What is the basic operating principle of a flash tank?

Answer 1

What You Should Know About Flash Tanks

by J. E. TROCOLLI • Sarco Co., Inc. • Allentown, PA (Actual Specifying Engineer/71)

High-pressure steam systems

require flash tanks -

here is a guide to determining sizes

Condensate temperatures in high-pressure steam systems

generally are only slightly less than the saturated

temperature of the steam. When hot condensates are discharged

into lower-pressure areas, condensate temperature

immediately drops to the saturated temperature of

the low-pressure area. As the result of the drop in temperature,

heat released evaporates a portion of the condensate,

generating flash steam.

To return condensate to the boiler or to discharge it to

the sewer, it is necessary to separate flash steam from

the condensate. This is accomplished by discharging condensate

through steam traps into a vented tank, referred

to as a flash tank.

Flash steam produced in the flash tank may be vented

to the atmosphere or piped to a low-pressure main. Condensate

remaining may then be returned to the boiler or

discharged to drain.

Tanks must be large enough to ensure dryness of the

released steam and to avoid carryover by the steam of

water in droplet form. When using horizontal flash tanks,

the required area is found by multiplying the diameter of

the tank by its length. This measures the tank's capacity

to handle condensate. Table 1 illustrates the required area

for each 1,000 lb. of condensate/hour with varying steam

and flash pressures.

How big a tank?

Problem:

An absorption machine condenses 12,000

lb. of steam/hr. Assuming the flash tank is vented to the

atmosphere, determine the size of the flash tank required

with a steam pressure of 12 PSIG.

Solution:

Enter table 1 at 12 PSIG steam pressure,

moving horizontally to 0 PSIG. Find .75, which is the number

of square feet required for each 1,000 lb. of condensate.

Since 12,000 lb. of steam are generated, it may be

determined that by multiplying 12 x .75, 9 sq. ft. of surface

will be required, or that the diameter of the tank in

feet times its length in feet must equal 9. Thus, a tank 2

ft. by 4.5 ft. may be used.

Problem:

A dryer operating at 100 PSIG condenses

18,000 lb. of steam/hr. The flash tank is to discharge its

flash into a 5-PSIG heating main. Determine the size of

the flash tank required.

Solution:

Again, using table 1, enter at 100 PSIG

initial pressure. Move horizontally to the 5-PSIG column

and find 1.92.

Then:

18 x 1.92 = 34.6 sq. ft.

A flash tank 4 ft. by 9 ft. will be satisfactory for the

application.

Sizing vent lines

If flash steam is to be discharged to the atmosphere,

a properly sized vent line must be provided. To determine

the proper size, first find the area of the flash tank,

using the method described above.

Problem:

Determine the size of the vent line using

table 2. If, as in the first problem, tank size is 2 ft. by 4.5

ft. (9 sq. ft.), refer again to table 2, where 9 sq. ft. falls on

7.4-to-12 line. For this range, a 2-in. vent line would be

satisfactory.

Problem:

Determine the size of the vent line if the

flash tank size is 4 ft. by 9 ft. or 36 sq. ft. In table 2, 36 is

in the 27-to-36 line, and, in this case, a 3

1/2 in. vent would

be needed.

Figure 1 shows a typical flash-tank piping diagram

in which the flash is discharged to the atmosphere.

If it is desired to utilize the flash steam by discharging

it into a low-pressure main, refer to figure 2. In this

instance, it will be necessary to properly size the line

connecting the flash tank to the low-pressure main.

Using table 3, determine the percent of flash. Multiply

this percentage by the condensate load in lb./hr. to

determine the number of pounds of steam that are flashed.

Determine what steam velocity will be acceptable in the

line. If a low noise level is desired, a relatively low velocity

must be selected - 4,000 to 6,000 FPM.

Where noise is not a factor, a velocity of 12,000 FPM

or higher may be used. After velocity has been determined,

the required pipe size can be found in table 4.

Problem:

10,000 lb./hr. of condensate is discharged

into a flash tank from a 125-PSI steam system. Flash

steam is to be piped into a 10-PSIG low-pressure heating

main. Determine the size pipe required for connecting

the flash tank to the steam main.

Enter table 3 at 125-PSI initial pressure. Move horizontally

to the 10-PSIG column and find 12.2 percent

flash. Then the amount of flash steam/hr. is found this

way:

10,000 x 12.2 percent = 1,220 lb./hr.

SECTION ENG - PAGE 0100

FLASH TANKS

TABLE 1 FLASH TANK IN SQ. FT. = DIAMETER x LENGTH OF HORIZONTAL

TANK FOR 1,000 LB. CONDENSATE PER HOUR BEING DISCHARGED

TABLE 2 VENT LINE SIZE TABLE 3 PERCENT FLASH

FOR HORIZONTAL

FLASH TANKS

½"

FLASH TANKS

TABLE 4 STEAM VELOCITY CHART

FORM FLASH TANKS

Revised 4/04 SHIPCO® IS A REGISTERED TRADEMARK OF SHIPPENSBURG PUMP CO., INC.

