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PROCESS PIPING   &   PIPING   FITTINGS

 

In any Petrochemical, Chemical, Fertilizers  or Power generation plant, one

can see kilometers of various piping carrying various process fluids from

one end to other. The process piping acts as blood carriers in our body. It

plays very important role as any minor failure of process piping or pipe

fitting of critical service in continuous process plant means shutdown of

the plant. The role of the maintenance engineers is to design and select

appropriate piping material and rating to avoid such failures.

Piping design includes selection of applicable codes/standards,

environmental requirement and service parameters and the supports

required. Calculation of process piping requires data like, type of fluid,

process flow rate, system working pressure and maximum pressure,

temperatures, stresses due to other equipments, requirement of valves,

type of valves, Special loops if required, number of drains, factor of safety,

corrosion allowance, erosion if any, maximum flow velocity allowed and

requirement of insulation etc. Depending upon these factors piping

material, piping sizes, supports system, type of expansion joints etc. are

considered then location of valve skids are decided before final designing

of the piping.

Piping specification covers the service, ratings, material of construction,

fabrication process, type of fittings, fabrication consumables, erection

consumables, fastners, installation and service performance requirement.

Vent, drain and sampling points are considered according to service

requirements.

The potential for damage due to corrosion in piping must be addressed in

the design stage itself and appropriate material and service class shall be

selected. Extra measures shall be taken to avoid external as well internal

corrosion by providing coating or lining or painting etc. Underground

piping is in general protected by providing cathodic protection.

 

Internal Wear:

Depending on the service conditions, internal erosion and wear of the

piping also posses unseen threat of system failure. If the prevailing

conditions are known then material selected shall be accordingly suitable

and piping design shall be such that minimum wear takes place. In this

type of services, short radius elbows and returns are avoided.

 

Thermal expansion:

For high temperature service piping, compensation for the resultant

expansion and contraction due to temperature changes should be

considered both in piping and supports. Suitable expansion loops shall be

given as per standard so that line strength is not exposed to sever

stresses. Similarly the additional load which shall be born by the piping

shall be known when these are to be connected to equipment nozzles and

vice versa (Piping load to be born by equipment shall also be conveyed to

equipment manufacturer.)

Another important factor in process piping system is consideration of

presence of condensate. This condensate shall be drained intermitantly in

the system and it should not gets accumulated in the vertical loop of the

piping. The accumulated condensate in a piping can stop the flow of the

fluid in the piping. During the stoppage and start-up of the process plant

this condensate plays an important role and shall be taken care otherwise

it can lead to failure of the system.

 

Standards

There are various standards and codes for designing process piping. In

petro chemical industries and refineries in general ASME standard is

followed. e.g. ASME B 31.3 is for piping and B16.5 is for Steel pipe flanges.

Pipe fittings shall be as per ASTM A-234. These standards include the

minimum design requirement for various pressure piping system,

thickness of pipe, supports type of expansion joints etc. In case of

complex and heavy wall piping combination of two or more standards are

also followed depending upon the severity of the system. The most

stringent check of all the standards shall be considered applicable in case

of more than one standard is followed.

For all process piping size 2” and above, butt joints are made but below

1.5” NB piping, socket welded joints may be made as per design

requirements. Threaded joints are normally permitted if specified in the

piping specifications.

All pipes of class 300lbs and above shall be of seam-less construction un-

less or otherwise specified in the piping specifications. Galvanised piping

is generally recommended  to be used where the service temperature is

below 60 °C.

 

 Piping system is subjected to total loads i.e. internal pressure due to

flowing fluid and self weight of the pipe. Load due to fluids are normally

the sustained loads but occasionally additional loads or force are also born

by the system. These are intermittent loads e.g. hydrostatic test etc.

Piping systems are also subjected to other dynamic loads while in service

e.g. water hammering due to accumulated condensate and energy

released by pressure safety devices. These dynamic loads may also cause

vibration in a piping system due to impact of fluctuating force or pressure

acting on the system.

Piping codes gives the required design criteria for a particular piping

system and in general following factors are to be considered for any

piping design .

