Flash Tanks for Steam and Boiler Systems
Low in initial cost, flash tanks are easy to apply and aid in condensate drainage of steam equipment
By Roy C.E. Ahlgren
Associate Member ASHRAE
In the old days, steamfitters and plant engineers knew the advantages of having a properly designed flash tank in the right place and for the right purpose. In trying to reduce steam system operating costs today, many are turnmg once again to this surprisingly versatile and energy efficient device.
This article describes how flash tanks work, how they can be employed to improve system operation and save energy, and how they should be designed for safe and trouble-free operation.
Operation
Flash tanks can be an important part of many steam systems. They get their name from the sudden evaporation or "flashing" that occurs when hot water at some higher pressure is suddenly released to a lower pressure. We can think of the heat that causes this flashing as the difference between the enthalpy of the liquid at the higher pressure and the enthalpy of saturated liquid at the lower pressure.
The enthalpy of evaporation (the heat required to change the liquid to steam at the lower pressure) determines how much of the liquid can be converted to steam by the heat available In other words,
% of flash steam =
energy required for change of phase
% flash =
enthalpy of evaporation at the lower pressure
For example, one pound of boiling water (saturated liquid according to the steam tables) at 100 psig/338°F (689 kPa/170°C) is flowing froma pipe into a tank at atmospheric pressure. What fraction, X, of the water will flash into steam?
From the steam tables, we know that each pound of water at saturation conditions has a given maximum heat content (liquid enthalpy) or total enthalpy for steam. The data from the example and the steam tables can be summarized as shown in Table 1.
If we ignore heat losses, the enthalpy of the high pressure water must be equal to the total enthalpy of the low pressure water and steam entering the tank:
This says that 0.113 lb (0.06 kg) of steam at 0 psig/212°F (0 kPa/100°C) will be formed from each pound of water as it drops in pressure from 100 psig/338°F to 0 psig/212°F (689 kPa/170°C to 0 kPa/100°C). In general terms,
% flash =
enthalpy of evaporation at the lower pressure
So, if a given flow of water, 2,000 lb/hr (0.25 kg/s) at 200 psig/338°F (689 kPa/170°C), is flashing to steam at atmospheric pressure, the flow of flash steam will be:
and the remainder of the flow will be:
The production of flash steam is influenced by other components in the system too. For example, thermostatic steam traps open only after the condensate has subcooled, or dropped below the saturation temperature, so the amount of flash steam for a given drop in pressure will be reduced. For one pound of condensate, each Farenheit degree drop in temperature below saturation reduces the enthalpy of the condensate by approximately one Btu.
Uses for the flash tank
Flash tanks provide one or more of the following: a common lower pressure point for collecting condensate from steam equipment operating at different pressures; a means to cool hot condensate to allow the use of low temperature rated pumping equipment; and a source of low pressure steam for heating or process use.
One of the simplest flash tank applications uses high pressure condensate to raise the average temperature of a mixture of low pressure condensate and make-up feed water in a vented receiver. The high pressure condensate should be introduced by means of a perforated pipe below the water line so the flash steam can be completely condensed as it mixes with the volume of water. If the flow rates and temperatures result in an average temperature below 212°F (100°C), vent losses will be minimal.
Often several steam condensing units are operated in parallel, each with it's own temperature regulating valve. The steam pressure in each unit will vary widely, depending on the setpoint and load. If these units were simply connected to a common condensate return pipe, back pressure in the pipe from the unit discharging condensate at a higher pressure would cause condensate to back up in the unit operating at lower pressure. Installation of a flash tank vented to the atmosphere reduces the possibility of back-pressure and allows all of the units to drain, increasing their productivity and minimizing the potential for heat exchanger damage due to condensate flooding.
Although the flash steam is often vented directly to the atmosphere, many have recognized the significant waste of energy and water associated with this practice and installed another heat exchanger (called a vent condenser) to condense the flash steam recovering the heat and water.
Flash tanks usually drain by gravity to a receiver and pump combination. Because condensate flashed to atmospheric pressure is cooled immediately to 212°F (100°C), only a little more cooling is required to allow use of conventional, low temperature rated, condensate pumps. These pump receivers are usually vented to the atmosphere and rated to handle condensate below 200°F (93°C). A vented flash tank is shown in Figure 1.
