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Hydraulic Cylinder Applications in the Chemical Industry

Hydraulic cylinders are a type of hydraulic actuator that transmits power in a linear motion. Commonly referred to as linear actuators, they used in the chemical and petrochemical industries for lifting applications, valve actuation, and in maintenance tools. Heavy duty hydraulic cylinders are used with equipment as varied as bulldozers, portable cranes, excavators, machine lifts, forklifts, lift gates, jacks, flange spreaders, hydraulic hand pumps, industrial presses, bearing pullers, among others. In conjunction with a fluid flow control system of an oil-drilling operation or a chemical plant, they are used to actuate gate or emergency shut-off valves.

Construction

In general, hydraulic cylinders are constructed with three main components:

¨      Cylinder Barrel

¨      Piston Rod

¨      Rings and Seals

The cylinder barrel, containing hydraulic fluid, is the 2-chambered, external enclosure, similar to a forged pipe, that the piston rod slides within. The rubber o-rings or high-temperature, cast iron metal seals of the piston prevent hydraulic fluid from leaking out of the bottom chamber of the barrel. Mounting attachments or flanges connect the piston rod to the object being worked upon. As pressure is applied to the cylinder, hydraulic fluid forces the piston to move up or down, causing linear movement of the object, such as a gate valve. Hydraulic cylinders can be constructed in a tie rod or welded-body styles. The tie rod style binds the cylinder’s components together with tie rods that bolt the entire unit together.  The welded-body style welds the cylinder’s end caps to the barrel. While an advantage of the tie rods is that they can be removed for repair, tie rods do not work well at high pressures. The advantage of the welded-body style is that they are more compact and more easily customized.

Types

The major types of hydraulic cylinders are single acting, double acting, differential, rephasing, position sensing and plunger. The single-acting type is the simplest type of hydraulic cylinder. It has only one port where hydraulic fluid enters the cylinder to apply force to move the piston. However, to return the piston back to its home position, some type of external force is required. The double-acting type does not require this external force; it is constructed with both retracting and extending ports. In general, the double-acting type is used in applications where a high level of hydraulic power is need in both directions of travel. The differential type is a modification of the double-acting hydraulic cylinder. For the retracting (pulling) operation, it works similar to all cylinders. However, for the extending (pushing) operation, instead of a dual port, the hydraulic fluid is diverted back to the bottom of the cylinder to speed of the extending (pushing) operation. Differential cylinders require a special control system. The rephasing type is more of a connection style than a unique type of cylinder design. They connect cylinders in series or parallel in a synchronized configuration. The advantage of rephasing is the need for flow dividers or mechanical torque tubes is eliminated. By doing so, they are more compact and less costly. Position-sensing, hydraulic cylinders are commonly used in hot and cold rolling mills to control the gauge or thickness of the steel plate. These cylinders contain a magnetic, Hall Effect device that can sense the position of the piston and produce an electrical signal proportional to the position that is ultimately used by the hydraulic system in a closed loop control system. The plunger type does not have a piston; rather, it has a thick plunger that can only be used for pushing operations.