Steam Traps
A steam trap removes condensate, noncondensable gases and air from a steam piping network. No single type of trap is suitable for all applications, and most systems require more than one type.
Noncondensable gases, such as carbon dioxide and air, must be removed because they dilute the steam and lower the condensation temperature, as well as reducing efficient heat transfer. Removal is accomplished by either installing vents on top of terminal equipment or through a thermostatic sensor in a steam trap.
Thermostatic traps are the most commonly used. The element operates by measuring the steam temperature and determining if it is below the saturation temperature for a given pressure. If it is, then non condensable gases are present, which must be vented. The trap opens, venting the gas. Proper condensate removal is essential for efficient plant and process operation.
All steam traps shall be able to:
1. Vent air and other gases from piping and equipment
2. Prevent the flow of steam into the condensate piping
3. Allow only condensate into the condensate piping network
Condensate removal can be accomplished either manually or automatically. The manual removal of condensate will require that a valve placed in the network be constantly opened and closed by operating personnel. This method is inefficient and wasteful.
An automatic steam trap is a valve that opens in the presence of condensate or noncondensable gases and closes in the presence of steam in order to remove condensate from the steam piping network and prevent steam from entering the condensate piping network.
There are three main groups of automatic steam traps, grouped together by their mode of operation: mechanical, thermostatic, and thermodynamic.
Mechanical Traps. Mechanical traps are buoyancy operated and sense the density difference between steam and condensate through the use of a float or bucket as the operating member. Mechanical traps are usually preferred in applications that do not require much air venting, such as high-pressure steam mains and other noncycling systems. Condensate subcooling is not required, and the trap cycles either wide open or dead shut. Mechanical traps are preferred where immediate removal of the condensate is required or where a cooling leg upstream of the trap could interfere with operation of the equipment. This is the typical situation in industrial traps draining condensate from high-pressure steam mains or heat exchangers that can be damaged by flooding.
One disadvantage of all mechanical traps is that the size of the discharge orifice is controlled by the buoyancy power of the float, which is constant. As the pressure of the steam system increases, the size of the orifice should decrease. This requires that the size of the valve seat be different for various pressures.
Loose Float Trap. The loose float trap, illustrated in Fig. 1, is the simplest of traps. When condensate enters the chamber, the float B is lifted off its seat A and allows condensate to discharge. It has no moving parts except for the float and is very inexpensive. Disadvantages are that it cannot vent air, and it is difficult to obtain good seating. A hand cock C has been added to vent air. This type of trap
is no longer used but may be found in older facilities.
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| FIG 1 |
Float and Lever Trap. The float and lever trap, illustrated in Fig. 2, overcomes some of the problems of the loose float by providing a float arm C connected to an outlet valve D for more effective seating. This type of trap is no longer used in modern systems but may be found in older facilities
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| FIG 2 |
Float and Thermostatic Trap. The float and thermostatic element trap is illustrated in Fig. 3 When condensate enters the trap, the float rises and allows condensate to discharge. A thermostatic element is provided to automatically vent air out of the chamber. This element is open only when air is present to cool the element, thereby allowing air to vent. As the temperature of the chamber rises when
no air is present, the element is closed.
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| FIG 3 |
Open Bucket Trap. The open bucket trap, illustrated in Fig. 4, uses a bucket instead of a float. The bucket floats when condensate is present and falls when empty. When condensate enters the trap, it first fills the body outside the bucket.
The floating bucket holds the seat closed. After the body is full, condensate spills over to fill the bucket causing it to sink drawing the seat open and allowing condensate to discharge.
These traps are mechanically simple and strong, and they are capable of withstanding shock and corrosive condensate. Disadvantages are reduced air-venting capability, heavy weight in relation to their discharge capacity, and susceptibility to damage by freezing.
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| FIG 4 |
Inverted Bucket Trap. The inverted, bucket trap, illustrated in Fig. 5, uses an inverted bucket instead of a float. The bucket is normally at the bottom, causing the seat to be open. When condensate enters and fills the chamber, it is able to discharge through the open seat. When a small amount of steam enters the trap, it escapes through the vent. More steam entering causes the bucket to rise, closing the seat. As the steam escapes through the vent, the bucket will again drop to the
bottom, opening the seat.
These traps are mechanically simple and strong, capable of withstanding shock, corrosive condensate, and superheated steam. Disadvantages are the low air-venting capacity and the need to have a steady pressure. Air vent capacity can be increased by over sizing the condensate discharge piping
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| FIG 5 |
Thermostatic Traps. In this type of trap, a bimetallic or fluid-filled element operates
a valve that opens in the presence of condensate and closes in the presence of steam. There are a large number of variations for this type of valve.
The working pressure of the steam does not affect the operation of the trap. Instead, it is the difference in temperature between the steam and the condensate which sets up the difference between the pressure inside and outside the element, that opens and closes the seat.
Thermostatic traps are intended for relatively low condensate removal capacities and typically used in low-pressure steam heating equipment, in equipment that can tolerate condensate collection upstream of the trap in a ‘‘cooling leg’’ where it can reach the subcooled temperature that opens the trap. While the condensate is subcooling, some sensible heat is made available. Sometimes this sensible heat can be useful, for example, in tracing temperature-sensitive components.
Bellows Trap. Often referred to as a balanced-pressure trap, the bellows type is illustrated in Fig. 6 . The principle of operation is the expansion and contraction of a bellows filled with a liquid that has a lower boiling point than water.
The valve seat is open when cold, allowing air to vent and condensate to discharge. As condensate enters and before it reaches the boiling point, the element closes. As the condensate cools, the element opens, discharging the condensate.
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| FIG 6 |
Bimetallic Trap. The bimetallic trap operates by the action of a composite strip of metal that bends when the temperature changes. The principle of operation is illustrated in Fig. 7. The seat is open when cold, allowing the free passage of air and condensate. When steam temperature is approached, the element bends and closes the seat.
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| FIG 7 |
Liquid Expansion Trap. Another type of thermostatic trap is the liquid expansion
type, illustrated in Fig. 8
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| FIG 8 |
Thermodynamic Trap. The thermodynamic trap, often referred to as a disk trap, is illustrated in Fig. 9. This device has only one moving part—the disk. When air or cold condensate enters the trap, it lifts the disk off its seat and is discharged.
When steam or hot condensate enters the trap, some of it flashes into steam upon exposure to a lower pressure. The increased velocity of this vapor flow decreases the pressure on the underside of the disk causing it to close.
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| FIG 9 |
Piston Trap. A piston trap, also called an impulse trap, is illustrated in Fig. 10. As cooler condensate enters the body, pressure raises the piston to open the seat, allowing air and condensate to discharge. As the condensate nears steam temperature, some flashes into steam, which passes through the gap. The flash steam with the greater pressure forces the cylinder closed, stopping flow.
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| FIG 10 |
Labyrinth Trap. Another type of kinetic trap is the labyrinth, illustrated in Fig.11
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| FIG 11 |
Selecting and Sizing the Steam Trap
General. Each trap design has specific characteristics. Because there is such a variety of trap designs, one of the key decisions in condensate drainage design is the choice of the right type of trap. Often, this choice is easy; for example, low pressure heating system radiators are almost always equipped with a thermostatic trap because the characteristics of that type of trap match the condensate drainage
and air venting requirements of low-pressure heating equipment. Sometimes, either of two different types of traps could equally well be applied to a given condensate drainage situation because either set of trap characteristics meets the drainage and venting requirements. There are some advantages and disadvantages to both, but either one could do the job. For example, a high-pressure steam main could be equipped with either a thermodynamic or an inverted bucket trap.
The choice between them then becomes a nontechnical matter, for example, cost, personal preference. In some cases, a given kind of trap simply would not be able to do the job; for example, a thermodynamic trap requires a significant pressure drop between the steam-condensing device and the condensate pipe. Such a trap installed in a low pressure heating system would not have a great enough pressure differential to operate. General selection criteria are given in Table 1. General characteristics are given in Table 2.
Liberal and oversized steam traps do not always provide an efficient and safe steam main drain installation. The following points should also be considered by the design professional:
1. Method of heat-up to be employed
2. Providing suitable reservoirs, or collecting legs, for condensate
3. Ensuring adequate pressure differential across the steam trap
4. Steam trap load safety factor
5. Flow of condensate from the selected trap
Heat-Up Method.
The type, size, and installation of the steam trap used to drain steam mains depends upon the method used in bringing the system up to normal operating pressure and temperature. The two methods of system heat-up commonly used are the supervised heat-up and the automatic heat-up.