Flash legs are really small flash tanks that have been built into the steam piping to dispose of the condensate from high pressure steam mains. The flash leg is an oversized pipe with end-plates welded to it. High pressure condensate passes through a steam trap into the flash leg where it immediately flashes, not to atmospheric pressure, but to the pressure of the low pressure steam main. The flash steam is vented directly to the low pressure main and the remaining condensate passes through another trap into the low pressure condensate main. A flash leg is shown in detail in the ASHRAE Handbook - Systems.(1)
If several steam loads are operating at constant pressures, we may connect them to a flash tank operating at a pressure lower than the load's but higher than atmospheric. In this way, the flash steam can be recovered under pressure and the flash tank becomes a source of low pressure steam.
Pressure in the flash tank may be maintained by connecting the steam vent from the flash tank to a pressureized system such as a deaerator or low pressure steam main. In that case, a check valve must be installed to prevent backward flow if the flash tank pressure should drop. A back-pressure valve could be used to control the maximum pressure in the tank and a relief valve is always required to protect the system. The steam piping between the flash tank and the application point should be generously sized to minimize velocity and carryover of water droplets.
Ideally, the system should be designed so that the maximum rate of flash steam flow will not quite provide all the low pressure steam required for the application. Under these conditions, a pressure reducing valve (PRV) from a steam supply main would be used to make up the total amount needed and carry the entire load when the flash tank is not in service or when high pressure condensate is not available. When flash steam is available, the PRV will close, reducing load on the boiler. A pressurized flash tank is shown in Figure 2.
In designing a pressurized flash tank, be aware that the water leaving the flash tank may flash again in the piping as pressure on it is reduced. Because of flashing, the slection of condensate pipe size involves some complexity.
One way to size this pipe is to calculate the amount of flash steam that would form if the condensate went immediately from flash tank pressure to destination pressure. Then assume that this flashing occurs all at once, and size the condensate pipe as if it were a stam pipe carying only the flash steam. The pressure drop due to the liquid portion is negligible.
If we fail to consider flash steam in the condensate pipe, the pipe may be badly undersized. this further condensate flashing results in a significant waste of energy if vented pump receivers are used, so special closed condensate systems have been developed to keep pressure on the condensate as it leaves the flash tank to prevent additional flashing and, thereby, save energy.
The receiver in a closed condensate unit operates at the same pressure as the flash tank so condensate flows to it by gravity. Because there is no pressure reduction and no vent, there is no loss through condensate cooling or vented flash steam and steam traps between the flash tank and the closed condensate equipment are not required. Specially designed pumps, which can handle the high temperature condensate without cavitating, are required for this kind of equipment.
In all of these applications, the steam traps at the condensing equipment must be selected and installed to be able to handle their condensate load under the worst case condition of highest back-pressure and lowest steam pressure.
Another common use for flash tanks is to recover heat from the continuous blowdown or surface blow from the boiler. An additional liquid-to-liquid heat exchanger can extract still more energy from the boiler water leaving the flash tank by cooling it to an environmentally acceptable temperature before discharging it to waste. Boiler water is introduced into the flash tank by a perforated pipe below the waterline to aid in reducing carryover of boiler contaminants.
Designing the flash tank
Calculate the amount of condensate entering the flash tank. This will be the sum of the steam condensing capacities of all equipment to be connected to the flash tank. Do not use trap capacities because traps are usually sized by multiplying the condensate load by a safety factor to allow for unknowns and flow variation. For boiler blowdown heat recovery, multiplying the maximum blowdown percentage by boiler capacity and the number of boilers.(5)
Calculate the flow rate of flash steam formed by using the formula and data from the steam tables. Multiply this percentage by the flow rate of water into the tank to get the flow rate of flash steam.
The tank steam volume must be large enough for the instantaneous flash. We can calculate the required volume using specific volume figures from the steam tables (3) In this example, one pound of steam at 0 psig will occupy 268 cubic feet.
