A Working Guide to Process Equipments Upto 22062016

7/25/2019 A Working Guide to Process Equipments Upto 22062016

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A Working Guide to Process

Equipments


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Steam and Condensate Systems

It is best to think about steam reboilers as steam condensers.

For thermosyphon (natural flow) reboilers, it is necessary to obtain small

pressure drop on process side and it is easily obtained by placing the

process fluid on shell side.

Normally steam is kept in tube side.

All steam reboilers mainly depend upon the latent heatof the steam. So

it is best to use the saturated steam.

For a small change in Temp change in latent heat might be large as

compared to

For a small


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Gas Compression

Heat: Thermal unit required to raise unit temp in unit mass of water

Energy: Weight*Dist unit required to move unit mass by unit distance

Before the term Thermodynamic was coined, it was called as Heat in

motion

Thermodynamics was developed by heating air under different

conditions

1 Btu is 740 ft-lb worth of work


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Most efficient engine ever made has produced only 39% of thermal

energy into mechanical energy.

For most of the industrial processes, centrifugal compressors are

preferred over reciprocating ones except in case of low molecular

weight (and hence low density) gases i.e. hydrogen etc.

Reciprocating compressors can easily be installed and engineered

Both suction and discharge valves are spring loaded check valves.

Volume trapped between cylinder head and piston before piston starts

moving away from cylinder head is called starting volumetric clearance.


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Indicator card for compressor: It shows the actual graph between

pressure (measured by pressure transducer which is screwed at cylinder

head end) and volume (which is measured through piston movement

which in turns sensed by magnetic pick-up)


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Area between dotted line and solid line is compression work lost to heat

and ratio of dotted line area and solid line area is called adiabatic

compressor efficiency.

Indicator card is the only real way to monitor reciprocating compressors

performance.

High temp in compressor discharge shows hat mechanical power of

piston is being wasted in increasing thermal energy of gas which could

have been utilized for compression purpose; this is the case of adiabatic

inefficiency.

There are two type of compressor efficiencies:

Adiabatic Efficiency (Applies on both centrifugal and reciprocating)

Volumetric Efficiency (Applies only on reciprocating)

Even a small amount of liquid might be the cause of valve failure in

reciprocating compressor.

High temp in discharge may cause the plate (in spring loaded valves)

cracking or spring failure.

The cause of high discharge valve temp is primarily valve leakage due to

recompression.

Valve leakage is caused by combination of pulsation and fouling deposits(Salts, Sulphur compounds etc that may deposit in valve assemblies).

These may inhibit the proper seating of movable plate of valve.

High discharge temp trip is given in order to avoid overstressed piston

rod failure.


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Relief valves, corrosion and safety trips

Most common and catastrophic accidents in process industry are due to

corrosion type failures.

We usually operate pressure vessels 10% below the PSV setting.

Ultrasonic testing (UT) or Sonaray is being used to check the thickness of

process lines on stream.

Corrosion coupons are being used to check corrosion c=status in anu

process equipments and it calculates in term of Mils/Annum corrosion

rate.

Corrosion probes are also used to monitor the corrosion rate in

Mils/year, it is an electronic method. It measures the change in electrical

conductivity of probe or sometimes hydrogen activity too.


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Fired Heaters

Two equations are of paramount importance:

Heat transfer by radiation

Heat transfer by convection

Temp driving force for radiation section is always very high as

compared to the convective zone.

We use finned tube in convective zone in order to increase heat

transfer area and so the heat transfer rate.

Tubes in firebox area (in radiation zone) are made up of High chrome

steel which can withstand with high temperatures.

It is advisable to know about what types of tubes have been used in

furnace and what their temp tolerance is.

The only way to prevent convective zone tubes from overheating is

dont let the flame to reach to the convective zone.


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If incomplete combustion occurs, then heater may set up for afterburn

as soon as the raw flue gas finds the oxygen.

In case of incomplete combustion, flue gases are pollutants to

atmosphere and automatic temp control of furnace becomes unsteady.

The point at which fuel consumption is lowest and heat transfer is

highest is called point of absolute combustion.

Point of absolute is practical term which is analogous to complete

combustion which is theoretical term.

In case of burning Fuel oil with high C/H ratio, incomplete combustion

(oxygen starvation) would result in black color of stack gases but not

case of hydrogen burning.


