The present invention relates to a remanufacturing process for extending the working life of cylinder liners used in fluid pumps in the oil, gas, and water drilling industries. The process is carried out by performing a number of manufacturing steps during which a worn cylinder liner is reformed and improved to produce a usable liner which provides an extended period of service.
There are three basic types of fluid pumps used in oil and gas exploration drilling rigs and in water well drilling equipment. In the oil and gas drilling rigs, these are referred to as "mud pumps" and are generally of two types: (a) a duplex pump which has two reciprocating pistons which force fluid into the discharge line and (b) a triplex reciprocating pump in which three pistons act to force fluid into the discharge line. "Multiplex pumps" is a generic term which is occasionally used to include triplex pumps and those having up to six cylinders. These fluid pumps can be single acting in which fluid is discharged on alternate strokes or double acting in which each stroke discharges fluid.
The fluid pumps involved in this invention are of the horizontal reciprocating type in which one end is termed the fluid end and the other end is designated the power end. The fluid end consists of a pump housing in which are fitted a number of cylinders corresponding to the number of pistons which are operated within the pump. The power end of the pumps contains a power source and connecting rods designed for supplying reciprocal driving force.
Each pump cylinder contains a liner within which the piston operates over its power stroke. Other fluid end pump parts are rods, valve pots, seats, gaskets and piston rubbers. The liner and some of these parts are referred to as expendable elements
The cylinder liner is subject to high wear rates due to a large number of factors such as the geological formation being drilled, the solids content in the fluid, the abrasive properties of the solids, and pH of the fluid, the pump pressure, strokes per minute of the piston, and the materials used in the various pump parts.
The pump cylinder liner in a duplex pump typically has an average life of 1200 to 1500 pump hours, that is about 90 to 100 days, and the average life of the cylinder liners in the triplex pumps is about 500 to 900 hours or about 50 to 60 days of service life at a normal duty cycle. The usual maintenance practice for fluid pumps is then to replace the fluid end expendable parts at the end of their respective service lives. This practice, however, has grown relatively expensive since about 50% of the total maintenance cost is in the cylinder liners which range in price from about $400 to $1400. Thus, replacement of liners in a large sized triplex pump could run three times the larger amount. The average liner costs are; however, less since these are estimated to be $750 for each duplex liner and $650 for a triplex liner.
About 90% of the liners sold in the U.S. at the present time are known as "premium liners" in which a alloy steel outer hull has an inner surface layer of a wear resistant chromium alloy. Such hard surfaced cylinder liners are priced in the mid to upper end of the above price range. Other "nonpremium" liners are fabricated from mild steel and have only a heat hardened inside diameter, I.D., surface. The replacement of the cylinder liners at the end of each 50 to 100 days of the service cycle results in high pump maintenance costs.
The liner replacement market in the U.S. and Canada is about 110,000 to 120,000 liners per year. Statistical maintenance data has shown that the average number of liner changes per year for a triplex pump is 7.2. At this level of use 23 liners/year are required for a cost of approximately $15,000.00 while the additional maintenance charges amount to another $15,000.00. Part of the high liner replacement cost is due to the chromium content in the scrapped premium liners which is not recovered. Each of the worn liners contains about 12 to 15 pounds of chromium. The wear resistant chromium alloy is about 28% chromium with other elements being magnesium, vanadium, and molybdenum. The chromium alloy inner surface has a hardness of 62 to 65 on the Rockwell C scale and is a wear and corrosion resistant metal surface which is heat treated.
The premium pump cylinder liners with the chromium alloy inner surfaces are manufactured by the three principal processes of (a) bi-metal centrifugal casting; (b) shrinking a wear-resistant chromium iron or stainless steel alloy sleeve into the liner and (c) chrome alloy plating the inner surface of an alloy steel liner hull. Other seldom used methods of producing premium liners are those in which hard surface treatments are used for the steel hulls. These consist of coating the inner surface of the liners with a hard metal using such techniques as plasma arc spray or powdered metallurgy surfacing. These treatments produce a wear-resistant layer of about 0.020 inch in thickness with an additional thickness of an impregnation layer within the hull metal. The alloy steel liner hulls used for these manufacturing processes are usually fabricated from 4130 to 4150 steel alloy. The mild steel nonpremium liners are usually made of 0.50 percent carbon steel in which the inner surface has been heat hardened. Such nonpremium liners do not have hard metal inner surface layers.
An interesting property of the bi-metal cast and sleeve liners which are both centrifugally cast is that the chromium alloy is believed to be more dense and of a composition closer to the theoretical specifications at the cast interface. This results in the metal just inward toward the liner axis from the interface area being even more wear resistant than the inner-most I.D. surface of a new premium liner.
The pump liner is designed to fit into the cylinder which is formed in the fluid end housing of the pump and is retained in the housing by a liner cage and retainer arrangement. The reciprocating piston consists of a metal piston core having one or more flanged hubs and a piston rubber which is retained in the gap between the flanges. The outside surface of the piston rubber contacts the inner surface of the liner. Various types of wear occur on the liner and the piston body. Streaking of the liner bore and the piston rubbers is generally caused by aggregate, sand or other abrasive or foreign materials in the drilling fluid. A pitted liner indicates corrosive conditions, usually found for acidity levels below 7.2. pH. A concentration of wear on one side of the piston; or liner can occur if various working conditions such as loose cross head slides, worn pump bores, snuffing boxes and junk rings misalignment and unequal tightening of the liner rod packing have occurred. Pump failure is generally indicated by excessive clearance between the piston flange and the liner wall.
