7.1 Introduction
The enthalpy of steam and condensate was discussed in detail in Chapter 1.0 Heat Engineering Concepts. It was made clear that condensate discharge has a major influence on the efficiency of a steam system and that its importance should not be underestimated. Steam traps form a vital link between efficient steam usage and the condensate system. Together with a condensate system that meets all the specified requirements, they represent the crucial key to cost-effective steam use.
7.2 Steam trap specifications
Steam traps should have the following properties and satisfy the following requirements:
- Efficient and problem-free discharging of condensate without loss of live steam.
- Immediate discharge of air and inert gases when the steam system is started up.
The steam trap should have these properties at all times, even if the steam or back pressure fluctuates, and it should meet the specified requirements throughout the pressure range. All steam traps should additionally comply with the following demands:
- The seat and plug of the steam trap must withstand the abrasive action of the flashing condensate.
- The steam trap should be compact and take up as little space as possible in order to restrict heat emission losses to a minimum.
- The steam trap should be robust, reliable and insensitive to water hammer.
In many cases, the desirable properties are so diverse that they are impossible to reconcile:
- A steam trap is required to keep the steam space of a heat exchanger free of condensate.
- On the other hand, it can be useful to allow backing-up (waterlogging) in the heat exchanger in order to utilise the sensible heat of the condensate (e.g. in trace heaters for heating instruments by setting the steam trap to sub-cooling).
- Discharging of large amounts of condensate at low differential pressures (e.g. for heating and draining a steam pipe on start-up).
- Discharging of small amounts of condensate without steam loss at high differential pressures (e.g. for draining a steam pipe in service).
There are numerous applications where the requirements defined for the steam traps are actually mutually exclusive. In short, there is no such thing as a universal steam trap to suit every situation.
7.3 Classification of steam traps
The three basic types of steam trap are classified in DIN EN 26704:
- Mechanical steam traps – controlled by the condensate level
- Thermostatic steam traps – controlled by the condensate temperature
- Thermodynamic steam traps – controlled by changes in the fluid state
7.4 Mechanical steam traps
The most important members of this group are:
- Float-type steam traps (sealed float)
- Inverted bucket steam traps (float open at the bottom)
Fig. 7-1: Operation of a float-type steam trap, ARI Type CONA®S 631 / CONA®SC 634
The operating principle of a float-type steam trap (Fig. 7-1) is based on the density difference between steam and condensate and on the buoyancy force of a ball float. The float opens and closes the drain valve or the drain gate by means of a system of lever arms. The water level in the trap rises as the condensate builds up. The float rises with the condensate and the drain valve opens; when the condensate level falls again, the float sinks and the valve closes. The float trap thus acts as a modulating level controller that guarantees continuous and immediate discharging of condensate. The drain orifice should be drilled as low down as possible, depending on the model. The seat and plug are always located below the minimum condensate level; this provides a water seal which gives protection against live steam loss. This latter property and the ability to discharge condensate continuously are the most important advantages of this trap type. The temperature of the condensate in the steam trap is the same as the saturated steam temperature of the corresponding pressure at the outlet of the heat exchanger and the inlet of the trap. No steam is lost, even if the trap is completely emptied, because in this case the float goes to the bottom of its travel, and the drain valve is closed.
Pressure range
The majority of float traps have a limited range as regards the maximum permissible pressure difference. As a result, not every trap will operate efficiently in a particular differential pressure range. If the differential pressure is too high, the buoyancy force and the length of the lever arm will be insufficient to open and close the plug. A smaller seat bore must be selected for higher maximum differential pressures, to ensure that the valve can still be operated at high pressures. Float-type steam traps are offered with different regulators (seat areas) for different pressure ranges for this reason.
Fig. 7-2 shows the interaction of the buoyancy force of the float with the closing force of the valve.
Fig. 7-2: Interaction of the forces in a float trap
Where:
P1 = Pressure in the steam trap
P2 = Pressure downstream of the steam trap
A = Cross sectional area of the discharge port
m = Mass of the float
F = Buoyancy force (Archimedes)
L = Lever length up to the float
I = Lever length up to the valve
CONA®SC 634: In order to open the valve, the moment created by the buoyancy force of the float and the lever length must be greater than the moment acting on the valve plug and seat area as a result of the pressure difference.