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In case of NG burning, if we continue to operate on wrong side of

absolute combustion the color of stack gases may appear starting from

pale yellow to dark yellow to light orange to dark orange to brown to

finally black.

In case of COT automatic control which cascades into Fuel consumption

control, during bad operation, fuel will increase to raise or maintain the

COT but actually it makes it worse and cools the firebox because there is

already shortage of oxygen.

Regardless of Oxygen content in Stack gases, try to optimize the fuel gas

consumption vs COT.

Appearance of firebox and flame:

1. Bright and clear firebox denotes more than enough O2

2. Hazy, smoky or yellow firebox denotes less O2

3. To be exactly right there should be slight haze and flames

should be compact and not towards firebox walls in search

of oxygen.

Flame color depends upon fuel, gas often burns blue but oil burns

yellow. Yellow color denotes the thermal cracking of fuel which is

nothing wrong; in fact it is flame shape which matters not exactly the

color.

When combustible material or unburned material reignites in convective

zone, a dramatic increase in Stack gas temp can be observed.


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The oxygen in convective zone will always be higher than that in firebox

because of leakages in the convective zone and these leaked air

collectively known as Tramp air.

Tramp air depends upon mechanical integrity of fired heater and draft

balance over firing rate.

So, oxygen measured in stack will be the sum of unused oxygen in

firebox plus oxygen due to tramp air. Therefore, analyzers in convective

section or in stack might be so misleading due to tramp air that it may

cause dangerous situation in furnace operation wrt air flow adjustment.

Oxygen analyzers in the firebox area are much more reliable than that in

stack.

Draft: Comparison of two pressures at same elevation and traditionally

quoted in inches of water.


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If you open the inspection port in furnace or peep hole during positive

draft, you may liable to singe your eyebrows.

Air registers and stack damper work together as a team to optimize the

heater draft.

Balancing the draft means maintaining -0.1 to -0.2 in wc (-2.5 to -5.0

mmwc) pressure just below the shock tubes (interface point between

radiation and convection zone), on the same we have to maintain the

enough air in order to operate the furnace on the good side of absolute

combustion.


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If we close the damper gradually, inside pressure of convective section

will start increasing and hence decreasing the draft which in turns for a

fixed opening of air registers, will decrease the combustion air. So to

accommodate this shortage of less combustion air we will have to

increase the opening of air registers. This is overall what we call draft

balancing.

To save fuel wastage against air leakages, try to optimize draft through

closing the stack damper. O2difference between combustion chamber

and stack must be as minimum as possible.


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Patch up of leakages in furnace can be done by heavy duty aluminum

tape, insulating mud or silicone sealers and can be weld up any loose

sheet metal.

Function of the burner is to mix oxygen, in the form of air, with the

fuel so that fuel can be burned most efficiently. Fuel will burn at the

end of a tube with no burner at all but burning will be far from

efficient.

Air entering through Primary Air Register is much more able to mix

efficiently than that entering from Secondary one. So first try to

maximize the use of primary air register and then adjust flame with

secondary register.

Close openings near burner like sight port, pilot lights and other

openings, if any because air can only mix efficiently if it comes

through burner.


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When operating on reduced firing rates, close down some burners, if

possible because burners work efficiently when operating close to

design. Also dont forget to close the air registers of closed burners.

A typical preheater reduces the fuel requirement by 10%. But this

system requires higher temperature in radiation zone, so if there is

any furnace which is operating below its design firebox temp then

installation of APH is a good choice.

Symptoms of APH leakage:

Increased O2 content in flue gas

Low flue gas outlet temp

Increased delta T of flue gas than that of combustion air

Effects of APH leakage:

Reduced thermal efficiency of APH


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Decreased combustion air to burners which can cause

afterburn and then this afterburn can destroy the APH

itself.

FD Fan may require higher drive horsepower.

APH are subjected to corrosive attack due to condensation of SO3,

therefore dont let down the flue gas outlet temp below the dew

point of sulphur tri-oxide and this can be achieved either partial

bypass of APH or increase combustion air.

Before lightening burners, ensure proper purging of combustible

mixture and check it with HC detectors until it finds zero value.

Pilot lights are the MUST before burning main burners.

