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Condensate Recovery Systems
Steam System Training 13-1
2003
Condensate Recovery Systems
Condensate When steam transfers its heat in a process, heat
exchanger, or heating coil, it reverts to a liquid phase called
condensate. Condensate is condensed steam, not water.
Condensate contains:
Water Boiler treatment chemicals Particulate Most importantly
energy
Condensate therefore, needs to be returned to the boiler to:
Improve energy efficiency Reduce chemical cost Reduce make-up
water costs Reduce sewer system disposal costs
Why Have Pumps? In gravity type systems the condensate lines do
not have the pressure to flow the condensate back to the boiler
operation; therefore there is a need to have a vented condensate
pumping system. Another common application is the main collection
point in the boiler operation, where there is a need to collect the
condensate and pump the condensate to the deaerator system. In low,
medium and high pressure systems there is a need for condensate
pumps depending the on the design of the system.
Types of Pumping Systems:
Electric (on-off operation) Electric (continuous flow operation)
Steam motive type pump (self actuating)
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Condensate Recovery Systems
Steam System Training 13-2
2003
Air motive type pump (self actuating) Electric type steam or air
motive pump
Applications for Each Type Electric (On-Off) As the condensate
level in the tank increases to a certain level, contacts close in a
float switch and start the condensate pump. The pump operates,
until the condensate level decreases to a point that the float
switch contact opens and the pump stops. This operation repeats as
the water level rises and falls.
The pump is allowed to operate at a nearly constant
head-capacity point and not over the entire pump curve as with
continuous operation.
Condensate capacities of 8,000 lbs per hour or less Single pump
or dual pump Gravity systems, low pressure or medium pressure
return system
Advantages: o Low cost o Simple operation
Disadvantages: o Low capacities o Surging in the condensate
lines during pumping mode
Electric (Continuous Flow) The condensate level is controlled by
a modulating valve, which regulates to keep a constant condensate
level in the tank. As the demand increases and the level start to
increase, the valve opens further to let more condensate flow
though the valve into the condensate return system. As the demand
decreases and the level begins to drop; the valve closes down and
reduces the amount of condensate being discharged.
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Condensate Recovery Systems
Steam System Training 13-3
2003
The condensate pump operates continuously and pumps against the
modulating valve. The flow corresponds to the pump performance
curve at various discharge heads relating to settings of the
modulating valve.
Continuous operation is more suited to centrifugal pump
operation, which allows for wide capacity changes over a smaller
change head pressure. Also, the horsepower does not increase as a
pump is operating against a nearly closed modulating valve, which
can occur during operation.
Condensate capacities above 8,000 lbs per hour (high capacities)
Single pump operation (most common) Gravity systems, low pressure
or medium pressure return system
Advantages:
High capacities Handles load variations
Continuous flow of condensate in the return system, therefore
no
surging in the condensate lines
Disadvantages:
More complicated Higher initial cost
Steam Motive Pump (Self Actuating) The operating force of this
type of pump is steam, and the consumption is very low. Since the
pump handles a low volume of condensate at each stroke, its
applications are somewhat limited.
The steam-powered pumps can be used in a closed loop system or a
vented system to atmosphere. In a closed loop system, a steam trap
must be installed at the discharge of the pump unit.
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Condensate Recovery Systems
Steam System Training 13-4
2003
A vented receiver or collection header is an essential part of
the installation, as any flash steam must be separated from the
condensate before it reaches the pump assembly.
Condensate capacities of 8,000 lbs per hour or less Gravity
systems, low pressure or medium pressure return system
o Advantages: Low cost Simple operation No electric is required
Used in explosion proof areas
o Disadvantages: Low capacities Needs a fill head Mechanical
failures of the mechanism Venting on flash steam in the chamber is
limited
Air Motive Pump (Self Actuating) The operating force of this
type of pump is compressed air, and the consumption is very low.
Since the pump handles a low volume of condensate at each stroke,
its applications are somewhat limited.
It is not recommended that these types of pumps be used in
groups to handle larger condensate loads.