CONA®S 631: In order to close the valve, the moment created by the mass of the float and the lever length must be greater than the moment acting on the valve plug and seat area as a result of the pressure difference.
This relationship is expressed by the following formula:
When specifying the steam traps for a system, it is important to remember that the difference between the normal flow rate and the maximum possible flow rate can often be considerable. When the system is started up, the steam consumption – and hence the amount of condensate – can be high at comparatively low pressures. On the other hand, there may be relatively little condensate at high pressures during the normal process.
The specification for a float-type steam trap should always take account of the following aspects:
- When the (float) trap is ordered, the maximum amount of condensate to be discharged should be specified in addition to the service pressure and the differential pressure.
- The trap should be rated for this peak capacity. For example, it must have a capacity of 60/10 x 400 = 2400 kg/h in order to handle a maximum flow of 400 kg of condensate in 10 minutes.
- If a defective DN 50 trap needs to be replaced, it is not sufficient simply to take another DN 50 trap from the store. The size and designation of the controller must be checked as well, because it is they that determine the seat area and hence the trap's capacity.
Air venting
Air venting is not only important because air can cause corrosion but also because air in the steam system has a negative effect on heat transfer.
Since there is always a minimum residual condensate level in a float trap and the obturator valve orifice is underwater, air that collects in the system upstream of the steam trap can only be discharged via the trap. Float traps for steam systems have an air venting function for this reason.
Continuous air venting
The simplest method of continuous air venting is to connect a bypass pipe from the inlet to the outlet of the steam trap or to drill a short-circuit path. The steady loss of steam is a small price to pay in return for eliminating all air from the system. However, this is only an acceptable solution if the steam that is discharged can be put to meaningful use with the help of a flash vessel. A needle valve can be installed in the bypass pipe to restrict the steam loss to a minimum (Fig. 7-3). This valve should only be opened when the system is started up or vented. If you forget to close it again, it will represent an additional source of leakage or faults.
Fig. 7-3: Float trap with a needle valve for air venting, ARI Type CONA®S 631
Automatic air venting
The majority of manufacturers provide their float traps with automatic internal air venting in the form of thermostatic devices. Another alternative is to vent the trap by means of external, thermostatic traps mounted on the float trap body.
Fig. 7-4: Float trap with a external bimetallic air vent, ARI Type CONA®S 631 with mounted ARI Type CONA®M 614
The float trap shown in Fig. 7-5 features an integrated thermostatic capsule for air venting. The capsule opens when cold and does not close until all air has been discharged and the corresponding saturation temperature almost reached.
Fig. 7-5: Float trap with a bimetallic air vent (vertical installation), ARI Type CONA®SC 634
The integrated thermostatic air vents are based on the principle that when air is present in the trap, the temperature of the air / steam mixture is lower than the saturation temperature of the corresponding steam pressure.
The float trap shown in Fig. 7-6 vents air automatically by means of a bimetallic element integrated into the valve mechanism.
In many cases, the buoyancy force of the ball float is not sufficient to overcome the pressure (and hence force) created by the pressure acting over the areas of the orifice. This problem has been solved in the configuration below where opening of the valve is assisted by steam pressure. The valve closes counter to the flow direction if condensate is forced back into the trap, in other words it doubles as a non-return valve. No condensate is returned to the heat exchanger from the condensate system if the plant is not in service.
Fig. 7-6: Float trap with a bimetallic air vent (horizontal installation), ARI Type CONA®S 631
Fig. 7-7: Regulator unit with in-service air venting, ARI Type CONA®S 631
Applications
Float-type steam traps are usually more expensive to purchase than other types owing to their size and design. They are best suited for draining heat exchangers in process applications where the product temperature has to be precisely controlled within defined limits. The following three features represent their main advantages:
- The characteristic of a float trap follows the saturation curve, in other words the trap discharges the condensate boiling hot and without waterlogging (Fig. 7-8).
- A float trap modulates continuously according to the condensate level in the trap.
- A float trap is capable of discharging large amounts of condensate without losing steam.
