6.1 Condensate as a cost factor
There are several reasons why around 25 % of the valuable condensate is not returned to the energy process via the boiler house.
- Users are unaware of the potential money saving.
- Users are unaware of the influence of steam traps on the overall energy balance.
Calculation of the condensate loss
The amount of condensate that is lost may be approximated by the amount of make-up water supplied to the boiler. This value can be read off directly on the water meter downstream of the softener or the demineralisation plant.
If this is not possible, you can also calculate the percentage of returned condensate. This calculation is based on the conductivity or the chloride content of the make-up water and the feedwater.
Percentage of condensate returned = 
Clmw = Chloride content (or conductivity) of the make-up water
Clfw = Chloride content (or conductivity) of the feedwater (excluding the deaerator)
Example:
Clmw = 50 mg/l
Clfw = 30 mg/l
Continuous blowdown percentage = 
Condensate costs
Condensate of (saturated) steam at a pressure of 10 bar has a heat content (enthalpy according to the steam table at 10 bar) of 762 MJ/tonne. Assuming a gas price of €7.50 /GJ and a boiler efficiency of 90 %, the heat content of the condensate alone costs about €6.35 /tonne. If the steam pressure is 3 bar, the costs for the heat content (561 MJ/tonne) of the condensate amount to €4.20 /tonne. If the condensate is not returned to the boiler house, this figure is increased by water costs of approximately €1.- /m3 for treatment and effluent costs.
Flash steam costs
Flash steam is often discharged into the atmosphere without thinking. The heat content that is wasted is 2675 MJ/tonne.
Assuming a boiler efficiency of 90 % and a price of €7.50 /GJ for natural gas, one tonne of discharged flash steam costs as much as €21.60.
Once again, water costs of approximately €1.- /m3 must be added to this figure.
6.2 Efficient condensate systems
Calculating efficient condensate systems and the energy recovery from condensate is quite complicated.
The fact that condensate does not behave like water, but rather resembles a two-phase flow (flash steam with condensate), is not properly appreciated. This is a common cause of many condensate and transport problems, e.g. in particular:
- Incorrectly sized pipes
- Unacceptable two-phase flow velocities
- Incorrect choice of the steam trap type
- Poor heat transfer efficiency if condensate backs up in the heat exchanger
- Pipe leakage due to corrosion, erosion or water hammer
- Inexpert installation
- Merging of several condensate pipes into one
- Condensate lift downstream of a steam trap
Incorrectly calculated pipes
The most frequent mistake is made when the condensate system is designed. The pipe sizes are erroneously calculated based on a pure water flow rather than the actual twophase flows. The discharged condensate flashes because the pressure downstream of the steam trap is lower than in the heat exchanger.
The flash steam that is produced has a volume 300 to 1500 times greater than that of the original condensate. It is therefore essential to consider the volume of the flash steam when calculating the condensate pipe.
Percentage of flash steam
The percentage of flash steam can be determined as follows:
a) Calculation
Percentage of flash steam 
where:
hf1 = Heat content of the condensate upstream of the steam trap
hf2 = Heat content of the condensate corresponding to the pressure in the condensate system
hfg = Latent heat of vaporisation corresponding to the pressure in the condensate system
Example:
| Pressure upstream of the steam trap | 8 bar | hf1 = 721 kJ/kg |
| Pressure in the condensate system | 3 bar | hf2 = 561 kJ/kg |
| Latent heat of vaporisation at | 3 bar | hfg2 = 2163 kJ/kg |
Percentage of flash steam 
b) Using a diagram
The percentage of flash steam can also be determined using a diagram (refer to Fig. 6-2).
c) Using a rule of thumb
Percentage of flash steam = temperature difference across the steam trap x 0.2.
Example:
Steam pressure upstream of the steam trap 10 bar (condensate temperature = 180 ºC)
Pressure in the condensate system 5 bar (temperature = 150 ºC)
Percentage of flash steam = (180 - 150) x 0.2 = 6 %
If steam flashes at 10 bar into an atmospheric condensate system (100 ºC), the flash percentage is (180 - 100) x 0.2 = 16 %.
