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LINE SIZING CRITERIA PHILOSOPHY

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Piping shall be sized for the controlling case of flow rate, operating pressure and temperatures. Consideration

should also be taken to start-up and unusual conditions which impact line size. For gas and two-phase lines,

attention shall be given to the effects of change in operating conditions and composition with time.

In general all line sizing shall comply with API RP 14E.

Equipment capacity shall be considered in the sizing of lines.

All gas and liquid - two-phase lines and single phase gas lines shall be sized to satisfy the following

criteria:

The erosion velocity shall not be exceeded.

The pressure drop shall not exceed 0.5 psi /100 foot of pipe (except for flow lines)

For lines in corrosive service where corrosion inhibitor is being used, the gas velocity shall not exceed

45 feet I second. This is not applicable for corrosion resistant alloys and Company shall be consulted

for allowable maximum velocities in the event special alloys are utilized

The worst case combination of pressure and flow over the life of the facility as stated in Process Design

Basis shall be the sizing basis.

No provision for future increased capacity shall be made.

The erosion velocity shall be calculated as outlined in API RP 14E with a constant "C" of 100 for continuous

service and 125 for intermittent service. For duplex stainless steel piping materials a "C" value of 200 may be

used with Company approval.

Compressor recycles and station bypass lines shall be sized on the basis of intermittent service. The Company

may, at its sole discretion, relax the sizing criteria on a case by case basis.

Operating velocities for piping in gas service (continuous duty) shall be limited to below its erosion velocity

and noise level. Company may approve higher operating velocities for specific piping systems on a case by

case basis.

Calculation sheets shall be prepared for the piping configuration of each pump showing the calculations for

suction and discharge conditions, static head, static pressure, vapor pressure at pumping temperature, and the

minimum available NPSH (Net Positive Suction Head) including the pressure drop in equipment, piping,

valves, and fittings.

Pressure drop due to piping components other than straight pipe shall be expressed for calculation purposes



as equivalent lengths and added to the overall length of straight pipe in the system. Equivalent lengths for

valves and fittings shall be determined as outlined in the reference 5 (Flow of Fluids through Valves, Fittings,

and Pipe", Crane Technical Paper No. 410) for hydraulic calculations.

All piping shall be sized I designed to eliminate I minimize the impacts of pressure surge due to transient

operating conditions and events.

Pipe roughness shall be based on the following values:

Table 1.

Pipe roughness

Material Roughness

Carbon Steel 0.046

Carbon Steel Flare vent Headers (Heavily Corroded) 0.46

Stainless Steel Flare Headers (Hot Rolled, Longitudinally

Welded)0.046

Duplex/Stainless Steel (New Seamless, Cold Drawn) 0.041

Galvanized Carbon steel 0.15

Epoxy Lined Pipe 0.15

Glass Reinforced Epoxy (GRE) 0.005

1. Sizing Criteria for Single Phase Liquid Lines

The sizing of single phase liquid lines shall be based on considerations of both pressure drop and velocity in

accordance with the guidelines given below. In determining maximum allowable velocities, consideration shall

also be given to noise and erosion/corrosion aspects.

In systems where sand may be present, a minimum velocity of 3 ft/s shall be maintained for all pipe sizes to

minimize sand depositions in the lines. This may be relaxed for the liquid export pipeline where pigging and

other corrosion control methods are available.

a. Calculation Method

Pipe size will be based on pressure drop calculated using the Darcy equation. Friction factor values should be

taken from a friction factor chart (modified Moody diagram - refer API RP 14)

For pump suction lines handling boiling fluid, lines shall be sized such that the total frictional pressure drop

does not exceed 2 ft of head of the fluid.

Table 2.

