Maintenance Engineering -- Steam Traps

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Steam-supply systems are commonly used in industrial facilities as a general heat source and as a heat source in pipe and vessel tracing lines used to prevent freeze up in non-flow situations. Inherent with the use of steam is the problem of condensation and the accumulation of non-condensable gases in the system.

Steam traps must be used in these systems to automatically purge condensate and non-condensable gases, such as air, from the steam system. However, a steam trap should never discharge live steam. Such discharges are dangerous as well as costly.

---1 Inverted-bucket trap.

CONFIGURATION

Five major types of steam traps are commonly used in industrial applications:

inverted bucket, float and thermostatic, thermodynamic, bimetallic, and thermo static. Each of the five major types of steam trap uses a different method to determine when and how to purge the system. As a result, each has a different configuration.

Inverted Bucket

The inverted-bucket trap is a mechanically actuated steam trap that uses an upside down, or inverted, bucket as a float.

The bucket is connected to the outlet valve through a mechanical linkage. The bucket sinks when condensate fills the steam trap, which opens the outlet valve and drains the bucket. It floats when steam enters the trap and closes the valve.

As a group, inverted-bucket traps can handle a wide range of steam pressures and condensate capacities. They are an economical solution for low- to medium pressure and medium-capacity applications such as plant heating and light processes. When used for higher-pressure and higher-capacity applications, these traps become large, expensive, and difficult to handle.

Each specific steam trap has a finite, relatively narrow range that it can handle effectively. E.g., an inverted-bucket trap designed for up to 15-psi service will fail to operate at pressures above that value. An inverted-bucket trap designed for 125-psi service will operate at lower pressures, but its capacity is so diminished that it may back up the system with unvented condensate.

Therefore it’s critical to select a steam trap designed to handle the application's pressure, capacity, and size requirements.

Float-and-Thermostatic

The float-and-thermostatic trap shown is a hybrid. A float similar to that found in a toilet tank operates the valve. As condensate collects in the trap, it lifts the float and opens the discharge or purge valve. This design opens the discharge only as much as necessary. Once the built-in thermostatic element purges non-condensable gases, it closes tightly when steam enters the trap. The advantage of this type of trap is that it drains condensate continuously.

---2 Float-and-thermostatic trap.

Like the inverted-bucket trap, float-and-thermostatic traps as a group handle a wide range of steam pressures and condensate loads. However, each individual trap has a very narrow range of pressures and capacities. This makes it critical to select a trap that can handle the specific pressure, capacity, and size requirements of the system.

The key advantage of float-and-thermostatic traps is their ability for quick steam-system startup because they continuously purge the system of air and other non-condensable gases. One disadvantage is the sensitivity of the float ball to damage by hydraulic hammer.

Float-and-thermostatic traps provide an economical solution for lighter condensate loads and lower pressures. However, when the pressure and capacity requirements increase, the physical size of the unit increases and its cost rises.

It also becomes more difficult to handle.

---3 Thermodynamic steam trap.

Thermodynamic, or Disk-Type

Thermodynamic, or disk-type, steam traps use a flat disk that moves between a cap and seat. On startup, condensate flow raises the disk and opens the discharge port. Steam or very hot condensate entering the trap seats the disk. It remains seated, closing the discharge port, as long as pressure is maintained above it. Heat radiates out through the cap, thus diminishing the pressure over the disk, opening the trap to discharge condensate.

Wear and dirt are particular problems with a disk-type trap. Because of the large, flat seating surfaces, any particulate contamination such as dirt or sand will lodge between the disk and the valve seat. This prevents the valve from sealing and permits live steam to flow through the discharge port. If pressure is not maintained above the disk, the trap will cycle frequently. This wastes steam and can cause the device to fail prematurely.

The key advantage of these traps is that one trap can handle a complete range of pressures. In addition, they are relatively compact for the amount of condensate they discharge. The chief disadvantage is difficulty in handling air and other non condensable gases.

---4 Bimetal trap.

---5 Thermostatic trap.

Bimetallic

A bimetallic steam trap, which is shown, operates on the same principle as a residential-heating thermostat. A bimetallic strip, or wafer, connected to a valve disk bends or distorts when subjected to a change in temperature. When properly calibrated, the disk closes tightly against a seat when steam is present and opens when condensate, air, and other gases are present.

Two key advantages of bimetallic traps are (1) compact size relative to their condensate load-handling capabilities and (2) immunity to hydraulic-hammer damage.