Steam at 5 bar has a specific volume of 0.375 m3/kg, whereas the specific volume of atmospheric steam (1 bar) is 1.69 m3/kg
In other words, if 1000 kg/h of condensed 10 bar steam is fed to a 5 bar condensate system, 60 kg/h of flash steam with a volume of 60 x 0.375 = 22.5 m3/h is produced.
160 kg/h of flash steam with a volume of 160 x 1.69 = 271 m3/h is produced if the condensate is supplied to an atmospheric system.
Only the volume of the flash steam is important for calculating the pipe diameter. The volume of the condensate is so small compared to that of the flash steam that it can be ignored for the purpose of the calculation (Fig. 6-1).
Fig. 6-1: Ratio of flash steam to condensate volume
Fig. 6-2: Quantity of Flash Steam Graph
Example 1:
Condensate at 10 bar saturation temperature flashes into an atmospheric condensate system. The percentage of flash steam is 15.2 %.
According to the rule of thumb, this is equivalent to (180 - 100) x 0.2 = 16 %.
Example 2:
Condensate at 5 bar saturation temperature flashes into a 2 bar condensate system. The percentage of flash steam is 6 %.
According to the rule of thumb, this is equivalent to (152 - 120) x 0.2 = 6 %.
Unacceptable two-phase flow velocities
Downstream of the steam trap there is a mixture of flash steam and condensate in the condensate pipe, with 95 % of the total volume typically accounted for by steam.
The condensate is entrained at the same velocity as the flash steam. Whereas the flash steam continues to flow smoothly when it reaches a pipe bend, the water particles have a tendency to keep moving in the same direction owing to their greater specific density. As a result, they hit the bend in the pipe, initially creating a barrier and then suddenly shooting through it like a wave (the correct technical term for this phenomenon is "plug flow"). Erosion occurs owing to this unacceptably high flow velocity. It can be assumed that around 75 % of all leakage in condensate pipes is on bends.
A maximum velocity of 20 m/s is recommended for condensate pipes in the literature on steam traps. In worst-case conditions, this value can be critical. The water droplets in a continuous flow can have an impact velocity of more than 60 km/h.
Apart from ball float steam traps, all other trap types work intermittently. It is not uncommon for an intermittently operating steam trap to remain shut for up to half its service time (and possibly even longer). If the condensate is discharged in bursts, the velocity can increase to 120 km/h for short periods.
Efficient velocity
To ensure operation without erosion, approximately 10 m/s should be taken as a starting basis when calculating condensate pipe sizes with float-type steam traps or 6-8 m/s for systems with other trap types. Individual calculations provide further information.
Which calculation method is NOT acceptable?
Unfortunately, it is common to base these calculations on water. The result is then corrected and a permissible velocity of 0.3 to 0.5 m/s is assumed.
The correction is reasonably accurate at pressures up to around 3 bar, even though adequate account has still not been taken of the flash steam volume. The nominal diameter of the pipe can deviate by up to three sizes, depending on the condensate flow rate: Example: Size DN 40 is required instead of DN 80.
Important:
The majority of problems in condensate systems are attributable not to defective steam traps but to incorrectly calculated condensate pipes. If a condensate header does not contain any flash steam, the diameter is determined by calculating the condensate pipe as a water pipe with a permissible water velocity of 1.5 to 2 m/s. The volume of the flash steam must be considered when calculating condensate pipes.
Determination of the condensate pipe diameter
The fundamental formula for calculating the diameter of a condensate pipe is as follows:
where:
Q = Water, gas or steam flow rate in m3/s
D = Inside diameter of the pipe in mm
v = Flow velocity in m/s
In practice, it is advisable to specify the flow rate in kg/h and the pipe diameter in mm. The formula must therefore be adapted as follows in order to calculate the required diameter (Q = 1/4 π x D2 x v):
where:
D = Diameter of the condensate pipe in mm
Q = Condensate flow rate in kg/h
X = Proportion of flash steam (decimal)
vg = Specific volume (m3/kg)
Example:
1000 kg/h of condensed steam at 11 bar (hf = 781 kJ/kg) flashing in the 4 bar condensate system (hf = 604 kJ/kg, vg = 0.4622 m3/kg and hfg = 2133 kJ/kg)
The percentage of flash steam is as follows: (781 - 604)/2133 x 100 % = 8.3 % or 83 kg/h
The volume of flash steam is as follows: 83 x 0.4622 = 38 m3/h
97.6 % of the volume of a condensate / flash steam mixture is accounted for by flash steam. The corresponding pipe diameter for a velocity of 8 m/s is as follows:
Assuming discharge into an atmospheric condensate system (vg = 1,694 m3/kg), (781 - 418)/2258 x 100 % = 16 % of the condensate, in other words 160 kg/h, flashes. In this case, the condensate pipe will have the following diameter:
A size DN 100 or DN 125 pipe should be selected, depending on the local situation.