 -                     Material

-                     Allowable stresses

-        Allowable dead loads

-        Allowable live loads

-                     Min pipe wall thickness (para 304.1.2 of ANSI B31.3)

-                     Min. Thread depth in case of pipes less than 1.5” ND

-        Maximum deflection

-        Seismic loads

-        Thermal expansion

-        Corrosion allowance

-        Factor of safety

 Codes for piping pipe fittings and valves are separate. But all the details shall be specified in the construction isometrics. Construction isometrics shall contain following information’s: Pipe Number

Pipe ClassInsulation ClassSupport classPipe SizeMain dimensions for assemblyDeatil dimensions for valves & spool piecesPoint of referenceInstrumentsPipe supportsPart List 

For process piping design ASME B31.3 includes selection of material,

design, fabrication, assembly, erection, examination, inspection and

testing.

Welding joints of pipes and fittings also plays an important role. For any

process piping, PQR (procedure qualification record), WPS(Welding

Procedure Specifications) and QAP (Quality assurance plan) are made with

reference to the standards. These documents gives the details of type of

welding, type of electrodes, qualification of welder, preheat and post weld

heat treatment cycles, inspection and testing to be done etc for any

piping.

Quality assurance plan and Procedure qualification records are generally

developed in workshops anf for this many destructive tests are also

performed along with Non-destructive tests. Based on these recorded

documents, NDT (Non Destructive Test) which consists of  DPT (Dye

Penetrating Test) Radiography/ X-Ray , Ultrasonic(UT)  MP(Magnetic

Particle Test) test  etc are to be carried out on actual job.

Piping material are selected based on the piping specifications. It may be

carbon steel (CS), stainless steel (SS) or alloy steel (AS) of different

grades. Pipe wall thicknesses are also specified in piping specifications.

Pipe thicknesses are related to various  Schedule which gives standard

thicknesses of specified size of pipe. These schedules are, 5, 10, 20, 30,

40, sd, Xs, 60, 80, 100, 120, 140, 160 and XXS. All sizes of pipes are not

available in all schedules.

In selection of a pipe, the most important factors are system pressure,

temperature and type of fluids (corrosive or non corrosive). In high

temperature services, where plane carbon steel pipes may not be suitable

then alloy steel grades are selected depending on their service class. In

corrosive fluid services in general stainless steel of various grades are

used however for conc. Acids SS piping is not suitable in that case cast

iron piping is found most suitable. These days lined pipings are becoming

popular for corrosive fluids.

In general in piping classes higher than 300#, seamless pipes are being

used. However depending on economics and service criticality, welded

pipes are also used. Welded pipes may be ERW (Electric Resistence

Welded) or spiral welded or fusion welded type.

Regarding the pipe fittings, standards and codes specified the types of

fittings to be used in a particular class of piping. Pipe fittings includes

Elbow (45° & 90°, SW & BW), Bends (Short Radius and Long Radius, 90° &

180°), Tee (Equal and un-equal, Socket Weld & Butt Weld), Flanges (Slip-

On Raised Face , Weld Neck Raised Face & SW), Socket, reducer sockets,

Couplings, half-couplings, Reducer (Conc. & Eccentric SW & BW) , Socko-

let, threado-let, weldo-let, nippo-let, Blind flange, spectacle blinds etc.

Generally in all piping class in sizes below 11/2” NB all fittings are used in

Socket Weld versions and in Socket weld fittings the min. class available is

3000#. Above this class, 6000# and 9000# are available. All piping above

11/2” NB size butt joints are used for welding un-less or other wise

specified for particular service. For drinking water service, Galvanised

piping is used with threaded joint fittings.

In case of Flanges, there are many classes of flanges specified by rating

150lb, 300lb, 600lb, 900lb, 1500lb, 2500lb etc. For special purpose in

other standards, other classes of flanges are also used. These rating

depend upon the service class or  pressure & Temperature only.

Type of flanges:

SORF-Slip On Raised Face

WNRF-Weld Neck Raised Face

WNRJ-Weld Neck Ring Joint

SWRF-Socket Welded Raised Face

Flanges are manufactured as per ASME B 16.5 and  ASME B 16.10. B-16-

10 covers the dimension of flanges, PCD of bolt circle, dia of holes, face to

face dimension, thickness of flange, OD of flange, bore of flange etc. While

ASME B16.50 standards covers pressure & temperature ratings,

tolerances, marking, testing, deviation and acceptable limits etc. This is

internationally accepted practice.

