away the glue in the plastic fittings and any swarf remaining on the pipe end may get drawn into the joint and result in a void. To combat this, all swarf must be removed, the pipe chamfered on the outside edge to an angle of approximately 45 degrees and then cleaned of any debris.

Following these simple tips will save a huge amount of time and anguish when working with pvc, abs and c-pvc pipe and fittings.

Read More

Plastic pipe use in swimming pools

There are three main types of piping used in the construction of swimming pools, these are; rigid pvc pipe, flexible pvc pipe and rigid abs pipe. Domestic pools tend to use either 1 ½” or 2” pipe systems (or 50mm / 63mm in metric). Due to larger flow rates, commercial swimming pools tend to operate with much larger pipe diameters, which are then reduced to smaller sizes when they branch off and approach their destinations, such as filtration equipment.

Swimming pool pipework, especially in the domestic environment, has traditionally be manufactured in white from pvc (polyvinyl chloride) or abs (acrylonitrile butadiene styrene). The white colour was chosen to provide an attractive looking product where it is viewable by the pool users, however in reality, white plastic is more readily attacked by ultra violet rays from the sun and will have a shorter operational life. 
The process of gluing pvc pipe and abs pipe is a very simple one, and can be carried out by non skilled practically minded people with the right tools. The required tools are a sharp wood cutting saw, a stable bench, a flat medium file, some disposable paper cloth, latex gloves, a tape measure, a strong polythene bag (the type used to freeze food), a fine permanent marker pen and a paint brush approximately 40 – 50% the diameter of the pipe’s internal diameter.

The pipe length needed is measured with the tape, remembering to add on the depth of the sockets that are being glued into, and marked with the marker pen. The pipe is then cut square and after cutting, any loose strands of plastic rubbed off with your hand or the file. The outside edge of the pipe must then be chamfered at an angle between 40 and 60 degrees. This has two purposes, to allow the pipe to enter the socket more smoothly, but also to prevent the sharp, cut edge of the pipe, from pushing the glue out of the joint. The pipe and socket should then be cleaned with a small amount of acetone cleaner to remove any oils or other debris. If the pipe or fitting is an old one, some emery cloth should also be used to roughen the surface a little before cleaning.

Before applying the solvent pvc or abs cement it is a good idea to put a small mark on the pipe at a distance equivalent to the of the socket depth. This acts as a guide to how far up the pipe to apply the glue, and also acts as a check that the pipe is fully in the fittings when assembled. Solvent cement glue is applied first to the socket, ensuring there is a good film around the entire fitting and then to the pipe, ensuring that the chamfered angle is also covered. Push the pipe and the fitting together in one swift movement. Do not twist the pipe as this will affect the integrity of the joint. Once the pipe is in place, hold for a few seconds, or the pressure of the glue, which has been forced between the pipe and fitting, may push them apart. Then wipe any excess glue off the joint. Assembling ABS pipe if more difficult than PVC pipe as the former bonds in a fraction of a second, where the latter has a two to three second window where, if a mistake has been made, it can be pulled apart again. A pvc fitting can be forgiving to a very slight turn of 5 degrees or so in the fitting in the first couple of seconds to assist with alignment, but this is not the case for ABS.

As soon as you are finished gluing, wrap the end of the brush in the polythene bag, this will prevent the brush going hard between joints.

Read More

Plastic pipe classifications

There are three main systems if industrial plastic pressure pipe classification. These are broadly categorized as follows: The USA follows an imperial system with pressure ratings that conform to an ASTM (American Society for Testing and Materials) standard, the UK also have an imperial system, but this one conforms to a British Standard and most other countries, unless they have adopted the UK or USA systems in the past, conform to a metric ISO (international standards organisation) standard.  Some countries such as the UK are a mix between two systems – in this case the imperial (BS) and the metric.

These standards are common across ranges of pvc pipe and abs pipe.

The ASTM and BS systems are mostly compatible except for two sizes of plastic pipe; 2 ½”, where the UK system measures 75mm and the USA systems measures 73mm, and 5” where the UK system measures 140mm and the USA system 141.3mm> the other critical difference in the USA and UK systems is for threaded fittings. In the USA, threaded plastic fittings generally have an NPT (National Pipe Thread) thread and those from the UK generally as BSP (British Standard Pipe) thread. Oddly enough the metric system has adopted the BSP system for its threads. NPT and BSP plastic pipe threads are generally not compatible, and efforts to marry them together will result in stripped threads and leaking joints in the pipe system. Plain fittings from the metric classification are not compatible with either system (with the exception of 125 and 140mm sizes for the UK system). Most manufacturers supply adaptor fittings to take metric on one side and UK imperial on the other.

