Water Hammer

Water hammer, or surge pressure, is a term used to describe dynamic surges caused by pressure changes in a piping system. They occur whenever there is a deviation from the steady state, i.e. when the velocity of the fluid is increased or decreased, and may be transient or oscillating. Waves of positive or negative pressure may be generated by any of the following:

opening or closing of a valve

pump start-up or shutdown

change in pump or turbine speed

wave action in a feed tank

entrapped air

The pressure waves travel along at speeds limited by the speed of sound in the medium, causing the pipe to expand and contract. The energy carried by the wave is dissipated and the waves are progressively damped.

The pressure excess to water hammer must be considered in addition to the hydrostatic load, and this total pressure must be sustainable by the piping system. In the case of oscillatory surge pressures extreme caution is needed as surging at the harmonic frequency of the system could lead to catastrophic damage.

The maximum positive or negative addition of pressure due to surging is a function of fluid velocity, bulk modulus of elasticity of the fluid, pipe dimensions and the modulus of elasticity of the pipe material.

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Pipe Brackets – loose or fixed?

Pipe support for plastics pipes

Plastic pipe systems should be installed using supports designed for use with plastics and should then be installed taking care not to damage or over stress the pipe.

What is a loose pipe bracket?

A loose pipe bracket is a bracket which allows axial movement of the pipe, to allow stress free compensation of temperature changes and compensation of any other operating condition changes.

What is a fixed point?

A fixed pipe bracket is a bracket which prevents the pipe from moving in any direction. The aim of which is to control system stresses caused by temperature changes.

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Chemical and weathering resistance of PVC-U

The outstanding chemical resistance of PVC-U extends to high concentrations; resistance against the influence of most mineral acids, bases and salt solutions and also sodium hypochlorite solutions is very good. Resistance to aliphatic hydrocarbons and elemental chlorine is also good.

PVC-U, in general, shows weakness against aromatic or chlorinated solvents, esters and ketones. Use with gases is also not recommended.

If the use of oils, varnish or fats is being considered, a prior investigation is advisable. These specifications are also valid – with exceptions – for adhesive joints, which normally are implemented by applying strongly dissolving gap-filling solvent cement to the PVC-U.

PVC-U is very resistant to weathering. Long-term influence of direct sunlight as well as the effect of wind and rain damage on the material is only superficial. Despite its very good weathering resistance regarding ultraviolet radiation, PVC-U loses some of its impact strength. In extreme applications it can be advantageous to protect the material from direct sunlight exposure.

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Using plastics means saving energy

Plastics constitute only a small percentage of the entire crude oil usage. But crude oil resources are limited. Already today we have to fall back upon raw materials which can be recycled and extend alternative energy sources. In this context we talk about re-usable raw materials.

All working processes need energy (heat, pressure, motor power). In comparison with metals, manufacturing plastics requires less energy. The production of 1 dm3 material requires an amount of energy which is given in kilograms oil equivalent per litre material in the chart below.

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Comparing thermoplastics, thermosets and elastomers

Thermoplastics

  • repeated melting
  • fusible
  • the amount of crystallites determines the density and mechanical properties
  • under strong mechanical stress they tend to creep and show lasting deformation
  • the strength value decreases with increased heating
  • can be transformed and deformed several times

Thermosets

  • do not melt
  • cannot be fused
  • number of links is decisive for mechanical properties can be deformed under mechanical load, but regain their original form after load is removed
  • only behave elastically in a relatively narrow upper temperature range, therefore more heat stable
  • can only be deformed once

 Elastomers

  • do not melt
  • cannot be fused
  • number of links is decisive for the rubber hardness
  • can be strongly deformed under mechanical stress
  • remain elastic down to low temperatures

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Why use Compression Fittings?

Compression fittings are popular because they do not require soldering, so they are comparatively quick and easy to use. They require no special tools or skills to operate. They are also used in situations where a heat source, in particular a soldering torch, is prohibited, or where it is difficult to remove remains of water from inside the pipe which prevent the pipe heating up to allow soldering.

Compression Fittings are ideal for new installations, repairs and extensions of existing water pipelines and can be integrated perfectly into numerous types of industrial applications. The compression fitting also lends itself to temporary solutions in pipe networks because it is so easily assembled and disassembled. All it takes is one simple operation to assemble and what’s more this fitting can be used repeatedly. Other applications can be found in mining, recreation parks, golf courses, camping sites, agriculture, greenhouses and irrigation systems.