Fig. 7-8: Control characteristic of a float trap
Float traps are also suitable for critical drainage tasks or as level controllers in condensate flash vessels and condensate coolers
Another point in favour of the float design is that its functionality is not in any way impaired by variations in the pressure or flow rate or higher back pressure. If a change in the operating state causes the differential pressure and the condensate flow rate to exceed the operating and performance limits of the installed regulator, it is possible to switch to another regulator with a different cross-section in the same trap body.
Installation requirements
Owing to the design of the float trap, there is always a small amount of residual liquid in the cover, with the associated risk of freezing when the system is removed from service. It is therefore advisable to provide the trap with an automatic drain valve, particularly in outdoor installations susceptible to frost or systems that are frequently shut down. (Fig. 7-9)
Fig. 7-9: Float trap with a start-up drain valve, ARI Type CONA®S 631 with CONA® 665
Both the float traps and the inlet and outlet pipes should be insulated to ensure safe working conditions, to prevent freezing and to reduce heat losses through radiation. On the other hand, insulating the supply pipe also creates a risk of a steam lock occurring in the pipe between the heat exchanger and the steam trap. The steam lock is formed when part of the condensate in the pipe evaporates upstream of the trap due to a sudden pressure drop in the exchanger. Especially if the piping arrangement is less than ideal and includes rising bends, these steam locks can be very stubborn and a cause of waterlogging in the heat exchanger. It is a good idea to install the float traps as close as possible to the steam user and to lay the supply pipe with a gradient in the direction of flow.
Float traps can also be used to drain compressed air systems. Owing to the low temperatures, a thermostatic air vent must be removed and plugged off for these applications otherwise compressed air would escape continuously
Fig. 7-10:Float trap with a start-up drain valve, ARI Types CONA®S 630 and CONA®SC 636
Inverted bucket steam traps
Inverted bucket steam traps resemble a float trap that is open at the bottom. Like float traps, they discharge the condensate at saturation temperature though not continuously. Opening and closing of the outlet valve are directly controlled by the inverted bucket via a lever mechanism. A small air vent (bleed) hole is provided in the top of the bucket to prevent air from collecting underneath it.
Method of operation
To understand the operating principle of an inverted bucket steam trap, it is a good idea to start at the point in the operating cycle where the body of the trap is completely filled with condensate (refer to Fig. 7-11). The bucket rests on the bottom and the outlet valve is fully open. The pressure difference across the trap causes the condensate to be discharged directly into the header or the atmosphere. If steam flows under the bottom of the bucket, the bucket becomes buoyant, rises and starts to close the valve. The valve remains shut as long as the buoyancy force is greater than the weight of the bucket. When the weight exceeds the buoyancy force again, the bucket begins to sink and the outlet valve is opened.
Fig. 7-11: Operation of an inverted bucket steam trap
Since it would take far too long for the steam bubble to condense underneath the bucket, a small hole is provided in the top to allow steam to escape into the top part of the body. When more condensate flows under the bottom of the bucket, the buoyancy force decreases and the bucket starts to sink. Finally, when the weight of the bucket exceeds the buoyancy force, the valve is opened by means of the lever arm and the condensate is discharged. The larger the pressure difference, the higher the level in the trap must rise in order to open the valve.
Starting up an inverted bucket steam trap
Extreme care is essential when starting up an inverted bucket steam trap. If there is no condensate inside the trap, as is the case when the system is put into service for the first time or following a repair, the bucket hangs right down and the outlet valve is fully open. A water seal, i.e. the presence of condensate in the body, is necessary for the inverted bucket to work. If the inlet valve upstream of the trap is opened when there is only steam in the pipe and no condensate, the steam will escape into the condensate system through the open trap. If the steam contains condensate, the latter is entrained due to the high velocity of the mixture and the condensate level in the trap builds up either very slowly or not at all. It is therefore advisable to collect the required amount of condensate in the trap prior to starting it up. This is done by opening the inlet valve but keeping the outlet valve downstream of the trap closed for the time being.
