Chemical Process Machinery Area: [Weekly Topic] Issues with rotating equipment encountered at work? (2011.7.11~7.17)

I. Temperature control valves 1. Structure and working principle of radiator temperature control valves The temperature in a user’s indoor space is controlled through the radiator thermostatic control valve. The radiator thermostatic control valve consists of a thermostatic controller, a flow control valve, and a pair of connectors; the core component of the thermostatic controller is the sensor unit, namely the temperature bulb. The thermal bulb can change its volume in response to changes in the surrounding temperature, which in turn causes the valve element to move, thereby regulating the amount of water flowing through the radiator and thus adjusting its cooling capacity. The set temperature of the thermostatic valve can be adjusted manually; it automatically controls and regulates the water flow to the radiator according to the set parameters, thereby achieving the goal of controlling the indoor temperature. Thermostatic valves are generally installed in front of the radiator, and by automatically adjusting the flow rate, they help to achieve the room temperature desired by the residents. Thermostatic valves are divided into two-way thermostatic valves and three-way thermostatic valves. Three-way temperature control valves are mainly used in single-tube systems with bypass pipes; their flow division coefficient can vary within the range of 0 to 100%, offering a large degree of flow regulation. However, they are relatively expensive and have a more complex structure. Two-way temperature control valves are used in some double-pipe systems and in some single-pipe systems. The two-way temperature control valve used in dual-tube systems has high resistance ； The resistance for single-tube systems is low. The temperature sensing element of the thermostatic valve and the valve body are generally assembled as a single unit, with the sensing element itself serving as the sensor for the indoor temperature at the site. If necessary, a remote temperature sensor can be used ； The remote temperature sensor is placed in the room where temperature control is required, while the valve body is located somewhere in the heating system. 2. Selection and design of thermostatic valves: Thermostatic valves are the primary devices used for regulating flow in heating systems; other control valves serve only as auxiliary devices. Therefore, thermostatic valves are essential. A heating system cannot be considered a heat metering billing system if it does not have a temperature control valve. In the design of temperature control valves, proper selection is very important. The purpose of selecting a temperature control valve is to determine the KV value (flow coefficient) based on the design flow rate (under a known heat load) and the allowable pressure drop ； Then, the diameter (model) of the temperature control valve is determined by the KV value. Therefore, design catalogs or manufacturer samples must provide the relationship between KV values and diameter; otherwise, it is not convenient for designers to use them. In the selection and design of temperature control valves, it is by no means sufficient to simply choose a valve with the same diameter as the pipeline. Rather, it is necessary to create ideal pressure difference operating conditions for the selected temperature control valve during the selection process. The normal operating pressure difference of a thermostatic valve is between 2 and 3 mH2O, with a maximum not exceeding 6 to 10 mH2O. To this end, it is necessary to specify the range of preset values for the temperature control valve in order to prevent noise generation and ensure its proper operation. When there are two or more valve sizes available for the same KV value, it is preferable to choose the valve with the smaller diameter, in order to improve the control performance of the temperature control valve. II. Electric control valve: An electric control valve is a device used for flow regulation in computer-based monitoring systems. It is generally used in unattended heat stations. An electric control valve consists of a valve body, a driving mechanism, and a transmitter. A temperature control valve is a device that performs self-acting flow regulation through a temperature sensor, and it does not require an external power supply ； Electric control valves generally require a single-phase 220V power supply, and are typically used as actuators in computer-based monitoring systems to regulate flow. Electric control valves or temperature control valves are the main devices for flow regulation in heating systems, with the rest being auxiliary devices. III. Balance valves: Balance valves are divided into manual balance valves and self-acting balance valves. Whether it is a manual balance valve or a self-acting balance valve, their function is to increase the resistance at the upstream end of the heating system, thereby preventing the actual flow rate from exceeding the designed flow rate ； In other words, its function is to overcome the excess head pressure at the upstream end of the heating system, allowing the electric control valve or temperature control valve to operate under an allowable head pressure. Therefore, both manual balance valves and self-acting balance valves are auxiliary flow control devices for thermostatic valves or electric control valves; yet they are extremely important. If not selected properly or designed appropriately, neither the electric control valve nor the thermostatic valve can function properly. 1. Manual balance valve 1.1. Working principle of the manual balance valve: The manual balance valve is adjusted manually once, and it is unable to change its resistance coefficient automatically in response to changes in the system conditions; hence it is called a static balance valve. The manual balance valve acts on resistance; it functions as a manually adjustable orifice to balance the resistance in the piping network system, thereby achieving resistance equilibrium across various circuits. It is capable of addressing the issue of steady-state imbalance in the system: when the operating conditions differ from those designed, the flow rate of water may be higher or lower than the value specified in the design. Since balance valves are used to balance the system’s resistance, they can distribute the new flow rate according to the proportions calculated in the design, ensuring that the flow rates in each branch increase or decrease proportionally, thus still meeting the flow requirements under the current load. 1.2 Issues to consider in the selection and design of manual balance valves (2) (1) The valve’s characteristic curve determines its regulating capabilities; for example, in the case of globe valves, if it is considered that flow rate changes within the range of 95%–100% are insignificant, then a flow rate variation from 0% to 5% already covers the entire range of possible flow rates. Such valves cannot be used for balancing flow rates in hydraulic systems. Since the theoretical characteristic curve of a valve is determined under maximum pressure differences, in actual operating conditions, as long as the valve’s characteristic coefficient is not 1, the pressure difference across the valve is large when it is open to a small degree, while it is smaller when it is open to a large degree. This results in the value of dG/dC increasing at small opening degrees and decreasing at large opening degrees, causing the actual operating curve of the valve to shift in the direction of faster opening. The smaller the characteristic coefficient, the greater this shift. For valves with linear characteristics, such shifts in performance lead to a reduction in the effective range of opening degrees for precise regulation; therefore, it is better to use a characteristic curve that follows a lower chord arc, such as an equal percentage characteristic. For valves with equal percentage characteristic curves, the actual operating curve may approach a linear characteristic when the valve authority is between 0.3 and 0.5. (2) Usually, when a valve is opened to a small degree, the flow velocity through the valve is too high, resulting in vigorous turbulent vortical zones behind the valve. The pressure in these vortical zones is very low; when this pressure drops below the saturation pressure corresponding to the water temperature, vaporization occurs, leading to steam hammer: severe noise, vibration of the valve and pipes, and damage to the valve, pipes, and pipe supports. To prevent such accidents, it is necessary first to consider, in the design of the valve flow channels, creating a narrow throttling passage between the valve plug and the valve seat at low opening degrees, thereby restraining the formation of intense turbulent vortices ； Secondly, when selecting valves, try to increase their valve coefficient as much as possible to avoid operating the valves at low opening degrees. Additionally, whenever pressure conditions are not a concern, it is advisable to install the alkaline balance valve on the return water pipe where the water temperature is lower.

