RESULTADOS

Many manufacturers of thermoplastic piping material use the standard dimension ratio (SDR) method of rating pressure pipe. SDR is the ratio of the pipe O.D. to the minimum thickness of the wall of the pipe. It can be expressed mathemati-cally as:

SDR⫽ where SDR⫽ Standard Dimension Ratio

D⫽ Pipe outside diameter in inches t⫽ Pipe minimum wall thickness in inches

For a given SDR the ratio of the O.D. to the minimum wall thickness remains constant. An SDR 11 means the O.D. of the pipe is eleven times the thickness of the wall. This remains true regardless of diameter. For example, a 14-inch O.D. pipe with a wall (t) of 1.273-inch is an SDR 11 pipe. An 8-inch O.D. pipe with a wall (t) of .785-inch is also an SDR 11 pipe. Common SDR ratios are SDR 9.3, SDR 11, SDR 13.5, SDR 15.5, SDR 17, SDR 19, SDR 21, SDR 26 and SDR 32.5. For high SDR ratios, the pipe wall is thin in comparison to the pipe O.D. For low SDR ratios, the wall is thick in comparison to the pipe O.D. Given two pipes of the same O.D., the pipe with the thicker wall will be stronger than the one with the thinner wall.

Pipes with a high SDR rating have low-pressure ratings and pipe with low SDR ratings have high-pressure ratings because of the relative wall thickness.

The pressure rating of thermoplastic pipe is mathematically calculated from the SDR and the allowable hoop-stress. The allowable hoop-stress is commonly known as the long-term hydrostatic design stress. This is the stress level that can exist in the pipe wall continuously with a high degree of confidence that the pipe will operate under pressure for at least 50 years with safety. The American Society for Testing and Materials (ASTM) and the Plastics Pipe Institute (PPI) has adopted

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GENERAL DESIGN PROCEDURES 4.31

the following formula relating SDR and hydrostatic design stress as the standard for the industry.

P⫽ where P⫽ Pressure rating (psi)

D⫽ Pipe OD (inches)

t⫽ minimum wall thickness (inches) S⫽ Hydrostatic Design Stress SDR⫽

From the formula you can see that all pipes of the same SDR (regardless of diameter) will have the same pressure rating for a given design stress.

Water Hammer/Pressure Surge. The effects and calculations for water hammer or pressure surge were covered in detail in chapter 3. The following section contains some key design considerations for handling the effects within thermoplastic pip-ing systems.

Since all moving objects have mass and velocity, any flowing liquid has momen-tum and inertia. When flow is suddenly stopped, the mass inertia of the flowing stream is converted into a shock wave or high static head on the pressure side of the pipeline. Some of the more common causes of hydraulic transients are

1. The opening and closing (full or partial) of valves 2. Starting and stopping of pumps

3. Changes in turbine speed 4. Changes in reservoir elevation 5. Reservoir wave action 6. Liquid column separation 7. Entrapped air

Thermoplastic piping materials are well suited to handle occasional surge sures. Some PE manufactures provide pipe that can withstand occasional surge pres-sures up to 2.5 times the rated pressure capability of the pipe without a cumulative effect. This is due to the long-term modulus of the material being only a fraction of the short-term modulus.

In general, good system design will eliminate quick opening/closing valves on anything but very short lines. The design engineer should use judgment with regard to the addition of surge pressures to operating pressures when selecting pipe SDRs. The following rules of thumb may be of help:

• Occasional shock pressures can be accommodated within the design safety factor. Due to the short time duration of the surge pressure, occasional shock

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wave surge pressures to 2.5 times the SDR pressure rating at 73.4°F are usu-ally allowable

• If surge pressure or water hammer is expected in a system, keep the flow velocity on the low side of the velocity range

If surge pressure or water hammer is expected, maximize the time required to shut off a valve or reduce flow. A shutoff cycle 6–10 times the time period 2L/S is suggested to minimize surge pressures by gradually slowing the fluid flow. If constant and repetitive surge pressures are present, the excess pressure should be added to the nominal operating pressure when selecting the pipe SDR.

