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1、地上高密度聚乙烯(HDPE)管一、背景有时，我们会在需要在主要地形上铺设管道的工程上工作。管道可以简单地放置在地面上，也可以悬挂或“支撑”在支撑结构中。由于临时 管道系统的经济考虑、岩石的存在和爆破沟槽的本钱、土地权利、文化资源或防止管 道埋置的地役权，可能需要地面安装。聚乙烯(PE)管具有良好的接头完整性、韧性、柔韧性和低重量，使其在许多“地 面”应用中实用。本技术说明介绍了塑料管道协会推荐的聚乙烯(PE)管道地面安装 的设计标准和现行工程方法。本技术说明将讨论极端温度、化学暴露、紫外线辐射和 潜在机械冲击或载荷的影响。文中还讨论了在地面和悬挂式或悬挂式聚乙烯管道装置 的工程设计考虑因素。二
2、、设计准那么已经开发了一个Excel电子表格来帮助进行转换和分析。电子表格位于西澳州工 程网页上。影响安装在地面上的聚乙烯管性能的设计标准包括：温度、耐化学性、紫外线照 射、潜在的机械冲击或载荷、火灾损失三、温度地上管道安装暴露在温度的大范围波动中。相比之下，地下设施的温度通常相对 稳定。有三种主要的温度变化会影响安装在地面上的任何管道材料。这些变化是：太 阳光照射、季节性极端温度和昼夜温度变化。一般情况下，聚乙烯管可在温度低至-70 华氏度和高至150华氏度下平安使用。但温度对聚乙烯管材的工程性能有影响。聚乙烯管的压力性能取决于制造过程中所用聚合物的长期静水强度(LTHS)。随 着聚乙烯管暴
3、露温度的升高，热释光率降低了 50%。相应地，随着工作温度的降低， 特定管道的额定压力增加了 150虬有关特定聚乙烯管温度响应特性的信息，可从相应的管道制造商处获得。四、耐化学性聚乙烯管不会因化学、电子或电流作用而生锈、腐烂、凹陷或腐蚀。对聚乙烯管 道造成潜在严重问题的唯一化学环境是强氧化剂和某些碳氢化合物。浓硫酸和硝酸是 强氧化剂，而柴油和燃料油是典型的碳氢化合物。hydrostatic strength (LTHS) of the polymer used in its manufacture. The LTHS decreases as much as 50% as the temper
4、ature to which the PE pipe is exposed increases. Correspondingly, the pressure rating of a specific pipe is increased as much as 150% as the service temperature decreases.Information regarding the temperature responsive nature of a specific PE pipe is available from the respective pipe manufacturer.
5、Chemical ResistancePolyethylene pipe will not rust, rot, pit or corrode because of chemical, electronic or galvanic action. The only chemical environments that pose potentially serious problems for polyethylene pipe are strong oxidizing agents and certain hydrocarbons. Concentrated sulphuric and nit
6、ric acids are strong oxidizers while diesel and fuel oils typify the hydrocarbons.Environments that contain oxidizing agents may affect the performance characteristics of PE pipe. The continued exposure of polyethylene to strong oxidizing agents may lead to crack formations or a crazing of the pipe
7、surface. Usually, occasional or intermittent exposure to these agents will not significantly affect the long-term performance of a PE pipe.Ultraviolet ExposurePolyethylene pipe utilized outdoors in above ground applications is subjected to extended periods of direct sunlight. The ultraviolet compone
8、nt in sunlight can produce a deleterious effect on the pipe unless the material is sufficiently protected. Polyethylene pipe produced with a minimum of 2% concentration of finely divided and evenly dispersed carbon black is protected from the harmful effects of UV radiation.Mechanical Impact or Load
9、ingAny piping material that is installed in an exposed location is subject to the rigors of the surrounding environment. It can be damaged by the movement of vehicles or other equipment, and such damage generally results in gouging, deflecting or flattening of the pipe surfaces. If an above ground i
10、nstallation must be located in a region of high traffic or excessive mechanical abuse, the pipe requires extra protection. The PE pipe may be protected by building a berm or by encasing the pipe where damage is most likely.Design criteria for the installation of buried flexible thermoplastic pipe sh
11、ould be used for those areas where the above ground PE system must pass under a roadway or other access and/or where an underground installation of a portion of the system is necessary.In general, in an installation in which any section of PE pipe has been gouged in excess of 10% of the minimum wall
12、 thickness, the gouged portion should be removed. When the PE pipe has been excessively or repeatedly deflected or flattened, it may exhibit stress-whitening, crazing or cracking, or other visible damage. Any such regions should be removed and replaced with new pipe material.Fire DamageA major consi
13、deration for the use of above ground PE pipe is the potential damage from fire. PE materials will sag, deform, and/or burn when subjected to high temperatures associated with fire.The potential for wildfire along the path of any above ground pipe installation needs to be addressed in the operation a
