Lec6a Process Piping Design Part2

8/7/2019 Lec6a Process Piping Design Part2

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Piping Design Engineer

Engineer must understand certain process basics

When handling utilities

the designer knows that steam cools and forms condensate

he is aware that this condensate must be drained off

How the condensate is removed??

Possibly with a steam trap at selected low points in the steam system

Design engineer

also should know how to handle two-phase flow

equilibrium liquids

hot vapor by-passes


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Phase Equilibrium Liquids

Equilibrium liquids require the piping designers attention

Small amount of pressure drop

EL will start flashing

resulting in two-phase flow

increased line velocity

fluid that is difficult to control and impossible to measure

although flashing doesnt do any harm

Piping designer must first recognise

what liquids are in equilibrium?

when flashing can be tolerated?


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Phase Equilibrium Liquids

Where do you find equilibrium liquids??

any tray draw-off

tower bottoms

two phase flow

reboiler liquid draw-off biggest piping problems occur at reboiler liquid drawoffs


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Incorrect Piping of Reboiler Liquid

Problem of routing the reboiler

liquid from the area behind theweir

Through a pair of orifice flanges

(meter run)

Through a level control valve

Into the main pipeway or rack


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Incorrect Piping of Reboiler Liquid

Liquid is drawn @ nozzle A runshorizontally

Rises @B (here the liquid must pushagainst the head of liquid in the riserB)

Press drop is induced in the system

Flashing will start

The meter run, C can not properlymeasure two-phase flow

The control valve can not controlproperly

Piping fabricated and installed in thismanner

would have to be dismantled andrebuilt in the field

COSTLY MISTAKE

Nozzle

Riser

Orifice meter

Control

valve


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Correct Method of Piping Equilibrium Liquid

Liquid is drawn through nozzleA stays horizontal through

meter run B and the controlvalve

Then rises vertically at C.

Flashing still occurs but aftermeasuring and control

functions

If excessive flashing occurswhat one must do??

Line size can be increased tokeep the velocity low

By keeping both the meter runand the control valve below theliquid level in the reboiler

flashing is prevented in this

method


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Two-phase Flow Scenario (improper)

Two-phase flow causes piping

designers the most problems

Pipes friction reacts more on the

liquid portion, as the vapor tends to

flow at a greater velocity.

Real problem occurs when two-phase

flow must be divided into separate

piping systems

Major portion of flow would be routed to B

This would cause exchanger A to have less

pressure drop and exchanger B to have

more pressure drop

(although both HEs are designed for same

duty and pressure drop)


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Two-phase Flow Scenario (improper)

Since A has less pressure drop, the

liquids velocity will direct it to B

vapor will take the path of least resistance

and will go to exchanger A

the net result is

A is getting vapor

B is getting liquid

HTR of HEs is designed for equal flow

of liquid and vapor

Consequences: Exchangers will not perform as designed, the process unit

will not perform and must be shut down for correction

If piping designer does not recognise that the piping was for two-phase

flow, then these problems are likely to prevail.

How to modify??

(to make it proper)


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Two-phase Flow Scenario (proper)

Two-phase flow enters the horizontal pipe

midway between the exchangers

Pressure drop is same to either one

Absence of a path of least resistance

Flow will be equal to both exchangers

Features


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Piping at Fired Heaters (proper)

This a very common piping system

The heater inlet is all liquid flow

and the outlet is two-phase flow

In majority of the cases (nine out of

ten cases), this installation

completely unnecessary

excessively expensive

WHY???


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Piping at Fired Heaters The liquid heater feed (item1) is divided into 4 streams

to match the four passes of

heater tubes

each stream has a globe valve

(item 2) and flow indicator

(item 3, orifice)

each heater pass outlet has atemperature indicator

To operate the heater

flow is regulated with

the globe valve to ensure

that each pass has the

same flow

T indicates the desired

outlet temperature

The piping is correct up to

this point


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Piping at Fired Heaters

Where is the inaccuracy in this

installation??

In the downstream of the TI located

in each pass outlet

Someone has coined the magic

term two-phase flow and decided tohave symmetrical piping

symmetrical piping means

money and piping problems

can be doubly expensive if

the heater outlet is alloy

material

Symmetrical piping is necessary

for two-phase flow if there is no

method of control and distribution

must be made


13/25

Piping at Fired Heaters

In this installation distribution made

while the fluid is all liquid, controlled by

the globe valve and metered by the flow

indicator

to ensure that streams A, B, C, and D are

all equal flow.

If the flow is equal going into the heater

it must be equal coming out

Where to consider symmetrical piping??

Low pressure systems (such as a

crude charge heater service)

High pressure installations

the outlets should be combined in

the most economical manner and

routed on its way

outlet pass differential pressure

drop is minor and no consequence


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Hot Vapor By-pass

Tower overhead is routed to an air cooler (fin-fan)

Condensed vapor is directed to the accumulator

To maintain pressure on the accumulator, a hot

vapor by-pass will be installed

Hot vapor is by-passed around the cooler and isrouted to the pressure control valve, which allows

pressure to enter the accumulator as required

Hot vapor by-passes should never be pocketed

The pressure control valve should be installedabove the top of the accumulator

The by-pass piping must continuously drain from

point A to point B


15/25

Hot Vapor By-pass

As this by-passed vapor cools due to rainfall on

the line or cool air cooling the line, condensate

will form

With small amount of differential pressure

between points A and B

a pocket of liquid head might not be

overcome

the hot vapor by-pass may not work.

Generally, there is less than 10 psig differential

between A and B

Pressure control valve (butterfly type) is specified

to keep the pressure drop to minimumbut this will consume 2-3 psig

Line loss will consume another 2-3 psig

leaving about 4 psig

this will not overcome much liquid head


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Static Head or Pressure Head or Head

Liquids at rest causes a pressure equal in all directions and perpendicularto any surfaces in contact with liquid

This pressure is due to the weight of the liquid

Plus the pressure at the top level of the liquid

The liquid height is called Static Head or Pressure Head or Head

Head is important when calculating

hydrostatic pressure of vessels

Piping systems


17/25

Impact of water head

A tower system designed for 50 psig

This tower requires a hydrostatic

pressure of 75 psig and this test pressure

must be measured at the highest point in

the system (point A)

To attain this pressure at A the fieldbrings in a hydrotest pump and fills the

system full of water.

Piping designer knows this pump is

located located at grade.

Engineer should calculate the height

from the pumps pressure gauge to point

A and add this to the required test

pressure.


18/25

Impact of water head

In this case 54 psig must be added to 75

psig, totalling 129 psig

Which is the pressure that must be

obtained at which pressure gauge to

satisfy the 75 psig hydrostatic

conditions

The vessel designer must also considerthe static head

While calculating the tower head and

shell thickness.

This additional weight is considered inthe design of the support for the tower.

This can be critical when vessels are

located high in a steel or concrete

structures


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Flow of Oil in a Branched Pipeline

The pipeline handles an oil with specific gravity of 0.92 and kinematic viscosity

of 5 centistokes (cS) at a total rate of 12,000 cuft/hr. All three pumps have the

same output pressure. At point 5 the elevation is 100 ft and the pressure is 2 atm

gage. Elevations at the other points are zero. Line dimensions are presented in

Table 1.

Table Line dimensions

Line L (ft) D (ft)

14

24

34

45

1000

2000

1500

4000

0.4

0.5

0.3

0.75

Using the schematic for branched pipeline network shown in Figure 1, calculate

the flow rates in each of the lines and the total power requirement.

1


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Branched pipeline


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Branched pipeline


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Branched pipeline


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Lec6a Process Piping Design Part2