The permissible clearance prior to replacement of the fluid end expendable parts depends upon the operating pressure. In low pressure service (less than 850 psi) the total clearance may be 3/32 inch or more. At medium to high pressure (850 psi to 1600 psi) a 1/16 inch clearance is regarded as the limit. At higher operating pressures of from 1600 psi to 3200 psi a clearance of 0.04 inch is considered to indicate a "worn out" piston and/or liner. At medium to high pressures the indication of normal wear in a duplex pump is between 0.06 to 0.08 inches over nominal I.D. and in a triplex liner is 0.035 to 0.06 over nominal. The clearance dimension must be taken in the center of the liner since the wear occurs in the mid-portion rather than at the ends of the liners. The continued use of the pump beyond these clearance dimensions will result in a short service life for the piston rubbers.
As the piston fails, there is high velocity fluid slippage between the piston flange and the liner bore. With a slow failing of the piston or if a failed piston is allowed to operate in the pump, this jetting fluid will cause wash cut damage to the piston flange and liner bore and the dragging of the piston flange in the bore at 180.degree. from the wash cut. The pump efficiency and the length of service life rapidly decrease with increasing piston flanges-to-liner clearance. For example, if the clearance is 0.040 at 3000 psi, a set of replacement rubbers can be expected to last only 50 percent as long as they would on a new piston in a new liner with a clearance of 0.010 inch.
The wearing of the piston and the liner can not be eliminated due to the abrasive nature of the fluids, the corrosive properties of the same, and the high operating pressure utilized in fluid pumps in the drilling industry. The required replacement of the cylinder liners results in a continual high cost of operation for these fluid pumps.
U.S. Pat. No. 2,163,885 to MacClatchie et al describes fluid pump liners and a method of fabricating the liners in which flanges are joined to cylindrical pipe sleeves. There is no disclosure of removing the worn liners from the fluid pumps in order to remanufacture the same. Rather the worn liners are simply replaced. This patent does not therefore describe a process for remanufacturing and thereby increasing the working life of a cylinder liner.
U.S. Pat. Nos. 2,412,587 and 2,435,837 both to Larson are directed to increasing the inner diameter (I.D.) of master and wheel cylinders and then inserting a tubular sleeve in order to produce the original I.D. dimension. There is no disclosure in this patent of remanufacturing a removable liner for the brake system cylinder.
It is known from U.S. Pat. No. 1,842,441 to Yount to rebore engine block cylinders and insert cylinder sleeves or liners and to then bore the new liners to the desired size for the piston to be installed. There is no use of an extracted cylinder liner in this process.
U.S. Pat. No. 2,686,091 to Young describes various methods of making pump liners using concentric sleeves, the inner one of which is usually hardened for greater resistance to wear and for longer life. There is no mention in this patent of remanufacturing the liners by reboring or grinding.
Other U.S. Pat. Nos. in this general art are: 2,133,403 to Rubin; 2,832,653 to Wilson; 3,220,101 to Roy; 3,238,604 to Reinharz; 4,153,983 to Stockton and 4,227,292 to Kipling. The latter of these patents describes a process for remanufacturing a brake master cylinder in which the interior bore of the cylinder housing is enlarged and a sleeve pressed therein which is subsequently honed to a smooth finish. The sleeve is not honed outside of the brake cylinder and the various process steps preclude such a step. These patents do not describe remanufacturing processes wherein worn pump cylinder liners are extracted from pump housing and thereafter remanufactured in a manner which allows increasing of the working life of the cylinder liner.
It has been previously known during an earlier period when nonpremium mild steel liners were extensively used in mud pumps that the liners were sometimes removed from the pumps and bored out to the next commercially available piston size. Such metal boring and reuse of liners is not possible and to applicant's knowledge has never been practiced with the currently used premium liners because the wear-resistant metal alloy inner layer in such liners is so hard that destruction of the liner and/or the bore tool would occur.
The present invention provides a process of remanufacturing pump cylinder liners used in drilling fluid pumps in order to extend the working life of the liners and thus reduce operating and maintenance costs. New pump liners are manufactured with bores sized 3 inches to 8 inches and in stepped 1/4 inch increments, although most production is of the one-half inch increment sizes. The American Petroleum Institute (API) has a 0.010 inch tolerance on these parts. Examples are as follows:
______________________________________ Liner Size Nominal Dimension, inches ______________________________________ 3" 3.000 to 3.010 31/4" 3.250 to 3.260 31/2" 3.500 to 3.510 33/4" 3.750 to 3.760 4" 4.000 to 4.010 41/4" 4.250 to 4.260 41/2" 4.500 to 4.510 43/4" 4.750 to 4.760 5" 5.000 to 5.010 ______________________________________
The outer diameters of the liners are established by the design of the particular pump and vary between the triplex and duplex pump types. The liner cage and retainers also vary depending upon the horse power of the pump, the length of the stroke and other design features.