Steam wastage due to the absence of a water seal not only occurs when the trap is started up. If there is a sudden drop in pressure or if no condensate accumulates, the condensate that is needed to form the seal may flash to steam during operation. When the level falls, the bucket sinks to the bottom and the trap remains wrongly in the open position. It is hence a good idea to increase the length of the supply pipe to the trap slightly and to refrain from insulating it.
Applications
The main advantage of the inverted bucket steam trap is its insensitivity to dirt, because the outlet valve is located on the top of the trap. Dirt can thus collect on the bottom without damaging the valve seat or plug. The disadvantage of this trap type is that it can only vent air very slowly. Any air trapped under the bucket (for instance, when the system is started up) can only escape gradually through the small hole in the top. This is an important aspect in a system that is started up and shut down frequently. Live steam likewise escapes through the bleed hole during operation. The steam loss owing to the hole in the bucket is approximately 0.4 kg/h, equivalent to €65.- /year. Generally speaking, float-type steam traps – or bimetallic traps for pipe drainage and trace heating – should be preferred to inverted bucket traps if large amounts of condensate are involved, depending on the application.
7.5 Thermostatic steam traps
Introduction
This group comprises two main types:
- Steam traps controlled by a bimetallic element or thermoelastic action (bimetallic steam traps)
- Steam traps controlled by steam pressure (balanced pressure traps, liquid expansion traps)
Thermostatic steam traps are controlled according to the condensate temperature. A temperature difference must exist between the saturated steam and the condensate for a thermostatic trap to open and close. The valve only opens, in other words, after the condensate inside the trap has cooled down to a few degrees below the saturation temperature and closes before the temperature increases to that of the steam pressure. All thermostatic traps operate intermittently. The opening and closing temperature can be influenced by the trap's adjustment and design.
7.6 Bimetallic steam traps
Method of operation
The principle of a bimetallic steam trap is based on the thermoelastic action of bimetal elements. The bimetallic element consists of two strips of metal with different coefficients of expansion. The top strip has a higher coefficient than the bottom strip.
Fig. 7-12 shows two cold bimetal elements; the layers with the lower coefficient of expansion are facing one another. When the element is heated, these layers are deflected against one another (Fig. 7-13.). The higher the temperature, the greater the deflection.
Fig. 7-12: Cold bimetallic plates
Fig. 7-13: Heated bimetallic plates
If several bimetal elements are stacked in pairs and connected by a valve stem, the changes in the temperature of the inflowing condensate cause the bimetal elements to be deflected, so that the plug opens and closes. Fig. 7-14 shows a cold bimetallic regulator. The bimetal elements are lying flat, one on top of the other, and the valve is fully open. Condensate and air are discharged swiftly under the influence of the pressure difference between the steam and condensate systems.
When the bimetal elements are heated by hot condensate, they are deflected and the stack expands. If the temperature rises to a few degrees below the saturation temperature, the expansion of the bimetal elements causes the valve to close (refer to Fig. 7-15). The closing force (FS) of the bimetallic regulator is greater in this state than the opening force (FÖ) generated by the system.
Principle of a bimetallic steam trap:
Fig. 7-14: Valve open
Fig. 7-15: Valve closed
The expansion of the bimetal elements thus not only results in the closure of the valve but also produces the force that is necessary to keep the valve permanently shut at saturation temperature. In the closed state, the force of the steam or system pressure acts on the valve and attempts to open it.
Fig. 7-16: Interaction of the forces in a bimetallic steam trap
The following is true: Fö = (p1 - p2) x Av
| Where: | Fö = Opening force of the valve |
| p1 = Pressure upstream of the steam trap | |
| p2 = Back pressure in the condensate system | |
| Av = Active surface area of the valve |
To keep the valve shut, the bimetal stack must apply a closing force (FS) that is greater than the opening force (FÖ). The closing force (FS) decreases as the condensate cools down. When a defined temperature is reached, the system pressure acting on the valve area overcomes the closing force due to the expansion of the bimetal elements and causes the valve to open. Since the valve plug moves in the opening direction, the spring pressure of the bimetal stack also increases. A new balance is created between the opening and closing forces, depending on the condensate temperature. The majority of bimetallic steam traps are set to open at approximately 15 degrees below boiling point and close again at around 5 degrees below saturation temperature.