14、nd maintenance plan. Items may include the use of fire retardant vegetation along the pipeline route and established fire breaks.DESIGN METHOD: ALLOWABLE DESIGN PRESSUREThe exposure of above ground pipe to sunlight can result in extremely high outside surface temperatures. In the majority of cases,
15、the water flowing in the pipe is substantially cooler than the exterior of the exposed above ground pipe and water flowing through the pipe tends to moderate the surface temperature of the exposed pipe. This can result in a pipe wall temperature that is only slightly above the temperature of the wat
16、er flowing through the pipe. However, in pipeline systems with occasional f1ow, the temperature increase can be much higher. The site specific design needs to determine the allowable pressure rating of the PE pipe based upon the expected maximum service temperature.Example 1What is the pressure capa
17、bility for a SDR 11 series of PE 3408 pipe designed to operation at 100 o F?From the manufacturer data, the pressure capability rating for SDR 11, PE 3408 pipe with water at 74. 3 o F is 160 psi.Table 1 - Pressure Capability Design Factors, PE 3408 PipeService Apparent Modulus Apparent Long-Term Pre
18、ssureFrom Table 1, the 100 o F (38 o C) pressure design factor is 0. 78, therefore the design pressure capacity, P (100 o F) would be the allowable design pressure multiplied by the design pressure factor:DESIGN METHOD: EXPANSION AND CONTRACTIONThe expansion and contraction for an unrestrained PE pi
19、pe can be calculated by the following equation:A 1000 ft section of 2 SDR 11 material (PE 3408) is left unrestrained overnight. If the initial temperature is 70 oF, determine the change in length of the pipe section after a night time temperature of 30 oF.As shown in Example 2 the change in pipe len
20、gth for PE pipe can be significant. However, this calculated change in length assumes both an unrestrained movement of the pipe and an instantaneous drop in temperature. Actually, no temperature drop is instantaneous and the ground on which the pipe is resting creates a retarding effect on the theor
21、etical movement due to friction. Practical field experience of polyethylene pipe has shown that the actual contractions or expansions that occur because of temperature change is approximately one-half the theoretical amount.Field experience has also shown that changes in physical length are often fu
22、rther mitigated by the thermal properties or heat-sink nature of the flow stream within the pipe. However, conservative engineering design warrants that consideration is given to the effects of temperature variation when the f1ow stream is static or even when the pipe is empty.When PE pipe is expose
23、d to temperature changes and restrained from moving, the specific anchor(s) must resist the stresses developed in the pipe wall.Typical devices include: tie down straps, concrete anchors, thrust blocks, etc.DESIGN METHOD: LONGITUDINAL STRESS VS. TEMPERATURE CHANGEAsuming the same conditions as Examp
24、le 2, what would the maximum theoretical force developed on the unrestrained end of a PE pipe if the other end is restrained? The cross sectional area of the pipe wall is approximately 1. 5 in2, the temperature change is instantaneous, and the frictional resistance against the soil is zero.For the c
25、onditions where the temperature change is gradual the actual stress level is approximately half that of the theoretical value. This would result in an actual force of about - 600 lbs or 50% of the theoretical force (FT) of 1200 lbs.INSTALLATION CONSIDERATIONSThere are two basic types of above ground
26、 installations. One type is on grade or stringing out over the naturally occurring grade or terrain. The other type is suspending/supporting the pipe from various saddles and/or support structures. Each of these installations involves different design methodologies.On Grade InstallationsAs discussed
27、 previously, pipe subjected to temperature variation will expand and contract in response to the variations. The designer has two options available to counteract this phenomenon. The pipe may be installed in an unrestrained manner, allowing the pipe to move freely in response to temperature change.
