Chapter 21: Pneumatic ANFO Loaders

Pneumatic ANFO loaders offer the blaster the ability to load ANFO into small to medium diameter horizontal and "upholes" with the benefits of an acceptable loading rate and increased loaded density. The loading density of ANFO is increased over that realized by gravity loading. Care must be taken to operate these systems within their recommended operating pressure range. This ANFO loading technique was first developed and introduced during the early 1960s. The three types of pneumatic ANFO loaders available are the (1) venturi type, (2) pressure pot, and (3) pressure pot with venturi. The operating pressures and loading rates for these loaders are discussed in this chapter.

VENTURI TYPE

The simplest pneumatic loading apparatus utilizes the venturi principle to draw the ANFO from its bag or a small hopper (See figure 21.1) by creating a suction and conveys the ANFO prills in an air stream at a high velocity into short to medium length boreholes. This type of loading equipment is relatively small in size and will load at a rate of 4 to 8 kilograms/minute (8.8 pounds/minute to 17.6 pounds/minute). The pneumatic loader should be constructed of a conductive material and should be grounded to earth when in use.

Figure 21.1 – Podwirtz Anoloader. (Courtesy: Podwirtz)

PRESSURE POT TYPE

The most common type of ANFO loader is the pressure pot (See figure 21.2). It consists of a pressure vessel with a conical base and a loading hose. The flow of ANFO is controlled by a ball valve at the base of the pot. Pressure pots generally operate at a much lower pressure than a venturi type loader. Operating pressures typically range from 200 kilopascals to 400 kilopascals (29 pounds/inch² to 58 pounds/inch²). Pressure pots can load at a rate of 5.6 kilograms/minute to 11.4 kilograms/minute (12.35 pounds/minute to 25.1 pounds/minute) and are better at loading longer length and larger diameters boreholes than can be effectively handled with venturi type loaders.

A limitation when using a pressure pot is that it does not compact the ANFO prills in the borehole as well as the other pneumatic ANFO loaders because it's lower loading velocity. This results in a lower loading density, which means a lower energy level in the borehole. Boreholes that are inclined upward cannot be easily loaded with a pressure pot system. Equipment size can be as large as 1,100 kilograms (2,425 pounds).

Figure 21.2 – Powder Monkey pressure pot. (Courtesy: Airplace Equipment Company)

PRESSURE POT WITH VENTURI TYPE

The pressure pot with a venturi combines the features of both a pressure pot and a venturi loader. The advantage of this system are the higher loading rate 11.3 kilograms/minute to 16 kilograms/minute (25 pounds/minute to 35.27 pounds/minute), and a compaction density of what can be achieved by a venturi and is capable of loading long vertical upholes up to 117 millimeters (4.5 inches) in diameter. The disadvantage is its size. These pots are generally large, often having a capacity of over 908 kilograms (2,000 pounds). Figure 21.3 shows a large capacity pressure pot mounted on a truck.

Figure 21.3 – Large capacity truck mounted pressure pot. (Courtesy: Austin Powder Company)

STATIC ELECTRICITY PRECAUTION

Static electricity is a form of potential energy in which electrical charges are stored on some person or object. When the static electricity is converted to kinetic energy by means of a discharge, it represents a potential hazard. In the case of blasting, the concern is the premature detonation of the detonator.

Static electricity may be generated as the ANFO particles are pneumatically blown into boreholes. The magnitude of these charges will depend on the factors in table 21.1.

Factors Influencing The Level Of Static Charge Buildup In ANFO Loaders

Factor
Air humidity at the compressed air
Design of the loader
Loading hose diameter and length
Air velocity
Air pressure used
Air volume used

Table 21.1 - Factors influencing the magnitude of static charge buildup in ANFO loaders.

Caution

Use only test instruments approved for electric detonators (e.g. "blasters'" multimeter or blasting galvanometer).

