Filters and valves
HMA plant owners need to be informed about filters and valves to ensure that they get the ones most appropriate for their asphalt heating and storage system. This is especially true if all of the piping is to be fabricated at the job site by an independent contractor. Some of the factors that may affect their choice are as follows: the type of material being controlled, its temperature, protection from damage, maintenance requirements, how often asphalt samples are taken, etc.
A strainer is a type of filter used to remove debris or foreign material from hot oil, liquid asphalt and heavy fuel. One strainer should be used in the hot oil line to protect hot oil pump(s). Another should be used ahead of the unloading pump to protect it. And another should be used ahead of the metering system to protect it. Where a heavy fuel preheater is used, a filter should be used ahead of its pump.
Strainers have a filter basket, either of perforated metal or wire mesh. A common mesh opening is 3/64-inch for hot oil and 9/64 for asphalt. A Y-strainer is the type most commonly used (Figure 48). In order to replace or clean the basket of a Y-strainer used in a hot oil line it is necessary to shut down the heater and to shut off the line to the filter.
Figure 48. Y-Strainer Is Commonly Used, But Requires Shut Down For Maintenance.
A duplex strainer is commonly used where a heavy fuel preheater is installed (Figure 49). It has two independent strainers and a valve that allows switching from one to the other. One can be serviced while the other operates, eliminating the need to shut down the system.
Sock filters are very effective and are highly recommended for use with hot oil heaters to protect the hot oil pump (Figure 50). They are especially recommended for new installations and when replacing an old heater with a new one. It is appropriate to use a sock filter in addition to a strainer in the hot oil line. The extra protection is worthwhile. The filter is long and shaped somewhat like a sock. It is made of a fine fabric mesh which collects dirt and debris from the hot oil flowing through it.
Figure 49. Duplex Strainer For Heavy Fuel Eliminates Need To Shut Down.
Figure 50. Sock Filter Protects Hot Oil Pump.
A sampling valve is a special type of valve that enables an operator to easily take samples of liquid from a tank (Figure 51). One should be installed in each asphalt storage tank, especially when asphalt samples must be taken frequently. In a horizontal asphalt tank the valve should be installed in one end, about 24 inches above the bottom. In a vertical asphalt tank it should be installed in the side, about the same height above the bottom. The valves are usually manually operated and feature a screw stem operated by a hand crank. They are virtually clog-free because of the way the valve is designed and mounted in the tank. (They do not need a hot oil jacket.) Moreover, they are usually leak-free and provide free-flow.
Figure 51. Sampling Valve Facilitates Frequent Asphalt Sampling.
Gate valves and other types of valves are sometimes used for taking asphalt samples, even though such valves are not designed for sampling. They often clog and require use of a torch to heat them and their connections to get the material to flow. But valves specifically designed for sampling virtually eliminate the hazards and difficulty associated with valves not designed for that purpose.
Plug valves are commonly used in piping of hot oil heaters and asphalt storage tanks. They can be used in lines for hot oil, asphalt, or fuel oil. They are not recommended for gas. Two-way versions of the valve can be used to either shut off or regulate flow. Three-way versions can be used to switch flow from one circuit to another. When used for asphalt, the valve should have a hot oil jacket. Plug valves use a slotted cylindrical plug that rotates 90 degrees to control the flow. An actuator, such as a pneumatic cylinder, can be used to operate the valve through its 90 degree stroke. Plug valves can also be used in hydraulic and pneumatic circuits.
Gate valves are sometimes used in piping of hot oil heaters and asphalt storage tanks. They can be used in lines for hot oil, asphalt, fuel oil, or gas to shut off flow. A gate, similar to a guillotine, moves to close passage through the valve as it is actuated. However, gate valves are not recommended for use in lines carrying liquid that normally contains foreign matter or debris that can accumulate in their grooved seats and prevent complete shut off. When used for asphalt, the valve should have a hot oil jacket. Most gate valves are manually operated, requiring multiple turns of a wheel on a threaded shaft. Pneumatic or hydraulic actuating cylinders cannot be used to operate a gate valve.
Globe valves can be used in piping for hot oil heaters. They are used in lines for hot oil, fuel oil or gas to regulate flow. They are not recommended for piping carrying liquid asphalt. The valve has a globe-shaped plug that moves to restrict the flow as its spindle is actuated. Most globe valves are manually operated, requiring multiple turns of a wheel on a threaded shaft. A variety of configurations and plug shapes are available. Some plugs are conical-shaped. Others are parabolic-shaped. Pneumatic or hydraulic actuating cylinders cannot be used to operate a globe valve.
Check valves are commonly used in fuel oil and hot oil piping of heaters, asphalt storage tanks and hot mix storage silos. A check valve allows liquid or gas to flow only in one direction. A hinged gate in the valve automatically unseats and opens when pressure is applied to one side of it. The gate automatically seats and remains closed when pressure is applied to its opposite side. Some check valves use a spring-loaded ball instead of a hinged gate.
Ball valves are often used in piping of hot oil heaters and asphalt storage tanks. They are used in lines for hot oil, asphalt, fuel oil, or gas. Two-way versions of the valve are often used to either shut off or regulate flow. Three-way versions are sometimes used to switch flow from one circuit to another. All use a slotted ball that rotates 90 degrees to control the flow. An actuator, such as a pneumatic cylinder, can be used to operate the valve through its 90 degree stroke.
Butterfly valves are used in the fuel supply lines of modulating burners that operate on gas. The valve has a hinged gate that is controlled by a modulating motor. The valve controls the firing rate or modulation of the burner in response to the temperature sensors and controls. A butterfly valve cannot be used to totally shut off the fuel supply to the burner.
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