The application and development of HPHT Hydraulic Cubic Press in China have a history of over 40 years. For a long time, single-acting intensifiers or ultra-high pressure pumps have been adopted as the intensification devices. Domestic single-acting intensifier technology is very mature and widely used by most press manufacturers. However, due to their disadvantages such as large volume, high cost, and stroke limitations, they are increasingly unable to meet the requirements of large-scale development and long-process pressure holding for HPHT Hydraulic Cubic Press. Although ultra-high pressure pumps offer the advantage of long-term intensification and pressure holding, their short service life, frequent replacement, and high installed power in practical industrial applications have limited their widespread promotion. In recent years, the developed double-acting reciprocating intensifier, capable of automatic continuous intensification, possesses unparalleled advantages over single-acting intensifiers and ultra-high pressure pumps, and has been widely promoted and applied domestically.

1. Working Principle of Double-acting Reciprocating Intensifier

A single-acting intensifier, driven by low-pressure oil, can only intensify during a single stroke, with the return stroke being a non-working state. The intensification time is limited by the stroke, thus preventing long-duration intensification. In contrast, a double-acting reciprocating intensifier, driven by low-pressure oil, can intensify during both strokes. As long as there is low-pressure oil driving it, it can intensify continuously. Figure 1illustrates the schematic diagram of a double-acting reciprocating intensifier. It is mainly composed of an intensifier cylinder, an automatic reversing valve, and four ultra-high pressure check valves.

The automatic reversing valve (a two-position four-way valve) controls the reciprocating motion of the intensifier cylinder. When the lower position of the automatic reversing valve is connected to the system, oil from the oil source enters the large chamber on the upper side of the intensifier cylinder via port P, and flows into the small chamber on the upper side of the intensifier cylinder via check valve I, generating a downward thrust. The effective action area is the cross-sectional area of the large chamber on the upper side of the intensifier cylinder. This force drives the piston downward.

The automatic reversing valve is hydraulically driven to reverse. It utilizes the unequal effective areas of the control chamber's small piston and large piston, with the oil source pressure biasing the small piston, and the large piston controlling the pressure to be either zero or the oil source pressure. This mechanism controls the reciprocating motion of the automatic reversing valve spool, changing the working position of the automatic reversing valve. As shown in Figure 1, a control oil groove is opened in the middle of the intensifier cylinder piston. When the piston moves down to the lowest end, the oil source pressure communicates with the large piston of the automatic reversing valve through the control oil groove. Although the acting pressures on the large and small pistons are equal at this time, the acting area of the large piston is greater than that of the small piston, causing the automatic reversing valve to switch to the upper position, and the intensifier cylinder begins to move upward for intensification. When the piston moves up to the highest end, the large piston of the automatic reversing valve communicates with port T through the control oil groove. At this point, the force exerted by the small piston is greater than that by the large piston, causing the automatic reversing valve to switch to the lower end, and the intensifier cylinder begins to move downward for intensification... From this, it can be seen that as long as pressurized oil is continuously supplied, the intensifier will automatically reciprocate and continuously output the intensified liquid.

Currently, double-acting reciprocating intensifiers have been successfully applied in Φ650mm cylinder diameter presses and Φ750mm cylinder diameter presses. The maximum working pressure can reach 120 MPa, and the pressure control accuracy can reach ±0.01 MPa. The intensification ratio is 7:1. The overpressure speed of the intensifier can be controlled by regulating the flow or pressure entering the intensifier.

This hydraulic circuit uses two variable pumps as power sources. The oil circuit controlling the forward and reverse movements of the six cylinders is the same as the original oil circuit. The oil circuit controlling the intensifier consists of a proportional relief valve, an electro-hydraulic directional valve, and a safety valve. The two variable pumps are combined as a large displacement variable pump and a small displacement variable pump. During overpressure, the large pump is activated (or both pumps are activated simultaneously). The electro-hydraulic directional valve is energized, and the output flow to the double-acting reciprocating intensifier is adjusted by closed-loop control of the large pump motor speed via a frequency converter. This in turn controls the overpressure speed. Alternatively, the overpressure speed can be controlled by closed-loop control through a proportional relief valve. When overpressure is complete, the electro-hydraulic directional valve is de-energized, and the double-acting reciprocating intensifier stops working. During pressure compensation, the small pump is activated, the electro-hydraulic directional valve is energized, and pressure compensation accuracy is controlled by the proportional relief valve. When pressure compensation is complete, the electro-hydraulic directional valve is de-energized, and the intensifier stops working. A safety valve is set in the circuit to ensure that the intensifier does not exceed the equipment's safe pressure in case of electrical component failure. Pressure relief can be achieved directly from high pressure using the same relief valve as an ultra-high pressure pump.

Advantages of Double-acting Reciprocating Intensifier in Cubic Press Applications As a new intensification device, the double-acting reciprocating intensifier offers unparalleled advantages over single-acting intensifiers and ultra-high pressure pumps in cubic press applications. These advantages are specifically reflected in the following aspects:

(1) Small size, light weight, low cost. The currently applied double-acting reciprocating intensifier weighs only 85kg and has dimensions of 775×150×205 (mm). Compared to single-acting intensifiers and ultra-high pressure pumps, its manufacturing cost is lower.

(2) Unrestricted overpressure time. As long as the low-pressure oil source is continuous, the double-acting reciprocating intensifier can continuously intensify, meeting the requirements for long-duration overpressure.

(3) Simple hydraulic circuit and control. Driven by low-pressure oil, it provides automatic continuous intensification. Control over the double-acting reciprocating intensifier's start, stop, and intensification speed can be achieved by controlling low-pressure hydraulic components, without the need for other auxiliary electronic components and ultra-high pressure hydraulic components.

(4) Low noise. The movement of the double-acting reciprocating intensifier is plunger-type sliding, with no rigid mechanical connections. This results in lower noise compared to ultra-high pressure pumps.

(5) Low installed power. Only two variable piston pumps are required to complete the overpressure and pressure compensation control for the double-acting reciprocating intensifier.