Within each pipework classification there are subcategories which are mostly indicative of working pressure ratings. The USA system has two different ASTM standards, the SDR system and the Schedule system. In the SDR system, subcategories have largely the same pressure ratings, no matter what the pipe size, whereas in the Schedule systems (such as schedule 40, 80, 120) the pressure rating decreases with an increase in pipe diameter. Nevertheless, the schedule system is the more common of the two systems and that which is most often specified for pipe fitters.

The UK plastic pipe classification system operates much like the USA ASTM/SDR system, and used a series of letters, most commonly B,C,D,E and a number, 7 to indicate a pressure rating that is consistent throughout the size range. For example 2” class C pvc pipe has a pressure rating of 9 bar, as does every other size of class C pipe.

The metric system of plastic pipe classification uses a more straight forward set of abbreviations. For example PN10 pipe has a pressure rating of 10bar, PN6 pipe, a pressure rating of 6bar and so on.

Pipe fittings, generally only come in one available pressure rating, with metric being 16 bar up to 160mm and then 10 bar to 450mm, UK imperial pipe fittings being 15bar up to 6” and then 9 bar for fittings between 6” and 12”. 

In the ASTM system, the classification of pipe fittings is rather more convoluted; the pressure rating of the fitting is associated to its schedule and so, as with the pipe, changes with size. An SDR classification is not available for plastic pipe fittings.

Read More

Industrial Plastic Pipe manufacturing

Plastic pipe is a commodity manufactured in many countries throughout the world. Due to the different processes involved between pipe and fittings manufacture they are rarely carried out by the same company.

Pipe manufacture is carried out by extrusion machines. The granular plastic media is fed from a hopper at one end of the machine and supplied into an enclosed auger. The auger is enclosed in a heated tube which is electronically controlled to ensure the plastic is heated to it optimum molten temperature. If the plastic is too hot it will burn or be extruded too fast through the machine resulting in a weak pipe structure. If the molten material is too cool, the auger will struggle to push enough through the die at the end, resulting in the plastic pipe having a cavitated structure, which will significantly weaken the end product. Temperatures depend on the material, colour and other additives in the plastic mix. ABS pipe is typically extruded at temperatures between 215 and 230 degrees centigrade. PVC pipe is extruded at slightly lower temperatures, typically between 190 and 220 degrees centigrade. The auger typically rotates at 120rpm and pushes the molten plastic towards a screen that removes any unwanted particles and ensures that only smooth, molten plastic is used for making the pipe. The screen and plate assembly has a secondary purpose on the plastic, and transforms the rotating direction from the auger, to a linear one, which is more suitable for the die. The pressure at this point is very high, in the region of 340 bar and so additional reinforcing plates need to be used to ensure the integrity of the screen is maintained.

The die is the part of the system that gives the end product of plastic pipe its shape. All plastic pipe is manufactured to an outside controlled diameter. The outside diameter is the critical diameter for most pipe systems as the method of system construction requires that the pipe is pushed into moulded sockets. In this way, the wall thickness can be increased and decreased for different pressure ratings, but all pipes of all pressures will fit all fittings. The internal diameter of the pipe will vary slightly, the amount depending on the quality of the manufacturing process and the quality of the raw materials. The internal wall of the pipe is not as smooth as the outside. Pipe that is well manufactured will have only a slight, barely noticeable difference, but poorly manufactured tube will sometimes display ripples in the internal bore. The atmosphere in the room where the pipe is manufactured is very carefully controlled to ensure that the plastic is uniform. After extrusion, the plastic needs to be cooled, in a controlled manner, to ensure both the retention of accuracy of he dimensions and the uniformity of quality. This process usually uses long water, temperature controlled baths, where the fluid is kept moving and the pipe cools slowly as it travels along the length of the bath.

At the end of the process, the pipe is cut to length. Some pipes are manufactured with a socket end, and this is achieved after extrusion and cooling, by reheating the end of the pipe and flaring it with a special tool, before once again, cooling it.

Metric plastic pipe is usually manufactured in 5m lengths whereas imperial is more often in 6m lengths. The 5m lengths are often preferred as they can fit in a standard iso container, where the 6m cannot.

Read More

Moulding of plastic pipe fittings

The manufacture of plastic fittings, from thermoplastic materials such as pvc, cpvc, abs, polyethylene, polypropylene and pvdf is carried out by injection moulding.

The injection moulding process involves a metal die, which is cut out to be the reverse image of the fitting. Molten plastic, maintained at the perfect temperature, is first filtered to get rid of any impurities, and then pumped at high pressure (as it is still quite viscous). The mould contains air release holes which must be strategically placed to ensure that no air pockets remain after all the plastic has been injected. The plastic fitting will have cavities and be not fit for purpose if all the air is not expelled. The air vents in the mould often leave a tail coming off the fitting which is cut off flat as it exits the mould. 

