Asphalt heating and storage systems at HMA plants require a variety of pumps. They are used to pump asphalt, hot oil and fuel. There are several issues concerning pumps.
Two asphalt pumps are usually required at all HMA plants. One is used to unload asphalt from delivery trucks. It is known as an unloading pump (Figure 52). The other is used to supply asphalt from the storage tank to the drum mixer or batch plant. It is known as a supply pump. Plants with the asphalt metering system known as “a pump pushing a pump” has another pump, which is used passively for metering (Figure 53). Asphalt pumps should be carefully selected to avoid maintenance problems and down time.
Figure 52. Typical 3-Inch Asphalt Unloading Pump.
Figure 53. Metering Package Using “A Pump Pushing A Pump”.
The positive-displacement pump is the industry standard for pumping asphalt. Only those jacketed for hot oil heating should be used. Full-jackets are strongly recommended. In any case, the packing boxes should be heated. Otherwise, the packing will be short-lived and leaking will be a constant problem.
An asphalt unloading pump is used to transfer liquid asphalt from the tank of a delivery truck to a storage tank at a HMA facility. On portable plants the pump is usually mounted on the gooseneck of a portable storage tank. On relocatable plants the pump is sometimes mounted on the same skid as the hot oil heater. On stationary plants the pump is usually independently mounted at a location that will conveniently serve two or more storage tanks.
Unloading pumps used usually range in size from 3 to 5 inches, providing 200 to 450 gpm respectively. The time it takes to unload tanker trucks delivering liquid asphalt cement to a HMA facility depends upon the size of the unloading pump and connecting lines.
A 3-inch pump unloads 200 gpm and takes 30 minutes to unload a tanker with 6,000 gallons of liquid asphalt cement. But a 5-inch pump unloads 450 gpm and takes only 13.3 minutes to unload the same tanker. Thus, a 5-inch pumping system saves 16.7 minutes per truck load.
A facility that runs 150,000 tons of hot mix a year using 5 percent liquid AC can save about $4400 by switching from a 3-inch pump to a 5-inch pump. This is based on a savings of $13.88 per truck load, 320 truck loads and a trucking cost of $50.00 per hour.
But unloading time may not be a concern if you do not operate your own delivery trucks. And if the delivery truck has a 3-inch unloading line there is no advantage to having a 5-inch pump.
Supply pumps range in size from 2 to 5 inches, and are usually picked according to the rating of the drum mixer or batch plant. The size used most frequently is a 2-1/2 inch pump. Modified asphalt needs larger pumps and lines than used for virgin asphalts because it has a higher viscosity.
A problem can occur with a supply pump when its inlet is higher than the outlet of the storage tank. This is a condition that sometimes occurs with underground storage tanks. This makes the pump lift the liquid to its inlet before it can be pumped. Lifting the liquid creates a negative pressure or a vacuum at the inlet of the pump. Pumps should not be subjected to a vacuum in excess of 15” Hg (inches of mercury). Excessive vacuum will cause cavitation, which can severely damage the pump if allowed to continue. The solution is to either place the pump at a lower level or place the storage tank at a higher level.
Hot oil pumps
A potential problem at virtually every asphalt plant is oil leaks from the hot oil pump. The high operating temperatures (about 400 degrees F) takes its toll on pumps. Gear pumps often leak soon after the pump is put into service, regardless of the type of seal or packing used. For years leaking pumps were believed to be unavoidable and were accepted as a routine maintenance problem.
But now environmental concerns have given more importance to solving the problem. Leaking oil must be contained so it cannot contaminate the soil and ground water. The federal government has put strict limits on the amount of oil allowed to spill on the ground. Oil leaks also need to be eliminated because they are a fire hazard. So, preventing oil leaks is an important goal.
Heatec has switched from gear pumps to centrifugal pumps as a solution to solving this problem (Figure 54). Our field trials revealed that seals in certain centrifugal pumps can go for years without leaking. The centrifugal pump Heatec uses has other advantages. It runs much quieter. It is air-cooled and runs cooler. And when seals wear to the extent that they leak, there is only a small leakage, which gradually increases over a period of time. Thus, seals can be replaced at a convenient time before maximum leakage occurs.
Figure 54. Centrifugal Pump Minimizes Hot Oil Leaks.
The use of a centrifugal pump brings on special design considerations. The pump is more sensitive to pressure variations at its inlet. So, if the system is not designed to limit these variations, the pump may not achieve its rated performance. Moreover, inadequate flow at its inlet can cause the pump to cavitate, thereby damaging the pump. The system must also be designed to avoid excessive starting loads and overloads.
← previous page next page →