Steam is used in a large number of industrial applications. It is used for powering turbines to generate electricity and for food, chemical and other product processing. Steam is also used for cleaning, heating, and humidification. Most applications require some control, and steam control valves become an important part of the process. The major application of valves with steam is to reduce the pressure of the inlet steam for process applications. In addition to controlling pressure, a steam valve can control temperature. Industrial processes frequently use steam at lower pressures.
Since steam at low pressure has higher latent heat, (which increases energy efficiency) and steam pressure and temperature are related, temperature is controlled through the control of steam pressure. Lower steam pressure will also provide improved plant safety.
Steam is sometimes considered antiquated due to its connection to steam locomotives and early industrial factories, but it remains a useful conveyor of heat energy. Steam can be produced by any number of methods, many of which are considered environmentally sound.
Though most types of steam valves can control steam flow, special service conditions exist with steam regarding temperature and pressure. The most commonly used steam valves are listed here.
· Ball steam valves provide tight shut-off and predictable control. They have a high range of use due to the design of the regulating element, without the complications of side loads typical of butterfly or globe valves.
Advantages include ease of operation and high flow, high pressure, and high-temperature capabilities.
Disadvantages include low cleanliness and an inability to handle slurries.
· Butterfly steam valves control the flow of steam with a disk which turns on a diametrical axis inside a pipe, or by two semi-circular plates hinged on a common spindle, permitting flow in only one direction. They are used as throttling valves to control flow. Butterfly steam valves offer a rotary stem movement of 90 degrees or less in a compact design. Unlike ball valves, butterfly steam valves do not have pockets in which fluids and gas may become trapped when the steam valve is closed.
· Globe steam valves are linear motion valves with rounded bodies, from which their name is derived. They are widely used in industry to regulate fluid flow in both on/off and throttling service.
Advantages include precise throttling and control, as well as high-pressure limits.
Disadvantages include poor cleanliness.
· Gate or knife steam valves are linear motion valves in which a flat closure element slides into the flow stream to provide shut-off. Gate steam valves are usually divided into two types: parallel and wedge-shaped.
Parallel gate valves use a flat disc gate between two parallel seats (upstream and downstream).
Knife valves are of this type, but with a sharp edge on the bottom of the gate. Large sizes are commonly available.
Disadvantages include pressure limitations, lack of cleanliness, and low shut-off.
· Diaphragm steam valves use a 'pinching' method to stop the valve flow using a flexible diaphragm. One of the major advantages of using diaphragm steam valves is that the valve components can be isolated from the process fluid, making them ideal for sanitary applications.
Several factors must be taken into consideration when specifying a steam valve.
Environment, pressure, temperature, connection, actuations, and other service variables must be accounted for. Actuators, connection, and valve body and component material are selected after an appropriate valve type and size are determined by desired pressure and flow requirements.
Typical connections for steam valve are threaded, flanged, or compression fitted. Materials of construction can be aluminium, brass, bronze, cast iron, copper, steel, and stainless steel.
Steam valve actuation is achieved through typical valve actuation methods such as:
· Manual devices are opened or closed via a hand-operated wheel or crank. An example of a manual mechanical valve is shown at right.
· Electric devices utilize electric actuators.
· Electric and pneumatic valves are commonly available and use solenoids or air pressure to activate the valve.
· Pneumatic valve is activated by a pneumatic actuator or cylinder to open and close valve.
· Thermally actuated valves are activated by a change in temperature in the handled media. Typically, temperature and pressure relief occurs once a present temperature is reached.
Steam is a valuable medium to supply heat to industrial and chemical processes. Steam is also used for building heating, to supply mechanical energy, to control temperatures and pressures, to remove contaminants, and drive mechanical equipment. Once conveyed from the boiler to the point of application, steam heats by direct heating (coming in contact with the product to be heated) or indirect heating through a heat exchanger.
Processing: Heating applications for steam can be found in food processing factories, refineries, and chemical plants. Saturated steam is used as the heating source for process fluid heat exchangers, reboilers, reactors, combustion air preheaters, and other types of heat transfer equipment.
Cleaning: Steam is used to clean a broad range of surfaces. High temperature steam will sterilize food preparation equipment. Steam cleaning has long been used to remove grease and dirt from industrial equipment as maintenance or in preparation for painting.
Power: Steam provides the energy to drive the turbines connected to generators in gas, coal, nuclear and certain solar plants. Both superheated and saturated steam is used in these applications.
Atomization: Steam can be used to atomize a liquid for the purpose of separating liquids. One instance is to break up fuel into small droplets to maximize combustion efficiency.
Miniaturization: Steam can add water to a process while at the same time introducing heat energy.
Heating: Many large commercial and industrial facilities use low pressure saturated steam as the predominant heat source for indoor heating. HVAC units and air handlers can use existing plant steam to humidify the air flow for indoor comfort and protection of goods and equipment.