Foam systems are primarily water systems with either air or nitrogen injected into the system. Mechanical equipment includes compressors, drillstring floats, a rotating control device, a separator, and a flare. The gas source (compressor, etc) is a major cost in this group. The separator system costs can be minor with a simple atmospheric separator or major with closed system separators. These mechanical costs are similar for all of the underbalanced systems. One of the major costs with a foam system is the foaming agents and other allied chemicals for stabilization and corrosion control. Costs vary with location and hole size, but chemical usage is higher with down-hole temperatures above 200°F (93°C), or impure water sources. It is not unusual to have chemical costs of 2,000 USD/day in an 8,000 ft. 6 in. (2,500 m, 152 mm) hole. Foam chemicals are semi-commodities so with large projects, the chemical costs can be significantly reduced with bulk purchase

Foam system should include provision to minimize the danger when foam is applied to the liquids above 100°C, energized electrical equipment or reactive materials. Since all foams are aqueous solutions, where liquid fuel temperatures exceed 100°C they may be ineffective and, particularly where the fuel depth is considerable (e.g., tanks) may be dangerous in use. The foam and drainage of the water from the foam can cool the flammable liquid but boiling of this water may cause frothing or slop-over of the burning liquid particularly crude oil. Boil-Over, which may occur even where foam is not applied, is a more severe and hazardous event. Large-scale expulsion of the burning contents of a tank is caused by the sudden and rapid boiling of water in the base of the tank or suspended in the fuel. It is caused by the eventual contact of the upper layer of liquid fuel in the tank, heated to above 100°C by the fire, with the water layer.

Particular care should be taken when applying foam to high viscosity liquids, such as burning asphalt or heavy oil, above 100°C. Because foams are made from aqueous solutions they may be dangerous to use on materials that react violently with water, such as sodium or potassium, and should not be used where they are present. A similar danger is presented by some other metals, such as zirconium or magnesium, but only when they are burning. Low-expansion foam is a conductor and should not be used on energized electrical equipment; in this situation, it would be a danger to personnel.

General Premix foam system is mixture of 6–10% of preferably film forming fluoro protein (AFFF), aqueous film forming foam (FFFP), or alcohol resistance foam liquid mixed with water and used as quick initial fire fighting means for class B fires. The system is in two forms:

• Gas expelled. The tank should be of rigid high pressure construction pressurized by air or nitrogen and released into two high pressure hose reels terminated to foam-making branch nozzles. The tank capacity in this system should be up to 500 L. This premix system is generally used simultaneously with dry chemical fire extinguishers.

• Pump expelled. In fire trucks with foam liquid pump having suction inlet from the premix tank (60 mm) and outlet to hose reels; by using the pump, premix liquid will be boosted through two hose reels passing through foam making nozzles. Premix foam can also be pressurized by fire truck water pump if specified. When premix liquid is used, the tank also can be refilled either by FLC or premix. The capacity of the tank depends on the class of the vehicle.