Simple determination of the condensate pipe diameter
information is presented in table form in Fig. 6-3.
The first column shows the steam pressure while the horizontal indicates the back pressure in the condensate system. The table specifies the pipe diameter for 100 kg/h at defined pressure drops and a flow velocity of 10 m/s. If the required velocity is less than 10 m/s, e.g. 5 m/s, the calculated diameter must be multiplied by . In this case, DN 50 becomes DN 80, for example.
The nominal diameter of condensate pipes can be determined directly using the table in Fig. 6-3.
Fig. 6-3: Nominal diameter of condensate pipes
If necessary, the diameters should be corrected according to the actual condensate flow rate using the factors given in Fig. 6-3.
Example:
Steam pressure = 9 bar, condensate flow rate = 1500 kg/h
Back pressure in the condensate system = 2 bar
The value 18.2 can be read off in Fig. 6-3 for 9 bar steam pressure and 2 bar back pressure. The correction table shows the factor 3.9 for 1500 kg/h. The pipe should therefore have a diameter of 18.2 x 3.9 = 71 mm. A size DN 80 pipe should be selected. A size DN 65 pipe might also be suitable, depending on the local conditions (short pipe, continuous flow).
Condensate discharge when multiple drain points are connected to a single header
If several condensate pipes flow into a single header and from there to the flash vessel, the pipe diameter must be adapted to take account of this situation.
Fig. 6-4 shows three different condensate pipe sizes (DN 80, DN 50 and DN 40) with a single header.
Fig. 6-4: Drainage from multiple heat exchangers into a single header
The condensate pipe from heat exchanger 1 is size DN 80 up to the point where heat exchanger 2 is connected. The diameter of the downstream condensate pipe section must be calculated as follows:
Heat exchanger 3 has a size DN 40 drain at the next connection point. The diameter of the final pipe section is as follows:
High condensate header
If the condensate header is higher than the trap of the steam user, for example at a pipe bridge, the condensate must overcome the gravity head from the user to the header.
The condensate is accompanied by flash steam bubbles, between which so-called "condensate plugs" are formed. These plugs fall through the steam bubbles due to gravity, causing small water hammer pulses. This process may be repeated several times.
To avoid this phenomenon, a flow velocity 12 to 15 m/s higher should be selected in rising condensate pipes. The condensate can then be drained without any problems.
Incorrect choice of the steam trap type
It is unfortunately a common occurrence for inappropriate steam trap types to be selected for certain applications or for the traps to be incorrectly installed. One common mistake, for instance, is to use thermostatic steam traps as level controllers or to install thermodynamic traps in a system with high back pressure (> 60 % of the steam pressure).
If the traps or the control valve trim are replaced, you must take account of the size of the controller for the steam trap. Controllers in high pressure traps have smaller orifices than low pressure traps. This means that a high pressure trap may not be adequately sized for a low pressure application.
If the traps or the control valve trim are replaced, you must take account of the size of the controller for the steam trap. Controllers in high pressure traps have smaller orifices than low pressure traps. This means that a high pressure trap may not be adequately sized for a low pressure application.
Poor heat transfer if condensate backs up in the heat exchanger
If the condensate drainage in the heat exchanger stalls (backs up), the heat transfer surface of the process apparatus (e.g. the heat exchanger, trace heater or jacket) is completely or partially flooded with condensate. No steam, or not enough steam, is able to condense on the heat transfer with the result that no heat transfer, is compromised.