For example, 2,000 lb/hr (0.25 kg/s) of condensate at 100 psig (689 kPa) flashes to atmospheric pressure. As before:
Steam flow rate = 0.133 x 2,000 lb/hr = 266 lb/hr (0.03 kg/s)
Steam volume =
In addition to providing adequate volume for expansion of the steam, a vent pipe of proper size must be installed to avoid pressure build up. use steam flow tables, formulas or a nomogram to select a pipe that will vent the steam at a velocity of about 4,000 ft/min (20.3 m/sec) or less. This will limit flow noise and allow the flash steam to vent off withouit seignificant increas in pressure.(3)
In no case should the vent size be less than 2 in. (50 mm). The ASHRAE nomogram for steam pipe sizing(2) at 0 psig indicates that a 2-1/2 in. (65 mm) vent pipe could carry 266 lb/hr (0.03 kg/s) of flash steam at low velocity and without bnuilding up significant pressure in the tank.
The tank water volume depends on the condensate flow rate and density. For the example above:
The density of water decreases a bit at typical flash tank temperatures. In this example, it's about 59.8 lb/ft3 (957 kg/m3), and:
The total volume entering the tank each second will then be:
Therefore, this flash tank should have a volume of approximately two cubic feet.
In this example, no allowance has been made for varitations in condensate flow, or the fact that flash steam separating from the condensate at high velocity may carry over significant amounts of the liquid or boiler contaminants. Some sources have recommended that arbitrary safety fators be applied to the steam and water volumes, but these are hard to justify and they lead to wide differences in the tank size recommended.
A better method for finding the flash tank size is available. it depends on the disengaging area as well as the volume of the tank. The disengaging area is the surface through which the steam must flow to separate from the liquid, so this concept is especially useful when liquid is introduced to the tank below the water line.
The disengaging area is interpreted as the rectangle that is formed by the intersection of the water surface and the inside of the tank when the tank is exactly half-full, or the length of the tank times its diameter for a horizontal cylindrical tank. For a vertical cylindrical tank, the disengaging area is simply the area of the circle formed by the intersection of the water surface and the inside of the tank. The disengaging area is shown in Figure 3.
The flash tank disengaging area required is shown in Figure 4. you can use the disengaging area curves without looking up values from the steam tables to ensure that a tank will have a certain maximum steam separation velocity. these curves take into account the amount of flash steam generated under different conditions of pressure and pressure drop as well as different steam separation velocities.
Use high velocities for lowest intitial cost or when some provision for liquid carryover (such as steam separator) has been provided in the low pressure steam main. Lower velocities will limit the amount of carryover and are particularly important for boiler blowdown applications. Use of lower velocities will lead to larger tan ksizes, an advantage when the flow of liquid entering the tank is expected to be highly variable.
Enter at the bottom of Figure 4 with the initial pressure of the liquid entering the tank. Follow a vertical line up to the intersection with the flash tank pressure curve in the desired velocity range, then across to read the disengaging area required per 1,000 lb/hr of high pressure liquid.
For our earlier example, 100 psig (689 kPa) condensate flashing to 0 psig requires about 0.5ft2 per 1,000 lb/hr to maintain separation velocities below 2 ft/sec (0.6 m/s). that is approximately the separation velocity commonly used in packaged boilers to ensure dry steam.
For our example:
Knowing A, the area required, we can select a commercially available tank diameter and calculate the minimum length by using the formula:
or we can calculate the diameter required in a vertical tank by:
Some flash tank designs are specially suited for high separation velocities. In a tank wher ethe high pressure liquid is introduced above the waterline, the actual disengaging area is the sum of the surface areas of all the water droplets spraying into the tank. So, tank size can be reduced by using the curves that allow higher steam separation velocities, up to a maximum of 10 gt/sec (3 m/s).
Some manufacturers supply a vertical cylindrical tank with a tengentially-mounted condensate inlet pipe to give the water a whirling motion that aids in separating the steam and water and also allows higher velocities. For our example, the disengaging area could be reduced from 1.0 ft2 to 0.2 ft2 simply by allowing the higher separation velocity.
Flash tank shape is not critical so horizontal or vertical cylindrical tanks may be used. notice that the level of the water is the vertical tank is not as important as it is in the horizontal tank because the same disengaging area exists from top to bottom. That's the reason for the discharge piping loop around the horizontal tank - it ensures that the water level will remain at half-full.