28、The other option is to use a restrained system where the pipe may be anchored by some means that will control any change of the pipes physical dimensions.Unrestrained (Free) Movement of PipeAn unrestrained pipe installation requires that the pipe be placed on a bed or right of way that is free of ma
29、terial that may abrade or otherwise damage the exterior pipe surface. The object is to let the pipe wander9 freely without restriction as expansion/contraction occurs without potential for damage from abrasion or point loadings. This installation method usually entails snaking the PE pipe along the
30、right of way.An otherwise free moving PE pipe must eventually terminate at or connect to a rigidstructure (i.e. inlet structure, trough, pump, etc). Transitions from free moving PE pipe to a rigid pipe appurtenance must be stabilized to prevent stress concentration within the transition connection.
31、Some common methods used to restrain the pipe adjacent to a rigid termination/connection are: An earth covered section of pipe. A reinforced concrete thrust block. Mechanical pipe anchor.This circumvents the concentrating stress effect of lateral pipe movement at termination points by relieving the
32、stresses associated with thermal expansion or contraction within the pipe wall itself. Equations 2 and 3 can be used to determine expected pipe expansion/contraction and the design stress on anchors.In many instances, it is desirable to control the zone of pipe movement on an unrestrained reach of p
33、ipe. This is especially important on sloping land since unrestrained pipe will move down slope as expansion/contract ion occurs. In addition, this technique can beused to limit the zone of horizontal movement where the ground surface must be modified to create a suitable surface on which to lay the
34、pipe. This can be accomplished by placing posts along a desired pipe route and allowing the pipe to move freely within the designated zone. Posts, at a spacing of approximately 300 ft, have adequately served this purpose. A closer spacing is generally needed for bends.On sloping ground, sufficient r
35、estraints need to be placed along the pipeline to ensure that the stress created in the pipe wall does not exceed the allowable stress limits for the pipe material. The long-term downhill movement of the pipe during expansion and the need for the pipe to pull itself uphill during contraction can be
36、great. When pipe goes straight upslope or down slope, the maximum distance between anchors, for straight reaches of pipe will be determined as follows:Restrained Movement of Pipe. Designs for a restrained above ground installation must consider the means by which the movement will be controlled, the
37、 anchoring or restraining force needed to control the anticipated expansion, and contraction stresses. Common restraint methods include earthen berms, pylons, angered anchors, concrete cradles or thrust blocks.The earthen berm technique may be either continuous or intermittent. The pipeline may be c
38、ompletely covered with a shallow layer of soil over its entire length or it may be stabilized at specific intervals. An intermittent earthen berm installation entails stabilization of the pipe at fixed intervals along the length of the pipeline. At each point of stabilization, the above ground pipe