Recommended Velocity and Maximum AP for Carbon Steel Liquid Lines



HYDROCARBONS

Type of Service Velocity ft/sMaximum AP psi/100

ft

General

Recommendation

Liquid Density, lb/ft3

100 5.0 - 8.0 5

50 6.5 - 10.0 4

20 10.0 - 16.5 1.3

Gravity Rundown Lines 3.3 - 8.2 0.15

Liquid Feed to Towers 4.0 - 6.5 0.3 - 0.5

Liquids to Reboilers 3.0 - 6.0 0.1 - 0.25

WATER LINES

GeneralRecommendation 2.0 - 16.0 0.3 - 2.0

Service Diameter, inch

1" 2.0 - 3.3

2" 3.3 - 5.0

4" 5.0 - 7.0

6" 7.0 - 9.0

8" 8.2 - 10.0

10" 10.0 - 12.0

HYDROCARBONS

Type of Service Velocity ft/s Maximum AP psi/100 ft

12" 12.0 - 14.0

16" 12.0 - 15.0

20" 12.0 - 16.0

Pump Suction See Table 10 See Table bellow

Pump Discharge 6.0-16.5, refer See Table bellow

also to Table

Must read

PROCESS BLOCK DIAGRAM OF NGL PLANT

Design Aspects For Flare Systems



bellow

Boiler Feed 8.0 - 15.0

Cooling Water 12.0 - 15.0 2

Table 3.

Allowable Pressure Drops and Velocities in Piping for Centrifugal Pumps

Pipe NB inch

Pump

Maximum Velocity ft/s Allowable P psi/100ft

Pump

Suction

Pump

Discharge

Pump Suction

Pump

DischargeBoiling

Liquid

Sub-

cooled

Liquid

8" 6 16.5

0.10-

0.30

0.50-

1.00 1.6-2.0

b. Liquids Flowing by Pressure

In general, these lines shall be sized in accordance with Table 9.Lines where flashing may occur upstream of

control valves shall be sized to prevent vapour flash ahead of the valve. Where the available pressure drop is

low the line should be sized such that the actual pipe frictional pressure drop does not exceed 50% of the

available pressure drop.

c. Gravity Flow Lines

Gravity flow lines shall be sized using a maximum pressure drop of 0.15 psi/100 ft equivalent length (based on

1 :500 slopes). This may be exceeded for sub-cooled liquids with large available heads. Caution should be

taken to prevent flashing in barometric legs. Pipes with a diameter below 8 inches shall be designed for 50%

full of liquid, while pipes with a diameter of 8 inches and greater shall be designed for 75% full of liquid.

d. Static Accumulation

The maximum velocity in lines entering/leaving tanks containing hydrocarbons, and not installed with a gas

blanket will be limited to 3 ft/s in the first 30 ft of line to/from the tank in order to reduce the accumulation of

static charge. This criterion applies to tanks containing flammable fluids such as diesel.

e. Other Corrosive Liquids

To minimize corrosion, pipes containing corrosive liquids shall be sized for the maximum velocities and

pressure drops in Table 4.


Top



Table 4.

Guidelines for Lines Containing Corrosive Liquids

Description

Maximum AP Maximum Velocity (ft/s)

(psi/100ft) NB 4" & smaller NB 6 " & larger

Pump suction 0.5-1.0 3-3.6 5 5

Pump discharge 5 2 5

f. Drain

To avoid deposits of solid particles, sewage, closed and open drains shall be designed for the minimum fluid

velocities given below:

Table 5.

Guidelines for Sizing Drain Piping

Service Minimum Velocity (ft/s)

Sanitary

drains 2.6

Oily

water drains (Open drains) 3

Other

drains 3.6

However, a minimum size of NB 4" shall be used for the header and NB 3" for the sub-headers.

g. Tank Overflows

Overflows are sized for the maximum net inflow to the tank taking full advantage of the available head above

the overflow.

For overflows through vertical internal pipes, use the Francis formula:

Q=K*D*H1.5

Q = flowrate (ft3/min)

D = pipe internal diameter (ins)

H = liquid height above top of overflow pipe (ins)

K = constant= 1.26 (for above units)

h. Fire Water Lines

Refer to NFPA-20 "Standard for the Installation of Stationary Pumps for Fire Protection" and NFPA-24

"Standard for the Installation of Private Fire Service Mains and Their Appurtenances", minimum velocity



criteria of fire water lines shall be 10 ft/s and the line shall not exceed the maximum velocity of 20 ft/s. No

pressure drop criteria restriction for fire water lines.