On the other, a tank with adequate disengaging area does not necessarily have sufficient volume to maintain the tank pressure within acceptable limits.
The tank volume required is shown in Figure 5. This figure allows us to quickly estimate the tank volume required without using steam tables or arbitrary safety factors. Returning to our example, enter the chart at 100 psig, then across to read 0.99 ft3 per 1,000 lb/hr.
For our example:
Using the disengaging area chart in Figure 4, we determined that a minimum area of one square foot is required for low velocity. The actual volume and disengaging area of the tank will depend on the selection of the other tank dimensions as shown in Table 2 for some readily available tank sizes.
At low flow rates, standard saize and shape tanks that meet the volume requirement will usually meet the disengaging area requirement too. At higher flow rates, or where a tank is to be built to order, applying the disengaging area criterion becomes more important.
Size the low pressure condensate piping using standard water pipe sizing methods if the condensate will not be subject to further flashing. Condensate from a pressurized flash tank will flash again as pressure on it is reduced and vent losses will occur if a closed condensate system is not used.
Additional considerations
Because of heat losses from the piping and tank, the actual amount of flash steam generated will always be less than the calculated amount. Because it is good practice to provide a pressure reducing valve to augement the flash steam from a pressurized flash tank anyway, this small discrepancy need not be of concern. The condensate piping to the flash tank and the tank itself should be well insulated to reduce these losses.
In determining the feasibility of using flash steam, the timing of its availability and its requirement in the process should be considered. if flash steam will not always be available, then the augmenteing pressure reducing valve must be capable of handling the entire low pressure steam demand. If the peak production of flash steam occurs at the time of minimum requirement, then another applicaiton must be found, or some way to exhauset the steam to a vent condenser or the atmosphere must be provided.
Finally, there are several features that should be included in every flash tank installation:
**A condensate bypass around the flash tank should be provided to return condensate directly to the pump receiver when the flash tank is out of service. Adequate provision for venting the receiver and cooling the condensate must be made or a suitable closed condensate recovery system must be provided.
**All pressure tanks should be ASME stamped and equipped with relief valves of proper size and capacity as well as a vacuum breaker if cold water could be introduced in to the hot tank.
**A pressure gauge should be provided on the flash tank.
**Properly sized steam traps should be provided on a pressurized tank.
**Each condensate feed branch to the flash tank should be equipped with a check valve to avoid backfow. The steam piping may also need a check valve if it is connected to pressurized equipement.
**The flash tank should be equipped with a thermostatic air vent to aid in start-up and to remove any non-condensable gases that may accumulate during operation.
Conclusions
Flash tanks operate with no moving parts, using simple thermodynamic principles. Therefoer, they are low in initial cost and easy to apply in many condensate handling situations to aid in condensate drainage and proper operation of steam equipment.
By simultaneously generating flash steam and cooling the liquid, the flash tank can supply steam for useful purposes, thereby reducing load on the boiler and allowing condensate transfer by use of readily available pumping equipment.
As energy costs increase, flash tanks as an addition to the steam and condensate system will become increasingly popular.
This article was offered in the hope that it will serve to help you design better steam systems. Although a great deal of research and study has gone into the article, no guaranty or warranty either written or implied is given for the fitness or usability of these ideas.
Local building codes and regulations should be carefully checked to ensure that the information agrees with the codes and regulations. Local conditions regarding atmospheric and other conditions should be checked to ensure the usability of the dtails and design data.
References
1. ASHRAE 1980 Handbook-Systems. Chapter 13, figure 38. Atlanta, Georgia.
2. ASHRAE 1989 Handbook-Fundamentals. Chapter 33, figure 10. Atlanta, Georgia.
3. ITT Fluid Handling Division. 1978. Basic Steam Pipe Sizing Charts. Training Manual TES 378. March. Morton Grove, Illinois.
4. ITT Fluid Handling Division. 1982. High Temperature Condensate Return Systems. Training Manual TES 582. May. Morton Grove, Illinois.
5. Kendrick, Lee. 1986. Design Manual for Heating, Ventilation, Plumbing and Air Conditioning Systems. 4th ed. Arlington, Virginia: Technical Standards Publications.
Printed in ASHRAE Journal, August, 1991
posted by DHernandez @ 9:17 AM
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