CHAPTER C3
STEAM SYSTEMS PIPING
Daniel A.Van Duyne,P.E.
Senior Design Engineer,Northeast Utilities
Millstone Nuclear Power Station,Waterford,Connecticut
Formerly Assistant Chief Engineer,Mechanical Di v ision
Stone&Webster Engineering Corporation
Boston,Massachutts
INTRODUCTION
General
In this chapter we willfirst review the basics of piping system design for steam systems applications;
we will then consider specifics for underground steam piping and steam piping ud in power plants.
Definitions and Terminology
Condensate.Condend steam.
Trunk line distribution system.Distribution system with a large-diameter line leav-ing the boiler plant;as lateral branches are installed off it for rvice,the trunk line gradually diminishes in diameter.
Main and feeder network distribution system.Distribution system that receives its supply of steam through a high-pressure feeder main leading from the plant through the network;the size of the feeder main required in this ca is not as large as in a trunk-line system with the same boiler-plant steam pressure.
Protecti v e conduits(typically for underground steam lines).Enclosures for under-ground steam mains and rvices to(1)protect the pipe and insulation from damage due to earth pressure and impact loadings,(2)allow free longitudinal expansion and contraction while held in proper alignment,and(3)prevent groundwater epage or flooding by providing either drains or a completely waterproof structure.
Light water reactors(LWR).Nuclear power reactors of either the pressurized water reactor(PWR)or boiling water reactor(BWR)type.
C.83C.84PIPING SYSTEMS
ASME Class11piping.Includes main steam piping up to and including thefirst stop valve outside the reactor containment for BWRs and it is designated as ASME Class1.
ASME Class21piping.Includes main steam piping up to and including thefirst stop valve outside the reactor containment for pressurized water reactors(PWRs) and BWR main steam piping after thefirst isolation valve outside the reactor containment and it is designated as ASME Class2.
ASME B31.12piping.Includes main steam piping downstream of thefirst stop valve outside the reactor containment in PWRs and piping external to boilers in fossil power plants,and it is constructed to the ASME B31.1Code for Pressure Piping. ASME B31.33piping.Includes steam process piping in industrial plants and it is designed to the ASME B31.3Code for Process Piping.
BEP.Boiler external piping.跨界歌王第三季
DNBR.Departure from nucleate boiling ratio.
FFWT.Final feedwater temperature.
HARP.Heater above the reheat point.
IP.Intermediate pressure(turbine).
IV.(Governor-operated)intercept valve.
kPa.Pressure in kilo pascals.
LP.Low pressure(turbine).孕妇能吃马蹄吗
MPa.Pressure in mega pascals.
NSSS.Nuclear steam supply system.
Pa.Pressure drop or pressure in pascals.
psi.pressure drop in pounds(force)per square inch,lb f/in2.
psia.pressure in pounds(force)per square inch,absolute.
psig.pressure in pounds(force)per square inch,gauge.
PWHT.Postweld heat treatment.
VWO.Valves wide open(steam turbine).
Nomenclature
A.An additional thickness to provide for material removed in threading,corrosion or erosion allowance,and material required for structural strength of the pipe,as appropriate,in(mm).See Table C3.1for lected values of A.
d.Inside diameter of pipe,in(mm).In using Eq.(C3.4),the value of d is for the maximum possible inside diameter allowable under the purcha specifications.
D o.Outside diameter of pipe,in(mm).For design calculations,the outside diame-ter of pipe as given in ASM
E B36.10M and ASME B36.19M4,5and specifications shall be ud in obtaining the value of t m.
L.Length of pipe,ft(m).
P.Internal design pressure,psi(Pa).愚人节几号
P a.Calculated maximum allowable internal pressure,psi(Pa),for straight pipe which shall be at least equal the design pressure.P a may be ud for piping products with pressure ratings equal to that of straight pipe.6For pipe products where the
TABLE C3.1Selected Values of A1,2
Type of pipe A,in(mm)
Cast-iron pipe centrifugally cast or cast horizontally in0.14(3.56)
green sand molds
Cast-iron pipe,pit cast0.18(4.57)
Threaded steel,wrought-iron or nonferrous³⁄₄in(19mm)0.065(1.65)
nominal and smaller
Threaded steel,wrought-iron or nonferrous1in(25mm)Depth of thread
nominal and larger
Grooved steel,wrought-iron,or nonferrous Depth of groove plus1/64in.