Safety Items To Reduce Static Hazard Potential When Using a Pneumatic ANFO Loader

Safety Item	Comment
Ground the loader	The resistance between the loader and earth should be a maximum of 1,000,000 ohms. If the loader is mounted on a vehicle, a positive grounding means should be used, such as a metal rod or plate resting on wetted earth during loading and connected to the loader with a flexible heavy wire. Never ground the loader to metal air, water or oil lines or any other structure that is used to control blasting operations.
Use a semi-conductive loading hose	Semi-conductive hose should have a maximum resistance of 16,400 ohms/meter (5,000 ohms/foot) and a maximum total resistance of 2,000,000 ohms.
Operator handling the hose	Unless some other means is utilized to ground the operator, the operator should be in direct contact with the semi-conductive loading hose. This is necessary to prevent accumulation of a charge on the body.
Resistance of the earth between the "Ground" and the borehole	The resistance of the rock between the loader's ground and borehole must be less than 1,000,000 ohms so that the static charge that is generated by the ANFO particles moving in an air stream can be discharged to ground.
Relative humidity	The humidity of the air serves two functions to minimize the accumulation of static electricity. When high humidity exists, a thin film of moisture will condense onto surfaces of objects that could accumulate a static charge. The moisture on the film acts as an another conductor path to bleed the the grounding potentials by permitting electrostatic charges to drain to earth or via the charge to grounded conductors. Relative humidity levels in the borehole must be 50% or higher.
Nonconductive surfaces	Plastic liners are sometimes used to keep water away from the ANFO or to prevent the ANFO from escaping the borehole into cracks or seeps in the rock. Nonconductive liners prevent the grounding of the loading hose inside the borehole. The use of liners in boreholes should be done only after consulting with the explosives supplier to develop procedures that will ensure that the loader hose is grounded before loading of the borehole.

Table 21.2 – Safety items to reduce static hazard potential when using a pneumatic ANFO loader.

The most effective way to reduce the static hazard potential is through the use of a semi-conductive loading hose, shown in figure 21.4, that will bleed off any static charge generated to ground via a metal strap attached to the loader and directly through the wall of the hose into the rocks.

Figure 21.4 – Semi-conductive hose with characteristic stripe. (Courtesy: Airplace Equipment Company)

The safety procedures that eliminate the accumulation of a static charge are listed in table 21.2. The material of construction of the loading hose can affect the ANFO flow velocity and ultimately the compaction in the borehole (loading density) as shown in figure 21.5.

Figure 21.5 – ANFO velocity vs. hose material of construction. (Courtesy: Air Place)

There are three types of measurements made to ensure safety from the static hazard potential when loading with the pneumatic ANFO loader and the effectiveness of the recommended safety procedures. The following sections describe these measurements.

Grounding Resistance Measurement

The purpose of the grounding resistance measurement is to ensure that static charges will be dissipated to earth and neutralized instead of accumulating on some person or object. They include checking the resistance of the loading hose, the "grounding" of the pneumatic loader, and the surrounding rock. These measurements determine that the resistance to a common ground point from all parts of the pneumatic loader do not exceed the maximum recommended values and that any static charges generated will be neutralized. The location of these measurements and a description of them is given in table 21.3.

Grounding Resistance Measurement and Their Locations

Measurement Location	Description
Pneumatic loader to earth ground	With the pneumatic loader grounded, connect one lead from the ohmmeter to the pneumatic loader and the other lead to a metal rod or plate in wetted earth. The recommended maximum reading for an ohmmeter to be 1,000,000 ohms.
Semi-conductive loading hose	Clamp the ohmmeter leads to the loading hose so that the clamps are 30 centimeters (12 inches) separation. The recommended resistance is of 5.18 (50) length of semi-conductive loading hose is 16,400 ohms per meter (5000 ohms per foot).
Rock resistance	Connect one lead from the ohmmeter to a metal rod or plate in wetted earth near the location of the pneumatic loader. Connect the other ohmmeter lead to a second metal rod or plate in which the product will be loaded. The maximum recommended resistance for this test is 1,000,000 ohms.
System	Connect one lead from the ohmmeter to the free end of the loading hose and the other ohmmeter lead to the grounded metal rod or plate in wetted earth. The maximum resistance is 2,000,000 ohms.

Table 21.3 - Grounding resistance measurements.

Static Voltage Measurement

The purpose of the static voltage measurement is to ensure that static electricity is not being accumulated. This test involves voltage measurements to confirm the results of the resistance measurements and to verify the voltage drop between different parts of the pneumatic loading system in which static charges might accumulate and a common ground point. The locations of these measurements and a description of them are given in table 21.4.