2. Sizing Criteria for Single Phase Vapor Lines

Table 6.

Recommended Velocity and Pressure Drop for Single Phase Vapour Lines

(Excluding Vent lines)

Operating Pressure (psig) Maximum Velocity (ft/s) Pressure Drop (psi/100 ft)

General Recommendation

495 0.5

Suction 66

Discharge 98

Anti-surge recycle 246

Notes:

- Reciprocating compressor lines should be sized on a conservative basis with maximum velocities of 40 ft/s.

- The velocity of gases containing wet carbon dioxide is not to exceed 60 ft/s where film forming corrosion

inhibitor is being used. At velocities above this limit corrosion inhibitor loses its effectiveness.

- For continuous flow (including lines in flare systems) the vapour velocity should not exceed the erosion

velocity as calculated in below. The calculated velocity shall not exceed either the maximum recommended

velocity or the erosion velocity. For intermittent flow where pressure drop is not a consideration (eg.

Compressor recycles lines) the velocity should not exceed 130 ft/s.



In addition to the velocity and pressure drop criteria, the maximum allowable vapour velocity relating to the

acceptable noise levels should not normally be exceeded. This velocity is calculated by the formula:

Vmax = 198.5()0.506

Vmax = Maximum allowable vapour velocity (m/s) based on a noise limit of 85 dBA@ 1 m

p = Gas density at flowing conditions (kg/m3)

When the vapour velocity exceeds Vmax for noise limits, consideration should be given to the routing of the

pipe work, and addition of insulation on the piping, before significantly increasing the line size.

- Lines in vacuum service to have pressure drops less than 0.09/100 ft.

3. Sizing Criteria for Gas/Liquid Two Phase Lines

a. flow patterns

Two phase flow in pipes can be classified by flow patterns which describe the type of flow exhibited, with

different flow patterns identified for horizontal and vertical flow. The flow patterns are determined from

standard flow pattern maps.

b. rosion

High velocities in 2 phase lines can cause rapid wear by erosion. The velocity at which erosion may occur is

calculated by the formula given in API RP14E, as follow:

Where

Ve : erosion velocity (m/s)

C : constant (empirical)

: 122 for carbon steel lines in continuous service

: 150 for carbon steel lines in intermittent services

: 250 for duplex and stainless steel lines in non-erosion (continuous) service

: 400-450 for duplex/stainless steel in intermittent service

m :Mixture density (kg/m3) at operating pressure and temperature (gas density should be used

for single phase gas lines).

Mixture density can be calculated from the following:

Where

: liquid volume fraction

m : gas density (kg/m3)



1 : liquid density (kg/m3)

Maximum velocity shall be maintained below erosion velocity.

c. line sizing

The following criteria can be used to size two phase lines. Gas gathering and long flowlines are excluded from

this procedure and should be sized using proprietary software. Calculate the erosional velocity and determine

the minimum pipe size required to avoid fluid erosion.

Check the flow pattern to ensure that slug flow is avoided - annular mist flow is preferred. However, if

lug flow cannot be avoided due to erosion velocity constraints, then the line size will be accepted.

Calculate the pressure drop as noted in this document.

4. Sizing Criteria for Relief and De-pressuring Lines

For preliminary engineering the guidelines given below should be followed. A preliminary network analysis will

also be undertaken for all principal relieving/de-pressuring cases in order to ensure that the maximum

allowable back pressure at each individual relieving device is not exceeded. Calculations will be performed in

accordance with API RP 520 and API Std 521.

a. relief valve inlet lines

The inlet line pressure drop is to be less than 3% of set pressure calculated at set pressure conditions

assuming flow based on installed relief valve area for conventional and balanced valves. (Note - not for

modulating type pilots) However pressure drop in the inlet line for a pilot valve should be calculated and

considered when determining the relief valve capacity. The inlet line shall not be smaller than the inlet flange of

the relief valve.

b. relief valve outlet lines

The discharge line shall not be smaller than the outlet flange of the relief valve. The computation of the

allowable pressure drop using the required relief capacity is as follows:

Conventional Valves size the discharge line from conventional valves to limit the pressure drop to less

than 10% of the set pressure (gauge).