(0.40mm)
Plain-end steel or wrought-iron pipe or tube for sizes0.05(1.27)
1in(25mm)and smaller
Plain-end steel or wrought-iron pipe or tube for sizes0.065(1.65)
over1in(25mm)
Plain-end nonferrous pipe or tube0.00
pressure rating may be less than that of the ,flanged joints and reinforced branch connections where part of the required reinforcement is in the run pipe), the design pressure shall be ud instead of P a.
⌬P.Pressure drop,psi(Pa).
S.Maximum allowable stress for the material at the design temperature,psi(Pa). t.Specified or actual wall thickness minus,as appropriate,material removed in threading,corrosion or erosion allowance,material manufacturing tolerances,bend-ing allowance(e Table C3.2),and material to be removed by counterboring, in(mm).
卡夫丁峡谷t m.Minimum required wall thickness,in(mm).If pipe is ordered by its nominal wall thickness,the manufacturing tolerance on wall thickness must be taken into account. W.Steamflow,lb m/min(kg/hr).
三文鱼鱼头Y.Density of steam,lb m/cu ft(kg/m3)[ud in Eqs.(C3.2)and(C3.2M)].
y.A coefficient having values as given in Table C3.3.For pipe with a D o/t m ratio
TABLE C3.2Minimum Thickness Prior to Bending*18
Induction and Rotary Ram and Radius Furnace incremental draw roll
of bend bending bending bending bending 6Dn 1.06t m 1.06t m 1.09t m 1.08t m 5Dn 1.08t m 1.08t m 1.14t m 1.10t m 4Dn 1.14t m 1.10t m 1.20t m 1.13t m 3Dn 1.25t m 1.14t m 1.26t m 1.17t m 2Dn 1.22t m
1.5Dn 1.30t m
*t m is determined by Eq.(C3.3)or(C3.4).TABLE C3.3Values of Coefficient y1,2
Temperature900(482)9501000105011001150(621)ЊF(ЊC)and below(510)(538)(566)(593)and above Ferritic steels0.40.50.70.70.70.7 Austenitic steels0.40.40.40.40.50.7
Note:The value of y may be interpolated between the50ЊF(28ЊC)values shown in the table.For nonferrous materials and cast-iron,y equals0.4.
春联的种类
less than6,the value of y for ferritic and austenitic steels designed for temperatures of900ЊF(482ЊC)shall be as calculated from Eq.(C3.1),as
follows:
Types of Systems
Steam Distribution Systems.While there are nofixed standards for the design of steam distribution pi
ping systems,most of the systems fall into one of two general class:(1)a trunk-line distribution network system and(2)a main and feeder distribution network system.
In Ca1,the diameter of the trunk line leaving the boiler plant is large,and as lateral branches are installed off it for rvice,the diameter of the trunk line is gradually reduced as the needs for carrying capacity are diminished.
In Ca2,the main and feeder network distribution system receives its supply of steam through a high-pressure feeder main leading from the plant through the network.Advantage is taken of the pressure drop available for the transportation of large volumes of steam to the low-pressure network.
Figures C3.1and C3.2show typical steam distribution systems.
Since piping is the largest individual factor in the lection and design of a steam distribution system,the major items that must be resolved are as follows:(1) pipe size,(2)wall thickness,(3)materials lection,(4)types of joints,(5)proper insulation,(6)a protective conduit for the pipe and insulation from water and mechanical damage,(7)drainage of condensate,(8)provision for thermal expansion with controlling anchorage,and(9)safety provisions.
Underground Steam Piping.Underground piping for steam distribution has been a highly specializedfield of engineering peculiar to the district-heating industry; today underground steam piping is also ud to carry process steam.With the advent of groups of buildings such as in housing developments,institutions,and industrial plants,central heating systems and steam distribution problems are no longer restricted to the district-heating industry.Steam piping systems may be buried and not readily available for enlargement,replacement,and repair.Such piping must be protected from ground elements and excessive heat loss;thus it is important that before such a system is installed every pha of its design and operation be understood.Figures C3.1and C3.2show typical steam distribution systems which would be fed by underground steam lines.