Condensate capacities of 8,000 lbs per hour or less Gravity
systems, low pressure or medium pressure return system
Advantages:
CONDENSATE PUMPS
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Condensate Recovery Systems
Steam System Training 13-5
2003
Low cost Simple operation No electric is required Used in
explosion proof areas
Disadvantages: Uses compressed air, which is typically a higher
cost utility Low capacities Needs a fill head Mechanical failures
of the mechanism Venting on flash steam in the chamber is
limited
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Condensate Recovery Systems
Steam System Training 13-6
2003
Electrical Steam or Air Motive Type Pump The operating force of
this type of pump is steam or compressed air. This type of unit
uses electrical level sensors to activate the steam or compressed
air motive force.
Condensate capacities of 8,000 lbs per hour or less Gravity
systems, low pressure or medium pressure return system
Advantages:
No electrical pump Disadvantages: More complex than all other
designs Low capacities Needs a fill head
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Condensate Recovery Systems
Steam System Training 13-7
2003
Selecting the Correct Pumping System Capacity required
Maximum Minimum Normal
Tank sizing Required discharge pressure NPSH required due to the
temperature of the condensate Control of the flow of condensate
Flash and neglect steam venting Location and installation
Capacity Required The plant needs to document the required
capacity of the condensate pumping system. Condensate pumps are
used in a variety of process and heating applications. The maximum
load is never usually achieved and there is typically a great
variance between the normal high condensate flow and the minimum
condensate flow. Therefore, careful consideration must be given
when defining the condensate capacity.
Sizing of Receivers The receiver should be sized for capacity
sufficient to allow condensate storage for a minimum of 15
minutes.
Example: Given condensate load:
4,000 lbs. per hour
4,000 div by 8.3 div by 60 = 8.03 gpm
8.03 gpm x 15 = 120 gallon storage tank
The tank material is typically a heavy wall steel tank or a
stainless steel. In some cases, the tank is coated with a
corrosion-resistive material. It is recommended that the tank be
stamped ASME, even if the tank is vented to atmosphere, to provide
a more desirable tank construction for industrial applications.
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Condensate Recovery Systems
Steam System Training 13-8
2003
NPSH
What Causes Cavitation As liquid enters the eye of the impeller
in a centrifugal pump, its pressure is reduced. If the absolute
pressure at the impeller eye drops down to the vapor pressure of
the fluid, vapor pockets begin to form. As these vapor pockets
travel in the fluid along the vanes of the impeller, pressure
increases and the pockets collapse.
This collapse is called cavitation. Cavitation is not only noisy
but also damages the pump impeller, shaft and seal, and, over time,
may reduce pumping capacity. NPSH refers to the minimum suction
pressure, expressed in feet of water column, that is required to
prevent the forming and collapsing of these vapor pockets.
The diagram shows the change in system pressure (Ps) as the
fluid travels through the impeller. To prevent cavitation, Ps must
remain above the vapor pressure.
The top curve shows system pressure (Ps) remaining above fluid
vapor pressure as it passes through the pumps; cavitation cannot
occur. The bottom curve shows Ps falling below the vapor pressure
as it enters the impeller eye. This will cause cavitation. The
cutaway view of a pump on the right shows the passage of flow
through the impeller.
Net Positive Suction Head (NPSH) A critical factor that should
be investigated in the selection of a condensate pump is the NPSH
due to the high temperatures that do occur in condensate
returns.
NPSH is determined by factors:
Temperature Altitude Static head Capacity
NPSH = Barometric Pressure, Ft. + Static Head on suction,ft. -
friction losses in suction piping, ft. - Vapor Pressure of liquid,
ft.
Defined as a suction pressure minus vapor pressure expressed in
feet of liquid at the pump suction. Results from the height of
water above the pump suction.
Suction Head = Total Pressure of Liquid Entering the Pump
Suction
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Condensate Recovery Systems
Steam System Training 13-9
2003
Two Values of NPSH There are two values of NPSH: NPSHR and
NPSHA. NPSHR (required) is the amount of suction head required to
prevent pump cavitation, is determined by the pump design, and is
indicated on the pump curve. It varies between different makes of
pumps, between different pumps of the same design and varies with
the capacity and speed of any one pump. This is a value that must
be supplied by the maker of the pump.
