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The steam produced by hot water boilers is widely used in industrial applications. Industrial processes such as drying, mechanical work, power generation and process heating are typical steam applications. The steam valve is used to reduce the inlet steam pressure and to precisely adjust and control the steam and temperature supplied to these processes.
Unlike most other industrial process fluids, steam has specific characteristics, making it difficult to control with valves. These characteristics can be its high volume and temperature as well as its condensing capacity, which can quickly reduce the volume by more than a thousand times. If you use the valve as a process control tool, there are several considerations when using steam.
The following are the 7 most serious mistakes in valve applications that you must not make when using steam. This list does not cover all precautions for steam valve control. It describes common operations that often result in damage or unsafe conditions when trying to regulate steam.
Everyone knows that steam will condense, but when discussing the process control of steam pipelines, this obvious feature of steam is often forgotten. Most people think that the production line is always in high temperature and gaseous state, and the valve is designed for this.
However, the steam line does not always run continuously, so it will cool and condense. And condensation is accompanied by a significant reduction in volume. Although steam traps effectively treat condensed steam, the valve operation on the steam line must be designed to treat liquid water, which is usually a mixture of liquid and gas.
When steam forces incompressible water to accelerate suddenly and is blocked by valves or fittings, water hammer will occur in steam pipes. Water can move at high speed, causing noise and pipe movement in mild cases, or explosive effects in severe cases, causing damage to pipes or equipment. When operating with steam, the valve on the process pipeline should be opened or closed slowly to prevent sudden bursting of the fluid.
Valves designed for steam applications must operate under the design conditions of pressure and temperature. The steam quickly expands to a large volume. A 20 K increase in temperature will double the pressure in the valve, which may not be designed for such pressures. The valve must be designed for the worst case (maximum pressure and temperature) in the system.
A common mistake in valve specification and selection is the wrong type of valve for steam applications. Most valve types can be used in steam applications. However, they provide different functions and controls. Ball valves or gate valves provide precise flow control, which is more achievable than butterfly valves. Due to the large flow rate, this difference is critical in steam applications. Other types of valves that are common in steam applications are gate valves and diaphragm valves.
A similar error in the choice of valve type is the choice of actuator type. The actuator is used to open and close the valve remotely. Although an on/off actuator may be sufficient in some applications, most steam applications require adjusting the actuator to precisely control pressure, temperature, and volume.
Before selecting a valve for steam applications, take some time to estimate the expected pressure drop across the valve. The 1.25-inch valve can reduce the upstream pressure from 145 psi to 72.5 psi, while the 2-inch valve on the same process stream will reduce the 145 psi upstream pressure to only 137.7 psi.
Although using smaller valves is cost-effective and tempting, especially when sufficient, they are unfortunately prone to noise. They are also related to vibration that reduces the life of valves and pipe fittings. Consider a larger-than-required valve to manage noise and vibration. The steam valve also has a special noise reduction device.
Another error in valve sizing is the one-stage reduction in pressure. It causes the high steam velocity at the valve outlet to wear the surface in a process called erosion. If the supply steam pressure is several orders of magnitude higher than the local requirement, please consider reducing the pressure in two or more stages.
The last point of valve size is the critical pressure. This is the point where a further increase in upstream pressure will not increase the steam flow through the valve. It indicates that the valve is too small for the required process application. Keep in mind that the size of the valve should not be too large to avoid “swing”, which can happen when a slight change in valve position causes a significant change in the control function, especially under partial load.
The design of steam valves and their processes can be tricky. The specifications for handling the volume differences between water and steam, condensation, water hammer, and noise can be confusing. Many people make these common mistakes when designing a steam system, especially on the first try. After all, making mistakes is a natural part of learning. Knowing the information fully can help you avoid errors that can lead to increased costs and downtime for steam applications.
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Post time: Oct-08-2021