At a steam-side temperature control condensate backup is probably detectable, whilst comparing the temperature in front of the steam trap with the steam temperature behind the control valve regulating the heat exchanger. If condensate is being drained correctly, these two temperatures are very similar. A small temperature difference is permissible; the term 'stall' is used if this difference increases to between 15 and 25 ºC.
The temperature differences can be measured with a surface or infrared thermometer. If the infrared method is used, both measuring points should have the same colour. It is possible to measure a higher temperature under the same conditions on carbon steel than on polished, stainless steel or on surfaces with an aluminium structure.
The main causes of stall are as follows:
- The capacity of the steam trap is too small.
- The pressure difference across the steam trap is too low because the pressure in the condensate system is too high. Possible reasons: Pipe diameter too small, live steam loss from steam traps, bypass valve open.
- As set-point is approached, low steam temperature (and hence pressure) steam is required in the heat exchanger. This pressure may not be sufficient to overcome the back pressure in the condensate system.
- The filter in the steam trap or the upstream strainer is clogged.
- The heat exchangers are incorrectly sized (generally oversized, with too great a heat transfer area). Too much pressure is lost, so that the pressure upstream of the trap is unacceptably low.
Pipe leakage due to corrosion, erosion or water hammer
Condensate pipes are renowned for leakage. There are three main causes:
- Erosion
- Corrosion
- Water hammer
Erosion
Erosion is caused by high velocities of the condensate / flash steam mixture. Entrained water particles act like an abrasive cleaner in the pipe bend. The change of flow direction in valves results in strong turbulences that damage not just the plug and seat but also the valve body. The mixture velocity can be reduced by increasing the pipe diameter.
Erosion in condensate systems is usually accompanied by corrosion if the condensate is aggressive. To prevent erosion, the pressure in the condensate system is sometimes increased artificially by installing an excess pressure valve at the end of the condensate pipe, for example upstream of the inlet into the condensate vessel. Less flash steam is produced in the header as a result: it is important to ensure that the pressure maintained by the valve does not reduce, or even halt, the capacity of the steam trap. This will reduce plant performance.
Corrosion
Corrosion in condensate systems is mainly caused by the oxygen (O2) or carbon dioxide (CO2) contained in the steam. Oxygen corrosion occurs as pitting or shallow abrasion if condensate collects in an atmospheric vessel. As long as a steam cushion forms in the collection vessel, no oxygen can penetrate. However, if sub-cooled condensate reaches this vessel, ambient air will enter via the vent. Preventing the ingress of oxygen into the condensate vessel is described later in this chapter.
Carbonic acid corrosion in condensate systems is mainly encountered when the feedwater is only treated with a softener. The feedwater contains carbon dioxide compounds, such as free carbon dioxide (CO2) from the ambient air or carbonic acid bound to the water (H2CO3). Only the free carbon dioxide is expelled in the deaerator. The bound carbonic acid is only split into carbonic acid gas and water at a higher temperature in the boiler. The carbonic acid gas is supplied to the user together with the steam and is then dissolved in the condensate. The pH may be reduced to less than 4.
Corrosion is particularly likely at those points where condensate cools down and is aerated. Carbonic acid corrosion is visible on the pipe or apparatus bottom and is uniformly distributed over a wide area. Chemicals that increase the pH (neutralising agents) are often added as a preventive measure. Amines that build up a protective layer can also be metered into the condensate system. However, they only make sense in the part of the system that actually carries condensate.
The latter method can occasionally lead to unpleasant side-effects in pipes and steam boilers, for example if old corrosion residues are detached and filters or drain valves become clogged as a result. If amines are used, the information provided by the supplier must always be observed.
Corrosion residues can also become detached if the feedwater is treated by demineralisation instead of softening.
If a steam or condensate system is temporarily removed from service, for example at weekends, it is advisable to provide draining at the lowest points in the system, e.g. by installing drain valves.
Inexpert installation
A few examples of inexpert installation are described in the following.
If a thermostatic steam trap is insulated, it works slowly because an insulated supply pipe and globe valve prevent essential cooling. Thermostatic or thermodynamic steam traps should never be insulated for this reason. A perforated plate is often mounted as a shield in such cases in the interests of work safety.
Steam traps that are installed in the wrong direction of flow are a particularly common mistake.