NPSHA (available) is the amount of suction head available or
total useful energy above the vapor pressure at the pump suction.
This is determined by the system conditions. NPSH typically is
measured in ft of liquid.
Pounds Pressure versus Feet of Head Each pound of pressure
developed by a pumping system is equal to 2.31 feet of head.
Therefore, 10 pounds of pressure (PSI) will lift water vertically
23.1 feet.
This can be calculated for any setting using the following
formula:
Pounds per sq. in. = Head in Feet 2.31 Head in Feet = Pounds per
sq. in. x 2.31
TABLE 1NPSHA (at sea level) at various temperatures.
Temp., F. Vapor pressure of water, psia
Vapor pressure of water, ft
Positive head, ft
220 17.186 39.7 0
218 16.533 38.2 0
216 15.901 36.7 0
214 15.289 35.3 0
212 14.696 33.95 0
210 14.123 32.6 1.35
208 13.568 31.3 2.65
206 13.031 30.1 3.85
204 12.512 28.9 5.05
202 12.011 27.7 6.25
200 11.526 26.6 7.35
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Condensate Recovery Systems
Steam System Training 13-10
2003
(Note: Water Temperature Rating) (Warning) One of the most
common pump systems is the floor mounted horizontal tank with one
or more pumps mounted to the side of the tank. It must be
understood that this design is usually operated at temperatures
below 205 degrees F. If temperature is higher the pumps will
cavitate and malfunction.
Another type of electric pump system is one with the tank
elevated above the pumps. This arrangement provides the necessary
NPSH for the pumps, thus relieving a lot of the problems with high
condensate temperatures. This design is able to operate with higher
condensate temperatures and is the preferred way of pumping
condensate.
Intermittent (On/Off) VS Continuous Operation When designing
condensate return pump systems there are two ways to operate the
pumps, on/off and continuous flow.
8,000 lbs or less On-Off operation 8,001 lbs or more Continuous
flow
To select flow rate for condensate pumps (on-off), multiply the
required flow by 3 to determine pumping capacity of a pump
operating 1/3 of the time.
Example: Given condensate rate = 4000 pounds per hour
Pumping rate (GPM) 4000 Lbs. /Hr.
8.33 x 3 60
= 24.1 GPM
Select a pump for approximately 24 GPM
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Condensate Recovery Systems
Steam System Training 13-11
2003
VENT - RECEIVERS
Continuous flow should have a by-pass orifice or recirculation
valve. These devices will recirculate a required flow back to the
receiver, if the control valve on the discharge side of the pump
closes to prevent the possibility of pumps overheating or
cavitating.
Minimum By-Pass Sizing --Consult your pump manufacturer for
assistance.
Shown here is a condensate pumping system with continuous
operation using a modulating control valve at the discharge side of
the pump.
Vent Sizing for Condensate Tanks The vent for the condensate
tank should be sized for the amount of flash steam. Please refer to
the flash steam tables in the first chapter. Also, the vent must
have added capacity for live steam that may occur from a poorly
managed steam system.
The vent should be a lazy discharge of vapors without any
velocities.
The most common failure of a condensate pump system is the
failure to size the vent properly. If the vent is under sized, the
tank will pressurize.
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Condensate Recovery Systems
Steam System Training 13-12
2003
Vent Sizing Example: Load: 20,000 lbs per hour
System: 100 psig modulating
Tank: Vented to atmosphere
Flash %: 13.3
Flash load: 2,660 lbs per hour
Neglect factor: 40 steam traps
20% will be failed
8 steam traps at 500 lbs per hour each
4000 lbs per hour of neglect steam
(size vent to handle 6,660 lbs per hour of steam to atmosphere
at low velocities)
Energy Loss of the Vent The energy loss from allowing the flash
and neglect to go to atmosphere can be calculated at:
Steam loss: 6,660 lbs per hour
Cost of steam: $5.00 per thousand lbs of steam
Hourly dollar loss: $33.00
Daily dollar loss: $792.00
Yearly dollar loss: $ 277,200.00
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Condensate Recovery Systems
Steam System Training 13-13
2003
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Condensate Recovery Systems
Steam System Training 13-14
2003