Home Log In / Sign Up Contact Us

A Comparison Between Intensifier and Crank Drive Pumps

Dr. John H. Olsen

As different as the two technologies seem, intensifier and crank drive pumps share the same pumping principle: A plunger is pushed into a closed chamber to raise pressure and expel fluid through an outlet check valve; as the direction of the plunger is reversed, low pressure fluid enters the chamber through an inlet check valve. In both cases the continuously reciprocating plunger provides the pumping action (see figure 1). The difference between the two technologies is the means by which the plunger is moved. The crank pump uses a crank similar to the one in an automobile engine; the intensifier drives the plunger with a hydraulic cylinder, usually with oil.


Figure 1

Pump Efficiencies

At high pressures, liquid is compressible. At 40,000 psi, water is about 10 percent compressible. That means the plunger must move enough to fill 10 percent of the chamber volume before the pressure reaches 40,000 psi. At that point the outlet check will open against the pressure in the output line. At the end of the stroke, when the plunger reverses and the outlet check closes, any water that is trapped in the cylinder expands, pushing on the plunger until the plunger has moved far enough to drop the pressure to the inlet pressure and open the inlet check valve. The energy put into the plunger motion by this expanding water can be recovered or not depending upon the drive type.

In the crank pump this expansion energy is recovered in the same way that it is recovered from the expanding hot gasses in an internal combustion engine: it goes back into the kinetic energy of the rotating components. In the intensifier pump it gets dumped into the oil of the hydraulic circuit, which causes heating. That heat must then be removed, usually by an oil to water heat exchanger. As a result, intensifiers operate at about 70 percent efficiency whereas crank drive pumps operate at efficiencies of 95 percent and higher. Intensifiers also require extra water for cooling purposes, resulting in significantly higher electric and water costs to pay for energy wasted as heat.

Power Delivered to the Nozzle

While it is possible to put an excessively large electric motor on either type of pump, the amount of power drawn from the motor depends solely on nozzle size and operating pressure. Two identically-sized nozzles that operate at the same pressure and flow the same amount of water will draw also the same power from the pump. Put another way, a 50 horsepower motor gives no benefit whatsoever in running a nozzle sized for drawing 20 or 25 horsepower. In fact, in locations where the electric bill is based in part on either the current drawn or potential load (rather than just the energy consumed), the user must pay extra for the unused capacity of the larger motor.

Pump Size and Parts Costs

Other differences between the two pump types arise from the relative operating speeds of the plunger. Crank drive plunger speeds are about 30 inches per second, while intensifier plunger speeds are only about 6 inches per second. For comparable output flows, the intensifier plungers, cylinders, and check valves must be larger (and therefore more expensive) than corresponding crank drive parts. Overall, a hydraulic system is much more expensive and complex than a crank; initial costs and parts maintenance costs are significantly lower for the crank drive pump.

The Pressure Ripple Effect

Because of its low plunger speed, the intensifier pump delivers one or two large discharges per second, whereas the crank drive pump delivers 30 small discharges per second. Thus the pressure output of the crank pump is very smooth, eliminating the need for an accumulator. The crank drive pump does not produce defects from pressure ripple, nor does it require a large accumulator vessel that can cause a safety concern. Even with an accumulator, each shift of the intensifier features a pressure dip of about 2000 to 5000 psi. In order to achieve comparable cutting quality, the intensifier must run at a pressure 2000 to 5000 psi higher than the crank drive.

Pressure Control

While the two pumps are comparable in the area of pressure control, each goes about the job differently. The intensifier's output pressure is controlled by varying the stroke (hence flow) of the hydraulic pump. The crank drive output pressure is controlled by varying the RPM of the electric motor through a variable speed drive. The intensifier has a quicker response to load changes and can be used to run independent nozzles turning on and off at random. The direct drive can also run multiple nozzles, but they must be turned on and off simultaneously.

Noise Reduction

A crank drive pump operating at about 600 RPM generates far less noise than the hydraulic system of an intensifier. Quiet intensifier pumps are possible only by providing costly sound control measures.

Maintenance Issues

There are quite a few mechanics who can understand and work on crank drive pumps because of their simplicity and close similarity to internal combustion engines. On the other hand, technicians familiar with hydraulic pumps, valves, filters, pressure controls, and heat exchangers are far more rare and unlikely to be found in an ordinary machine shop.

Equality through Modern Technology

Throughout the 1970s and 1980s, crank drive pumps dominated the market for pressures 20,000 and below because of their low cost and reliable operation. Intensifiers were used for 30,000 and above because, at that time, all pumps were plagued by three problems that favored the slow operations of intensifiers at the higher pressures: metal fatigue, check valve wear, and seal life.

Metal fatigue refers to failure of metal from repeated loading and unloading that causes initiation and propagation of cracks. The life of any given component depends on what material it is made from, the stress levels it experiences during operation, and the number of cycles of load applied. For steels, a stress level just below that which causes failure at 10,000,000 cycles will never cause failure. An intensifier achieves 10,000,000 cycles in about 3000 hours; a crank drive pump reaches the same level in about 300 hours. However, by using modern materials and stress control techniques, both types of pump can be designed for infinite fatigue life at pressures below 55,000 psi. Therefore, metal fatigue is no longer an issue limiting crank pumps.

Check valve wear is another problem solved by modern technology. Metal seats wear as a result of adhesive wear - a condition where metal particles transfer from one surface to another. Wear life depends upon the number of open and close cycles and operating pressure. Modern ceramics have the strength necessary for check valve components. In addition, adhesive wear between metal and ceramics is now so low that check valve life is virtually limitless on crank drive pumps.

Although seal life formerly limited crank drive pumps to pressures in the low 40,000 range, continuing advances in this area have allowed the pressure range of crank drive pumps to rise. Today crank drive pumps successfully operate at 55,000 psi with dynamic seal life of 500 hours or more.

Comparison Summary

Feature	Crank	Intensifier
Parts Costs	least
Pressure Ripple	least
Initial Cost	least
Electric Power Costs	least
Maintenance Simplicity	easiest
Noise	least
Service Interval	comparable	longest
Pressure Control	same	same
55 Ksi Operation	yes	yes