ARNOLD, K. (1999). Design of Gas-Handling Systems and Facilities (2nd ed.) Episode 2 Part 9 ppt

Courtesy of Flexible Valve Corp.Check Valves Used to restrict reversal of flow, check valves should not be consid-ered as positive shut-off valves when flow is reversed, since the seati

Figure 15-7 Diaphragm valve (Courtesy of Flexible Valve Corp.)

Check Valves

Used to restrict reversal of flow, check valves should not be

consid-ered as positive shut-off valves when flow is reversed, since the seatingelement is always in the flow stream and subject to erosion (Figures 15-9

to 15-13) A section of a line should not be considered isolated if the onlybarrier to flow is a check valve On the other hand, because they dorestrict backflow to very low levels, check valves installed in appropriatelocations can protect equipment and minimize damage in case of a leak

in the upstream line Some of the advantages and disadvantages of thevarious check valve configurations are as follows:

• Swing

1 Suitable for non-pulsating flow

2 Not good for vertical upward flow

3 Available in wafer design for mounting between flanges

• Split Disk

1 Mounted between flanges

2 Springs subject to failure

• Lift Plug and Piston

1 Good for pulsating flow

2 Can be used in vertical upward flow

3, Easier to cut out in sandy service than full-opening swing

4, Subject to fouling with paraffin and debris

Figure 15-8 Needle valve (Courtesy of Anderson Greenwood and Co.)

Figure 15-9 Swing check valve (Courtesy ofJudd Valve Co., inc.}

• Ball

1 Does not have a tendency to slam shut on flow reversal

2 Usually for sizes 1-in and smaller

3 Can be used in vertical lines

Valve Selection and Designation

Table 15-3 summarizes and compares the different valve types cussed in this chapter and highlights important properties that impactvalve selection

dis-It is beneficial to designate valve types in schematic drawings of thefacilities The designation should indicate the type of valve (ball, gate,etc.) the type of end connection (flange, socketweld, threaded, etc.), thepressure rating class (ANSI 150, ANSI 600, API 2000, etc.) and thematerials of construction Table 15-4 shows a sample designation system.Using this system, the designation VBF-15-1 would indicate an ANSI

150 flanged ball valve The specific attributes would then come from apipe, valve, and fitting specification, such as Table 15-2, or from a sepa-rate valve specification for VBF-15-1, as shown in Table 15-5

Comparison of Valve Properties

Yesfor low AP

ANSI 150

Not All

YesNo

YesAdjustable

choke only

ThrottleNoOn/OffNoOn/OffSome

YesGas Low APYesYesNo

YesAdjustablechoke only

Where UsedIsolationubiquitousIsolationRareControl,wellhead isolation,

double block & bleed

Isolation/Control

Controlbypass, ventInst/Control

To restrict reversal of

flowIsolationControl

PigYes(Full)NoYes

No

NoNoRoddableSwing checkValves onlysome casesNo

Pressure DropLowLowLow

Courtesy of Paragon Engineering Services, Inc.

Table 15-4 Sample Valve Designation System

Each valve designation has four (4), and possibly five (5), parts

(1) This part of each valve designation is always V, which stands for "valve,"(2) The second letter identifies valve type:

(4) The fourth part of each valve designation is a 2-, 3-, or 4- digit number

indicating the highest ANSI or API class for which the valve can be used:

(5) The fifth part of a valve designation, when used, is a modifier that

distinguishes between two or more valves that have the same type andpressure rating but that are considered separately for some other reason

Courtesy of Paragon Engineering Services, Inc.

CHOKES

Chokes are used to control flow where there is a large pressure drop.They can either be adjustable, where the opening size can be varied man-ually as shown Figure 15-14 and 15-15 or have a fixed size orifice Due

to the erosive nature of the fluid flow through a choke, they are structed so beans, discs, and seats can be easily replaced

con-Table 15-5 Sample Valve Table

Valve Designation: VBF-15-1

Service: Hydrocarbons, Non-corrosive Glycol

Type: Ball Valve

Body Material: Carbon Steel

Trim Material: Hard Plated Carbon Steel Ball

End Connection: RF Flanged

Valve Operator: Lever through 8", Gear Operated 10" and larger

Body Construction: 2"-4": Floating Ball, Regular Port

6" and larger: Trunnion Mounted Ball, Regular PortTrim Construction: Renewable Seats, Removable Stem, Fire Safe

Valve Comparison List

Manufacturer Manufacturer's Fig No Nominal Sizes

Maximum allowable working pressure (MAWP): Highest pressure to

which the system can be subjected during operation Thus, pressure isestablished by a relief device set pressure and must be less than or equal

to the material strength limitations of equipment This pressure

establish-es piping class for fittings and pipe wall thicknestablish-ess requirements, both ofwhich are discussed in Volume 1

Normal operating pressure: Anticipated process operating pressure

used to determine pipe diameter requirements and pressure drop tions for various operating conditions

limita-Figure 15-10 Wafer check valve (Courtesy of TRW Mission Drilling Products

Division.}

Figure 15-11 Lift check valve (Courtesy of Jenkins Bros.}

Figure T5-12 Piston check valve (Courtesy of Whealtey Pump and Valves, Inc.]

