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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.
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