Cyclic Overpressure

There are many causes to short-term cyclic overpressure. Activities such as a stuck relief valve, a plugged discharge line or repetitive freeze/thaw cycles can cause ex-tended rises in pressure. In steel, concrete, PVC or fiberglass pipe, the effect of the increased pressure can be devastating. With many of the thermoplastic piping mate-rials the pipe wall is able to stretch (strain) with the freezing water and the pipe will return to its original condition after the frozen water has thawed. Although some residual strain may be evident, the physical properties of the pipe resin are not ad-versely affected and the performance of the pipe at normal operating conditions is not affected. Due to the innate elastic characteristics of many thermoplastic pip-ing materials, they are capable of withstandpip-ing these types of cyclic loadpip-ings with-out damage to the pipe’s performance. The effects of extended and repeated over pressure can be tolerated within specific limits. Many thermoplastic pipes have an inherent ability to recover from the strain of overpressures. If the recovery period at a normal level of stress is equal to or greater than the duration of the over pres-sure, the pipe can be subjected to the stresses for short periods without affecting their long term strength, endurance, and performance.

The basic limitation on short-term overpressure cycles is to stay within the elastic limits of the pipe material. If the system pressure exceeds 2.5 times the rated pressure of the pipe for any length of time, permanent strain or deformation of the pipe occurs. As a result, the expected service life of the pipe can be dramatically reduced. When overpressure cycling is expected as a regular condition of opera-tion, the highest pressure anticipated the majority of the operating times should be considered as the operating pressure and it should be treated as though it would persist continuously for the design life of the system.

Longitudinal Stress from Internal Pressure

When a fully restrained pipeline such as a buried or anchored pipeline is pressur-ized, longitudinal stresses develop in the pipe wall. The longitudinal stress is cal-culated as follows:

GENERAL DESIGN PROCEDURES 4.33

SL⫽ where SL⫽ Longitudinal tensile stress, psi

␮ ⫽ Poisson’s ratio

P⫽ Internal operating pressure, psi D⫽ Pipe outside diameter, inches

t⫽ Pipe wall thickness, inches

Thermal Expansion and Contraction

Thermal expansion and contraction are key items of concern in the design of a ther-moplastic piping system. Design parameters should be developed and incorporated into the installation specifications. Thermoplastic piping materials have a higher coefficient of expansion than some other common pipeline materials, however, the forces generated by thermal stresses are much lower because the modulus of elas-ticity is lower and it is capable of stress relaxation.

There are many methods available to the designer to control expansion or con-traction. One method is to install the pipeline when it is within 10 to 15 degrees Farenheit of its operating temperature. Other methods of controlling expansion/

contraction are pertinent to certain types of installations and are briefly discussed.

Supported Pipelines

A common practice is to install the pipe in a warm condition in a straight line while it is in an expanded state. As the pipeline cools it develops a tensile stress and the pipeline remains straight between supports. As the pipe warms to its installation tem-perature due to seasonal change or operating conditions, it returns to its installation condition and straightness. In this manner, sag between supports is minimized.

Overland Pipes

Controlling the expansion and contraction of overland surface lines is difficult be-cause the uneven soil friction between the pipe and the ground does not allow dis-tributed lateral deflections to occur uniformly. In the worse case, all deflection may occur in one area where friction is low and the pipe may kink. This condition will most likely occur in empty lines or where large, sudden operating temperature changes occur. If overland pipelines are installed in a snaked pattern, thermal expansion/contraction can be controlled through control of lateral deflection. Dur-ing pipeline warmDur-ing, the “S” configuration becomes slightly greater. As the pipe cools, the pipeline becomes straighter. Surface lines that are continuously operated full of fluid normally experience small, slow temperature variations and are easy to control. The weight of the fluid also increases friction and reduces deflection.

How-␮P(D ⫺ t)

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PLASTIC PIPING HANDBOOK 4.34

ever, it may necessary to anchor the line at intervals to direct and limit the deflec-tion in any one segment of the pipeline.

Buried Pipelines

Buried pipeline installations offer a significant degree of restraint due to soil fric-tion. This is further controlled because the pipe usually lies in a slight “S” curve in the trench as it is installed. Because the temperature of the soil is fairly constant, temperature changes that do occur take place over a yearly season. Due to the enor-mous heat sink capability of the earth, the magnitude of any temperature change is reduced and the time required to effect that temperature change extended.

A buried process pipeline operating at a specific temperature may develop some initial thermal stress upon start-up. This is usually restrained by soil friction and dissipated with time by stress relaxation. As the pipeline continues operation, it tends to bring the soil envelope surrounding the pipeline into equilibrium with the operating temperature. If a minor temperature change does occur, its effect is further minimized by the massive thermal inertia within the pipe wall and in the soil surrounding the pipeline.