Influence of back pressure on the opening and closing torque
The factor (p1 - p2) in the above formula decreases as the back pressure in the condensate system increases. The opening force (FÖ) is therefore reduced and the closing force (FS) must also be reduced in order for the valve to open. The condensate in the steam trap must continue to cool down, so that the expansion of the bimetal elements decreases. The trap remains closed until this new balance is established. Put another way, an increase in back pressure is accompanied by increased condensate sub-cooling. "Sub-cooling" is defined as the difference between the condensate temperature and the saturation temperature. In contrast to a float-type steam trap, the characteristic of a bimetallic trap is influenced by the back pressure.
This is aptly illustrated by an example:
A bimetallic regulator is set to the standard sub-cooling temperature, i.e. approximately 15 degrees. Assuming a pressure p1 = 10 bar (a) [Ts = 180 ºC] upstream of the steam trap and atmospheric pressure [p2 = 1 bar (a)] downstream of the trap, the trap will open at approx. 165 °C. If the back pressure rises to 4 bar (a) [Ts = 144 ºC], the trap opens at 15 degrees below the saturation temperature (159 °C) for 10 - 4 = 6 bar, that is to say at approx. 144 °C
This higher condensate sub-cooling at back pressure can be critical in certain applications (trace heating). In this case, it is advisable to correct the setting on the bimetallic regulator by adjusting the valve clearance. In other applications, the higher condensate sub-cooling brings with it the often desirable effect of reducing flashing. Owing to the smaller pressure difference at higher back pressure as well as the lower opening temperature, the percentage of flash steam downstream of the steam trap is smaller.
Combination of thermostatic and thermodynamic valve control
If the condensate in the steam trap cools down sufficiently for the opening force to start to overcome the closing force of the bimetal elements, the valve opens slightly. In order to open it further, the condensate needs to continue cooling. Since this is not normally the case, condensate will exit at very high velocity through the small gap between the valve plug and the seat. This hypercritical velocity will lead to erosion due to cavitation on the seat and plug, preventing the valve from closing tightly. Modern bimetallic steam traps get round this problem by combining the thermostatic with the thermodynamic principle. Chapter 4.0 Pipes includes a description of a full-lift safety valve. The stem of this valve is connected to a valve disc. If the system pressure rises enough to overcome the spring force that is keeping the valve shut, the outflowing medium lifts the disc and opens the valve completely. This principle is also employed in the regulator shown below (Fig. 7-17). The valve obturator takes the form of a plug rather than a simple ball. When the valve begins to open, the outflowing condensate hits the plug. The pressure inside the chamber (pz) rises spontaneously owing to the small gap area. The plug acts like a piston, causing the valve to open fully. Condensate and dirt can thus be discharged over a larger effective area.
When the condensate temperature rises, the deflection of the bimetal elements increases and the valve begins to close. The flow velocity increases according to the thermodynamic principle (Bernoulli) in the ever smaller gap between the seat and the plug. The pressure inside the chamber (pz) falls and the evaporating condensate downstream of the valve plug unloads the stem, thus supporting the closing movement through the increased deflection of the bimetal elements. The valve closes and remains closed until the condensate cools down and the cycle repeats. This intermittent mode of operation reduces the time for which condensate flows out at critical velocities between the seat and the plug, with the result that the service life of the bimetallic regulator is extended, especially at high service pressures.
Fig. 7-17: Principle of thermodynamic valve opening
Design of the bimetal elements
Fig. 7-8 shows the control characteristic of a ball float trap. The opening characteristic coincides exactly with the saturation curve, in other words boiling hot condensate is discharged at the corresponding pressure. The opening characteristic of a bimetallic steam trap, on the other hand, is always a few degrees below the saturation curve. It can be seen from Fig. 7-18 that the characteristic curve of a simple bimetallic trap is linear rather than following the saturation curve. In practice, this means that sub-cooling is not constant over the pressure range. The linear characteristic of a simple bimetallic regulator is presented in Fig. 7-18 in relation to the saturation curve. The trap operates very close to the saturation temperature in the low and high pressure ranges but with relatively high sub-cooling in the mid-range area.
