In the severe service pressure-reducing control valve art, two distinct technologies have been available. Both technologies utilize an annular device that is inserted into the pipeline and a plug that causes the flow in the pipeline to be radially diverted from the axial flow path through a plurality of passages to be discharged from outlet ports in the exterior surface of the device to effect the reduction in pressure, after which the fluid flow is returned to a downstream pipeline.
A pressure-reducing valve trim commonly referred to as the drilled hole cylinder also known as a single stage, multiple flow passage cartridge is in the form of an annular metal tube or sleeve, typically fabricated from 300 and 400 series stainless steel through which a plurality of radial openings extend. This device can include a plurality of annular sleeves in a concentric assembly, each sleeve having a plurality of radial openings extending a radial fluid path from the axial opening of the innermost tube to the adjacent sleeve through appropriate channels.
A principal drawback of this type of pressure reducing trim is that due to mechanical space limitations, the number of concentric sleeves is restricted to three or, at most, four. Under severe conditions in gas service, with a sufficiently great fluid pressure differential, the gas moves at supersonic velocity and one or more stages of the valves can become choked. For liquid service the choked flow occurs as the liquid expands after passing through a radial orifice, the velocity of the fluid increases with a corresponding decrease in pressure. When the pressure drops below the vapor pressure of the fluid, the material is at least partially vaporized. This condition leads to excessive noise, vibration and deterioration of the fluid flow rate. It is also known that the second and third stages in the valves of this configuration have limited pressure-reducing capability.
A second type of pressure-reducing valve is the multiple-path, multiple-stage, stacked disk drag valve, also known as a multi-labyrinth trim valve. This device operates on the basis of a high energy loss as the fluid passes through it. In this valve, the pressure reduction is achieved by passing the fluid through multiple directional changes in a tortuous path. The plurality of tortuous paths are defined by a series of annular metal disks that are braised together.
The stacked disk device is annular with a central axial opening through which the high pressure fluid flow enters. A movable plug is positioned in the central channel or opening formed by the annular disks to block the flow of fluid and cause the fluid to be diverted through the plurality of generally radial tortuous passages. The design of this valve avoids the problem of a choked flow at every stage. One particular advantage of this trim design is that the number of pressure-reducing stages is not limited to four stages and rapid increase in fluid velocity, i.e., the conversion of pressure into velocity, is not required to achieve the desirable pressure drop. The axial or longitudinal spacing of the disks, as well as the radial passage inlet ports provide a regular array on the interior surface of the central opening of the device.
The construction and methods of operation of the high energy loss control valves are well known in the art. Their structure and functioning are disclosed in U.S. Pat. Nos. 3,513,864 and 3,514,074 issued in 1970.
One limitation of the drag valve is that the metal disks are made from relatively soft materials, and when used in applications in which metallic particles or process debris are transported in the fluid, this debris can become imbedded in the inlet of the drag valve radial flow paths and as the plug is lowered to control the flow rate through the valve, extensive damage results to the valve trim. The inlet passages are sheared and the plug can jam, rendering the valve inoperable. Since the valve trim is one-piece braised assembly, repairs to damage are difficult and the replacement costs are high.
The tube or sleeve pressure-reducing device is generally the more robust in resisting damage and/or loss in operating characteristics due to debris because of its simple design. However, the pressure reduction capabilities are limited by choked flow produced by this device due to cavitation in liquid service and sonic velocity in gas applications.
In view of these and other limitations of the prior art, it is an object of the present invention to provide a pressure-reducing valve of improved construction for use in severe service conditions on high pressure fluid pipelines.
A particular object of the inventor is to provide a valve trim for automatic pressure control valves that has a high tolerance to debris in the process fluid stream and improved pressure-dissipating characteristics not found in valves of the prior art.
Another object is to provide a pressure-reducing valve that comprises a hardened metal barrier to be utilized with fluids that are carrying metal particles or other debris that cause damage to and/or would otherwise adversely effect the functioning of a drag type valve.
A further object of the invention is to provide an improved pressure-reducing valve that is easy to construct and that will withstand severe service conditions due to its rugged construction.