When a density sensitive trap (inverted bucket or ball float) with a threaded connection is installed, it is vital to ensure that the level control can operate in the correct plane. If the trap is turned away from the perpendicular in the pipe axis, for instance, or if it is installed at an angle, the level control will no longer be able to move freely.
6.3 Water hammer
A distinction is made between hydraulic and thermal water hammer.
Hydraulic water hammer
Hydraulic water hammer occurs, for example, if a globe valve is suddenly closed during normal flow through a water pipe. The water flow immediately comes to a standstill and the velocity energy of the moving water is converted to destructive pressure energy. The liquid downstream of the closed globe valve is still in motion and continues to flow for a while. However, since no more liquid is flowing in its wake, a vacuum is created in this section of the pipe. As soon as the flow velocity drops to zero downstream of the valve, the movement is reversed under the influence of the vacuum. A water hammer pulse occurs in the direction of the globe valve as a result.
These pressure waves may be repeated several times, during which the system pressure can significantly exceed the normal service pressure. Depending on the magnitude of the pressure peaks, the water hammer can have a destructive effect on system components, for example gaskets may be blown out.
Thermal water hammer
Thermal water hammer is mainly a problem in condensate systems. Sub-cooled condensate is fed in a condensate pipe that is partially filled with flash steam. The flash steam condenses. The volume of the flash steam is abruptly reduced and a vacuum is created locally. As a consequence of this vacuum, condensate flows towards it from all sides at high velocity. Once again, a water hammer pulse occurs here when the vacuum is removed and the liquid comes to a halt.
Sub-cooled condensate must never be fed into a pipe containing flashing condensate!
Water hammer is a common phenomenon at condensate drains and in process apparatus that is controlled on the condensate side. A condensate cooler installed downstream of a heat exchanger also increases the risk of water hammer. The condensate drained from this kind of configuration is always sub-cooled. However, sub-cooled condensate is only allowed to be returned via a separate line and sprayed into the condensate vessel from above. Another alternative would be to mix it with warm condensate using a perforated supply pipe (nozzle pipe).
Avoiding or reducing the risk of water hammer
In addition to the above-mentioned causes, condensate discharge from a drain trap into an atmospheric condensate header also results in water hammer. Imploding steam bubbles close to the port lead to serious damage to the pipe material.
Water hammer is additionally possible when a cold plant is put into service. It is a good idea to drain the condensate using a start-up drain valve while the plant is heating up. One positive side-effect of start-up drain valves is that if a plant stoppage occurs, the residual condensate is automatically drained and the system aerated.
It is advisable to feed condensate in on the top side of the header, if possible in the parallel flow direction as is usual with steam pipes (Fig. 6-5).
Fig. 6-5: Condensate inlet on the top side of the header
If condensate must be supplied from a low level to a higher condensate pipe, water hammer can be avoided by installing a condensate lock as a damping mechanism. The steam cushion that is formed in the top part of the condensate lock cushions pressure waves (refer to Fig. 6-6).
Fig. 6-6: Condensate lock used as a damping mechanism
6.4 Condensate flashing
Introduction
The temperature of the condensate that forms is identical to the saturation temperature of the steam, for example 152 ºC at 5 bar
Fig. 6-7 shows a typical installation:
- Heat exchanger controlled on the steam side
- 5 bar steam pressure downstream of the control valve
- Condensate temperature upstream of the trap = 152 ºC
- Heat content of the condensate = 640 kJ/kg
- Condensate discharged into the atmosphere
Fig. 6-7: Tube bundle heat exchanger
When it exits from the steam trap, the condensate is abruptly flashed from 5 bar to atmospheric pressure. The maximum temperature of water or condensate in the atmospheric state can be up to 100 ºC with a heat content of 420 kJ/kg.
The heat content of the 5 bar condensed steam, on the other hand, is 640 kJ/kg. In this example, the excess heat contained in the condensate amounts to 640 - 420 = 220 kJ/kg. This surplus results in partial evaporation of the condensate, also referred to as flashing.
Evaporation = flashing
In this example, the flash steam percentage can be calculated as follows: 220/2258 x 100 % = 9.7 % (Enthalpy of Evaporation (hfg) at 1 bar = 2258 kJ/kg)
If this flash steam is supplied to a steam system at low pressure, it is possible to restrict the losses. How to utilise the flash steam, for example in order to save costs, is described in the next section.