Fig. 7-18: Characteristic of a simple bimetallic steam trap
The characteristic can be shifted closer to the saturation curve or closer to sub-cooled condensate by altering the valve clearance. If it crosses the saturation curve, steam is lost in the areas above the curve. This risk is particularly great in the low pressure range as well as close to the maximum pressure.
The CONA®B bimetallic trap differs from this simple type in a number of key respects, offering intermittent operation, less wear, a longer service life and almost constant subcooling over the entire pressure range for which the regulator is designed (refer to Fig. 7-19).
Fig. 7-19: Bimetallic steam traps, ARI Type CONA®B
The bimetal elements in the trap shown here are not flat but stepped (Fig. 7-12, Fig. 7-13, Fig. 7-19). The stamped elements provide a larger heat contact surface (90 % compared to 55 % for flat elements), which means the trap reacts very quickly to temperature changes and are less sensitive to dirt. The stamped elements additionally support the spring characteristic of the bimetal stack.
By combining the thermostatic and thermodynamic principles, it is possible to achieve intermittent operation with spontaneous opening of the valve, followed by a long cooling phase. This function was described in detail in the previous section and the special design of the valve plug is illustrated in Fig. 7-19.
The integration of a spring in the bimetallic regulator results in a steeper characteristic in the low pressure range and guarantees almost constant sub-cooling throughout the pressure range of the regulator. The valve lift is calculated and adjusted so that the characteristic remains below the saturation curve and no live steam is lost (refer to Fig. 7-20).
Fig. 7-20: p-t saturation curve with characteristic of a ARI Type CONA®B
Sub-cooling setting
Some designs allow the steam trap to be adjusted externally while it is operating. This is particularly important with simple bimetallic traps where the characteristic or the subcooling degree needs to be adapted to the respective service pressure. If this adjustment is carried out by a person without the appropriate technical know-how, it inevitably increases the risk of steam leakage and undesirable backing-up (waterlogging). Fig. 7-21 shows bimetallic steam traps that are configured for 15 Kelvin sub-cooling in the factory. These traps are designed to operate automatically with almost constant sub-cooling over the full pressure range. There is consequently no need to alter the setting during operation. Higher sub-cooling (30 Kelvin, 40 Kelvin, etc.) can be set in the factory if required.
Fig. 7-21: Bimetallic steam traps with adjustable sub-cooling, ARI Types CONA®B 600/601
The option of setting a steam trap to different sub-cooling degrees has the advantage that the heat from the hot condensate can be utilised commercially, for example for trace heating processes. At the same time, multiple sub-cooling settings enable the percentage of flash steam to be reduced.
Check valve function
If the steam pressure in the heat exchanger drops below the pressure in the condensate system as a result of reduced heat demand, or if a user connected in parallel is shut down, condensate could flow back or be drawn into the exchanger. The bimetallic steam trap in Fig. 7-21 features an integrated check valve that reliably prevents return flows of this kind.
Air venting
A bimetallic steam trap is open in the cold state and remains open until the condensate temperature rises to a few degrees below the saturation temperature of the steam pressure. When a steam pipe, a trace heater or another steam user is started up, the air it contains is discharged swiftly together with the cold condensate. Bimetallic traps are thus suitable both for start-up venting and as thermostatic air vents in continuous operation.
Applications of bimetallic steam traps
Bimetallic steam traps can be used for a wide range of drainage and venting tasks, such as pipe drainage, trace heaters, drainage systems, heat exchangers or air heaters, in which the condensate back-up due to the design does not impair the process or lead to water hammer. Traps with adjustable sub-cooling are sometimes recommended in trace heaters used for frost protection in order to utilise the sensible heat of the condensate. If a bimetallic trap is used to drain steam pipes, the globe valve upstream of the trap and the trap itself should not be insulated. Bimetallic steam traps are excellent air vents in heating systems with free air access that are frequently started up or shut down. They are also ideally suited as air vents when large steam spaces are put into service, for example in autoclaves. Bimetallic traps are extremely robust and insensitive to water hammer, making them an attractive alternative for all applications where water hammer would be a serious problem.
