管道公称尺寸(NPS)历史与应用.docVIP

管道公称尺寸(NPS)历史与应用接触过ASME管道工程的工程师都应该知道NPS的意思，它的全称是Nominal Pipe Size。它是北美用于高、低压及温度的管道尺寸标准体系之一。其管道尺寸用两个非尺寸的编号来标识：即用以英寸单位的NPS和管表号（用Sched.或Sch.）表示。与NPT（national pipe thread）的简称来源相混淆有关，NPS常常被错误地叫做美国国家管道尺寸（National Pipe Size）。对于其它的管道尺寸标准请有兴趣的同行参见相应的管道（材料）尺寸标准，这里不作详细说明。欧洲的相当于NPS的术语是DN（diamtre nominal/nominal diameter），它的尺寸单位毫米表示。还有一个与NPS频繁相交互使用的术语是NB （nominal bore）。历史背景1927年3月，美国标准协会受权了一个委员会去标准化铸钢、铸铁钢和非标管（tube）的尺寸。当时只有一小部分的管壁厚系列被应用，即基于今天在使用的钢管尺寸IPS（iron pipe size）体系：标准重量（STD）、特加重壁厚（XS）和双倍特加重（XXS）管。但是这三个尺寸不满足应用的需要，于是此委员会在行业中进行了调查，制订了基于管壁厚分级差距更小的标识管壁厚的管表号系统（虽然IPS和NPS编号在意义上相同）。最初的目的是使每一个管表号编号与给定的压力等级相关联，但是通常使用中的此编号与管壁厚背离太大，当时的目的没有达到。曾在1939年希望将STD，XS和XXS三个壁厚标识逐步淘汰而采用管表号编号，但时至今日这三个仍在使用。自最初的管表号制订以来，基于行业应用和来自API、ASTM及其它标准，对管表号进行了多次修订和补充。不锈钢管的应用曾在20世纪中期很普遍，由于其抗腐蚀的优越性，更薄壁厚管被允许使用。截止1949年，基于压力要求修订的最接近BWG编号的薄壁厚管表号5S和10S被制订，后来又制订了加注“S”的此类其它管表号。由于它们薄壁性，此小尺寸的“S”管在ASME Standard 36.19M-2004规范上要求不能螺纹连接，只能采用电熔焊接连接。应用从NPS和管表号上不能看出相应的管外径（OD）和壁厚，只能通过ASME标准B36.10M 和B36.19M来获得。例如，NPS14 Sch40，通过查标准可知其OD为14英寸、壁厚为0.437英寸。但是，并不是所有的NPS和OD对应的值都是一样的，这样就产生的混乱。对于NPS1/8到NPS12英寸，其NPS和OD值是不相同的。例如，NPS12管的OD实际上是12.75英寸；要知道其真实的OD，只能通过标准查知。（注：对于Tube管，其NPS总是与实际的OD相同）NPS14及以上尺寸，NPS与OD值是相同的。换句话说，一个NPS14的管子，其真实的OD也正好是14英寸。NPS1/8到NPS12英寸的管子尺寸其NPS与OD不同的原因是，当时壁厚标准最初对这些管的NPS值用同一内径（ID）表示。后来经过发展，ID也有所变化，于是对这些尺寸的管用NPS既不与ID相同，也不与OD相同。对于一个给定的NPS，其OD总是保持不变，其壁厚随着管表号增大。对于一个给定的管表号，其OD随着NPS而增大，但壁厚保持不变或增大。如果应用ASME B31.3中的公式，可以发现随着NPS的增加而保持不变的管表号其压力等级是会降低。在有些规定书上使用的管表号为：标准壁厚（STD）、特加重（XS）、双倍特加重（XXS），尽管这种表示方法已属于一种老的被称为钢铁管尺寸（IPS）的标识方法。IPS编号与NPS编号方法是一样的。对于NPS1/8到NPS10（包括NPS10）之间的尺寸，STD与SCH40S是一样的；XS与SCH80S是一样的；不同的是XXS，其与SCH160通常更厚一些。管表号数字后加注的“S”，比如“NPS Sch 10S”最常用于不锈钢管的壁厚标识中，但也有一些不锈钢管的壁厚也用不加“S”的碳钢合金钢（对应标准ASME B36.10M）标准选用，因此说加注了“S”只是区别ASME B36.19M中不同于ASME B36.10M。对于PVC和CPVC管来说它们都采用NPS的尺寸标识。Nominal Pipe SizeNominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures.1 Pipe size is specified with two non-dimensional numbers: a nominal pipe size (NPS) based on inches, and a schedule (Sched. or Sch.). NPS is often incorrectly called National Pipe Size, due to confusion with national pipe thread (NPT). For other pipe size standards, see pipe (material) - sizes. The European designation equivalent to NPS is DN (diamtre nominal/nominal diameter), in which sizes are measured in millimetres.2 The term NB (nominal bore) is also frequently used interchangeably with NPS.HistoryIn March 1927, the American Standards Association authorized a committee to standardize the dimensions of wrought steel and wrought iron pipe and tubing. At that time only a small selection of wall thicknesses were in use: standard weight (STD), extra-strong (XS), and double extra-strong (XXS), based on the iron pipe size (IPS) system of the day. However these three sizes did not fit all applications. The committee surveyed the industry and created a system of schedule numbers that designated wall thicknesses based on smaller steps between sizes,3 although IPS and NPS numbers remain equivalent.The original intent was that each schedule would relate to a given pressure rating, however the numbers deviated so far from wall thicknesses in common use that this original intent could not be accomplished.34 Also, in 1939, it was hoped that the designations of STD, XS, and XXS would be phased out by schedule numbers, however those original terms are still in common use today (although sometimes referred to as standard, extra-heavy (XH), and double