Figure 15-13 Ball check valve (Courtesy of Wheatiey Pump and Valves, Inc.]

Figure 15-14 Plug and seat choke (Courtesy of Willis Control Division, Cameron

Iron Works, Houston.)

Figure 15-15 Rotating disc choke [Courtesy of Willis Control Division, Cameron

Iron Works, Houston.)

(text continued from page 441)

Future operation pressures: Sizing of lines must consider operating

pressures expected as the reservoir depletes Diameter requirement lations should be made using both initial and future conditions to deter-mine the governing case Often in gas and two-phase lines the greatestflow velocity occurs late in life when flowing pressures are low eventhough flow rates may be lower than initial conditions

calcu-Process Temperatures

Design temperature: Highest or lowest (depending upon which is

con-trolling) temperature to which a line can be subjected during operation

Normal operating temperature: Anticipated process operating

temper-ature used to determine pipe diameter for various operating conditions

Process Liquid Flow Rates

Liquid lines in production facilities are generally in either continuous

or slugging service Continuous duty lines should be sized to handle theaverage daily flow rate of the facility An additional capacity is oftenadded for surges Lines in slugging service should be sized to accommo-date actual flowing conditions Design flow rates should be the maxi-mum capacity that a line will accommodate within the design limits ofvelocity and pressure drop, both initially and in the future

Process Gas Flow Rates

The sizing procedure for gas piping must take both high-pressure andlow-pressure flow conditions into consideration if the operating pressure

of the line changes over time

Two-Phase Flow Rates

Whenever two-phase flow is encountered in facility piping it is usually inflowlines and interfield transfer lines Some designers size liquid linesdownstream of control valves as two-phase lines The amount of gasinvolved in these lines is low and thus the lines are often sized as single-phase liquid lines Oversizing two-phase lines can lead to increased slug-ging and thus as small a diameter as possible should be used; consistentwith pressure drop available and velocity constraints discussed in Volume 1

Viscosity: High viscosity crudes may flow in the laminar flow regime

which causes high pressure drops This is especially true of emulsions ofwater in high-viscosity crudes where the effective velocity of the mixturecould be as much as ten times that of the base crude (see Volume 1)

Solids: Some wells produce large amounts of sand and other solids

entrained in the fluid Where solids are contained in the stream, sufficientvelocity should be provided to assure they do not build up in the bottom

of the pipe, causing higher than anticipated pressure drops or potentialareas for corrosion However, if the velocity is too high, erosion mayoccur, (See Volume 1.)

Gases such as H2S and CO2 (acid gases) in the production streams aresometimes encountered These gases are not only corrosive to piping, butmany are harmful and possibly fatal upon contact with humans Specialcare should be exercised in designing piping containing acid gases.Velocities above 30 to 50 ft/s should be avoided in piping containing acidgases to avoid affecting the ability of corrosion inhibitors to protect themetal Special metallurgy may be needed to combat H2S corrosion (SeeChapter 8.)

Handling Changing Operating Conditions

Each production facility has three categories of equipment whosedesign depends upon operating conditions:

1 Vessels and other mechanical equipment are the most difficult tochange or alter after installation

2 Piping is the next most difficult

3 Instrumentation is the least difficult

Often the facility is designed with equipment and piping that can dle the complete range of operating conditions, and with control valvesselected so that their internals ("trim") can be substituted as operating

han-conditions change Sometimes the piping must be designed to allowaddition of future pieces of equipment This is especially true for com-pressors and water treating equipment that may not be needed initially.The key to arriving at the most flexible system design lies in forecast-ing future operating conditions Many engineers are not aware of theimplications of future conditions and their effect upon initial design aridlong-term operation Often some information is available on potentialfuture scenarios, but the facility design engineer elects to design for aspecific '"most likely" forecast This is unfortunate, as the designershould at least consider the sensitivity of the design and economic conse-quences to the whole range of possible forecasts.

