General guidelines for selection of a primer are:
The detonation pressure of a primer must be above the level necessary to cause the main charge to detonate at or above its normal velocity. The density and confined detonation velocity can be used as indicators of detonation pressure if detonation pressure values are not available. A primer that has a density of approximately 1.2 g/cc with a confined detonation velocity greater than 15,000 ft/s will normally be adequate when priming non-cap sensitive explosives, materials such as ANFOs, blasting agents, and most water gels. This combination of density and velocity produces a detonation pressure of about 60 kbar. For explosives such as emulsions, which detonate at higher velocities, more energetic primers will produce better results. A density of primer of 1.3 g/cc with a confined detonation velocity greater than 17,000 ft/s will be adequate to more quickly achieve the explosive’s normal velocity. This combination of density and velocity produces a detonation pressure of about 80 kbar.
The diameter of the primer should be larger than the critical diameter of the explosive used for the main column charge.
The primer must be sensitive to the initiator. A wide variety of the products are used as primers and each have different sensitivities. Some may be initiated by low energy detonating cord, while others may be insensitive to these initiators. It is important that the operator understand the sensitivity of the primer to ensure that detonation in the main column charge will properly occur.
The explosive in the primer must reach its rated velocity of detonation within the length of the cartridge. That is, the primer length should be sufficient so that the steady state velocity can be reached. If this is achieved, then additional cartridges of primer explosive serve no useful purpose.
For most blasting applications, where there is no decking, no more than two primers per blasthole are needed. The second primers, although technically not needed, is commonly used as a backup system should the first primer fail or fail to shoot the entire charge.
Boosters are used to intensify the explosive reaction at a particular location within the explosive column. Boosters are sometimes used between each cartridge of detonating explosive to ensure a detonation transfer across the ties of the cartridge, but this is normally a poor use of a booster as it is seldom needed and adds to the cost. The selection of an explosive in a cartridge that would not require a booster between each cartridge may be a more economical solution.
In general, boosters are used to put more energy into a hard layer within the rock column. They are sometimes also used to intensify the reaction around the primer, which will put more energy at the primer location. This is commonly used when primers are near the bottom of the hole, since the bottom of the hole is the hardest place to break. Using a booster at the hole bottom normally allows the increase in the burden dimension and better breakage at the toe of the shot. Boosters can be made of similar explosive materials as primers. Their sole function is to place more energy at point locations within the explosive column.
Effects of detonating cord on energy release. Cap sensitive explosives, such as dynamite, are initiated by detonating cord. Non-capsensitive explosives such as ammonium nitrate, emulsions, and water gels can be affected in many ways by detonating cord passing through the explosive column. If the detonating cord has sufficient energy, non-cap-sensitive explosives may detonate or burn. A burning reaction, rather than a detonation, releases only a fraction of the explosive’s available energy. The blast becomes underloaded because of this low energy release and it can result in ground vibration levels increasing while blastholes may vent and produce flyrock.
To prevent the main explosive charge from burning or deflagrating, detonating cord should not be not too large for the borehole diameter. Acceptable cord grain loads that are not predicted to cause deflagration are given in Table 11.
If the detonating cord is too small to cause an appropriate reaction in the explosive, it can cause the explosive to be damaged. The damage that results is called dead pressing or precompression. Dead pressing increases the explosive density causing it not to detonate. This occurs when the detonating cord is of sufficient energy to crush out the air spaces within the explosive or to break the air-filled microspheres placed in some products. These air pockets are needed to provide locations to form hot spots for detonation. The adiabatic compression of air is necessary for detonation to proceed throughout the explosive.
When the explosive is partially compressed or damaged by pre-compression, it may detonate or burn releasing only a fraction of the available energy. This effect can be confusing since the explosive may be totally consumed yet little rock breakage results. Commonly, the blaster who suffers this type of problem believes that the problem is because of hard, tough rock. To obtain a better understanding of this problem, look at the energy loss that results from passing a detonating cord though an explosive column in Figure 38.
Figure 38 (Bhushan. Konya, Lukovic. 1986) shows the energy loss for ANFO, which is damaged by detonating cord. Slurry can also suffer similar damage. Even a four-grain detonating cord can cause a significant energy loss in ANFO with approximately 38% of the useful energy is lost with as little as a four-grain cord in a 2-in. diameter blasthole.
The general recommendation is not to use any detonating cord in small diameter holes unless the holes are loaded with Dynamite.