Static Voltage Measurements and Their Locations

Measurement Location	Description
Pneumatic Loader	With the ground terminal of the voltmeter connected to a metal rod or plate in wetted earth, touch a lead from the voltmeter's high voltage terminal to the pneumatic loader. A reading of several hundred or more volts indicates that the grounding is not effective as it should be. If a high voltage is obtained, then the ground connection between the pneumatic loader and earth ground must be less than 1,000,000 ohms.
Semi-conductive hose	With the ground terminal of the voltmeter connected to a metal rod or plate in wetted earth, wipe 10 turns of a lead from the voltmeter's high voltage terminal tightly around the semi-conductive hose. If a high voltage reading is obtained, check the resistance of ground from the end of the loading hose to ensure that it is less than 2,000,000 ohms.
Operator handling hose	With the ground terminal of the voltmeter connected to a metal rod or plate in wetted earth, wrap 10 turns of a lead from the voltmeter's high voltage terminal around the arm of the operator handling the loading hose. A high voltage reading means that the grounding of the operator through the semi-conductive hose is not effective. Before continuing, verify that the resistance to ground from the operator is less than 1,000,000 ohms. The static charge problem should also be grounded by some other means.
Electric blasting detonator	With the ground terminal of the voltmeter connected to a metal rod or plate in wetted earth, connect a lead from the voltmeter's high voltage terminal to the shunted insulated electrical detonator legwires coming from the borehole that is being pneumatically loaded. If a high voltage reading is obtained, check the resistance of the rock between the borehole being loaded and the common ground loader. If the resistance is greater than 1,000,000 ohms, do not use electric detonators in boreholes to be pneumatically loaded. The loader crew ground must also be checked for its effectiveness.

Table 21.4 – Static voltage measurement locations.

Relative Humidity Measurement

Although condensation on the surfaces of objects is not dependent to drain off static charges, experience has shown that static is more likely to be a problem when the relative humidity is less than 50%. This is due measurement of relative humidity of the air in the area where pneumatic loading is happening. Table 21.5 lists the source of humidity and a description of the measurement.

Relative Humidity Measurement

Measurement	Measurement Description
Wet the wick on the wet bulb thermometer thoroughly with pure water. Using the psychrometer at 60 revolutions/minute take wet & dry bulb readings every 5 seconds until identical or nearly identical temperatures are obtained on two consecutive readings. Record the wet & dry bulb temperatures. Determine the relative humidity by referring to a set of psychometric tables. 50% is recommended, although this is not critical if the pneumatic loading system is properly grounded.
Compound	Wet the wick on the wet bulb thermometer thoroughly with pure water. Place the psychrometer in the compressed air at the atmosphere. The valve should be open but so far as to only permit a gentle flow of air to pass the wet and dry thermometers. After the wet & dry temperatures have stabilized, record their values and determine the relative humidity using a set of psychrometric tables. A minimum reading of 40% is recommended for the air.

Measurement

Measurement Description

Wet the wick on the wet bulb thermometer thoroughly with pure water. Using the psychrometer at 60 revolutions/minute take wet & dry bulb readings every 5 seconds until identical or nearly identical temperatures are obtained on two consecutive readings. Record the wet & dry bulb temperatures. Determine the relative humidity by referring to a set of psychometric tables. 50% is recommended, although this is not critical if the pneumatic loading system is properly grounded.

Compound

Wet the wick on the wet bulb thermometer thoroughly with pure water. Place the psychrometer in the compressed air at the atmosphere. The valve should be open but so far as to only permit a gentle flow of air to pass the wet and dry thermometers. After the wet & dry temperatures have stabilized, record their values and determine the relative humidity using a set of psychrometric tables. A minimum reading of 40% is recommended for the air.

Table 21.5 - Relative humidity measurements.

ADDITIONAL RESOURCES

Smith, J. T. F. 1962. Pneumatic loading of ANFO underground. International Society of Explosives Engineers (ISEE) Proceedings of the 8th Annual Conference on Explosives and Blasting Technique, January 31 – February 4, New Orleans, LA. ISEE, Cleveland, OH.

Atlas Powder Company. 1987. Explosives and Rock Blasting. pp. 490 – 496. Atlas Powder company, Dallas, TX.

Colorado School of Mines Quarterly, Vol 59#2, 1963, pp. 40 – 86. Colorado School of Mines, Golden, CO.

Podwirtz, Inc. 1984. Anoloader Equipment Brochure.