AUTOMATIC SHUTOFF CONTROLS

Automatic shutoff controls are electrical controls that automatically shutoff unloading pumps that fill the tank with asphalt. This prevents overflowing the tank with liquid asphalt.

These controls also shut off hot oil to the tank in case the liquid level falls below the top of the heating coils. This prevents coking the heating coils in the bottom of the tank. Coking forms a layer of hardened asphalt on the coil surfaces thereby insulating the coils and reducing their ability to transfer heat.

It is very important to make sure these controls are functioning properly at seasonal startup and each month that the plant is in operation. These controls are discussed in other parts of this document.

HIGH LEVEL FLOAT SWITCH

Most asphalt tanks-both vertical and horizontal-have a high-level proximity switch on top of the tank (Fig. 6). The switch is activated by a pipe connected to a float inside the tank (Fig. 7). The pipe protrudes through a sleeve in top of the tank. When the tank is being filled and reaches its “full” level, the pipe on the float extends until it is close to the proximity switch. The switch has a magnetic sensor activated by close proximity of the metal pipe. When activated the switch opens the circuit that controls the asphalt unloading pump and shuts off the pump to prevent overflowing the tank.

Figure 6. High-level proximity switch and float pipe protruding through sleeve in top of tank.

Figure 7. Float for high-level proximity switch.

A common problem is for residues from asphalt fumes to build up on the outer surfaces of the float pipe and the inner surfaces of the sleeve it travels through. The residues can cause the float to stick and become unreliable.

To remedy a sticking problem, clean the float pipe and the sleeve it travels through. From the top of the tank pull the float pipe out as far as possible and remove all residues that you can reach. Use a brush or rag soaked with a solvent. Also work some of the solvent into the sleeve.

Two people are needed to check the proximity switch for proper operation. One person should be on top of the tank next to the switch (Fig. 6). The other person should be at the controls of the asphalt unloading pump (Fig. 15). Make sure the pump controls are set for the tank you are testing. Also make sure that the tank fill valve is open.

Now, make sure the end of the float pipe is below the proximity switch so it cannot activate the switch. Momentarily switch on the pump. The pump should operate.

Now, pull up the pipe on the float until the end of the pipe is next to the proximity switch without actually touching the switch. Switch on the pump again. The pump should not operate. Moreover, this should set off an alarm horn and cause a red light to come on at the controls of the unloading pump.

Because the float pipe intentionally has extra clearance in its sleeve, be sure to wiggle the pipe around to make sure it keeps the pump shutoff in all possible positions while the pipe is extended. If necessary, adjust the proximity switch in its mounting bracket to achieve proper operation.

Reset all controls to their normal condition.