 

Some ASME Standards are given below

ASME/ANSI B16.1 - 1998 - Cast Iron Pipe Flanges and Flanged Fittings

ASME/ANSI B16.3 - 1998 - Malleable Iron Threaded Fittings

ASME/ANSI B16.4 - 1998 - Cast Iron Threaded Fittings

ASME/ANSI B16.5 - 1996 - Pipe Flanges and Flanged Fittings

ASME/ANSI B16.9 - 2001 -Wrought Steel Butt welding Fittings

ASME/ANSI B16.10 - 2000 - Face-to-Face and End-to-End Dimensions of

Valves

ASME/ANSI B16.11 - 2001 - Forged Steel Fittings, Socket-Welding and

Threaded

ASME/ANSI B16.12 - 1998 - Cast Iron Threaded Drainage Fittings

ASME/ANSI B16.20 - 1998 - Metallic Gaskets for Pipe Flanges-Ring-Joint,

Spiral-Would, and Jacketed

ASME/ANSI B16.21 - 1992 - Nonmetallic Flat Gaskets for Pipe Flanges

ASME/ANSI B16.28 - 1994 - Wrought Steel Buttwelding Short Radius

Elbows and Returns Some Sketch of Piping Fittings given Bolow :  

 

Weldolet: Weldolets are used for making branches in the piping system.

The size of the weldolet specifies the size of the main line, size of the

branch line, schedule of the main line and schedule of the branch line

along with MOC.

 

Nippolet: Nippolets are also used for making branches in the piping system

but for small sizes and in particular for making drains and vents. The

rating of the Nippolet shall confirm to rating of the main line, along with

MOC.

Threadolet: Threadolets are also used for making branches in the piping

system but for small sizes and in particular for making openings for fixing

Thermo-well etc. The rating of the Threadolet shall confirm to rating of the

main line, along with MOC.

 Letrolet: Lettrolets are used for making branches in the piping system at

the Elbows for small sizes and in particular for fixing thermo well etc threaded

letrolets are used. The rating of the letrolet shall confirm to rating of the main line,

along with MOC. Butwelded letrolets are also known as elbolets.

 

 

Sweepolet Sweepolet are used for making branches in the piping system

without reinforncing. The size of the sweepolet specifies the size of the

main line, size of the branch line, schedule of the main line and schedule

of the branch line along with MOC. Both joints are Butt Weld joints

 

Standard form of Branching

 

 

 

 

 

VALVES:

Valves are basically used to allow or restrict flow of fluid in the piping as

per process requirement. There are various type of valves which are used

for specific services in industries. End connection of these valves may be

with flanges or welded end. Welding end may be Butt weld type or socket

weld type. For socket weld valves the min. class for any type of valves is

800lb. However for flanged valves the classes are based on the flange

classes. E.g. 150#, 300#, 600#, 900#, 1500# and 2500# etc.

 

GATE VALVE:-Gate Valves are specifically intended for use as isolation

applications. These valves creating minimum pressure drops across them

and they are used for ON and OFF applications. Although they can be used

for throttling but the service life gets reduced due to wear out seats.

 

GLOBE VALVE :-   Globe valves open more rapidly than a gate valve as

the disc only needs to move a small distance from its seat to allow full

flow but due to change in direction in flow, resistance to flow increases

and this also generates turbulence in the flow. This results in a higher

pressure drop across globe valves than gate valve. These valves are used

to control the flow in the process lines.

 

 

 

MATERIAL SPECIFICATION GATE/GLOB VALVES

Name of Part Specification

 1 Body 

 ASTM A105

 2 Bonnet/Cap  ASTM A105

 3 Stem  AISI 410

 4 Gate/Disc 13% Cr. Steel with suitable Seating surface

 5 Body Seat    Ring*

 13% Cr. Steel with  suitable Seating surface

 6 Stem Packing  Graphite

 7 Handwheel  Ductile Iron or Steel

 8 Bolts/Studs  ASTM A193 Gr.B7

 9 Gasket  Spirally wound SS 304 with Graphite filler

  International standards & codes followed for design of valve AP1-600 & API-602 for valve design, ASME B16.10 for face to face dimension of flange valves, API 598 for pressure testing of valves etc.

 

For any valve following are the main parts which plays important roll in its

performance.

