Harwood
Engineering Company, Inc.
Harwood
Engineering Company, Inc.
Using a non-lubricated mechanically-driven unit improves gas purity — although the problems of the pressure limitations and the speed control remain.
As long as the diaphragms do not rupture, the diaphragm units will not add to the gas impurity except for particles scuffed from soft packing sometimes used at the edge of the diaphragm, but when the diaphragms rupture, oil will be injected into the gas stream.
Speed control of the diaphragm unit is again, difficult, as it is driven by oil displacement from a mechanical pump and has a rather narrow and preferred RPM range to operate in.
When either a mechanically driven unit or a diaphragm unit goes down, the maintenance problems are usually lengthy and expensive to correct.
A cross-section view of a modern high pressure intensifier for gas service is shown in Figure 1. Note that the packings are carried on a floating button. Since the piston speed is relatively slow, the packings are not lubricated. The floating button design allows the packing to seek its own center, and minimizes alignment problems. This, in turn, minimizes the wear on the piston packings. The packing arrangement illustrated is a modern version of an old concept2, being a conventional packing backed up by a set of antiextrusion wedge rings. An expanded view of some high pressure packings is given in Figure 2. It shows the conventional packing 2 backed up by a wedge ring 3 to stop extrusion. Variants of this packing concept have been used to compress helium to 30 Kbar.
A glance at the design of the hydraulically driven unit will show that it is designed with the idea of ease of maintenance in mind. There are no check valves built into the intensifier; these are external. By keeping the check valves external, several advantages accrue:
Inherent to the intensifier approach is large displacement and high volumetric efficiency. Hence, to move a given quantity of gas discharged, there are fewer strokes and slower piston speeds than with the common mechanical design. For example: typically, a hydraulically driven gas compressor will run at 12 strokes per minute, 6 strokes on each end of the unit; this means that the stress load on the high pressure cylinders will be applied 6 times in a minute, that the inlet and discharge check valves will have to function only 6 times in a minute; considerably less service than a mechanically driven unit, running at typically 300 RPM or so, where the typical mechanically driven or diaphragm unit runs. In 24 hours of pumping, for example, the hydraulically driven pumps will have made 8,640 strokes, while the mechanically driven unit at 300 RPM will have made 43,200 strokes; significantly more, especially if one is operating in the higher pressure ranges, where fatigue problems can or might be expected. Problems from fatigue and mechanical cycling of parts are therefore minimized by the intensifier pump compressor design.
Operating at 12 strokes per minute allows the gas to be compressed at something closer to isothermal compression than the faster moving units will; this will allow the packings to run cooler, further improving packing life over what could be expected from small displacement machines.
The consequences of packing failure are less damaging with this machine than with any of the others described. All that happens is a lack of ability to generate high pressure. There is not any contamination of the gas by oil as is the case with the diaphragm or piston units. The characteristics of the hydraulic drive are such as to preclude any other damage to the intensifier or other equipment in the process.
Referring to Figure 1, the intensifier assembly, note that between the driving cylinder and the high pressure cylinder there is a yoke (#1) that isolates the high pressure piston (#2) from the driving cylinder. With this arrangement, the possibility of the driving oil contaminating the high pressure cylinder or the gas being compressed is eliminated.
Should the discharge of the hydraulically driven unit become blocked from either deliberate safety interlocking of valves or other process failure, the hydraulic drive will not allow damage to the intensifier. The driving pressure of the hydraulic drive will rise to the set pressure of the hydraulic relief valve, which is set at a safe level for both the intensifier and the drive pump.
The only possible added contaminant to the process gas being compressed is scuffed soft packings, as in the diaphragm units which have an o-ring seal. The packings are tight; they do not perpetually bleed as do the piston ring designs of many mechanical compressors. The worst thing that shreds from packings can do in the intensifier pump is make a check valve fail — an infrequent occurrence.
It is in the aspect of pressure and flow control that the hydraulically driven compressor excels. Three earlier papers (Ref. 1, 4, 5) describe the use of standard fluid power hydraulic controls to control speed, pressure and flow characteristics of the hydraulically driven compressor. Microprocessing and computer control devices have already been developed to work with fluid power controls. Consequently, the fitting of modern computer control to hydraulically driven compressors is quite routine. In fact, existing hydraulically driven equipment of the intensifier-type can be readily retro-fitted with computer based controls.
A large advantage in using the computer control is that it allows very complex programs of process control to be run with quite simple and reliable hydraulic circuits. An example is given in Figure 3.
The hydraulic schematic shown, simplified to match the space allowed for it, illustrates graphically how the complexity of control is concentrated in the computer control. Note that the hydraulic drive is deliberately designed with a simple, fixed displacement pump, an air operated flow control valve, and an ordinary electro-hydraulic valve (not servo-type valve) for maximum reliability with a minimum of maintenance. The hydraulic circuit shown is an old and proven system used in mill conditions with great success. The maintenance required is as elementary as possible, being principally the periodic change of an oil filter cartridge and scheduled oil changes. Circuits of this type have lasted for over 20 years in mill environments and worse. It is important to keep things simple, rugged, and as maintenance-free as possible.
As an example of the operation of the computer-based controls, should it be desired to control the rate of rise of the pressure, either on its own or in comparison with other variables (such as process temperature), the desired functions are programmed into the computer. The maximum driving pressure is set by valve EV, from command by the computer, which in turn receives data from the high pressure transducer, HP-TRNS; the drive transducer, TRNS-D; the feed pressure transducer, TRNS-F; and the other variables, such as temperature. Comparing these data quickly, the computer sends a net signal to valve EV, which limits the driving pressure by valve EV's action on valve RR. Such controls are useful in test work, and because of their flexibility and adaptability, we foresee their growing acceptance in fields such as hot, and to a lesser extent, cold isostatic processing, as well as in the process industries.
Should it be desired to control flow instead of pressure, the computer could as easily send its command signals to the air actuated pressure compensated flow control valve through an electrical/pneumatic signal converter. In fact, the computer control will run the hydraulically driven equipment as a function of any programmable variable or combination of variables; pressure, temperature, flow, strain, whatever is desired. It will do it quickly, and reliably, using components and interfacing equipment developed for other industries and proven in many non-high pressure applications.