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The production of plastics

Plastics are manufactured by compounding together large number of similar basic components (monomers) through chemical bonding.

The plastic industry only consumes approximately 6 % of the petroleum products originating from refineries.

To produce plastics three different processes are used:

Polymerisation

Polycondensation

Polyaddition

Polymerisation is the most frequently used procedure for the synthesis of plastics. Polymerisation means the lining up of macromolecule chains without separation of foreign matter.

For example polyethylene, polybutene, polypropylene, polyvinylchloride and other plastics are all produced by means of polymerisation.

During polycondensation similar and dissimilar monomers are lined up to become macromolecule chains, at the same time separating a by-product, e. g. water, hydrochloric acid, etc.

Polycondensation is applied, for example, to produce phenolic resins and polyamids.

During polyaddition macromolecules are created from chemically different molecules, however without separating a by-product.

The procedure is used for the production of polyurethanes and epoxy resins.

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Keeping Wind Farm Systems Cool

The power generated by the wind turbine is in AC mode and to prevent energy loss in the transfer of power the current is converted to DC through the transformer.

During the AC to DC current conversion there is heat generated and cooling is required to minimise the energy lose and prevent the turbines from catching fire. Typically during this current transfer about 1% energy loss occurs.

Material Requirements on the distribution system for wind turbines

  • Dielectric strength 

    Combination of ac and dc voltage stress put high requirements on purity and insulation properties of the material

  • Mechanical strength

    High and stable mechanical strength is required to give freedom in choice of geometries and shapes for details of the piping system.

  • Machinability

    Good machinability allows for complex geometries and narrow tolerances to ensure long term tightness of the system

  • Weld ability

    Good and stable welding properties with material strength comparable to the base material gives high freedom in design of the system

  •  Material stability

    With a required design life time of plus 30 years, in an environment with high temperature and high mechanical and electrical stress, the material must show a high degree of stability in order to ensure long life.

  • Fire retardancy

    The handling of large amounts of power gives risk for excess heat and requires a fire retardant material to limit the consequences of a system malfunction.

    The cooling manifold is typically supplied in stainless steel.  Given the corrosion for stainless steel near the sea it can be replaced by a PVDF assembly. Some of the advantages of PVDF include excellent mechanical /electrical properties, effective shaping and welding properties and excellent machinability.

    The current from the wind turbines is fed into a distribution centre as a DC current and then fed into the electrical network as an AC current for the retail market.

    Electrical cables reach temperatures of 400C and need to be cooled to minimise energy lost. They can be cooled with running cold water through a surrounding PE100 pipe through heat transfer. As the water heats up it is passed through a heat exchanger and cooled again in a continuous closed loop.

     

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Chemical, weathering, and abrasion resistance for PE

Due to its non-polar nature as a hydrocarbon of high molecular weight, polyethylene shows a high resistance against chemical attack. PE is resistant to acids, alkaline solutions, solvents, alcohol and water. Fat and oil swell PE slightly. PE is not resistant against oxidising acids, ketones, aromatic hydrocarbons and chlorinated hydrocarbons.

For additional information, please refer to the detailed list of chemical resistance tool on the GF Piping Systems website.

If polyethylene is exposed to direct sunlight over a long period of time, it will, like most natural and plastic materials, be damaged by the short wave UV portion of sunlight together with oxygen in the air, causing photo-oxidation. Because of this, GF black polyethylene grades are effectively stabilised against UV light by adding carbon black.

PE also has excellent resistance against abrasion. As a result, PE piping systems are used in numerous applications for transporting solids and slurries.

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How to select the right valve for your pipeline

The medium transported basically determines the selection of valves. Pressure and temperature are important criteria. The valve best suited for a particular pressure and temperature can be determined from the technical data of the respective valve.

The selection of the material for the valve seals needs to be selected. This is best decided by consulting the chemical resistance list.

Also to be considered is compressible media.

Suitable materials for compressible media are those that under standard conditions and at low temperatures do not tend toward brittle fractures owing to their ductility.

Such materials include polyethylene (PE) and acrylonitrile-butadiene-styrene (ABS). All other raw materials such as polypropylene (PP-H), polyvinyl chloride (PVCU/-C) or polyvinyliden fluoride (PVDF) are to be limited to ≤ 0.1 bar with respect to the operating pressure of gases. Higher pressures are possible if secondary containment piping systems are used (for environmental protection, brittle effects, gas shocks, intoxication).

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