 

1.       Body of the Valve

2.       Bonnet of the Valve

3.       Stem of the Valve

4.       Wedge  & Disc

5.       Seat

6.       Stem gland Packing

7.       Gland Bush

8.       Yoke

9.       Gland stud & Nut

10.     Gland Flange

The material of construction of the above parts varies with the service

class of the valves and is generally standardized by the manufacturers.

 

The word “TRIM” in case of Valves means combination of seat and disc

(wetted portion) of valve and the gland packing. Trim number specify the

particular combination of disc/stem/and seat with gland packing will be

used. These are the most vulnerable parts of the valve and this defines

the performance of valve.

 

CHECK VALVES :-   Check valves or Non-Return Valve are very - very

important part of piping. It is used to allow fluid to flow in one direction

only i.e. when the upstream of a check valve is more than the

downstream pressure then it will allow forward flow only but vice versa is

not possible.

Check valves are in generally of two type. One is swing type check valve,

others are lift or plug type check valve. Type of valves to be used depends

upon the service and operating conditions of the piping.

 

SWING CHECK VALVE OR NRV         Disc type Check Valve or

NRV         

           

 

DISC CHECHK VALVES :-  The disc type check valve mainly use a spring

loaded disc which is seated on a seat due to spring force. The differential

pressure required to open the check valve is determined by the type of

the spring used. Where the differential pressure across the valve is small

no spring is used simply a disc with a stem works as check valve.

otherwise   spring or Heavy duty spring are used.

 

STEM SEALING :-   In order to prevent leakage of the process fluid from

and along the Stem of the valve, a barrier must be placed around the

stem for sealing. Stem sealing is generally achieved by providing gland

packing (usually a low cost item) or metallic flexible bellow (Expensive).

Bellows are more effective for achieving ‘Zero’ leakage, whereas gland

sealing is not a Zero leakage arrangement and also require to be changed

periodically.

 LINERA MOVEMENT VALVE STEM OPTION

Linear movement valves are available with a number of different stem

arrangement:

Rising/non-rising stems- In rising stem valves, stem will move vertically

upward when the valve is opened by turning of wheel. Where as in non-

rising stem valves, stem rotates in its position only when the valves is

being operated.

In rising stem valves, the length of exposed stem directly indicates the

degree of valve opening, which in turn roughly reflects the amount or

percentage of flow through the valve. Valves with rising stems do however

require more space above the bonnet to accommodate the rising stem in

the fully opened  condition. The use of non-rising stem type valves is

recommended where contamination is to be controlled. E.g. in dairy

product plants etc.

In-side/Out-side stem screw – Rising and non/ rising stem type globe

valves are known as Inside/Outside stem threads. On a stem the actuating

threads on the stem are situated outside the valve body and are not

exposed to the process fluid. As screw threads are particularly susceptible

to corrosion outside screw should always by used on fluids with corrosive

or erosive properties. They are also beneficial where the valve is

frequently exposed to large temperature variations, as the expansion and

contraction of the stem may cause binding of the threads inside the body.

 

Pipe Sizing:- The objective of the pipe lines is to allow or supply fluid at

correct pressure and flow to the desired point of usage. Pressure drop

across the distribution system of a fluid is an important parameter.

Additionally there exist some losses due to friction between the fluid and

pipe wall. Total pressure drop  losses depends upon following parameters:

L= Length of the pipe in meter (M)

D= Dia. of the pipe in meter (M)

U= Mean velocity of the fluid in Kg/ms

= Density of the fluid in Kg/m3

Ks= Roughness of the pipe wall

The roughness of the pipe inside wall plays an important role. The head

loss due to friction can be calculated through D’Arcy’s equation and

Reynolds number chart with the help of formula :

 

          hf =4FLu2/2Gd  or hf =fLu2/2gd

 

Where hf =head loss(m)

              F =Friction factor

              L =Length of pipe(m)

              u=Flow velocity of fluid (m/s)

              g =Gravitational constant(9.8)

              D =Dia of pipe(m)

 The Optimum size of pipe should be selected for better flow of fluid and

less friction and minimum pressure drop. Oversize or undersize pipe, both

leads to disadvantages more than the advantages. In case of undersize

pipe lines, greater risk of erosion due to high velocity will always be there.

Water hammer and starvation in down stream equipments (in case of

steam) and noise will be there due to increase in velocity.

Pressure drop can be calculated from the pressure factor chart. Velocity is

an important factor in deciding of pipe line size for particular service.

There are many chart and formula for calculation these