Heat recovery with flash steam
The recovery of energy from flash steam facilitates a significant reduction in costs, as well as CO2 emissions. The potential benefit of heat recovery is described in the following with the help of two diagrams and calculations. Fig. 6-8 shows how the flash steam from the condensate is discharged into the atmosphere by several heat exchangers operating at 10 bar.
Fig. 6-8: Discharge of the flash steam into the atmosphere
Saturation temperature of 10 bar steam: 180 ºC
Percentage of flash steam: (180 - 100) x 0.2 = 16 %
Live steam flow rate: 1500 kg/h
Loss due to flash steam: 240 kg/h
Assuming a gas price of €7.50 /GJ, this represents a yearly loss of €38,400.- for 8000 hours operation.
Fig. 6-9 shows how flash steam is produced and supplied to a low-pressure steam system.
fig-6-9-recovery-of-the-flash-steam-in-a-low-pressure-steam-system
1500 kg/h of condensed steam is flashed from 10 bar to 2.5 bar (127 °C saturation temperature) into a downstream steam system.
The amount of flash steam formed is as follows: (180 - 127) x 0.2 = 10.6 % or 160 kg/h. The residual condensate (1340 kg/h) is flashed to the atmosphere in the second flash vessel. (127 - 100) = 5.4 % or 72 kg/h is discharged in the process.
The discharge steam loss is reduced to 72 kg/h with a value of €11,500.- /year instead of €38,400.- – an annual cash saving of €26,900.- .
6.5 Condensate flash vessels
There are two different types of flash vessel (or flash tank):
- Atmospheric flash vessels
- Pressure expansion vessels
Atmospheric flash vessels
Atmospheric flash vessels are used to flash condensate from low-pressure steam systems. The flash steam is discharged into the atmosphere because the low steam pressure is unsuitable for commercial applications. If there is no condensate, or if the temperature of the condensate remains below 100 ºC, ambient air can enter the vessel freely and the condensate is aerated. The risk of oxygen corrosion (pitting) is increased in the vessel and in the downstream pipes. A safety valve that regulates the pressure to 0.2 bar can be installed in the discharge pipe to prevent the ingress of air. If not enough condensate is supplied, a small amount of live steam is blown in via a control valve to prevent vacuum in the vessel and ensure that slight gauge pressure is maintained.
Fig. 6-10: Flash vessel with steam cushion
Pressure expansion vessels
The mixture of condensate and flash steam collects in the pressure expansion vessel (Fig. 6-11), where the steam is separated from the condensate. The flash steam is supplied to a low-pressure steam system while the condensate is returned to the boiler house.
Fig. 6-11: Pressure expansion vessel
Requirements specified for pressure expansion vessels
Pressure expansion vessels are connected to the condensate system on the steam side. Strictly speaking, the term "expansion vessel" is inaccurate. The condensate is flashed as soon as it exits from the steam trap, providing it is supplied to a system with a lower pressure than the steam pressure in the heat exchanger. The separation of the condensate from the flash steam takes place in the flash steam vessel.
The sizing of the vessel with its connection ports and its protection have to meet various requirements in order to guarantee the corresponding quality of the flash steam. The flash steam that is fed into the steam system must not be wet – if it is, condensate will be entrained with it. To guarantee optimal separation of the condensate from the flash steam, the vessel should ideally work like a cyclone. The mixture of condensate and flash steam must be supplied tangentially above the water level for this purpose. Owing to the high specific mass of the water, it is spun against the vessel wall and most of the flash steam is separated from the condensate.
An additional separation can be achieved with the calculation by assuming a maximum steam velocity in the vessel of approximately 1 m/s. In addition, the path travelled by the steam to the outlet point should be as long as possible in order to ensure a sufficient residence time. Due to the low steam velocity and the high specific mass of the condensate, the separation is almost perfect and the flash steam reaches up to 97 % dryness.