Increasing combustion air temp by 100 deg through APH would result

in flame temp rise by same amount (100 deg)


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Three temps in furnace:

Heater inlet temp (= convection inlet temp)

Convection outlet temp = Radiation inlet temp

Radiation outlet temp (= heater outlet temp = COT)

In case of high firebox temp (limiting to refractory conditions) and

lower convective side heat absorption, combustion air may be

increased in order to lower the radiant temp and to increase the flue

gas rate resulting in high convective side heat absorption. This is what

we call heat balancing. Increased airflow is not being used for


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combustion but to transfer heat from radiation section to convective

section and also, here oxygen requirement to reach absolute

combustion becomes irrelevant as we are operating with plentiful

amount of oxygen.


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It is good idea to check what portion of combustion heat is being

transferred to process side.

Heat of combustion = Amount of fuel consumed * Net heating value

Heater Efficiency = Func ( Stack temp, Excess O2, Ambient heat loss)

Process heat duty = Sensible Heat + Vaporization Heat

Three major products combustion:

H2O

CO2

BTU Flue Gas = H2O +CO2+ N2 + O2(Excess O2) : Around 80% is N2

A typical Excess O2in flue gas is 2 to 6%.

Combustion Heat distribution:

Convection heat (provided to combustion air)

Radiation heat to tubes (provided directly to process fluid)

Radiation heat to refractory walls

In majority of heaters, large portion of combustion heat is of 3rd

type

that would glow the refractory walls and then it returns the heat to

tubes.

When process fluid flow is interrupted, generally fuel is also tripped

off but due to stored energy in refractory walls it takes time to lower

the temp but tubes may be overheated in this case because there is

no fluid to take off this heat.

Typical firebox temp is 800 deg C thus tube skin temp may approach

to 700 deg C in case of process flow loss even though fuel supply is

tripped off. This may lead to mechanical deterioration of tubes as well

as coking inside tubes. One way to combat this problem is with steam,

open high pressure purge steam asap when there is loss of process


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flow, this will not only avoid coking but will help to take away some

heat from tubes to control skin temp.

If sudden loss of feed followed by premature restoration of flow

occurs repeatedly over a period of time, coking may occur inside

tubes plugging the tubes partially and hence increasing the delta P

that ultimately leads to shut down the furnace. This problem is called

shuttering feed interruption.

ALWAYS try to raise or lower the heater temp in controlled way as per

SOP and keeping the all design considerations in mind.

It is good practice to place one color vs temp chart near peep hole of

furnace.

Heater design P & T: Design pressure of tube is not the inlet pressure

but pumps dead head or shut-in pressure. Similarly, design temp is

not COT but maximum skin temp (estimated at EOR) which is also

called TMI (Tube Metal Indication).

A typical process heater tube dia is 4 in to 10 in and thickness quarter

to half inch. High chrome content (13%) can withstand with high temp

as compared to lower chrome content (3%).

For added corrosion and temp resistance, Ni or Mo content is also

being increased; tube with high Ni content is classified as 300 Series

stainless steel.

When temp of tube goes beyond 700-760 deg C, tube tends towards

plastic deformation and inside pressure of tube forces tube to expand

in this case, this is called high temp creep and tubes may be called to

be bulged.

Tubes rarely fail because of external oxidation and it rarely burn-up,

they fail because of high temp creep which leads to expansion and


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then burst. Thus fundamental cause of tube failure is localized high

temp, which is also called hot spots.

In any case, if combustible process fluid spills out of tube then it will

be to fuel rich to explode (limiting O2 content will prevent any

unexpected fire in heater). But what we can expect is flame and dark

smoke which will come out through stack and that wont be as

dangerous as it appears.

In this situation (tube failure), dont try to stop fuel and/or process

fluid immediately as it will be dangerous because now air: fuel ratio

will start increasing and once it comes under explosive region then

will be too difficult and dangerous to handle. Correct way to prevent

this kind of fire is to immediately start firebox purge steam as it will

help to sweep oxygen content from furnace and then fuel and process

fluid can be shut off safely.

Consider a orifice flow meter whose one tapping point got plugged

then what will happen is it will record less pressure in plugged side

which in turns reflect in higher delta P and hence high flow indication

will be shown as compared to actual flow. If this kind of problem

occurs in heater process fluid then there is a chance of overheating

and hence, in general too, such critical flow control valves should be

set a lower limit of say 20% or any appropriate.


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Pumps

Centrifugal pumps are dynamic machines, which means they convert

velocity into feet of head.