extra-heavy (XXH), respectively). Since the original schedules were created, there have been many revisions and additions to the tables of pipe sizes based on industry use and on standards from API, ASTM, and others.3Stainless steel pipes, which were coming into more common use in the mid 20th century, permitted the use of thinner pipe walls with much less risk of failure due to corrosion. By 1949 thinner schedules 5S and 10S, which were based on the pressure requirements modified to the nearest BWG number, had been created, and other S sizes followed later. Due to their thin walls, the smaller S sizes can not be threaded together according to ASME code, but must be fusion welded.5ApplicationBased on the NPS and schedule of a pipe, the pipe outside diameter (OD) and wall thickness can be obtained from reference tables such as those below, which are based on ASME standards B36.10M and B36.19M. For example, NPS 14 Sch 40 has an OD of 14 inches and a wall thickness of 0.437 inches. However the NPS and OD values are not always equal, which can create confusion.For NPS to 12 inches, the NPS and OD values are different. For example, the OD of an NPS 12 pipe is actually 12.75 inches. To find the actual OD for each NPS value, refer to the tables below. (Note that for tubing, the size is always the actual OD.) For NPS 14 inches and up, the NPS and OD values are equal. In other words, an NPS 14 pipe is actually 14 inches OD. The reason for the discrepancy for NPS to 12 inches is that these NPS values were originally set to give the same inside diameter (ID) based on wall thicknesses standard at the time. However, as the set of available wall thicknesses evolved, the ID changed and NPS became only indirectly related to ID and OD.For a given NPS, the OD stays fixed and the wall thickness increases with schedule. For a given schedule, the OD increases with NPS while the wall thickness stays constant or increases. Using equations and rules in ASME B31.3 Process Piping, it can be shown that pressure rating decreases with increasing NPS and constant schedule.4Some specifications use pipe schedules called standard wall (STD), extra strong (XS), and double extra strong (XXS), although these actually belong to an older system called iron pipe size (IPS). The IPS number is the same as the NPS number. STD is identical to SCH 40S, and 40S is identical to 40 for NPS 1/8 to NPS 10, inclusive. XS is identical to SCH 80S, and 80S is identical to 80 for NPS 1/8 to NPS 8, inclusive. Different definitions exist for XXS, but it is generally thicker than schedule 160.The S designation, for example NPS Sch 10S, most often indicates stainless steel pipes. However some stainless steel pipes are available in steel designations, so strictly speaking the S designation only differentiates B36.19M pipe from B36.10M pipe.1Both polyvinyl chloride pipe (PVC) and chlorinated polyvinyl chloride pipe (CPVC) are made in NPS sizes.SourcesOberg, Erik; Franklin D. Jones, Holbrook L. Horton, and Henry H. Ryffel (2000). ed. Christopher J. McCauley, Riccardo Heald, and Muhammed Iqbal Hussain. ed. Machinerys Handbook (26th Edition ed.). New York: Industrial Press Inc. ISBN 0-8311-2635-3. Nayyar, P.E., Mohinder L. (2000). A1. in Mohinder L. Nayyar, P.E. Piping Handbook (7th ed.). New York: McGraw-Hill. ISBN 0-07-047106-1. Engineering Department (1988). Flow of Fluids Through Valves, Fittings, and Pipe (Technical Pap