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When the pressure in an oil reservoir is depleted to the point where the well will no longer flow, some artificial means of lift must be used to bring the oil to the surface. The most generally accepted means of artificial lift is the Subsurface Sucker Rod Pump, actuated by a pumping unit and a sucker rod string. There is two basic types of Subsurface Pumps, the Tubing Pump and the Insert Pump. The Tubing Pump Barrel is threaded directly onto the bottom of the production tubing, and is installed near the bottom of the well immersed in the oil within the well casing. The plunger is run into the tubing on the sucker rod string. Generally a standing valve puller is run between the bottom of the plunger as well as the separate standing valve. This makes the removal of the standing valve optional when pulling the sucker rods and plunger. The plunger, traveling valve, and standing valve can be serviced simply by pulling the rods, but it requires removal of the tubing to service the pump barrel. An Insert Pump is run into the well on the rods as a complete unit. A seating assembly on the pump engages a seating nipple on the tubing when the pump is either run in or installed. After the well has pumped up, the additional pressure of oil in the tubing combined with the flow line pressure holds the pump firmly in place. The entire Rod Pump is removable, and therefore may be serviced merely by pulling the sucker rod string.
The old API Standard 11A defined elements which were primarily threads and critical dimensions of various parts to insure interchangeability of these parts among all pump manufacturers. The new API Standard 11AX defines elements, but in addition defines pitch lengths of parts so that the pull tube, pull rod and maximum stroke lengths of any designated API pump are all now interchangeable.
This made it possible to designate a complete API pump assembly. The Letter Designations are the portion of the complete API part number most frequently used, and we will attempt to describe the use of these and the actual well conditions calling for the various types of pumps indicated in the letter designations. The first letter is either R or T, the R standing for Rod (commonly called insert) and the T for Tubing pumps. Usually Insert pumps are the first choice because they can be serviced simply by pulling the sucker rod string. Tubing pumps require pulling the tubing in order to service the pump barrel. Since Tubing Pumps do have a larger bore and consequently greater capacity, they are used where a large volume of production is the first requirement. The second letter is either H, or W. The H stands for a heavy-wall barrel of one-piece construction. This barrel is externally threaded, permitting a pump to be spaced so that the plunger may stroke out both ends of the barrel if desired.
The heavy wall designation may be applied to either Insert or Tubing pumps. With Insert pumps, the API sizes are 1 1/16” and 1 1/4” for 2 3/8” tubing, 1 1/2” and 1 3/4” for 2 7/8” tubing and 2 1/4” for 3” tubing. In Tubing pumps, API sizes are 1 3/4” for 2 3/8” tubing, 2 1/4” for 2 7/8” tubing and 2 3/4” for 3 1/2” tubing.
The W stands for Thin Wall Barrel. The Thin Wall Barrel is 1/8” wall and is internally threaded. It is therefore, not possible to stroke the plunger out either end as it is in the smaller bore, but externally threaded heavy wall or tubing pump barrel. The Thin for 2 7/8” tubing, and 2 1/2” bore for 3 1/2” tubing. A 1 1/4” bore for 2 3/8” tubing is also frequently furnished.Wall Barrel is not used in Tubing Pumps. API sizes are 1 1/2” bore for 2 3/8” tubing, 2” bore
The third letter (which is omitted for tubing pumps as the position of the hold down for these pumps is not variable)designates the location of the seating assembly, which can be either A, B or T as illustrated below:
• A – Stationary barrel top anchor. This pump has the hold-down at the top of the barrel, so the entire barrel and standing valve of the pump extend below the pump seating nipple.
• B – Stationary barrel, bottom anchor pump. This pump has the hold-down on the bottom. The standing valve and entire pump are above the hold-down inside the production tubing.
• T – Traveling barrel bottom anchor pump. This pump has the hold-down on the bottom of a section of hollow pull tube below the plunger. The standing valve is at the top of the plunger. The entire pump is above the hold down and remains inside the production tubing.
Each of the three types has inherent advantages that require its use under certain pumping conditions. Each has at least one limitation that makes its use undesirable under other conditions. In many ways, any one of the three types may do equally well, but under difficult producing conditions selection of the proper type is of greater importance. In most cases, the experience of the operators in a given field will be the determining factor in selecting the proper type of pump for a well. The fourth letter is either C or M. A seating assembly employing seating cups is designated as C. A cup seating assembly has the lowest first cost, and is a reliable means of seating a pump (one time only). When a cup seating assembly is used, a slight interference fit between the cups and the seating nipple holds the pump down until the well pumps up.
A cup seating assembly has the disadvantage that when the pump is unseated, portions of the cups frequently will tear away from the mandrel. A strainer nipple should be used to keep this debris from entering the pump. In addition, on a rod part fishing job when a cup seating assembly is used, it is usually desirable to retrieve the pump to change cups, even though the parted rod was at a shallow depth. This concern can be addressed with the use of steel or brass friction rings; this configuration can be seated and unseated a number of times without the danger of seating assembly failure. It is referred to as “FR” in the API designation nomenclature.
A seating assembly employing a metal seal is designated M. In the case of the API Mechanical Bottom Lock seating assembly, the seal is generally brass or soft steel. Either of these can be reseated several times without replacing. A spring lock holds the pump down while the well is pumping up. In the case of the API Mechanical Top Lock seating assembly, these generally rely on precision ground stainless steel rings which serve as both the no-go and the sealing ring when they engage a second stainless ring in the top lock seating nipple in the tubing string. A spring lock holds the pump down while the well is pumping up. The Mechanical Top Lock, like the Bottom Lock, can be reseated numerous times without replacing.
The basic types of pumps and letter designation covered by this specification are as follows:
Complete pump designations include: (1) nominal tubing size, (2) basic bore diameter, (3) type of pump, including type of barrel and location and type of seating assembly, (4) barrel length, (5) plunger length, (6) length of upper extension (when used), and (7) length of lower extension (when used) as follows: