Correct selection of steam trap/drain valve

When selecting a steam trap/valve, it is not advisable to simply choose based on the maximum discharge volume. Special attention should be paid to the principle of selecting a steam trap based solely on the diameter of the pipe. The selection must be based on the principles of steam trap selection and combined with the specific situation of the cold clean water system. Generally, the selection should be based on the following three aspects.

1. Firstly, based on the requirements of the heating equipment for removing cold clean water, select and determine the type of steam trap.

For heating equipment that requires fast heating speed and strict temperature control, it is necessary to keep the equipment free of cold clean water. As long as there is water, it must be drained. Therefore, it is best to choose a mechanical drain valve/trap that can drain saturated water. Because it is a drain trap/valve that discharges water whenever there is water, it can promptly eliminate the adverse consequences caused by accumulated water in the equipment, quickly improve and ensure the required heating efficiency of the equipment.

For heating equipment with large heating surfaces and lax requirements for heating speed and temperature control, water accumulation may be allowed, such as steam heating drainage, process tracing pipeline drainage, etc., and thermal static type steam traps/valves should be selected as the best option.

For medium and low-pressure steam transmission pipelines, the cold clean water generated in the pipeline must be quickly discharged, otherwise it may cause water hammer accidents. The increase in moisture content in steam leads to a decrease in steam temperature, which cannot meet the process requirements of steam equipment. Therefore, it is recommended to use mechanical steam traps/valves for medium and low pressure steam transmission pipelines.

2. Then, based on the size of the drainage volume, select and determine the performance parameters of the drain valve/trap.

In addition to matching the pressure, temperature, and other parameters of the steam trap/drain valve with the equipment conditions used, the drainage capacity under various pressure differentials of the steam trap/drain valve is an important factor in selecting the steam trap/drain valve. If the drainage volume of the selected and installed steam trap/valve is too small, it will not be possible to timely discharge all the cold clean water that has reached the steam trap, causing the cold clean water to be blocked and flow back, ultimately leading to blockage and significantly reducing the heating efficiency of the equipment. On the contrary, selecting a drain valve/trap with a large drainage capacity will result in premature wear and failure of the valve closure components. As the valve body increases, its manufacturing cost will also increase, which is uneconomical. Therefore, the amount of cold and clean water generated in equipment or pipelines must be accurately measured or calculated based on formulas to provide conditions for the correct selection of steam traps/valves.

Therefore, when determining the drainage capacity of a steam trap/valve, the "safety factor K" should be determined based on the characteristics of various steam consuming equipment and the discharge form of the steam trap/valve.

Generally, it should be calculated according to the following formula:

The hourly discharge of the steam trap is equal to the amount of clean water generated by the equipment or pipeline per hour multiplied by the safety factor K

The amount of cold and clean water generated by the equipment per hour is the steam consumption of the equipment per hour. The safety factor K is not theoretically specified or can be calculated, but is a data obtained from experience. The determination of the safety factor K is mainly influenced by two factors: one is the drainage performance of the steam trap/valve. Due to the limited applicability of current steam traps/valves to system pressure differentials and flow changes, and the fact that the steam trap capacity provided by manufacturers to users is expressed in terms of continuous discharge per hour, in most cases, steam traps/valves discharge intermittently. On the other hand, there are various types of steam using equipment. The load characteristics of cold clean water generated during the operation of steam using equipment are also different. Consider the large amount of cold clean water generated during the start-up of steam using equipment and the changes in its cold clean water discharge during normal operation; At the same time, different situations such as whether the equipment is running continuously or intermittently should also be considered. If the drainage characteristics change significantly and the equipment operates intermittently, the safety factor should be selected as too high, otherwise it is better to choose too low. Taking into account various factors, it is recommended to set a safety factor of K=2-4 for different steam using equipment

Installation precautions for steam traps/valves

Whether the installation of steam traps/valves is appropriate has a direct impact on the normal operation of steam traps/valves and the production efficiency of equipment. The installation of steam traps/valves must follow the proper installation requirements in order to achieve the best working efficiency of the steam traps/valves and equipment.

1. Before installing the drain valve, it is necessary to blow the pipeline with pressurized steam to remove debris from the pipeline.

2. A filter should be installed before the drain valve/trap to ensure that the drain valve is not blocked by pipeline debris, and the filter should be cleaned regularly.

3. Valves should be installed before and after the drain valve/trap to facilitate easy maintenance of the drain valve/trap at any time.

4. The direction of condensate flow should be consistent with the arrow mark on the installation of the drain valve.

5. The drain valve should be installed at the lowest point of the equipment outlet to promptly remove cold clean water and avoid steam resistance in the pipeline.

6. If there is no position to install a drain valve/trap at the bottom of the device, a backflow elbow (cold clean water lifting joint) should be added at the bottom of the outlet to raise the cold junction water level before installing the drain valve, in order to avoid steam resistance.

7. The outlet pipe of the drain valve/trap should not be immersed in water. If immersed in water, a hole should be drilled at the bend to break the vacuum and prevent sand from being sucked back.

8. Mechanical drain valves should be installed horizontally.

9. Do not install steam traps in series.

10. Each device should be equipped with its own drain valve/trap

11. A subcooling pipe of at least one meter without insulation is required in front of the thermal static type drain valve/trap, while other types of drain valves/traps should be placed as close to the equipment as possible.

12. When selecting a drain valve/trap for drum drying (with siphon tube type) equipment, please indicate that a drain valve with an anti steam resistance device should be used to avoid equipment steam lock

13. If there is cold water recovery after the drain valve, the outlet pipe of the drain valve should be connected to the main pipe from above the recovery main pipe to reduce back pressure and prevent backflow.

14. If there is cold water recovery after the drain valve, pipelines of different pressure levels should be recovered separately.

15. The cold water recovery main pipe after the drain valve cannot climb the slope, which will increase the back pressure of the drain valve.

16. A check valve should be installed before the cold water from the drain valve enters the recovery main pipe to prevent the cold water from flowing back.

17. Install a drain valve/trap on the steam pipeline, and set up a cold water collection well close to the radius of the main pipeline. Then, use a small pipe to lead it to the drain valve/trap.

18. If the mechanical drain valve/trap is not used for a long time, the drain screw should be removed to drain the water inside to prevent freezing.

19. If steam leakage is found in the drain valve/trap, it is necessary to promptly drain and clean the filter screen. Check it frequently according to actual usage, and repair it promptly in case of any malfunction. It should be inspected at least once a year to remove impurities inside.

The drain valve/trap is considered a small accessory in the entire steam system, but it has a significant impact on the system's operation and economic performance. Therefore, the maintenance and repair of the drain valve is also crucial. Only by fully valuing the important role of the drain valve/trap in production can it be effectively maintained. Frequent maintenance is necessary to keep the drain valve/trap in good working condition, in order to ensure optimal performance and improve economic efficiency.

Table 1: Comparison of Three Forms of Drain Valves/Traps