The Importance of Fit

  • By Don-Nan
  • 10 Nov, 2017

When attempting to efficiently move production fluid from reservoir to surface, one key determining factor is fluid seal within the sucker rod pump. The right combination of plunger and barrel is essential to achieving fluid seal.

The amount of space between the plunger OD and the barrel ID is the fit. Proper fit is required to create fluid seal. If the fit is too tight, there won’t be enough lubrication between the components and the pump will become damaged very quickly in operation; too loose of a fit will result in lost production as excessive fluid slips down around the plunger on every upstroke, making a continuous loop within the pump.

In order to achieve proper fit, an assessment will be performed to establish pump operating depth, casing and tubing size, fluid properties (viscosity and solid content), size and type of surface pumping unit, strokes per minute, and production goals. This assessment will indicate which barrel and plunger combination is best suited for production.

After the appropriate combination has been selected, the barrel and plunger will be further evaluated. The barrel’s internal wall must be inspected for any pits or imperfections that can cause a disruption in fluid seal. Pits typically only emerge in barrels that have seen extensive field use in corrosive downhole conditions. In order to test a barrel, an ID air micrometer is used. The micrometer measures air flow utilizing the resistance created between the inner barrel wall and the micrometer barrel plug. The air resistance will remain constant unless a pit is encountered. After the barrel has been found to be free of any weak points, the plunger may be evaluated.

ID Air Micrometer for Sucker Rod Pump Barrel

Calculation of the plunger size will depend on the length and inner diameter of the barrel combined with fluid viscosity. A thicker (more viscous) fluid will require a looser fit, whereas a thinner (less viscous) fluid will need a tighter fit. A slight amount of fluid slippage is required to keep the plunger properly lubricated. The happy medium between too little and too much clearance is fluid seal. After careful consideration of all relevant factors, the right plunger will be selected to achieve the fit needed for efficient production.

OD Micrometer for Sucker Rod Pump Plunger

Before installing a plunger into a barrel, a technician will inspect the plunger with an OD micrometer, accurate to within .001 inches. Whether assembling a pump from new or previously used components, it is standard operating procedure for Don-Nan technicians to inspect all components at the time of assembly.

Once a pump has successfully been through the design, inspection, and assembly process, it will undergo final inspection and be deployed to the field.

For assistance on pump design, please contact the   Don-Nan Tech Team   .

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