A float-type steam trap must be used to drain a flash vessel. In the event of sudden pressure relief, for example if there is an abrupt increase in steam consumption in the low-pressure steam system, the pressure expansion vessel boils empty. The condensate in the vessel is completely flashed as a result. To avoid this problem, the float-type steam trap should be mounted higher than the supply pipe. This manometric loop ensures that the trap is always full of condensate. Flash steam is consequently unable to reach the trap and the service life is significantly extended.
The pressure expansion vessel must be fitted with a safety valve! The set pressure is determined by the maximum permissible pressure load at the maximum vessel temperature. The vessel must be capable of discharging both the maximum possible flash steam flow rate from the maximum inflow of condensate and the steam flow through any open steam traps.
6.6 Multi-stage condensate flashing
Different steam pressures can occur in a plant with several steam systems. If this is the case, it is advisable to use multi-stage flashing.
Fig. 6-12 shows how condensate from a 20 bar steam system is flashed in a flash vessel connected to an 11 bar steam system. The condensate from the 11 bar heat exchanger is then flashed in a flash vessel connected to the 3 bar steam system. The condensate from the 3 bar heat exchanger is supplied to an atmospheric flash vessel, from which flash steam is discharged into the open. This steam cloud can be reduced if required by means of a heat exchanger with cold condensate or feedwater, for example using a vapour cooler.
Fig. 6-12: Multi-stage flashing
6.7 How to avoid the flash steam cloud
The flash steam cloud that inevitably emerges from an atmospheric flash vessel unfortunately cannot be avoided altogether. However, there are several tricks that can be employed to reduce it. Some of them are very effective, while others occasionally resorted to in practice are technically irresponsible. A few of the most suitable methods for reducing the steam cloud are described in the following.
Condensing with feedwater or condensate
One proven technique for condensing the flash steam cloud without water hammer is to spray feedwater or cold condensate into the vent pipe. The cloud can be partially or completely suppressed in this way. Sub-cooled condensate produces better results than hot condensate. The following points must be taken into account:
- There must be no water hammer
- The temperature of the make-up water / condensate mixture to the deaerator must not exceed 95 ºC
Another condition is that the flow velocity in the vent pipe must not be too high. It is consequently advisable to select the pipe section to lead into the vent pipe with a larger diameter. This section should have a diameter 2.5 times that of the vent pipe and be between 1 and 1.5 metres long.
Spraying the water in a cascaded condenser counter to the flow of flash steam, with modulating control of the water, represents a particularly elegant solution (Fig. 6-13).
Fig. 6-13: Reduction of flash steam by spraying with feedwater or condensate
Reducing the steam pressure
In steam users without temperature control, the steam pressure should be reduced to the minimum permissible value. The lower the steam pressure and the closer the saturation temperature to 100 ºC, the lower the percentage of flash steam.
Example:
A trace heater (product temperature to be maintained = 80 ºC) requires 500 kg/h of steam. Steam pressure = 6 bar, saturation temperature = 159 ºC Condensate drainage in an atmospheric condensate system Total amount of flash steam lost = (159 - 100) x 0.2 = 11.8 % or 59 kg/h
However, a steam pressure of 2 bar is sufficient to maintain the product at a temperature of 80 ºC. If a pressure reducing valve (Fig. 6-14) were to be installed to reduce 6 bar steam pressure to 2 bar, the flash steam loss would be only 4 % instead of 11.8 %. An annual saving of 312 tonnes of steam, equivalent to €6,300.- , could be achieved in this way. The payback period would be less than twelve months.
Fig. 6-14: Pressure reducing valve, ARI Type PREDU®
Steam traps with additional sub-cooling
Steam traps with variable sub-cooling should be used for trace heaters. They help reduce flashing and hence afford better protection against frost.
Thermostatic steam traps are ideal for this task. Bimetallic traps feature adjustable condensate sub-cooling. Balanced pressure steam traps may be fitted with have different capsules with different sub-cooling characteristics for greater flexibility.
Thermostatic traps do not open when the saturation temperature of the steam is reached, but will remain closed for a while longer, so that condensate backs up. The trap only opens when the temperature drops to 30 °C to 40 °C below saturation. The condensate that has backed up continues to exchange heat with the product pipe.
Fig. 6-15 shows a steam trap that has a sub-cooling characteristic. Thermostatic steam traps with a membrane regulator are often used for trace heating (Fig. 6-16).
