condensate recovery systems

Condensate Recovery Systems

Steam System Training 13-1

2003


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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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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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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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.


Advantages:

CONDENSATE PUMPS



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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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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.


Advantages:

No electrical pump Disadvantages: More complex than all other designs Low capacities Needs a fill head



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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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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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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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(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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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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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

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