See‘‘Protective Conduits for Underground Lines’’for more information about
FIGURE C3.1Typical steam distribution system for a housing development.
protective enclosures and‘‘Drainage of Condensate’’for condensate drainage con-
siderations for underground piping.
Fossil-Fueled Power Plants.The main steam system conducts superheated steam
from the steam generator(economizer plus evaporator and superheater)to the
turbines as shown in Fig.B1.1in Part B of this handbook.Figure C3.3shows a
衫组词simplified schematic of a main steam system.The main steam system may also
provide steam for various auxiliary rvices.
The piping layout,for a steam power plant,in addition to paying due respect
to economic factors,should consider(1)the layout’s mechanical design or ability
to function properly and efficiently with respect to the mechanical equipment which
it rves,(2)the layout’s convenience from an operating standpoint,and(3)the
layout’s appearance as a coordinated part of the plant.Although the relative impor-
tance of the basic points obviously falls in the order named,each has an important
bearing on the acceptability of any arrangement,as will be discusd in the follow-
ing ctions.
B31.11states that when boilers are connected to a common header,the connection
from each boiler having a manhole opening shall befitted with two stop valves
having an ample free-blow drain between them.The discharge of this drain shall
be visible to the operator while manipulating the valve.The stop valves shall consist
preferably of one automatic nonreturn valve(e next to the boiler)and a cond
valve of the outside-screw-and-yoke type,or two valves of the outside-screw-and-
yoke type shall be ud.When a cond stop valve or valves is required,it shall
have a pressure rating at least equal to that required for the expected steam tempera-
ture and pressure at the valve,or the pressure rating at least equal to85percent
of the lowest t pressure of any safety valve on the boiler drum and for the expected
temperature of the steam at the valve,whichever is greater.All valves andfittings
FIGURE C3.2Typical high-pressure steam-distribution system in a tower office building. on steam lines shall have a pressure rating of at least100psig[700kPa(gauge)]
in accordance with the applicable American National Standard.
FIGURE C3.3Main steam system.
Availability of steam generators has incread to the extent that single units are ud almost universally to supply the steam to turbines of sizes up to approximately 1000MW.In fossil-fueled power plants exhaust steam from the high-pressure turbine is piped back to the steam generator,wh
ere it is reheated in a special ction before being returned to the inlet of the intermediate pressure or reheat turbine.
车位出租合同The cold reheat system(CRS)and the hot reheat steam(HRS)system shown in Fig.C3.4are treated here in one ction becau they are parts of the same
overall system ud for reheating steam.The CRS conducts steam from the
outlet
FIGURE C3.4Reheat steam system.of the high-pressure turbine to the reheater inlet and provides steam for the auxiliary steam and extraction steam systems.The CRS is the normal auxiliary steam source, but at low loads auxiliary steam is usually augmented from the main steam system via a pressure-reducing station.
The exhaust steam from the high-pressure turbine is transported in single or multiple leads into the reheater inlet.Each reheater may have a desuperheater for reheater outlet temperature control,and safety valves at the reheater inlet.A cross-connection(cross-tie)may be ud between the two cold reheat leads to provide steam to the associated feedwater heater and the auxiliary steam system without creating a pressure imbalance at the reheater inlet.This system has provisions for isolation of the reheater for hydrostatic testing.
The extraction steam system conducts steam from the high-pressure turbine, cold reheat line,intermediate-pressure turbine,and the low-pressure turbines to the feedwater heaters.This extraction steam is required for feedwater heating. Feedwater heating increas cycle efficiency;in addition,the extraction steam system may provide steam for the feedwater pump turbine.In nuclear u
nits,the system also removes moisture from the turbine to provide protection for the lower-pressure turbine blades and to increa turbine efficiency.
The hot reheat system conducts superheated steam from the reheater outlet header to the intermediate-pressure turbine or to the inlet of the low-pressure turbine.One or more hot reheat leads may be furnished.Each lead has a safety valve for reheater protection.A cross-connection between the two hot reheat leads is often furnished to ensure equal pressure at each reheat stop and intercept valve before entering the turbine.