Balanced Valves Higher pressure drops may be used to affect considerable cost savings. Limit the

pressure drop to 50% of the set pressure (gauge) and to no more than the rating of the internal bellows.

Where there are a number of valves discharging into the same manifold, caution should be exercised to ensure

that the backpressure in the manifold allows all relief valves to discharge properly.

The recommended maximum Mach number to be used is 0. 75, calculated at rated flow rate and downstream

end of header conditions. Back pressure based on installed critical area shall be checked to confirm it is

below the system design pressure.

Note that Mach number for vapour lines is calculated from the following equation:

Where:



Vs : sonic velocity ( m/s)

: ratio of specific heats for the gas (cplcv)

R : individual gas constant= 8314/M (J/kg K) M = gas molecular weight

T : absolute temperature (K)

c. Flare Headers and Sub-Headers

The equivalent length of headers and sub-headers should be determined by reference 5. The maximum Mach

number used shall be 0. 75, calculated at rated flow rate at the downstream end of line conditions.

d. flare stacks

Flare Stack Diameter is generally sized on a velocity basis although pressure drop should be checked. For

stable flare burning in pipe flares, API std 521 recommends 0.5 Mach for a peak short term infrequent flow,

with 0.2 Mach maintained for the more normal and possibly more frequent conditions. Requirements for pipe

flares and other proprietary designs should be discussed with the vendor.

e. blowdown Inlet lines

The maximum velocity should be limited to 200 ft/s.

f. blowdown outlet lines and high pressure vents

Vent stack diameter is generally sized on a velocity basis although pressure drop should be checked. Since

there is no requirement to maintain a stable flame, the vent tip can be sized for sonic velocity. This minimizes

the size and provides gas dispersion. An allowance must be made in the pressure drop calculation for the

pressure discontinuity which occurs at the tip when sonic velocity is established.

g. atmospheric tank vents

Non-refrigerated atmospheric tank vents are sized on the basis of maintaining an operating pressure that the

tank can safely withstand. Flow rates are determined by consideration of thermal inbreathing and out-

breathing, maximum fluid inflow or outflow, and vapour production resulting from fire exposure. Refer to API

Std 2000 for design criteria.

h. vent stacks

If sonic vent tip is applied, the tip shall be sized with 1 Mach for a peak vent load. Venting line and stack size

requirements for proprietary designs should be discussed with the vendor.

5. References

API Std 520 Part I - "Sizing, Selection and Installation of Pressure-Relieving Devices in Refineries", 8th

Edition 2008

API RP 520 Part II - "Sizing, Selection and Installation of Pressure-Relieving Devices in Refineries", 5th

Edition 2003

API STD 521 - "Pressure-Relieving and De-pressuring Systems'', 5th Edition 2007 4. ASME 831.3 -

"Process Piping"



ASME 816.5 - "Pipe Flanges and Flanged Fittings"

"Flow of Fluids Through Valves, Fittings, and Pipe", Crane Technical Paper No. 410.

API STD 2000 - "Venting Atmospheric and Low-Pressure Storage Tanks", 5th Edition, 1998

GPSA "Engineering Data Book", FPS Version, 1ih Ed. 2004

API STD 617 - "Axial and Centrifugal Compressors and Expander-compressors for Petroleum, Chemical

and Gas Industry Services", ih Edition 2002

API RP 2350 - "Overfill Protection for Storage Tanks In Petroleum Facilities", 2nd Edition 1996

Category: Oil and Gas

Tags: Sizing Criteria for Gas Liquid Two Phase Lines Sizing Criteria for Relief and De-pressuring Lines Sizing Criteria for Single Phase Liquid Lines

Sizing Criteria for Single Phase Vapor Lines

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