39、is encased with earthen fill for a distance of five to ten feet. Other means of intermittent stabilization listed above provide equally effective restraint of the pipeline.A pipeline that is anchored intermittently will deflect laterally in response to temperature variations and this lateral displac
40、ement creates stress within the pipe wall. The relationships between these variables are determined as follows:As a rule, the frequency of stabilization points is an economic decision. For example, if lateral deflection must be severely limited, the frequency of stabilization points increases signif
41、icantly. On the other hand if substantial lateral deflection is permissible, fewer anchor points will be required and the associated costs are decreased. Allowable lateral deflection of PE is not without a limit. The upper limit is determined by the maximum permissible strain in the pipe wall itself
42、. This limit is a conservative 5% for the majority of above ground applications as determined by Equation 6.Equations 5 and 6 are used to determine the theoretical lateral deflection or strain in overland pipelines. Actual deflections and strain characteristics may be significantly less due to the f
43、riction imposed by the prevailing terrain, the weight of the pipe, the flow stream and that most temperature variations are not normally instantaneous.Example 5含有氧化剂的环境会影响聚乙烯管的性能。聚乙烯继续暴露在强氧化剂下可能导 致裂纹形成或管道外表开裂。通常，偶尔或间歇接触这些药剂不会显著影响聚乙烯管 的长期性能。五、紫外线照射在室外地面上使用的聚乙烯管会受到长时间的阳光直射。阳光下的紫外线成分会 对管道产生有害影响，除非材料得到充
44、分保护。用最小浓度为2%的分散均匀的细炭 黑生产的聚乙烯管可防止紫外线辐射的有害影响。六、机械冲击或载荷任何安装在暴露位置的管道材料都要受到周围环境的严格限制o它可能因车辆或 其他设备的移动而损坏，这种损坏通常会导致管道外表的刨削、偏转或压扁。如果地 面安装必须位于交通繁忙或过度机械滥用的区域，那么管道需要额外保护。聚乙烯管可 通过建造护道或在最有可能损坏的地方包裹管道来保护。埋地软质热塑性塑料管安装的设计标准应适用于地面PE系统必须穿过道路或其 他通道和/或系统的一局部必须在地下安装的区域。一般来说，在任何一段聚乙烯管被凿除超过最小壁厚10%的装置中，凿除的局部 应被移除。当聚乙烯管过度或反
45、复偏转或压扁时，可能出现应力白化、开裂或其他可 见损伤。任何此类区域都应移除，并用新的管道材料替换。七、火灾损失使用地上聚乙烯管的一个主要考虑因素是火灾可能造成的损害。聚乙烯材料在与 火有关的高温下会下垂、变形和/或燃烧。在运行和维护计划中，需要解决任何地上管道安装路径沿线可能发生的野火。项 目可能包括沿管道路线使用阻燃植物和已建防火带。八、设计方法：允许设计压力地上管道暴露在阳光下会导致非常高的外外表温度。在大多数情况下，管道中流 动的水比暴露在地面上的管道的外部要冷得多，流经管道的水倾向于调节暴露管道的 外表温度。这可能导致管壁温度仅略高于流经管道的水的温度。然而，在偶发流动的 管道系统中
46、，温度升高可能会更高。现场具体设计需要根据预期的最高使用温度确定 PE管的容许压力额定值。例1Engineering Technical Note #12 ABOVE GROUND HDPE PIPEJanuary 2009 Assume that a 10 0D, SDR 11, PE pipe is strung out to grade and anchored at 100 ft intervals. What is the maximum theoretical lateral deflection possible, given a 50 oF temperature increa
47、se? What strain is developed in the pipe wall by this temperature change?From the calculations in Example 5, it is apparent that lateral deflections that appear significant may account for relatively small strains in the actual pipe wall. The relationship between lateral deflection and strain rate i
48、s highly dependent on the selected spacing interval for the restraints.Supported or Suspended PipelinesWhen PE pipe installations are supported or suspended, the temperature and corresponding deflection characteristics are similar to those discussed above for unsupported pipelines with intermittent
49、anchors. There are two additional parameters to be considered as well: 1) beam deflection and 2) support or anchor configuration. Since this type of installation is not often used on NRCS projects with PE pipe, information on these procedures are shown in the reference sited at the conclusion of this note.Anchor and Support DesignProper design of anchors and supports is as important with PE piping as it is with othe