Steam traps controlled by steam pressure (balanced pressure traps)
The most important representatives of this thermostatic trap class are designed either with a membrane capsule for pressure balancing or with a bellows for liquid expansion. Only the membrane capsule type in common use today is described here.
Method of operation
The steam traps shown in Fig. 7-22 work according to the vaporisation principle. A control liquid is trapped inside a stainless steel capsule between the membrane and the capsule body. The boiling point of this liquid must be equal to or less than the boiling point of water at the corresponding pressure. The opening and closing movements are caused by imbalance between the condensate pressure in the steam trap and the steam pressure of the liquid in the membrane capsule. The liquid enclosed in the capsule vaporises as the temperature in the steam trap rises, resulting in a gauge pressure that forces the membrane against the valve seat and shuts the trap a few degrees below the saturation temperature. The trap opens again when the control liquid condenses as the condensate cools down.
Fig. 7-22: Balanced pressure steam traps, ARI Types CONA®M 611/613
Fig. 7-23 a and Fig. 7-23 b illustrate the principle of a balanced pressure steam trap in more detail. The trap's opening and closing temperature at the corresponding pressure can be influenced by varying the composition of the control liquid. By altering the filling, in other words, it is possible to obtain membrane capsules for different sub-cooling degrees.
Fig. 7-23 a: For cold condensate ARI Type CONA®M 611/613
Fig. 7-23 b: For hot condensate ARI Type CONA®M 611/613
The membrane capsule with the standard filling opens and closes at 10 Kelvin below saturation temperature. This is also referred to as 10 Kelvin sub-cooling. Membrane capsules with higher sub-cooling are especially suitable for trace heaters and – as mentioned in the previous chapter – as a way to reduce flashing.
Membrane capsules for different condensate sub-cooling degrees
Various membrane capsules with different sub-cooling degrees and different fillings are available for the CONA®M series and can be replaced individually (refer to Fig. 7-24). Capsule 1 discharges the condensate at about 3 to 5 Kelvin below the saturated steam temperature and thus has a similar characteristic to mechanical traps. Capsule 2 has approximately 15 Kelvin and capsule 3 approximately 25 Kelvin sub-cooling.
Fig. 7-24: Characteristic with different membrane capsules, ARI Type CONA®M 611/613
The number of the capsule is stamped on the rating plate and on the capsule itself for easier identification and to avoid confusion.
Applications
Balanced pressure steam traps are the solution of choice in trace heaters as well as for draining and venting small vessels with double jacket heating. They are fast and efficient air vents. They are also used as aerators for steam spaces (vacuum breakers) in models without a check valve function. Like their bimetallic counterparts, balanced pressure traps should not be insulated.
7.7 Thermodynamic steam traps
Steam traps with non-pressure balanced valve disc
Thermodynamic steam traps are controlled according to the thermodynamic state of the fluid. The simplest form of thermodynamic trap consists of no more than a body with an integrated valve seat, a valve disc and a screw cap (refer to Fig. 7-25).
Fig. 7-25: Thermodynamic Steam Trap, ARI Type CONA®TD 640/641
The two seat rings in the body are hardened (like the valve disc) and their surface has a fine ground finish. The inlet orifice is centrally positioned in the seat. A small, circular duct is provided around this orifice, with three holes connected to the outlet orifice. When the valve disc is resting on the seat, it simultaneously closes both the inlet orifice and the duct with the outlet holes.
Method of operation
The principle of the thermodynamic steam trap is based on Bernoulli's law, which states that the sum of the static pressure (potential energy) and dynamic pressure (kinetic energy) is constant at all points in a fluid flow (gas or liquid). If the static (or gauge) pressure falls, the velocity (or dynamic pressure) rises and vice versa.
Pressure changes occur when condensate passes the valve disc at saturated steam temperature and is partially flashed due to the lower pressure in the condensate system. The operation of a thermodynamic steam trap is determined by the change in static and dynamic pressure. Its action can best be explained with the help of an example:
If cold condensate flows into the trap on start-up, the valve disc is forced upwards and the condensate is discharged via the outlet holes. The trap is fully open. As the start-up phase progresses (Fig. 7-26 a), the condensate becomes hotter and the pressure rises. Part of the static pressure is then converted to velocity in the chamber between the seat and the disc.