Supervised Heat-Up. In the supervised heat-up method, manual drain valves
are installed at all drainage points in the steam main system. The valves are fully opened to the condensate return before steam is admitted to the system. After most of the heat-up condensate has been discharged, the drain valves are closed, allowing the steam traps to drain the normal operating load. Therefore, the steam traps are sized to handle only the condensate formed due to radiation losses at the system’s operating pressure. This heat-up method is generally used for large installations, having steam mains of appreciable size and length, and where the heat-up generally occurs only once a year, such as in large systems where the system pressure is maintained at a constant level after the start-up and is not shut down except in emergencies. A typical installation detail is illustrated in Fig. 12
Automatic Heat-Up. In the automatic heat-up method, the steam boiler brings the system up to full steam pressure and temperature without supervision or manual drainage. This method relies on the traps to automatically drain the warm-up load of condensate as soon as it forms. This heat-up method is generally used in small and medium-sized installations that are shut down and started up at regular intervals, as in heating systems or in dry cleaning plants, where the boiler is usually shut
down at night and started up again the following morning. A typical installation detail is illustrated in Fig. 13.
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| FIG 12 |
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| FIG 13 |
Adequate Pressure Differential across the Trap.
The trap cannot discharge condensate unless a pressure differential exists across it, that is, a higher pressure at the inlet compared to a lower pressure in the condensate line. The collecting leg should be of sufficient length to provide a hydrostatic head at the trap inlet so that the condensate can be discharged during warm-up, before a positive steam pressure develops in the steam main. For mechanical traps, not only the minimum differential, but also the maximum allowable differential—that is, the trap ‘‘seat pressure rating’’—must be considered. In draining devices like heat exchangers controlled by temperature-regulating valves that could possibly operate in a vacuum at part
load, install a vacuum breaker to ensure that pressure upstream of the trap cannot fall below atmospheric, and ensure that adequate hydrostatic head is available
Steam Trap Load Safety Factor.
After determining the actual amount of condensate expected to enter a trap, it is accepted practice to assign a safety factor to increase the amount of condensate. This safety factor is obtained from the manufacturer of the trap. As a guide, Table 3 provides recommended safety factors. Sizing Condensate Pipes to Carry Flashing Condensate
1. Determine the pounds of flash steam from Table 4.
2. Multiply total high-pressure condensate flow by the pounds of flash steam to determine the flash steam flow rate.
3. Using Fig. 14, size the condensate pipe as if it were a steam pipe carrying nothing but the flash steam flow.
This procedure will oversize the condensate pipe to accommodate the flash steam without generating excess return line pressures. For example, determine the trap
requirements for 1000 ft of 10-in horizontal main with a maximum operating pressure of 250 psig. Assume that the supervised heat-up method will be used.
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| FIG 14 |
Maximum Condensate Flow Rate
Maximum Flow Rate from Any Individual Trap. When the trap is installed at a piece of terminal equipment, the PPH steam use of the equipment will discharge the same PPH of condensate. Therefore, 100 PPH of steam will produce 100 PPH of condensate discharged from the trap. The maximum flow rate actually discharged will be determined from published literature for the specific trap selected. For traps placed in steam mains to drain system condensate, the maximum flow rate depends on several variables. Refer to sizing the steam trap as previously discussed.
Diversity Factor. When several traps discharge into a common line, it is not good practice to design a system in the belief that they will all discharge at the same time. Therefore, some diversity factor must be assigned. If the traps are operating on a light loading, a maximum factor of 20 percent should be used. For a medium loading, a 35 to 40 percent factor should be used, and for heavy loads, a 70 percent factor is recommended. Only when using a modulating trap, such as a float and thermostatic trap, can the drainage flow rate be the same as the calculated condensate load
Trap selection and safety factor for steam mains (saturated steam only).
Select trap to discharge condensate produced by radiation losses at running load. Sizing for start-up loads results in over sized traps which may wear prematurely.
Size drip legs to collect condensate during low-pressure, warm-up conditions table 5
Condensate loads of insulated pipe can be found in Table 6 . All figures in the table assume
the insulation to be 75% effective. For pres-sures or pipe sizes not included in the table, use the following formula:
Where:
C = Condensate in lbs/hr-foot
A = External area of pipe in square feet (Table 6, Col. 2)
U = Btu/sq ft/degree temperature difference/hr from figure 15
T1 = Steam temperature in °F
T2 = Air temperature in °F
E = 1 minus efficiency of insulation (Example: 75% efficient insulation: 1-.75 = .25 or E = .25)
H = Latent heat of steam
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| FIG 15 |