Their biggest disadvantage is the need for constant adjustment or calibration, which is usually done at the factory for the intended steam operating pressure. If the trap is used at a lower pressure, it may discharge live steam. If used at a higher pressure, condensate may back up into the steam system.

Thermostatic or Thermal-Element

Thermostatic or thermal-element traps are thermally actuated by using an assembly constructed of high-strength, corrosion-resistant stainless steel plates that are seam-welded together.

Upon startup, the thermal element is positioned to open the valve and purge condensate, air, and other gases. As the system warms up, heat generates pressure in the thermal element, causing it to expand and throttle the flow of hot condensate through the discharge valve. The steam that follows the hot condensate into the trap expands the thermal element with great force, which causes the trap to close. Condensate that enters the trap during system operation cools the element. As the thermal element cools, it lifts the valve off the seat and allows condensate to discharge quickly.

Thermal elements can be designed to operate at any steam temperature. In steam-tracing applications, it may be desirable to allow controlled amounts of condensate to back up in the lines to extract more heat from the condensate.

In other applications, any hint of condensate in the system is undesirable. The thermostatic trap can handle either of these conditions, but the thermal element must be properly selected to accommodate the specific temperature range of the application.

Thermostatic traps are compact, and a given trap operates over a wide range of pressures and capacities. However, they are not recommended for condensate loads over 16,000 lbs/hour.

PERFORMANCE

When properly selected, installed, and maintained, steam traps are relatively trouble-free and highly efficient. The critical factors that affect efficiency include capacity and pressure ratings, steam quality, mechanical damage, and calibration.

Capacity Rating

Each type and size of steam trap has a specified capacity for the amount of condensate and non-compressible gas that it can handle. Care must be taken to ensure that the proper steam trap is selected to meet the application's capacity needs.

Pressure Rating

As discussed previously, each type of steam trap has a range of steam pressures that it can effectively handle. Therefore, each application must be carefully evaluated to determine the normal and maximum pressures that will be generated by the steam system. Traps must be selected for the worst-case scenario.

Steam Quality

Steam quality determines the amount of condensate to be handled by the steam trap. In addition to an increased volume of condensate, poor steam quality may increase the amount of particulate matter present in the condensate. High concentrations of solids directly affect the performance of steam traps. If particulate matter is trapped between the purge valve and its seat, the steam trap may not properly shut off the discharge port. This will result in live steam being continuously exhausted through the trap.

Mechanical Damage

Inverted-bucket and float-type steam traps are highly susceptible to mechanical damage. If the level arms or mechanical linkages are damaged or distorted, the trap cannot operate properly. Regular inspection and maintenance of these types of traps are essential.

Calibration

Steam traps, such as the bimetallic type, must be periodically recalibrated to ensure proper operation. All steam traps should be adjusted on a regular schedule.

----1 Common Failure Modes of Steam Traps

INSTALLATION

Installation of steam traps is relatively straightforward. As long as they are properly sized, the only installation imperative is that they are plumb. If the trap is tilted or cocked, the bucket, float, or thermal valve won’t operate properly.

In addition, a non-plumb installation may prevent the condensate chamber from fully discharging accumulated liquids.

OPERATING METHODS

Steam traps are designed for a relatively constant volume, pressure, and condensate load. Operating practices should attempt to maintain these parameters as much as possible. Actual operating practices are determined by the process system rather than the trap selected for a specific system.

The operator should periodically inspect them to ensure proper operation.

Special attention should be given to the drain line to ensure that the trap is properly seated when not in the bleed or vent position.

TROUBLESHOOTING

A common failure mode of steam traps is failure of the sealing device (i.e., plunger, disk, or valve) to return to a leak-tight seat when in its normal operating mode. Leakage during normal operation may lead to abnormal operating costs or degradation of the process system. A single ¾” steam trap that fails to seat properly can increase operating costs by $52,000 to $62,000 per year. Traps that fail to seat properly or are constantly in an unload position should be repaired or replaced as quickly as possible. Regular inspection and adjustment programs should be included in the Standard Operating Procedures (SOPs).

Most of the failure modes that affect steam traps can be attributed to variations in operating parameters or improper maintenance. ----1 lists the more common causes of steam trap failures.

Operation outside the trap's design envelope results in loss of efficiency and may result in premature failure. In many cases, changes in the condensate load, steam pressure or temperature, and other related parameters are the root causes of poor performance or reliability problems. Careful attention should be given to the actual versus design system parameters. Such deviations are often the root causes of problems under investigation.

Poor maintenance practices or the lack of a regular inspection program may be the primary source of steam trap problems. It’s important for steam traps to be routinely inspected and repaired to ensure proper operation.

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