SHIPPENSBURG PUMP COMPANY, INC.,

P.O. BOX 279, SHIPPENSBURG, PA 17257 • PHONE 717-532-7321 • FAX 717-532-7704 • WWW.SHIPCOPUMPS.COM

PRINTED IN THE U.S.A. • BEIDEL PRINTING HOUSE, INC., 717-532-5063 PERMISSION TO REPRINT BY SPIRAX SARCO INC.

Since low noise level is important, a velocity in the

4,000 to 6,000 FPM range must be selected. Enter table

4 at 1,220 lb./hr., moving horizontally to a flash-steam

pressure of 10 PSIG. Then move up to 4,000 to 6,000

FPM velocity. Here, the chart shows that a 3-in. pipe will

handle about 6,000 FPM, or a 4-in. line would handle

about 3,500 FPM.

When vent lines cannot be extended to discharge

outside the buildings, it is important that the condensate

be cooled below the dewpoint to prevent the exhaust

from condensing and wetting walls, machinery, floors and

so on.

Estimating temperature

Since the dewpoint depends on several factors, including

relative humidity and temperature (which are variable),

the temperature to which the condensate must be

cooled should be estimated for individual cases.

The following is the recommended procedure:

Calculate the size of the flash tank in the method

described above:

Estimate the dewpoint, assuming unfavorable conditions,

and, when making the estimate, take ventilation into

account - it is a factor in determining relative humidity:

Once the dewpoint is known, estimate the quantity

of cooling water that will be required and finally:

Determine the pipe size and size of temperature

regulator valve required. A self-contained regulator with

a normally closed valve to open when the temperature

rises is recommended. All such controls have an ample

range over and under the calibration point so that setting

may be adjusted after installation.

Problem:

Calculate the quantity of cooling water required

to cool condensate in a flash tank vented to an

enclosed space, assuming the following data:

Steam pressure is 100 PSIG;

Condensate is entering the flash tank at 1,550 lb./hr.

at 335°F;

Ambient temperature of the space into which vent

discharges is 75°F; and

Cold water temperature is 50°F.

Assuming that ventilation at the above temperature

will be sufficient to have not more than 70 percent relative

humidity, the dewpoint will be 64.5°F (determined

from psychrometric chart).

To allow 1°F for safety, condensate should be cooled

from 338°F to 63.5°F. When the installation is completed,

further adjustment can be made by resetting the regulator.

The heat to be extracted from the condensate is equal

to:

1,500 lb./hr. x (338-63.5)°F = 410,000 BTUH

The quantity of cooling water required:

410,000 BTUH

(63.5 -50)°F

= 30,500 lb./hr.

or 3,670 GPH = 61 GPM

A 1.5-in. pipe to supply the water and a 1.25-in. temperature

regulator are recommended for this application.

Flash tanks separate flash steam from the condensate

by venting the flash steam to the atmosphere or

piping it to a low-pressure main, while returning the remaining

condensate to the boiler or discharging it to the

drain. If flash steam is discharged to the atmosphere, a

flash tank and a properly sized vent line must be determined;

if discharged to low-pressure mains it is necessary

to calculate the correctly sized line connecting the

flash tank to the low-pressure main. Also, the proper temperature

for cooling the condensate must be determined

for projects in which vent lines cannot be extended to

discharge outside the buildings.

Different situations require individual solutions to determine

the correctly sized flash tank, connecting pipe

and cooling temperature needed, but the calculation examples

offered here provide the means to determine the

necessary installations and accessories required.

Figure 1

A typical flash tank piping

diagram discharging to atmosphere.

NOTE: Omit trap if condensate is

discharged into vented pump receiver.

Figure 2

A typical flash tank piping

diagram with flash discharging to lowpressure

steam system.

Figure 3

This diagram depicts a

combination flash tank installation with

subcooling condensate.

VENT

PRESSURE

RELIEF VALVE

VENT IF

DESIRED

TO LOW

PRESSURE MAIN

HIGH PRESSURE

FROM

CONDENSATE

HIGH PRESSURE

SYSTEM

FLASH TANK

STRAINER

FLOAT AND

THERMOSTATIC

TRAP

FIN TUBE

COOLING LEG

REQUIRED

TO LOW PRESSURE

RETURN LINE OR

DRAIN

VENT TO

ATMOSPHERE

HIGH PRESSURE

FROM

CONDENSATE

HIGH PRESSURE

SYSTEM

FLASH TANK

STRAINER

FLOAT AND

THERMOSTATIC

TRAP

FIN TUBE

COOLING LEG

REQUIRED

TO DRAIN

CONDENSATE

FROM HIGH

PRESSURE

STEAM SYSTEM

TEMPERATURE

REGULATOR

STRAINER

COLD WATER

SUPPLY

VENT TO

T ATMOSPHERE

WATER LEVEL

COIL

DRAIN

TO SEWER

FLASH TANKS