Before the plastic fitting can be released from the mould it must be cooled. If it is not cooled sufficiently then product will distort as it is automatically ejected from the machine. The die therefore has a complex structure of cooling tubes and vanes through which temperature controlled water is pumped prior to injection. The cooling process is critical to ensure that the quality of the plastic is uniform. If the plastic cools too quickly it can lead to weak areas and hairline cracks in the future. This is especially so with pvc and abs. If the fitting cools too slowly the machine cannot manufacture fittings fast enough.

The controlled measurements on a plastic pipe fitting are those that will marry up with the associated pipe. For socket fittings the internal diameter is critical whereas for spigots it is the external diameter. Some fittings have an integral spigot / socket and these are very difficult to manufacture accurately as the cooling rate must be very tightly controlled.

As there are different ranges of pipe, namely imperial and metric, used in different countries, most manufacturers have shims that they can add, or take away from their moulds to adapt to imperial or metric sizes. The cost of making the die is very high, especially for large plastic fittings where they can cost tens of thousands of pounds and so they are designed to double up as imperial and metric. manufacturers typically do not change the identification part of the fitting which leads to a lot of confusion, as. A pvc plastic pipe fitting, manufactured in a plant where they are using a die for both imperial and metric systems, may have a marking of 63mm/2” stamped on it. Users of plastic pipe who are not familiar with the different systems often assume that 63mm and 2” are the same whereas in fact a 2” fitting has been moulded with a die that has shims added and actually measures 60.3mm. This is the same for all sizes of pipe fitting. Polyethylene and polypropylene tend not to suffer these problems as they are generally only manufactured in metric and so do not need shims or a secondary sizes marked on them.

Pipe fittings are usually manufactured in just two colours; pvc and abs are made in grey or white, with the majority being grey. The manufacture of different colours is a costly process as it can take several hours, or even days to ensure a machine is totally empty of one colour, before starting to produce another. Polyethylene pipe is manufactured in black, yellow and blue. The three colours are cost effective due to the large quantities of these three colours used. Black is for general use, blue for potable water and yellow for gas supply.

Read More

Chemical resistance of plastic pipe

Plastic pipe is manufactured from many different compounds with the most common being pvc (sometimes referred to as pvcu or upvc, where the “u” stands for unplasticised, meaning it is rigid), cpvc (or corzan) abs and polyethylene. The choice as to which plastic to use is often dependant on the chemical formula of the substance that is passing through the pipe and fittings.

Pvc pipe is a pipework system with a very good level of chemical resistance to both acids and alkalis, and it is also very good with salts. Although pvc pipe is suitable for highly concentrated acids, bases and hypochlorites, when used in such applications it is recommended that a special chemical resistant solvent cement is also used. Where pvc is not suitable, is in the use of chlorinated and aromatic hydrocarbons. 

CPVC (or as it is sometimes referred to pvc-c pipe) is suitable for most mineral acids, salts, paraffinic hydrocarbons, alcohols and salts. CPVC pipe is a modified pvc polymer that is used to form a plastic pipe with a much higher chemical and temperature resistance than standard pvc pipe. The higher temperature rating also makes if the plastic pipe of choice over normal pvc when using warm or hot chemical solutions. This is because the chemical resistivity of all plastics reduces with increasing temperature. 

Abs pipe is manufactured from a styrene and so is a completely different type of compound and has a very different chemical resistance profile.  The styrene nature of abs pipe makes it very impact resistant down to temperatures as low as -40, because of this ABS is often chosen over pvc for glycol circuits. Abs is only resistant to weak acids but can handle weak to medium alkalis. This property also renders it suitable for chilled food areas, soft drinks and high purity water. ABS is also more abrasion resistant than most other thermoplastics and so if often the choice for abrasive slurries.

Polyethylene pipe (sometimes denoted PE, LDPE, MDPE or HDPE, where the LD, MD and HD stand for Low Density, Medium Density and High Density respectively) has a very good all round chemical resistance. PE pipe will withstand almost all acids and other caustic compounds and, at 20 degrees centigrade, it also resists all organic and inorganic solvents.  Where PE pipe is not suitable, is in applications involving  strong oxidising acids such as ozone gas, or liquids laden with very high concentrations of ozone.  Like ABS Pipe, PE pipe is also suitable down to low temperatures and can be used for glycol circuits etc. ABS usually takes precedence because of the ease of installation as it uses a simple solvent cement process rather than a heat weld system. It should be pointed out that smaller diameter PE pipe can be connected by compression fittings, but these use polypropylene and other rubberized compounds for the sealing rings which may have an adverse effect on the chemical resistance of the pipe system.

Read More

Air locking in plastic pipe systems

Air locking is a problem in many poorly designed plastic pipe systems. The consequences of air locking, especially for continuous production lines and more specifically aquaculture, where the pipe is an integral part of the life support system, can be very costly.