If there is sudden reduction in velocity then it must convert either in feet

head in case of open system (fig 1) or in pressure in case of closed

system (fig 2).


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In fig 1, we can easily see the sudden increase in water level once some

pumps suddenly stop but in second case if shut off valve closes then

increase in pressure can be noted in PG which is attached to the riser

tube.

In fig 1 if sump is running (water is being withdrawn continuously) then

the sump level would be slightly less than the lake level and that levelwill be called head loss through running pipe but that difference would

have zero in case of no flow.


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If pressure in pump suction falls below the bubble point of the liquid

then it starts vaporizing and this phenomena is called cavitation.

To avoid cavitation one must prime the pump before start up and then

open discharge valve slowly hence giving sufficient time to convert

suction pressure energy into kinetic energy.

Above depicted is Overhung, Single stage pump. Overhung means it

has only inboard, but no outboard, bearing. Single stage means it has

only one impeller. Multistage pump may have 5 or 6 impellers.

The inboard side means the end which is closest to the driver.

Main components of pump:

Shaft: Used to spin the impeller

Coupling: Attaches the shaft to driver (Motor or Turbine).

Bearings: Support the shaft.

Seal: Prevents liquid to leak from the casing around the shaft


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Impeller wear ring: Prevents internal leakage from discharge to

suction

Impeller: Accelerates the liquid.

Volute (working part of centrifugal pump): Converts velocity

imparted by impeller on liquid to feet of head.

The function of impeller is to increase the velocity or kinetic energy of

liquid. Liquid enters and leaves the impeller at the same pressure. The

pressure at the impellers vane tip is same as suction pressure.

When high velocity liquid enters into volute (also called Diffuser) its area

gets widened and hence velocity gets converted in feet of head (not in

pressure)

psi = pounds per square inch

Centrifugal pump generates same head regardless of liquid being

pumped having viscosity below 40 cp or 200 SSU (Saybolt Seconds

Universal)

As motor we are using for pump is fixed speed machine, the rpm of

pump impeller would be same and by property of centrifugal pump head

will also remains same irrespective of fluid. Power driven will be affected

by the liquid being pump as power requirement = feet * pounds and


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here pounds depend upon density keeping the volumetric flow rate

same.

Pump curve: With increase in volumetric flow, feet of head (and thus

discharge pressure) decreases.

Pump curve has two main areas: flat and steep, we normally design and

operate pump to run towards the end of the flat portion.


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Driver horsepower for pump varies in proportion of product of vol. flow

and feet of head. First consider the flat portion of pump curve, vol. flow

increases and feet of head decreases marginally but keeping the product

of both on increasing and for this reason Amp meter shows increasing

indication. Now for the steep portion of pump curve, with increase in

vol. flow feet of head decreases dramatically and hence product of both

goes on decreasing which reflects in Amp meter by decreasing

indication.

So, Amp meters are best to showcase pump performance and hence

should be located near start/stop switch.

As a rule of thumb, decreasing 10% of impeller dia will decrease the

driver horsepower by 25% and thus may increase the lifetime of motor

by 10 years!

So, if C/V d/s of pump is at almost at closing position then it is good idea

to trim down the impeller dia to reduce the unnecessary power

consumption and also to reduce the huge delta p across C/V.

On the other hand, increasing impeller dia by 10% will require increased

driver power by 25% and hence in most cases, it may require new motor

and breaker to support the larger impeller.

There are three (3) types of limits of centrifugal pumps:

Impeller limit (or, pump limit)

Driver limit (Trip on high amps)

Suction pressure limit (NPSH)

In most process plants in America, motors are of three phase and rpm of

3600 or 1800. In EU, it is 3000 or 1500.

3-phase motor can also be used as electrical power generator whereas

2-phase motor, without modification, cant.


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Motor trip points:

Increasing flow when pump is operating on flat portion of curve

Increasing density of liquid

Increasing dia of impeller

Motor winding deterioration (usually happens on aging)

Dirt build up on motors cooling air fan guard screen

Operators forget to clean motor fan screen which causes heat buildup

within the motor resulting in high amps requirement.

Next to the amp meter on the motor breaker, there should be a tag or

penciled number showing full limit amp (FLA). Above FLA, motor should

be tripped off and if not there might be burning of motor windings.