Selecting Pipe Sizes

Basic steps in piping design are:

1 Establish operating conditions, i.e., flow rates, temperatures, sures and compositions of fluid over the life of the system This mayinvolve several cases

pres-2 Using velocity as the limiting criterion, calculate allowable pipeinternal diameter ranges using the criteria of Chapter 9, Volume 1

3 If more than one standard pipe size is indicated, calculate the wallthickness for each standard pipe size based on required maximumallowable working pressure and select a standard wall thickness foreach size

4 Calculate maximum and minimum capacities for each size usingvelocity limits as criteria

5 Estimate the pressure drop for each size and compare to the able pressure drop

avail-6 Arrange the information from the previous steps and determinewhich pipe size is best suited to all operating conditions

7 As piping drawings are developed, re-evaluate those lines whereestimated pressure drop was a criterion in size selection, taking intoaccount the actual piping configuration and effects of control andpiping components

8 Proceed with design of pipe supports and stress analysis, if required

It is also a good practice to verify design conditions and piping lations just prior to release of the drawings for construction Systemrequirements sometimes change significantly during the course of a pro-

calcu-ject In most facility piping situations experienced designers can selectsize quickly without a formal tabulation of the steps just described Incertain cases, especially where pressure drop is an important considera-tion, a formal tabulation may be required,

GENERAL PIPING DESIGN DETAILS

Steel Pipe Materials

Most production facility piping is fabricated from ASTM A-106 Grade B

or API 5L Grade B pipe, which is acceptable for sweet service and tures above ~20°F Between -20 °F and -50 °F, ANSI B31.3, "ChemicalPlant and Petroleum Refinery Piping," allows this material to be used if thepressure is less than 25% of maximum allowable design and the combinedlongitudinal stress due to pressure, dead weight, and displacement strain isless than 6,000 psi Below -50°F it is required that the pipe be heat treatedand Charpy impact tested Volume I, Chapter 9 discusses the various com-mon piping codes and methods for calculating maximum allowable pres-sure for various steels Some common low-temperature steels include:

tempera-Steel Minimum Temp, without Special Testing

A-333 Grade ! - SOT

Figure 7-1 shows regions of H2S concentration and total pressurewhere the provisions of NACE MR-01-75 govern A-53 Grade B, A-106Grade B, A-333 Grade 1, and API 5L Grades B and X-42 through X-65are acceptable for use in the sulfide-stress cracking region

Minimum Pipe Wall Thickness

From the standpoint of mechanical strength, impact resistance, and rosion resistance, some operators prefer to establish a minimum wall thick-ness of approximately 0.20 in Thus, they establish the following minimum

cor-pipe schedules (standard wall thickness), even though pressure ment calculations would indicate that smaller thicknesses are allowed:

contain-% in and smaller — Sch 160

2, 2!i and 3 in — Sch 80

4 and 6 in — Sch 40

ANSI B 31.3 requires threaded pipe that is VA in and smaller be at least

Sch 80 and that 2 in and larger be at least Sch 40

Pipe End Connections

Pipe, valve, and fittings tables must specify which size of each class ofpipe is threaded, flanged, or socket welded ANSI B31.3 provides no spe-cific guidance except that it suggests that threads be avoided where cor-rosion, severe erosion, or cyclic loading is anticipated

API RP 14E recommends:

• Pipe 1 [ A in or less should be socket welded for:

Hydrocarbon service above 600 ANSI

Hydrocarbon service above 200°F

Hydrocarbon service subject to vibration

Glycol service

• Pipe 2 inches and larger should be flanged for:

Hydrocarbon service

Glycol service

« Utility piping 2 inches and smaller may be threaded

A common practice onshore is to use threaded connections on 2-in.pipe or smaller, no matter what the service It is also common to seethreaded connections on 4-in pipe and smaller in low pressure oil service.Figure 15-16 shows three types of flange faces Raised-face (RF) andflat-faced (FF) flanges use a donut-shaped flat gasket to create the pres-sure seal Ring-joint flanges (RTJ) use a ring that fits into the circularnotches in the face of the flange to effect the pressure seal RTJ flangescreate a more positive seal and are used for all API class flanges and forhigher pressure ANSI classes However, they are difficult to maintain, asthey require the mating flanges to be spread to remove the ring Raised-face flanges tend to form a tighter seal than flat-faced flanges and areused in steel piping Flat-face flanges are used in cast-iron piping and inbolting to cast-iron and ductile-iron pumps, compressors, strainers, etc

Figure 15-16 Typical flanges.

Bolting a raised-face flange to a flat-faced, cast-iron flange can createbending moments in the less ductile cast-iron flange, which could cause

it to crack

The ANSI specifications allow the use of both RF and RTJ flanges.API RP 14E recommends RTJ flanges for ANSI Class 900 and higherand recommends RTJ flanges be used in 600 ANSI service subject tovibration Onshore it is common to use RF flanges for ANSI classesthrough 2500

The hesitancy to use RF flanges at higher pressures may stem from anera when plain He-in, asbestos gaskets were the only type available Mod-ern spiral-wound polytetrafluoroethylene (PTFE) filled with internal ringgaskets with 316 stainless-steel windings may create as positive a sealwith RF flanges as is obtainable from RTJ flanges

RTJ gaskets are normally cadmium-plated, soft iron or low carbonsteel Soft iron is used for ANSI 600 and 900 classes, and 304 or 316stainless steel for higher classes

Branch Connections

Where a branch connection is connected to a main run of pipe, it isnecessary to specify the type of fitting required ANSI B31.3 provides a