Systems can air lock for a number of reasons but the phenomenon is more common in pipelines which are fed by a low pressure head and / or running at low water velocities. Pipelines that run at high pressures and high velocities have more mixing effect, and even if there is some air trapped in a part of the pipework, it will usually be pushed through the system and out at a valve or purge point.

Air locking can occur at various times of operation and all types of plastics such as pvc pipe, abs pipe etc. are just as prone. Some users install clear pipe systems so that they can observe the air collecting in the pipes and fittings, but this is often only short lived, as the internal bore of the pipe gets coated with particles or biological fouling, which limits the vision. System start up is a time when air locks often occur as parts of the pipe and fittings are full of air at the start of the process. The only way for an air lock to occur once a pipeline is fully flooded is for air to be drawn into the pipe system.

Air can be drawn in through a number of ways. An incorrectly assembled section of plastic pipe may contain a slight leak. If the water velocity, and angle past this leak are at certain values, then the hole will not leak water, but instead, air will be sucked into the plastic pipe in a venturi effect. This very often occurs at threaded joints, especially where there is a size reduction in the pipe, which results in a higher velocity and a lot of turbulence. Air can also get entrained into your plastic pipe systems through incorrectly seated seals in pumps, filters and other devices. Pumps are the most common source however, as there is true suction at some points in the system, assisting the draw in of air.

Once in the pipework, entrapped air rises to the top of the pipe, with small bubbles coalescing into large slugs of air. If the water velocity and pressure in the pipe are high enough the slugs will be drawn along the pipe and will not have a chance to collect. If however, the pipe system is poorly designed, water velocities are low, pressure is low and the pipe either undulates, or has high and low points in the system rather than a continuous gradient, the air can collect and form a barrier to water passing.

Once a system has been installed, if it is prone to air locking, there are a number of methods to alleviate this. In low head systems, plastic pipe fittings can be installed at the point of the air lock and rise up to a level higher than that of the head. Air will then escape through this route. Where this is not possible, special air release valves can be added to the pipe. These types of  valves use an internal float that opens a small valve in the presence of air and closes when all air has been released. Air release valves are only suitable for use in plastic pipe systems where there is a clean liquid (of the valve becomes blocked) and can only be used in a pressure line e.g.; NOT on the suction side of a pump or the fitting will act as a venturi and draw even more air into the pipe system.

Read More

Plastic Ducting uses

Plastic ducting is usually made out of either pvc-u (unplasticised polyvinyl chloride) or polypropylene. Pressure pipe has a much thicker wall and is far heavier than ventilation or ducting pipework, which has a much thinner wall and is subject to different standards and ratings.

Plastic ducting is not suitable for use with liquids or solids such as granular materials as it does not have sufficient strength to prevent distortion and breakage. The main use for this pipe is in the fume extraction and ventilation industries where it is used to carry gasses such as air or contaminated air.
The choice of materials used for ducting is dependent on the application. PVC ducting is the cheapest to buy, systems are easier to construct through a solvent weld method and it is available in a larger number of fittings.  PVC does however have its limitations: it is restricted to a maximum operating temperature of 50 degrees centigrade and because the plastic begins to increasingly soften with temperature, it requires more frequent supporting as the temperature increases. The properties of pvc, namely the fact that it is very easy to bond to, makes it subject to contamination and so unsuitable for some clean room applications such as the food industry. The molecular structure of PPS Polypropylene on the other hand is a very different and is such that other compounds find it very difficult to adhere to it. This makes it the plastic of choice for clean room applications. Polypropylene ventilation systems can however build up a significant amount of static, which can lead to the attraction and build-up of dust and other charged particles. For situations where ultra clean and antistatic pipework is important, such as in the semiconductor industry, PPS ducting is available in a special antistatic form. This is the most expensive of all types of plastic ducting.

Where pvc pipe is solvent welded together, polypropylene should, for permanent installations, be heat welded. Some users prefer not to heat weld and after adding in extra supports, use a silicone or other type of sealant to bond fittings together. This may be suitable for some applications, but care needs to be taken that the system is properly supported and siliconed joins do not impart any strength. Silicone is also easy to bond to and so if used as part of the assembly process, may compromise the suitability of the system.

Plastic ducting, especially pvc, often finds uses I other industry sectors. The film and media sectors use it to create false columns, where it is painted or has material glued to it – for trees etc. The lightweight nature of the pipe makes it easy handle and to move around sets. Smaller bore ducting is sometimes used as cable duct, where the smooth inner surface makes it easy to pull multiple cables through long distances. It also finds a use as protective packaging for expensive machine parts, and when mounted vertically, the manufacture of small plastic water and chemical tanks and air and water filtration systems.
The main manufacturer of plastic ducting in Europe is HOKA, a company based in Germany. The UK distributor for their ducting products are Pisces Pipe Systems.
In summary, plastic ducting is a versatile component that has many uses, other than straight forward ventilation works.

Read More