Fig. 7-26 a: "Pressure build-up" state
The increase in kinetic energy causes the pressure to drop and the condensate begins to flash. At the same time, there is a sharp rise in the volume and velocity. The condensate and the flash steam flow along the underside of the valve disc at high velocity. This velocity increase results in a pressure drop underneath the disc. Part of the flash steam enters the chamber above the disc and is drawn towards the inside of the cap.
Fig. 7-26 b: "Flashing" state
The steam pressure that is built up above the valve disc forces it against the seat and shuts the valve (refer to Fig. 7-26 b). The trap remains closed under the influence of the difference between the active surface area above the disc and under it.
Fig. 7-26 c: "Closed" state
Heat is dissipated into the atmosphere via the cap of the body (refer to Fig. 7-26 c). As a result, the steam in the chamber above the valve disc condenses. The pressure above the disc decreases and is no longer sufficient to close it against system pressure. The valve disc opens and the cycle repeats.
Applications
Thermodynamic steam traps are likewise used for trace heaters and pipe drainage. They are insensitive to water hammer and suitable for a broad pressure range. However, the thermodynamic principle also suffers from several restrictions and disadvantages in practice:
Thermodynamic traps should not be used if high back pressure occurs, i.e. greater than 60 % of the upstream pressure. The reason for this is that with a pressure difference less than this not enough condensate is flashed under the valve disc, so the pressure drop is not large enough to close the valve. Thermodynamic traps are not suitable for applications with fluctuating pressure, high back pressure or significant variations in the amount of accumulated condensate, as is often the case with heat exchangers that are regulated on the steam side.
- Thermodynamic traps are poor air vents. Air is contained both in the system and in the trap on start-up. According to Bernoulli's law, the valve disc is forced against the seat if air attempts to escape at high velocity. The trap remains closed and does not vent the air.
- Thermodynamic traps must be opened from time to time in order to renew the steam cushion above the disc and build up the required closing pressure. If the steam does not contain any condensate, for example with superheated steam, live steam will be released via the trap.
- With outdoor installations, the condensation of the steam cushion above the valve disc is easily influenced by the ambient conditions. Rain or wind will cause the opening frequency of a thermodynamic trap to increase, leading inevitably to wear.
7.8 Fixed orifice devices
Design and method of operation
Fig. 7-27 shows a fixed orifice device. The condensate enters the trap via a calibrated orifice and exits it again via an expansion chamber connected to the condensate pipe.
Fig. 7-27: Fixed orifice steam trap
The orifice plate and the expansion chamber are sized according to the maximum expected amount of condensate and the design service pressure (pressure upstream of the trap). Liquid flows freely through the orifice. Hot condensate that passes the orifice flashes in the expansion chamber. Owing to the enormous volume of the flash steam, the flow of steam and non-condensables is reduced and possibly prevented altogether. This blocking function is influenced by the dimensions of the expansion chamber and the length of the orifice. A labyrinth or multi-stage orifice can be obtained by connecting several chambers and orifice plates in series. Labyrinth and orifice traps are fixed (static) and cannot adapt to changing operating conditions. The fixed hole size means the maximum flow rate is restricted. If a larger amount of condensate builds up, waterlogging will occur. If the orifice is too large or if the steam does not contain much condensate, live steam will escape as well.
Applications
The potential applications for orifice traps are limited. This drainage method only works reliably under constant operating conditions. There should be little or no variation in the heat load, upstream pressure, pressure difference or condensate amount. Orifice traps must not be used to drain steam pipes. When a pipe is put into service, a large amount of condensate is formed at low steam pressure, so that an orifice with a large hole is required. Actually in service, on the other hand, only a small amount of condensate occurs at a higher steam pressure, so that a small hole suffices. Impurities must be reckoned with when a system is started up for the first time and the small orifices of this trap type can easily become blocked. Orifice traps are thus relatively susceptible to dirt. In addition, they have no automatic air venting function. Air must be vented manually on the heat exchanger, especially if the system is started up and shut down frequently.