Actually when we start a pump, an initial torque which is required to

start spinning pump shaft gives an instantaneous reach of motor amps

beyond its FLA but due to time delay of say 15-30 seconds motor cant

be tripped off immediately and when discharge valve is opened

sufficiently, amps gets normal.

During cavitation in centrifugal pump, low suction pressure will cause

erratically low discharge pressure, discharge flow and hence low amp

draw and a sound like shaking a bucket full of nut and bolts will also

appear.


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Caution: When a pump completely loses its suction then it runs with

steady discharge pressure and steady discharge flow A Zero Steady

value. This is not cavitation but a Must Avoid situation; here impeller is

spinning in empty casing with no purpose.

If a liquid flows through 8 in suction line to 2 in impeller eye then its

velocity increase by a factor of 16 and kinetic energy by 264. This change

in suction pressure to kinetic energy is called required net positive

suction head(required NPSH).

Now lets assume if C/V d/s of pump increases its opening then its

discharge flow (and hence velocity) increases so does the suction flow/

velocity and when this factor (16 and 264) applies on this increased

value of suction velocity and KE then required NPSH also increases.

Therefore, required NPSH increases with increase in

flow/velocity/Control valve opening.


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The unit of NPSH = feet of liquid head

Available NPSH = Suction pressure Vapor pressure of liquid at suction

pressure

When required NPSH equals available NPSH, pump cavitates.

When liquid is in equilibrium with its vapor then liquid is said to be at its

bubble point and vapor at dew point.

On comparing Fig 25.1 and 25.2 we see if we operate pump up to 250

flow rate then it is ok but if go beyond 250 required NPSH then exceeds

the available NPSH and hence cavitation will take place. Operation of above pump at 300 flow rate would require and additional

6ft liquid head to avoid cavitation. This could have been possible if vessel

was located 6 ft above than its current elevation or if we were able to

raise liquid level in current vessel by 6 ft.

If we increase the vessel pressure by control valve then also we cant

avoid cavitation because now current VLE will show that current


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(modified composition) liquids vapor pressure has now also been

increased with the same amount as at the suction, hence keeping the

available NPSH (=Suction pressureVapor pressure) unchanged.

Decreasing temp will work in same way as described in last point of

pressure increase.

BUT, if we cool down the suction line of pump without cooling the liquid

in the vessel which is in equilibrium with its vapor then we can succeed

to avoid cavitation because this subcooled liquids vapor pressure will

decrease which in turn will increase the available NPSH (=Suction

pressureVapor pressure).

When designing vessel height, pump running NPSH requirement +

starting NPSH requirement + frictional losses must be taken into

account, so that summation of these three must not equal the available

NPSH.


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Overcoming starting NPSH requirement: Though increasing vessel

pressure suddenly by some amount would have no effect on available

NPSH but this is true only for running pump. It will always take time (say

close to residence time of vessel) for new composition liquid in the

vessel to reach pump suction and during this period if we can start the

pump (which could not be run due to starting NPSH limit) successfully

the starting NPSH requirement can be taken care of by this temporary

increase in available NPSH. However, the available NPSH will settle at its

original value once new liquid start reaching to impeller eye irrespective

of the increased vessel pressure. Points to remember for this operation:

first, increase in pressure should be sudden and second, start-up

procedure of pump must be quick, discharge valve can be kept crack

open, for example.

NPSH limitation due to plugged or undersized draw off nozzle:


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Consider a case where draw off nozzle has maximum flow capacity of

100 kg/hr and we are operating pump at 90 kg/hr. Now if we want to

increase the flow rate up to 110 kg/hr and we have more than sufficient

available NPSH that can even accommodate up to 150 kg/hr. What will

happen in this case is when flow reaches to a value of say 109-110 kg/hr,

however pump may allow a discharge flow of 110 kg/hr but nozzle wont

and level in suction line will start creeping down and when it comes

down to a level which equals the required NPSH, pump will start

cavitating. Underdesigned nozzle is a rare case we have force of highly

expert project and process engineers but that well designed may be


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plugged and those plugs are responsible for restricted flow not those

highly qualified innocent engineers!

Sub cooled liquids allow us to operate the pump at considerably low

suction pressure even at sub atmospheric pressure sometimes, as in

case of tank storage where liquids always are well below from their

boiling points.

Documents

A Working Guide to Process Equipments Upto 22062016