Intensifiers

Fig. 1. Axial-piston style of hydraulic intensifier uses two separate fluid systems. Pressurized fluid applied to center piston produces force on smaller piston, pumping high pressure fluid with each stroke; meanwhile, opposite side of piston draws fluid in for use on return stroke.

Intensifiers come in a broad range of styles and shapes, and have numerous options. The models at left do not require an external power source, include an integral dump valve, have a corrosion-resistant coating, and contain no dynamic seals. Models such as shown below can boost shop air at up to 100:1 ratios, and are quite economical.

Intensifiers operate on the ratio-of-areas principle in a linear actuator. A common rod connects the pistons of two cylinders of different bore, Figure 1. Lower-pressure fluid, acting on the larger piston, exerts a force that is transferred mechanically by the rod to the smaller piston. The smaller piston generates a higher pressure in the fluid in its bore: the pressure ratio is inversely proportioned to the areas ratio.

Theoretically, there is no limit to the outlet pressure that can be achieved. Practically, the increased viscosity of oil at higher pressures imposes an upper limit, as does heat of compression. Air containing oil may diesel at higher temperatures.

An intensifier operates at a constant power level; outlet flow decreases as outlet pressure increases. This is analogous to transformers, which very voltage and amperage at the same electrical power level, as well as gear trains, which vary torque and speed at the same mechanical power level.

Nomenclature

Intensifiers come in two basic types: those that stroke or cycle once, and those that reciprocate continuously. The driving (inlet) and outlet fluids can but need not be the same. The possible combinations create a confusion of unofficial terminology.

A single-stroke intensifier, which uses air as the driving fluid and oil as the outlet fluid, is often called a booster. A reciprocating intensifier with gas as the outlet fluid might be called a booster compressor. All meet the basic intensifier definition, and their intensification principles are the same.

Single stroke or one-shot intensifiers are constructed much like standard cylinders. The two pistons are mounted on the ends of the same rod. Often, the smaller piston or ram is the rod itself. Seals around the rod separate the low- and high-pressure chambers.

The simplest one-shot intensifier construction has only one port in the high-pressure chamber. This port connects to a low-pressure fluid supply line and to a high-pressure outlet line. The supply and outlet lines are separated with check valves as shown. When the intensifier retracts, low-pressure supply fluid is drawn into the fluid compression chamber through the inlet line; when the intensifier extends, the ram forces high-pressure fluid into the outlet line. The large piston can be spring- or air-retracted; or the intensifier might be mounted vertically with low-pressure chamber underneath so the ram returns by gravity.

A slightly more complex design is another one-shot model that has separate supply and outlet ports at the high pressure end. This design minimizes the possibility of trapping air in the high-pressure fluid. The small piston area and stroke length determine the high-pressure flow volume per cycle for one-shot intensifiers. Here, the effective stroke does not begin until the ram enters the lower seal. There is no intensification during retraction, so high-pressure flow is intermittent.

Reciprocating intensifiers

Although one-shot intensifiers can be made to cycle continuously, high pressure fluid still flows only on the extension stroke. For many applications, that is satisfactory. Two one-shot reciprocating intensifiers feeding the same circuit with staggered strokes would smooth pulsations to some extent.

If the low-pressure piston is mounted centrally on a double-ended rod with a ram and high-pressure chamber at each end, the outlet piping can be combined fore more nearly continuous flow. This design also can be controlled to reciprocate continuously.

Some advanced intensifier designs incorporate an oscillating pump unit consisting of a low-pressure piston, a high-pressure piston, and a bi-stable reversing valve to convert a portion of inlet flow to a higher pressure outlet flow. They automatically compensate for consumption on the high pressure side. These units are self-contained and require no external valving except to control flow to the inlet. Dumping pressure from the high-pressure side can be done through an integral pilot-operated check valve.

Note that fluid flow and pressure will fluctuate with any of these arrangements. If continuous high-pressure fluid flow is required, a rotary pump is the answer.

Installation and design checklist

Typically, intensifier manufacturers list the intensification ratio (outlet pressure/driving or inlet pressure), outlet flow (per stroke for one-shots,and per unit time for reciprocating intensifiers), and maximum allowable pressures.

After high pressure and flow requirements are determined for an application, intensifier manufacturers can help size the intensifier for the job. Other considerations for satisfactory performance include:

proper sizing of external components in the intensifier circuit. The design inlet pressure must be available to the intensifier. Undersized lines, valves, or FRLs are bound to create problems
fluid conductors.Low-pressure lines should be sized for intended pressure and flows, or the intensifier will operate slowly. High-pressure line size is less important, unless the same lines carry low-pressure fluid during rapid advance period of a cycle. Minimal use of fittings and bends is always good design practice
control valving. Valves should be designed into the low-pressure system whenever possible to avoid restrictions and leakage on the high-pressure side
air exhausts. These should be sized to avoid back pressure. Severe exhaust restrictions may stall a continuously reciprocating intensifier
high-pressure fluid. Check with the fluid manufacturer for possible viscosity problems at the design high pressure. Special intensifier fluids are available
component location. Air entrapment is a common problem in poorly-designed intensifier circuits. If the oil reservoir is located the highest elevation and the work cylinder at the lowest, then the system will purge itself. Preferably, the intensifier should be mounted vertically with the high-pressure end pointing down. Cylinder ports should be up. If the machine configuration requires some other arrangement, air bleed valves should be designed into the high-pressure circuit
contamination. High pressure accelerates damage from contaminated fluid. Good filtration practices should be followed
reservoirs. If an intensifier circuit also uses fluid from a pump, both components should be supplied from the same reservoir. If separate reservoirs are required, they should be connected to avoid pumping one of them dry, and
intensifier protection. Particularly for testing and pressing circuits, where a part might fail and allow the intensifier to stroke unloaded, a high-pressure line restrictor will protect the intensifier from damage while stroking with no load.

Variations

There are several variations on the areas ratio principle using the general intensifier configuration. These include:

tandem. Two large, low-pressure pistons in a tandem cylinder actuate the same ram for high intensification
two-stage. A double-end intensifier has two different sized rams. Output from the larger ram is piped to the smaller one for two-stage intens-ification, and
debooster. High pressure, controlled to stroke the small piston, produces greater flow at lower pressures from the large piston chamber

Like their cylinder cousins, intensifiers are simple, rugged reliable fluid power components.For the right application, intensifiers can reduce installation costs, simplify control, lower operating costs, and provide long service life.