Nuclear-Fueled Power Plants.Only the stationary light water reactor,either of the PWR or BWR type,is discusd in this chapter as being typical of nuclear practice.In the typical boiling water reactor the reactor is the steam generator. Water is circulated through the reactor core,producing steam which is parated from recirculation water,dried in the top of the reactor vesl,and directed to the steam turbine.
The pressurized water reactor us two clod systems—a primary system includ-ing the reactor and its cooling system,and a condary system including a turbine-generator,a condensate system,and a feedwater system.The two systems communi-cate with each other at the steam-gener
ator tube interface,where pressurized water of the primary system transfersfission-reaction heat to the steam-generator feedwa-ter in the condary system,producing steam to drive the turbine generators.PWR power plants typically utilize two,three,or four primary loops,each containing a steam generator which transfers the energy from primary coolant to the feedwater on the condary side.
The nuclear steam supply system(NSSS)customarily consists of tho compo-nents in contact with the reactor coolant,and specialized auxiliary machinery.
The main steam system in LWRs transports steam from the outlet of the reactor/ steam generators to the turbine stop valves.The main steam system also provides steam for various auxiliary rvices.Additionally,it provides a means of controlled heat relea from the NSSS during periods of station electrical load rejection.
In PWRs,each steam generator supplies steam to a line which connects to a common main steam manifold.Valves in each line permit isolation of individual steam generators.Safety valves on each line provide pressure-relief protection for the steam generators.The main steam manifold supplies steam to the high-pressure turbine throttle valves,the moisture parator reheaters,and auxiliary steam loads.
The main steam system includes a turbine bypass system that provides a direct
FIGURE C3.5Typical NSSS performance curves.
steam path to the condenr and to the atmosphere in order to prevent unnecessary reactor trips during load rejections up to100percent of full electrical load.
The turbine bypass system also permits testing NSSS at power levels up to55 percent without having the turbine loaded.It is also ud to maintain reactor coolant temperature during hot standby and shutdown operation and is ud to conduct controlled cooldown of the plant to the point where the residual heat removal system can be placed in operation.
For the PWR and BWR,the main steam system design pressure is typically about1200psia(8280kPa)(e Fig.C3.5)and600ЊF(316ЊC).Actual design condi-tions depend upon the specific reactor design.Main steam line sizing is determined by performing an economic analysis of pressure drop,pipe cost,and so on,with a maximum velocity of15,000fpm(250fps)(4570m/min,76m/s);e‘‘Preventing Turbine Overspeed’’for moreflow velocity data.Preliminary pipe sizing is bad on a3percent pressure drop from the steam generator outlet to the turbine stop valves.Wall thickness is calculated using the equations prented in‘‘Design Pressure.’’
For PWR and BWR main steam lines,turbine water induction problems are of a great concern as the main steam is saturated or contains some moisture.Prevention criteria are to be as specified in ASM
E Standard No.TDP-2,7‘‘Recommended Practices for the Prevention of Water Damage to Steam Turbines Ud for Electric Power Generation,’’Nuclear Fueled Plants.
Each type of nuclear plant has isolation valves outside of the reactor containment. BWR plants also have isolation valves in main steam lines inboard of the reactor containment.Since the BWR main steam is radioactive,it must be shielded,and special considerations must be made in its design to minimize crud traps(small deposits of radioactive contaminants inflow passages).When welding pipes to-gether,the u of backing rings is avoided.
In LWR nuclear plants,reheating is achieved in a combined moisture parator and reheater unit.High-pressure turbine exhaust steam pass through the moisture parator portion of the unit,where most of the moisture is removed mechanically. The steam is then reheated in one or two stages by passing over the bundles of tubes containing high-temperature heating steam.For single-stage reheat and for the cond stage of two-stage reheat,the heating steam is supplied from the main steam system at a point ahead of the turbine stop valves.Pipelines carrying live steam to reheaters belong to the main steam system.For two-stage reheat,the
first-FIGURE C3.6Steam header in industrial plant.(Courtesy
of Val v e World.)