Also, our compressors have not been operating properly; they experience many problems, mainly because the designed operating conditions differ from the actual operating conditions.

There are indeed many problems encountered at work; whether it’s related to processes, production, or equipment, these issues can all be resolved. The only thing is the length of time required to handle them and whether it affects production. The difficulty lies in the fact that it’s hard to communicate about personnel issues. All the scheming and deception among one another. Watching these people fight openly and secretly all day long for promotions and personal gains, it’s amazing they never get tired.

There are many rotating devices encountered in work; pumps are the simplest. The operation of compressors is relatively straightforward, though oil leakage poses a problem. In my environment, the most challenging device is the catalytic flue gas extractor – excessive dust in the flue gas has the greatest impact on this equipment.

In fact, some problems with rotating equipment arise from improper connections between moving and stationary components. Last week, when I went to a customer’s factory to repair equipment, I encountered such an issue: it was specified at the time of installation that flexible bellows connections should be used, but he opted for rigid connections to save time; as a result, the outlet of our company’s pump was severely damaged! Furthermore, since the hard connections are difficult to assemble and disassemble, proper maintenance cannot be carried out, and the interior gets blocked as a result of resin curing.

1. The old centrifugal pumps experience significant vibration, and this issue cannot be resolved. 2. There are many leaks in the lubricating oil of the machinery. 3. New employees receive only short training periods; monitoring the operating equipment and handling problems requires continuous learning. 4. Some of the equipment is outdated and has a high failure rate, but it is difficult to replace them: dizzy:

The turbines in our plant are operated with a back pressure machine and a condenser connected in series. During operation, the back pressure machine constantly exerts pressure on the main thrust bearings; sometimes, even when the process parameters, bearing temperatures, vibrations, and oil pressure are all normal, the system shuts down suddenly due to shaft displacement. Upon inspection, it was found that the babbitt material in the main thrust bearings of the back pressure machine had disappeared. We increased the diameter of the balance pipe by one size and added a drain at the lowest point, with exhaust and drainage valves that remain open at all times; yet the problem still persists.

We have two air separation compressors. Due to electrical issues, when one compressor is started and the second one is attempted to be started, it causes a power outage throughout the entire factory. When the first compressor stops operating, the oil pump also stops, and the circulating water ceases to flow, which is very harmful to the compressors