Fig. 6-15: Variable bimetallic steam trap, ARI Type CONA®B
Fig. 6-16: Steam trap with a sub-cooling capsule, ARI Type CONA®M
Fig. 6-17 shows a 7 bar trace heater without sub-cooling while Fig. 6-18 depicts a steam trap that has been set to 40 ºC sub-cooling.
Without sub-cooling, (165 - 100) x 0.2 = 13 % of the flash steam is lost. With sub-cooling, the figure is reduced to (125 - 110) x 0.2 = 5 %.
Fig. 6-17: Trace heater and steam trap without sub-cooling
Fig. 6-18: Trace heater and steam trap with 40 °C sub-cooling
The saving for a trace heater used for frost protect systems is considerable, especially if the condensate is discharged to the atmosphere. With trace heaters like the ones in Fig. 6-17 and Fig. 6-18, which consume 2 t/h of steam during 4000 h/a in service, the total saving amounts to around €14,000.- per year.
Cooling the condensate
At process temperatures below 100 ºC (e.g. in air heaters for drying plant or central heating systems), the condensate can be cooled very efficiently by connecting a condensate cooler in series with the heat exchanger.
However, mistakes are often made here in the implementation. Fig. 6-19 shows two faulty designs.
In design A, the condensate is flashed downstream of the steam trap. The mixture of flash steam and condensate hits the tube plate of the condensate cooler at too high a velocity. The result: erosion and leakage.
In design B, the condensate is not cooled at all. The heat exchange surface is simply enlarged due to the series connection.
Fig. 6-19: Faulty designs for condensate cooling
Fig. 6-20 shows a more appropriate design. A float-type steam trap is installed downstream of the condensate cooler, but at a slightly higher elevation. The trap can therefore be used to control the water level and the cooler is always full of condensate. The cooled condensate should be supplied to the condensate vessel or the boiler house in a separate pipe. If not, there is an acute risk of water hammer in case of discharge into a condensate pipe with flash steam.
Fig. 6-20: Optimally designed condensate cooling
Thermal Compressors
Vapour compression with ejectors is occasionally used if sufficient amounts of live steam are available (Fig. 6-21). An ejector allows the flash steam to be compressed from 1 bar to 2.5 bar by using 10 bar live steam. In this example, 3 kg of live steam is required to compress 1 kg of flash steam.
This method should only be chosen if a large amount of steam has to be reduced from a higher pressure to a lower pressure.
Fig. 6-21: Thermal compression
Thermal compression is often used for drying cylinders in the paper industry. The main reason for its popularity is that when condensate is drained from the cylinders, a small amount of live steam is deliberately allowed to escape together with the flash steam to enable the condensate to be drained from the drying cylinders. This mixture of flash and leakage steam is increased to the required pressure (generally only a slight amount) after each drying stage.
Typical planning errors
The same unsuitable methods are often selected to avoid the steam cloud, unfortunately with similarly unsatisfactory results in the majority of cases.
Condensate vessel without a vent pipe
The basic idea behind this method is that if there is no vent pipe, there is also no flash steam. As long as the mixture temperature of the condensate streams to the vessel remains below 100 ºC, no steam is generated. In the worst case, a vacuum is created inside the vessel, although this is not a problem providing the system is designed correctly (Fig. 6-22).
Fig. 6-22: Condensate vessel without a vent pipe
If, on the other hand, condensate reaches the condensate vessel at a temperature of more than 100 ºC, some of it will flash to steam. The pressure inside the vessel then gradually increases to the vapour pressure of the hot condensate. It is therefore essential to mount a safety valve on the condensate vessel. Ultimately, the pressure in the vessel is almost identical to that in the heat exchanger. The pressure difference is then too small and only an very small amount of condensate can be discharged to the vessel. The heat exchanger fills up with water as a result (i.e. it "floods"). The condensate pump is only able to transport a very small amount of condensate (compared to the amount of flash steam produced).
Other measures
Two other measures are commonly implemented to prevent the steam cloud:
- The mixture of returning condensate and flash steam is fed in below the condensate level.
- The mixture is sprayed into the vessel's vent pipe.
However, neither of these solutions have any effect on the vessel's heat balance and the flash steam is not reduced in size.